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

Abstract

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 a–f)) 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 a–d do not interact by π–π interactions, which are restricted by the 10-phenyl ring oriented perpendicularly to the isoalloxazine skeleton. On the other hand, N3H⋅⋅⋅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 CH⋅⋅⋅O bonds and weak π–π 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.