Metal-free catalysis

Metal-Free Catalysis by Oxygen-Containing Carbon Nanotubes... 

Catalysis of Free Radical Reactions by Colloidal Metals.117... 

The iridium catalyst was found to be sufficiently soluble for catalysis when in the form of the substrate complex, but precipitated quantitatively once all substrate had been consumed. Supercritical fluid extraction at that stage yielded the solvent- and metal-free product in crystalline form leaving the active and selective catalyst behind for... 

Berkessel, A. and Thauer, R. K. (1995) On the mechanism of catalysis by a metal-free hydro-genase from methanogenic archaea Enzymic transformation of H2 without a metal and its analogy to the chemistry of alkanes in superacidic solution. Angew. Cbem., Int. Ed. Engl., 34, 2247-50. 

While the studies outlined in Eqs. (11.1)-(11.8) summarize the most important frontiers for metal-free synthesis, they also highhght a common limitation for the organocatalysis field. To date, there remain relatively few (less than 10) activation mechanisms that have been established to be amenable to organic catalysis. Accordingly, a primary objective for the advancement of the field of asymmetric organocatalysis has been the design and/or development of concepts that enable organic substrates to function as catalysts for a wide variety of new and established reactions. 

Since 2000, the field of asymmetric catalysis has bloomed extensively (and perhaps unexpectedly) with the introduction of a variety of metal-free catalysis concepts that have collectively become known as organocatalysis. Perhaps more impressive... 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

Remarkably, the catalyst loading could be decreased to 0.01 mol% without a considerable loss in reactivity and selectivity (one example 90% yield, 93% ee). A substrate/catalyst ratio of 10,000 to 1 has not been achieved in asymmetric metal-free catalysis before. 

Our own group is also involved in the development of domino multicomponent reactions for the synthesis of heterocycles of both pharmacologic and synthetic interest [156]. In particular, we recently reported a totally regioselective and metal-free Michael addition-initiated three-component substrate directed route to polysubstituted pyridines from 1,3-dicarbonyls. Thus, the direct condensation of 1,3-diketones, (3-ketoesters, or p-ketoamides with a,p-unsaturated aldehydes or ketones with a synthetic equivalent of ammonia, under heterogeneous catalysis by 4 A molecular sieves, provided the desired heterocycles after in situ oxidation (Scheme 56) [157]. A mechanistic study demonstrated that the first step of the sequence was a molecular sieves-promoted Michael addition between the 1,3-dicarbonyl and the cx,p-unsaturated carbonyl compound. The corresponding 1,5-dicarbonyl adduct then reacts with the ammonia source leading to a DHP derivative, which is spontaneously converted to the aromatized product. 

Yonker, C. R. and Linehan, J. C., A high-pressure NMR investigation of reaction chemistries in a simple salt hydrate,. Supercrit. Fluids, 29, 257 2004. Mehnert, C. R, Supported ionic liquid catalysis, Chem. Eur. ]., 11,50,2005. Giernoth, R. and Bankmann, D., Transition-metal free synthesis of perdeuter-ated imidazolium ionic liquidsby alkylation and H/D exchange, Eur. J. Org. Chem., 2008 (in print). DOT 10.1002/ejoc.200700784. 

Metal-free catalysis Organocatalysis removes concerns over whether a pharmaceutical or agrochemical might be contaminated with metal impurities from the catalyst. 

The wide assortment of catalytic asymmetric Strecker reaction methodologies devised to date can be divided into two major categories based on the nature of catalyst utilized 1) Lewis acid-promoted and 2) metal-free (or organo-catalytic) systems. Both classes of catalysis will be discussed and key results will be highlighted. 

General acid-base catalysis is often the controlling factor in many mechanisms and acts via highly efficient and sometimes intricate proton transfers. Whereas log K versus pH profiles for conventional acid-base catalyzed chemical processes pass through a minimum around pH 7.0, this pH value for enzyme reactions is often the maximum. In enzymes, the transition metal ion Zn2+ usually displays the classic role of a Lewis acid, however, metal-free examples such as lysozyme are known too. Good examples of acid-base catalysis are the mechanisms of carbonic anhydrase II and both heme- and vanadium-containing haloperoxidase. 

The main emphasis has so far been placed on metal-containing catalysts, both in the field of dendritic catalysis and in organic catalysis in general. However, the current trend is increasingly towards catalysis with purely organic compounds. A number of example of such metal-free - and dendritic - catalysts,... 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

In the metal-free epoxidation of enones and enoates, practically useful yields and enantioselectivity have been achieved by using catalysts based on chiral electrophilic ketones, peptides, and chiral phase-transfer agents. (E)-configured acyclic enones are comparatively easy substrates that can be converted to enantiomeri-cally highly enriched epoxides by all three methods. Currently, chiral ketones/ dioxiranes constitute the only catalyst system that enables asymmetric and metal-free epoxidation of (E)-enoates. There seems to be no metal-free method for efficient asymmetric epoxidation of achiral (Z)-enones. Exocyclic (E)-enones have been epoxidized with excellent ee using either phase-transfer catalysis or polyamino acids. In contrast, generation of enantiopure epoxides from normal endocyclic... 

In metal-free catalysis enantioselective ring-opening of epoxides according to Scheme 13.27 path B has been achieved both with chiral pyridine N-oxides and with chiral phosphoric amides. These compounds act as nucleophilic activators for tetrachlorosilane. In the work by Fu et al. the meso epoxides 71 were converted into the silylated chlorohydrins 72 in the presence of 5 mol% of the planar chiral pyridine N-oxides 73 (Scheme 13.36) [74]. As shown in Scheme 13.36, good yields... 

In recent years there has been considerable progress both in the base-catalyzed isomerization of meso-epoxides and in the metal-free catalysis of enantioselective opening of meso epoxides. The former approach has proven its potential in sev-... 

Because the ferrocene moiety of 3 and 4 fulfills a purely structural role and does not participate directly in the catalysis, the nucleophilic catalysts 3 and 4 are considered metal-free and are thus included in this account. 

The C-H transformation in the synthesis of fine chemicals can be separated in reactions employing catalysis by organometallic compounds and metal-free synthesis. Organometallic catalyzed functionalization is usually performed in liquid or gas/liquid reactions whereas the metal-free synthesis of fine chemicals often occurs as a gas-phase reaction. 

In Co- and Ni-mediated polymerizations of dienes the molar mass is commonly regulated by metal-free agents such as hydrogen, cyclooctadiene, 1,2-butadiene etc. In Nd-catalysis these additives do not result in a reduction of molar mass. What is the reason for this difference and are there metal-free chain modification agents which work with Nd-catalysts ... 

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