Proton addition to silylstyrenes: Overcoming the predilection for protiodesilylation

Normally, organosilyl nucleophiles such as vinylsilanes and allylsilanes undergo protiodesilylation reactions with protons. To favour addition reactions under these conditions, the ligands on silicon have been modified such that the leaving group ability and, simultaneously, the β-effect of the silyl group is reduced. In the case of allylsilanes, the use of dichlorosilyl groups does not significantly favour addition over substitution processes at the olefin. However, with vinylsilanes bearing a second π- nucleophile, a dichlorosilyl group can be used to regioselectively direct the formation of two bonds (CH and CC) sequentially in a process in which the silicon is not lost from the molecule, but may ultimately be cleaved leading to the formation of diols. Thus, benzyldichlorostyrylsilane 7, after cyclization to 9 in the presence of triflic acid, is converted into diol 12. The synthetic utility of this process is restricted by the relatively low reactivity of the styryl π-system and the necessarily reactive electrophiles needed to initiate the process. The effect of changing from electron-donating groups to clectronegative groups on silicon on reaction mechanism is discussed.