My PhD dissertation - Institut Fresnel

3.3 Assessment of the Relative Ease of Alkyl Expulsion
from Phosphonium Salts
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The new method developed during this thesis work for quantitatively determining the
relative stabilities of various carbanions, and particularly the alkyl ones, was based on the use of
the alkaline decomposition of phosphonium halides. On the basis of their precursor work on this
field, G.W. Fenton and C.K. Ingold [30] postulated that the ease of expulsion of one of the
phosphonium substituents was governed by the stability of its related anion. These authors, and
more recently H.R. Hays and R.G. Laughlin [52] treated a series of mixed phosphonium salts
bearing phenyl, allyl, benzyl or various alkyl groups such as methyl, ethyl, propyl, dodecyl or
tridodecyl under alkaline conditions and evaluated the ratios of the different products formed. The
decomposition of dodecyltrimethyl or tridodecylmethyl phosphonium hydroxide [52] gave in both
cases, only one phosphine oxide and its associated alkane, corresponding to the expulsion of the
most stable methyl carbanion. On the other hand, when decomposing phosphonium salts bearing
different but more similar groups such as ethyltripropyl or triethylpropyl phosphonium salts, G.W.
Fenton and C.K. Ingold [30] obtained a mixture of tripropyl and ethyldipropyl phosphine oxide or
of diethylpropyl and triethyl phosphine oxide, respectively, and the corresponding alkanes.
Unfortunately, since the quantification of the reaction products was relying on gas evolution
measurements and isolation of the oxides by distillation, the data collected were only qualitative
or, to some extent, gave a vague idea of the relative ease of expulsion for the different alkyl
substituents. From these results, the different author involved in these studies proposed an
approximate scale of stability for various carbanions:
benzyl, allyl > phenyl > methyl > ethyl, higher primary alkyls
The aim of this new method was to provide accurate and quantitative data for a series of
primary but also previously unstudied secondary and tertiary alkyl substituents to probe relative
stabilities of alkyl carbanions and discuss the different parameters influencing their leaving
ability. As a matter of fact, even if few primary alkyl anions have been already described [29, 30, 52] ,
be it only qualitatively, no reports concerning secondary or tertiary alkyl anions stabilities in
solution have so far been published. The lack of informations for the alkaline decomposition of
tetraalkyl phosphonium salts probably partially accounts for the chemical properties of
alkylphosphine compounds which are fairly difficult to handle and easily prone to oxidation.
Furthermore, the collected results of this study were analyzed by the light of various mechanistic
discussions concerning the alkaline decomposition of phosphoniums halides.