The iridium complex 80 bearing a tricyclic bisoxazoline-fused imidazole-derived
N-heterocyclic carbene ligand (Scheme 53) catalysed the asymmetric TH of (E)chalcones G1, G2, G4, G5, G7, G8, G10, G11, and G18–G20, as well as that of the
heterocyclic α,β-unsaturated ketones G27 and G31. In iPrOH, at 75
C, at a catalyst
loading of 1 mol%, using NaOH (8 mol%) as a base, moderate to good yields
(36–91%) in the corresponding full hydrogenated alcohol, were obtained within 3 h.
Probably, the existence of an equilibrium between conformational isomers in the
iridium catalyst 80 at the temperature of the reaction, originates a poor enantioface
discrimination during the reaction resulting in poor e.r.’s (52.5/47.5–62.5/37.5).
From detailed mass spectrometric studies, a mechanism consisting of two catalytic
cycles working in tandem was proposed [201].
Hydrogenation of the (E)-chalcone mediated by the bis(carbene)catalyst 9a
(Scheme 2) was performed in iPrOH, at 95
C using KOH (10 mol%) as a base.
The catalytic reaction was selective for the C¼C bond and required only 30 min to
Scheme 52 (continued)
Recent Advances in Iridium-Catalysed Transfer Hydrogenation Reactions
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