Catalysis transition metal catalysts

Addition of HCN to unsaturated compounds is often the easiest and most economical method of making organonitnles. An early synthesis of acrylonitrile involved the addition of HCN to acetylene. The addition of HCN to aldehydes and ketones is readily accompHshed with simple base catalysis, as is the addition of HCN to activated olefins (Michael addition). However, the addition of HCN to unactivated olefins and the regioselective addition to dienes is best accompHshed with a transition-metal catalyst, as illustrated by DuPont s adiponitrile process (6—9). 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

The wide electrochemical windows of ionic liquids, in combination with their ability to serve as solvents for transition metal catalysts, opens up new possibilities for a combination of electrochemistry and transition metal catalysis. A very exciting first example has recently been published by Bedioui et al. [27]. 

Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

An unusual ee-screening system for enantioselective transition metal catalysts is based on IR-thermo-graphy,55 which had previously been used in heterogeneous catalysis.36,81,82 Using an AIM-2562 IR... 

The use of dendrimers as supports to anchor transition metal catalysts has attracted considerable attention over the past decades [48] (see also Chapter 4 of this book). Several groups studied the use of dendrimers immobilised on insoluble supports [49], and this type of material meet the requirements for catalysis in interphases. Alper reported the use of diphosphine functionalised polyamidoamine (PAMAM) dendrimers... 

The possibility of adjusting solubility properties is of particular importance for liquid-liquid biphasic catalysis. Liquid-liquid catalysis can be realised when the ionic liquid is able to dissolve the catalyst, especially if it displays partial solubility of the substrates and poor solubility of the reaction products. Under these conditions, the product phase, which also contains the unconverted reactants, is removed by simple phase decantation. The ionic liquid containing the catalyst can then be recycled. In such a scenario the ionic catalyst solution may be seen as part of the capital investment for a potential technical process (in an ideal case) or at least as a working solution (only a small amount has to be replaced after a certain time of application). A crucial aspect of this concept is the immobilisation of the transition metal catalyst in the ionic liquid. While most transition metal catalysts easily dissolve in an ionic liquid without any special ligand design, ionic ligand systems have been applied with great success to... 

Comparing the different subareas of catalysis, all have common characteristics, but also significant differences are visible. Our area of main interest is homogeneous catalysis based on transition metal catalysts. In general, these homogeneous catalysts (most often the precatalyst) are molecularly defined. 

Not all C-H activation chemistry is mediated by transition metal catalysts. Many of the research groups involved in transition metal catalysis for C-H activation have opted for alternative means of catalysis. The activation of methane and ethane in water by the hexaoxo-/i-peroxodisulfate(2—) ion (S2O82) was studied and proceeds by hydrogen abstraction via an oxo radical. Methane gave rise to acetic acid in the absence of external carbon monoxide, suggesting a reaction of a methyl radical with CO formed in situ. Moreover, the addition of (external) CO to the reaction mixture led to an increase in yield of the acid product (Equation (ll)).20... 

Like so many other reactions, the ene reaction has been given new life by metal catalysis. The use of metals ranges from common Lewis acids, which simply lower the barrier of activation of the hetero-ene reactions to transition metal catalysts which are directly involved in the bond-breaking and -forming events, rendering reactions formal ene processes. This review is meant to serve as a guide to the vast amount of data that have accumulated in this area over the past decade (1994-2004). If a particular subject has been reviewed recently, the citation is provided and only work done since the time of that review is included here. Finally, the examples included within are meant to capture the essence of the field, the scope, limitations, and synthetic utility therefore, this review is not exhaustive. 

In contrast to the maturity of asymmetric synthesis utilizing chiral transition metal catalysts, asymmetric phase transfer catalysis is still behind it and covers organic reactions to lesser extent. Thus, it is further necessary in wide range to explore efficient asymmetric phase transfer catalysis keeping its superiority of easy operation, mild reaction conditions, and environmental binignancy. 

Phase transfer catalysis has more recently been applied to the important area of organometallic chemistry(18). Reported applications include both the preparation(19) and the use of transition metal catalysts in isomerizations(20), carbonylations(21) and reductions(22). 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

More synthetic interest is generated by the potentially very useful hydration of dienes. As shown on Scheme 9.6, methylethylketone (MEK) can be produced from the relatively cheap and easily available 1,3-butadiene with combined catalysis by an acid and a transition metal catalyst. Ruthenium complexes of several N-N chelating Hgands (mostly of the phenanthroline and bipyridine type) were found active for this transformation in the presence of Bronsted acids with weakly coordinating anions, typically p-toluenesulfonic acid, TsOH [18,19]. In favourable cases 90 % yield of MEK, based on butadiene, could be obtained. 

The author hopes that this chapter has convinced the readers of the value of homogeneous catalysis for the synthesis of organophosphorus compounds and for organo-heteroatom compounds in a broader sense. Hydrosilylation and hydroboration are indispensable modern synthetic reactions in this category. The H-P addition reactions herein described joins them as a third member. Although this chapter does not cover, the addition reactions of the S-P and Se-P bonds in thiophosphates [39] and selenophosphates [40] to alkynes also proceed in the presence of transition metal catalysts. In view of the wide use of phosphorus compounds, the new procedures will find practical applications. 

The majority of chemical methods for the asymmetric hydrogenation of unsaturated systems rely on the use of transition metal catalysts or stoichiometric amounts of metal hydride. The chemical importance of this transformation has led to the development of some of the most powerful and efficient methods in catalytic asymmetric synthesis. Routinely used on the milligram to multi-tonne scale, they represent one of the biggest success stories of asymmetric catalysis [120]. 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

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