Heterogeneous Photocatalytic C(sp)-H Activation of Formate for Hydrocarboxylation of Alkenes.

Light-driven carboxylation offers a promising approach for synthesizing valuable fine chemicals under mild conditions. Here we disclose a heterogeneous photocatalytic strategy of C(sp)-H activation of formate for hydrocarboxylation of alkenes over zinc indium sulfide (ZnInS) under visible light. This protocol functions well with a variety of substituted styrenes with good to excellent yields; it also works for unactivated alkenes albeit with lower yields. Mechanistic studies confirm the existence of CO⋅ as a key intermediate. It was found that C(sp)-H activation of formate is induced by S⋅ species on the surface of ZnInS via hydrogen atom transfer (HAT) instead of a photogenerated hole oxidation mechanism. Moreover, both cleavage of the C(sp)-H of HCOO and formation of a benzylic anion were found to be involved in the rate-determining step for the hydrocarboxylation of styrene.