Transfer Hydrogenation of Imines and Alkenes and Direct Reductive Amination of Aldehydes Catalyzed by Triazole-Derived Iridium(I) Carbene Complexes

A series of new iridium(I) triazole-based NHC complexes [(cod)Ir(NHC)L]BF4 (L = PPh3, pyridine) were prepared and showed good activity for transfer hydrogenation on CO, CN, and CC double bonds in 2-propanol with K2CO3. The phosphine series was shown to be more active than the pyridine series in the case of imine transfer hydrogenation. A neopentyl wingtip substituent on the NHC gave the best catalytic activity with the following competitive order: aldehyde > ketone > imine. In a substrate containing both aldehyde and ketone functionalities, only the aldehyde was reduced. Of great interest, the transfer hydrogenation of polarized and nonpolarized CC bonds was also proved possible. In a useful organic synthetic application, direct, one-pot reductive amination of RCHO with R‘NH2 to give RCH2NHR‘ was shown for a variety of cases.