Organic Chemistry

762 CHAPTER 18 Carbonyl Compounds II

Tutorial:

Wittig reaction–synthesis

Evidence has accumulated that the Wittig reaction is a concerted cycload-

dition reaction, with the nucleophilic carbon of the ylide attacking the electrophilic

carbon of the carbonyl compound. It is called a cycloaddition reaction be-

cause, of the four electrons involved in the cyclic transition state, two come from the

carbonyl group and two come from the ylide (Section 29.4). Elimination of tri-

phenylphosphine oxide forms the alkene product.

The phosphonium ylide needed for a particular synthesis is obtained by an re-

action between triphenylphosphine and an alkyl halide with the appropriate number of

carbon atoms. A proton on the carbon adjacent to the positively charged phosphorus

atom is sufficiently acidic to be removed by a strong base such as butyl-

lithium (Section 12.11).

If two sets of reagents are available for the synthesis of an alkene, it is better to use

the one that requires the less sterically hindered alkyl halide for synthesis of the ylide.

Recall that the more sterically hindered the alkyl halide, the less reactive it is in an

reaction (Section 10.2). For example, it is better to use a three-carbon alkyl halide and

a five-carbon carbonyl compound than a five-carbon alkyl halide and a three-carbon

carbonyl compound for the synthesis of 3-ethyl-3-hexene because it would be easier to

form an ylide from 1-bromopropane than from 3-bromopentane.

+ (C 5 H 6 ) 3 P CHCH 2 CH 3

+ (C 5 H 6 ) 3 P CCH 2 CH 3

O

CH 2 CH 3

3-ethyl-3-hexene

H

C

CH 3 CH 2 C CHCH 2 CH 3

CH 2 CH 3

O

CH 2 CH 3

C

CH 3 CH 2

CH 3 CH 2

preferred method

SN 2

1 pKa = 352

SN 2

RR

P(C 6 H 5 ) 3 P(C 6 H 5 ) 3

C CH 2

+

+

O

+

C

R

R

R

CH 2

−

O O P(C 6 H 5 ) 3

R C CH 2

a [2 + 2]

cycloaddition

reaction

p

[2+2]

[2+2]

C

CH 3

CH 3

+ (C 6 H 5 ) 3 P CHCH 3

+ (C 6 H 5 ) 3 P C + (C 6 H 5 ) 3 P O

CHCH 3

a phosphonium ylide

+ (C 6 H 5 ) 3 P O

triphenylphosphine

oxide

O

CH 3

CH 3

O

H 3 C

H 3 C

C

H 3 C

H 3 C

C

triphenylphosphine

+ CH 3 CH 2 Br

+

CH 2 CH 3

CH 3 CH 2 CH 2 CH 2 Li

(C 6 H 5 ) 3 P(C 6 H 5 ) 3 P

+−

(C 6 H 5 ) 3 P CHCH 3

Br− a phosphonium ylide

− +

SN 2