One pot synthesis of propargylamines by three component amine-aldehyde-acetylene (A³) coupling catalyzed by neutral Ag(I) and Au(I) and cationic Pd(II) and Ni(II) complexes of a pincer N-heterocyclic carbene

The amine−aldehyde−acetylene (A3) coupling yielding propargylamines was successfully catalyzed by a series of neutral silver and gold complexes, [2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine]M2Cl2 [M = Ag (2), Au (3)], and cationic palladium and nickel complexes {[2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine]MCl}X [M = Pd, X = Cl (4), BF4 (5); M = Ni, X = Cl (6), BF4 (7)], stabilized over a new (1R)-(−)-menthyl substituted bis(imidazole) based [CcarbeneNCcarbene] type pincer ligand. The Au(I) (3), Pd(II) (4) and Ni(II) (6) complexes were conveniently synthesized by transmetallation reaction from the Ag(I) analog (2), which was obtained from the N-heterocyclic carbene ligand precursor (1) by treatment with Ag2O. The anion exchange reaction of Pd(II) (4) and Ni(II) (6) with AgBF4 yielded the corresponding Pd(II) (5) and Ni(II) (7) complexes respectively. Mass spectrometry studies suggested the formation of a catalytically active metal acetylide species namely, [NHC]M2(C[tbnd]CPh)Cl [M = Ag (2a), Au (3a)] and {[NHC]M(C[tbnd]CPh)}X [M = Pd, X = Cl (4a), BF4 (5a); M = Ni, X = Cl (6a), BF4 (7a)] (where NHC = 2,6-bis-(1-(1S)-menthyl-3-methylene-imidazol-2-ylidene)-pyridine). which mediates the formation of the propargylamine with a protonated Schiff imine salt produced in situ from an aldehyde and an amine in the single pot reaction.