Chapter16 Chem 241b

7/30/2019 Chapter16 Chem 241b

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"You cannot reason a personout of a position he did not

reason himself into in the first

place." Jonathan Swift


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Chapter 16

Conjugated double bonds

Lycopene (red dye in tomatoes, berries)

NH

HN

O

O

Indigo (Blue in Blue Jeans)

NN

NN

OO

MeO

H3C

H3C CH3

CH3H3C

OO

R

Mg

Chlorophyll

NN

NN

H3C

CH3

CH3

Fe

H3C

CO2H

HO2C

Heme

2,6,10,14,19,23,27,31-Octamethyl-dotriaconta-2,6,8,10,12,14,16,18,20,22,24,26,30-tridecaene

NH

HN

O

O

Tyrian Purple (Mollusks)

Br

Br


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3

Conjugated Alkenes

Conjugated

Not Conjugated

Red Shift in electronic spectra More stable than corresponding non-conjugated mole

sp2-sp2s bonds are shorter and stronger than sp3-sp2

CO2H

H

H

HH

Linolenic Acid

CO2H

5.5 kJ/mole more stable than linolenic acid (belo


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4

Identifying Conjugated Organics

O

NN

1,3-cyclohexadiene

1,4-cyclohexadienebenzene

cyclopentadiene1,3-butadiene1,4-pentadiene 1,3-pentadiene

1,2-diphenylethane 1,2-diphenylethyne trans-1,2-diphenylethene trans-azobenzene

allylbenzeneor 3-phenylpropeneor 3-propenyl benzene

1-propenylbenzenecyclohexa-2,5-dienone3-methylenecyclohexene


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Conjugated C=C Bonds and Electronic Spectra


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Conjugated Alkenes

Crocetin (from Saffron; 422 nm)

HO2CCO2H

Lycopene (505 nm)NH

HN

O

O

Indigo Blue (605 nm)

430 nm 500 nm

560 nm

580 nm

400 nm

420 nm

465nm

530 nm

600 nm620 nm

640 nm

680 nm

720 nm

NN

NN

OO

MeO

H3C

H3C CH3

CH3H3C

OO

R

Mg

Chlorophyll a (675 nm)


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Structure Determination in Conjugated

Systems: UV Spectroscopy The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to

the lowest unoccupied molecular orbital (LUMO)


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Conjugated Alkenes

430 nm 500 nm

560 nm

580 nm

400 nm

420 nm

465nm

530 nm

600 nm

620 nm

640 nm680 nm

720 nm

# Double Bonds max

1

2

3

4

5

6

11

177 nm

224 nm

275 nm

310 nm

342 nm

380 nm

455 nm


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Butadiene: Molecular Orbitals

4 p-orbitals

HHH

H

H

HH

H

H

H

H

30 kJ/mole

1

2

3

4

1,3-Butadiene Ethylene

185 nm215 nm


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Conjugated C=C Bonds and Electronic Spectra


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430 nm 500 nm

560 nm

580 nm

400 nm

420 nm

465nm

530 nm

600 nm620 nm

640 nm

680 nm

720 nm


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Conjugation, Color and the Chemistry

of Vision b-Carotene is converted to Vitamin A, which is converted to

11-cis-retinal:


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Conjugation, Color and the Chemistry

of Vision 11-cis-retinal is converted to rhodopsin in the rod cells of the

retina.

Visual pigments are responsible for absorbing light in eye and

triggering nerves to send signal to brain


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O

OH

H2N

4-Aminobenzoic acid

PABA

HO

NN

N

HO NN

N

2,2_-methanediylbis[6-(2H-benzotriazol-2-yl)-4-(2,4,4-trimethylpentan-2-yl)phenol]

Bisoctrizole

O

O

N

2-ethylhexyl 2-cyano-3,3-diphenyl-2-propenoate

Octocrylene

O O

O

1-(4-Methoxyphenyl)-3-(4-tert-butylphenyl)propane-1,3-dione

Avobenzone

SUNSCREENS


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Special Properties of Conjugated

Dienes

Length of the central single bond is shorter than non-

conjugated similar molecule

Comparison of 1,3-Butadiene and Butane with C-13

NMR

H2C=CH-CH=CH2 CH3-CH2-CH2-CH3

148 pm 153 pm

1,3-Butadiene Butane

Shorter

Bond


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Why more stable (thermodynamics) than non-

conjugated?

p-p overlap is better than hyperconjugation

1

2

3

4

Conjugated

LUMO

HOMO

LUMO

HOMO

Isolated


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1

2

3

4

Conjugated

LUMO

HOMO

LUMO

HOMO

Isolated

More reactive (kinetics) than non-conjugated

LUMO is lower

moreelectrophilic

HOMO is

highermorenucleophilic


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Br

H

Br

H

HBr

H

Br

Br

H

Ea

Ea

HBr

H

Br

H

Br

HBr

Faster reactions, less G for reactions (start lower)


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1

2

3

4

Conjugated

LUMO

HOMO

LUMO

HOMO

Isolated

30 kJ/mole barrier to rotation

Adds Double bond character to C2-C3

Substracts Double bond character to C2-C3


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Diene Preparation

H H

Cyclohexene 3-

Bromocyclohexene

1,3-

Cyclohexadiene

NBS

CCl4

Br+K -OC(CH3)3

HOC(CH3)3

Based Induced Elimination of HX


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Electrophilic Additions to

Conjugated Dienes: Allylic

Carbocations

Electrophilic addition to 1,3-Butadiene yields a

mixture of two products: 1,2 addition

1,4 addition


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Non-conjugated alkene addition

reactions

CH3

C+

CH3

CH3

HCl

EtherCH3

C

CH3

CH2

CH3

C

CH3

CH3

Cl

2-

Methylpropene

Tertiary

Carbocation2-Chloro-2-

methylpropane

CH2

CH

CH

2

CH

CH2

HCl

EtherCH

3CH

CH

2

CH

CH3

Cl Cl


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At room temperature, electrophilic addition

to a conjugated diene leads to a product

mixture where the 1,2 adduct predominates

over the 1,4 adduct.

