Cooperative catalysis addition

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).

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... 

In addition to the synthesis of specific ligands and the resulting catalysts new ideas and concepts begin to combine with transition metal chemistry to open up new areas in homogeneous catalysis, like catalysis under supercritical conditions, colloidal catalysis , organometallic electrocatalysis and multi-metallic catalysis " ( cooperative catalysis ). 

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... 

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). 

Iminium-based organocatalysis is somewhat less explored than enamine-based organocatalysis and has been mostly used in the activation of a,/S-conjugated aldehydes and ketones. Therefore, this type of catalysis has unsurprisingly been the subject of a limited number of studies under the umbrella of the metal-organic cooperative catalysis concept. In 2011, the Cdrdova group [55] reported the first enantioselective and chemoselective /3-silyl addition to a./S-unsaturated aldehydes using copper salts and chiral pyrrolidine derivatives as catalysts. As proposed, the chiral secondary amine forms an iminium salt with... 

Scheme 19.19 As3mimetric Michael additions catalysed by the cooperative catalysis of [(salen)Al]20 and Er-Pybox.

In 2007, Shibasaki et al. reported a three-component domino reaction induced by cooperative catalysis between copper and zinc. This novel domino reaction occurred between acetophenone, diethylzinc and an allenic ester, providing the corresponding chiral lactone in high yield and excellent enantioselectivity of 96% ee, as shown in Scheme 3.1. The process began with the addition of diethylzinc to the allenic ester, which was followed by an aldolisation with acetophenone, and a lactonisation reaction gave the final product. Initially, the Cu(ii) was reduced to Cu(i) in the presence of... 

Similar hypothesis of this kind of cooperative catalysis mechanism has been proposed by Matsunaga, Shibasaki, and co-workers in their heterobimetallic gal-lium/ytterbium-Schiff base complex catalyzed asymmetric oc-addition of isocyanides to aldehydes [99]. Research along this line will be reviewed in another chapter of this book [100]. 

Inspired by the success of intramolecular addition and tautomerization of aldehydes with a pendant alkyne through cooperative catalysis of a secondary amine and an An complex, in 2008, Yang et al. reported a cascade reaction with the combination of a copper complex and an achiral secondary amine catalyst for the synthesis of attractive carbocycles [48]. This chemistry merged a pyrrolidine-promoted Michael addition via iminium ion intermediates and a Cu-catalyzed cycloisomerization protocol (Scheme 9.54). Various ketones and alkyne-tethered active methylene compounds could be converted into densely functionalized cyclopentene derivatives. Although the asymmetric version was not given, the chemistry described here was amenable for the implementation of asymmetric synthesis of such functionalized molecules by a combination of chiral amines and suitable Au complexes. 

In 1992 Murahashi, Hosokawa, and co-workers described the anti-Markovnikov oxidative addition of amides and carbamates to electron-deficient olefins by applying a palladium and copper cooperative catalysis under oxygen atmosphere [41]. The proposed mechanism involved a ff-bonded palladium(II) intermediate resulting from the addition of the nucleophile to the olefin, and subsequent ) -palladium hydride elimination to yield the functionalized alkene. Interestingly, both lactams and cyclic carbamates gave predominantly the corresponding E-enamide derivatives. Acyclic amides, conversely, afforded ElZ mixtures of products. The addition of a catalytic amount (5 mol%) of hexamethylphosphoric triamide (HMPA) was found notably beneficial for the reaction of 5-membered lactams and reduced the reaction time of such particular oxidative amidations (Scheme 2). 

The above report by Jones and coworkers strongly indicates that it is necessary to keep the amount of silanols constant for the investigation of the cooperative catalysis of both organic acid and base groups. Additionally, it can be said that surface silanol groups possess potential usefulness for the promotion of amine-catalyzed reactions. The detailed role of silanol groups in the cooperative catalysis will be discussed in the next section. 

The basic kinetic properties of this allosteric enzyme are clearly explained by combining Monod s theory and these structural results. The tetrameric enzyme exists in equilibrium between a catalytically active R state and an inactive T state. There is a difference in the tertiary structure of the subunits in these two states, which is closely linked to a difference in the quaternary structure of the molecule. The substrate F6P binds preferentially to the R state, thereby shifting the equilibrium to that state. Since the mechanism is concerted, binding of one F6P to the first subunit provides an additional three subunits in the R state, hence the cooperativity of F6P binding and catalysis. ATP binds to both states, so there is no shift in the equilibrium and hence there is no cooperativity of ATP binding. The inhibitor PEP preferentially binds to the effector binding site of molecules in the T state and as a result the equilibrium is shifted to the inactive state. By contrast the activator ADP preferentially binds to the effector site of molecules in the R state and as a result shifts the equilibrium to the R state with its four available, catalytically competent, active sites per molecule. 

Unactivated aUsynes 209 undergo the addition of aryhnagnesium reagents under cooperative iron and copper catalysis, yielding trisubstituted alkenyhnagnesium species. They can be trapped with electrophiles, giving tetrasubstituted alkenes such as 210. In some... 

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