Aggregation Effects in Visible-Light Flavin Photocatalysts: Synthesis, Structure, and Catalytic Activity of 10-Arylflavins

A series of 10-arylflavins (10-phenyl-, 10-(2',6'-dimethylphenyl)-, 10-(2',6'-diethylphenyl)-, 10-(2',6'-diisopropylphenyl)-, 10-(2'-tert-butylphenyl)-, and 10-(2',6'-dimethylphenyl)-3-methylisoalloxazine (2?af)) was prepared as potentially nonaggregating flavin photocatalysts. The investigation of their structures in the crystalline phase combined with 1H-DOSY NMR spectroscopic experiments in CD3CN, CD3CN/D2O (1:1), and D2O confirm the decreased ability of 10-arylflavins 2 to form aggregates relative to tetra-O-acetyl riboflavin (1). 10-Arylflavins 2?ad do not interact by pp interactions, which are restricted by the 10-phenyl ring oriented perpendicularly to the isoalloxazine skeleton. On the other hand, N3?H...O hydrogen bonds were detected in their crystal structures. In the structure of 10-aryl-3-methylflavin (2?f) with a substituted N3 position, weak C?H...O bonds and weak pp interactions were found. 10-Arylflavins 2 were tested as photoredox catalysts for the aerial oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde (model reaction), thus showing higher efficiency relative to 1. The quantum yields of 4-methoxybenzyl alcohol oxidation reactions mediated by arylflavins 2 were higher by almost one order of magnitude relative to values in the presence of 1.