Organic Synthesis in Supercritical Fluids

ORGANIC SYNTHESIS IN SUPERCRITICAL FLUIDS Supercritical Fluids 

Considering the importance of the Diels-Alder reaction in organic synthesis and its proven sensitivity to solvent, a number of studies have been carried out using 

SCFs as solvents for this reaction, for example, Paulaitis and Alexander (1987), Kim and Johnston (1987), Ikushima et al. (1990, 1991, 1992), Isaacs and Keating (1992), Kaupp (1994). The results of these and other studies confirm the expectation that there is no discontinuity in the reaction rate in the transition from fluid to sc CO2 at constant density, but at the same time there is no significant rise in the reaction rate either. An example of a reaction that shows a reasonable increase in the rate constant is the cycloaddition of p-benzoquinone and cyclopentadiene in sc CO2 (Isaacs and Keating, 1992). The rate constant increases with pressure and is about 20% higher than in diethyl ether. 

An interesting result is that the selectivity in a parallel Diels-Alder reaction can be controlled by pressure, as demonstrated in the cycloaddition of methyl acrylate and cyclopentadiene in sc CO2 (Kim and Johnston, 1987). 

The endojexo selectivity increased with pressure, although the maximum increase was only about 2.5%. 

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