My PhD dissertation - Institut Fresnel

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However, in the eventuality of an effective concerted mechanism as discussed
previously, the front and back strains occurring in the transition state could distort the results
obtained. Indeed, internal strains if not negligible, could either accentuate or counterbalance
the "negative partial charge stabilization" parameter which was aimed in this work. The
results presented in chapter 3.3.5 will possibly help to address this crucial issue and provide
new mechanistic insight on the alkaline decomposition of phoshonium salts.
3.3.3 Qualitative and Quantitative Analysis of the Reaction Products
For each experiment, stoichiometric amounts of phosphonium salt and internal standard
(tributylphosphine oxide or triisopropylphosphine oxide) were submitted to the alkaline reaction
conditions in a thermostated bath at 110 °C. A possible side reaction in alkaline conditions was
the β-elimination of hydrogen from the phosphonium salt, leading to the formation of an alkene
and a corresponding tertiary phosphine. Although such process had never been reported for
phosphonium salts and does not seem favored [30] under these conditions, it was deemed of capital
importance to check the complete absence of these undesired by-products which could distort the
results collected. Thus, during the whole reaction time, the gas evolutions were passed through a
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solution of bromine 139 in chloroform to detect the presence of alkenes. After the excess of
elementary bromine had been neutralized, the organic layer was systematically analyzed by gas
chromatography to detect any traces of possible dibrominated alkanes. Fortunately, this safety
measure followed the expectations and never revealed any detectable traces of such by-products.
Due to the volatility of the alkane products formed during the alkaline decomposition
of phosphonium salts, the relative amounts of those species could not be quantified by gas
chromatography. Furthermore, the quantitative analysis of the phosphorus derivatives formed
was first attempted, but gave non-reproducible results, probably due to an uncomplete
combustion of the phosphine oxide in the flame ionization detector (FID) which is known to
be the most accurate gas chromatography quantification technique. Thus, the ratios of both
possible phosphine oxides formed in the course of the decomposition reactions were
systematically quantified by 31 P NMR analysis. The assignment of each peak in the NMR
spectra of the crude mixture was effected by comparison with the spectra of each single
authentic phosphine oxides prepared independently. In this context, the NMR spectroscopy
appeared to be a useful and convenient method for this type of study since the 31 P NMR
spectra did not suffer any parasite signal making the interpretation or measurement inaccurate
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and the spectral window was wide enough to avoid any superimposition of peaks 140 .