Page 197 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

5B.9 THE WITTIG AND RELATED REACTIONS: PHOSPHORUS YLIDES 177

R 1 R 1
−
Ph P C + C + X
3
H Ph 3 P H
H X H
R 1
(5B.40)
C + + −
H − Li C +
H PPh 3 + R 1 PPh
H 3
Ylide
Li

The ylidic carbon may be viewed as a carbanion surrogate and it attacks the carbonyl sub-
strate like a typical strong nucleophile. The zwitterionic product is called a betaine, which
is defined as a neutral molecule containing a heteroatom-based positively charged group
with no attached hydrogens, such as a quaternary ammonium or phosphonium group, and
a nonadjacent negatively charged center or group, such as carboxylate. The betaine then
cyclizes to form a four-membered ring (oxaphosphetane) with a P–O bond:

+ +
P −
Ph 3
O Ph 3 P O Ph 3 P O
− C
C C C C C (5B.41)
R 1 2 H
H R H R 1 R 1 H
H R 2 2
H R
Betaine Oxaphosphetane
The strained four-membered ring falls apart under the reaction conditions, yielding an
alkene and Ph PO as the final products.
3

+ −
Ph P O
3
Ph P O
3
R 1 + H
(5B.42)
C C
R 1 H C C
H R 2 2
H R
Once again, formation of Ph PO is the driving force for the last part of the mechanism. Note
3
the ambiguity in the stereochemistry of the alkene product (as indicated by the squiggly
bonds), which results from the fact that the betaine intermediate equilibrates to some extent
with both the starting materials and the heterocyclobutane intermediate. With sterically
unhindered ylides, a Z (or cis) stereochemistry is generally the preferred stereochemical
outcome.
A major deficiency of the Wittig reaction is the difficulty of separating the desired alkene
product from the triphenylphosphine oxide byproduct. The fact that triphenylphosphine is
used as a stoichiometric reagent also leads to a poor atom economy, which is defined as: