Hydrogenation chiral heterogeneous

Chiral heterogeneous hydrogenations have been much studied. The area is not without complication. Results vary widely and depend on a number of... 

Chiral heterogeneous catalysts, although have lower enantioselectivity and stability than homogeneous catalysts, are often preferable because of their handling and separation properties (5). Aim of this work was to shed light on the enantioselective hydrogenation of a,p-unsaturated acids or their... 

To summarize, chiral heterogeneous catalysts were prepared from rhodium-diphosphine complexes and aluminum-containing mesoporous materials. The bonding occurred via an ionic interaction of the cationic complex with the host. These catalysts were suitable for asymmetric hydrogenation of functionalized olefins. The catalysts can be recycled easily by filtration or centrifugation with no significant loss of activity or enantioselectivity. 

FIG. 1. Examples of asymmetric synthesis, (a) Addition of a Giignard reagent to a chiral a-keto ester, (b) Chiral hydroboration with di-3-pinanylborane. (c) Chiral heterogeneous hydrogenation. 

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

In conclusion, chiral heterogeneous catalysts are prepared from chiral Rhodium diphosphine complexes and Al-MCM-41. The bonding supposedly occurs via an ionic interaction of the cationic complex with the host. Also a slight reduction of weak acidic sites of Al-MCM-41 has been observed. These catalysts are suitable for the hydrogenation of functionalised olefins. The organometallic complexes remain stable within the mesopores of the carrier at reaction conditions. The catalyst can be recycled by filtration or centrifugation. 

Hydrogenations using heterogeneous catalysts usually require above ambient T and Hj pressures > 10 Pa. Homogeneous catalysts are often more selective and involve lower T (e.g., 25°C) and Hj (ca. 10 Pa). Highly useful selectivity in hydrogenation is shown in reactions of H2 with prochiral substrates in the presence of chiral catalysts, e.g., op-... 

For the enantioselective reduction of olefins, there are few alternatives to homogeneous hydrogenation because neither transfer hydrogenations with hydrogen donors such as HCOOH/NEt3 [28] nor chiral heterogeneous catalysts [12 c] are ready for larger-scale applications. 

The possibility of performing enantio-selective hydrogenation over heterogenous catalysts is based on the fact that a chiral additive can govern the way a... 

Rh-MeBPE(33)-Al-MCM-48 catalyst exhibited 95% ee for the hydrogenation of methyl a-acetamidoacrylate, while its homogeneous counterpart showed 90% ee. These chiral heterogeneous catalysts can be reused at least four times without any activity loss, with a TON in excess of than 4000. 

Scheme 10.3S Hydrogenation of ethyl nicotinate using a chiral heterogeneous catalyst.

The topic of this chapter is enantioselective hydrogenation over chiral or chirally modified solid catalysts. Diastereoselective hydrogenation of chiral compounds and asymmetric hydrogenation with heterogenized (supported, embedded) homogeneous transition metal complexes will not be discussed. 

Smith et al. have found a new type of chiral heterogeneous catalyst based on Pt, Pd, Rh or Ru dispersed onto a >eto-cyclodextrine-epichlorohidrin copol5mier (CDP). The ability of these catalysts to catalyze the enantioselective hydrogenation of C=C bonds was tested with the hydrogenation of dimethylitaconate into dimethyl methylsuccinate and ( )-2-methyl-pent-2-enoic acid into 2-methylvaleric acid (Scheme 3.8.). 

Klabunovskii, E.I., and Vedenyapin, A.A. (1977) Mechanism of as3Tnmetrizing action of chiral heterogeneous hydrogenation catalysts modified with optically active ligands, Zh. Fiz. Khim. (J. Phys. Chem., russ.), 51, 3005-3012, Chem. Abstr. 1978,88,61803c. 

Smith, G.V., Notheisz, F. (1999) Enantioselective hydrogenations, in Heterogeneous Catalysis in Organic Chemistry, Acad. Press, pp. 97-117. Augustine, R.E., Tanielyan, S.K., Doyle, L.K. (1993) Enantioselective heterogeneous catalysts 1. A working model for the catalyst modifier sub-strate interactions in chiral pyruvate hydrogenations. Tetrahedron Asymm. 4, 1803-1827. 

An early approach toward chiral heterogeneous catalysts was the deposition of the catalyticaUy active metal or metal oxide particles onto intrinsically chiral supports such as quartz [24], cellulose [25], or synthetic chiral polymers [26-28]. Hydrogenation and dehydration reactions were tested, but enantioselective performance was found to be poor. In a recent review, Mallat et al. [29] attributed this poor enantioselectivity to the fact that only a small fraction of the metal atoms would... 

Enantioselective hydrogenation over heterogeneous catalysts in the presence ofa mixture of two chiral catalyst modifiers often results in the values of enantiomeric excess that do not follow a linear relationship with concentrations of modifiers. The mechanism is present in the following graph... 

Four general methods have been used for obtaining chiral ligands resolution of a racemic mixture, use of a chiral naturally occurring product 33), and asymmetric homogeneous or heterogeneous hydrogenation. 

Among the various strategies [34] used for designing enantioselective heterogeneous catalysts, the modification of metal surfaces by chiral auxiliaries (modifiers) is an attractive concept. However, only two efficient and technically relevant enantioselective processes based on this principle have been reported so far the hydrogenation of functionalized p-ketoesters and 2-alkanons with nickel catalysts modified by tartaric acid [35], and the hydrogenation of a-ketoesters on platinum using cinchona alk oids [36] as chiral modifiers (scheme 1). 

The enantioselective hydrogenation of C=C bonds is included in Chapter 3, where the general nature of heterogeneous enantioselective hydrogenation is fully explored, so here we examine only the narrow subject. Enantioselective hydrogenation of C=C bonds occurs only on prochiral double bonds (at least one of the carbons substituted with two different groups other than hydrogen). A chiral environment is required. 

New modifiers have traditionally been discovered by the trial-and-error method. Many naturally occurring chiral compounds (the chiral pool38) have been screened as possible modifiers. Thus, the hydrogenation product of the synthetic drug vinpocetine was discovered to be a moderately effective modifier of Pt and Pd for the enantioselective hydrogenation of ethyl pyruvate and isophorone.39 Likewise, ephedrine, emetine, strychnine, brucine, sparteine, various amino acids and hydroxy acids, have been identified as chiral modifiers of heterogeneous catalysts.38... 

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