CHEM 3390 STRUCTURAL TRANSFORMATIONS O …home.cc.umanitoba.ca/~hultin/chem3390/labprocs/Labman2014.pdf · synthesis. These techniques include a re -exposure to distillation, recrystallization,

CHEM 3390 STRUCTURAL TRANSFORMATIONS

IN ORGANIC CHEMISTRY

LABORATORY MANUAL FALL 2014

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CONTENTS General Information - Fall 2013 ...................................................................................................................... 3

Introduction ................................................................................................................................................................................. 3 2013 Schedule of Experiments ..................................................................................................................................................... 3

Laboratory Notebooks .................................................................................................................................... 4 Writing up the Lab Notebook and the Formal Report: ................................................................................................................ 5

Lab Notebook ................................................................................................................................................................ 5 Formal Write-up ............................................................................................................................................................ 5

A Few Notes about Lab Techniques and Procedures ........................................................................................ 7 Books in the library ...................................................................................................................................................................... 7 Websites ...................................................................................................................................................................................... 7 Laboratory Safety ........................................................................................................................................................................ 7 Safety Data .................................................................................................................................................................................. 8

Using the 300 MHz NMR Spectrometer ........................................................................................................... 9 NMR Sample Preparation ............................................................................................................................................................ 9 Using the Bruker AV 300 in Automation Mode With the Sample Changer ................................................................................ 10

Putting your sample into the sample changer. ........................................................................................................... 10 Setting up and submitting your job on the spectrometer console. ............................................................................ 10 Note about choosing experiments. ............................................................................................................................. 11

Getting NMR Data from the Spectrometer ................................................................................................................................ 12 Processing and Plotting Spectra ................................................................................................................................................ 12

Note about NMR data on the Bruker spectrometer ................................................................................................... 12 Step 1: Transfer the data to your computer ............................................................................................................... 12 Step 2: Use the Spinworks program to process and plot ............................................................................................ 13

Determining an unknown molecular structure using NMR ............................................................................ 14 The Pinacol Coupling and the Pinacol Rearrangement .................................................................................. 15

Introduction ............................................................................................................................................................................... 15 The Pinacol Coupling ................................................................................................................................................... 15 The Pinacol Rearrangement ........................................................................................................................................ 16

Procedure ................................................................................................................................................................................... 16 Acid-catalyzed Pinacol rearrangement: ...................................................................................................................... 17

Preparation of Acetophenone Oxime and the Beckmann Rearrangement .................................................... 18 Acetophenone oxime from acetophenone ................................................................................................................................. 18

Procedure .................................................................................................................................................................... 18 Points for your discussion ........................................................................................................................................... 18

Beckmann Rearrangement of Acetophenone Oxime ................................................................................................................. 18 Procedure .................................................................................................................................................................... 19 Points for your discussion ........................................................................................................................................... 19

Regioselectivity and stereoselectivity in epoxidations: mCPBA and H2O2 oxidations of carvone. ................... 20 Procedures ................................................................................................................................................................................. 20

mCPBA Epoxidation of R-carvone ............................................................................................................................... 20 Epoxidation of R-carvone by alkaline hydrogen peroxide ........................................................................................... 21 Identification of epoxide products .............................................................................................................................. 21

Dr. P.G. Hultin August 2013

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Hydration Reactions of Alkenes .................................................................................................................... 22

Part I: Oxymercuration-demercuration of an alkene ................................................................................................................. 22 Procedure .................................................................................................................................................................... 22

Part II: Hydroboration-Oxidation of an alkene .......................................................................................................................... 23 Procedure .................................................................................................................................................................... 23

Asymmetric catalytic dihydroxylation of an alkene: regio- and stereoselectivity. ......................................... 24 Introduction ............................................................................................................................................................................... 24

Background.................................................................................................................................................................. 24 The reaction ................................................................................................................................................................ 24 The AD-mix reagents ................................................................................................................................................... 25

Procedure ................................................................................................................................................................................... 26 Performing the reaction .............................................................................................................................................. 27 Workup and product isolation .................................................................................................................................... 27 Product analysis .......................................................................................................................................................... 28

Points to consider in your report................................................................................................................................................ 28 Chemo- and Stereoselective Reduction Reactions of a Polyfunctional Molecule ............................................ 29

Carbonyl Reduction using NaBH4 or NaBH4/CeCl3 ..................................................................................................................... 29 Procedure .................................................................................................................................................................... 29 Points for discussion. ................................................................................................................................................... 30

Appendix: Multidimensional NMR Spectroscopy ........................................................................................... 31 COSY or Correlation Spectroscopy ............................................................................................................................................. 32 HSQC: Carbon-hydrogen connectivity ........................................................................................................................................ 33 HMBC: more carbon-hydrogen connectivity .............................................................................................................................. 34


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CHEM 3390 STRUCTURAL TRANSFORMATIONS IN ORGANIC CHEMISTRY

GENERAL INFORMATION - FALL 2014 Laboratory Teaching Assistant Navneet Chehal Lab/Office: Room 558/558A Parker

INTRODUCTION The laboratory section of CHEM 3390 provides students with experience in many of the basic experimental techniques used in organic synthesis. These techniques include a re-exposure to distillation, recrystallization, and extraction as taught in the Introductory Organic courses (CHEM 2210/2220). Students will also run their own 1H and 13C NMR and IR spectra. The laboratory runs in a more "research" oriented fashion than did the 2210/2220 labs, and students must take greater initiative in preparing and running their experiments.

There will be no experiments during the week of the midterm test.

The laboratory is a very important part of the course. It counts as 30% of your final grade and there may be questions based on chemistry from the laboratory on the final exam. You should expect to put a significant amount of time and effort into the laboratory.

2014 SCHEDULE OF EXPERIMENTS

September 10/11 Lab Check-in, NMR Training, Identification of Unknowns

September 17/18 Pinacol Coupling, Prepare Oxime

September 24/25 Pinacol Rearrangement, Isolate Oxime

October 1/2 Beckmann Rearrangement

October 8/9 Epoxidation of Carvone (Come to the lab the afternoon before to start your mCPBA reaction).

October 15/16 Oxymercuration

NO LAB IN WEEK OF OCTOBER 22/23 DUE TO MIDTERM

October 29/30 Hydroboration

November 5/6 Sharpless Dihydroxylation

November 12/13 Sharpless Dihydroxylation

November 19/20 Reduction of Carvone

November 26/27 Lab Clean-up and Checkout


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LABORATORY NOTEBOOKS All students in the laboratory program must purchase a bound laboratory notebook (available at the bookstore) for recording each experiment. The basis of all scientific work is the accurate observation of the system under study and the accurate (and readable) accounting of the experiment and observations. You are expected to record ALL your observations and conclusions in the laboratory notebook. Part of each experiment should be written up prior to coming to the laboratory. Actual observations and results are to be written into the laboratory notebook during the experiment.

• RESULTS ARE NOT TO BE WRITTEN ON ODD SCRAPS OF PAPER! • YOU MUST HAVE YOUR LAB BOOK INITIALLED BY THE DEMONSTRATOR BEFORE LEAVING THE LAB!

Note: The reading of the laboratory notebook and report provides the major criterion for marking the laboratory. Seventy-five percent of the final lab mark will be based on evaluation of the laboratory write-ups, including yield and purity of product, discussion of infrared and/or nmr spectra, and inclusion of the actual spectra. Twenty-five percent of the final mark will be assigned by the demonstrator and lecturer based on observation of the student’s performance in the lab (cooperation with other students, neatness, technique, independence and comprehension).

• ALL LABS WILL BE MARKED BY NAVNEET AND SHOULD BE HANDED IN AT ROOM 558 PARKER. • LAB NOTEBOOKS ARE USUALLY DUE FOR MARKING ONE WEEK AFTER COMPLETION OF THE EXPERIMENT. • LAB REPORTS HANDED IN LATE WILL BE PENALIZED BY 50% OF THE ASSIGNED MARK.


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WRITING UP THE LAB NOTEBOOK AND THE FORMAL REPORT: There are a number of key features of a well-kept lab notebook and a good experimental record - please observe them.

1. Date experiment was conducted 2. Title of experiment 3. Reaction scheme 4. Literature reference to product(s) and procedure(s) if any 5. Table of reagents and products. 6. Details of procedure actually used (in sufficient detail to be repeated) 7. Characteristics of the product(s) [i.e. mp, bp, colour] 8. Analytical and spectral data tabulated 9. Signature of experimenter (you) and a witness (the T.A.)

Comments: Items 1-5 must be written into the lab notebook prior to the actual laboratory session. Item 6 requires a description of any unusual equipment used, amounts of reagents (in weight or volume units as appropriate for reagents, substrates and solvents, and also moles for reagents and substrates), the sequence of experimental operations, and the method used to isolate and purify the product(s). Any colour or temperature changes should be noted.

The notebook must be bound and the pages must be numbered in consecutive order. Several blank pages for an index should be left at the beginning of the book, and entries made as you go along. ENTRIES MUST BE MADE IN INK and any errors simply crossed out with a single line so that the words remain legible. If a page will not be used, draw a large X through it. Write-ups are to be in sufficient detail to allow someone else to repeat the experiment using only your notebook.

LAB NOTEBOOK LEFT-HAND PAGE: Notes on the nature and mechanism of the reaction. Table of reagents, including molecular formula and molecular weights, with amounts to be used expressed in grams or mL and moles. Theoretical yield in grams of product(s). Literature values for physical constants (i.e. mp and/or bp) and the literature reference for such values.

RIGHT-HAND PAGE: Headed with the date of the experiment and the equation which defines the reaction being carried out. Provide a concise but detailed account of your actual experimental method (including any errors or changes). The yield of your product is to be expressed in grams, moles and % of theoretical. The physical constants of your product are also to be listed. The signature of the TA must be obtained when you have completed the experiment. Get the approval of the TA or Dr.Luong on all spectra before proceeding to write up the experiment.

Samples of all compounds prepared must be kept in a sample vial which has been labelled with the tare weight of the vial plus label, the structure of the product, physical properties of the product, and the lab notebook page reference. This is conveniently done using your initials followed by a page number; PGH-15A refers to the lab notebook of P.G. Hultin, page 15, the first product obtained on that page. All NMR and IR spectra should also be identified using this sample name. The printed spectra should be labelled with the structure of the compound and the notebook page reference.

The TA will be marking your notebooks and looking for adherence to the format description mentioned above. The formatting of the lab manual is worth about 5% of the laboratory grade.

