J. Am. Chem. SOC.
A Theoretical Study of the Wittig Olefination Reaction:
the Wittig Half-Reaction of
Unstabilized Ylides with Aldehydes’
Frank Mari, Paul M. Lahti,* and William E. McEwen*
Contribution from the Department
with our previous report (Man, F.; Lahti, P. M.; McEwen,
Wittig half-reaction (H3P-CH2
the oxaphosphetane), use of the MNDO-PM3 molecular orbital method on the
reaction of (C6HS)$=CHCH3 and the mythical analogue H3P-CHCH3 with aldehydes (acetaldehyde, propargylaldehyde,
and glyoxylonitrile) to give the respective
and E-diastereomeric oxaphosphetanes has provided evidence that the mechanisms
are best described as very asynchronous cycloadditions (borderline two-step mechanisms). The geometries of the transition
states of these model reaction are such that the P, C, C, and
0 atoms that interact in the bond-breaking-forming process
lie in the same plane. The C-C distance in the transition state varies from
whereas the P-O distance varies
Analysis of the evolution of the bond orders of these reactions shows that each C-C bond is about
formed in the respective transition states, whereas no P-O bond has been formed to any significant degree.
of localized molecular orbitals througout each reaction leads to the same conclusions. For the formation of the
the enthalpies of activation AH* were calculated to be
kcal/mol higher than the
for the formation of the
oxaphosphetanes. The enthalpies of reaction, AHr, were calculated to be
kcal/mol less negative for the formation of the
oxaphosphetanes when compared to the formation of E oxaphosphetanes. In addition, values of
less and the values of AHr
kcal/mol less negative for reactions with H3P=CHCH3
than for the respective ones of
with the same aldehydes. Thus, the presence of the phenyl groups bonded to phosphorus makes an important
contribution to the steric strain of formation of each oxaphosphetane and to the transition state leading to it. The calculated
decreases as the electrophilicity of the aldehydes is increased, which is in
agreement with experimental evidence.
The nature of the transition states suggests that along the reaction coordinate for real Wittig reactions carried out in polar
solvents or in the presence of metal ions, the reactants do not have to be restricted to the ‘syn” approach.
of the reactants is considered as a possibility to explain
stereoselectivity of some Wittig reactions.
stereoselectivity of alkene formation is perhaps
the most striking mechanistic feature of the salt-free Wittig re-
actions of certain phosphorus ylides2 (Scheme