We have developedametal-free process for the aerobic photooxygenation of sulfides to sulfoxides mediated by riboflavin tetraacetate or riboflavin (vitamin B 2 )p hotocatalysts andv isible light (450 nm) in an acetonitrile-water (85:15 v/v) mixture. Theo ptimised solvent systeml eads to both singletoxygena nd electron-transfer pathways in photooxygenation,t husa llowing oxidation of electron-poor and electron-rich thioanisoles,d ialkyl sulfidesa nd sterically hindereds ulfides.B esides having ab road substrate scope,t he method has very shortr eaction times and requiresl ow catalyst loading (downt o 0.1 mol%). These properties are due to the high photocatalyst stability and the extremely high quantum yields (1.3 for thioanisole oxygenation). Moreover, the method is chemoselective,p roducing only sulfox-ides without overoxidation to sulfones.T aking into account the broad substrate scope,h ighs electivity and high efficiency,t his method distinguishes itself from those previously reported. Other advantages include easy work-upo ft he reaction mixture,t he availability andb iodegradability of the photocatalysts and mild reactionc onditions.W ed emonstrated, on ap reparative scale,i ts practical applicationi nt he synthesiso ft he psychostimulant modafinil, in the selective oxidation of methionined erivatives,a nd in the detoxification of mustard gas.

We report a system with ethylene-bridged flavinium salt 2b which catalyzes the aerobic oxidation of toluenes and benzyl alcohols with high oxidation potential (E ox > +2.5 V vs SCE) to give the corresponding benzoic acids under visible light irradiation. This is caused by the high oxidizing power of excited 2b (E(2b*) = +2.67 V vs SCE) involved in photooxidation and by the accompanying dark organocatalytic oxygenation provided by the in situ formed flavin hydroperoxide 2b-OOH.

Chemoselective photooxidation: Tetra‐O‐acetylriboflavin and blue light mediate the selective oxidation of sulfides to sulfoxides without overoxidation to sulfones. Reaction is very effective in ethanol/water (95:5,v/v) mixture reaching a quantum yield of Φ=0.60. The photooxidation reaction scope includes aliphatic and aromatic sulfides including sterically hindered substrates.

A new application of flavin derivatives in visible light photocatalysis was found. 1-Butyl-7,8-dimethoxy-3-methylalloxazine, when irradiated by visible light, was shown to allow an efficient cyclobutane ring formation via an intramolecular [2+2] cycloaddition of both styrene dienes, considered as electron-rich substrates, and electron-poor bis(arylenones), presumably proceeding via an energy transfer mechanism.