Diels-Alder reactions supercritical fluids

A number of Diels Alder reactions have been investigated in supercritical media and some of them will be illustrated. Most of the research has been focused on the influence of the pressure, which greatly influences the density of the fluid, on the kinetic aspects and on the product distribution of the reaction. 

Whilst it is obviously valuable to measure the solubility of reagents in the SCF, it is important to be aware that the solubility in a multicomponent system can be very different from that in the fluid alone. It is also important to note that the addition of reagents and catalysts can have a profound effect on the critical parameters of the mixture. Indeed, at high concentrations of reactants, the mole fraction of C02 is necessarily lower and it may not be possible to achieve a supercritical phase at the temperature of interest. Increases in pressure (i.e. further additions of C02) could yield a single liquid phase (which would have a much lower compressibility than scC02). For example, the Diels-Alder reaction (see Chapter 7) between 2-methyl-1,3-butadiene and maleic anydride has been carried out a pressure of 74.5 bar and a temperature of 50 °C, assuming that this would be under supercritical conditions as it would if it were pure C02. However, the critical parameters calculated for this system are a pressure of 77.4bar and a temperature of 123.2 °C, far in excess of those used [41]. 

As mentioned earlier, physical chemistry plays a significant role in providing understanding of phenomena important to developing supercritical fluid technology for chemical synthesis applications (see Figure 3). A previous study of Diels-Alder reactions with cyclopentadiene and ethyl acrylate in our group illustrates this important role. As mentioned above, of paramount interest was the intrinsic rate of this bimolecular reaction as well as the endo to exo selectivity in scCOa this... 

Homogeneous reactions carried out in supercritical fluids are reactions in supercritical water, organo-metallic reactions and Diels Alder reactions. Reactions in supercritical water are well studied and will be described in the following section. 

Examples of the use of activation volume for the reactions in supercritical fluids is the unimolecular decomposition of a-chlorobenzyl methyl ether in 1,1-difluoroethane Jc = 113.4°C), studied by Johnston and coworkers [54] and the Diels-Alder reaction of isoprene and maleic anhydride in supercritical carbon dioxide [56], described in section 3.1.2.1. In the latter study the large variation in the rate coefficient at 35°C and near-critical pressures was quantified as AV = - 1.39 X lO cm moF in the highly compressible region, as compared with - 38.4 cm mol at 200 bar. Paulaitis and Alexander interpreted their results as a solvent effect stemming from an induced quadrupole moment in the carbon dioxide molecule. 

S Kim, KP Johnston. Adjustment of the selectivity of a Diels-Alder reaction network using supercritical fluids. Chem Eng Commun 63 49-59, 1988. 

ME Paulaitis, GC Alexander. Reactions in supercritical fluids. A case study of the thermodynamic solvent effects on a Diels-Alder reaction in supercritical carbon dioxide. Pure Appl Chem 59 61-68, 1987. 

BL Knutson, AK DUlow, CL Liotta, CA Eckert. Kinetics of a Diels-Alder reaction in supercritical propane. In KW Hutchenson, NR Foster, eds. Innovations in Supercritical Fluids Science and Technology. ACS Symposinm Series No. 608. Washington, D.C. American Chemical Society, 1995, pp 166-178. 

While studies of reactions in supercritical fluids abound, only a few researchers have addressed the fundamental molecular effects that the supercritical fluid solvent has on the reactants and products that can enhance or depress reaction rates. A few measurements of reaction rate constants as a function of pressure do exist. For instance, Paulaitis and Alexander (1987) studied the Diels Alder cycloaddition reaction between maleic anhydride and isoprene in SCF CO2. They observed bimolecular rate constants that increased with increasing pressure above the critical point and finally at high pressures approached the rates observed in high pressure liquid solutions. Johnston and Haynes (1987) found the same trends in the... 

The Diels-Alder cycloaddition reaction of maleic anhydride with isoprene has been studied in supercritical-fluid CO2 under conditions near the critical point of CO2 [759]. The rate constants obtained for supercritical-fluid CO2 as solvent at 35 °C and high pressures (>200 bar) are similar to those obtained using normal liquid ethyl acetate as the solvent. However, at 35 °C and pressures approaching the critical pressure of CO2 (7.4 MPa), the effect of pressure on the rate constant becomes substantial. Obviously, AV takes on large negative values at temperatures and pressures near the critical point of CO2. Thus, pressure can be used to manipulate reaction rates in supercritical solvents under near-critical conditions. This effect of pressure on reacting systems in sc-fluids appears to be unique. A discussion of fundamental aspects of reaction kinetics under near-critical reaction conditions within the framework of transition-state theory can be found in reference [759],... 

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