Photoredox-Catalyzed Formal [4 + 2] Cycloaddition of Pyridinium Masked Enols with Unactivated Olefins

Org Lett. 2025 Jun 29. doi: 10.1021/acs.orglett.5c02037. Online ahead of print.

Abstract

Herein, we developed a photoredox-catalyzed formal [4 + 2] cycloaddition of pyridinium-masked enols with unactivated olefins utilizing Ir(dF(Me)ppy)2(dtbbpy)PF6 as catalyst under blue light (455 nm) irradiation. A variety of pyridinium-masked enols and unactivated alkenes were examined to synthesize tetralones under optimal conditions. Through control and deuterium labeling experiments, it was suggested that the in situ generated α-carbon radical first attacked the alkene, forming a γ-carbonyl radical intermediate. This intermediate subsequently underwent intramolecular radical cyclization and deprotonation to produce the desired tetralones. This protocol facilitates the synthesis of functionalized tetralones under mild conditions, significantly enhancing the synthetic application of pyridinium-masked enols.