PROTON ADDITION TO SILYLSTYRENES - OVERCOMING THE PREDILECTION FOR PROTIODESILYLATION

Normally, organosilyl nucleophiles such as vinylsilanes and allylsilan es undergo protiodesilylation reactions with protons. To favour additi on reactions under these conditions, the ligands on silicon have been modified such that the leaving group ability and, simultaneously, the beta-effect of the silyl group is reduced. In the case of allylsilanes , the use of dichiorosilyl groups does not significantly favour additi on over substitution processes at the olefin. However, with vinylsilan es bearing a second pi-nucleophile, a dichlorosilyl group can be used to regioselectively direct the formation of two bonds (C-H and C-C) se quentially in a process in which the silicon is not lost from the mole cule, but may ultimately be cleaved leading to the formation of diols. Thus, benzyldichlorostyrylsilane 7, after cyclization to 9 in the pre sence of triflic acid, is converted into diol 12. The synthetic utilit y of this process is restricted by the relatively low reactivity of th e styryl pi-system and the necessarily reactive electrophiles needed t o initiate the process. The effect of changing from electron-donating groups to electronegative groups on silicon on reaction mechanism is d iscussed.