Air-stable, convenient to handle Pd based PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation) themed precatalysts of N/O-functionalized N-heterocyclic carbenes and its utility in Suzuki-Miyaura cross-coupling reaction

Several new air-stable, convenient to handle and easily synthesized Pd based PEPPSI ( Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) type precatalysts supported over N/O-functionalized N-heterocyclic carbenes (NHC) namely, trans-[1-(benzyl)-3-( N-t-butylacetamido) imidazol-2-ylidene] Pd( pyridine) Cl-2 ( 2), trans-[1-(2-hydroxy-cyclohexyl)-3( benzyl) imidazol-2-ylidene] Pd( pyridine) Cl2 ( 3) and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene] Pd( pyridine) Br-2 ( 4), have been designed. Specifically, the Pd-NHC complexes, 2, 3 and 4, were conveniently synthesized from their respective imidazolium halide salts by the reaction with PdCl2 in pyridine in presence of K2CO3 as a base. A new imidazolium chloride salt, 1-(benzyl)-3-( N-t-butylacetamido) imidazolium chloride ( 1) was synthesized by the alkylation reaction of benzyl imidazole with N-t-butyl-2-chloroacetamide. The molecular structures of the imidazolium chloride salt, 1, and the Pd-NHC complexes, 2, 3 and 4, have been determined by X-ray diffraction studies. The density functional theory studies of the 2, 3 and 4 complexes were carried out to in order to gain insight about their structure, bonding and the electronic properties. The nature of the NHC-metal bond in these complexes was examined using Charge Decomposition Analysis (CDA), which revealed that the N-heterocyclic carbene ligands are effective r-donors. In addition, the catalysis studies revealed that the Pd-NHC complexes, 2, 3 and 4, are effective catalysts for the Suzuki-Miyaura type C-C cross-coupling reactions.