Section 18.10 The Wittig Reaction 763
The Wittig reaction is a very powerful way to make an alkene because the reaction
is completely regioselective—the double bond will be in only one place.
The Wittig reaction also is the best way to make a terminal alkene such as methylene-
cyclohexane because other methods would form a terminal alkene only as a minor
product if at all.
The stereoselectivity of the Wittig reaction depends on the structure of the ylide.
Ylides can be divided into two types:Stabilized ylideshave a group, such as a car-
bonyl group, that can share the carbanion’s negative charge; unstabilized ylidesdo not
have such a group.
Stabilized ylides form primarily Eisomers, and unstabilized ylides form primarily
Zisomers.
+ (C 6 H 5 ) 3 P CH
+ (C 6 H 5 ) 3 P CHCH 2 CH 3
Z alkene
C
H
CCH 3
O
R
O
C
H
R
O
C
HH
RCH 2 CH 3
C
C
HC
RH
C
O
CH 3
E alkene
+
O
(C 6 H 5 ) 3 PCHCCH 3 (C 6 H 5 ) 3 PCHCCH 3 (C 6 H 5 ) 3 P CHCH 2 CH 3
a stabilized ylide an unstabilized ylide
− +
O−
+ −
+
CH 3
Br
CH 3 CH 2
minor
HO−
CH 3
OH
CH 3
H 2 SO 4
∆
CH 2 Br CH 2 OH + CH 2
minor
HO−
100%
O + (C 6 H 5 ) 3 P CH 2 CH 2 + (C 6 H 5 ) 3 P O
methylenecyclohexane