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.