New chiral diphosphoramidite rhodium(I) complexes for asymmetric hydrogenation

Chiral 1,5‐cyclooctadiene rhodium(I) cationic complexes with C 2 ‐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 H 2 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.