Other Catalyst Metals

Other Metal Catalysts. - Hydroformylation catalysts containing Co and Rh have been known for many years and these metals are the only catalysts used industrially. Recently two catalyst systems have been developed that have certain advantages over the conventional catalysts. Pt/Sn chloride complexes show very high selectivity to n-aldehyde. and Reppe-type systems which hydroformylate alkenes in the presence of CO/H2O rather than CO/H2.  

Hayashi, Y. Kawabata, T, Isoyama, and I. Ogata, Bull. Chem. Soc. Jpn., 1981, 

This rate law led the authors to suggest a mechanism where the low dependence on alkene concentration is explained by fast dissociation of one end of the phosphine ligand to give a vacant site for alkene co-ordination and where the co-ordination of more than one CO ligand is inhibited by the bidentate phosphine. The first-order dependence on H2 partial pressure is similar to that observed in Rh or Co catalysis where hydrogenolysis of an acyl complex is the rate determining step. 

Pt/Sn systems with chiral diphosphine ligands have been studied for the asymmetric hydroformylation of alkenes, and although in earlier work the optical yields were generally low ( 20%) a recent paper by Pittman describes a system which gives the highest optical yields in asymmetric hydroformylation reported. Thus (5 )-(+)-2-phenylpropanal was produced in 95% optical yield from styrene on treatment with the PtCl2 complex of (—)DBP-DIOP (9) and SnCU (Sn Pt = 2.5) at 40°C in benzene under 

4 MPa H2/CO (2.4 1). Efficiency to aldehydes under these conditions was 70% (balance ethylbenzene) with a n i ratio of 1 4. 

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether 

Albrow, K. Biswas, A. Crane, N. Chaplin, T. Easun, S. Gladiali, B. Lygo and S. Woodward, Tetrahedron Asymmetry, 2003, 14, 2813-2819. 

Borner, J. Gimeno, S. Gladiali, P. J. Goldsmith, D. Ramazzotti and S. Woodward, Chem. Commun., 2000, 2433-2434. 

Capito, L. Bernardi, M. Comes-Franchini, F. Fini, M. Fochi, S. Polli-cino and A. Ricci, Tetrahedron Asymmetry, 2005, 16, 3232-3240. 

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Nonregenerable aluminum chloride catalyst is employed with various carriers ia a fixed-bed or Hquid contactor. Platinum or other metal catalyst processes that utilize fixed-bed operation can be either regenerable or nonregenerable. The reaction conditions vary widely, between 40—480°C and 1035— 6900 kPa (150—1000 psi), depending on the particular process and feedstock. 

The organotin sdanolate can then react with the polydimethylsiloxane diol by either attack on the SiOC bond or by sdanolysis of the SnOC bond (193,194). Other metal catalysts include chelated salts of titanium and tetraalkoxytitanates. Formation of a cross-linked matrix involves a combination of the three steps in equations 24—26. 

In either the acid catalysis or the niekel carbonyl (or other metallic catalyst) method, if alcohols, thiols, amines, etc. are used instead of water, the product is the corresponding ester, thiol ester, or amide, instead of the carboxylic acid. 

Raney nickel, named for its inventor, Murray Raney, is widely used in the industry, chiefly because it is inexpensive and exhibits a wide range of catalytic activities. Essentially, it is prepared by an NaOH leaching of A1 from a 50-50 alloy of Ni and Al. Various standard forms of Raney nickel are used, and discussions of these are readily available 6,7 Tablel.l lists some essentials of the preparation.8 The Raney process is used to prepare several other metal catalysts. 7,9,1°... 

Raney predicted that many other metal catalysts could be prepared with this technique, but he did not investigate them [8], Copper and cobalt catalysts were soon reported by others [4,5], These catalysts were not nearly as active as Raney s nickel catalyst and therefore have not been as popular industrially however they offer some advantages such as improved selectivity for some reactions. Skeletal iron, ruthenium and others have also been prepared [9-13], Wainwright [14,15] provides two brief overviews of skeletal catalysts, in particular skeletal copper, for heterogeneous reactions. Table 5.1 presents a list of different skeletal metal catalysts and some of the reactions that are catalyzed by them. 