FORMAL WRITE-UP A large proportion of the marks for each lab depend on the quality of your discussion. You may type (preferred) or neatly hand-write your reports on 8½ × 11 paper. Chemical structures may be hand drawn in appropriate places in the text, or inserted using a computer


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program (preferred).1 The formal write up will include all of items 1-5, and 7-9 from your rough work in the lab book, as well as a detailed discussion of your observations and results. The mechanism of each reaction must be explained in detail, the purpose of all reagents should be made clear, and the significance of all spectral signals in assigning the structure of the product(s) should be described. You will be judged on the accuracy and sophistication of your discussion. You should bring in outside sources of information to support your discussion when necessary, but ensure that they are properly referenced. Attach the original plots of all spectra to your reports.

The mark breakdown of how much each report is worth in the overall lab grade will be discussed by Navneet on the first day of labs.

1 Some drawing programs are freely available. See http://www.acdlabs.com/resources/freeware/chemsketch/ for example. Links to some popular programs can be found at http://openwetware.org/wiki/Chemical_structures_drawing_software. There are also some Apps for iPhone/iPad on the iTunes store. The best one is ChemDraw, but it is NOT free (only $10 though)

A scientist who does not keep good records of his or her work is wasting time. Experimental work means nothing unless it is documented and reproducible.

University researchers are in the business of producing and sharing knowledge. You may think that the compound you make is your product, but it isn’t. The real products of university research are information and knowledge. If the information isn’t clearly recorded during your experiment, you have in effect thrown out your product and wasted your time.

If you work in an industrial lab, good lab notebooks can be the difference between a billion-dollar patent and nothing at all. In the event that a patent is challenged (which is nearly inevitable if the patent claims something really useful) the lab notebooks are the key evidence brought into court. If there is the slightest doubt that you did what you said you did when you claimed you did it, your patent is likely to be thrown out. You can imagine the effect this would have on your company, and more specifically on your career!


http://www.acdlabs.com/resources/freeware/chemsketch/

http://openwetware.org/wiki/Chemical_structures_drawing_software

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A FEW NOTES ABOUT LAB TECHNIQUES AND PROCEDURES

Unfortunately many students in CHEM 3390 did not get a thorough understanding of laboratory techniques in their introductory courses. This becomes a serious issue in the 3390 laboratory, both from a safety perspective and from a practical point of view. Failing to understand the purpose of key experimental operations generally is a guarantee of a poor outcome. We can’t give you a complete handbook of laboratory operations here, but some good sources of information are provided below.

Reading these references cannot guarantee that your experiments will work, or that following the procedures in these references will guarantee that you will be safe in the laboratory. We will provide you with information, with answers to questions if you ask, and we will supervise your work to the best of our ability. Even so, success and safety ultimately depend on you. Think about the chemistry you are doing and how it works at every step along the way. If you are not sure about anything, ask!

BOOKS IN THE LIBRARY • Pavia, D.L.; Lampman, G.M.; Kriz, G.S.; Engel, R.G. Microscale and macroscale techniques in the organic laboratory. QD 261

M57 2002. • Zubrick, J.W. The Organic Chem Lab Survival Manual. A Student’s Guide to Techniques. 3rd Ed. QD 261 Z83 1992. (Newer

editions are available but this one is just fine too). • Vogel, A.I. Vogel’s Textbook of Practical Organic Chemistry. 5th Ed. QD 261 V63 1989. (This is a classic book and although it

may look a bit intimidating it contains much valuable information).

WEBSITES • Not Voodoo http://chem.rochester.edu/~nvd/. Excellent guides to the how and why of every aspect of doing an organic

experiment. We will assume that every CHEM 3390 student has read and understood the relevant material from this website BEFORE each experiment.

• MIT OpenCourseWare http://ocw.mit.edu/courses/chemistry/5-301-chemistry-laboratory-techniques-january-iap-2012/study-materials/ or YouTube at https://www.youtube.com/view_play_list?p=B208D0FA80AD438F. Videos about how to do an experiment.

• Royal Society for Chemistry Interactive Lab Primer http://www.chem-ilp.net/. Descriptions of equipment and procedures, with videos.

LABORATORY SAFETY A chemical laboratory is similar to a woodworking or machine shop. In a workshop you will find powerful tools that assist you to manipulate and alter wood or metal in desirable ways. Most of these tools can also injure you very severely, or even kill you, if not used properly.

In the laboratory many of our tools are reactive substances that permit us to alter other substances in desirable ways. Misused, these chemical tools can also cause injury or death.

In safety terms, power tools and chemical reagents are hazardous. That is, they have the potential to cause harm by their very nature. Hazards cannot be removed – what use would a blunt saw be? On the other hand risks can be removed. Risk refers to the likelihood that people will be exposed to the hazard, and safety training is about managing risk. Power saws have blade guards to keep fingers away. In the laboratory we handle chemicals in fume hoods; we wear eye protection, gloves and lab coats to reduce our risk of exposure. In both the workshop and the laboratory it is not the tool, but the situation and the way that the tool is used that creates risk. The onus is on the worker to use the tools appropriately so as to reduce or eliminate risk. As a student, you are still learning how to do this. The first thing you must do is identify the hazards associated with your tools.


http://chem.rochester.edu/%7Envd/

http://ocw.mit.edu/courses/chemistry/5-301-chemistry-laboratory-techniques-january-iap-2012/study-materials/

http://ocw.mit.edu/courses/chemistry/5-301-chemistry-laboratory-techniques-january-iap-2012/study-materials/

https://www.youtube.com/view_play_list?p=B208D0FA80AD438F

http://www.chem-ilp.net/

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SAFETY DATA The Internet provides easy access to MSDS (Material Safety Data Sheet) information. An MSDS lists the hazards associated with a particular substance. It is important to be aware of the hazards, but do not let them frighten you. A hazard isn’t dangerous in itself – it only becomes dangerous if you are exposed to it. MSDSs do not tell you how to use a chemical substance safely – they are about hazards, not about risk. Thus while an MSDS will state that diethyl ether is a highly flammable liquid with a very low flash point, it will not tell you how to transfer diethyl ether solutions to or from a heated reaction vessel without causing a fire. You must extrapolate from the information in the MSDS to the operation you are proposing to carry out. Using information about hazards, you can devise a procedure that minimizes or eliminates risk. As students, this means that if you have any doubt or concern about safe handling you must ask the TA for guidance before you begin.

The Sigma-Aldrich Company website provides MSDSs for every product they sell. This covers the majority of the substances you will encounter in CHEM 3390. http://www.sigmaaldrich.com/canada-english.html. Use the search function to find the substance you are interested in. Go to that substance’s page and choose the MSDS link on the left side of the browser window.

Fisher Scientific’s website also provides MSDSs for the chemical products in their catalogue. http://new.fishersci.com/wps/portal/CMSTATIC?pagename=msds tells you how to get the MSDS for a given product once you have found it on the online catalogue.

Canadian Centre for Occupational Health and Safety provides MSDS information at http://ccinfoweb.ccohs.ca/msds/search.html.


http://www.sigmaaldrich.com/canada-english.html

http://new.fishersci.com/wps/portal/CMSTATIC?pagename=msds

http://ccinfoweb.ccohs.ca/msds/search.html

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USING THE 300 MHZ NMR SPECTROMETER The Bruker Avance 300 NMR spectrometer is located in Room 338 Parker. This is in the nook directly opposite the elevator on the third floor.

NMR SAMPLE PREPARATION You will be acquiring and processing your own NMR spectra during the CHEM 3390 laboratory. Sample preparation is an important part of the process but it is a step that unfortunately students often fail to appreciate. A poorly-made sample is unlikely to yield high-quality NMR data.

We will use 5-mm NMR tubes. These glass tubes are machined to a high level of precision and although they are surprisingly strong, they are prone to chipping around their top edges. Treat them carefully while handling, filling, emptying and cleaning them. Although the tubes come with a coloured plastic cap, you should treat the caps as disposable items and use a fresh cap for every sample. Tube caps are very hard to clean properly and lead to contamination.

You will be given a step-by-step demonstration of sample preparation in the first lab session and complete instructions are available from the NMR lab web site. Only the key points are summarized here.

Ensure your NMR sample tube is clean and dry – no solvent residues or debris should be present. Do not use broken, chipped or cracked tubes. They may be broken by the sample changer. Do not use tubes shorter than 16.5 cm (6.5”).

1. Weigh your samples. You need at least 5 mg (0.005 g) for a 1H spectrum and at least 10 mg (0.010 g) for a 13C spectrum. We strongly recommend that you use 15-20 mg (0.015-0.020 g) of sample for all your NMR.

Since you don’t have any idea what 20 mg of sample looks like, you must weigh the sample material into a clean 1-dram vial. Do not weigh directly into the NMR tubes. Did we mention you should actually weigh the samples? No, really, use the balance to weigh your samples.

2. Dissolve the sample material in ca. 0.3 mL of CDCl3 solvent.

3. Filter the solution through a pipette filter into the NMR tube.

4. Rinse the sample vial with about 0.15 mL of CDCl3 and pass the rinse through the pipette filter into the NMR tube.

5. Repeat the rinse and filter sequence.

6. You should now have a usable NMR sample in the tube. The solvent level should be about 4.0 cm from the bottom of the tube, which will be roughly 0.5 mL. More is not better. Less is not better.

7. Put a cap on the tube and label the tube.

8. Wipe the tube with a Kimwipe to remove any debris, finger grease etc from the outside surface.


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USING THE BRUKER AV 300 IN AUTOMATION MODE WITH THE SAMPLE CHANGER Our 300 MHz spectrometer is equipped with a robotic sample changer. NMR spectra for CHEM 3390 will be run entirely in automation so once you have placed your sample(s) in the changer and set up your job(s) on the computer you can leave and return later to collect your results and sample(s).

You can access the instrument to set up jobs any weekday afternoon between 1:30 pm and 5:00 pm. Jobs submitted for automation will be run on a first-come-first-served basis and will run overnight if necessary. You can collect your spectra and samples at any time during normal working hours. DO NOT LEAVE SAMPLES IN THE CHANGER OR IN THE NMR LAB LONGER THAN NECESSARY!

