New chiral diphosphoramidite rhodium(I) complexes for asymmetric hydrogenation

Abstract

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.

Rh(I) complexes with new C2‐symmetric chiral diphosphoramidite ligands: Catalytic activity for asymmetric hydrogenation of olefins

“…Previously, we observed a significant positive effect of H 2 pressure on the enantioselectivity of the hydrogenation of dimethyl itaconate catalyzed by the cationic rhodium complex bearing diastereomeric diphosphoramidite ligand ( R c ,R c ,S a ,S a )– L2 . In that case, an ee value of 88% ( S ) was reached at 60 bar.…”

“…Indeed, complete reduction of substrate 9 was achieved in 6 h under the same catalytic conditions (Entry 4). Moreover, the rhodium catalyst derived from diphosphoramidite L4 containing unsubstituted biphenol fragments turned out more active than that derived from diphosphoramidite L3 containing bulky ortho tert ‐butyl groups …”

“…Some years ago, we reported a family of C 2 ‐symmetric bidentate diphosphoramidite ligands containing the inexpensive chiral source (1 R ,2 R )‐diaminocyclohexane as the backbone and two biaryl units (Figure ). We tested these P,P ligands in transition metal catalyzed asymmetric reactions such as conjugate addition, hydrosilylation, Diels‐Alder and hydrogenation …”

Origin of the Selectivity and Activity in the Rhodium-Catalyzed Asymmetric Hydrogenation Using Supramolecular Ligands

“…The influence of H 2 pressure on the enantioselectivity provides indirect information on the mechanism. − For this reason, we studied the influence of the hydrogen pressure in the range 1–40 bar on the enantioselectivity of the hydrogenation reaction of substrates S3 , S4 , and S5 and by using complex 1 as the catalyst (Figure ). …”