New chiral diphosphoramidite rhodium(I) complexes for asymmetric hydrogenation

Chiral 1,5-cyclooctadiene rhodium(I) cationic complexes with C2-symmetric chelate diphosphoramidite ligands containing (R,R)-1,2-diaminocyclohexane as the backbone and two atropoisomeric biaryl units were easily synthesized and fully characterized by multinuclear one- and two-dimensional NMR spectroscopy and elemental analysis. These complexes were used as catalysts in the asymmetric hydrogenation of dimethyl itaconate, methyl 2-acetamidoacrylate and (Z)-methyl-2-acetamido-3-phenylacrylate. The rhodium complexes derived from diphosphoramidite ligands that contain two (R) or (S) BINOL (2,2′-dihydroxy-1,1′-binaphthyl) units proved to be efficient catalysts, giving complete conversion and very good enantioselectivity (up to 88% ee). An uncommon positive H2 pressure effect on the enantioselectivity was observed in the hydrogenation of dimethyl itaconate catalyzed by Rh-complex with diphosphoramidite ligand that contains two (S)-binaphthol moieties. Copyright © 2014 John Wiley & Sons, Ltd.