Direct excitation strategy for deacylative couplings of ketones.

ABSTRACT

The homolysis of chemical bonds represents one of the most fundamental reactivities of excited molecules. Historically, it has been exploited to generate radicals under ultraviolet (UV) light irradiation. However, unlike most contemporary radical-generating mechanisms, the direct excitation to homolyze chemical bonds and produce aliphatic carbon-centered radicals under visible light remains rare, especially in metallaphotoredox cross couplings. Herein, we present our design of the dihydropyrimidoquinolinone (DHPQ) reagents derived from ketones, which can undergo formal deacylation and homolytic C-C bond cleavage to release alkyl radicals without external photocatalysts. Spectroscopic and computational analysis reveal unique optical and structural features of DHPQs, rationalizing their faster kinetics in alkyl radical generation than a structurally similar but visible-light transparent radical precursor. Such a capability allows DHPQ to facilitate a wide range of Ni-metallaphotoredox cross couplings with aryl, alkynyl and acyl halides. Other catalytic and non-catalyzed alkylative transformations of DHPQs are also feasible with various radical acceptors. We believe this work would be of broad interest, aiding the synthetic planning with simplified operation and expanding the synthetic reach of photocatalyst-free approaches in cutting-edge research.