Heterogeneous heterogenization, chiral catalysts

In most cases homogeneous chiral catalysts afford higher enantioselectivities than heterogenous catalysts. Nevertheless, the development of heterogeneous chiral catalysts has attracted increasing interest because workup of the reaction, and recovery of often valuable chiral auxiliaries by simple filtration, is more convenient than in the case of homogeneous catalysts. 

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

The production of enantiomerically pure products is of great importance in chemical industry. The most desirable way to obtain these products is by chiral catalysis. Homogeneous complexes can often be used as chiral catalysts however, because of their difficult regenerability, the development of heterogeneous chiral catalysts by immobilization of these complexes is difficult but highly desired. 

One of the important conclusions of the early attempts was that it is fruitful to place the functionality near an optically active support. Already in 1958, Isoda and coworkers reported for the first time the enantioselective hydrogenation with a Raney nickel catalyst modified with optically pure amino acids. Optical yields reported at that time were from low (2.5%) to moderate (36%) values (for references see [12]). Subsequently, in 1963, Izumi and coworkers [100] initiated an extended study of the modified Raney nickel system with TA. As a result of their initial researches, this system was the first heterogeneous chiral catalyst to give high enantioselectivities in the hydrogenation of / -ketoesters (95%) [101,102],... 

As it can be seen, the investigations of heterogeneous chiral catalysts started in the late 1950s in Japan and has known a worldwide renaissance in the last few years. Because of a multitude of catalysts discovered and developed in the recent years, combinatorial methods have become an important focus of research in asymmetric catalysis [168,169], In the last few years, efficient techniques have been developed for the high throughput parallel screening of chiral catalysts [170-172], However, parallel screening based on product analysis has potential pitfalls, since the e.s. of a... 

However, many of heterogenized chiral catalysts suffer from the leaching of the active metal or the chiral auxiliary into the solvent and from the decrease of e.s. Another problem associated with catalysts made from metal complexes that are attached to either a modified polymer or metal oxide surface is that the techniques used for their preparation are rather specific and are driven by the nature... 

What are the main drawbacks of a heterogeneized chiral catalyst and how can they be solved Question 10... 

Special mention should be made of organobimetallic catalytic phases where one or more of the organic fragments have chiral characteristics. In this case, by using SOMC/M techniques, heterogeneous chiral catalysts of very good quality in terms of chemo- and enantioselectivity, as well as good stability and reuse capability, have been obtained [45, 46]. 

Figure 6.18 Enantioselective hydrogenation of ethyl pyruvate on heterogeneous chiral catalysts. Activity as a function of time for the following catalysts ( ) Pt/(-)-MenSnBu3, ( ) Rh/ (-)-MenSnBu3 and (A) Ni/(-)-MenSnBu3. (Reproduced from Reference [45].)...

Heterogeneous chiral catalysts are useful because of the easier separation of catalyst from the product and recovery process than homogeneous chiral catalysts. Heterogeneous chiral catalysts supported on polystyrene-type resin catalyse the highly enantioselective addition of dialkylzincs to aldehydes.17... 

Other fundamental strategies are grafting of homogeneous catalysts on organic supports or the use of supported liquid phases3. Recent reviews reveal the potential of heterogeneous chiral catalysts created by the methods mentioned4-6. 

The enantioselective C—C and C—X bond-formation catalyzed by inorganic supported heterogeneous chiral catalysts is summarized in Table 2.3, and selected examples are further described in this section. 

Soai and co-workers have continued their studies as described in two recent communications. One report details the synthesis of enantiomerically enriched ferrocenylamines by the catalytic, asymmetric dialkylzinc alkylation of a ferro-cenylimine [35b], and the other report introduces the use of heterogeneous chiral catalysts in the enantioselective diethylzinc alkylation of a phosphinoyl imine [35c]. 

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. 

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

Heterogeneous catalysts, in the general sense of catalysts placed in a phase different from that of the reagents and products, present clear advantages from a practical point of view, including ease of recovery and potential recycling and reuse. The latter point is especially important when the catalyst cost is high, as is the case for chiral catalysts [1]. 

An alternative that has received a great deal of attention in recent years is the immobilisation of a chiral catalyst on a nonsoluble support (polystyrene resins, silica gel, zeolites, etc.), thereby creating a chiral heterogeneous catalyst. Unlike homogeneous catalysts, these supported complexes can be recovered from the... 

An excellent enantioselective heterogeneous metal catalyst should be one in which the active metal sites are set in a chiral arrangement fulfilling the Ogston principles. Assuming no surface atom rearrangement, such a catalyst should furnish high enantioselectivities. 

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