Enhanced Activity of [Ni(NHC)CpCl] Complexes in Arylamination Catalysis

Seven new air- and moisture-stable
nickel complexes bearing flexible
bulky NHC (N-heterocyclic carbene) ancillary ligands (NHC = IPr*,
IPr*Tol, IPr*OMe, IPent) are reported. Using
experimentally determined crystal structures, the steric environments
of [Ni­(NHC)­CpCl] complexes were analyzed. A survey of their catalytic
activity in Buchwald–Hartwig arylamination has been performed.
In comparison to less sterically demanding analogues (NHC = IMes,
SIMes, IPr, SIPr), an increase in the ligand bulkiness was found to
correlate to a dramatic enhancement of the C–N bond formation
efficiency. Finally, the catalytic activity of the most active precatalyst,
[Ni­(IPr*OMe)­CpCl], was further explored and the scope and
limitations of this complex were examined.