Extracoordinate intermediates

Nucleophilic substitution at silicon, as described in previous sections, usually proceeds through five- or possibly six-coordinated silicon intermediates or transition states. For silicon bearing the common alkyl or aryl ligands, the barrier to formation of such extracoordinated intermediates is quite low. In these examples bimolecular substitution is so facile that unimolecular substitution, even in the most favourable cases, is so slow by comparison as to be unobservable. 

Mechanisms for the substitution at silicon in solution have been a subject of several reviews (7-15). Since expansion of the coordination number of silicon is a common feature, particular interest is directed to mechanisms involving intermediates or transition states with silicon having a coordination number of 5 or 6. So far, little attention has been devoted in the review literature to mechanistic pathways that do not invoke extracoordination. Knowledge of these mechanisms, however, has often become necessary for the understanding of chemical behavior of organosilicon compounds. In this article we discuss mechanistic pathways involving heterolytic cleavage... 

Route 1, attack of the nucleophilic reagent leading to a hexacoordinate intermediate or transition state, has been given preference (11,12,247,306-308). A similar pathway has been also postulated in organophosphorus chemistry (309-311). This mechanism may account for many experimental observations. It does not, however, seem to be generally accepted as the most favorable route (see, for example, Refs. 243 and 290). Activation toward nucleophilic attack intuitively should be less effective in the extracoordination structure [Eq. (72), B] than in the silylonium ion (C), because of the higher positive charge and lower steric requirements of the Si center in the latter. 

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