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.