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.