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

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.

在二氧化硫/氧气气氛下，在二氧化钛和可见光（λ≥400 nm）存在的情况下，首次实现了烷烃的催化光磺氧化反应。对于正庚烷和环己烷，反应在纯液体中进行，对于金刚烷则在冰醋酸中进行。二氧化钛表面与二氧化硫的电荷转移（CT）络合在400 - 420 nm处产生一个CT带，这是原本只吸收紫外光的二氧化钛具有可见光活性的原因。光电子转移产生的初级电荷通过烷烃的解离电子转移以及氧还原产生的羟基自由基夺氢产生烷基自由基。一旦形成，烷基自由基就会引发自由基链式反应，这与在无催化剂情况下经典的紫外诱导磺氧化反应中所熟知的情况相同。该反应表现出受产物强吸附抑制的特征。因此，用甲醇洗涤催化剂后，初始光催化活性完全恢复。时间分辨光电压测量表明，光催化剂失活与二氧化钛从n型变为p型有关。少量的水和自由基清除剂会抑制产物的形成。该反应具有高化学选择性，因为只形成了微量预期的副产物，如硫酸盐、酮和醇。因此，除了在可见光诱导电荷产生中的基本作用外，二氧化钛表面还能实现化学选择性的C - S键形成。