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

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Abstract

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.

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