Mononuclear palladium-allyl complexes bearing one N-heterocyclic carbene (NHC) ligand have been synthesized. These complexes offer a straightforward entryway into a number of catalytic cycles by simple action of a base.

The synthesis and structural characterization of a series of palladium complexes bearing N-heterocyclic carbenes (NHC) as supporting ligands are described. The reaction of commercially available [Pd(allyl)Cl]2 and isolated or in situ generated NHC leads to monomeric palladium complexes where one NHC is bound to the metal center, as indicated by spectroscopic and single-crystal X-ray diffraction studies. The relative reactivity trend for these complexes as catalysts in aryl amination is discussed in terms of ligand steric properties, which vary as a function of imidazole-nitrogen substituents and perturbation resulting in modulation of ring planarity. The concept of buried volume is used to quantify the steric demand of each NHC in the corresponding complexes.

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Nucleophilic N-heterocyclic carbenes have been conveniently used as catalyst modifiers in amination reactions involving aryl chlorides, aryl bromides, and aryl iodides with various nitrogen-containing substrates. The scope of a coupling process using a Pd(0) or Pd(II) source and an imidazolium salt in the presence of a base, KO(t)Bu or NaOH, was tested using various substrates. The Pd(2)(dba)(3)/IPr.HCl (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) system presents the highest activity with respect to electron-neutral and electron-rich aryl chlorides. The ligand is also effective for the synthesis of benzophenone imines, which can be easily converted to the corresponding primary amines by acid hydrolysis. Less reactive indoles were converted to N-aryl-substituted indoles using as supporting ligand the more donating SIPr.HCl (5, SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene). The Pd(OAc)(2)/SIPr.HCl/NaOH system is efficient for the N-arylation of diverse indoles with aryl bromides. The general protocol developed has been applied successfully to the synthesis of a key intermediate in the synthesis of an important new antibiotic. Mechanistically, palladium-to-ligand ratio studies strongly support an active species bearing one nucleophilic carbene ligand.

A number of palladium-N-heterocyclic carbene (NHC) complexes were found to be active catalysts for the arylation of ketones. A large number of substrates, both aryl halides and ketones, are compatible with the reaction conditions. The ketone arylation reactions are achieved with low catalyst loading in short reaction times using aryl chlorides and triflates as reactive partners. [reaction: see text]