Abstract
Reductive transformations of substances that are difficult to reduce continue to pose challenges for photoredox catalysis. Promising photoreduction catalysts include flavin and deazaflavin derivatives; however, even their reductive abilities are limited for the range of substrates considered "inert". In this work, we present 5-deazaalloxazines, a new group of deazaflavin analogues that are predisposed to catalyze reductions due to their low reduction potential (down to -1.65 V vs. SCE) even in the ground state. We studied three series of 5-deazaalloxazines ([i] 5-unsubstituted, [ii] 5-aryldeazaalloxazines, and [iii] 5-trifluoromethyl-5-deazaalloxazines) to determine their photophysical and electrochemical properties and their ability to participate in model photoreduction reactions. From 31 compounds, we selected 1,3-dimethyl-7,8-dimethoxy-5-(o-tolyl)-5-deazaalloxazine [3a(o-MePh)], as it showed, among other things, the highest efficiency in photodehalogenation of p-fluoroanisole and was photostable and absorbed in the visible light region, thereby allowing photoreactions using a 400 nm LED. Practical applicability was demonstrated in the C─P coupling reaction of electron-rich aryl halides (including chloroanisoles and p-fluoroanisole) with trimethyl phosphite, providing an arylation reaction to form dimethyl arylphosphonates, and in the release/deprotection of amines from the corresponding tosyl and triflylamides.