Chemistry 125: Lecture 51February 15, 2010

More Addition to Alkenes:Organometallic Reagents

and Catalysts This

For copyright notice see final page of this file

Mechanism for Acid-Catalyzed Hydrolysis of Acetal

RO

ROCH2

+H

HOH

:

:

RO

ROCH2

+ H ROH

RO-CH2

+

HO

ROCH2+

H

First remove RO, and replace it by HO.

HO

ROCH2

Now remove second RO, then H (from HO) +H

:HO

ROCH2

+ H

RO=CH2

+

cation unusually stable;thus easily formed

ROH

H-O-CH2

+

O=CH2 O=CH2

ROH

ROH

RO

ROCH2 O

H

H

:Overall Transformation:

H2O + Acetal Carbonyl + 2 ROHH+

(pp. 785-787)

(hemiacetal)?

?SN1

E1

HOO=CH2CH2

HO-OO

H

H

O-O

OH2CO

H

H H

HH2C=O

Ozonide is a Double AcetalSo Double Hydrolysis

and hydrogen peroxide

Gives Two Carbonyl Compounds

which oxidizes aldehydes to carboxylic acids!

Sec. 10.5b pp. 440-441

Add a reducing agent like (CH3)2S (or Zn) to destroy HOOH and save RCH=O.

Or go with the flow and add more HOOH to obtain a good yield of RCOOH.

3-membered ring with O-O bond is

even worse.

What Happens to HOOH + RCHO?

OC

H

R

O OH-OH

O

C

H

R

O

- O

C

H

R

O OH-

HOH

-

O

CR

OR

B

R

R

O OH

-Cf.

Problem: Try drawing an analogous acid-catalyzed mechanism in which HOOH attacks the protonated carbonyl, then H+ is lost from one O of the HOOH fragment in the product and added to the other before rearrangement.

OHOH- is a bad leaving group from C,

but O-O bond is very weak.Hydride Shift

“Nucleophilic”Additionto C=O

The nucleophilic addition of methyl lithium to carbonyl groups* is

formally quite different from these additions of electrophiles

to alkenes, but the following transition state analysis reveals a marked mechanistic similarity.

* which will be discussed in more detail later.

Transition StateMotion

Li-CH3

O=CH2

Li CH3

O CH2

Transition StateOrbital Mixing

Li-CH3

O=CH2

HOMOLUMO+2

* LUMO HOMO

Orbital Variety from Metals

overlaps with alkene *overlaps with alkene

LUMOHOMO

OsO4 / PermanganateSec. 10.5c p. 443

Os or Mn-

Os analogue of cyclic acetal

H2O

Diol + O2Os=O

OsO4 is poisonous and expen$ive!Use as a 1% catalyst by adding oxidant.

H2O2 (1936) “NMO” (1976)

ChiralAmineLigand

Sharpless(1988)

R

R

R

R

H H H H*LUMO HOMO

orthogonal

Sections 10.2a (410-413), 10.10 (452)

Catalytic Hydrogenation

HOMO/LUMO : Concerted

H HH H*LUMOHOMO

C C

H H

C C

H H

C CC C

*LUMOHOMO

(“works” with Pt/C Catalyst! Sec 4.9A, 168ff)

HOMO-HOMO repulsive emptyPd

Pd HOMO (4d)

Ethylene LUMO ()HOMO ()

HOMO-4Ethylene-PdComplex

…(4d)10 (5s) 0 (5p)0

13%

40% 4dxy

47% C-H

HOMO ()

Pd HOMO (4d)

Ethylene

UMO (5s)UMO (5p)

(4d)10 (5s) 0 (5p)0

HOMOEthylene-Pd

Complex

+6% 5s5% 5p

15% 4dz2

67%

Sigma Bond Analogue“Oxidative” Insertion (crummy PM3 calculation)

H-H+Pd

10

5

0

kcal/mole

H2 dissociates on bulk Pd surface (and moves and dissolves)(entropy help)

H

Catalytic Hydrogenation “oxidative insertion” “oxidative insertion”

Pd

C C

Pd

C C

Pd

H H

Pd

H H

“reductive elimination” “reductive elimination”

Pd

HCC

Pd

HCC

Pd

HCC

H

Pd

HCC

H

Pd addition concerted; H replaces Pd twice syn addition

Catalytic HydrogenationStereochemistry

syn addition (p. 412)

Stereochemistry(Loudon, Sec. 7.9 E p. 313)

No yields

specified!

No literature

reference!

pp. 20-22 of H. O. HouseModern Synthetic Chemistry (1972)

J. Chem. Soc., 1354 (1948)

H2 / Pt

R’ = Ac R’ = Ac

H

Catalytic Hydrogenation

Pd

H

Pd

HCC

Pd

HCC

H

Pd

HCC

H

C CC

CH H H

H

CCC

Pd

H

Pd

HCCCH

Pd

HCCCH

CC

alkene isomerized

10

12

34 5

6

7

89

10

12

34 5

6

7

89

??

VII VIII

Alkene Metathesis

C

C

Grubbs Catalyst

Ru

C

C

C

Ru

CC

C

Ru

C

C

C

Ru

C

C

C

Ru

C

Nobel Prize 2005

Tall Fred Ziegler(not Karl Ziegler)

with Robt. Grubbs

TouristsZiegler Grubbs

Host

ROMPRing-Opening Metathesis Polymerization

Ru

CRuC

RuCn

n

metathesis

metatheses

Catalytic Hydrogenation

Ti

RCC

Ti

RCC

Ti

RCC

H

Pd

HCC

Pd

HCC

Pd

HCC

H

Pd

HCC

H25 x 106 tons(2004)

-(CH2-CH2)n-

n = 800-8000

Ziegler-Natta Polymerization

45 x 106 tons(2007)

-(CH2-CH)n-CH3

n up to 105

isotactic

All head-to-tail, but stereorandom (atactic)All head-to-tail, and stereoregular (isotactic)

End of Lecture 51Feb. 15, 2010

Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).

Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.

Materials from Wikimedia Commons are denoted by the symbol .

Third party materials may be subject to additional intellectual property notices, information, or restrictions. 

The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0