NUCLEOPHYLIC SUBSTITUTIONS AT SILICON; EVIDENCE FOR ION-PAIR DISSOCIATION AS CONTROLLING FACTOR OF THE STEREOCHEMISTRY AND A SIMPLE MECHANISTIC PROPOSAL

Article abstract of DOI:10.1016/S0040-4020(01)88903-1

Stereochemical and kinetic data are reported for reactions between organolithiums or LiAlH4 and some chiral organosilanes.They rule out a mechanism involving complexation control, such as the S_Ni-Si process proposed by Sommer et al.Electrophylic assistance to cleavage of a Si-X bond does not control the stereochemistry, but acts as an additional factor which can facilitate the inversion by increasing the ability of the leaving group to depart.The results reveal the dominant influence of ion-pair dissociation, and thus of the electronic character of the nucleophile: the consequence of the use of either hard reagents with a localized negative charge, such as alkyllithiums, or softer reagents with a more delocalized negative charge, support this dependence.A simple mechanistic interpretation of the data is proposed, based on a description of nuclepohilic substitutions at silicon as a frontier orbital process.