苄醇的氧化是现代有机合成中的重要转变。已经开发了大量的光氧化还原方案来实现醇的有氧氧化为羰基化合物。最近,几个小组描述了紫外线(UV)或紫色光可以在没有外部催化剂的情况下引发苯甲醇的好氧氧化,并描述了涉及·O2-作为关键活性氧(ROS)的光诱导的不同机制。然而,基于全面的机械调查,包括对照实验,自由基猝灭实验,EPR研究,紫外-可见光谱,动力学研究,和密度泛函理论计算(DFT),我们在这里阐明HOO•,通过H2O2消除α-羟基过氧自由基[ArCR(OH)OO•]释放,作为苯甲基醇的自催化光氧化的真正的链载体。澄清了先例文献中描述的机械歧义,就关键的ROS及其演变而言,限速步骤,和主要的自由基级联反应。这项工作强调了对涉及醛(或酮)生成的紫外线驱动的氧化反应进行更严格的机理分析的必要性。
The oxidation of benzylic alcohols is an important transformation in modern organic synthesis. A plethora of photoredox protocols have been developed to achieve the aerobic oxidation of alcohols into carbonyls. Recently, several groups described that ultraviolet (UV) or purple light can initiate the aerobic oxidation of benzylic alcohols in the absence of an external catalyst, and depicted different mechanisms involving the photoinduction of •O2- as a critical reactive oxygen species (ROS). However, based on comprehensive mechanistic investigations, including control experiments, radical quenching experiments, EPR studies, UV-vis spectroscopy, kinetics studies, and density functional theory calculations (DFT), we elucidate here that HOO•, which is released via the H2O2 elimination of α-hydroxyl peroxyl radicals [ArCR(OH)OO•], serves as the real chain carrier for the autocatalytic photooxidation of benzylic alcohols. The mechanistic ambiguities depicted in the precedent literature are clarified, in terms of the crucial ROS and its evolution, the rate-limiting step, and the primary radical cascade. This work highlights the necessity of stricter mechanistic analyses on UV-driven oxidative reactions that involve aldehydes\' (or ketones) generation.