Enantioselectivity in hydrogenation

Fig. 5.5. Suggested basis of enantioselectivity in hydrogenation of a-methylstilbene by a phosphinoaryl oxazoline-iridium catalyst. Reproduced from Chem. Eur. J., 9, 339 (2003), by permission of Wiley-VCH.

Substituents on imino nitrogen influence both reactivity and enantioselectivity in hydrogenation of imino compounds. Figure 1.32 shows two successful examples. An f-BINAPHANE-Ir complex effects asymmetric hydrogenation of A-aryl aromatic imines.On the other hand, an Et-DuPHOS-Rh complex (see Figure 1.2) is effective for hydrogenation of A-acyUiydrazones. ... 

The Ru-BINAP system has shown excellent enantioselectivity in hydrogenation of (Z)-N-acyl-1-alkyl idenetetrahydroisoquinolines 54. Thus, a series of chiral isoquinoline products 55 can be efficiently synthesized.44... 

In order to verify the existence of a chiral intermediate with kinetic methods too, we studied the effect of concentration of the chiral auxiliary on rate and enantioselectivity in hydrogenation of isophorone and acetophenone with Pd/C-(S)-proline. 

The phosphines of type (229) deserve special comment they are prepared by a simple but novel route from an optically active cyclic sulfate (Equation (54)). Some complexes based on a rhodium center have been found to have excellent enantioselectivity in hydrogenation reactions. 

Also, they compared enantioselectivities in hydrogenation of MAA over Raney Ni-Tart at 60°C and 10 bar hydrogen in AcOEt solutions to enantioselectivities of amorphous Ni-boride (Ni-B) and Ni-phosphide (Ni-P) catalysts (prepared by reduction of Ni-salts with NaBH4 or NaH2P02, respectively) and found that a mean crystallite size of 6-8 nm produced ee values of 55%, which are comparable to Raney Ni prepared from Ni-Al alloy or from decomposition of Ni-formate. Ni-B and Ni-P catalysts have very small crystallite sizes and exhibit lower enantioselectivities (Figure 5.4.). The Figure also contains data (as curve 4) from the paper by Gross and Rys... 

Other protein-rhodium conjugates containing cationic rhodium catalysts have also been prepared using bis(diphenylphosphino-ethyl)amino derivatives 5-7 and solutions of these bis(phosphine) ligands in the presence of carbonic anhydrase, a-chymotrypsin and bovine serum albumin (47). However, the exact nature of the complexes formed has not been discerned in any of these cases, and these latter enzyme-transition metal complexes evidently do not exhibit enantioselectivity in hydrogenation of a-acetamidoacrylic acid. 

The results of this study confirmed the heuristic value of the well-known quadrant rule [36], which accounts for the origin and bias of enantioselectivity in hydrogenation with complexes of ligands with C2 symmetry (Fig. 2.6) [35]. 

Enantiocontrol of hydrogenation of geometry-fixed cyclic imines seems easier than that wifli flexible, acyclic imines. The chiral titanocene catalyst 34 exhibits excellent enantioselection in hydrogenation of various cyclic imines [346, 352]. For example, 2-phenyl-l-pyrroUne (R = C5H5, = CHj), a five-membered imine, is hydrogenated with an R catalyst to give (R)-2-... 

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