THE CHEMICAL REACTIVITY OF THE ARYL-SILICON BOND.

A study was made of the effect of varying the non-cleaved groups, R1, on the rate of cleavage of p-methoxyphenyltriorgansilanes, p-MeO.C6H4.S1R3, by aqueous methanolic perchloric acid. With triphenyl-p-methoxyphenylsilanes, m- or p-X.C6H43 SiC6H4.Me-p, electron-releasing substituents facilitate and electron-withdrawing groups retard reaction the effects are small, and in accord with the beta-constants of the substituents. Because of steric hindrance, tri-o- is 670 times less reactive than tri-p-tolyl-p-methoxyphenylsilane. The relative rates of cleavage of trialkyl-compounds, p-MeO.C6H4.SiR3, are R Me, 1 Et, 0.48 n-Pr, 0.42 i-Pr, 0.06, and this order is attributed to steric effects. Chloro- and ethylamino-methyl-p-methoxyphenyl- dimethyl-silane the latter being present almost wholly as its conjugate acid in the reaction medium are cleaved 8 and 50 times, respectively, less readily the trimethyl-compound, because of the increased electron-withdrawal from silicon. Steric and polar factors combine to make tribenzyl-p-methoxyphenylsilane 11 times less reactive than the trimethyl-compound. With the compounds MexPh3-xSi.C6H4.OMe-p, the ease of cleavage falls as x increases, the triphenyl- being 61 times less reactive than the trimethyl-compound. Author