Disilenes electrophilic addition

Kinetic isotope effect measurements support this interpretation. A small kinetic isotope effect (kR/kD = 0.71) is observed for p-methoxyphenol in agreement with a ratedetermining nucleophilic attack, while a large kinetic isotope effect (kR/kR = 5.27), observed for p-trifluoromethylphenol, strongly supports a mechanism in which a phenolic H (or D) is transferred to 39 in the rate-determining step. Unfortunately, 39 is a symmetric disilene so that diastereoselectivity could not be determined. It will be interesting to examine whether the diastereoselectivity will be effected by the change in the addition mechanism from electrophilic to nucleophilic. 

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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