Homogeneous Hydrogenation and Isomerization

For the hydrogenation of ethylene and acetylene with chloroplatinic acid and stannous chloride as catalyst, a number of steps have been outlined which are characteristic of the mechanism 305). These steps are (1) competitive formation of a rr-ethyleneplatinum and a hydroplatinum complex (2) formation of the hydro-rr-ethyleneplatinum complex (3) rearrangement by insertion to form an ethylplatinum complex and (4) attack of the protonic hydrogen on the metal-carbon bond to form ethane and the catalyst. The reduction of acetylene to ethane proceeds via the intermediate formation of ethylene. 

Complexes Used as Catalysts for the Hydrogenation of Double Bonds 

Bailar and co-workers have used several complexes as catalysts for reduction of linolenic ester to linoleic or oleic ester without any reduction to the saturated stearic ester. A considerable portion of this work was carried out using a mixed catalyst consisting of tin(II) chloride and a platinum(II) complex. However, catalytic work has also been carried out using palladium and nickel complexes, the former again being used along with tin(II) chloride J91). The experimental details have been recently reviewed (f90) so that this article is concerned with the conclusions and mechanistic aspects rather than with the direct results. 

NiHCN(PPh3)2 although conclusive proof was not presented (256). 

Studies on Soybean Oil Methyl Ester and Methyl Linolenate Using Mixtures of PtCIziPPha) and SnCl2.2H20 

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The ready hydrogenation and isomerization of methyl oleate and palmitoleate with Fe(CO)s confirm the results of Ogata and Misono (18) with monounsaturated aliphatic compounds. In the isomerization of monoolefins Manuel (15) suggested the occurrence of equilibria involving either 7r-olefin HFe(CO)3 and a-alkyl Fe(CO)3 complexes, or TT-olefin Fe(CO)3 and 7r-allyl HFe(CO)3 complexes. The formation of olefin-iron tetracarbonyl complexes has been reported (19). The reaction of butadiene and Fe2(CO)9 has been observed to lead to the formation of butadiene-Fe(CO)4 and butadiene-[Fe(CO)4]2 complexes in which one or both double bonds are pi-bonded to the iron (16). A mechanism involving both monoene-Fe(CO)4 (I) and allyl-HFe(CO)3 complexes (II) is postulated for the isomerization of methyl oleate (Scheme II) and for its homogeneous hydrogenation. 

The mechanism for homogeneous hydrogenation of methyl linoleate by Fe(CO)s based on kinetic evidences and radioactive tracers involves monoene- and diene-Fe(CO)4 and diene-Fe(CO)3 complexes as important intermediates. Contrary to our previous postulate (7) the free conjugated diene is only a minor intermediate. Confirmatory evidence is needed for the occurrence of oleate- and linoleate-Fe(CO)4 complexes during hydrogenation and isomerization with Fe(CO)r>. Also, the species of iron carbonyl hydrides formed during hydrogenation should be elucidated. 

Together with hydrogenation and isomerization, epoxidation completes the trio of commercially significant applications of enantioselective homogeneously catalyzed reactions. Stereospecific olefin epoxidation is distinctive in that it creates two chiral centers simultaneously. The enantioselective epoxidation method developed by Sharpless and co-workers is an important asymmetric transformation known today. This method involves the epoxidation of allylic alcohols with tert.-butyl hydroperoxide and titanimn isopropoxide in the presence of optically active pure tartrate esters (Eq. 3-14). 

R. W. Adams, G. E. Batley and J. C. Bailar, Jr., Homogeneous Catalytic Hydrogenation and Isomerization of Olefins with Dichlorobis(triphenylphosphine)platinum(II)-Tin Chloride Catalyst, Inorg. Nucl. Chem. Letters 4 455 (1968). 

Two reviews deal with the relation between homogeneous and heterogeneous catalysis. One of these reviews is of a general physical and theoretical nature, while the other is concerned with specific classes of compounds — alkenes, alkynes, and fats — and concentrates on their hydrogenation and isomerization. Homogeneous catalysis by ruthenium complexes has been reviewed. The application of molecular orbital symmetry rules, to organic as well as to organometallic reaction mechanisms, has been discussed, and a set of rules similar to, but simpler to apply than, the Woodward-Hoffmann rules has been described. ... 

Metallacarboranes. These are used in homogeneous catalysis (222), including hydrogenation, hydrosilylation, isomerization, hydrosilanolysis, phase transfer, bum rate modifiers in gun and rocket propellants, neutron capture therapy (254), medical imaging (255), processing of radioactive waste (192), analytical reagents, and as ceramic precursors. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

A series of anchored Wilkinson s catalysts were prepared by reacting the homogeneous Wilkinson catalyst with several alumina/heteropoly acid support materials. These catalysts were used to promote the hydrogenation of 1-hexene. The results were compared with those obtained using the homogeneous Wilkinson and a l%Rh/Al203 catalyst with respect to catalyst activity and stabihty as well as the reaction selectivity as measured by the amount of double bond isomerization observed. The effect which the nature of the heteropoly acid exerted on the reaction was also examined. 

Nickel is frequently used in industrial homogeneous catalysis. Many carbon-carbon bond-formation reactions can be carried out with high selectivity when catalyzed by organonickel complexes. Such reactions include linear and cyclic oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions [1], Many of the complexes that are active catalysts for oligomerization and isomerization reactions are supposed also to be active as hydrogenation catalysts. 

Fig. 13.9 Facsimile of a molecular and kinetic scheme of the one-pot glucose into mannitol bio-chemo cascade, showing the many different species that undergo interconversion during the overall process, involving enzymatic isomerization, homogeneous mutarotation and heterogeneous hydrogenation. For simplicity, the various sugar-borate species have been omitted [23, 24],...

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

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