Catalysts cooperative

Selective aromatic functionalization has been a permanent object of research since the ninenteenth century. Catalysis has offered a powerful tool to achieve this goal. Over the years we have worked out a complex catalytic system consisting of an inorganic compound such as a palladium salt and an organic molecule containing a strained double bond such as norbomene (1,2). We have seen that these two catalysts cooperatively react with an aromatic iodide, an alkyl iodide and a terminal olefin. The following equation reports an example (L = solvent and/or olefin) (3). 

Many of the trial results have indicated that the coexistence of an acid and alkaline center is conducive to dehydration of ethanol to ethylene. Hassan suggested the catalytic mechanism of ethanol dehydration catalyzed by solid acid or alkaline catalysts in 1982 (Abd El-Salaam and Hassan 1982). He claimed the acid and alkaline center of the catalyst cooperated in the process of ethanol dehydration. Although the reaction was mainly catalyzed by the acid center, the existence of a modest alkaline center could promote this reaction. Hassan considered ethanol was absorbed in the acid and alkaline center on the surface of the catalyst and then formed adsorption-state compounds, which could further dehydrate to ethylene and release the acid and alkaline center. The mechanism of dehydration of ethanol to ethylene catalyzed by activated alumina suggested by Cosimo et al. (1998) is shown Fig. 4. 

Keywords Alkyne activation Bimetallic Catalysis Catalyst Cooperativity... 

Various situations are analyzed where the two metal centers play a role in one of the coordination modes A-E. There are many cases in which bimetallic catalysis can occur with the two metals acting cooperatively, for instance, in the dimerization of alkynes at two ruthenium metal centers, where a ruthenium-vinylidene species is formed, which is able to subsequently activate the second alkyne reactant through a C-H cleavage on the second ruthenium center. The coupling of these two moieties occurs on this dinuclear platform to provide the enyne product molecule. Examples are also presented where bimetallic catalysts cooperatively activate substituted alkynes in the catalyzed formation of heterocycles. 

Catalytic asymmetric synthesis is a major focus in the field of synthetic organic chemistry. In asymmetric catalyses, achiral additives can enhance the enantioselectivity, and achiral cocatalyst can cooperatively participate in the event of enantioface selection. We have reported on the unusual reversal of enantioface selectivity that the two / -affording chiral catalysts cooperate to give the opposite 5 enantiomer. That is, the enantioselective addition of /-Pr2Zn to pyrimidine-5-carbaldehyde 1 was carried out catalyzed by a mixture of two chiral catalysts DMNE and 2-[(l-phenylethyl)amino]-ethanol (PEAE) (Scheme 15) [55]. The reaction using (11 ,2S )-DMNE alone afforded (l )-5-pyrimidyl alkanol 1, and (/ )-PEAE alone also catalyzed the production of (/ )- with the same enantioface selectivity. 

Wacker chemistry normally gives branched products via Mar-kovnikov olefin hydration, yet linear alcohols are highly desirable. Grubbs has now found conditions that give the long-sought anti-Markovnikov olefin hydration, based on a multistep scheme in which a pair of catalysts cooperate. 

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. 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

Coolwater Coomassie Brilliant Blue Cooperite Cooper pairs Coordination Coordination catalysts... 

Aluminum Chloride-Based All lation. The eadier alkylation processes were variations of the Eriedel-Craft reaction on an aluminum chloride catalyst complex in a Hquid-phase reactor (27), including those developed by Dow Chemical, BASE, Monsanto, and Union Carbide in cooperation with Badger. The Union Carbide-Badger process was the one most widely used during the 1960s and 1970s, with 20 plants built worldwide. 

Wulff and coworkers have applied their aluminum catalyst 2 containing a vaulted biphenanthrol ligand (VAPOL, Section 2.1) to the Diels-Alder reaction between methyl acrylate and cyclopentadiene [25] (Scheme 1.32). In this Diels-Alder reaction auto-induction is observed, because of a cooperative interaction between the product... 

Jacobsen developed a method employing (pybox)YbCl3 for TMSCN addition to meso-epoxides (Scheme 7.22) [46] with enantioselectivities as high as 92%. Unfortunately, the practical utility of this method is limited because low temperatures must be maintained for very long reaction times (up to seven days). This reaction displayed a second-order dependence on catalyst concentration and a positive nonlinear effect, suggesting a cooperative bimetallic mechanism analogous to that proposed for (salen)Cr-catalyzed ARO reactions (Scheme 7.5). 