PUTTING YOUR SAMPLE INTO THE SAMPLE CHANGER. 1. Ensure that your sample is properly prepared with 4 cm. sample depth. The NMR tube must not be cracked or chipped at

the top because it could be crushed by the robotic gripper if it is weakened by damage. 2. Good laboratory practice requires that all samples be properly labelled. The tubes can be labelled with a permanent marker,

but a labelling surface such as 3M Scotch “Magic Tape” works better. In order not to interfere with the sample holder, all labelling must be placed at least 13 cm. from the bottom of the tube. Any stick-on labels must be flush to the tube and not

flapping like a flag. 3. Wipe the outside of the sample tube with a Kimwipe and place the tube in one of the blue holders. The tube should slide into the holder with only moderate resistance, and should fit snugly. If the sample tube is either very tight in the holder, or is very loose, do not proceed. Remove your sample, discard that sample tube and get another. 4. Using the depth gauge, adjust the sample position in the holder to the stop at the bottom of the depth gauge (Figure 1). Note that the zero line of the gauge should be approximately midway between the bottom of the tube and the meniscus of the solvent. 5. Remove the holder and tube from the depth gauge and place it into one of the available positions in the sample changer. Note the position number you used.

SETTING UP AND SUBMITTING YOUR JOB ON THE SPECTROMETER CONSOLE. 1. The spectrometer is controlled from an ordinary PC and the software that runs it looks much like ordinary programs you are

already familiar with. When you come to the NMR room, the PC should be logged on to the Sample Changer account, and it should be running in the ICONNMR automation mode. If not, please consult one of the NMR centre staff or your TA. DO NOT LOG OUT OF WINDOWS OR SWITCH INTO ANOTHER WINDOWS ACCOUNT.

2. Check the current ICONNMR user (shown at the lower right of the automation window). If it is not “student,” click on the “Change User” icon.

a. Select userid “student” and enter the password if requested. 3. In the sample set-up grid, the term “holder” refers to an individual sample in the sample carousel. Make sure that the holder

number in the set-up grid matches the number for the sample in the carousel. If you are running multiple experiments on the same sample, they should all be set up under the same holder entry.

4. Double click on a holder line and you will be presented with a series of information fields. Fill in the following information. a. Do not change the Disk entry. b. Provide a name for the sample. You may use letters, numbers, dashes (-) and underscores (_) in the filename. Do

not use characters like / “ ‘ * ?, or a blank, as these all have special meaning to the operating system. Student

FIGURE 1: USE OF THE NMR SAMPLE DEPTH GAUGE


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samples should be in the format “PGH42a” or “pgh42a”, that is, the initials of the chemist followed by the page number in the lab notebook and a letter indicating which compound from that page if necessary.

c. You can usually leave the experiment number (No) entry at the default. d. Select the correct Solvent from the pull-down menu. e. Choose which NMR experiment you want to do from the pull-down menu. f. The Orig/Title field can be used to provide optional commentary information on the sample that will be printed on

the spectrum. Note that solvent, experiment and user are automatically printed on the spectrum and do not need to be repeated in the Orig/Title field. You can (and this is encouraged) use something like: “reduction of ketone km143b”. Click on the little pencil icon to activate this field, and click on the Set Title button when done.

g. Leave the Pri fields alone. 5. To do another experiment with the same sample, click on the Add button near the bottom of the automation window. This

will insert a new line into the list for the holder you are working on. Note that the program will automatically increase the experiment number, and it will retain the solvent information. You will only have to select what type of NMR experiment you want the instrument to run, and perhaps change the Title for the new experiment.

Most 2D experiments require a proton reference spectrum. If you select one of these 2D experiments and have not already selected PROTON as an experiment, a proton experiment will be added automatically to your list for that holder.

6. When you have set up all of your experiments on a particular sample, click on the line with the holder number (it will be highlighted in blue) and then click the Submit button near the bottom of the screen. Your spectra will now be automatically run, processed, and plotted.

7. DO NOT SHUT DOWN THE PROGRAM OR THE COMPUTER. You can leave once your sample(s) have been submitted and come back later to pick up your sample and spectra.

8. Once your sample is finished, your may remove your sample from the carousel and return the blue sample holder (spinner) to the case to the right of the spectrometer. DO NOT REMOVE SAMPLE SPINNERS FROM THE NMR LAB.

9. Samples left in the carousel overnight will be removed by NMR lab staff in the morning and placed in the fume hood beside the door.

NOTE ABOUT CHOOSING EXPERIMENTS. A number of experiments have several versions, varying only in the number of scans acquired. For example, there are two basic proton jobs called PROTON and PROTON128, with PROTON128 being used only for very dilute samples. CHEM 3390 students should only need to use the standard PROTON experiment (16 scans) to obtain 1H spectra. There are several 13C experiments to choose from: CARBON256 should be sufficient if you have 20-30 mg of compound in your sample.

You can change the number of scans of any experiment but you should consult with NMR staff or the TA before doing so. Click on the button in the Par column. Enter the desired number of scans (NS) in the box provided. Do not change the TD value. Note that the number of scans must be a multiple of the experiment’s phase cycle, and this will vary with the experiment. For most experiments, a multiple of 8 is a safe choice. Consult with the NMR staff if you are not sure. For many 2D experiments, 2 or 4 scans are also acceptable.


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GETTING NMR DATA FROM THE SPECTROMETER

PROCESSING AND PLOTTING SPECTRA The spectrometer will produce a small plot of your spectrum when it is finished acquiring the data. This plot is useful to verify that everything is ok, but it isn’t good for analyzing the data. For this, you will want to re-process and re-plot so that you can see the details of the spectrum more clearly. While it is certainly possible to do all this data manipulation on the console computer, we don’t want the console tied up with this kind of work when it should be acquiring more data for other people. Therefore, all data processing will be done on another computer.

We assume that everyone in the class has access to their own computer. If you do not, there are computers in the Chemistry Department that can be used for re-processing and plotting.

NOTE ABOUT NMR DATA ON THE BRUKER SPECTROMETER The data from each NMR sample are identified by the name you entered when you set up the NMR experiment, which should be your initials followed by the notebook page for the experiment. Thus, if you obtained a product called PGH-35, you would use this as the name for the sample. All NMR data related to this sample will be stored in a folder on the computer called PGH-35. Each NMR experiment will be in a separate sub-folder, numbered sequentially 1, 2, 3… in the order you acquired the data.

When you copy NMR data to another computer, take the entire sample folder, or at the very least, an entire experiment folder. Thus, it is ok to take the entire PGH-35 folder, or if you want to you can take PGH-35/1 or PGH-35/2, but do not just take one or two files from an experiment folder. This will not be enough information for processing the data.

STEP 1: TRANSFER THE DATA TO YOUR COMPUTER We do NOT permit USB memory sticks to be used in the NMR computer.

The preferred method for data transfer is to map the NMR disk drive to your own computer and then copy the data from one to the other. This will work as long as your computer is on campus and connected through the U. of Manitoba network. It will not work from off-campus.

Windows: Open “Computer”. Choose “Map Network Drive”. In the box marked “Folder” enter \\avance300.chem.umanitoba.ca\student (note the backslashes). A dialog will open asking for user name and password. The user name is student and you will be given the password during the first lab. When you enter this information, click “OK” and an Explorer window will open showing the nmr folder. If you look in My Computer, you will see that the nmr drive has been assigned to a drive letter on your computer. You can then drag and drop or copy and paste your data folder from the nmr drive to a location on your own drive. When you are finished, you should “disconnect” the nmr drive from My Computer.

Mac OSX: In the “Finder”, open the “Go” menu and choose “Connect to Server”. A dialog box will open asking for the name of the server. Enter smb://avance300.chem.umanitoba.ca/student (note the forward slashes; also note the prefix “smb:”) and press “Connect”. A dialog asking for a login name and password will appear. The user name is student and you will get the password at your first lab. Enter this information and click “OK”. A new icon will appear on your desktop and in your finder sidebar - that is the nmr drive. Double-click on the icon to open and use the drive. When you have copied your data, remember to disconnect the nmr drive by dragging the icon to the trash, or clicking on it and choosing “eject”.

Your data are available off-campus using FTP – “File Transfer Protocol” and both Windows and Mac computers have tools for using FTP. This approach is a bit more complicated than mapping a drive and we assume that most students will simply get their data while on campus. If you want to use FTP from off-campus and don’t know how, ask us for guidance. The FTP address for the NMR is davinci.chem.umanitoba.ca and the user name is student. You will be given the password during the first lab.


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STEP 2: USE THE SPINWORKS PROGRAM TO PROCESS AND PLOT Spinworks is a program for NMR processing that was developed by Dr. Kirk Marat, the NMR Facility Manager in our Department. It can be downloaded from: http://www.umanitoba.ca/chemistry/nmr/spinworks/index.html. Spinworks currently runs on Windows and there is also a LINUX/Mono version available (speak to Dr. Marat about this). Mac users who have one of the common Windows environments for the Mac can also run Spinworks. Popular choices are Parallels (http://www.parallels.com/products/desktop/) and VMware Fusion (https://www.vmware.com/products/fusion/) but note that these are not free.

Spinworks is not supported by the University of Manitoba IST Help Desk, and it is not installed on computers in the various public computer rooms on campus. Dr. Marat, Dr.Hultin and the lab TA can all help you with NMR processing. If you have problems do not hesitate to ask for help, and if you think there is a bug in the program or have other technical questions or difficulties, please email Dr. Marat ([email protected]).

There is not space here to describe the use of Spinworks in detail. You will be given a brief introduction to it during the first lab session, and it comes with a very good manual in PDF format.


http://www.umanitoba.ca/chemistry/nmr/spinworks/index.html

http://www.parallels.com/products/desktop/

https://www.vmware.com/products/fusion/

mailto:[email protected]

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DETERMINING AN UNKNOWN MOLECULAR STRUCTURE USING NMR You will be given a sample of an organic material. Prepare an NMR sample from this material and acquire its 1H and 13C spectra on the Bruker AV300 spectrometer. Optionally you can also obtain COSY and HSQC spectra (see descriptions of these experiments in the appendix to this lab manual). Based on these spectra, propose a structure for the unknown material. Assign every carbon atom and proton to an appropriate signal in the spectra if this is possible.

Your report for this lab will be short (2-3 pages of text). It should include the proposed structure, a listing of the peak assignments and a brief explanation of your reasons for making those assignments (i.e. – chemical shift, integral, coupling, etc). Be sure to assign every possible nucleus in the structure to the peaks identified in the spectra. You must attach copies of your NMR spectra to your report, including expanded plots of complex regions. One acceptable format for summarizing NMR assignments in tabular form is illustrated below. This tabulation is somewhat more detailed than we expect you to provide, but it illustrates the type of information that is important.

