Homogeneous Hydrogenation of Organic Substrates

Hydrogenation reactions of unsaturated organic compounds are clean reactions. Frequently, they proceed quantitatively without formation of side products and there is no waste except the trace amount of catalyst. Thus, in terms of ecology and atom economy [34], they are ideal reactions. 

Hydrogenations with the Wilkinson catalyst RhCl(PPh3)3 are operationally simple. They are usually carried out at ambient temperatures. In many cases a blanket of hydrogen (1 bar or 0.1 MPa) is sufficient and no hydrogen pressure is necessary. Solvents are usually methanol, ethanol, acetone, THF or benzene [20]. Chloroform and carbon tetrachloride should be avoided because they may undergo H/Cl exchange [40]. 

Terminal olefins are easily hydrogenated. Their hydrogenation is much faster than the hydrogenation of double bonds in cyclic systems or internal double bonds. c/5-Olefms are hydrogenated faster than tran -olefins. Conjugated diole- 

The catalyst [CoH(CN)5] is soluble in water. It is selective for the reduction of olefinic double bonds in a,y9-unsaturated systems. Reduction of NO2 groups only occurs at elevated pressures. Hydrogenolysis of C-Hal bonds is observed [20]. Progress as far as water-soluble hydrogenation catalysts is concerned has also been made with Wilkinson-type catalysts by using phosphine ligands with sulfonic acid substituents [44]. 

In cluster catalysis there is always the possibility that the cluster breaks down and fragments or monomers are the actual catalysts. Os3H2(CO)io [45] in the hydrogenation of alkenes, and Ru4H4(CO)i2 [46] and its phosphine derivatives in the hydrogenation of alkynes, alkenes, and even ketones, seem to catalyze as clusters. The dimer Rh2(OAc)4 [47] is an active hydrogenation calatyst for alkenes. 

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