My PhD dissertation - Institut Fresnel

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depending on the alkyl substituent expelled. It is then obvious that both possible transition
states would not present comparable front and back strains, and thus the probability for each
alkyl group to be cleaved would be, to some extent, influenced by these steric factors. This
argument can be illustrated by the example of dibutyldi-tert-butylphosphonium iodide
alkaline decomposition:
For the sake of clarity, the above mentioned scheme do not show all steric strains
relationships, but only the significant ones showing the difference between both possible
transition states. In order to afford a much reasonable comparison between transition states,
the bulkiest substituent (e.g. tert-butyl group) was arbitrary placed in axial position to reduce
the steric congestion, since it is known that this type of P–C bond is slightly longer and
weaker than the equatorial ones. On the scheme presented above, the transition state leading
to elimination of tert-butane shows more important steric strains than the one one favoring
cleavage of butane. Following this hypothesis, one can legitimately believe that cleavage of
the sterically less hindered substituent, resulting from the less encumbered transition state,
will be favored in the rate determining step of alkaline decomposition of phosphonium salt.
As mentioned previously, the stabilisation of a carbanion in condensed phase is
mainly due to solvation of the negative charge. In this way, bulky carbanions are usually less
efficiently solvated than small ones, and thus are more basic. Steric strains depicted in this
chapter, even if negligible, can cause a distorsion between the effective stability of the alkyl
carbanions and the one measured in this study. However, since stabilities of the cabanions
considered are in presumably governed by their steric bulk, it appears difficult to evaluate the
real proportion of steric strains' influence in the expulsion of alkane from a phosphonium salt.