Hydrogenation and Hydroformylation Reactions in Alternative Solvents

There are many different homogeneous catalysts available that can be used for hydrogenation and hydroformylation reactions. The most widely used metals that are present in these catalysts are the transition metals, especially Co, Ni, Ru, Rh and Ir [1], 

Chemistry in Alternative Reaction Media D. Adams, P. Dyson and S. Tavener 2004 John Wiley Sons, Ltd ISBNs 0-471-49848-3 (Cloth) 0-471-49849-1 (Paper) 

The first step in the catalytic cycle is the dissociation of a phosphine ligand from Wilkinson s catalyst which produces a highly reactive trigonal planar rhodium centre, compound B. Oxidative addition of H2 to B affords C which then undergoes association of the C=C compound to afford D. One of the hydride ligands undergoes transfer to the C=C bond affording a coordinated alkyl as 

C bonds using Rh(PPh3)3Cl. Step 1, step 3, ligand association step 4, 

In the hydroformylation reaction, the fundamental steps that occur are essentially the same, albeit in a different sequence, to those that occur in the hydrogenation reaction, except that there is an additional CO insertion step. In the reaction, a mixture of linear and branched chain products are produced, the resulting composition being defined as the n.iso ratio. The n.iso ratio obtained is dependent upon the catalyst used and the reaction conditions, and in general, the linear products are of higher value to industry. 

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The use of alternative solvents in hydrogenation and hydroformylation reactions has developed at an incredible rate over the last few years. Many elegant systems have been designed which offer cleaner alternatives to those carried out in conventional organic solvents. Apart from the attractiveness of the separation process, catalyst lifetimes can be extended which represents another major advantage. In some cases, conventional organic solvents are completely removed from the system. 

Various other biphasic solutions to the separation problem are considered in other chapters of this book, but an especially attractive alternative was introduced by Horvath and co-workers in 1994.[1] He coined the term catalysis in the fluorous biphase and the process uses the temperature dependent miscibility of fluorinated solvents (organic solvents in which most or all of the hydrogen atoms have been replaced by fluorine atoms) with normal organic solvents, to provide a possible answer to the biphasic hydroformylation of long-chain alkenes. At temperatures close to the operating temperature of many catalytic reactions (60-120°C), the fluorous and organic solvents mix, but at temperatures near ambient they phase separate cleanly. Since that time, many other reactions have been demonstrated under fluorous biphasic conditions and these form the basis of this chapter. The subject has been comprehensively reviewed, [2-6] so this chapter gives an overview and finishes with some process considerations. 

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