Bidentate ligands asymmetric hydrogenation

Whereas bidentate phosphites and phosphonites are excellent ligands for rhodium-catalyzed asymmetric hydrogenation, bidentate phosphoramidites (not shown) gave very poor results with low reaction rates and enantioselectivities. Results of a number of other monodentate phosphoramidites in the asymmetric hydrogenation of. V-acetyl dehydrophenylalanine derivatives are shown in Table 14.7.31-35-36-39-40... 

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. 

Selected derivatives of the ligands and complexes described above have been tested in catalytic applications. Early tests with bidentate P,P or P,N ligands such as 4, 6, and 8 in Rh-catalyzed asymmetric hydrogenation were disappointing, with ee values below 20%. However, as was demonstrated mainly by the Fu group, phosphaferrocene derivatives do have the potential for successful applications in asymmetric catalytic reactions, provided the phosphaferrocene is endowed with sufficient steric bulk. Examples are depicted in Eig. 1.5.8 the Cp derivative 32,... 

Weis, M., Waloch, C., Seiche, W. and Breit, B. (2006) Self-assembly of bidentate ligands for combinatorial homogeneous catalysis Asymmetric rhodium-catalyzed hydrogenation. J. Am. Chem. Soc.. 128. 4188-4189. 

Do , Do = PAHAt, P(OR)2 Scheme 2.8 Self-assembly of chiral monodentate to chiral bidentate ligands through complementa hydrogen-bonding on the basis of an A-T base pair analogue for combinatorial asymmetric catalysis. 

For recent review about bidentate ligands in asymmetric hydrogenation, see Tang,... 

J. Am. Chem. Soc., 126. 4494. (i) Takacs, J.M., Chaiseeda, K. and Moteki, S.A. (2006) Rhodium-catalyzed asymmetric hydrogenation using self-assembled chiral bidentate ligands. Pure Appl. Chem., 78, 501. (j) Duckmanton, P.A., Blake, A.J. and Love, f.B. (2005) Palladium and rhodium ureaphosphine complexes Exploring structural and catalytic consequences of anion binding. Inorg. Chem., 44, 7708-7710. 

Sandee, A.J., v.d. Burg, Lidy, and Reek. J.N.H. (2007) UREAphos supramolecular bidentate ligands for asymmetric hydrogenation. Chem. Commun., 864-866. 

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