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

Abstract

The compounds o-XC₆H₄·SiMe₃ with X = Br, I, Bu^t, 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, k_rel, relative to that of phenyltrimethylsilane, are as follows: (X=) Bu^t, 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 k_rel for the ortho-substituents against [σ_p+r(σ_p+–σ_p)] 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.