Copper Photoredox-Catalyzed Allylation of Imines with Allylic Bromides

A copper photocatalyzed procedure has been developed for the allylation of imines with allylic bromides by using the heteroleptic complex [Cu(neocuproine)(DPEphos)]PF₆ as the photocatalyst. The transformation takes place in the presence of N,N-diisopropylethylamine as a sacrificial reducing agent and does not require stoichiometric metal reagents or precious transition metal catalysts. The reaction can be performed with a range of N-benzylidene anilines with different substituents on the benzylidene moiety. It can be extended to N-benzhydrylidene aniline, but no conversion occurs with other ketimines or N-benzylidene benzenesulfonamide. Besides allylic bromides, the alkylation can also be carried out with benzyl bromide. Mechanistic experiments indicated that the imine is not reduced to the radical anion during the transformation. Thus, a mechanism is proposed where N,N-diisopropylethylamine is oxidized to form an α-aminoalkyl radical, which reacts with allyl bromide through a halogen-atom transfer to generate the allyl radical. The latter then adds to the imine to form an N-centered radical, which is reduced to the product homoallylamine.