The rapid synthesis of organic compounds in microwave ovens

Abstract

This work demonstrates that organic compounds can be synthesized up to 1240 times faster in sealed Teflon vessels in a microwave oven than by conventional (reflux) techniques. It is shown that all polar molecules absorb microwave energy rapidly and that the rate of energy absorption varies with the dielectric constant. The rates of reaction of polar molecules in nonpolar solvents are not increased appreciably by the microwave method. Also, the homogeneity of the reaction does not affect the rate enhancement. The rate enhancement arises predominantly because the oven superheats the solvent rapidly. Finally, pressure (temperature) measurements have shown that the maximum rate enhancement is achieved when the proper power level and volume of solvent are used. It appears that rate enhancements of approximately 200 are possible for many reactions if the reaction conditions are optimized.RICHARD N. GEDYE, FRANK E. SMITH et KENNETH CHARLES WESTAWAY. Can. J. Chem. 66, 17 (1988).Dans ce travail, on demontre que les composCs organiques peuvent &tre synthCtisCs 1240 fois plus rapidement dans des rkcipients de TCflon scelles, dans des fours a micro-ondes, que par les techniques conventionnelles de reflux. On dCmontre que toutes les molCcules polaires absorbent rapidement 1'Cnergie des micro-ondes et que les vitesses d'absorption de 1'Cnergie varient avec la constante diklectrique. La mCthode des micro-ondes ne provoque pas en une augmentation substantielle des vitesses des rCaetions impliquant des molCcules polaires dans des solvants qui ne sont pas polaires. De plus, I'homogCnCitC du milieu reactionnel n'influence pas le taux d'augmentation des vitesses des rkactions. L'augmentation des vitesses des reactions provient principalement du fait que le four provoque une surchauffe rapide du solvant. Enfin, des mesures de pression (tempkrature) ont permis de dtmontrer que le taux maximal d'augmentation des vitesses de rCaction est atteint lorsqu'on utilise un niveau de puissance et un volume de solvant appropriCs. I1 semble que l'on puisse atteindre des taux d'augmentation des vitesses de rCactions d'environ 200 si les conditions des rCactions sont optimisCes.[Traduit par la revue] Introduction Although several chemical applications of microwave heating have been reported (1-5), it is only recently that microwave ovens have been used in organic synthesis (6, 7). In a preliminary communication (6) we described the use of microwave ovens for synthesizing esters from carboxylic acids, carboxylic acids from alkyl benzenes and amides, and ethers from alkyl halides. In fact, these organic reactions occurred up to 1240 times more rapidIy in sealed Teflon containers in the microwave oven than by classical reflux methods. Since then, Giguere et al. (7) reported dramatic reductions in reaction time in microwave syntheses using the Diels-Alder, Claisen, and ene reactions.The present paper extends the scope of this new synthetic method and reports our investigations of several of the factors that affect the rate enhancements found in organi...

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