In the Laboratory
Fries Rearrangement Accelerated by Microwave Radiation in the Undergraduate Organic Laboratory Inder R. Trehan, Jasvinder S. Brar, Ajay K. Arora, and Goverdhan L. Kad Department of Chemistry, Panjab University, Chandigarh, India 160014 Herein we present an opportunity for undergraduate stu(a) dents to carry out a reaction using microwave radiation. The rapid rate of reaction prevents student boredom and inactivity. The rate increase appears to be due primarily to rapid attainment of high reaction temperatures. However, the possibility of an additional modest acceleration in rate due to some effect (b) specific to the microwave wavelengths employed has not been completely excluded. We have found that the Fries rearrangement of aryl esters to ortho- and para-hydroxy acetophenones can be carried out in dry open media in ordinary glassware using a commercial microwave oven. The Fries rearrangement normally requires a Lewis acid (1a) and long reflux times or photochemical condiFigure 1. Fries Rearrangement. tions (1b). Microwaves are used extensively in a variety of applications (2). Recently, their use in rapid synthesis of organic compounds (3) and for the induction of reactions in dry media (4, 5), has been reported. The reactants are absorbed on the surface of a solid support, which absorbs the microwaves. Such supported reagents not only induce reactions efficiently under safe conditions but also avoid hazards due to high pressures when using solvents in sealed Teflon bombs. Doing the Fries rearrangements on K-10 montmorillonite (from Fluka) in dry open media in 5–10 minutes represents a step forward from conventional techniques. We present here two reactions to illustrate the suitability of microwave acceleration in an undergraduate teaching laboratory setting. In the first reaction, the acetate of β-naphthol was exposed to microwave irradiation at power level 9 in an ordinary household microwave oven for 10 minutes and 70% conversion into ortho, para acetylated product in ratio of ortho:para 9:1 was observed (Fig. 1a). The second example used the steroid estrone. At power level 9 and a time of 10 minutes the ortho isomer was formed in 65% yield (Fig. 1b). Experimental Procedure In a typical experiment, 1 mmol of the aromatic ester was dissolved in a suitable low-boiling solvent (e.g. dichloromethane) in a 500-mL beaker. One gram of K-10 montmorillonite clay was added and stirred for 2 minutes. The solvent was evaporated by warming the beaker. The mixture was then exposed to microwave radiation in a microwave oven of 2450 MHz, model No. MX2100, output 650
324
W. The progress of the reaction was monitored by thin layer chromatography. After exposure to microwave radiation the beaker was cooled and a low- boiling solvent was added to the reaction mixture. The organic layer was washed with water in a separatory funnel to remove the clay, dried over calcium chloride, and evaporated. The product was characterized by IR and 1 H NMR spectroscopy. Apart from saving time, the use of the microwave oven eliminates the use of heating mantles, oil baths, reaction flasks, inert atmosphere, and difficult work-ups. Acknowledgment Experiments were aided by a grant from the CSIR, New Delhi, India. Literature Cited 1. (a) Martin, R.; Demerseman, P. Synthesis 1992, 738. (b) For a review see, Bellus, D. Adv. Photo. Chem. 1971, 8, 109. 2. (a) For recent reviews on microwave-assisted chemical reaction see Abramovitch, R. A. Org. Prep. Proceed. Int. 1991, 23, 683. (b) Mingos, M. P. ; Baghurst, D. R. Chem. Soc. Rev. 1991, 20, 1–41. 3. (a) Banik, J. P.; Loupy, A.; Pigeon, P.; Ramdan, M.; Jacqualt, P. J. Chem. Soc. Perkin Trans. 1 1993, 397. (b) Banik, B. K.; Manhas, M. S.; Kaluza, A.; Barakat, K. J.; Bose, A. K. Tetrahedron Lett. 1992, 33, 3603 and references cited therein. (c) Bari, S. S.; Bose, A. K.; Chaudhary, A. G.; Manhas, M. S.; Raju, V. S.; Robb, E. W. J. Chem Educ. 1992, 69, 938–939. 4. Ipaktaschi, J.; Bruck, M.; Chem. Ber. 1990, 123, 1591–1593. 5. Varma, R. S.; Varma, M.; Chatterjee, A. K. J. Chem. Soc. Perkin Trans. 1 1993, 999 and references cited therein.
Journal of Chemical Education • Vol. 74 No. 3 March 1997
