Handbook of Solvents - George Wypych - ChemTech - Ventech!

1446 Aydin K. Sunol and Sermin G. Sunol
21.1.4.1.1 Homogeneous reactions catalyzed by organometallic compounds
Homogeneous catalysts have advantages over heterogeneous catalysts such as possibility of
carrying out the reaction at milder conditions, higher activity, and selectivity, ease of spectroscopic
monitoring, and controlled and tunable reaction sites.
Organic reactants and products are not soluble in water while most catalytic materials
are soluble in water. Therefore, homogenization of organic systems utilized environmentally
undesirable organic solvents. An alternative to environmentally unacceptable organic
solvents is a supercritical solvent that has added advantages over organic solvents such as
increased reaction rate, higher selectivity and easy separation of reactants and products as
well as of the catalyst after the reaction. Since the properties of supercritical solvents can be
adjusted by manipulating the operating conditions, reaction rate and selectivity are better
tunable in reactions carried out in supercritical solvents.
Carbon dioxide is the supercritical solvent that is most commonly used in homogeneous
catalytic reactions. In addition to being environmentally acceptable (nontoxic, nonflammable),
inexpensive, and available in large quantities, carbon dioxide does not
participate in most reactions. It also has an ambient critical temperature. Although, supercritical
carbon dioxide is more effective in dissolution of non-polar, nonionic and low molecular
mass compounds, addition of co-solvents enhances the solubility of many otherwise
insoluble compounds in supercritical carbon dioxide. A recent review by Noyori et al. 83 discusses
homogeneous catalytic reactions under supercritical conditions.
When homogeneous reactions are carried out under supercritical conditions, gas/liquid
interfacial transport is eliminated, which is an advantage for reactions such as hydrogenation,
where diffusion of gas into the liquid may be limiting the reaction rate. In
asymmetric hydrogenation reactions, hydrogen and the supercritical solvent are miscible
and this results in better enantioselectivity. In Diels-Alder reactions, the advantage of the
supercritical solvent is the higher selectivity obtained rather than increased rate of the reaction
due to the solvent. Most of the oxidation reactions are carried out in supercritical water.
Some heterogeneously catalyzed reactions are also carried out in supercritical carbon dioxide.
Recently, homogeneously catalyzed reactions carried out in supercritical carbon dioxide
have been reported. Examples of homogeneous catalytic reactions carried out under
supercritical conditions are summarized in Table 21.1.10.
Table 21.1.10. Homogenous reactions in supercritical carbon dioxide catalyzed by
organametallic compounds
Reaction Catalyst
Isomerization
1-hexene to 2-hexene Iron catalyst
Hydrogenation
CO2 to formic acid Ruthenium(II) phosphine complex
Asymmetric hydrogenation of tiglic acid Ruthenium catalyst
Asymmetric hydrogenation of enamides Cationic rhodium complex
Cyclopropene Manganese catalyst