Visible-Light-Induced Sulfoxidation of Alkanes in the Presence of Titania

Abstract

The first catalytic photosulfoxidation of alkanes is accomplished in the presence of titanium dioxide and visible light (λ≥400 nm) under an atmosphere of SO₂/O₂. For n-heptane and cyclohexane the reaction is performed in the neat liquid, for adamantane in glacial acetic acid. Charge-transfer (CT) complexation of sulfur dioxide by the titania surface generates a CT band at 400–420 nm responsible for the visible-light activity of otherwise only UV light absorbing titania. The primary charges generated upon optical electron transfer produce alkyl radicals by dissociative electron transfer from the alkane and by hydrogen abstraction by OH radicals produced from oxygen reduction. Once formed, the alkyl radicals initiate a radical chain reaction as known from the classical UV-induced sulfoxidation in the absence of a catalyst. The reaction exhibits features characteristic for product inhibition by strong adsorption. Accordingly, the initial photocatalytic activity is fully restored after washing the catalyst with methanol. Time-resolved photovoltage measurements indicate that photocatalyst deactivation is connected with a change from n-type to p-type titanium dioxide. Small amounts of water and radical scavengers inhibit product formation. The reaction proceeds with high chemoselectivity because only traces of expected by-products like sulfates, ketones, and alcohols are formed. Thus, in addition to its basic role in visible-light-induced charge generation, the surface of titania enables also a chemoselective CS bond formation.