Asymmetric hydrogenation phosphine phosphoramidite ligands

Figure 9.12 Other phosphine phosphoramidite ligands for asymmetric hydrogenation.

Scheme 9.11 Rh catalyzed asymmetric hydrogenation of ortho substituted arylenamides with a triphosphorus bidentate phosphine phosphoramidite ligand 49.

As with other P(III) compounds, the binaphthol (BINOL) derivatives have been by far the most successful for a wide range of synthetic methods. Several reviews and reports have been published on the incorporation of BINOL-derived phosphoramidites in rhodium-catalysed asymmetric hydrogenations Novel chiral phosphine-phosphoramidite ligands, HY-Phos (61) and THNAPhos (62), have also been successful in rhodium-catalysed asymmetric hydrogenations. Asymmetric synthesis of the Roche ester and its derivatives (63) by rhodium-INDOLPHOS-catalysed hydrogenation was reported and a number of other BINOL-derivered phosphoramidates have been employed in iridium-catalysed hydrogenations and allylic alkyla-... 

Phosphoramidites are probably the most versatile ligands in this series as in amidites the substituents at the nitrogen atom are in close proximity to the metal centre and also the substituents could carry chiral centres. In Figure 4.22 we have depicted the simplest derivative, named Monophos , which is highly efficient for asymmetric hydrogenation but for a variety of other reactions as well. The ligand is much easier to make than most, if not all, chiral bidentate phosphine ligands and surely commercial applications will appear. 

The undesired hydrolysis of phosphoramidites can lead to an unexpected alteration of catalytic results. Thus, during Rh-catalyzed asymmetric hydrogenation of a-substituted ethenylphosphonic acid, by chance Ding and coworkers [40] observed the hydrolysis of the P-N bond with adventitious water (Scheme 2.134). The newly formed secondary phosphine oxides were more efficient than the parent phosphoramidite ligands in the respective catalytic reaction. When an equimolar amount of NHj was added to the reaction mixture before hydrogenation was started, the reaction did not occur. This clearly indicates that the phosphine oxide is formed from the phosphoramidite only in acidic media. 

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