Simple, efficient and reusable Pd-NHC catalysts for hydroamination

A series of chelating NHC-palladium complexes with different alkane-bridges of the type Pd[NHC-(CH2)(n)-NHC]X-2 (X = Br or Cl, n = 2-4) were synthesized, where NHC is a triazolyl-N-heterocyclic carbene donor ligand. The bromide complexes with n = 2 and 3 were characterized by X-ray crystallography. The effects of the length of the bridge and halide ligand on the catalytic reactivity in the hydroamination reaction were investigated. The best catalytic performance was observed with the propylene-bridged Pd-NHC bromide complex 2. The hydroamination of phenylacetylene or 4-methylphenylacetylene with various substituted anilines catalyzed by 2 yielded only Markovnikov addition products in good yield. Interestingly, this homogenous catalyst can be reused three times without significant loss in activity.