Electrophilic reactions, disilenes

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

Although the chemistry of disilenes has been developed considerably after the isolation of stable disilenes by West, Finle and Michl, rather little has been known about the mechanism, especially on the stereochemistry for the addition reactions to Si=Si bonds of nucleophiles or electrophiles. West and coworkers reported that the addition reaction of ethanol to ( )-l,2-di-t-butyl-l,2-dimesityldisilene in THE gave a 1 1 mixture of two diastereomers of alkoxysilanes (equation 9), suggesting a stepwise mechanism. However, the bulky substituents necessary to stabilize disilenes sometimes complicate the stereochemistry. In fact, for the addition reaction of water to the parent disilene (H2Si=SiH2), theoretical calculation predicted a concerted-type four-center-like transition state, leading to a yw-addition product . 

The reaction mechanism and the stereochemical diversity of the addition of water to disilene has been studied at the MP2/6-311-h+G level. Two pathways are feasible leading to syn and anti-addition. The syn addition proceeds via nucleophilic attack by water oxygen with a barrier of ca. 12 kJ mol k anti-Addition proceeds via intramolecular electrophilic attack by water hydrogen in a weakly bound disilene/water complex with antarafacial approach, in accordance with the Woodward-Hoffmann rules, and leads to an activation barrier of ca. 22 kJ mol ... 

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