Hydrogenation using homogeneous chiral

Table I. Hydrogenation of Unsaturated Organic Substrates Using Homogeneous Chiral Ruthenium and Rhodium Catalysts...

Examples of hydrogenation using homogeneous catalysts in moderate-scale industrial syntheses exist. Such catalysts are more selective and involve milder reaction conditions (i.e., 25°C and ca. 10 Pa Hj) than their heterogeneous counterparts . Asymmetric hydrogenation of a prochiral amino acid using chiral Rh catalysts yields L-dopa in high optical purity ... 

In the heterocyclic series several heteroarylalanines have been obtained by asymmetric hydrogenation using chiral homogeneous catalysts. For example, replacement of the phenyl ring by furan, thiophene, selenophene, pyridine, or indole yields fiirylalanines, thienyManines, 2- and 3-selenienyManines,... 

Chiral ionic liquids have also been used to aid enantioselective metal catalysed reactions. For example, homogeneous rhodium catalysed hydrogenations using tropoisomeric biphenylphosphine ligands have been reported using chiral ionic liquids derived from L-proline and L-valine. Enantioselectivities of up to 69% could be achieved and the catalytic system could be reused after extraction with SCCO2. 

The homogeneous chiral phosphine/DPEN-Ru catalyst can be immobilized by use of polymer-bound phosphines such as polystyrene-anchored BINAP (APB-BINAP) [57, 98], Poly-Nap [99], and poly(BINOL-BINAP) [100], poly(BINAP) [101]. These complexes hydrogenate T-acetonaphthone and acetophenone with S/C of 1000-10 000 under 8 10 atm H2 to give the corresponding secondary alcohols in 84-98% e.e. The recovered complexes are repeatedly used without significant loss of reactivity and enantioselectivity. Immobilization allows the easy separation of catalyst from reaction mixture, recovery, and reuse. These advantages attract much attention in combinatorial synthesis. 

Hydrogenations using heterogeneous catalysts usually require above ambient T and Hj pressures > 10 Pa. Homogeneous catalysts are often more selective and involve lower T (e.g., 25°C) and Hj (ca. 10 Pa). Highly useful selectivity in hydrogenation is shown in reactions of H2 with prochiral substrates in the presence of chiral catalysts, e.g., op-... 

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