Examples of the Reactions in SCFs

In recent years there has been much interest in the use of supercritical fluids (SCFs) as replacements for conventional liquid solvents, particularly in separation science, but also as reaction media. In addition to their environmental benefits, SCFs have further advantages over conventional liquid solvents, and these are briefly outlined in Section 2. The remainder of the chapter describes the use of SCFs as a medium for NMR spectroscopic studies. First we look briefly at motives for such NMR studies and the techniques employed. We then examine in more detail chemical shifts and nuclear spin relaxation in SCFs. The lower relaxation rates associated with SCFs and consequent sharper lines obtained for quadrupolar nuclei make SCFs excellent solvents. Section 8 describes some NMR studies of organometallic reactions in SCFs. Here the miscibility of supercritical solvents with gaseous reagents proves to be a tremendously useful feature in, for example, homogeneous catalysis. Finally we comment on future possibilities for NMR studies in SCFs. 

Many solvent properties are related to density and vary with pressure in a SCF. These include the dielectric constant (er), the Hildebrand parameter (S) and n [5], The amount a parameter varies with pressure is different for each substance. So, for example, for scC02, which is very nonpolar, there is very little variation in the dielectric constant with pressure. However, the dielectric constants of both water and fluoroform vary considerably with pressure (Figure 6.3). This variation leads to the concept of tunable solvent parameters. If a property shows a strong pressure dependence, then it is possible to tune the parameter to that required for a particular process simply by altering the pressure [6], This may be useful in selectively extracting natural products or even in varying the chemical potential of reactants and catalysts in a reaction to alter the rate or product distributions of the reaction. 

It would appear that water is a remarkable solvent for Diels-Alder reactions giving both rate and selectivity enhancements. There are now many examples of successful reactions being carried out in this solvent. However, water cannot be used for all reactions. Perfluorinated solvents have also been found to give beneficial rate enhancements over organic solvents as have ionic liquids. Interestingly, both ionic liquids and SCFs can be used to tune the selectivities of these reactions, ionic liquids by varying the solvent used and SCFs by altering the density of the solvent. 

Fluorous-derivatized phosphines have also been used for Heck reactions in scCC>2 together with palladium acetate [7]. The fluorous groups improve the solubility of the catalyst in the SCF compared to nonfluorous ligands. An example of a Heck reaction that uses a fluorous-derivatized phosphine to improve the solubility of a Pd(OAc)2 catalyst is shown in Scheme 10.5. 

Irradiation of ethyleneimine (341,342) with light of short wavelength in the gas phase has been carried out directly and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified in a fast flow photoreactor included hydrocyanic acid and acetonitrile (345), in addition to those found in a steady state system. The reaction of hydrogen radicals with ethyleneimine results in the formation of hydrocyanic acid in addition to methane (350). Important processes in the photolysis of ethyleneimine are nitrene extrusion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an intermediate in the photolytic formation of hydrocyanic acid from acetylene and ammonia in the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Supercritical fluids (SCFs) have fascinated researchers ever since the existence of a critical temperature was first noted more than 175 years ago [1]. Although initial studies focussed mainly on the physical properties of supercritical phases, their chemical reactivity was of interest from the beginning, too [2]. In fact, several well established industrial processes for the production of bulk chemicals occur under temperatures and pressures beyond the critical data of the reaction mixture, the Haber-Bosch process and the high pressure polymerization of ethylene being just the most outstanding examples. Based on the pioneering work of Kurt Zosel at the Max-Planck-Institut fiir Kohlenfor-... 

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