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170 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 and to a lesser extent to manufacture anthraquinone [13]; it should be noted that these reactions proceed without solvent. A substantial body of literature exists concerning Diels-Alder chemistry in supercritical flu- ids, CO2 in particular. For the most part, research on this particular reaction has been used (via analysis of the rate constants), to confirm the influence of concen- tration fluctuations (present near the critical point) on the rate of the reaction. In general, the rate reaches a maximum near Tc, dropping at both higher and lower pressures. However, this work is currently of scientific interest only, as control of a reaction in the neighbor- hood of a critical point is problematic at large scale. Tester et al. [238] report that most Diels-Alder rate constants in CO2 can be correlated using a simple Ar- rhenius expression provided that the pre-exponential term varies linearly with fluid density, similar to what Roberts [239] observed using propane as the solvent. Lewis acid catalysts are effective (if soluble), as shown by Matsuo et al. using a scandium triflate in CO2 [240]. Although the literature on Diels-Alder chemistry in CO2 at first glance appears uninteresting (from a green chemistry viewpoint), there are some pub- lications that merit closer scrutiny. For example, Ikushima et al. [241], published the results of a study of the cycloaddition of isoprene and methyl acrylate (Scheme 7), reporting that while one atmosphere conditions produced primarily the para isomer of the methyl acetoxy cyclohexene product, operation in CO2 produced significant amounts (at some pressures the major component) of the meta isomer. If true, such a result suggests that use of CO2 can alter product selectivities, and hence would significantly impact the Scheme 7. field of green chemistry in critical fluids. However, subsequent work by Danheiser and Tester [242] revealed that Ikushima et al. failed to note that multiple phases were present in the reactor, and that adequate sampling of the phases revealed that all conditions produced a 67–31 split of para and meta isomers. This again shows the importance of understanding the phase behavior of any reaction mixture under evaluation. Indeed, subsequent work by Danheiser and Tester on a wide range of Diels-Alder substrates revealed no effect of CO2 pressure on regioselectivity. Some additional observations on Diels-Alder chem- istry in CO2 include reports by Clifford et al. [243] that the endo:exo ratio of products in the reaction be- tween methyl acrylate and cyclopentadiene exhibits a maximum versus pressure in CO2. Totoe et al. [244] also observed differences in product selectivity be- tween toluene and CO2 in a 1,3 dipolar cycloaddition. In summary, although there have been some in- triguing reports on variations in selectivity in CO2 versus conventional solvents, most of the research on Diels-Alder chemistry in CO2 has been directed at deriving fundamental parameters rather than creating opportunities for green chemistry per se. The work by Danheiser and Tester should stand as a warn- ing to those involved in chemistry in supercritical fluids—one ignores phase behavior effects at one’s peril! 4.3. Lewis acid catalysis/Friedel-Crafts chemistry Friedel-Crafts chemistry is used extensively to per- form liquid-phase alkylations and acylations, although it should be noted that the largest scale industrial pro- cesses do not employ solvent and some have switched from the typical aluminum halide ‘catalyst’ to sup- ported acidic catalysts [13]. However, fine chemical syntheses often employ relatively toxic solvents dur- ing Friedel-Crafts reactions, and hence this reaction presents a viable target for use of CO2. Because Friedel-Crafts chemistry is usually performed in po- lar media, an obvious question is whether CO2 (with its low dielectric constant) can actually support such reactions. Further, the primary environmental draw- back to Friedel-Crafts chemistry is the need for large amounts of aluminum halide and hence, much recent research has focused on finding true catalysts for the various alkylations and acylations. Interestingly,PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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