Cooperative catalysis reaction

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],... 

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... 

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. 

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. 

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. 

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...

Hu et al. developed a system that involved cooperative catalysis by a chiral phosphoric acid and an achiral rhodium complex (Scheme 3.37) [81]. They applied the binary catalytic system to a three component coupling reaction among a diazoesters, primary alcohols, and aldimines. The steric bulkiness of the primary alcohol had a significant effect on both diastereo and enantioselectivities. The sterically demanding 9 anthracenemethanol was the best component to give p amino a alkoxy esters with excellent stereoselectivities under the combined and cooperative catalysis by phosphoric acid ll and the rhodium complex. 

These, and several other studies indicate that this reaction is not simple. Results suggest that, whilst basic sites are essential for side-chain alkylation the reaction proceeds more effectively via acid-base cooperative catalysis [81]. Experimented evidence strongly suggests that formaldehyde is an intermediate and that crystal stmcture [82], or at any rate microporosity [83], is also important in selectivity. This last point suggests that selectivity is improved by a preferred local geometry and this is suggested as a feature in the higher yields with KX rather than K.M catalysts. The low activity... 

By applying a new mode of cooperative catalysis involving the combination of a chiral Bronsted acid and a -symmetric biaryl saturated imidazolium precatalyst, Lee and Scheldt disclosed a highly enantioselective NHC-cata-lyzed [3 + 2] annulation reaction between a,p-alkynals and a-keto esters to generate the desired y-crotonolactones in high yields and excellent levels of enantioselectivity (up to 92% yield, 92% ee). The authors proposed that NHC-bound allenolate underwent addition to the a-keto ester activated by the chiral Bronsted acid derived co-catalyst (Scheme 7.43). 

The Chi group disclosed the first oxidative y-addition of enals to trifluo-romethyl ketones and enantioselective control via Lewis acid/NHC cooperative catalysis to give unsaturated 5-lactones. Enantioselective control involving the relatively remote enal y-carbon was achieved via Lewis acid and NHC cooperative catalysis (up to 81% yield and 94% ee). A reaction pathway... 

This review is not intended to be fully comprehensive but instead should serve to highlight current understanding of bimetallic cooperative catalysis as it applies to the activation of the alkyne triple bond. We have divided the review into four sections, separated by reaction type, which emphasise different aspects of the bimetallic alkyne activation mechanism. These four sections are as follows ... 

In 2010, Scheldt and coworkers reported cooperative N-heterocyclic car-bene/Lewis acid catalysis for highly stereoselective annulation reactions. This cooperative catalysis process integrating titanium(iv) and triazolium-derived NHCs allowed the synthesis of ds-cyclopentenes 69 with a broad substrate scope and high enantioselectivity (Scheme 20.33). 

Cooperative catalysis by combing chiral phosphoric acids and Lewis acids to promote the FCA reactions between arenes and chelating enones were recently developed [29]. It was observed by Luo and co-workers that simple swap of the counterions of Lewis acid 10X3 led to a regioselective shift between 1,2-addition and 1,4-addition in the reactions of indoles with (S,7-unsaturated a-ketoesters (Scheme 9.11). 

Thioamides can undergo direct enantio- and diastereo-selective aldol reaction with aldehydes, by means of a soft Lewis acid/hard Brpnsted base cooperative catalysis. 

Cooperative catalysis in multi-component reactions highly enantioselective synthesis of y-hydroxyketones with a quaternary carbon stereocenter, (b) X. Han, M. Gan, H. Qin, J. Ji, X. Zhang, L. Jiang, W. Hu, Synlett 2011, 1717-1722. Trapping of oxonium ylides with Michael acceptors highly diastereoselective three-component reactions of diazo compounds with alcohols and ben-zylidene Meldrum s acids/4-oxo-enoates. 

A different pathway to substituted quinoUne derivatives 442 involving Meldrum s acid 112 was reported by Verma et al. [183]. This report utilizes a huge library of amines and CH acids that undergo reaction with the ortho-alkynyl aldehyde 440 by cooperative catalysis of AgNOj and proline or... 

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