Air-Stable [(R3P)PdCl2]2Complexes of Neopentylphosphines as Cross-Coupling Precatalysts: Catalytic Application and Mechanism of Catalyst Activation and Deactivation

Abstract

Air-stable [(R3P)PdCl2]2 complexes with di-tert-butylneopentylphosphine (DTBNpP, 1a) or trineopentylphosphine (TNpP, 1b) ligands have been applied to Suzuki cross-coupling reactions, and the mechanism of their conversion to the active LPd0 catalyst species has been studied. Precatalysts 1a,b provide effective catalysts for Suzuki cross-coupling of aryl bromides at room temperature, even when the reactions are performed in air. The precatalyst systems provided much higher activity catalysts in toluene/water in comparison to acetonitrile/water. Precatalyst loadings could be decreased by a factor of 50 in toluene in comparison to acetonitrile, while achieving equal or better yields. In acetonitrile/water, ligand metalation to form a [(κ2-P,C-DTBNpP)PdX]2 palladacycle was found to compete with formation of the active Pd(0) species. The palladacycle shows low catalytic activity; thus, its formation represents a catalyst deactivation pathway. In toluene, clean formation of the active Pd(0) species occurs withou...