Elementary Steps in Olefin Metathesis: Nickelacyclobutanes via Cycloaddition to Nickel Carbenes

Olefin metathesis and cyclopropanation, major reactions in the synthetic toolbox, are predominantly catalyzed by platinum-group metals. First-row transition metal alternatives are desirable from the perspective of supply-chain sustainability. Although cyclopropanation by nickel catalysts is well established, limited evidence exists for the chemical feasibility of the cycloaddition step essential for inner-sphere cyclopropanation or olefin metathesis. We describe Ni(PP)(=CPh₂) complexes that provide this missing evidence. These complexes react with styrene and phenyl vinyl sulfone (PVS) to form β-H elimination products that imply nickelacyclobutane (NiCB) intermediates. For the PVS system, the NiCB could be observed directly by NMR analysis and X-ray crystallography. This is the sole example to date of an obervable NiCB generated via cycloaddition, beyond the fluorinated examples. Computations pinpoint a decreased energy barrier to cycloreversion, relative to reductive elimination or β-H elimination, as the core requirement to achieve metathesis. This work thus demonstrates that nickel complexes can engage in cycloaddition, the missing elementary step essential for olefin metathesis, and establishes the design parameters essential to close the catalytic cycle.