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

ABSTRACT

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 ß-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.