Thiophenol-Catalyzed Visible-Light-Mediated Oxidative Cleavage of C=C Bond of Alkene

Aromatic ketones are of great synthetic interest in pharmaceutical development, chemical biotechnology, industrial process and material science.Among various synthetic approaches, direct oxidative cleavage of unsaturated bonds of alkene is a classic and facile way to acquire aromatic ketone.In recent years, several visible-light induced alkene oxidation systems have been developed employing molecular oxygen and metal-free photocatalysts.However, these photocatalysts produce reactive oxygen species requiring the addition of oxidants, bases and additives to ensure high yields and selectivity.We try to explore a new photocatalyst combining molecular oxygen in this conversion.Thiophenol could be a good choice for the oxidative cleavage of alkene because it is inexpensive, abundant and easily available.Benefiting from these features, we set out to explore the possibility of thiophenol.Focus on our interest in the use of molecular oxygen, herein, a metal-free, visible-light-catalyzed oxidative cleavage of CC bond of alkene by p-toluenethiol under an O2 atmosphere has been developed.The desired product was obtained in 85% yield using 20 mol% p-toluenethiol as a photocatalyst, O2 as an oxygen source and methanol as the solvent with the irradiation of 30 W blue LED for 12 h.This reaction is very interesting because it is metal-free, requires only a small number of reagents, and can be completed with just a catalytic amount.Moreover, a broad range of ketones (34 examples, up to 90% yield) with various valuable functional groups were obtained.And a gram-scale experiment was also conducted to prove the prospect of this method and the late-stage functionalization of pharmaceutical drugs was demonstrated successfully.Mechanism studies revealed that p-toluenethiol absorbed the light and served as a photosensitizer to promote the transformation of O2 to the highly reactive 1 O2 via single electron transfer process, which then oxidatively cleaved the CC bond of the alkene.In conclusion, the approach offer many advantages including operation simplicity, high yields, easy scalability without any metals or external additives, which providing an alternative approach to antiquated oxidative cleavage methods.