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

Abstract

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)Cl₂ (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)Br₂ (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 σ-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.