Enantioselective heterogeneous catalysi support

Figure 5.2 Schematic representation of self-supporting strategy for enantioselective heterogeneous catalysis.

Torok B, Gyorgy S, Katalin B, Karoly F, Istvan K, Mihaly B (1999) Ultrasonics in heterogeneous metal catalysis sonochemical chemo- and enantioselective hydrogenations over supported platinum catalysts. Ultrason Sonochem 6(l-2) 97-103... 

Despite the convenience of handling and separation in heterogeneous catalysis, many other parameters have a strong influence on the stereochemical outcome pressure temperature modifier purity of substrates high substrate specificity catalyst preparation, which includes type, texture, and porosity of the support dispersion impregnation reduction and pretreatment of the metal.22 Reproducibility of catalyst activities and enantioselectivities can often be a problem attributed to variations in catalyst preparations and purity of the substrate.5-22... 

In heterogeneous catalysis, transition metal nanoparticles are supported on different substrates and are utilized as catalysts for different reactions [57], such as hydrogenations and enantioselective synthesis of organic compounds [58], oxidations and epoxidations [59], and reduction and decomposition [57],... 

Scheme 9.9 Heterogeneous catalysis of enantioselective hydrogenations with self-supported MonoPhos/Rh catalysts.

Torok, B., Szollosi, G., Balazsik, K., Felfoldi, K., Kun, I., Bartok, M. (1999) Ultrasonics in Heterogeneous Metal Catalysis. Sonochemical Chemo- and Enantioselective Hydrogenations over Supported Platinum Catalysts, Ultrasonics Sonochem. 6, 97-103. 

Fan, Q.H., Deng, G.J., Feng, Y., and He, Y.M. (2008) Enantioselective catalysis using dendrimer supports, in Handbook of Asymmetric Heterogeneous Catalysis (eds K. Ding and Y. Uozumi), Wiley-VCH Verlag GmbH, Weinheim. (c)... 

Scheme 5.22 Heterogeneous catalysis of enantioselective hydrogenations with self-supported MonoPhos/Rh catalysts 47a-i. (The data are cited from Refs [53, 54].)...

Nickel and other transition metal catalysts, when modified with a chiral compound such as (R,R)-tartaric acid 5S), become enantioselective. All attempts to modify solid surfaces with optically active substances have so far resulted in catalysts of only low stereoselectivity. This is due to the fact that too many active centers of different structures are present on the surface of the catalysts. Consequently, in asymmetric hydrogenations the technique of homogeneous catalysis is superior to heterogeneous catalysis56). However, some carbonyl compounds have been hydrogenated in the presence of tartaric-acid-supported nickel catalysts in up to 92% optical purity55 . 

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

Polymer-supported chiral catalysts have likewise been prepared in order to obtain access to reusable systems (cf. Section 3.1.1, especially Section 3.1.1.3). For example, copolymerized functionalized BINAP [160, 161] could be applied in the enantioselective hydrogenation of olefinic substrates (up to 94 % ee). Similarly, the copolymerization of vinyl-BINAPHOS with styrene derivatives led to a heterogenized auxiliary which made it possible to hydroformylate styrene and vinyl acetate (Rh catalysis) with selectivities and enantioselectivities close to those provided by the parent homogeneous catalytic system [162]. 

It is clear tliat the attachment of chiral catalysts to dendrimer supports offers a potential combination of the advantages of homogeneous and heterogeneous asymmetric catalysis, and provides a very promising solution to the catalyst-product separation problem. However, one major problem which limits the practical application of these complicated macromolecules is their tedious synthesis. Thus, the development of more efficient ways to access enantioselective dendritic catalysts with high activity and reusability remains a major challenge in the near future. 

I 2 Heterogeneous Enantioselective Catalysis Using Inorganic Supports... 

---