Visible-light induced photocatalyst-free synthesis of β-enaminones

Abstract

An environmentally friendly and efficient synthesis of β-enaminones through photo-acylation of vinyl azides is described herein, utilizing 4-acyl-1,4-dihydropyridines (4-acyl-1,4-DHPs) as dual-function reagents. These reagents simultaneously serve as reductants and radical initiators. The method accommodates a broad range of vinyl azides, demonstrating excellent functional group tolerance and enabling potential application. Comprehensive mechanistic exploration was conducted employing TEMPO trapping, on–off experiments, and isotopic labelling studies. Key to our understanding was the integration of dual-parameter Hammett analysis and kinetic isotope effect (KIE) studies. The Hammett analysis, using quadratic linear regression with dual radical parameters, σ_mb and σ_jj·, highlighted the involvement of radical mechanisms and anionic intermediates. In conjunction with KIE studies, these analyses revealed that the rate-limiting step in the transformation is the single-electron reduction of the conjugated enamine radical to the corresponding anion. This finding offers valuable insights into the reaction dynamics and could guide the development of related synthetic strategies.