Organosilanes nucleophilic substitution

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

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. 

Interestingly, the apicophilicity sequence closely parallels the rate of racemi-zation and die change of the stereochemical outcome in nucleophilic substitution from retention to inversion in reaction of tetracoordinated organosilanes ... 

Nucleophilic Substitution. Eabom reported that fluorosi-lanes are more reactive toward organometallic reagents than the corresponding chlorosilanes, and therefore more suited for preparation of sterically hindered organosilanes. Two examples of this enhanced reactivity are shown for the Tsi anion (eq 1) and a dianion (eq 2). The use of less-hindered reagents in stoichiometric amounts still allow for the sequential introduction of nucleophiles (eq 3).l ... 

Organosilanes bearing an electronegative heteroatom(s) at the a-carbon are susceptible to nucleophihc activation leading to silicon-carbon bond-cleavage, because of the electronic effect of the heteroatom. a-Heteroatom-substituted organosilanes are therefore quite valuable as protected carbon nucleophiles. The silicon-carbon bond is also readily activated by a transition metal complex. The reactivity is successfully utilized for catalytic carbon-carbon bond formation. 

One class of reaction of organosilanes that deserves further mechanistic study involves alkylsilanes functionally substituted near the c—si bond. These systems can involve electrophilic and/or nucleophilic attack at the points indicated... 

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