Aromatic reactivity. Part XXXIX. Effects of ortho-substituents in protodesilylation

Abstract

The compounds o-XC6H4·SiMe3 with X = Br, I, But, SO3H, CO2H, NO2, Ac, and Bz have been made by appropriate desilylations of o-bis(trimethylsilyl)benzene. The rates of cleavage of these compounds, and of those with X = H, F. Cl, and Me2N, by a mixture of aqueous sulphuric acid (3 vol.) and acetic acid (4 vol.) at 50° have been measured spectrophotometrically. Values of the rates, krel, relative to that of phenyltrimethylsilane, are as follows: (X=) But, 8·0; F, 7·3 × 10–2; I, 3·8 × 10–2; Cl, 3·4 × 10–2; Br, 2·5 × 10–2; CO2H, 5·2 × 10–3; SO3H, 2·6 × 10–3; NO2, 6·8 × 10–5; Me2N, 6·8 × 10–5. (In the last case the effective substituent in the reaction medium is thought to be the protonated Me2HN+.)A plot of log krel for the ortho-substituents against [σp+r(σp+–σp)] is a good straight line when r= 0·3. For the corresponding para-substituents, the appropriate value of r is 0·7, and these results are consistent with the view that the inductive are more important relative to the resonance components of the substituent effects for ortho- than for para-groups. The high reactivity of o-bis(trimethylsilyl)benzene is attributed to steric acceleration, but the reactivity of the o-fluoro-compound, which is anomalously low in terms of the analysis mentioned, is left unexplained.Cleavage of the aryl–silicon bond occurs when the amine o-Me2N·C6H4·SiMe3 is treated with methyl iodide in methanol.