ortho-Ethynyl group assisted regioselective and diastereoselective [2+2] cross-photocycloaddition of alkenes

Abstract

\n A highly regioselective and diastereoselective [2+2] cross-photocycloaddition between the electron-poor and electron-rich/electron-neutral alkenes under visible-light irradiation was developed. In the absence of an external photocatalyst, the substrates 1 and 4 reacted with rigid cyclic alkoxyethenes to generate (cis,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 ratio of d.r. 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.