Photooxidation of Benzyl Alcohols with Immobilized Flavins

Benzyl alcohols are oxidized cleanly and efficiently to the corresponding aldehydes under irradiation using flavin photocatalysts and aerial oxygen as the terminal oxidant in homogeneous aqueous solution. Turnover frequencies (TOF) of more than 800 h(-1) and turnover numbers (TON) of up to 68 were obtained. Several flavin photocatalysts with fluorinated or hydrophobic aliphatic chains were immobilized on solid supports like fluorous silica gel, reversed phase silica gel or entrapped in polyethylene pellets. The catalytic efficiency of the heterogeneous photocatalysts was studied for the oxidation of different benzyl alcohols in water and compared to the analogous homogeneous reactions. Removal of the heterogeneous photocatalyst stops the reaction conversion immediately, which shows that the immobilized flavin is the catalytically active species. The immobilized catalysts are stable, retain their reactivity if compared to the corresponding homogeneous systems and are easily removed from the reaction mixture and reused. TOF of up to 26 h(-1), TON of 280 and up to 3 reaction cycles without loss of activity are possible with the heterogeneous flavin photocatalysts.