ortho-Ethynyl group assisted regioselective and diastereoselective [2 + 2] cross-photocycloaddition of alkenes under photocatalyst-, additive-, and solvent-free conditions

A highly regioselective and diastereoselective [2 + 2] cross-photocycloaddition between electron-poor and electron-rich/electron-neutral alkenes under visible-light irradiation was developed. In the absence of an external photocatalyst, additive and solvent, the substrates 1 and 4 reacted with rigid cyclic alkoxyethenes to generate cis-anti-head-to-head heterocoupled [2 + 2] products in high yields with regiospecificity and good diastereoselectivity. Meanwhile, the reactions of 1 and 4 with (Z)- and (E)-1,2-diphenylethenes yielded the desired products in good yields with a high dr. It is important to note that no geometric isomerization of olefins was observed during the reaction. Mechanistic studies and DFT calculations suggested that 1 and 4 having ortho-ethynyl and cyano groups as a self-photocatalyst play a very important role in the reaction and a five-membered ring diradical intermediate (Int1) was formed via an intramolecular radical addition. The obtained regioselectivity and diastereoselectivity were confirmed by X-ray structural analysis of the representative products.