Deazaalloxazines – flavin derivatives that provide reductive photoredox catalysis with inert substrates

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 catalyse 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 (absorption/emission spectra, fluorescence quantum yield and lifetime, quantum yield of singlet oxygen production, triplet state lifetime and redox potentials) 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 between 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.