Density Functional Theory Evaluation of a Photoinduced Intramolecular Aryl Ether Rearrangement

Recently, a new approach of converting (hetero)­aryl ethers to C–C coupled products via a photoinduced intramolecular rearrangement has been reported. Although this reaction is photocatalyst-free, it requires excitation in the ultraviolet (UV) range. To help refine this process, three different 2-(hetero)­aryloxybenzaldehydes are selected from the available substrate scope in which the general mechanism based on experimental results is evaluated using density functional theory calculations. The reaction takes place in the triplet state after photoexcitation and includes three main steps: the addition of carbonyl carbon to the ipso carbon of the aryl ether followed by the C–O cleavage of the resulting spirocyclic intermediates and then the transfer of the formyl proton to afford 2-hydroxybenzophenone-type products. This agrees with the experiments, but the calculated pathways show considerable differences between the three substrates. Above all, either the first or the second step can be rate-determining but not the C–H activation. The important factor behind the differences is the spin-density rearrangement, which is mainly responsible for the barrier of the ether cleavage. Based on the obtained insights, the strategy to improve the ∼250 nm excitation has been briefly discussed, and promising molecules are proposed to improve the scope of this process.近日，有研究报道了一种通过光诱导分子内重排将（杂）芳基醚转化为C-C偶联产物的新方法。尽管该反应无需光催化剂，但需要在紫外（UV）波段进行激发。为优化该反应流程，研究人员从已报道的底物范围中选取了三种不同的2-(杂)芳氧基苯甲醛，并基于实验结果提出的通用反应机理，通过密度泛函理论（DFT）计算对其进行了验证分析。该反应在光激发后于三重态下进行，主要包含三步：首先是羰基碳加成至芳基醚的本位碳原子，随后对生成的螺环中间体进行C-O键断裂，最后通过甲酰基质子转移得到2-羟基二苯甲酮类产物。该机理与实验结果相符，但计算得到的反应路径在三种底物间存在显著差异。尤为关键的是，决速步可为第一步或第二步，而非C-H活化步骤。造成这些差异的核心因素是自旋密度重排，其主要决定了醚键断裂步骤的能垒。基于所得研究结论，本文简要探讨了优化约250 nm激发波长的策略，并提出了若干有望拓宽该反应底物适用范围的候选分子。