Experimental Evidence for Zerovalent Pd(NHC) as a Competent Catalyst in C–N Cross-Coupling (NHC = DiMeIHept<sup>Cl</sup>)
Volodymyr Semeniuchenko, Sepideh Sharif, Neha Rana, Nalin Chandrasoma, Wilfried M. Braje, R. Tom Baker, Jeffrey M. Manthorpe, William J. Pietro, Michael G. Organ
Abstract
Use of the branched N-heterocyclic carbene (NHC) ligand 1,3-bis(2,6-bis(3-methyl-1-(2-methylpropyl)butyl)phenyl)-4,5-dichloro-1,3-dihydro-2 H -imidazole-2-ylidene (DiMeIHept Cl ) facilitated the stabilization of several relevant intermediates for Pd(NHC)-catalyzed C–N cross-coupling reactions. Complexes [Pd(DiMeIHept Cl )] 2 (μ-N 2 ), [Pd(DiMeIHept Cl )] 2 (μ–η 2 -1,2-η 2 -4,5-C 6 H 6 ), and Pd(DiMeIHept Cl )(pyridine), representing zerovalent Pd(NHC) bearing labile ligands, were isolated and structurally characterized, along with divalent PdCl(Ph)(DiMeIHept Cl ) and PdCl(Ph)(DiMeIHept Cl )( n -propylamine). The former is a 14-electron Pd complex, which is stable under air and chromatography on silica gel or neutral alumina. One possible reason for this exceptional stability is the numerous dispersion interactions between the NHC alkyl chains and the Pd–Ph group. Detailed investigations of catalyst activation and oxidative addition confirmed that “Pd(NHC)” is formed from many known Pd(II)(NHC) precatalysts and provided activation rates for these different precatalysts.