Asymmetric cooperative catalysis

This final chapter summarizes the enzyme-catalyzed asymmetric reactions and introduces some new developments in the area of asymmetric synthesis. Among the new developments, cooperative asymmetric catalysis is an important theme because it is commonly observed in enzymatic reactions. Understanding cooperative asymmetric catalysis not only makes it possible to design more enan-tioselective asymmetric synthesis reactions but also helps us to understand how mother nature contributes to the world. 

Breinbauer, R. Jaeobsen, E. N. (2000) Cooperative asymmetric catalysis with... 

R.G. Konsler, J. Karl, E.N. Jacobsen, Cooperative asymmetric catalysis with dimeric salen complexes, J. Am. Chem. Soc. 120 (1998)... 

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... 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Asymmetric catalysis by bimetallic catalysts is currently a research field of great interest. There have been few studies of the interaction between bidentate Lewis acids and carbonyl groups [22a]. Reilly and Oh [22c] have reported the only example of such an asymmetric Diels-Alder reaction catalyzed by a 1 1 complex of N-tosyltrypto-phan [17-19] with l,8-naphthalenediylbis(dichloroborane), in which the two Lewis-acidic sites work in a cooperative manner (Eq. 22). 

Table 7-6. Asymmetric catalysis by multi-component ligand cooperation.

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. 

Fig. 13.4 General concept of rare earth bimetallic asymmetric catalysis (a) cooperative intramolecular catalysis, and (b) cooperative intermolecular catalysis.

Kanai M, Ikariya T, Ooi T, Ding K, Milstein D (2013) Recent topics in cooperative catalysis asymmetric catalysis, polymerization, hydrogen activation, and water splitting. In Ding K, Dai L-X (eds) Organic chemistry- breakthroughs and perspectives. Wiley-VCH, Weinheim,... 

Pauli DH, Abraham CJ, Scerba MT, Alden-Danfrath E, Lectka T (2008) Bifunctional asymmetric catalysis cooperative Lewis acid/base systems. Acc Chem Res 41(5) 655-663. doi 10.1021/ ar700261a... 

Abstract The concept of bifunctional acid catalysis is very helpful for inventing new catalytic asymmetric reactions. Compared with single functional acid catalysts, cooperative effect of two acid components has the potential to fine tune the reactivity as well as the selectivity of desired reaction pathways. This chapter focuses on some representative examples on the recent developments of bifunctional acid catalysis, including combined acid catalysis and other cooperative acid catalysis. 

Keywords Asymmetric synthesis Bifunctional acid catalysis Combined acid catalysis Cooperative acid catalysis Designer acid... 

In this chapter, we have successfully developed bifunctional chiral rhodium complexes bearing chiral phebox ligands that can be used in catalytic asymmetric reactions. The N,C,N meridional geometry with the rhodium-carbon covalent bond is the key character in the phebox complexes. The metal-phebox cooperative bifunctionality significantly contributes reactivity and selectivity in the catalytic asymmetric reactions. Furthermore, the prototype of the bifunctional catalyst can be explained to a wide range of asymmetric catalytic reactions promoted by the Lewis acids, hydrides, enolates, and bory active species. Their diversity further broadens the range of opportunities for asymmetric catalysis. 

Another example of this cooperative catalysis has been presented by Konsler et al.101 in the course of their asymmetric ring-opening (ARO) study. They found that the ARO of mew-epoxides with TMS-N3, catalyzed by Cr salen compound 132, showed a second-order kinetic dependence on the catalyst.102 They then proposed that there might be cooperative, intramolecular bimetallic catalysis taking place, with one metal activating the substrate mew-epoxide and... 

In addition, quite recently a direct catalytic asymmetric Mannich-type reaction has been achieved by the cooperative catalysis of ALB and La(0Tf)3-nH20. 

Figure 12. Proposed mechanism for cooperative catalysis in asymmetric ring opening of epoxide by dendimeric frame work.

Figure 6.23 Proposed transition states of the asymmetric 72-catalyzed cyanosilylation of ketones describe two alternative mechanistic pathways for cooperative catalysis Addition via thiourea-bound ketone (TS 1, preferred) and addition via thiourea-bound cyanide (TS 2).

A direct catalytic asymmetric Mannich reaction using unmodified ketones was reported using cooperative catalysis of a AlLibis((i )-binaphthoxide) complex ((.R)-ALB) and La(0Tf)3-nH20 [27,28]. It was also reported that enantiose-lective and diastereoselective catalytic nitro-Mannich reactions of N-phosphi-noylimines proceeded smoothly using the complex of ALB and ferf-BuOK [29, 30] [(Eq. (5)]. 

The nucleotide occupancy of the catalytic sites observed in the first crystal structure was exactly what Paul Boyer had predicted earlier in his binding-change model of cooperative catalysis (Boyer, 1993). Consequently, this first high-resolution structure of the Fj-ATPase immediately initiated a number of studies that ultimately led to the elucidation of the F -. TPase s rotational mechanism of cooperative catalysis. At the time, the F - ATPase structure represented the largest asymmetric structure solved to atomic resolution by x-ray crystallography, and this accomplishment, together with the visionary prediction of rotary catalysis, was subsequently awarded the 1997 Nobel prize in chemistry (to John Walker for the structure and Paul Boyer for the catalytic mechanism). However, whether the first (and many subsequent) structure (s) represented physiologically... 

If we first consider purely organic systems, one can mention the asymmetric reduction of ketmies with borane promoted by a chiral oxazaborolidine 1 developed by Corey, Bakshi, and Shibata (CBS reduction. Scheme 1) [13,14]. In this system, the nitrogen atom of the oxazaborolidine serves as Lewis base and coordinates BH3 thus improving its nucleoplulicity, while the endocyclic boron atom acts as the Lewis acid and activates the ketone toward the reduction. This seminal work constimtes an early example of metal-free catalysis and shows that cooperative effects can emerge from ambiphilic Lewis acid/base catalytic system. 

Scheme 22.11 Asymmetric desymmetrisation by cooperative catalysis with chiral general base catalyst 45 and achiral nucleophilic catalyst 54.

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