Enantioselective cinnamic acids

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... 

PYRPHOS and BDPP forming a chiral five- and six-membered chelate ring are useful for Rh-catalyzed enantioselective reduction of (Z)-oe-(acetamido)cinnamic acid with HCOOH/HCOONa [161]. 

Conceptually interesting is the discovery that products obtained by the aminohydroxylation itself can serve as ligands [23], Thus, the AA of styrenes 28 proceeds in high yields to the regioisomeric amino alcohols 29 and 30 in the presence of catalytic amounts of 31, being the AA product of cinnamic acid, with moderate, nevertheless significant enantioselectivity. 

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)-... 

A biocatalytic enantioselective addition of ammonia to a C=C bond of an a,)9-unsaturated compound, namely fumaric acid, makes the manufacture of L-aspartic acid possible on an industrial scale. This process, which is applied by, e. g., Kyowa Hakko Kogyo and Tanabe Seiyaku, is based on the use of an aspartate ammonia lyase as a biocatalyst [119]. Another comparable reaction is the asymmetric biocatalytic addition of ammonia to trans-cinnamic acid, which represents a technically feasible process for the production of L-phenyl-alanine [120]. 

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