No abstract

Iron(III) chloride-catalyzed Friedel-Crafts acylation and sulfonylation of aromatics have been carried out using a continuous-flow microwave (MW) reactor. The MW applicator is a monomode waveguide fitted with power and temperature controls. The flow-through system operates at atmospheric pressure, and homogeneous conditions were ensured by using an excess of the aromatic. Factors that influence the reaction yield (MW power, ratio and flow rate of reactants) were optimized for two different types of substrate, two polar aromatic ethers, anisole (1) and phenetole (2), and a nonpolar arene, mesitylene (3). Although the energy yield was mainly influenced by the nature of the aromatic, the temperature necessary to achieve a high yield was reached in all cases. Methoxybenzophenone (7), 4-chloroethoxybenzophenone (8), and mesityl phenyl sulfone (9) were obtained from 1-3 and the corresponding acid chloride in 85-95% yield with a 1.2 L h -1 flow rate.

Solvent-free sulfonylation of benzene and its activated or deactivated derivatives were carried out under microwave (MW) irradiation and a catalytic amount of iron(III) chloride, which, under these conditions, is more active than other metallic salts. With more reactive and/or nonvolatile reagents (anisole, xylenes, mesitylene) expeditious conditions (short reaction time at constant MW power without control of the temperature) were used. With less reactive and/or low-boiling reagents (benzene, toluene, halobenzenes), the rise in temperature and the increase of reaction time were controlled either by sequential MW irradiation or by a temperature order. It was shown that MWs cause preferential interactions with polar species present in the reaction, especially the aryl sulfone and its FeCl3-complexed form. A MW nonthermal effect was not observed when identical temperature gradients were produced by classical heating and MW irradiation, and if reaction temperature was strictly controlled.