My PhD dissertation - Institut Fresnel

VIII
Abstract
Organophosphorus compounds found applications in varied fields of the organic chemistry:
they are particularly present in natural compounds such as the deoxyribonucleic acid (DNA) and in the
field of catalysis in which their utility is also proven. Thus, this thesis work first focused on the
preparation of various phosphorus ligands utilizable in transition metal catalysed reactions.
Being given the importance of this type of reaction in organic synthesis, it appears
essential to have many ligands at disposal in order to fine-tune and to optimize the reaction
conditions as much as possible. Thus, the first objective of this work was to propose a simple and
general method for the preparation of monodentate phosphorus ligands. Based on the use of
phosphites in substitution for the usual phosphorus trihalides, this method was initially tested during
the preparation of different simple triarylphosphines. Thereafter, this method was extended to the
synthesis of new ligands having more complex structures.The yields obtained for the latter as well
as the variety of the substrates authorized by this method confirmed its value and suggested its
application to the synthesis of new heteroaromatic phosphines.
The preparation of bidentate phosphorus ligands based on chiral biphenyl motifs was
then addressed. For that purpose, the 2,2',6,6' -tetrabromobiphenyl was chosen as a pivot
substrate for the development of a modular synthetic approach. Indeed, by means of
successive halogen/metal permutational exchanges and after treatment with the suitable
electrophile, it was possible to functionalize this substrate, isolate each atropisomer in
enantiomerically pure form, and finally convert each one of those into chiral
biphenylbisphosphines. In addition, it was also possible, during the synthesis of these ligands,
to evaluate the rotational stability of some enantiomerically pure dilithiobiphenyls.
Finally, after having acquired some expertise in the handling of phosphorus
compounds, some phosphonium halides were prepared in order to evaluate the relative
basicities of some alkyl carbanions. It is postulated that the ease of expulsion for the different
substituents of a quaternary phosphonium salt treated in alkaline conditions is controlled by
the relative stabilities of their related carbanions. Following this idea, the proportions of the
products obtained from this decomposition were quantified in order to evaluate the stabilities
of some aliphatic carbanions. Furthermore, those results were discussed in the light of the
mechanistic arguments reported, up to date, in the literature.