Nickel-Catalyzed C–C Activation of Vinylcyclobutane with Visible Light: Scope, Mechanism, and Application to Chemically Recyclable Polyolefins

N-heterocyclic carbene (NHC)-supported nickel complexes were investigated for the oxidative ring-opening of vinylcyclobutane (VCB) and photocatalytic activity. Addition of VCB to in situ generated [(NHC)­Ni(0)] compounds furnished (NHC)­Ni­(VCB)2 that underwent oxidative addition and conversion to the corresponding Ni­(II) alkyl, allyl-metallacycles. The (NHC)­Ni­(C6H10) metallacycles were isolated, characterized, and exhibited high thermal and chemical stability. Irradiation with visible light at ambient temperature produced a mixture of ethylene and 4-vinylcyclohexene and 1,5-cyclooctadiene, cycloaddition dimers of butadiene, arising from formal retro-[2 + 2] cycloaddition. A mixture of hexadiene products arising from β-H elimination from the metallacycle was also observed. Free ethylene also underwent a secondary reaction to form cyclopropane products through formal [2 + 1] cycloaddition. A series of sterically and electronically modified NHC ligands was evaluated to establish the structure–activity relationship governing the rate of photocatalytic conversion of VCB and the resulting product distribution. Isotopic labeling experiments, resting state analysis, and independent synthesis of a range of nickel bis­(olefin) complexes provided insight into the mechanism of the reaction and origins of the organic product mixture. (NHC)­Ni-catalysis was also applied toward the retro-[2 + 2] depolymerization of (1,n′-divinyl)-oligocyclobutane to butadiene dimers.