Understanding photoenolization of O‐methyl acetophenone, and its subsequent [4 + 2] and 1,4 addition of its photoproduct from an aromaticity perspective

Abstract

AbstractPhoton‐driven enolization of aromatic ketones and aldehydes with an ortho alkyl group has been suggested to proceed through triplet state surface after photoexcitation. We have investigated the photoexcitation of 2‐methoxy‐6‐methylacetophenone using time‐dependent density functional theory (TDDFT) and complete active space self‐consistent field (CASSCF) theory, following which we have studied excited state proton transfer on the triplet surface and have computed the aromaticity of both the ground and excited state intermediates and transition states using nucleus‐independent chemical shifts (NICSs) based on multi‐reference wavefunction. The barrier for the crucial proton transfer for photoenolization on ππ* lowest Triplet surface is predicted to be a paltry 3.7 kcal/mol, as compared to the prohibitively high barrier of 38 kcal/mol on the S0 surface at M06‐2X/6‐311++G(d,P) level of theory. We show that a favorable change from anti‐aromaticity to aromaticity on the ππ* triplet surface makes proton transfer facile, whereas in the ground state reaction, reduction of aromaticity along the reaction coordinate is responsible for a higher barrier. Moreover, it was found that the product arising from photoenolization can serve as a more efficient diene than traditional dienes such as 1,3‐butadiene, 1‐methoxy‐1,3‐butadiene for Diels–Alder‐based [4 + 2] cyclization and 1,4 addition due to the significant gain in aromaticity in the transition state of the former case as compared to those of the traditional ones.