HN

O

O

NO

O1234

56

7 8 9 10

11

12

1H Chemical

Shift (δ, ppm) 1H Splitting

13C Chemical Shift (δ, ppm)

1 – – 172.1

2 4.96 ddd, 3J1 = 8.3 Hz, 3J2 = 7.2 Hz, 3J3 = 6.3 Hz 52.0

3 3.04 dd, 3J1 = 13.6 Hz, 3J2 = 6.3 Hz,

38.6 2.88 dd, 3J1 = 13.6 Hz, 3J2 = 7.2 Hz

4 – – 136.5

5

7.14 – 7.29 multiplet

128.4

6 129.4

7 126.8

8 – – 156.1

9 4.03 q, 3J = 7.2 Hz 61.2

10 1.17 t, 3J = 7.2 Hz 14.5

11 3.14 s 32.0

12 3.64 s 62.0

N-H 5.45 d, 3J = 8.3 Hz –


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THE PINACOL COUPLING AND THE PINACOL REARRANGEMENT

INTRODUCTION There are two reactions associated with the name “Pinacol”. One is a radical-mediated coupling of two ketones, to form a vicinal diol, and this is known as the Pinacol Coupling. The second reaction is the acid-catalyzed rearrangement of vicinal diols to form ketones, referred to as the Pinacol Rearrangement. In this experiment you will use both of these reactions.

THE PINACOL COUPLING The Pinacol Coupling can be carried out in several ways, but all the methods share some common features. The ketone group is reduced to a ketyl radical anion, most commonly by treatment with a reactive metal. Subsequently, two ketyl radical anions couple to form the vicinal diol product. This coupling is facilitated if the metal is divalent, in which case M2+ can act as a template to bring the two anions together. The reaction is completed by protonation during workup.

R R

O

R R

OM + M

R R

O2

O O

RR R

RH HO OH

RR R

R

Pinacol couplings can also be performed by photochemistry. This type of reaction works best with diaryl ketones like benzophenone, or electron-poor analogues of benzophenone dissolved in isopropanol solvent. In the reaction performed in this experiment, two molecules of benzophenone (1) will be reductively coupled by photochemical means. Benzophenone is colourless, and like aliphatic unsaturated ketones, it absorbs ultraviolet light with the formation of an electronically excited state (n→π*). This activated ketone (2), thought to be in the triplet state, abstracts a hydrogen atom from the solvent, isopropyl alcohol, to produce two radicals, benzhydrol (3) and hydroxyisopropyl. The hydroxyisopropyl radical transfers a hydrogen atom to neutral benzophenone, giving another benzhydrol radical and a molecule of acetone. Combination of the benzhydrol radicals produces benzopinacol, which crystallizes from the solution in dramatic fashion when the reagents are exposed to bright sunlight. Some useful background material can be found in Carey and Sundberg vol. A, sect. 13.3. This has been posted on the course web site.

O

1

hv

O

2

*

OH

HOH

3

OH

1OH O

3

3 3

HO OHbenzopinacolMW: 366.44MP: 189oC


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Obviously the Pinacol Coupling is most suitable for making symmetrical compounds. The “cross-coupling” of two different ketones usually leads to mixtures of all possible combinations. However, if both ketone groups are contained in the same molecule and constrained to remain in close proximity, intramolecular Pinacol couplings can be very effective ways to form rings.

THE PINACOL REARRANGEMENT The acid-catalyzed pinacol rearrangement was described in your second year course. It is related to other 1,2-shift reactions involving electron-deficient centres such as the Wagner-Meerwein rearrangement. The mechanism is quite simple, but you will find useful background material in Carey and Sundberg Volume B.

HO OHH

Ph

H2O

Ph Ph

OH

Ph- H2O

PhPh Ph

OH

Ph

OH

Ph

PhPh

Ph

- HO

Ph

PhPh

Ph

4

5

Again, this is a reaction that generally works best with symmetrical starting compounds because it is difficult to control which group migrates. Stereochemistry can play a big role, and different groups have differing tendencies to migrate in cationic rearrangements as well – the so-called “migratory aptitude”. In some cases very selective Pinacol rearrangements have been observed, but it is often hard to predict which possible product will be formed if there are several possibilities.

There is a variation on the Pinacol rearrangement known as the semi-Pinacol rearrangement that can be used to give this type of control. March discusses the Pinacol and semi-Pinacol rearrangements in some detail (Reaction 18-2).

PROCEDURE Photochemical Pinacol coupling of benzophenone:

In a small test tube dissolve 0.2 g of pure benzophenone in 2 mL of isopropanol, by warming on a hot water bath. Add a small amount of additional 2-propanol to keep the benzophenone in solution on cooling to room temperature, and 1 drop of glacial acetic acid (to prevent rearrangement of the product under the alkaline conditions of the glassware). Put a stopper in the tube, label the tube with your name and student number. Give the tube to the demonstrator, who will arrange to have it placed in a sunny location to receive sufficient UV rays.

When you return for the next lab session, obtain your tube from the demonstrator. Cool the tube in an ice-bath for 30 minutes to ensure complete precipitation of the product. Collect the solid product by suction filtration on a VERY SMALL Büchner or Hirsch funnel. Wash the product with a tiny portion of ice-cold isopropanol to remove coloured impurities. Suck the material as dry as possible before transferring it to a small weighed vial.

Remove about 0.05 g (weigh it!) of the product to use in the next step.


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Cover the vial with a Kimwipe, secured with a rubber band, and labeled with your name. Give the vial to the demonstrator. All the samples will be dried under high vacuum at least overnight.

You can come back later to weigh your product, calculate the yield, and determine a melting point. Obtain an infrared spectrum of the product and its 13C NMR spectrum. Comment on the purity of your product based on the melting point and spectra.

ACID-CATALYZED PINACOL REARRANGEMENT: In a test tube, place 50 mg of your benzopinacol, 0.25 mL of acetic acid, and one very small crystal of iodine (approx. 0.0005 g – the tiniest possible amount you can add!). Boil the solution until the benzopinacol crystals have completely dissolved, and then continue to heat for 5 minutes more. On cooling, the benzopinacolone separates as a stiff paste. Thin the paste with a little ethyl alcohol, and collect the product on your Hirsch funnel. Wash the product free of iodine and acetic acid using a very small amount of cold ethyl alcohol. Dry it under suction. Weigh the product, and calculate the yield. Determine the melting point, and obtain IR and 13C NMR spectra.

Note that 1H NMR spectra are not very useful to characterize these products. The IR and 13C NMR will be much more valuable guides to the identities of the compounds. Assign all the NMR signals to appropriate carbon atoms in the proposed structures. In the IR spectrum, assign the major absorption bands to the key functional groups present, and comment on differences between the spectra.


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PREPARATION OF ACETOPHENONE OXIME AND THE BECKMANN REARRANGEMENT Ketones react with primary amines to give imines (C=NR), with the loss of a molecule of water. If the primary amine is hydroxylamine (HONH2), the product is an oxime. Oximes rearrange when treated with strong electrophiles or other reagents that can convert the OH group into a good leaving group (e.g. acids, PCl5, acid chlorides or anhydrides etc.) to give amides (from acyclic ketone oximes) or lactams (from cyclic ketone oximes). This is called the Beckmann rearrangement. It can potentially form two different amides (or lactams) as illustrated below, depending on whether the group R1 or R2 migrates:

The mechanism of oxime formation is discussed in March2 and in most organic textbooks.

ACETOPHENONE OXIME FROM ACETOPHENONE PROCEDURE Hydroxylamine hydrochloride (6.25 g) is dissolved in water (35 mL) in a 125 mL Erlenmeyer flask. Dissolve sodium hydroxide pellets (2.5 g) carefully in 10 mL of water. Add this solution to the hydroxylamine solution. Acetophenone (2.5 mL) is then added. Slowly and with constant stirring, add the bare minimum amount of ethanol (approx. 30 mL) needed to obtain a clear solution.3 Heat the mixture to a gentle boil for about 10 minutes, and then allow it to cool to room temperature.

When it is cool, make sure the solution is still homogeneous. If it is, put in a stopper and label it with your name. Give it to the demonstrator to place in the refrigerator until the next lab period. If it has become cloudy or drops of oil have separated out, warm it up again and add a small amount of ethanol. Let it cool once more and check that it remains in solution at room temperature – this is very important.

When you return to the lab, collect the product on a Büchner funnel, wash it with about 2 mL of ice-cold water added drop wise using a pipette, and air dry under suction. It is essential not to use too much water, or you will wash your entire product away! Leave the product under suction until it is dry. Weigh, determine the melting point and obtain the IR, 1H and 13C NMR spectra for the dry product.

POINTS FOR YOUR DISCUSSION Why was it necessary to add a specific amount of NaOH to the hydroxylamine hydrochloride solution when setting up the reaction? Which isomer of the oxime did you obtain?4 Comment on the purity of your product by comparing the melting point and spectroscopic data with the literature values.

BECKMANN REARRANGEMENT OF ACETOPHENONE OXIME The Beckmann Rearrangement is a very old process, having been first reported in 1886.5 It is industrially important as a method for preparing caprolactam from cyclohexanone, which is crucial in the production of nylon-6. The Beckmann rearrangement often

2 Smith, M.B. March’s Advanced Organic Chemistry: Reactions Mechanisms and Structure, 6th Ed., reaction 18-17 pp 1613-1616.

3 It is important that you keep the volume to a minimum. You are trying to create a saturated solution so that as the product forms it will crystallize out spontaneously. The product is still somewhat soluble in ethanol, so too much ethanol will significantly reduce your yield.

4 It may be difficult to prove which isomer you have simply by looking at the spectra, but since these compounds have been prepared previously you can probably find authentic spectra for the two isomeric oximes for comparison. Be sure to cite any literature references you use in your report.

5 Beckmann, E. Chem. Ber. 1886, 19, 988-993; See Kürti, L.; Czakó, B. Strategic Applications of Named Reactions in Organic Synthesis, pp 50-51.

R1 R2

O NH2OH

R1 R2

NOH

electrophileR1 N

HR2

O+ R1 N

HR2

O


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requires strongly acidic conditions and high temperatures may also be necessary. Not surprisingly, such conditions frequently form substantial amounts of by-products.

We will be using a recent procedure for a catalytic Beckmann rearrangement instead. Deng and co-workers at Shanghai have reported that sub-stoichiometric amounts of p-toluenesulfonyl chloride in refluxing acetonitrile are highly effective conditions for the Beckmann rearrangement of various simple oximes.6 This process is simpler, quicker and cleaner than the previous reaction.

PROCEDURE Set up a small sand or oil bath7 on your heating/stirring plate and start it heating up to about 100 ºC. It can take a while to get to the desired temperature so get this going immediately.

