The electrophilic cleavage of allylsilanes : investigations into the mechanism of the electrophilic cleavage of allylsilanes and the synthesis of optically active silicon centred allylic silacyclopentanes

The mechanism of the reaction of allyltrimethylsilanes with electrophiles was investigated using a number of approaches. Firstly, the kinetics of the reaction of the relatively simple allylsilane, allyltrimethylsilane, with dichloroacetic acid were investigated by 1H NMR. Both addition and electrophilic cleavage reactions were observed, with no reversion to starting material occurring. A comparison was made with the reaction of the alkene, 4,4-dimethylpent-l-ene, where a rate enhancement of four orders of magnitude was observed on substituting the s-CMe3 group by a s-SiMe3 group. The second approach involved the synthesis of an optically active siliconcentred allylsilane, based on the relatively uncongested, unstrained silacyclopentane structure. Of the methods of synthesis attempted, the most successful was asymmetric synthesis via dimenth-3-oxysilane derivatives. Using this route, optically active 1-cis-crotyl-2-methyl-1-phenyl-1-silacyclopentane was synthesised. The electrophilic cleavage of this allylsilane with anhydrous hydrogen chloride proceeded with inversion (after the intermediate formation of an addition compound). The reactions of various allylsilanes with trifluoroacetic acid were also investigated. These reactions generally proceeded with y-addition relative to silicon, followed by rearrangement and cleavage in an SE’ fashion. However, for the silane derivatives of s-pinene (E- and Z-1,2-dimethyl-(2,10-pinen-10-yl)methylsilacyclopentane) the stabilisation offered by the formation of a tertiary carbenium ion outweighed that produced by the s-silyl effect. Thus, s-addition of a proton followed by rearrangement of the intermediate carbenium ion and ring opening of the pinene structure before addition of the anion was observed.