The [2 + 2] cycloaddition of enones provides access to acyl-substituted cyclobutanes. In 2008, Yoon and coworkers published a novel synthetic method for constructing cyclobutanes from enones through a photoredox process.1 This method could be applied to either intramolecular or intermolecular cycloadditions.2 Currently, many aspects of the mechanism can be qualitatively described from general chemistry knowledge. However, the mechanisms of photoredox-promoted reactions are intrinsically complex, involving 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. We describe a combination of experimental and theoretical studies of the dimerization of phenyl vinyl ketone and the hetero-coupling between phenyl propenyl ketone and methyl vinyl ketone. A relatively small kinetic isotope effect (KIE) at β-carbon was found in both cases by using the method developed by our group in 1995.3 This qualitatively supports the computational predictions that the first bond formation is the first irreversible step undergone by the starting materials. Small KIEs can be explained by electron transfer from activated enones to neutral enones. This electron transfer was gauged by measuring kinetics: the competitive reactions of methyl vinyl ketone with phenyl propenyl ketone or 4-chlorophenyl propenyl ketone. The kinetic models of the reactions involving significant electron transfer (Curtin-Hammett scenario) and in the absence of such electron transfer were simulated. Compared with these simulated models, experimental results show that the barrier of electron transfer is comparable to that of first bond formation.

Mechanistic study of photocatalytic [2+2] cycloaddition of α,β-unsaturated ketones

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.