Determine an appropriate TLC solvent and visualization method to monitor your reaction. You will need small samples of your ketoxime and acetophenone and perhaps p-toluenesulfonyl chloride (TsCl) dissolved in acetonitrile to use as reference standards. This can be done while the reaction is heating but do not delay – it may take longer than you think to find a good system.

In a 25 mL round-bottom flask, place ketoxime (2 mmol), 5-10 mol % of TsCl and 4 mL of anhydrous acetonitrile. Add a magnetic stirring bar and fit the flask with a condenser topped with a CaSO4 drying tube. Clamp the reaction flask in your hot sand bath and allow it to reflux vigorously until TLC indicates completion of the reaction (approximately 1 hour). Cool the reaction mixture and add saturated aqueous sodium hydrogen carbonate (5 mL) to quench the reaction. Continue stirring at room temperature for about 15 minutes to allow the TsCl to hydrolyze. Pour the mixture into a small separatory funnel and extract with generous portions of ethyl acetate. Wash the combined organic extracts with brine, dry with Na2SO4, and concentrate the organic solution on the rotary evaporator.

The resulting crude product can be purified by column chromatography on silica gel but we will probably not have time for this purification. Note whether your product is a solid or a liquid, its colour and what mass of product you have obtained. Obtain the 1H and 13C NMR spectra of the product, and also the IR spectrum if possible.

POINTS FOR YOUR DISCUSSION Based on the spectra, what was the primary product of your reaction? Were there other products formed as well and if so, what are they? Did your reaction reach completion?

From the spectroscopic data you obtained for your oxime you should be able to identify which stereoisomer you had. Given this information, is the result of this catalytic rearrangement consistent with the generally-accepted mechanism of the conventional Beckmann process?

How does the catalytic process work? Deng et al. suggest a possible catalytic cycle in Scheme 1 of their paper based on mechanisms that have been proposed for other catalytic Beckmann rearrangements.8 What do you think of Deng’s suggested mechanism? Nucleophiles do not usually attack tosylates at sulfur, although this kind of attack is not unknown. Can you find literature to support the penta-coordinate sulfur intermediate as shown in Deng et al.’s Scheme 1?

6 Pi, H.-J.; Dong, J.-D.; An, N.; Du, W.; Deng, W.-P. Tetrahedron 2009, 65, 7790–7793.

7 You can use either a sand bath or an oil bath depending on what is available. Sand baths are less messy than oil baths so I generally prefer them.

8 a) Zhu, M.; Cha, C.; Deng, W.-P.; Shi, X.-X. Tetrahedron Lett. 2006, 47, 4861–4863; b) Furuya, Y.; Ishihara, K.; Yamamoto, H. J. Am. Chem. Soc. 2005, 127, 11240-11241.


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REGIOSELECTIVITY AND STEREOSELECTIVITY IN EPOXIDATIONS: MCPBA AND H2O2 OXIDATIONS

OF CARVONE. We have seen that peroxycarboxylic acids such as mCPBA are electrophilic reagents that preferentially react with relatively electron-rich alkenes. On the other hand, electron-poor alkenes such as α,β-unsaturated ketones react only slowly with peroxyacids. The electrophilic nature of enones means that they can be epoxidized by alkaline hydrogen peroxide, which is a nucleophilic oxidizing agent.

In this experiment, based on a procedure by Mak et al.,9 we will explore the regioselectivity of the epoxidation of R-carvone by each of these reagents. We will also investigate the diastereoselectivity of the epoxidations.

O

R-carvone

O

O

OO

O

O

OOH

H H

HH

+

potentially four monoepoxides

O

H

O

O

potentially four diepoxides

[O]

PROCEDURES Each student will perform both the mCPBA and alkaline H2O2 epoxidation reactions. Note that the mCPBA reaction requires about 14 hours at 0 ºC to reach completion. Everyone must stop by the laboratory in the afternoon of the day prior to the lab session to get the reaction started. This should only take about 30 minutes. During the regular lab session, everyone should work up and isolate the products of the mCPBA reaction. You must also perform the peroxide reaction and isolate the products during the lab session, but this is not difficult or time consuming.

MCPBA EPOXIDATION OF R-CARVONE10

Caution: Wear gloves and safety glasses when performing this experiment. (R)-(-)-Carvone is a mildly toxic volatile organic liquid. Handle this compound in a fume hood and avoid contact with skin. m-Chloroperoxybenzoic acid (mCPBA) is shock sensitive. Commercial mCPBA contains approx. 25% of water as a stabilizer. This substance should be stored in a refrigerator and never be dried or ground.

9 Mak, K.K.W.; Lai, Y. M.; Siu, Y.-H. J. Chem. Ed., 2006, 83, 1058-1061.

10 a) Baldwin, J. E.; Broline, B. M. J. Am. Chem. Soc. 1982, 104, 2857-2865; b) Smitt, O.; Högberg, H.-E. Tetrahedron 2002, 58, 7691-7700.


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Dissolve (R)-(-)-carvone (0.5 g, 3.33 mmol) in CH2Cl2 (8 mL) in a 50 mL Erlenmeyer flask and cool the mixture to 0 ºC. Weigh out 1.1 equivalents of mCPBA11 into a 5-dram snap-cap vial, and dissolve this in CH2Cl2 (4 mL). Add this solution drop wise to the carvone solution over a period of 10 min while agitating gently to mix the reagent in. Stopper the flask, label it with your name and sample number and let the reaction mixture sit at 0°C for 13 to 16 hours.

When you return to the laboratory, check that the reaction is completed by tlc (you can develop the plate using 10:1 hexanes:ethyl acetate and visualize with iodine). Add 1 mL of 10% aqueous sodium sulfite solution and stir the mixture for 1 - 2 minutes. Filter the mixture, washing the solid residue with several small portions of CH2Cl2 and combine the washes with the filtrate. Wash the combined organic liquid successively with 10% Na2CO3 solution (3 x 15 mL) and saturated NaCl solution (15 mL). Dry the organic solution with anhydrous MgSO4 and filter the mixture. Concentrate the organic solution by rotary evaporation.

EPOXIDATION OF R-CARVONE BY ALKALINE HYDROGEN PEROXIDE12

CAUTION: In addition to the warnings given above, note that 30% H2O2 is corrosive and can cause painful blistering to skin. The 6 N NaOH solution is also corrosive. Handle these compounds very carefully and avoid contacting them with skin.

Dissolve (R)-(-)-carvone (0.72 g, 4.8 mmol) in 8 ml of methanol. Cool the mixture to 0°C and add 1.75 mL of 30% H2O2. With stirring, add 1 mL of 6 N aq. NaOH solution over a period of 1-2 minutes. Stir the mixture at 0°C for 15 minutes and then at room temperature for 20 minutes.

Dilute the mixture in 20 mL of CH2Cl2 and wash the organic solution with water (10 mL x 2). Wash the organic solution with saturated NaCl solution and then dry it with anhydrous MgSO4. Filter the mixture and concentrate the organic solution by rotary evaporation.

IDENTIFICATION OF EPOXIDE PRODUCTS • Obtain the 1H and 13C NMR spectra of your products. Assign as many of the signals as possible in the spectra and use these

spectra to verify the regioselectivity of the epoxidations. You may find it helpful to use COSY and HSQC to assign the NMR signals.

• Did each reaction produce only a single regioisomer? Were the reactions diastereoselective? If other isomers are present, determine the product ratios and identify what these other products are.

• Compare the spectra of the products produced by the two different methods and note similarities and differences. • You may also want to re-examine the purified products by tlc if the NMR indicates that more than one compound is present.

11 IMPORTANT: The mCPBA is only about 75% by weight, with the balance consisting of water and 3-chlorobenzoic acid. Check the actual value with the lab demonstrator. You must take this into account when calculating how much of the mCPBA to weigh out.

12 a) Muralidharan, K. R.; de Lera, A. R.; Isaeff, S. D.; Norman, A. W.; Okamura, W. H. J. Org. Chem. 1993, 58, 1895-1899; b) McChesney, J. D.; Thompson, T. N. J. Org. Chem. 1985, 50, 3473-3481.


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HYDRATION REACTIONS OF ALKENES The acid-catalyzed hydration of alkenes to yield alcohols is one of the basic reactions of organic chemistry. In the laboratory, however, it is not convenient to use the industrial method (dilute sulfuric acid). Also, several rearrangement pathways can interfere unless the conditions are carefully controlled.

Alkenes may be indirectly hydrated using mercuric acetate as a Lewis acid to activate the double bond. Subsequent reduction of the carbon-mercury bond results in formal addition of the elements of water across the alkene. This reaction is rapid, clean, and usually proceeds in high yield. It is applicable to virtually any alkene. One problem is the toxicity of mercury salts, and careful handling is necessary. The high cost of mercuric acetate is also a drawback. However, this cost can be partially offset by recovery of the mercury produced in the reaction. Indirect hydration of an alkene can also be achieved hydroboration followed by oxidation. The hydroboration-oxidation method of hydration has several advantages. The reaction occurs rapidly and cleanly (providing there are no other reactive functional groups). Additionally, the reaction occurs without rearrangement and the product results from anti-Markovnikov hydration.

This is a two-week experiment in which you will perform both an oxymercuration-reduction and a hydroboration-oxidation of the same alkene. You will be able to directly compare the products to see that the two methods provide complementary regioselectivity. You will write up the two-week experiment as a single lab report summarizing both reactions. This year everyone will work with the same alkene, 1-methylcyclohexene.

PART I: OXYMERCURATION-DEMERCURATION OF 1-METHYLCYCLOHEXENE PROCEDURE You will be measuring the liquid alkenes by volume and not by weight, using syringes. These syringes can only measure to the nearest 10 µL so you may need to adjust your stoichiometry slightly to take account of the round-off.

The oxymercuration-demercuration procedure is based on Brown, H.C.; Geoghegan, P.J. Jr. J. Org. Chem. 1970, 35, 1844. A similar procedure is described in Jerkunica, J.M.; Traylor T.G. Organic Syntheses, 1973, 53, 94.

Mercuric acetate (1.9 mmol) is placed in a 25 mL Erlenmeyer flask. To this is added 2.5 mL of water and a magnetic stirring bar. Stir until the solid dissolves. Next, with continued stirring add 2.5 mL of tetrahydrofuran. The rapid formation of a bright yellow, finely divided precipitate is observed. This precipitate is probably mercuric oxide.

Now add 2 mmol of the alkene. (Note: a very slight excess of the alkene is used to ensure that all of the mercuric acetate reacts.) Continue stirring until the yellow precipitate disappears and the solution becomes clear and colourless.

