Organosilicon compounds pentacoordinate species

The coupling of organosilicon compounds with organic electrophiles was not disclosed until 1988 by Hatanaka and Hiyama, when they demonstrated that through the addition of an appropriate silicophilic nucleophile, those desired pentacoordinate species can be generated in situ and transfer an unsaturated group. Nucleophilic fluoride sources were found to be the additive of choice, typically TASK, TBAF, and, in some cases, KF and GsF. These are the fundamental concepts of what is nowadays called the Hiyama reaction. The use of fluoride... 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

Pentacoordinate organosilicon species formed by coordination of a neutral two-electron ligand to tetracoordinate organosilanes constitute an interesting class of compounds. These systems can serve as models for the study of nucleophilic substitution at tetracoordinate silicon. 

ABSTRACT. Pentacoordinate organosilicon species behave as Lewis acid as well as nucleophile. This unique character provides several interesting applications in selective reactions useful for organic synthesis. Accordingly, new reduction of carbonyl compounds with pentacoordinate hydridosilicates, new regio- and enantioselective allylation of aldehydes with pentacoordinate allylsilanes, and the new cleavage reaction of C-Si bonds are discussed. In these reactions, it is demonstrated that unique characters of pentacoordinate silicon species as a Lewis acid play an important role. 

---