Modular Chelated Palladium Diaminocarbene Complexes: Synthesis, Characterization, and Optimization of Catalytic Suzuki−Miyaura Cross-Coupling Activity by Ligand Modification

A general, two-step procedure is reported for the modular synthesis of a series of palladium complexes
of chelating Chugaev-type diaminocarbene ligands via metal-templated addition of hydrazines to
alkylisocyanides. This method afforded high yields of (dicarbene)palladium dihalide complexes with
methyl, isopropyl, cyclohexyl, and tert-butyl substituents by addition of hydrazine to the corresponding
alkylisocyanide, and analogous backbone-substituted complexes were prepared by palladium-templated
addition of methylhydrazine to methylisocyanide. The complexes were fully characterized by IR, 1H
NMR, and 13C NMR spectroscopies. X-ray crystallographic analyses of four (dicarbene)palladium
dibromide complexes revealed structural similarities with complexes of imidizole-based N-heterocyclic
carbenes (NHCs), characterizing these chelating ligands as strongly donating, resonance-stabilized
diaminocarbenes. To examine whether these ligands are amenable to cross-coupling catalyst optimization
via systematic ligand modification, a set of 10 (dicarbene)palladium dihalide complexes was tested as
precatalysts in the Suzuki−Miyaura coupling of bromobenzene with phenylboronic acid. Substantial
variations in catalytic activity were observed, and a backbone-substituted palladium dicarbene complex
derived from methylhydrazine was identified as the most active precatalyst. Catalyst activities did not
correlate with ligand sterics, and subtle electronic perturbation of carbene donor ability by the alkyl
groups is proposed to be the origin of the differences in activity. The optimized catalyst was found to
give high yields in Suzuki−Miyaura cross-couplings of electron-poor aryl chlorides and a range of aryl
bromides, although elevated temperatures (120 °C) were necessary. Coupling reactions conducted open
to air showed little formation of homocoupling byproduct and minimal loss of yield in most cases,
identifying the optimized system as a rare example of an air-tolerant Suzuki−Miyaura catalyst. This
study highlights the importance of a modular ligand design in fine-tuning the activity of a homogeneous
catalyst.