Photocatalytic Oxidative [2+2] Cycloelimination Reactions with Flavinium Salts: Mechanistic Study and Influence of the Catalyst Structure

Flavinium salts are frequently used in organo‐catalysis but their application in photoredox catalysis has not been systematically investigated, yet. Here, we synthesize a series of 5‐ethyl‐3‐methylalloxazinium salts with different substituents in positions 7 and 8 and investigate their application in light‐dependent oxidative cycloelimination of cyclobutanes. Detailed mechanistic investigations with a coumarin dimer as a model substrate reveal that the reaction preferentially occurs via the triplet‐born radical pair after electron transfer from the substrate to the triplet state of an alloxazinium salt. The very photostable 7,8‐dimethoxyderivative turns out to be a superior catalyst with a sufficiently high oxidation power (E* = 2.26 V) allowing the conversion of various cyclobutanes (with Eox up to 2.05 V) in high yields. Even compounds can be converted whose opening requires overcoming a high activation barrier due to a missing pre‐activation caused by bulky adjacent substituents, that favour ring opening, e.g. all‐trans dimethyl 3,4‐bis(4‐methoxyphenyl)cyclobutane‐1,2‐dicarboxylate