8-Amino-5,6,7,8-tetrahydroquinolines as ligands in iridium(III) catalysts for the reduction of aryl ketones by asymmetric transfer hydrogenation (ATH)

Aqua iridium(III) complexes with 8-amino-5,6,7,8-tetrahydroquinolines CAMPY L1 and its derivatives as chiral ligands proved to be very efficient catalysts for the reduction of a wide range of prochiral aryl ketones, revealing a variety of behaviours in terms of reaction rate and stereoselectivity. As standard substrates, differently substituted acetophenones were studied and good enantioselectivity (86% ee) was achieved in the reduction of 1-(o-tolyl)ethan-1-one 6. Particularly interesting was the ATH reaction in the case of β-amino keto esters, precursors of β-lactams and azetidinones. The best results were obtained with [Cp∗Ir(H2O)(L1)]SO4 affording the corresponding diastereomeric alcohols in an (R,S)-configuration with an excellent 99% ee in the reduction of 2-(benzamido methyl)-3-oxo-3-(4-(trifluoromethyl)phenyl)propanoate 12.

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(R)-N-Methyl-5,6,7,8-tetrahydroquinolin-8-amineC10H14N2Ee = 99%[α]D20 = −20.8 (c 0.5, CH2Cl2)Source of chirality: l-(+)-Tartaric acidAbsolute configuration: (R)

(R)-N-2-Dimethyl-5,6,7,8-tetrahydroquinolin-8-amineC11H16N2Ee = 99%[α]D20 = −52.5 (c 1, CH2Cl2)Source of chirality: l-(+)-Tartaric acidAbsolute configuration: (R)