N,O- vs N,C-Chelation in Half-Sandwich Iridium Complexes: A Dramatic Effect on Enantioselectivity in Asymmetric Transfer Hydrogenation of Ketones

Cyclometalation of [Cp*IrCl2]2 with methyl (S)-2-phenyl-4,5-dihydrooxazole-4-carboxylate in the presence of NaOAc selectively led to a N,C- or N,O-chelated Cp*Ir(III) complex, depending on whether or not water was present in the reaction. While derived from the same precursor, these two complexes behaved in a dramatically different manner in asymmetric transfer hydrogenation (ATH) of ketones by formic acid, with the N,O-chelated complex being much more selective and active. The sense of asymmetric induction is also different, with the N,O-complex affording S while the N,C-analogue R alcohols. Further study revealed that the nature of the base additive considerably impacts the enantioselectivity and the effective HCOOH/amine ratios. These observations show the importance of ligand coordination mode and using the right base for ATH reactions.