Methyl-a-acetamidocinnamate

Scheme 8.5 Hydrogenation of methyl a-acetamidocinnamate with S/P ligands derived from carbohydrates.

In some cases an alternative sequence involving addition of hydrogen at rhodium prior to complexation of the alkene may operate.11 The phosphine ligands serve both to provide a stable soluble complex and to adjust the reactivity at the metal center. The a-bonded intermediates have been observed for Wilkinson s catalyst12 and for several other related catalysts.13 For example, a partially hydrogenated structure has been isolated from methyl a-acetamidocinnamate.14... 

Reaction using ligand 4 a, substrate methyl-a-acetamidocinnamate ... 

BF4 affords the immobilized [Rh(COD)(PPM)]BF4 complex 36. The supported Rh-PPM catalysts were then tested for their ability to hydrogenate methyl a-acetamidocinnamate (substrate Rh = 200 1, 25 °C, methanol, 0.1 MPa H2). 

The surface-active diphosphane 12 was applied in the hydrogenation of methyl a-acetamidocinnamate [Eq. (10)] with [RhCl(COD)]2 as the catalyst precursor in homogeneous methanolic solution and, alternatively, in ethyl acetate-water biphasic systems (96). 

Recently, carbohydrate amphiphiles have been tested in the asymmetric hydrogenation of (Z)-methyl a-acetamidocinnamate in water (98). With a rhodium(I)-BPPM complex, 50% of the reactant was converted in 5 min, and enantioselectivities up to 96% were observed. A comparison of amphiphiles with alkyl chains of different lengths showed that micelle-forming properties, hydrophilic-lipophilic balance, and the structure caused by hydrogen bonding in the head group may be responsible for these effects. 

However, for the Rh(COD) systems, the enantioselectivity in the hydrogenation of Z-methyl-a-acetamidocinnamate remained barely affected (Fig. 4) or decreased only slightly, depending on the chosen substrate. 

Rh (DIPHOS ) (Z-methyl-a-acetamidocinnamate ) ]BF4. A sample of 0.2 g of [Rh2(DIPHOS)2][BF4]2 and 0.1 g of Z-methyl-a-acetamido-cinnamate were stirred in ca. 10 mL of warm methanol until a clear red solution was obtained. After cooling to room temperature, diethyl ether was added slowly until the solution turned slightly cloudy, followed by addition of just enough methanol to clarify the solution. Red crystals of [Rh( DIPHOS) (Z-methyl-a-acetamidocinnamate)][BF4] were grown by diflFusing diethyl ether into the solution over a period of ca. 20 hr and subjected to x-ray structure analysis. 

We are grateful to the National Science Foundation for support of this research and to W. S. Knowles for generous gifts of samples of DIPAMP and of Z-methyl-a-acetamidocinnamate. The NMR facilities used in this research were supported in part through the University of Chicago Cancer Center Grant NIH-CA-14599. 

Efficient asymmetric hydrogenation of (Z)-methyl a-acetamidocinnamate using chiral rhodium(I) complexes was first performed by Oehme and coworkers. Using N-tert-butoxycarbonyl-4-diphenylphosphino-2-diphenylphos-phinomehlyl-pyrrolidine (BPPM) and 4,5-bis(diphenylphos-phinomethyl)-2,2-dimethyl-l,3-dioxolane (DIOP) as ligands, they achieved their efficient solubilization in water in the presence of sodium dodecyl sulfate or of Triton X-100 amphiphiles, together with reaction rates and enan-tioselectivities comparable to those obtained as references in methanol without surfactants (Figure 6). ... 

Figure 6 Asymmetric rhodium hydrogenation of (Z)-methyl-a-acetamidocinnamate in an aqueous medium as described by Oehme et al. Conditions (Z)-methyl-a-acetamidocinnamate...

In addition to dendrimers, hyperbranched polymers have been used by several groups as soluble supports for catalysts [7, 17]. These supports are polydisperse and randomly branched, and, since they are prepared in a single reaction step, are generally much cheaper materials. Nevertheless, it has been shown that catalysts immobilized on hyperbranched polymers may possess similar properties as dendritic systems [18]. Therefore, dendritic catalysts serve as ideal model systems for catalysts attached to hyperbranched polymers. We functionalized several hyperbranched polyethyleneimines (PEIs) employing the peptide coupling protocol in reactions with the pyrphos linker system. The pyrphos-rhodium complexes bound to the hyperbranched polymers were also found to be active catalysts for the hydrogenation ofZ-methyl-a-acetamidocinnamate [16]. As observed for the... 

CisHi3FeN06, Tricarbonyl-(methyl a-acetamidocinnamate)iron, 46B, 841 Cl 5H13M0NO2, (rj -N-Phenyliminoethyl) (T -cyclopentadienyl)dicarbonyl-molybdenum, 44B, 745... 

Low-temperature 1- and 2-D NMR spectroscopy following reaction of (Z)-methyl a-acetamidocinnamate with the solvent complex [Ru (R)-BINAP (MeCN)(S)2H](BF4) (S=THF, MeOH) has allowed determination of the... 

Glaser, R., and J. Blumenfeld Inhibition of the Asymmetric Hydrogenation of Z-Methyl-a-acetamidocinnamate Catalyzed by DIOP-Rhodium Catalyst in the Presence of Z-Adamantyl- or Bornyl-a-acetamidocinnamate. Tetrahedron Letters 1977, 2525. 

Similar heterogeneous chiral catalysts were prepared by the impregnation of mesoporous Al-MCM-41, Al-MCM-48, and Al-SBA-15 with rhodium diphosphine organometaUic complexes and were tested for the hydrogenation of dimethyl itaconate, methyl a-acetamidoacrylate, and methyl a-acetamidocinnamate [90]. The immobilized catalysts showed high activity and excellent chemo- and enantioselec-tivities, that is, up to >99% conversion, 99% selectivity, and 98% ee. 

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