Asymmetric hydrogenation catalysis with rhodium complexes

Amino-derived BDPP (2,4-bis[diphenylphosphino]pentane) has been used in asymmetric hydrogenation catalysis [15-17] (cf. Sections 6.2 and 6.9). NMR analysis showed that a ten-fold excess of HBF4 is sufficient to protonate reversibly all four amino groups in the [Rh(diene)(BDPP)]BF4 complex. Recycling of the catalyst after enantioselective hydrogenation of dehydroamino acid derivatives in methanol is achieved by acidification with aqueous FIBF4 followed by extraction of the product with Et20. Immobilization of the protonated BDPP rhodium complex on a Nafion support has been studied as well [18]. 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

One of the success stories of transition metal catalysis is the rhodium-complex-catalyzed hydrogenation reaction. Asymmetric hydrogenation with a rhodium catalyst has been commercialized for the production of L-Dopa, and in 2001 the inventor, Knowles, together with Noyori and Sharpless, was awarded the Nobel Prize in chemistry. After the initial invention, (enantioselective) hydrogenation has been subject to intensive investigations (27). In general, hydrogenation reactions proceed... 

The first report made on asymmetric catalysis in SCCO2 was also for hydrogenation. In 1995, the hydrogenation of a-enamides to a-amino acid derivatives in SCCO2 was reported using cationic rhodium complexes with the chiral (f ,i )-Et-DuPHOS... 

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