Dearylation of α,ω-diphenylpermethylated oligosilanes with triflic acid

Abstract

The relative rates of displacement of phenyl groups for a series of α,ω-diphenylpermethylated oligosilanes with the formula Ph(SiMe 2) n Ph ( n = 2−5) were studied. Triflic acid was utilized in the displacement reactions which occur as a two-step process with protonation at the ipso-carbon atom as the rate limiting step. The results showed the displacement of the firs phenyl group is more facile than the second group. The largest difference in reactivities is found for the disilane. Competitive displacement reactions between various oligomers were analyzed to establish the influence of the chain length of oligosilanes on the relative reactivities of the terminal phenyl groups. Both the first and second phenyl group displacement reactions are faster for the longer chain oligomers than the shorter analogs. The 13C NMR spectra of the oligosilanes indicate the highest election density on the ipso-carbon atoms in the disilane, in contrast to its lowest reactivity. This is interpreted by the increased stabilization of the positive charge int he transition state with an increase in the chain length. Thus, the reactivities of oligosilanes are governed by the structures of the transition states rather than the ground states.