At high temperatures, the product ratio

changes and the 1,4 adduct predominates

CH2

CH

CH

CH2 + HBr CH2 C

HCH

CH3

Br

CH3

CH

CH

CH

2

Br+

1,2 adduct 1,4 adduct

At 0oC: 71% 29%

At 40oC: 15% 85%


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Reacting Butadiene with

ElectrophilesH Cl

HH

H

HH

HCl-

Cl Cl

1

2

3

Non-Bonding

Anti-Bonding

Bonding

1/2 1/2or


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Allyl Cations


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Allylic and Benzylic Cations

HCl

Cl

Ether

H

H

H

HBr

Br

Br

majorminor

H

H

H H


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Diene polymerizations


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Vulcanization:


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Diels-Alder Reaction

O

O

O

O

O

O

Electron pooralkene

Electron richdiene

Net: 2 Pi bonds converted to sigma bondsin cycloaddition reaction yielding a six-membered ring

RT


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Dienes

DienophilesN

O

O

R

O

OMe

O

OMe

O

Me

O

Me

CN CN

NCO

OMe

O

MeO

CO2Me

O

O

O

R3SiO

OMe

O


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Diels-Alder Reaction:Diene Reactivity

s-cis"cis-like" arrangement about

the middle carbon-carbon singlebond

s-trans"trans-like" arrangement about

the middle carbon-carbon singlebond

NO Diels-AlderDiels-Alder Possible

Generally more reacti

than acyclic dienes

H

H

H

H

H

H

HH

s-cis

H

H

HH

s-trans

No Reaction

Sterically driven to s-transconformation

Locked in s-trans conformation


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H

O

O

O OO

O

H

H

H

H

O

H

H O

O

ONLY SYN ADDITION

O

O

OH

O

H

H O

O

O

O

OH

H

TWO MODES OF APPROACH

No way of telling which approach is used with this syst


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H

O

O

O OO

O

H

H

H

H

O

H

H O

O

ONLY SYN ADDITIONBUT ONLY THE ENDO (Bottom) DIASTEREOMER FORMSAT ROOM TEMP (KINETIC PRODUCT)

O

O

OH

O

H

H O

O

O

O

OH

H

TWO MODES OF APPROACH

O

H

H O

O

O

H

H O

O(R)

(S)(R)

(S)(R)

(S)

(R)

(S)

(R)

(S)

(R)

(S)

(R)

(S)

(R)

(S)

Meso

Meso

ENDO

EXO

RoomTemperature

200 C


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O

O

O

Ea

GO

H

H O

O

O

O

OHH

EXO

ENDO

O

O

O H

H

O

H

H O

O

O

O

O

Ea

G

Stability of exo product is greater

Higher temperature is required

to reach product for exo path

O

H

H O

O

O

H

H O

O

ENDO EXO

200 C

Running reaction at higher temp

gives thermodynamic product (e

Running reaction at room temp

gives kinetic product (endo)


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Diels-Alder Reactions

Me3SiO

OMeMe

O

Me OMeO

Me3SiOH

Me O

OH

1) Heat

2) Aqueousacid

O

OMe

H

H

O

OMe

H

H

CO2MeH

CN CN

NC

CNCN

CN


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Diels Alder in Synthesis

Look for cyclohexene = product of DA

H

H

O

O

OH

H

O

O

O

S id S h i


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38

The key DielsAlder reactions used to prepare the C ring of

estrone and the B ring of cortisone are as follows:

Steroid Synthesis


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190 C

RT

CO2Me

Me

Me MeMe

Me

Me

CO2Me

CO2MeCO2Me

Me

Me

MeO2C

Me

Me

CO2Me

CO2Me CO2MeMe

Me MeO2C

Me

Me

CO2Me


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Diels-Alder ReactionsO O

H

H

H

CN

NC

O

O CN

CN

H

N

O

O

OSi

Me Me

O

O O

OSi

O

MeMe

N

O

O

Room Temperature

> 90 CN

O

O

OSi O

MeMe

N

O

O

O

OSi

Me Me O

O

Glassy Polymer

Viscous Liquid


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N

NN

O

O

R

N

N N

O

O

R

H

H


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O

Benzene

155 C O

H

H

O

O

O

O

-CO

Diels-AlderPolymer


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NMR of Dienes

01234567

PPM

2.15

2.15

5.80

5.90

5.90

5.80

020406080100120

PPM

22.1

22.1

124.7

125.8

125.8

124.7


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NMR of Dienes

01234567

PPM

2.05

2.05

6.03

6.035.61

5.61

H

HH

H

020406080100120140

PPM

126.9

130.3

130.3

126.9

19.3

19.3


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Need to know

Diene nomenclature

Recognize conjugated versus non-conjugated systems

Know reaction and mechanism:

-addition of Br2 or Cl2 to dienes

-addition of HCl or HBr to dienes-Diels Alder cycloaddition (exo and endo)

Understand kinetic versus thermodynamics products for

diene reactions and Diels Alder cycloaddition reactions

Be able to synthesize dienes from alkanes throughbromination and elimination reactions.

Extra special: predict regiochemistry of Diels Alder

cycloadditions

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Chapter16 Chem 241b