Enantioselective hydrogenation cinnamic acids

Rhodium-catalyzed enantioselective hydrogenation of acctamido -cinnamic in water was also achieved using pyrphos bound to poly-acrylic acid as ligand.337 Roucoux described some Rh° nanoparticles which function as reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system.338... 

The use of 2 equiv. of MonoPhos (10 a) in the rhodium-catalyzed enantioselective hydrogenation of the key cinnamic acid derivative 15 resulted in the formation of 16 in 50% conversion and 20% ee after 5 h in isopropanol at 60 °C and 25 bar of hydrogen. Other phosphoramidites, such as the sterically demanding ligand 10 c, resulted in slightly better activity and enantioselectivity. In seeking a... 

The effect of a range of additives on enantioselective hydrogenation of the cinnamic acid precursor is shown in Scheme 36.15. One trend that emerges from this screen is the positive effect of the monodentate phosphines, in particular, tri-p-tolylphosphine. 

Table 41.9 The enantiomeric hydrogenation of (Z)-a-acetami-do cinnamic acid the effect of hydrogen concentration in the liquid phase on the conversion and the enantioselectivity.

Enantioselectivity is drastically reduced by carrying out the reaction at high initial hydrogen pressure. For example, the reaction of (Z)-a-(benzamido)cinnamic acid in ethanol under initial H2 pressure of 4 atm gives the saturated product in 96-100% optical yield the same reaction at 50 atm produces only a 71 % yield. 

Benzamido-cinnamic acid, 20, 38, 353 Benzofuran polymerization, 181 Benzoin condensation, 326 Benzomorphans, 37 Benzycinchoninium bromide, 334 Benzycinchoninium chloride, 334, 338 Bifiinctional catalysts, 328 Bifiinctional ketones, enantioselectivity, 66 BINAP allylation, 194 allylic alcohols, 46 axial chirality, 18 complex catalysts, 47 cyclic substrates, 115, 117 double hydrogenation, 72 Heck reaction, 191 hydrogen incorporation, 51 hydrogen shift, 100 hydrogenation, 18, 28, 57, 309 hydrosilylation, 126 inclusion complexes, oxides, 97 ligands, 19, 105 molecular structure, 50, 115 mono- and bis-complexes, 106 NMR spectra, 105 olefin isomerization, 96... 

Heterogeneisation of chiral rhodium complex of 1,2-diphosphines already known as very efficient catalysts for enantioselective hydrogenation32 was achieved through amine functionality borne by pyrrolidine molecule. The supported Rh complex revealed as its homogeneous counterpart very high enantioselectivity (<90%) in hydrogenation of a-(acetylamino)cinnamic acid and its methyl ester. 

In addition, a Rh-(R,R)-NORPHOS catalyst has been used to promote enantioselective transfer hydrogenation of the C=C double bond in (Z)-a-(acetylamino)cinnamic acid and in (Z)-a- and ( )-a-(benzoylamino)-2-butenoate by using 80% aqueous formic acid as the source of H2. Optical yields were improved by the addition of sodium formate representative results are presented in Table 2. Comparable, but generally somewhat lower, optical yields were obtained by using other Rh-(biphosphine ligand) catalysts, e.g., biphosphine ligand = (R,S)-(+)-BPPFA (2), (R)-(+)-PROPHOS(3), ot(R,R)-... 

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