Ambident Reactivity of Acetyl- and Formyl-Stabilized Phosphonium Ylides

The kinetics and
mechanism of the reactions of formyl-stabilized
ylide Ph3PCHCHO (1) and acetyl-stabilized
ylide Ph3PCHCOMe (2) with benz­hydrylium
ions (Ar2CH+, 3) were investigated
by UV–vis and NMR spectroscopy. As ambident nucleophiles, ylides 1 and 2 can react at oxygen as well as at the
α-carbon. For some reactions, it was possible to determine the
second-order rate constant for O-attack as well as for C-attack and
to derive the nucleophile-specific parameters N and sN according to the correlation lg k (20 °C) = sN(E + N) for both nucleophilic sites. Generally,
O-attack of benz­hydrylium ions is faster than C-attack. However,
the initially formed benz­hydryl­oxy­vinyl­phosphonium
ions can only be observed by NMR spectroscopy when benz­hydryl
cations with high Lewis acidity are employed. In other cases, rearrangement
to the thermodynamically more stable products arising from C-attack
occurs. The results derived from our investigations are employed to
rationalize the behavior of ambident nucleophiles 1 and 2 in reactions with carbon-centered electrophiles in general.
It is shown that the principle of hard and soft acids and bases (HSAB)
and the related Klopman–Salem concept of charge and orbital
control lead to incorrect predictions of regio­selectivity. We
also show that the rate of the Wittig reaction of ylide 2 with aldehyde 14 is significantly faster than the rate
of either C- or O-attack calculated using lg k (20 °C) = sN(E + N), thus indicating that the oxaphosphetane is
formed by a concerted [2 + 2] cycloaddition.