Two-center catalysis

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

Shibasaki M (2000) Asymmetric two center catalysis, in Stimulating concepts in chemistry (Vogtle F, Stoddart F, Shibasaki M ed.) Wiley-VCH, Weinheim, p 105... 

Just a few reviews on this quickly developping field a) D. H. Pauli, C. J. Abraham, M. T. Scerba, E. Alden-Danforth, T. Lectka, Bifunctional asymmetric catalysis cooperative lewis acid/base systems, Acc. Chem. Res., 2008,41, 655-663 b) M. Kanai, N. Katob, E. Ichikawab, M. Shibasaki, Power of cooperativity Lewis acid-Lewis base bifimctional asymmetric catalysis, Synlett, 2005, 1491-1508, c) M. Shibasaki, M. Kanai K. Funabashi, Recent progress in asymmetric two-center catalysis, Chem. Commun., 2002, 1989-1999. 

Over the last few years several examples have been reported in the field of asymmetric catalysis that are based on the interaction of two centers.6,119 Recently, Shibasaki and coworkers have developed an asymmetric two-center catalyst. Scheme 3.14 shows preparation of optically active La binaphthol (BINOL). This catalyst is effective in inducing the asymmetric nitro-aldol reaction, as shown in Scheme 3.15. 

Due to many impressive advances in metal-catalyzed transformations, both asymmetric and non-asymmetric, several efforts have been directed towards designing total synthesis routes that very heavily depend on various catalytic methods. These total syntheses benefit from the economic efficiency and environmental consciousness that are two of the inherent attributes of catalytic reactions. The total synthesis of wodeshiol 133 by Corey, discussed above (Scheme 19) is one such example. Two additional catalysis-based enantioselective total syntheses are briefly discussed below. In both efforts, all centers of asymmetiy are attained by a catalytic enantioselective method, and the synthesis is completed through the use of several other catalytic reactions. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

Figure 27-13 Proposed mechanism and transition state structure for the synthetic nucleotidyltransfer activity of DNA polymerase 3 (and other DNA polymerases). The chain-terminating inhibitor dideoxy CTP is reacting with the 3 -OH group of a growing polynucleotide primer chain. This -OH group (as -0 ) makes an in-line nucleophilic attack on Pa of the dideoxy-CTP. Notice the two metal ions, which interact with the phospho groups and which are held by three aspartate side chains. Two of the latter, Asp 190 and Asp 256, are present in similar positions in all of the polymerases. The active centers for the hydrolytic 3 -5 and 5 -3 exonuclease activities of some of the polymerases also appear to involve two-metal catalysis and in-line displacement. See Sawaya et al.27i...

Conceptually new multifunctional asymmetric two-center catalysts, such as the Ln-BINOL derivative, LnMB, AMB, and GaMB have been developed. These catalysts function both as Brpnsted bases and as Lewis acids, making possible various catalytic, asymmetric reactions in a manner analogous to enzyme catalysis. Several such catalytic asymmetric reactions are now being investigated for potential industrial applications. Recently, the catalytic enantioselective opening of meso epoxides with thiols in the presence of a heterobimetallic complex has... 

Binary pyrazolate complexes containing divalent metals bridged by two pz groups are known for many transition metals. The increasing interest for this class of complexes stems from the expectation that they may provide useful insights in the field of magnetostructural correlations as well as in multimetal centered catalysis. 

In this review, we have illustrated by different applications the importance of chiral organophosphorus compounds as catalysts opening up a new field in asymmetric catalysis. From the diversity of trivalent (pyramidal) A3a3 compounds and pentavalent (tetrahedral) A5a4, there are many possibilities for the design of asymmetric one- or two-center catalysts. It can be reasonably envisioned that new chiral phosphines, phosphine oxides, and phosphazenes, to-... 

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