Protiodesilylation of substituted 2-trimethylsilylthiophens

Abstract

First-order rate constants at 50 °C have been determined for cleavage of the thiophen compounds XC₄H₂S˙SiMe₃–2 in mixtures of (a) MeOH (5 vol.) with aqueous HClO₄(2 vol.) and (b) CH₃CO₂H (4 vol.) with aqueous H₂SO₄(3 vol.). Relative rates, k_rel, in the first medium are (X =) 5-OMe, 7 200; 5-Me, 36; H, 1.00; 5-Cl, 1.08 × 10^–1; 5-Br, 9.0 × 10^–2; 3-Br, 5.0 × 10^–2; 4-Br, 4.9 × 10^–3, and in the second medium, 5-Me, 36; H, 1.00; 5-Cl, 1.28 × 10^–1; 5-Br, 9.8 × 10^–2; 3-Br, 5.5 × 10^–2; 4-Br, 6.5 × 10^–3; 5-NO₂, 7.1 × 10^–7; 3-NO₂, 7.1 × 10^–7. Except for the nitro-compounds, the two sets of log k_rel values show excellent linear correlations with values for the corresponding XC₆H₄˙SiMe₃ compounds, with 3–, 4–, and 5-X in the thiophen ring being taken as equivalent to o-, m-, and p-X groups, respectively, in the benzene ring, the effects of substituents being a little larger in the thiophen than the benzene system; 3– and 5-O₂NC₄H₂S˙SiMe₃ are less reactive than would be expected from the correlations. Again with the exception of the data for the nitro-compound, the log k_rel values for the 4– and 5– substituted thiophens show very good correlation with σ_m⁺ and σ_p⁺ constants.