Mechanisms of Photoinduced Reactions from Kinetic IsotopeEffects

Abstract

Photochemistry lies at center of life due to its role in photosynthesis. Photochemical reactions can achieve chemical transformations without the need for additional reagents or catalysts, making them attractive alternatives to reactions that produce environmentally hazardous waste. Our work provides modern interpretations for photochemical reactions: photosensitized di-π-methane rearrangement and photoredox [2 + 2] cycloadditions. These reactions play a fundamental role in understanding key processes in photochemical reactions, including electron transfer and energy transfer. The di-π-methane rearrangement can be achieved by irradiation in the presence of photosensitizers. The qualitative mechanism can be understood on triplet surface involving cyclopropyldicarbinyl intermediates. We performed comprehensive experimental carbon kinetic isotope effect measurements and theoretical predictions to study the mechanism of the di-π-methane rearrangement. Reactions sensitized by low-energy sensitizers fit with statistical expectation with significant heavy-atom tunneling. On the other hand, the reactions sensitized by high-energy sensitizers display a great amount of dynamic effect. Quasiclassical dynamic trajectories suggest that the excess vibrational energy available from triplet energy transfer leads to hot and nonstatistical dynamics in the rearrangement. The use of photoredox catalysts in organic synthesis has received great attention in the last decade. A striking example of the synthetically valuable and complex photoredox reactions developed in recent years is the [2 + 2]-cycloaddition of enones that occurs with visible light in the presence of RuII(bpy)3Cl2 as photosensitizer, as developed by Yoon and coworkers. However, the mechanisms of photoredox-promoted reactions are intrinsically complex. A combination of photophysical steps, one or more electron-transfer steps leading to activated substrates, chemical conversions of radical ions, chain-transfer steps, and termination steps may be involved. Our results are explained as arising from a competition between C−C bond formation and electron exchange between substrate alkenes. This qualitatively supports the computational predictions that the first bond formation is the first irreversible step undergone by the starting materials. This study suggests the possibility of ways to control the chemoselectivity.