Activation and Reactivity of Vinylcyclobutane with Phosphine Nickel Catalysts

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A series of monodentate phosphine ligands was evaluated for the nickel-mediated activation of vinylcyclobutane (VCB) by oxidative addition to form the corresponding η 1,η 3 -metallacycles. The addition of VCB to equimolar mixtures of phosphine and Ni(COD) 2 (COD = 1,5-cyclooctadiene) produced a mixture of P 2 Ni(COD), P 2 Ni(VCB), and/or P 1 Ni(VCB) 2 . The accessibility of a VCB-derived P 1 Ni(II) alkyl,allyl-metallacycle correlated with an increasing number of alkyl substituents on the phosphine. Among trialkyl phosphines, PCy 3 and P i Pr 3 were the most effective for metallacycle formation. The VCB-derived metallacycles (Cy 3 P)Ni(C 6 H 10 ) and ( i Pr 3 P)Ni(C 6 H 10 ) were isolated and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The thermal and photochemical reactivity of (Cy 3 P)Ni(C 6 H 10 ) was explored and compared to related (NHC)Ni(C 6 H 10 ) metallacycles (NHC = N -heterocyclic carbene). Catalytic VCB-activation was observed with (Cy 3 P)Ni(C 6 H 10 ) 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 (Cy 3 P)Ni(C 6 H 10 ) 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. (Cy 3 P)Ni(C 6 H 10 ) 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.

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