Catalysis cooperative

The high enantioselectivity and broad substrate scope of the HKR are accompanied by an intriguing mechanistic framework involving cooperative catalysis between different catalyst species. Detailed mechanistic investigation into each of these pathways has produced new insights into cooperative catalysis and has resulted in synthetic improvements in the HKR and other ARO reactions [81],... 

Cooperative catalysis between multiple metal centers is considered to be common in enzymatic systems,96 and using this idea for designing catalytic systems... 

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. 

Flg.1. In the amino acid sequence of KO-42 is encoded its fold and its function as it controls the formation of a hairpin helix-loop-helix motif that dimerizes to form a four-helix bundle. On the surface of the folded motif a reactive site is formed that catalyzes hydrolysis, transesterification and amidation reactions of reactive esters, whereas unfolded peptides are incapable of cooperative catalysis. In addition the values, and thus the reactivities, of the histidine residues are controlled by the fold. The pK of each His residue of KO-42 is shown in the figure and deviate by as much as 1.2 units from that of random coil peptides which is 6.4... 

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

A similar Diels-Alder reaction was investigated at DFT-level by Houk and co-workers [57]. Instead of using TADDOL, they selected one methanol molecule, two methanol molecules and 1,4-butanediol in cooperative and bifurcated coordination as catalysts. It was found that cooperative catalysis is generally the favored route. 

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).

Preparation and catalysis of disubstituted cyclodextrin as an excellent enzyme model is demonstrated by the RNAase model reported by Breslow et al. (68, 83). The enzyme models 10 and II, derived from 1, show a bellshaped pH versus rate profile for the hydrolysis of the cyclic phosphate of 4-terf-butylcatechol, indicating the cooperative catalysis by two imidazole groups (Fig. 21). The reactions catalyzed by 10 and II give exclusively 12 and 13, respectively. This interesting specificity indicates that the geometry of the P—O bond cleavage is quite different from each other. Another interesting enzyme-like kinetic behavior that these hosts exhibited is successful demonstration of the so-called bell-shaped pH profile. 

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)]. 

Recently, Taillefer et al. reported an Fe/Cu cooperative catalysis in the assembly of N-aryl heterocycles by C—N bond formation [90]. Similarly, Wakharkar and coworkers described the N-arylation of various amines with aryl halides in the presence of Cu—Fe hydrotalcite [91]. Interestingly, Correa and Bolm developed a novel and promising ligand-assisted iron-catalyzed N-arylation of nitrogen nucleophiles without any Cu co-catalysts (Scheme 6.19) [92]. Differently substituted aryl iodides and bromides react with various amides and N-heterocycles. The new catalyst system consists of a mixture of inexpensive FeCl3 and N,N -dimethylethylenediamine (dmeda). Clearly, this research established a useful starting point for numerous future applications of iron-catalyzed arylation reactions. 

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... 

The cooperative catalysis of CpRu(PPh3)2(CH3CN)PF6 (18) and DBU has permitted chemoselective nucleophilic activation of acetonitrile in the presence of base-sensitive aldehydes to afford corresponding /1-hydroxynitriles (19) in good yield (Scheme... 

Murakami et al. have utilized Mayer vesides to study aldolase-type reactions [48]. Formation of [i-phenylserinc from glydne and benzaldehyde proceeded effectively by cooperative catalysis of a hydrophobic pyridoxal derivative (47) and Zn(n) ions in the bilayer vesicle formed with 32. The threo isomer was dominantly produced over the erythro form. A marked enantioselectivity was observed in the co-veside of 32 and 35 in combination with 47 and Cu(ii) the ee for formation of (2S,3R)-P-phcnylscrinc over its enantiomeric (2R,3S)-isomer was 58%. Enantioselectivity also arose with another bilayer assembly, formed with 32, 35, and 37 in the presence of Cu(ii), where the (2R,3S) isomer was dominant over the (2S,3R) species in 13% ee. The opposite enantioselectivity performed by the second system, as compared with that for 47, might reflect a different stereochemical environment around the quinoid intermediate that allows the attack of benzaldehyde. 

Another important research trend refers to cooperative catalysis, in which two or more metals cooperate in different steps of a process. 

The vinylphenol unit was similaily introduced to imidazole-containing copolymers cf. Section 4-4), but the incorporation resulted only in the increase of hydro-phobicity of the polymer catalyst On the other hand, some rate enhancements were observed due to phenolic compounds bound onto Bl-VP copolymers, and cooperative catalysis for die intraoanplex reactirxi was implied (71). 

Fig. 23.1 Example of cooperative catalysis. The catalytic triad in a serine protease enzyme rapidly cleaves amide bonds...

Are heterogeneous catalysts appropriate platforms to construct cooperativity Cooperative catalysis can occm in homogeneous catalysis. For example, Kanemasa and Ito [3] have reported a two-catalyst system that leads to the dual activation of an electrophile and a nucleophile by a Lewis acid and amine base, respectively. 

This example illustrates the utility of immobilizing different catalytic sites on a surface for accelerating reactions through cooperative catalysis. 

Fig. 23.7 The proposed mechanism for cooperative catalysis between acid and base groups in the aldol reaction. Reprinted with permission from [21]. Copyright 2006 WUey-VCH...

Huh S, Chen HT, Wiench JW, Pruski M, Lin V (2005) Cooperative catalysis by general acid and base bifunctionalized mesoporous silica nanospheres. Angew Chem Int Ed Engl 44 1826... 

Margelefsky EL, Dufaud V, Zeidan RK, Davis ME (2007) Organized surface functional groups Cooperative catalysis via thiol/sulfonic acid pairing. J Am Chem Soc 129 13691... 

Cooperative Catalysis by Metal Complexes and Chiral Phosphoric Acids... 

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