AROMATIC REACTIVITY .39. EFFECTS OF ORTHO-SUBSTITUENTS IN PROTODESILYLATION

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 Me 2HN+.) 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-Me 2N·C6H4·SiMe3 is treated with methyl iodide in methanol.