Nickel-Catalyzed C–C\nActivation of Vinylcyclobutane\nwith Visible Light: Scope, Mechanism, and Application to Chemically\nRecyclable Polyolefins

N-heterocyclic carbene (NHC)-supported\nnickel complexes\nwere investigated\nfor the oxidative ring-opening of vinylcyclobutane (VCB) and photocatalytic\nactivity. Addition of VCB to in situ generated [(NHC)Ni(0)] compounds\nfurnished (NHC)Ni(VCB)<sub>2</sub> that underwent oxidative addition\nand conversion to the corresponding Ni(II) alkyl, allyl-metallacycles.\nThe (NHC)Ni(C<sub>6</sub>H<sub>10</sub>) metallacycles were isolated,\ncharacterized, and exhibited high thermal and chemical stability.\nIrradiation with visible light at ambient temperature produced a mixture\nof ethylene and 4-vinylcyclohexene and 1,5-cyclooctadiene, cycloaddition\ndimers of butadiene, arising from formal retro-[2 + 2] cycloaddition.\nA mixture of hexadiene products arising from β-H elimination\nfrom the metallacycle was also observed. Free ethylene also underwent\na secondary reaction to form cyclopropane products through formal\n[2 + 1] cycloaddition. A series of sterically and electronically modified\nNHC ligands was evaluated to establish the structure–activity\nrelationship governing the rate of photocatalytic conversion of VCB\nand the resulting product distribution. Isotopic labeling experiments,\nresting state analysis, and independent synthesis of a range of nickel\nbis(olefin) complexes provided insight into the mechanism of the reaction\nand origins of the organic product mixture. (NHC)Ni-catalysis was\nalso applied toward the retro-[2 + 2] depolymerization of (1,<i>n</i>′-divinyl)-oligocyclobutane to butadiene dimers.