My PhD dissertation - Institut Fresnel

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Furthermore, the intensities of all signals were directly proportional to the amount of each
product present in the mixture, which allowed direct determination of the k1/2 values with
respect to the internal standard used. In order to get accurate results and devoid of any artifact,
the 31 P NMR spectra were recorded in a nondecoupling mode since it is known that the
Nuclear Overhauser Effect (NOE) causes changes in the peak intensities during the 1 H
decoupling process
[ ] 141 . On the other hand, for the peak intensities to be effectively
relevant to the exact amount of each product quantified, the 31 P nucleus relaxation time
(T1) had to be determined and set by classical NMR techniques so that all considered
nuclei went back to equilibrium. Under these conditions, the 31 P NMR analysis could
afford accurate and quantitative results.
To determine the standard deviation on each measurement, a series of phosphine
oxide pairs were prepared in different extreme ratios (ranging from 1 : 1 until 99 : 1) by
accurately weighting the substances. 31 P NMR analyses of each of these mixtures in the
above mentioned conditions revealed an average standard deviation of 0.5% (see
Experimental Part). Quantitative 31 P NMR analysis technique has been extensively used
during the last two decades for enantiomeric excess (ee) determinations of chiral alcohols,
diols and amines using phosphorus chiral derivatization agents (CDA)
[ - ] 142 146 . B.L.
Feringa et al. [143, 145] and A. Alexakis et al. [142, 144, 146] reported average incertitudes on
the measure in good agreement with those determined in this study when using this
technique of quantification. Finally, for the sake of testing the reproducibility of the
results, the data reported for each phosphonium halide were measured as an average of
three separate runs carried out in the same experimental conditions.
3.3.4 Starting Materials and Reference Substances
For the determination of relative alkyl anions stabilities, all the starting mixed
tetraalkylphosphonium halides had to be prepared by quaternarization of the
corresponding tertiary alkylphosphines with the appropriate alkyl halide. On the other
hand, all the phosphine oxides which could possibly form during alkaline decomposition
reactions also had to be prepared to serve as authentic reference compounds for 31 P NMR
assignment. These oxides were in most cases, synthesized by simple oxidation of the
tertiary phosphine already prepared.