The Wittig Reaction: Substitution of =O by =C

The Wittig Reaction: Substitution of =O by =C
Ph
Ph P CH 2
Ph
(carbon nucleophile)
Ph
Ph
P CH 2
Ph
a phosphonium ylide
Ph =
Ph 3 P
O
[Ad N ]
O
a betaine
This net substitution proceeds by
an addition-elimination sequence.
CH 2
+ Ph3 P CH 2
+
[A N ]
This elimination
involves breakdown of
the 4-membered ring
O
Ph 3 P
an oxaphosphetane
Ph 3 P=O
Phosphonium ylides (powerful nucleophiles)
add to aldehyde or ketone carbonyl groups via
normal [Ad N ]. This tetrahedral intermediate, a
betaine, (BAY-tah-een) has no leaving group,
yet it’s not stable. Because phosphorous has a
high affinity for oxygen, the oxyanion attacks
the positively charged P via [A N ] to make a 4-
membered ring (called a phosphaoxetane).
This strained phosphaoxetane reacts further to
split into Ph 3 P=O and an alkene. This
elimination is a little funky, but you should be
able to draw a reasonable set of curved arrows
(and identify the frontier orbitals).

Formation of Ylides
Ph 3 P + CH 3 I
triphenylphosphine
[S N 2]
H
Ph 3 P C H
H
methyltriphenylphosphonium
iodide
[pt]
I
Ph 3 P CH 2 +
+ LiI
CH 3 CH 2 CH 2 CH 3
butane
CH 3 CH 2 CH 2 CH 2 Li
butyllithium
(strong base)
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