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],... 

Kabanov et al.131) found that a copolymer of 4-vinylpyridine and acroleinoxime, 82 (PPox), is a powerful catalyst for the hydrolyses of PNPA, NABA, and 3-nitro-4-trimethyl-acetoxybenzoic acid 83 (NTBA). The activity of the copolymer was 103 times higher than that of the low-molecular-weight oxime, iso-butyraldoxime. They proposed the cooperative activation of the oxime- and pyridine-groups in the vicinity of pyridinium cation groups of the copolymer. 

When esterase models are designed, several important and fundamental problems have to be solved. Systematic studies on other interactions, such as hydrogen-bonding and charge-transfer type forces have not been fully performed. Furthermore, various cooperative actions between different kinds of interactions, e. g. the correlation between the attraction of substrate and repulsion of a product by a polyelectrolyte catalyst, has not yet been carried. 

The field of synthetic enzyme models encompasses attempts to prepare enzymelike functional macromolecules by chemical synthesis [30]. One particularly relevant approach to such enzyme mimics concerns dendrimers, which are treelike synthetic macromolecules with a globular shape similar to a folded protein, and useful in a range of applications including catalysis [31]. Peptide dendrimers, which, like proteins, are composed of amino acids, are particularly well suited as mimics for proteins and enzymes [32]. These dendrimers can be prepared using combinatorial chemistry methods on solid support [33], similar to those used in the context of catalyst and ligand discovery programs in chemistry [34]. Peptide dendrimers used multivalency effects at the dendrimer surface to trigger cooperativity between amino acids, as has been observed in various esterase enzyme models [35]. 

The use is described of a process involving both hydrolysis and pyrolysis to recover caprolactam from nylon 6 used in carpet fibres. By means of precise temperature control and the use of a catalyst, nylon 6 can be isolated from the PP backing. The process has been developed by the National Renewable Resource Laboratory, and interest has been shown by AlliedSignal who are considering a cooperative research and development project. 

The HKR reactions follow the cooperative bimetallic catalysis where epoxide and nucleophile activate simultaneously by two different (salen)Co-AlX3 catalyst molecules. The linking of two (salen)Co unit through the A1 induces the cooperative mechanism, albeit through a far less enantio-discriminating transition state than that attained with the catalyst la and la (Scheme2). 

Promotional effects of sulfide can evidently be explained, because exposure of reduced metals Is Increased on reduced sulfided catalysts. The role of cobalt Is less clear. It Is normally not fully reduced. It apparently does not promote greater exposure of Mo In any form detected, either In the presence or absence of sulfide. On the contrary. It evidently only decreases the concentration of exposed Mo atoms, although, at concentrations typically used, most. Mo atoms are unaffected by Co. Either some property of Co alone or some local cooperative effect of adjacent Co and Mo must explain promotion. Simple mechanical mixtures will not give the synergism observed, however (1-4). 

We thank Ms. Wei-Chee Tan for the preparation of the catalyst samples. This work was supported by the National Science Foundation (CTS-9403199), the international cooperative program NSF-CONICET (INT-9415590), and the Exxon Education Foundation We thank the University of Mar del Plata for a fellowship (WEA), as part of the international exchange program sponsored by the University of Oklahoma and the University of Mar del Plata. 

Very recently Chen and co-workers have applied the previously mentioned Ni-based dimetallic pre-catalyst 14 in the Negishi reaction. Remarkable results were obtained even when unactivated aryl chlorides were chosen as reaction partners providing an alternative to the more expensive Pd-based catalysts. The fact that dinuclear pre-catalyst 14 is more active than its mononuclear analogue 13 indicates a possible cooperative effect between the two metal centres [86] (Scheme 6.23). 

A proposed mechanism (Scheme 5-44) begins with deprotonation of dimethyl phosphite to give an Al-phosphito complex (32) which can react with the aldehyde via either a chelate or open transition state, the latter possibly involving cooperative action of two aluminum centers, consistent with the observation of co-catalysis. Following P-C bond formation, several possible rearrangements could regenerate the achve catalyst and form the product... 

V) and selectivity ( av 3.9). It appears fairly well accepted that in the very best cofacial porphyrin catalysts, bimetaUic cooperativity plays a critical role. The catalytic mechanism remains to be adequately elucidated, complicating rational improvement of these fascinating compounds. Cofacial porphyrins remain too unstable and prohibitively expensive for practical applications. 

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