Development of chiral C2-symmetric N-heterocyclic carbene Rh(I) catalysts through control of their steric properties
Résumé
Chiral square-planar Rh(I) complexes based on new C2-symmetric NHC ligands have been synthesized selectively in a few steps as single diastereoisomers. These chiral pre-catalysts were applied to the asymmetric transfer hydrogenation of 1-phe-nylpropanone and to the 1,2-addition of arylboronic acids to aldehydes. We demonstrated a proper functionalization of the aromatic rings connected to the nitrogen atoms of the NHC ligand improved significantly the asymmetric induction of the chiral Rh(I) NHC catalysts. Bulky substituents allowed a better control of the steric features of the catalyst quadrants because they behaved as confor-mational and chirality relays of the NHC chiral backbone.
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