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.