Activation and Reactivity of Vinylcyclobutane with Phosphine Nickel Catalysts

A series of monodentate phosphine ligands was evaluated for the nickel-mediated activation of vinylcyclobutane (VCB) by oxidative addition to form the corresponding η1,η3-metallacycles. The addition of VCB to equimolar mixtures of phosphine and Ni(COD)2 (COD = 1,5-cyclooctadiene) produced a mixture of P2Ni(COD), P2Ni(VCB), and/or P1Ni(VCB)2. The accessibility of a VCB-derived P1Ni(II) alkyl,allyl-metallacycle correlated with an increasing number of alkyl substituents on the phosphine. Among trialkyl phosphines, PCy3 and PiPr3 were the most effective for metallacycle formation. The VCB-derived metallacycles (Cy3P)Ni(C6H10) and (iPr3P)Ni(C6H10) were isolated and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The thermal and photochemical reactivity of (Cy3P)Ni(C6H10) was explored and compared to related (NHC)Ni(C6H10) metallacycles (NHC = N-heterocyclic carbene). Catalytic VCB-activation was observed with (Cy3P)Ni(C6H10) at 120 °C and furnished products from retro-[2 + 2] cycloaddition – ethylene, vinylcyclohexene, and cyclooctadiene – as well as 2,4-hexadiene arising from β-H elimination. Catalytic activation of VCB was observed upon irradiation of (Cy3P)Ni(C6H10) with blue light at 60 °C with improved selectivity for retro-[2 + 2] products over β-Η elimination and [2 + 1] cycloaddition products compared to NHC analogues. (Cy3P)Ni(C6H10) was also found to catalyze the retro-[2 + 2] depolymerization of (1,n′-divinyl)oligocyclobutane, but the presence of substituents on the cyclobutane ring resulted in an increased preference for β-H elimination.