New insights into the mechanism of synergetic photoredox/copper(i)-catalyzed carbocyanation of 1,3-dienes: a DFT study

This work presents a DFT-based computational study to understand the mechanism, and regio- and enantioselectivities in the synergetic photoredox/copper(i)-catalyzed carbocyanation of 1,3-dienes with alkyl redox-active esters. The calculated results show an unprecedented copper catalytic mechanism, where the reaction follows a catalytic cycle involving Cu(I)-only catalysis, instead of a Cu(i)/Cu(ii)/Cu(iii)/Cu(i) cycle as proposed in the experimental study. Moreover, it is found that the critical step involves the reaction of the cyanocopper(i) species with an allyl cation rather than the cyanocopper(ii) species reacting with an allyl radical as proposed in the experiment, and that the photocatalyst is regenerated via single electron transfer from the allyl radical to the oxidized photocatalyst. In the newly proposed photoredox/copper(i) catalysis, the reaction consists of four stages: (i) generation of the copper(i) active catalyst, (ii) formation of an allyl radical with oxidative quenching of the photoexcited species, (iii) generation of an allylcopper complex accompanied by the regeneration of the photocatalyst, and (iv) formation of the allyl cyanide product with the regeneration of the copper(i) active catalyst. The cyanation of the allyl cation is calculated to be the regio- and enantioselectivity-determining step. The excellent regio- and stereoselectivities are attributed to the favorable CH–π interaction between the substrate and catalyst as well as the small distortion of the substrate.