Synthesis of 1,3-Dialkylperhydrobenzimidazolinium Salts and Their Catalytic Properties in Heck Reactions

Abstract

Novel eight 1,3-dialkylperhydrobenzimidazolinium chloride salts 3a-h were prepared as precursors of N-heterocyclic carbenes by reacting N,N'-dialkylcyclohexanediamine, triethyl orthoformate and ammonium chloride. The salts were characterized spectroscopically and the complexes formed in situ from Pd(OAc) 2 and 3 have been tested as catalysts in homogenous Heck coupling reactions.The Palladium-catalyzed Mizoroki-Heck cross-coupling reaction of an olefin with aryl halides under basic conditions is one of the highly efficient tools for the carbon-carbon bond formation in organic synthesis. 1-3 Phosphines generally have been used as ligands for the palladium-catalyzed Heck and Suzuki coupling reactions. However, the phosphine ligands are expensive, toxic and sensitive to air and the moisture. 4-7 The strong electron donating property of N-heterocyclic carbene ligands leads to metal complexes with high stability against heat, air and moisture. These properties of N-heterocyclic carbene ligands make them alternatives to tertiary phosphines used in different transition metal-catalyzed reactions such as C-C and C-N arylation, olefin metathesis, hydrogenation, hydrosilylation and CO-ethylene co-polymerization reactions. 8-19 Apart from phosphine and N-heterocyclic carbene ligands,