Deciphering the Concerted PCET/Decarboxylation Pathway in Photocatalyst-Free Acylation of Activated Alkenes to 1,4-Dicarbonyls

Abstract

4-Dicarbonyl motifs are notoriously difficult to synthesize, yet the mechanistic underpinnings of conventional electron donoracceptor (EDA) strategies remain contentious. Here we unambiguously resolve this debate and disprove the hydrogen-bonding EDA (H-EDA) mechanism for decarboxylative acylation of activated alkenes with α-keto acids, establishing a concerted proton-coupled electron transfer (PCET) pathway as the exclusive operative mechanism. A combination of spectroscopic, electrochemical, photophysical, and computational studies provides definitive evidence against EDA/H-EDA formation and electron transfer, while DFT calculations reveal an exceptionally low activation barrier for concerted PCET (ΔG ‡ /ΔE ‡ = 5.1 -11.6 kcal/mol), consistent with high efficiency under mild conditions. This photocatalyst-and base-free visible-light protocol enables rapid assembly of diverse 1,4-dicarbonyl compounds, with broad substrate scope, exceptional functional group compatibility, and reliable gram-scale synthesis. This work not only delivers a practical sustainable route to 1,4-dicarbonyls but also corrects a pervasive mechanistic misunderstanding in catalyst-free photodecarboxylation, laying a general mechanistic foundation for rational development of visible-light-mediated transformations.