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.