Mechanism of Photocatalytic Cyclization of Bromoalkenes with a Dimeric Gold Complex

In recent years, dimeric gold complexes have been extensively used in photoredox reactions and have successfully mediated a series of traditionally challenging organic reactions. However, little is known about the function of the dimeric gold complexes in these reactions. In this study, we systematically studied the mechanism of the photocatalytic cyclization of bromoalkenes with the dimeric gold complex [Au2(dppm)2]2+ (dppm denotes bis­(diphenylphosphino)­methane). It is found that the dimeric gold complex acts as the radical initiator and terminator in this radical chain reaction. In the radical initiation step, the gold complex first promotes the electron transfer from amine to the bromoalkene substrate (via a reductive quenching mode) and then accepts the released bromide (from bromoalkene) to stabilize the reaction system. In the radical termination step, the dimeric gold complex mainly works as an unsynchronized bromine and electron donor. In the photocatalytic cyclization of bromoalkenes, the radical propagation step operates between the alkyl radical and the dehydrogenated amine radical. This study sheds light on the function of the dimeric gold complex in photoredox reactions and will hopefully benefit the future understanding of similar synthetic reactions.