In this chapter, the details of several types of Ir-complex-catalyzed cycloadditions have been summarized. Although, compared to other late transihon-metal com-plexes-such as those of Pd, Ni, Ru and Rh-the examples are few in number, some notable Ir-catalyzed cyclizations have recently been reported which cannot be achieved when uhlizing other metal catalysts. Until now it has not yet been possible to identify any dishnct explanation for the unique reachvity of iridium, and in parhcular its different reachvity compared to rhodium, which is located just above iridium in the Periodic Table of the elements. Nonetheless, many further developments of efficient and prachcal Ir-catalyzed cycloadditions are to be expected in the near future. 

By 1984, the palladium-catalyzed aUyhc alkylation reaction had been extensively studied as a method for carbon-carbon bond formation, whereas the synthetic utility of other metal catalysts was largely unexplored [1, 2]. Hence, prior to this period rhodium s abihty to catalyze this transformation was cited in only a single reference, which described it as being poor by comparison with the analogous palladium-catalyzed version [6]. Nonetheless, Yamamoto and Tsuji independently described the first rhodium-catalyzed decarboxylation of allylic phenyl carbonates and the intramolecular decarboxylative aUylation of aUyl y9-keto carboxylates respectively [7, 8]. These findings undoubtedly laid the groundwork for Tsuji s seminal work on the regiospecific rho-... 

Isomerization A refinery process used to produce branched compounds from straight-chain molecules. Butane, pentane, and hexane are isomerized to branched, higher-octane compounds through isomerization. Isomerization reactions are catalyzed through the use of aluminum chloride and other metal catalysts. 

Thermodynamics. At higher temperatures, Ni and other metal catalysts are believed to be less vulnerable to sulfur poisoning, since their sulfides are thermodynamically less stable. Figure 14 shows the change in AG with temperature for the following reaction. 

The use of other metal catalysts in attempted reductions of halo-naphthyridines has resulted in reduction of the ring or, at best, in the formation of mixtures. 

By Group IV metallocenes, lanthanides and other metal catalysts... 

The catalytic epoxidation proceeds via the formation of peroxytungstic acid. Similarly, other metal catalysts are effective in the H2O2 oxidation. Aqueous conditions are not appropriate for epoxidations since epoxides are prone to undergo acid-catalyzed hydrolysis36. Polymer-anchored catalysts are conveniently separated from the reaction mixture after catalyzed H2O2 epoxidations (equation 8)9. 

Another important factor in the catalytic properties of Fe and other metallic catalysts is the structure of these catalysts. It is a well-known fact that during ammonia synthesis, when it is exposed on the surface of a Fe catalyst s (111) plane, the catalyst is more active [14],... 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

Among the other metal catalysts for hydrogen oxidation, the Raney nickel deposited on the surface of graphite is widely known [10, 13], The modification of Raney nickel with such metals like Fe, Cr, Cu, La, Ti has been proposed [11] with the aim to increase its effectiveness. The effectiveness of the doping metals follows the series Cr>La>Ti>Cu>Fe. 

The ruthenium-catalyzed reactions by their diversity, selectivity, and interest for the production of fine chemicals, especially during the last decade, show that molecular ruthenium catalysts are not only versatile but that they have now become unavoidable tools in organic synthesis [127]. They also appear to be complementary to organic, enzyme, or other metal catalysts as they have the power to generate original activation pathways for the combination of a variety of simple and complex molecules. 

No more complete study of a single catalysed reaction has ever been published than that of Palmer and Constable,2 extending over eight years, on the dehydrogenation of alcohols by heated copper surfaces. Aldehydes are formed from primary and ketones from secondary alcohols by this reaction. The work has resulted in a considerable increase in our knowledge of the mode of formation and properties of the active centres on the surface of copper, and much of it may be applicable also to other metallic catalysts. The reactions are... 

The formal addition of carbenoids to the thiocarbonyl -bond [C + C=S] represents another major route used for the preparation of thiiranes. The reaction of thiocarbonyl compounds with diazoalkenes generally gives good yields of thiiranes (Scheme 11). The mechanism may involve an addition of a carbene across the thiocarbonyl group, especially in the presence of Rh(II) acetate, CuCl, CuS04, or other metal catalysts, but involvement of a zwitterion is also possible . 

Dimerisation Reactions with other Metal-Catalysts... 

Other donble bonds on the alkenyl chain can trap the initial gold carbenes leading to cyclopropanes. Thus by using An(I) catalysts, dienynes in eqnations (67) and (68) undergo totally stereoselective cyclizations to yield tetracyclic compounds under milder conditions s n than those required with other metal catalysts. ... 

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