Diels using supercritical carbon dioxide

Reaction medium. By virtue of its attributes supercritical carbon dioxide deserves its increasing applications as a reaction medium in synthesis. These include epoxidation" with oxygen and PhCHO, rhodium-catalyzed hydroboration, Pd(0)-catalyzed 1,4-hydroarylation of enones" and Heck reaction in the presence of Pd-C," carbonylation of aryl halides, Glaser coupling of 1-alkynes (mediated by CuCy using a solid ba.se (NaOAc)," as well as seandium(III) perfluorooctanesulfonate-catalyzed Diels-Alder and hetero-Diels-Alder reaetions. ... 

A variation in this technique has recently been reported, although not necessarily using the high pressures described above. Diels-Alder reactions can be done under pressure in supercritical carbon dioxide., although the synthetic application may be limited by slow reaction rates. Addition of a Lewis acid appears to overcome this problem, as in Kobayashi s use of scandium triflate in the reaction of methyl vinyl ketone with 2-methyl-1,3-butadiene to give a 93 7 mixture of 137/138. " 7a Rayner and co-workers had previously reported the use of scandium triflate in supercritical and observed that the maximum selectivity was obtained... 

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. 

RS Oakes, TJ HeppenstaU, N Shezad, AA Clifford, CM Rayner. Use of scandium tris(trifluoromethanesulfonate) as a Lewis acid catalyst in supercritical carbon dioxide efficient Diels-Alder reactions and pressure dependent enhancement of endo exo stereoselectivity. Chem Commun 1459-1460, 1999. 

J. Matsuo, T. Tsuchiya, K. Odashima,S. Kobayashi, Lewis add catalysis in supercritical carbon dioxide. Use of scandium tris(heptadecafluo-rooctanesulfonate) as a Lewis add catalyst in Diels-Alder and aza Diels-Alder reactions, Chem. Lett. 29 (2000) 178-179. 

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