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

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, <i>σ</i>_mb and <i>σ</i>_<i>jj</i>·, 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.