Cleavage of aryl-silicon bonds by electrophilic reagents.

A survey is presented of the literature on cleavage of ary1-silicon bonds by electrophilic reagents. The following compounds have been prepared: (i) a series of triaryl- and trialkyl-p-anisylsilanes (ten of them new), (ii) the compounds (p-anisyl)4-x Si(OEt)x, where x=l to 3, (all new), and (iii) four cyclic compounds (one of them new), in which silicon, joined to two aromatic nuclei, is part of the ring system. Rates of cleavage of these compounds by aqueous-methanolic perchloric acid have been measured at 50. For triaryl-p-anisyl- silanes, (X.C6H4)3 SiC6H4.OMe-p, and trialkyl-p-anisylsilanes, R3SiC6H4.OMe-p, increase in electron supply from X and R increases the rate of cleavage. The logarithms of the relative rates of cleavage for the (X.C6H4)3 SiC6H4.OMe-p compounds correlate satisfactorily with Hammett's ? constants for the various substituents, X. When X=o-Me, the rate of cleavage is very low, presumably because of steric hindrance to cleavage. For the series Me3-x Phx SiC3H4.OMe-p (x=0 to 3), stepwise replacement of methyl by phenyl causes a stepwise decrease in the rate of cleavage. The rates of cleavage of the compounds (p-anisyl)4-x Si(OEt)X (x=0 to 3) differ little from one another but are in the order x=0 < 1 3. When silicon is part of the ring system the Si-aryl bond appears to have increased stability. Rates of cleavage of the triaryl- and trialkyl-p-anisylsilanes and of the cyclic organosilicon compounds by bromine in 1.5 wt.% aqueous acetic acid at 25 have been measured. The logarithms of the relative rates for the cleavage correlate well with the logarithms of the relative rates for acid cleavage, but there are deviations, which are discussed. Rates of cleavage of triaryl- and trialky1-p-anisylsilanes by mercuric acetate in 20 wt.% aqueous acetic acid, at 25 have been measured. The logarithms of the relative rates for the cleavage correlate well with both the logarithms of relative rates for cleavage by acid and for cleavage by bromine; again some deviations occur and these are discussed.