Evidence for exchange processes associated with the hydrogenation of methyl (Z)‐α‐acetamidocinnamate by a silica‐bound chiral dirhodium catalyst

Abstract

Mass spectrometric studies show that hydrogen/deuterium exchange procaw precede the hydrogenation of the title com- pound.In a recent study') we have shown that both homogeneous (1) and silica-bound (2) (+ )-dicarbonyl-p-chloro-[p-[2-(triethoxysilyl)-ethanethiolato-S,S]bis( [S-methyl-2-(1 -methylethyl)cyclohexyl]dipheny1phosphine)dirhodium catalyze the asymmetric hydrogenation of methyl (Z)-a-acetamidocinnamate (3). The optical purity of the N-acetylphenylalanine methyl ester (4) formed, was found to be up to 98% ee at low conversions, but it dropped gradually when the reduction advanced. Loss in optical purity was also observed upon (i) lowering the hydrogen pressure below 7 atm, (ii) decreasing the catalyst: substrate ratio, and (iii) increasing the reaction tempera t urc.Since the homogeneous dirhodil;m catalyst decomposes in part during the hydrogenation process, we have initially attributed the diminishing in optical purity to a competing nonasymmetric hydrogenation induced by some of the liberated metallic rhodium. However, as the reduction of 3 by the silicon-bound catalyst (which does not deteriorate) was also found to give, with increasing reaction time, increasing amounts of racemic product, we considered hydrogen scrambling at the chiral carbon atom in 4 as an alternative cause for gradual loss in optical activity. Therefore, we decided to investigate the possibility of H/D exchange during the course of the hydrogenation process by using Dz in C H 3 0 H or C D 3 0 D as sol- vents.In a series of experiments, 0.42 mmol of 3 in a mixture of either 2.5 ml of C H 3 0 H and 5.0 ml of C6H6, or of 2.5 ml of CD@D and 5.0 ml of C6H6 was hydrogenated in the presence of 2.21 x lo-' mmol offully activated 3') at 120°C and 7.14 atm (initial pressure) of either H2 or D2. Mass spectral analyses of the Tesulting reaction mixtures after 4 and 16 h, revealed that both the product and the recovered starting material had undergone extensive H/D exchange both by D2 and by the labeled solvent.The 70-eV EI mass spectrum of the starting material 3 that consisted of peaks at rn/z (YO) = 219 (6), 177 (60), 176 (7), 118 (13) (52), 88 (loo), is also in line with well-known mass spectrometric fragmentation patterns, and the major fragmentations of 4+' are those given in Scheme 2. In addition. both 3+' and 4+' were found to form the C7H: ion (I) by several different fragmentation routes.

Supported Chiral Catalysts on Inorganic Materials

“…It has been assumed that the use of the longer and more flexible tether encouraged the formation of Rh complexes bound in a bidentate fashion to the surface. Immobilization of a homogeneous chiral catalyst sometimes affects the chirality of the products, as described by Blum et al, 23,24 who immobilized the homogeneous di-Rh complex 5 on silica to give the heterogenized di-Rh complex 6 (Figure 2). In the hydrogenation of methyl (Z)-R-acetamidocinnamate (3b), the homogeneous catalyst 5 led to the formation of dextro-rotatory product, methyl R-acetamidodihydrocinnamate (4b), while the silica-supported catalyst 6 gave levo-rotatory material.…”

New rhodium complexes anchored on modified USY zeolites. A remarkable effect of the support on the enantioselectivity of catalytic hydrogenation of prochiral alkenes

“…which have been shown to be usable. 4. 5 We wish to present our results on the preparation of new Rh-complexes with N-containing ligands, available from…”

5 Immobilisation of chiral catalysts: easy recycling of catalyst and improvement of catalytic efficiencies

“…17). 55,56 This difference in product chirality is presumed to be associated with the different orientations of phosphine ligands in the homogeneous and heterogeneous catalyst, Fig. 16 i.e., one side of the dirhodium complex in the immobilised complex may be blocked, which would force the substrate to approach the metal from a different direction.…”