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

Abstract

The design and synthesis of three new C 2 -symmetric chiral diphosphoramidite ligands starting from simple and cheap building blocks have been developed. Rhodium(I) cationic complexes bearing these chelate ligands have been prepared and applied in asymmetric hydrogenation of model olefins. A rhodium complex with a diphosphoramidite containing a chiral diamine configurationally stable and two fluxional chiral biphenyl units gave higher enantioselectivity with increasing hydrogen pressure (87% ee) in the hydrogenation of dimethyl itaconate. KEYWORDSasymmetric hydrogenation, C 2 -symmetric ligand, chiral rhodium complexes, diphosphoramidite | INTRODUCTIONThe development of a really worthwhile catalytic system for asymmetric synthesis is still a challenge for the chemical community. Presently, most efficient enantioselective catalysts are chiral ligand modified transition metal complexes. Therefore, the chiral environment generated by the enantiopure ligand coordinated to the metal is a crucial element in determining the outcome of the process.Although significant enantiomeric excesses were achieved with chiral metal complexes, the cost and availability of the ligands remain critical factors for larger scale applications. [1] Industrial demand for economical and modular chiral ligands have given rise to the development of rationally designed syntheses of chiral ligands derived from a simple and inexpensive enantiopure source. In this context, over the last years phosphoramidites have arisen as cheap and easily available chiral ligands for asymmetric catalysis. [2] Among catalytic asymmetric reactions, the hydrogenation of C¼C bond catalyzed by chiral ligand modified transition metal complexes is an important and efficient tool for the organic synthesis [3,4] and fine chemical production. [5] Several monodentate phosphoramidites as MonoPhos, SiPhos, DpenPhos, PipPhos, Morphos, PegPhos [6,7] as well as bidentate phosphine-phosphoramidites [8,9] have been applied in the rhodium catalyzed asymmetric hydrogenation of functionalized olefins with outstanding results. Although, diphosphoramidites are readily obtainable from inexpensive achiral/chiral diamines, only a few of them have been evaluated in the asymmetric hydrogenation. [10,2] Some years ago, we reported a family of C 2 -symmetric bidentate diphosphoramidite ligands containing the inexpensive chiral source (1R,2R)-diaminocyclohexane as the backbone and two biaryl units (Figure 1). We tested these P,P ligands in transition metal catalyzed asymmetric reactions such as conjugate addition, [11] hydrosilylation, [12] Diels-Alder [13] and hydrogenation. [14] Interestingly, we observed an uncommon positive H 2 pressure effect on the stereoselectivity in the Rh-catalyzed asymmetric hydrogenation of dimethyl itaconate using the diphosphoramidite (R,R,S a S a )-L2 which contains two (S)-BINOL (1,1′-binaphthalene-2,2′-diol) moieties.We also note that rhodium catalyst bearing the diphosphoramidite ligand L3 in which each BINOL-unit was replaced by a fluxional axially chir...