Asymmetric hydrogenation chiral phosphinite

If Q-symmetric ligands are employed in asymmetric hydrogenation instead of the corresponding C2-symmetric ligands, there coexist principally four stereoiso-meric substrate complexes, namely two pairs of each diastereomeric substrate complex. Furthermore, it has been shown that, for particular catalytic systems, intramolecular exchange processes between the diastereomeric substrate complexes should in principle be taken into account [57]. Finally, the possibility of non-estab-hshed pre-equilibria must be considered [58]. The consideration of four intermediates, with possible intramolecular equilibria and disturbed pre-equihbria, results in the reaction sequence shown in Scheme 10.3. This is an example of the asymmetric hydrogenation of dimethyl itaconate with a Rh-complex, which contains a Q-symmetrical aminophosphine phosphinite as the chiral ligand. 

The asymmetric hydrogenation of enol esters can also be catalyzed by chiral amidophosphine phosphinite catalysts derived from chiral amino acids, but the enantioselectivity of these reactions has thus far been only moderate.35... 

New Chiral Phosphinite Ligands Used For Asymmetric Homogeneous Hydrogenation in the Wilkinson-Type Catalyst System... 

Chiral bis(phosphinites) derived from (2/J,51 )-2,5-di(hydroxymethyl)tetrahydrothiophene have been prepared (Equation 73). These ligands have been used in the synthesis of rhodium complexes from Rh(COD)2X (COD = cyclooctadiene X = OTf, SbF6) and tested in the asymmetric hydrogenation of methyl a-acetamidocinna-mate. A maximum of 55% enantioselectivity was observed <19980M4976>. 

Fogassy, G., Tangier, A., Levai, A. (2003) Enantioselective hydrogenation of exocyclic alpha,beta-unsaturated ketones. Part 111. Hydrogenation with Pd in the presence of cinchonidine, y. Mol. Catal. A. Chem. 192, 189-194. Hapiat, F., Agboussou, F., Mortreux, A. (1994) Synthesis of new chiral arene Ru(ll) aminophosphine-phosphinite complexes and use in asymmetric hydrogenation of an activated keto compounds. Tetrahedron Asymm. 5, 515-518. 

Roucoux, A., Agboussou, F., Mortreux, A. and Petit, F. (1993) New alky-larylamido phosphine phosphinites as chiral diphosphines for asymmetric hydrogenation of acilated keto compounds. Tetrahedron Asymm. 4,2279-... 

Carbohydrate-based phosphinites (220) and chiral aminophosphane phos-phinites (DPAMPPs and PINDOPHOS) (221) were found to be excellent ligands for rhodium-catalyzed asymmetric hydrogenation. After the successful application of diphosphanes in asymmetric hydroformylation, promising results were published with aminophosphane phosphinites (40) and diphosphites (222). 

In 34 and 35, there is one more element of chirality in the phosphite or phosphinite moiety, such as the binaphthyl system [44]. These modifications could potentially influence the origin of the stereochemistry in the asymmetric catalysis process, and sometimes excellent results have been obtained (quantitative conversions and enantiomeric excess values higher than 99%) in the case of 34 (R = Ph, = H, R = Ph) that is better than other known catalyst systems for the asymmetric hydrogenation of unfunctionalized 1,1-disubstituted terminal alkenes [44]. 

Modification of the well known diop-Rh catalyst by the inclusion of o-methoxy-groups on the phenyl substituents results in a reversal of the R S ratio in the amino-acid products, relative to diop itself. Overall enantiomeric enrichments are comparable with those obtained with diop-Rh It has been shown that in some cases, chiral Rh -phosphinite complexes derived from sugars are more effective in asymmetric hydrogenations than the corresponding phosphine complexes. ... 

---