Chemoselective Anaerobic Dehydrogenation of Alcohols by Using Visible Light and a Positively Charged Deazaflavin Catalyst Regenerated by Acetonitrile Solvent

Abstract

Three series of novel deazaflavinum salts differing in their substitutions at positions 5 (R = H, phenyl or mesityl), 7 and 8 (R = OMe, Me, H or Cl) were synthesized as potential catalysts of a novel chemoselective visible light–mediated anaerobic oxidation of primary and secondary alcohols to carbonyl compounds. This mild procedure uses acetonitrile as a solvent, which acts simultaneously as a sacrificial electron acceptor (in place of the oxygen usually used in photooxidation reactions) and therefore the reaction does not need any additives. Structural and properties‐versus‐catalytic activity studies identified 5‐mesityl‐7,8‐dimethoxy‐3‐methyldeazaflavinium chloride (3a‐Cl) as the most potent catalyst. 3a‐Cl was effective in non‐deuterated acetonitrile (CH3CN), unlike its original 5‐phenyl analogue 2a‐Cl, which is efficient only in deuterated solvent (CD3CN). This difference arises because the regeneration of the 2a‐Cl catalyst is slower in CH3CN than in CD3CN. Our method using the optimized 3a‐Cl photocatalyst and CH3CN as a sacrificial oxidant and solvent in one is a useful addition to synthetic organic chemistry. Anaerobic conditions prevent side oxygenation reactions and overoxidations that usually occur in air or oxygen. This property makes this method suitable for dehydrogenations of alcohols that possess additional group(s) sensitive to oxygenation.