Controlled oxidation of organic sulfides to sulfoxides under ambient conditions by a series of titanium isopropoxide complexes using environmentally benign H2O2 as an oxidant

Abstract

Controlled oxidation of organic sulfides to sulfoxides under ambient conditions has been achieved by a series of titanium isopropoxide complexes that use environmentally benign H₂O₂ as a primary oxidant. Specifically, the [N,N′-bis(2-oxo-3-R₁-5-R₂-phenylmethyl)-N,N′-bis(methylene-R₃)-ethylenediamine]Ti(OⁱPr)₂ [R₁ = t-Bu, R₂ = Me, R₃ = C₇H₅O₂ (1b); R₁ = R₂ = t-Bu, R₃ = C₇H₅O₂ (2b); R₁ = R₂ = Cl, R₃ = C₇H₅O₂ (3b) and R₁ = R₂ = Cl, R₃ = C₆H₅ (4b)] complexes efficiently catalyzed the sulfoxidation reactions of organic sulfides to sulfoxides at room temperature within 30 min of the reaction time using aqueous H₂O₂ as an oxidant. A mechanistic pathway, modeled using density functional theory for a representative thioanisole substrate catalyzed by 4b, suggested that the reaction proceeds via a titanium peroxo intermediate 4c′, which displays an activation barrier of 22.5 kcal mol⁻¹ (ΔG^‡) for the overall catalytic cycle in undergoing an attack by the S atom of the thioanisole substrate at its σ*-orbital of the peroxo moiety. The formation of the titanium peroxo intermediate was experimentally corroborated by a mild ionization atmospheric pressure chemical ionization (APCI) mass spectrometric technique.