Late Transition Metal Catalyzed Intramolecular Hydroamination: The Effect of Ligand and Substrate Structure

Abstract

A series of cationic rhodium(I) and iridium(I) complexes of the type [M(N−N)(CO)2][BPh4] containing the imidazole-based bidentate nitrogen donor ligands (N−N), including bis(N-methylimidazol-2-yl)methane (bim) (1, a M = Rh, b M = Ir), bis(N-methylimidazol-2-yl)ketone (bik) (5, a M = Rh, b M = Ir), 1,1-bis(N-methylimidazol-2-yl)ethane (bie) (3, M = Rh), 2,2-bis(N-methylimidazol-2-yl)propane (bip) (4, M = Rh), and bis(N-methylbenzimidazol-2-yl)methane (bbnzim) (6a M = Rh, 6b M = Ir), were synthesized and characterized. Complexes incorporating the pyrazole analogue bis(1-pyrazolyl)methane (bpm) [M(bpm)(CO)2][BPh4] (2, a M = Rh, b M = Ir) were also prepared. The efficiency of each of the complexes as catalysts for the cyclization of 4-pentyn-1-amine to 2-methyl-1-pyrroline is reported. The influence of structural variations of the nitrogen donor ligand on the catalytic efficiency of cationic complexes of the type [M(N−N)(CO)2][BPh4] for the hydroamination reaction was found to be much less than the influence of the nature of the counterion. The scope of the substrates for the intramolecular hydroamination of alkynamines was also investigated using Rh and Ir catalysts with the bim and bpm ligands, using 3-butyn-1-amine (7a), 3-pentyn-1-amine (7b), 4-phenyl-3-butyn-1-amine (7c), 4-pentyn-1-amine (8a), 4-hexyn-1-amine (8b), 5-phenyl-4-pentyn-1-amine (8c), 5-hexyn-1-amine (9), and 6-heptyn-1-amine (10) as substrates. The rhodium and iridium catalysts under investigation preferentially catalyze the formation of five-membered rings and do not catalyze the formation of four- or seven-membered rings. The effect of the substituents on the alkyne on the efficiency of hydroamination differentiates Rh(I) and Ir(I) and suggests the nature of the reactive metal-bound alkynyl intermediate.