During this time, prepare a solution of sodium borohydride by dissolving 0.375 g of sodium borohydride in 17.5 mL of 3 M sodium hydroxide. Four students can share the same solution. A 2.0 mL aliquot of this solution is then slowly added to the reaction in the Erlenmeyer flask. Care is needed, because the reaction is exothermic. The temperature of the reduction should be maintained at about 25 °C by cooling the Erlenmeyer flask in a pan of cold water. Immediately after the addition, a precipitate of mercury forms, turning the solution grey. After a few minutes, the grey solution clears with the formation of a ball of metallic mercury. The reaction is allowed to stir for 1 hr to ensure completion of the reaction.

At this point, solid NaCl is added until the aqueous layer is saturated. Add ether (ca. 15 mL) and pour the contents into a 125 mL separatory funnel. Be certain to save the mercury! Separate the phases, and re-extract the aqueous layer once more with ether. The combined organic layers are then dried over anhydrous magnesium sulfate. The solution is then filtered and the solvent removed on the rotary evaporator. The liquid remaining is the product alcohol (plus excess alkene if too much is added). If the liquid is cloudy or contains drops of water, re-dissolve it in ether and dry it again properly.


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Weigh your product, and calculate the yield. Obtain an IR and the 1H and 13C NMR spectra. On the basis of the structural assignment, comment on the regioselectivity of this reaction.

PART II: HYDROBORATION-OXIDATION OF AN ALKENE In this reaction you will be using BH3·S(CH3)2, the dimethylsulfide complex of borane. This is one of the most stable and most useful borane complexes. However, it is very sensitive to water. All glassware must be dry.

WARNING: Borane-dimethylsulfide is POISONOUS and VERY SMELLY. Use it only in the fume hoods. The alkenes are also quite smelly. Only after the oxidation step is complete should you remove solutions from the fume hood.

PROCEDURE The hydroboration procedure is loosely based on Brown, H.C.; Zweifel, G. J. Am. Chem. Soc. 1959, 81, 247.

Set up and run your reaction in the fume hood.

Put a DRY magnetic stirring bar into a DRY 25 mL round bottom flask and seal the flask using a rubber septum. Make sure that the septum is tight and if necessary wrap an elastic band around it. Put 2.0 mmoles of 1-methylcyclohexene and 1.5 mL of DRY tetrahydrofuran into the flask using syringes. Note the comments above about the measuring precision of the syringes.

Cool the flask in an ice bath. Take the flask and ice bath to the fume hood where the demonstrator will dispense 0.5 mL of 2 M BH3·S(CH3)2 in diethyl ether into your reaction using a syringe. Return the apparatus to your hood and start the magnetic stirring. Allow the solution to warm to room temperature over about 20 min. and then stir at room temperature until TLC analysis indicates that the hydroboration is complete.

Still in the fume hood, cool the flask again in the ice bath. Remove the septum and add 0.5 mL of methanol DROPWISE WITH STIRRING to destroy any excess BH3·S(CH3)2. Add 0.5 mL of 10% NaOH in water, and then DROPWISE, add 0.5 mL of 30% H2O2 in water (careful: exothermic reaction). Let the solution warm to room temperature while stirring for 1 hour to complete the oxidation.

Wash the solution into a separatory funnel with dichloromethane and water (any sticky precipitate will dissolve in water). Separate the organic layer and extract the water at least once more with dichloromethane. Combine the organic extracts, and wash with brine. Dry the organic layer (MgSO4) and evaporate to dryness. The residue will be the desired alcohol plus any unreacted alkene (and water if you did not dry the solution properly - if it is wet, go back and dry it!).

Sometimes this workup procedure does not remove all the boron-containing materials, which can form a complex with your product alcohol. In order to ensure that all the boron by-products have been removed, you can pass your crude product through a plug of silica. Pack a fragment of Kimwipe into the bottom of a disposable pipette, and add about 2 cm of silica gel. Re-dissolve your crude product in a small amount of ether, and apply it to the silica gel. Wash the product through the silica into a clean flask with a little ether and then evaporate the ether.

Weigh and calculate the yield. Obtain both 1H and 13C NMR spectra and the IR spectrum. Make your structural assignments and comment on the regio- and stereochemistry of this hydration reaction.


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ASYMMETRIC CATALYTIC DIHYDROXYLATION OF AN ALKENE: REGIO- AND STEREOSELECTIVITY.

INTRODUCTION BACKGROUND As you have learned from your previous coursework, the oxidation of an alkene to a vicinal diol can be accomplished using osmium tetroxide, OsO4. Because this reagent is expensive, difficult to handle and very toxic, methods in which it is used in catalytic amounts have been developed. Since only very small amounts of osmium are present in such reactions, the process is less hazardous than a stoichiometric osmylation, and cheaper as well.

The rate-enhancing effect of amines on osmylation reactions has been known for some time. The biggest breakthrough in catalytic dihydroxylation reactions was the discovery that chiral amine additives not only make the reactions proceed faster, but enable osmium tetroxide to discriminate between enantiotopic faces of prochiral alkenes. The 2001 Nobel Prize to Prof. K. Barry Sharpless (along with Prof. R. Noyori and Prof. W.S. Knowles) in part recognized his role in developing the enantioselective dihydroxylation reaction as a practical synthetic tool.

In this experiment, we will use Sharpless’ catalytic dihydroxylation protocol to perform a regio- and diastereoselective dihydroxylation of the terpene alcohol, linalool. Note that although the chiral catalysts are usually used to achieve enantioselectivity in the dihydroxylation of prochiral alkenes, in our case we are using a chiral alkene substrate. Our reactions will therefore be diastereoselective. We will look at whether the presence of a stereogenic centre in the substrate influences the action of the chiral catalyst.

THE REACTION This experiment is based on a reaction published in 1993 by Vidari et al. (Tetrahedron Lett., 1993, 34(43), 6925-6928) which in turn uses a procedure by Sharpless et al. (J. Org. Chem., 1992, 57(10), 2768-2771).

Vidari reported that the dihydroxylation reaction selectively took place at the trisubstituted alkene, while only trace amounts of diol formed from the terminal alkene could be detected. He also observed that the two AD-mix reagents (see below) gave complementary stereochemical outcomes, but that the degree of diastereoselectivity was different. The goal of this experiment is to verify Vidari’s results and to offer an explanation for them based on your knowledge of the structure of the catalyst and the conformational properties of linalool.

During the dihydroxylation reaction, Os(VIII) is reduced to Os(VI). In order to use OsO4 in a catalytic amount, a stoichiometric oxidant is necessary to recycle the Os(VI) back to Os(VIII). Water is also necessary to hydrolyze the intermediate osmate ester, separating the diol product from the catalyst. Details of the reaction mechanism can be found in your textbook, but a general catalytic cycle is shown here.

HO HO

OH

OH

HO

OH

OH

+

"AD-mix-α"or

"AD-mix-β"

tBuOH/H2O0 oC


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A review article covering the asymmetric catalytic dihydroxylation (Noe, M.C.; Letavic, M.A.; Snow, S.L.; McCombie, S.W. Org. Reactions 2005, 66, 109-626) presents more sophisticated discussion of the mechanism, along with literally hundreds of examples. Those students interested in a deeper understanding may wish to consult this article but it is not required for this experiment.

THE AD-MIX REAGENTS Sharpless and his co-workers discovered that it was particularly convenient for small-scale dihydroxylation reactions to prepare a mixture of the catalyst and the stoichiometric oxidant that could be stored and easily weighed out with minimal handling. These mixtures were designated AD-mix-α and AD-mix-β, and are now sold by Sigma-Aldrich. The chiral ligand in each of the two mixtures is different. In the case of AD-mix-α, the ligand is DHQ2-PHAL, while in AD-mix-β it is DHQD2-PHAL. Inspection of the ligand structures reveals that they are isomers and nearly but not exactly mirror images of one another. These ligands are derived from the natural cinchona alkaloids quinidine and quinine. In the AD-mixes, the stoichiometric oxidant is potassium ferricyanide, K3Fe(CN)6. Osmium is present as potassium osmate dihydrate, K2[OsO2(OH)4], and K2CO3 is also added to keep the reaction medium slightly alkaline for optimal hydrolysis of the osmate intermediate.

The recipes for 1 kg of the AD-mixes are shown below:

Component Mass (g) MW Moles K3Fe(CN)6 699.6 329.24 2.12 K2CO3 293.9 138.21 2.13 K2[OsO2(OH)4] 1.04 368.45 0.0028 ligand 5.52 778.98 0.0071

Thus, each gram of AD-mix contains 2.12 mmoles of potassium ferricyanide, 0.0028 mmoles of potassium osmate, and 0.0071 mmoles of ligand.

R

R

OsO4 + Ligand

OOs O

R

R

O

OL

O OsOO

O

L

R

RHO

OH

Oxidant + H2O

Reduced oxidant

NNO

N

MeO

N

H

Et

O

N

OMe

NEt

DHQD2-PHALin AD-mix-β

N NO

N

OMe

N

HO

N

MeO

N EtEt

DHQ2-PHALin AD-mix-α

OHN

HO

N

quinine

OHN

HO

N

quinidine


P a g e | 26

Sharpless and his group studied the selectivity of this catalytic osmylation in great detail, and came up with a simple model to help chemists predict the stereoselectivity to be expected using the AD-mix reagents to oxidize trisubstituted and E-1,2-disubstituted alkenes.

This model does not attempt to represent the actual transition state for the reaction, but it does capture the essential steric requirements for attack on each face. Note that the two corners of the plane marked with 3-dimensional wedges are sterically hindered; the rear one will accommodate small alkyl groups like methyl or ethyl, while the front one can only accept H atoms. The other two positions are more or less unhindered although one accepts larger groups than the other.

PROCEDURE

SAFETY WARNING

Osmium salts are extremely toxic. Although there is very little osmium in the AD-mix powder, you must wear gloves at all times when handling AD-mix, or any mixture or solution containing osmium. If AD-mix powder is spilled, it must be cleaned up immediately. Do not allow the powder to become airborne where it can be breathed. You can work with osmium safely but you must be aware of its hazards and you must keep your work areas clean.

Do NOT flush osmium-containing wastes down the sinks!

In addition, the AD-mix powder is a strong oxidant. This means that it could potentially cause fires if it is carelessly mixed with organic materials. It must not be exposed to acid as it liberates toxic gas (HCN) when acidified.


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PERFORMING THE REACTION You will work in pairs for this experiment, with one partner using AD-mix-α and the other partner using AD-mix-β. Each pair will submit a single unified report presenting the data from both experiments.

