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 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, 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 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. Shining light on increased activity: Aryl rings that are perpendicularly oriented relative to the flavin chromophore substantially decrease aggregation relative to nonsubstituted derivatives. These 10‐arylflavin derivatives are more efficient photocatalysts for the oxidation of 4‐methoxybenzyl alcohol (see scheme).