Intramolecular Hydroamination Catalyzed by Cationic Rhodium and Iridium Complexes with Bidentate Nitrogen-Donor Ligands
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Abstract
Cationic rhodium(I) and iridium(I) complexes of the general formula [M(N−N)(CO)2]+[X]-, incorporating the bidentate heterocyclic nitrogen donor ligands (N−N) bis(1-methylimidazol-2-yl)methane (bim) and bis(1-pyrazolyl)methane (bpm), are efficient catalysts for hydroamination. The cyclization of an aliphatic aminoalkyne to the corresponding five-membered imine heterocycle was achieved in good yield with excellent regioselectivity using both rhodium and iridium catalysts. The nature of the counterion to the cationic catalysts has a significant effect on the rate of the hydroamination reaction and interionic contacts between the metal complexes and their counterions in solution were clearly visible by NMR spectroscopy using the nuclear Overhauser effect, indicating a strong anion/cation interaction in solution. The nature of the coligands also had a significant effect on the efficiency of the metal complexes as catalysts, and complexes containing phosphine coligands were less effective catalysts compared to those complexes containing CO coligands. The X-ray crystal structure of the new iridium(I) complex [Ir(bim)(CO)2]+[BPh4]- (2) is reported, as well as the synthesis and characterization of the Rh complex [Rh(bpm)(CO)2]+[BPh4]- (3).
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