Use a 25 mL Erlenmeyer flask equipped with a magnetic stirring bar and a stopper. Add 5 mL of tBuOH and 5 mL of water, followed by 1.4 g of either AD-mix-α or AD-mix-β. Wrap the flask in aluminum foil to exclude light, which degrades the catalyst. Allow the mixture to stir until all the solids dissolve; you should see two clear liquid phases13 and the lower (aqueous) phase should be yellow. It may take up to 30 minutes for all the AD-mix to dissolve.

While stirring, cool the mixture on an ice-bath to 0 ºC and then add 1 mmole of linalool. It is possible that some of the dissolved salts will precipitate as the mixture cools, but this does not harm the reaction. Stopper the flask and turn the stirring up to a vigorous rate (take care that the stir bar does not “dance” around but spins smoothly). Put a clear label on your flask to identify it from other samples.

The reaction will require at least 24 hours to achieve complete conversion. We will move the experiments into a fridge or cold room at the end of the lab period. NB: Vigorous stirring must be maintained throughout the reaction!

WORKUP AND PRODUCT ISOLATION Before you begin working up the reaction, obtain a tlc of the reaction mixture to verify the consumption of starting material. If the tlc is satisfactory, you can proceed to workup. Note that UV visualization does not work for these compounds. Also, while linalool shows up strongly by I2 staining, the product is quite faint by this method. Dipping or spraying the tlc plate in 20-30% aqueous H2SO4 and then heating will show up both starting material and product as dark spots.

While continuing to stir at 0 ºC, add 1.5 g of solid sodium sulfite. The cold-bath can then be removed and the mixture should be stirred at room temperature for about 30 minutes.

Filter the mixture into a 60 mL separatory funnel. Rinse the reaction flask with approximately 10 mL of ethyl acetate, and pour this through the filter into the funnel as well. Separate the layers, keeping the organic layer and returning the aqueous layer to the separatory funnel. Extract the aqueous layer 3 times with ca. 5 mL portions of ethyl acetate, adding the organic layers to the previous organic phase each time. When you are finished, you should have about 30 mL of combined organic extracts.

Put the combined organic extracts back into the separatory funnel, and wash with 5 mL of brine. Dry the organic phase with MgSO4, filter into a pre-weighed round-bottom flask, and evaporate the solvents on the rotary evaporator. The crude residue will be a mixture of the product(s), the ligand, and possibly any unreacted starting material. Obtain the mass of this crude residue.

We will do a rough purification of our products using a variation on chromatography known as solid-phase extraction. Prepare a short silica gel column in the barrel of a 10 mL polypropylene syringe as shown in Figure 2.

13 The solid AD-mixes are orange, but when they are dissolved the solutions are yellow. In practice, AD-mix-β may not fully dissolve. If the mixture is still an orange suspension after 30 minutes of stirring, proceed with the reaction anyhow.

cotton plug

half-full with silica gel

18-gauge needledisposable polypropylene syringe

FIGURE 2: PREPARATION OF A SOLID-PHASE EXTRACTION COLUMN

Approximate appearance of tlc eluted with 1:1 EtOAc:hexanes

S.M. Rxn


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Put six 18 × 150 mm test tubes into a test tube rack ready for use, and obtain a rubber septum that will fit into the test tubes.

Moisten the silica with 1:4 EtOAc/hexanes, and apply the crude product to the top of the silica. Put the “column” needle through your septum, and also connect vacuum via a needle through the septum. Put the septum onto the first tube. The basic set-up is shown in Figure 3. Add 5 mL of 1:4 EtOAc/hexanes to the top of the column and elute this into the first test tube, using gentle vacuum to obtain a steady flow rate. Collect a second fraction of 5 mL of 1:4, then switch to pure EtOAc and elute 4 more times with 5 mL portions of solvent, collecting each eluent in a new test tube. Analyze all the fractions by tlc – don’t discard anything! You should see any unreacted linalool in the first fraction and maybe the second one too, while the product should appear in one or more of the later fractions.

Combine all clean product-containing fractions in a pre-weighed round-bottom flask, and remove the solvent on the rotary evaporator. Obtain the mass of the product after all solvents have been removed and calculate your yield. Note that a skilled synthetic chemist would expect to obtain 80% yield or better from this reaction.

PRODUCT ANALYSIS Put a small amount (1-2 mg) of purified sample into an autosampler vial (obtained from the TA) and dilute it with 1 mL of CH2Cl2. Label your vial with your initials and the notebook page number (just as you do for NMR samples). Give the vial to the TA for analysis by gas chromatography. The GC will give you information about the ratio of the diastereomeric products, and also whether there is unreacted starting material present. The GC will be run overnight and the data will be available from your TA.

Obtain the 1H and 13C and COSY NMR spectra of your product. Assign as many as possible of the signals in the NMR spectra to the appropriate nuclei in the product structures; note that you probably will have two diastereomers present, which will mean that you will have two sets of signals. If there is unreacted starting material or other contamination also present, the spectrum can be very difficult to interpret!

Using the integrations of appropriate peaks in the 1H NMR spectrum, estimate the ratio of the two diastereomers.

POINTS TO CONSIDER IN YOUR REPORT Aside from general observations relating to the process of the reaction, the yield etc., you should address the following questions in detail:

• Is the major stereoisomer you obtained with each of the AD-mixes consistent with the predictions of Sharpless’ model? • Does the tertiary hydroxyl group in linalool exert any influence on the stereochemistry of the dihydroxylation reaction? • Why would the ratio of diastereomers obtained be different in reactions of linalool using AD-mix-α versus AD-mix-β? • Does the ratio of diastereomers measured by NMR match the value obtained using GC? • Explain the regioselectivity of the dihydroxylation. Why does only one alkene group react? • Can you identify the stereochemistry of each diastereomer from the NMR data? If not, why not? • Is there any evidence in either the GC or the NMR spectra of other byproducts? If so, what are they? • Based on the amount of potassium osmate you used in your reaction, and the overall conversion of the process, estimate the

“turnover number” for the catalyst. Is this an efficient process?

rubber septum

18 x

150 mm

test tube

vacuum

FIGURE 3: SET-UP FOR QUICK SOLID-PHASE EXTRACTION.


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CHEMO- AND STEREOSELECTIVE REDUCTION REACTIONS OF A POLYFUNCTIONAL MOLECULE The theme of CHEM 3390 has been selectivity. The ultimate test of selectivity is the ability to carry out one specific reaction on a given functional group in the presence of other groups that can potentially undergo a similar reaction. The terpene R-carvone has three reducible functional groups: the ketone, the conjugated alkene and the isolated alkene. This experiment will demonstrate how these groups can be altered at will using carefully designed and executed reactions. Note that we will only do the borohydride reduction portion because further development of the experiment remains to be done.

O

R-carvone

O

O

OH

+

O

OH

+

CARBONYL REDUCTION USING NABH4 OR NABH4/CECL3 Borohydride reagents act as nucleophilic hydride sources, and react with electrophilic centres in organic molecules. α,β-Unsaturated ketones have two possible electrophilic atoms, the carbonyl carbon and the β-carbon, due to the conjugation of the carbonyl and C=C bonds. As we have learned, hard nucleophiles tend to attack the carbonyl carbon, while soft nucleophiles favour attack at the β-carbon. This difference provides an opportunity for chemoselectivity.

Hard hydride agents such as LiAlH4 can give very good to excellent selectivity for 1,2-addition to simple enones,14 but LiAlH4 is not particularly chemoselective – that is, it reduces many other functional groups as well. Moreover, it presents hazards due to its pyrophoric nature. NaBH4 is a much safer hydride reagent but it is often not very selective for 1,2-addition over 1,4-addition to enones.14 However, the Luche Reduction15 provides a way of enhancing the 1,2-selectivity of enone reductions by NaBH4 that is operationally simple and relatively inexpensive. You can read about the Luche procedure in your textbook, but briefly it involves using CeCl3•7H2O in combination with NaBH4 in methanolic solution. As Gemal and Luche showed in their 1981 paper,15b the cerium(III) catalyzes the methanolysis of BH4

− to form methoxyborohydride species, which are markedly harder than the parent borohydride.

PROCEDURE This experiment is based on one devised by Prof. John Keller of the University of Alaska at Fairbanks.

You will perform both the conventional NaBH4 reduction and the Luche reduction of carvone in parallel. Note that it is possible that the reduction reactions may not completely consume the starting material since we are using limited amounts of NaBH4.

Put 1.0 mmol of R-carvone into each of two 10 × 150 mm test tubes and add methanol (2.5 mL). Shake gently to ensure that the carvone is dissolved. To one of the tubes, also add CeCl3•7H2O (1.0 mmol – remember to take the water of hydration into account). Be sure to label your tubes so you can tell which is the Luche reaction. Weigh out two 1.0 mmol portions of NaBH4. Carefully add one portion to each of your reaction tubes (watch for an exotherm and effervescence). Let the reactions proceed for 15 minutes, agitating the tubes from time to time to ensure mixing.

14 Johnson, M.R.; Rickborn, B. J. Org. Chem. 1970, 35, 1041-1045.

15 a) Luche, J.L. J. Am. Chem. Soc. 1978, 100, 2226-2227; b) Gemal, A. L.; Luche, J. L. J. Am. Chem. Soc. 1981, 103, 5454-5459.


P a g e | 30

Calculate the volume of 0.5 M HCl (aq.) needed to completely neutralize the NaBH4 and borate salts formed in the reaction. Using a pipette, add this amount of HCl solution to each reaction tube while stirring rapidly. Use a capillary to dab a tiny drop of each solution on a pre-moistened pH test strip to verify that the solutions are somewhat acidic before proceeding with the workup. If necessary add a small amount of additional HCl solution to obtain a mildly acidic solution.

You will have to perform the extraction of your products sequentially since you only have one small separatory funnel in your glassware kit. Put about 10 mL of water into your 50 mL separatory funnel. Use a pipette to transfer one of your quenched reaction mixtures into the funnel. Use about 10 mL of ether in several portions to rinse the reaction tube and pipette these washes into the separatory funnel also. Shake the layers thoroughly, and then separate them. Transfer the ether layer into an Erlenmeyer flask. Extract the water layer once more with 10 mL of ether. Combine the ether phases and dry with MgSO4. Filter into a clean pre-weighed 50-mL round-bottom flask (label it clearly) and evaporate the ether. This can be done on the rotary evaporator or by blowing a gentle stream of air16 into the flask.

Perform the same extraction procedure on the other reaction.

