Alkenes homogeneous catalytic hydrogenation

Besides heterogeneous and homogeneous catalytic hydrogenations, chemical reductions can also transform alkynes to cis alkenes. Interestingly, activated zinc in the presence of a proton donor (alcohol), although a dissolving-metal reagent, reduces disubstituted alkynes to cis alkenes 199... 

The dihydrido complex [RhH2Cl(PPh3)2] is a very important intermediate in the homogeneous catalytic hydrogenation of alkenes.20 The monohydrido complexes (Table 63) can be made by the oxidative addition of HY species to rhodium(I) complexes (equation 187). Similar complexes can be obtained when bulky tertiary phosphines are allowed to react with alcoholic solutions of hydrated rhodium trichloride.268 269... 

Alkyne hydrogenation remains a largely forgotten area of homogeneous catalytic hydrogenation. There are clear commercial benefits to be obtained in the conversion of alkynes to alkenes however, homogeneous catalysts are not usually sufficiently selective towards the alkene intermediate. 

The addition of dihydrogen to Vaska s compound (Scheme 7.3) is an excellent example of concerted, 3-center, syn addition in which the transition state is a three-membered ring consisting of the metal and the two hydrogen atoms. Addition of H2 to Wilkinson s catalyst, an early step in the homogeneous catalytic hydrogenation of alkenes (to be discussed in Chapter 9), serves as another well-known example (equation 7.29). 

An Organometallic Com pound That Occurs Naturally Coenzyme B,2 591 Organocopper Reagents 592 Palladium-Catalyzed Cross-Coupling 595 Homogeneous Catalytic Hydrogenation 597 Olefin Metathesis 600 Ziegler-Natta Catalysis of Alkene Polymerization 603 Summary 606 Problems 608... 

The literature of homogeneous catalytic hydrogenation is very extensive. There have been few wholly new developments in alkene hydrogenation where the emphasis has been on asymmetric syntheses. However, there have been some novel reports in the field of hydrogenation of arenes. 

In Section 5.9, we saw that alkenes can be converted to alkanes by catalytic hydrogenation by a variety of catalysts, such as palladium and platinum. These are heterogeneous catalysts. We also noted that homogeneous catalytic hydrogenation can be carried out by Wilkinsons catalyst, Ru[(PPh3)3Cl. We now return to that subject to discuss the reaction mechanism. We will find that hydrogenation by Wilkinson s catalyst occurs in a catalytic cycle that is strikingly similar to the catalytic cycles of the reactions we have discussed thus far in this chapter. The transition metal in the Wilkinson catalyst, however, is ruthenium, not palladium. 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

Hydroformylation is the most successful application of a homogeneous catalytic reaction to industrial processes [1], Aldehydes are conveniently produced by hydroformylation, in which CO and hydrogen are simultaneously added to an alkene (Figure 1). 

Despite its apparent noble character, gold catalysts have been recently found to be active in many homogeneous and heterogeneous catalytic processes such as oxidation reactions, nucleophilic additions, cross-coupling reactions, and alkene and imine hydrogenations (69—71). Corma and co-workers showed that Au... 

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