Alkenes hydrogenation with homogeneous

The mechanisms of hydrogenation of alkenes over finely divided metals such as nickel, platinum, and so on (Section 11-2) now are understood in a general way. However, these reactions are extremely difficult to study because they occur on a metallic surface whose structure is hard to define. In contrast, the mechanisms of hydrogenation with homogeneous catalysts are known in considerable detail and provide insight into their heterogeneous counterparts. 

Hydrogen donors that function poorly with homogenous catalysts include hydrazine hydrate, alkenes (e.g., cyclohexene), and ascorbic acid. This is somewhat surprising as they can be very effective in heterogeneous transfer hydrogenation. 

Quite remarkably partial hydrogenation of alkynes to trans alkenes is also possible with homogeneous rhodium complexes 168169... 

Aqueous biphasic catalysis is also used in homogeneous hydrogenations.117-119 In new examples Ru clusters with the widely used TPPTN [tris(3-sulfonatophenyl) phosphine] ligand120 and Rh complexes with novel carboxylated phosphines121 were applied in alkene hydrogenation, whereas Ru catalysts were used in the hydro-genation of aromatics. Aerobic oxidation of terminal alkenes to methyl ketones was carried out in a biphasic liquid-liquid system by stable, recyclable, water-soluble Pd(II) complexes with sulfonated bidentate diamine ligands.124... 

However, in 1972, one year before Manassen s and Joo s first publications, Parshall working at Dupont in Delaware reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCL when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100°C/5] It was even noted that the product could be separated by decantation or distillation. Parshall recognised the separation problem associated with homogeneous catalysis and in this paper states, An approach that seems under utilised is the use of molten salts as stable, non-volatile solvents from which organic products are readily separated by distillation. .. or decantation . An even earlier report by a group working for the U S. 

An important modern example of homogeneous catalysis is provided by the Monsanto process in which the rhodium compound 1.4 catalyses a reaction, resulting in the addition of carbon monoxide to methanol to form ethanoic acid (acetic acid). Another well-known process is hydro-formylation, in which the reaction of carbon monoxide and hydrogen with an alkene, RCH=CH2, forms an aldehyde, RCH2CH2CHO. Certain cobalt or rhodium compounds are effective catalysts for this reaction. In addition to catalytic applications, non-catalytic stoichiometric reactions of transition elements now play a major role in the production of fine organic chemicals and pharmaceuticals. 

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