Obtain the 1H and 13C NMR spectra of your products. Prepare a small sample of each product in GC autosampler vials for analysis.

POINTS FOR DISCUSSION. Using your NMR spectra, identify the product(s) of each reaction. You can compare with spectra from the literature to confirm your conclusions. Based on both NMR and GC results, what is the ratio of products formed? Is there a difference between the products formed by NaBH4 alone and those obtained in the Luche reduction of R-carvone? Does the Luche procedure in fact deliver superior 1,2-selectivity in this particular reaction? Note that reduction of the carbonyl group can give rise to two diastereomers. Which one predominates? Do the two procedures give the same major diastereomer?

16 Note: if you use compressed air, you must remove dirt and oil droplets from the air stream. To do this, pack a Pasteur pipette loosely with cotton. Connect this to the air hose, and adjust the air to provide enough flow to lightly perturb the ether in your flask without blowing it out. Clamp the pipette above your clamped flask in the fume hood and let the air evaporate your ether solution.


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APPENDIX: MULTIDIMENSIONAL NMR SPECTROSCOPY The biggest challenge in interpreting a set of NMR spectra to solve an unknown structure is establishing “connectivity”. That is, you must determine not only what kind of proton or carbon gives rise to a particular signal, but also what it is connected to. You have already learned to do this in simple cases because you know that coupled proton signals will show the same value of the coupling constant J and that you can match the integrations to the observed splitting patterns. You also know certain chemical shifts that are highly characteristic of specific neighbouring groups. Thus, the 1H NMR spectrum of a simple compound like ethyl acetate is trivial to assign.

The singlet at 2.04 ppm is obviously a methyl group next to C=O. The triplet at 1.26 ppm and the quartet at 4.12 ppm share the same coupling constant. It is logical to assume that they are next to one

another, and the splitting pattern is consistent with a CH2CH3 grouping. The position of the quartet at 4.12 ppm tells us it is next to the ester oxygen.

However, in the spectra of more complex molecules it is not always easy to tell what is coupled to what in the 1H spectrum. Likewise, in the 13C NMR spectrum of such a molecule it can be hard to relate specific carbon signals to specific structural fragments. This is especially true when you do not know what the structure is before you look at the spectra. Consider the spectra shown below, for example.

Once you get over the initial shock, you can see that all the signals are fairly well separated, and it should be possible to figure out what this compound is. There are a few 1H and 13C signals that are highly suggestive – can you see evidence for two methyl groups, an alkene and a carbonyl? If only we could determine what the environments of these important groups are, it would give us a way into the more complicated aliphatic portion of the spectra.

Fortunately, techniques for extracting this kind of information have been developed. It is not important at this stage to understand how these advanced NMR methods work in terms of the physics of nuclear spins. All we need for now is to know what the three most important multidimensional NMR techniques are and how they can be used to solve structural problems.


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COSY OR CORRELATION SPECTROSCOPY COSY is used to determine the relationships among proton signals. Basically, it maps two plots of the 1H NMR spectrum along the sides of a square, and links signals in each spectrum to one another. Multidimensional spectra are conventionally represented as contour plots. This is the same way that geographic data appear on a topographic map.

Because a COSY spectrum relates a single 1H spectrum to itself along the two sides, it is symmetrical along its diagonal. The 1H spectrum in fact forms the diagonal of the square. A COSY spectrum of 3-heptanone is shown here. You can see that contour spots along the diagonal correspond to individual signals in the 1H spectrum, but there are also off-diagonal contours. Pairs of off-diagonal peaks form squares (shown as dashed lines) with pairs of peaks on the diagonal. This tells us that those two signals in the 1H NMR

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Spectrum: Dr. G.A. Fahey, U. of Ottawa


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spectrum are from protons that are coupled to one another. We can track the coupling from one proton to the next, to see how the signals are linked.

The triplet at about 0.88 ppm is coupled to a multiplet at 1.28 ppm. This multiplet is coupled to another multiplet at 1.53 ppm (the cross-peaks are rather weak in this case) which in turn is coupled to a signal at 2.37 ppm. This latter signal is actually overlapping with another one at about 2.39 ppm, which is coupled to the remaining triplet at 1.02 ppm. It isn’t obvious what the splitting of the signals at 2.37 and 2.39 is, but we can tell that there are two different signals here from the integral.

The 1H NMR integration (not shown) is 4:2:2:3:3 so the upfield triplets must be methyls. The two multiplets are methylenes, and the downfield signal integrating for 4 protons is two distinct methylenes. The one at 2.37 ppm is a triplet, while the other is a quartet. From the COSY spectrum we can see that the 1H signals should be assigned as shown.

HSQC: CARBON-HYDROGEN CONNECTIVITY We conventionally acquire 13C NMR spectra with proton decoupling, so it is easy to forget that 1H and 13C nuclei in organic molecules are in fact coupled to one another. This fact is used to map the signals in a 1H spectrum to corresponding 13C signals. Heteronuclear single-quantum correlation spectroscopy is a method that can correlate proton signals to signals arising from any NMR-active heteronucleus (that is, a non-hydrogen nucleus; 13C and 15N are most common, but others are also used). The HSQC spectrum is set up to detect connections via one-bond coupling. Thus, it matches carbons with the proton(s) directly bonded to them.

HSQC spectra are also shown as contour plots, with the 1H spectrum usually along the top edge and the 13C spectrum along the side. Because this is not symmetrical, there is no diagonal in an HSQC contour plot. Instead, cross-peaks appear as isolated signals across the spectral map. The HSQC spectrum of 3-heptanone is shown below.

The 13C spectrum is plotted along the left side, and because carbonyl carbons (having no C-H linkages) will not appear in an HSQC plot, only the aliphatic region is shown. Interpretation is simple. A horizontal line will join any cross-peak to its corresponding 13C peak, while a vertical line links it to the matching 1H signal. From the spectrum of 3-heptanone, the fact that the 1H signal at about 2.4 ppm is actually due to two different methylenes becomes obvious, because it is correlated to two different carbon signals, at 35.9 and 42.2

CH2

CCH2

H2C

CH2

CH3

O0.88

1.28

1.53

2.372.39

H3C1.02

CH2

CCH2

H2C

CH2

CH3

O 0.8813.9

1.2822.4

1.5325.9

2.3742.2

2.3935.9

H3C

1.027.9


P a g e | 34

ppm. The methylene multiplets are linked to carbons at 25.9 and 22.4 ppm, while the methyl carbons are at 7.9 and 13.9 ppm.

A little thought will tell you that a CH2 group might have two 1H signals lining up with a single 13C peak if its protons are diastereotopic. You sometimes have to look closely if two signals are very similar in chemical shift in both dimensions, but generally the fact that 13C signals are spread over about 220 ppm means that HSQC is a good way to spread out overlapping 1H signals. The only problem with HSQC is that it cannot tell you about the non-protonated carbons.

HMBC: MORE CARBON-HYDROGEN CONNECTIVITY Although we will not likely need to use this method for our laboratory, it is included here for completeness. You will get a more complete description of HMBC in a later course.

Heteronuclear multiple-bond correlation spectroscopy helps us link nearby 1H signals to those pesky quaternary carbons that don’t have their own protons. Again, the technique is based on the fact that protons couple to 13C nuclei, even those that are not directly bonded. An HMBC experiment is sensitive to 2-bond (geminal) and 3-bond (vicinal) couplings.

The HMBC looks a lot like an HSQC but because each 1H and 13C signal may have several cross-peaks in an HMBC spectrum, these spectra are not as easy to interpret and must be approached systematically. Usually, it is best to get as much information from the 1-D spectra, the COSY and the HSQC as you can and then turn to the HMBC for the missing items. You will see signals for quaternary carbons if there are protons two or three bonds away that are coupled to them, and since these are usually the problem spots in assigning NMR spectra, it is probably best to start with the quaternaries and try to link them into signals you already know about. The HMBC provides confirmation of assignments you already know, and it links portions of the spectrum that are separated by heteroatoms and quaternaries.

The HMBC spectrum of 3-heptanone is shown on the previous page. There is a vertical band of weak signals in the middle of the

spectrum which you can ignore as it is an instrumental artefact. The sample is not completely pure either, and a set of cross-peaks due to the impurity are marked by a red dashed box.

C C

H

C

H

3J

2J


P a g e | 35

The 13C signal at about 213 ppm is obviously a ketone carbonyl. The blue dotted line at this chemical shift passes through three cross peaks. There is correlation to the methyl 1H triplet at 1.02 ppm but not to the one at 0.88 ppm. This means that the methyl hydrogens at 1.02 are 3 bonds away from the carbonyl carbon while those at 0.88 are farther away. The downfield cross peaks correlate both CH2 signals at 2.37 and 2.39 to the ketone carbonyl, not surprisingly since these are on either side of it.

Now, look at the methyl triplets. The relevant region of the HMBC spectrum is expanded in the diagram to the left. The triplet at 1.02 ppm shows a cross peak that links it to the 13C signal at 35.9 ppm. Remember, this is not the 13C chemical shift for the methyl itself. This is the 13C shift of a carbon either 2 or 3 bonds away from the methyl hydrogens. Our previous analysis of the COSY and HSQC gave us strong evidence that the signal at 35.9 ppm is from the CH2 on the “ethyl” side of the ketone, and this confirms it. The other

methyl triplet shows two cross peaks (look closely at the contours!) to carbons at 22.4 and 25.9 ppm. This again confirms its assignment as the end of a CH2CH2CH3 sequence. Note that the weak signal in the top left of the plot is from an impurity in the sample.

If we now look at the HMBC correlations for the downfield 1H signals we can confirm the rest of the assignment. The methylene 1H signals at 2.37 and 2.39 ppm are quite distinct in the HMBC. The signal at 2.39 is correlated to a 13C signal at 7.9 ppm, which we suspected was the CH3 of the “ethyl” fragment. The other methylene is correlated to a 13C signal at 25.9 – and it may also be correlated with the one at 22.4 because the cross peak is quite wide. This confirms the identity of this signal.

The spectra of 3-heptanone are not so complex that you really need to use advanced methods to assign all the signals. As you have seen, we knew pretty much everything without the HMBC. Where HMBC really becomes useful is when the spectra are much more complicated and where some sections of the molecule are isolated from others. You don’t need to worry about HMBC too much at this point but it is good to be aware of what it is and what it can do.

impurity


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CHEM 3390 STRUCTURAL TRANSFORMATIONS O …home.cc.umanitoba.ca/~hultin/chem3390/labprocs/Labman2014.pdf · synthesis. These techniques include a re -exposure to distillation, recrystallization,