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INTRODUCTION The first studies of the dynamic (reaction) stereochemistry of optically active acyclic R3Si*X were carried out in my laboratory about ten years ago by C. L. Frye. Since that time many enthusiastic collaborators have greatly expanded the field of organosilicon stereochemistry which now includes stereochemical studies of more than a hundred substitution reactions of R3Si*X. These studies have provided new insight into the nature of organosilicon reaction mechanisms. Indeed, it is perhaps appropriate to say here that the stereochemical data have afforded me many surprises and have compelled me to discard many of the mechanistic ideas which I held prior to 1958. Our mechanistic conclusions concerning some of the reactions of monofunctional R3SiX, based mainly on stereochemical data accumulated prior to 1964, but including also the implications of kinetic data when available, have been reviewedl. Those conclusions have received strong additional support from stereochemical work carried out during the past four years. Perhaps the major mechanistic conclusions we have reached concerning R3Si*X may be stated as follows: (1) The silicon atom is a highly sophisticated and sensitive reaction centre whose mechanism paths are many. (2) With very few exceptions, nucleophilic Substitution reactions of monofunctional acyclic organosilicon compounds do not involve expandedoctet (Si-V or Si-VI) intermediates, or else such intermediates, if formed, do not differ significantly in energy and structure from the rate-limiting transition state. In short, the stereochemical aspects of such reactions are consistent with the concept of such reactions as concerted processes involving essentially simultaneaus bond-making and bond-breaking processes. This is not to say that expanded-octet mechanisms are unavailable to R3SiX, but only that concerted displacement mechanisms in many (but not all) cases provide the path of lower free energy of activation. Perhaps the major evidence for this conclusion comes from the dependence of the stereochemistry of R3Si*X on the nature of the leaving group, X. (3) The SN2-Si Stereochemistry Rulel, 2 for R3Si*X states that good leaving groups X, whose conjugate acids (HX) have pKa less than ""' 6 undergo nucleophilic displacement from R3Si*X with inversion of configuration, regardless of the nature of the solvent, and providing only that the attacking reagent furnish an entering group Y that is more basic than X.

Mechanistic pathways of the Si-H bond - Stereochemical studies