My PhD dissertation - Institut Fresnel

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on the leaving alkyl group during the rate determining step of alkaline decomposition. Other
investigations on substituent effects were ran with o- and p-substituted-benzyltriphenyl phosphonium
salts in different conditions
[36, - ] 130 132 and showed ρ ranging from +2.2 to +5.6. Values of ρ ranging
from +4 to +5 are in good agreement with other known base-promoted reactions in which benzylic
carbanions in hydroxylic solvents are proposed as intermediates
[ , ] 133 134 . It was thus believed, at that
stage, that the relative ease of alkyl expulsion was governed by the stabilities of the corresponding
carbanions. However, B. Siegel [131] interestingly reported a Hammett analysis for variously
substituted Y-benzyltriphenyl phosphonium bromides in which σ – for substituents on the benzyl
group (Y) were graphed against log kY (kY being the third order rate constant of alkaline
decomposition of the salt). The plot, unfortunately, did not support a linear correlation which would be
expected if a carbanionic character was developed on the leaving group, and delocalized to the entire
aromatic ring, in the course on the expulsion of an alkyl carbanion. In contrast, a plot of those log kY
against σ gave a very good linear correlation (r = 0.99) with a slope ρ of +3.7. In view to those results,
it can be assumed that a polarization or partial negative charge is present on the leaving benzylic group
in the rate determining step but no proper negative charge is delocalized to the whole aromatic ring.
This assumption thus agrees with the hypothesis of a concerted expulsion-reprotonation of the alkyl
group.
Another argument favoring a concerted version of the alkaline decomposition of
phosphonium salts was suggested by a work reported by C. Eaborn et al.
[ ] 135 . Indeed, they assumed
that for a similar decomposition of silicon and tin compounds, the expelled anions were not
completely free but rather protonated as formed. When performing the reaction in MeOH−MeOD
(1:1) they observed kinetic isotope effects of kH/kD of 1.4 to 4.6 depending on the substrates. J.R.
Corfield and S. Trippett
[ ] 136 later performed alkaline decompositions of tetraphenylphosphonium
iodide and cumyltriphenylphosphonium iodide in H2O/D2O (1:1) and observed kinetic isotope effects
kH/kD of 1.22 and 1.21 respectively. These differences in reaction rates, even if lower than those
observed by C. Eaborn et al. [135] , can be explained by a mechanism of phosphonium salt
decomposition in which, in the rate-determining step, little breaking of the P−R bond has occurred
while there is a corresponding little transfer of a proton to the incipient carbanion. Furthermore, since
isotopic effects are dependent from the reaction solvents involved, the range of kH/kD values obtained
by J.R. Corfield and S. Trippett [136] can not be directly correlated to those obtained by C. Eaborn [135] .
δ
(1−δ)
H
C
P
O
O