Homogeneous hydrogenation of olefins and acetylenes

Homogeneous hydrogenation of olefins and acetylenes The variety of transition metal complexes which will catalyse the hydrogenation of unsaturated hydrocarbons under homogeneous conditions is exemplified in Table 30. Only two of these reactions, which have been studied in some detail, are discussed here. 

Butadiene to cis- and trans-2-butenes and 1-butene Methyl linoleate to isomers of [Co CN)3H]J- [51] 

Ethylcrotonate to ethylbutanoate PdCh + metal ions such  

Maleic and fumaric acids to saturated acids rCr(II) sulphate in aqueous dimethylformamide L Ru(II)a2aq [Co(CN)3H]3- [38] [45] 

Benzene solutions of (PPh3)3RIiCl (10 M.) will catalyse the reduction of olefins, except ethylene [47], and acetylenes [48], under very mild conditions and the catalyst is rather better than Adams catalyst. 

---

J. A. Osborne, F. J. Jardine, J. F. Young, G. Wilkinson, The Preparation and Properties of Tris(triphenylphosphi-ne)halogenorhodium(I) and some Reactions thereof including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and their Derivatives, J. Client Soc A. 1966,1711-1732. 

Osborn, J. A. Jardine, R H. Young, J. R Wilkinson, G. The preparation and properties of tris(triphenylphosphine)halogeno-rhodium(I) and some reactions thereof including catalytic homogeneous hydrogenation of olefins and acetylenes and their derivatives. /. Chem. Soc. A 1966,1711-1732. 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

Young JF, Osborn JA, Jardine FH, Wilkinson G (1965) Hydride intermediates in homogeneous hydrogenation reactions of olefins and acetylenes using rhodium catalysts. Chem Conunun 131 132... 

In benzene or similar solvents, tris(triphenylphosphine)halogenorhodium(I) complexes, RhX[P(C6H5)3]3, are extremely efficient catalysts for the homogeneous hydrogenation of nonconjugated olefins and acetylenes at ambient temperature and pressures of 1 atmosphere (6). Functional groups (keto-, nitro-, ester, and so on) are not reduced under these conditions. 

To investigate the oxidative addition of acyl halides to a metal to form the acyl complex and to find a better decarbonylation agent, we selected chlorotris(triphenylphosphine) rhodium (XI) as a model complex. This complex is known to catalyze the oxo reaction (8) and the homogeneous hydrogenation of acetylenes and olefins (7, 28). 

Evidence for such a scheme has been presented for hydrogenations conducted with simple Ru(II) salts, HCo(CO)4, [HCo(CN)s] , or [Pt(SnCl3)s] as catalysts 188-191). These homogeneous catalysts function best with terminal olefins and acetylenes. With dienes as substrates, the anionic catalysts selectively hydrogenate only one of the two double bonds. These complexes also catalyze olefin isomerization and, in the case of nonconjugated dienes, isomerization to a conjugated diene may precede hydrogenation. 

Photosensitized generation of hydrido-metal complexes in aqueous media provides a general route for H2-evolution, hydrogenation of unsaturated substrates (i.e. olefins, acetylenes), or hydroformylation of double bonds, see Scheme 2. Co(II) complexes, i.e. Co (II)-fn s-bipyridine, Co(bpy) +, or the macrocyclic complex Co(II)-Me4[14]tetraene N4, act as homogeneous H2-evolution catalysts in photosystems composed of Ru(bpy) + (or other polypyridine (Ru(II) complexes) as photosensitizers and triethanolamine, TEOA, or ascorbic acid, HA-, as sacrificial electron donors [156,157], Reductive ET quenching of the excited photosensitizer... 

Although the rates of hydrogenation of acetylenes and olefins barely differ or, as work with homogeneous catalysts has shown, are actually greater for C=C double bonds,124 partial reduction of the acetylenic bond is relatively easy. The reason for the selectivity is the preferential adsorption of acetylene derivatives on the catalyst surface it is therefore recommended that relatively small amounts of catalyst (sometimes 1-2% of the weight of the substrate)... 

The activity and selectivity of the bound Wilkinson catalyst parallels in many ways the homogeneous system. Terminal olefins are hydrogenated more rapidly than internal olefins, cw-olefins react faster than rran5-olefins, and olefins are reduced faster than acetylenes.There were some differences with the sterically hindered l(7)-p-menthene the polymeric catalyst, for example, produced appreciably more isomerized material. Furthermore, the attached complex was more selective in the hydrogenation of 1,3-cyclooctadiene. 

---