Reactive Intermediates in Carbon-Functional Organosilicon Reactions

Abstract

Addition, substitution and reduction were observed to occur as competing processes in the reaction between vinyltrichlorosilane and isopropylmagnesium bromide, whereas only addition and reduction were observed in the reaction between vinyltrichlorosilane and tert-butylmagnesium chloride. Unsuccessful attempts were made to prepare compounds containing silacyclopropane rings via Grignard cyclisation reactions of sterically hindered (2-bromoethyl) dialkylchlorosilanes. Sterically hindered (2-bromoethyl) trialkylsilanes were prepared and some reactions investigated. To account for the substitution product as well as the cleavage product from the solvolysis of (2-bromoethyl) tri-n-butylsilane, a mechanism involving attack of the carbonium ion intermediate as the rate-determining step is proposed. Rearrangement of (2-hydroxyethyl-2,2-d₂) tri-n-butylsilane with phosphorus tribromide and thionyl chloride was observed. The carbonium ion Bu3SiCH₂CH₂+ proposed as an intermediate in the rearrangement reaction was trapped using the silane transfer reaction. Observations indicate that if nucleophilic attack takes place at silicon, formation of the Grignard reagent from (2-halogenoalkyl)trialkylsilane is unlikely to occur; whereas if nucleophilic attack does not take place at silicon then Grignard formation is possible. No rearrangement was observed between chloromethyltri-n-butylsilane and a nucleophilic reagent such as sodium ethoxide or an electrophilic reagent such as aluminium chloride. An exploratory study of the pyrolysis of silacyclobutanes was undertaken with a view to the detection of silicon-carbon π-bonded intermediates.