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Free Catalyzed Stille Coupling Assisted by Microwaves for the Synthesis of Substituted Diarylmethanes
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1  UNS, Dpto de Química, Avenida Alem 1253, Bahía Blanca, Argentina.

Abstract: There is a wide variety of natural and synthetic benzophenones with antibacterial activity. In previous studies,[1] we reported the synthesis of substituted diarylmethanes under Barbier conditions by Stille coupling as precursors of substituted benzophenones. Catalyzed bond formation require hours to days to complete when conventional heating is used, under inert atmosphere. Microwaves have demonstrated that the reaction rate improves significantly and allows use closed vials without inert atmosphere.[2] The Stille coupling was one of the first reactions where the assistance of microwave was applied since some organotin compound have poor reactivity and, consequently, low yield of the desired product. We present here the study of the synthesis of diarylmethanes through Stille reaction under microwave irradiation, comparing the results with and without catalytic conditions and ligands,[3] based on the optimal results applying microwaves without catalyst that have been reported for Suzuki and Sonogashira coupling.[4],[5] In most cases, reactions were completed in short time and no homocoupling product was observed. Although reaction times are slightly higher and the yields were lower than those found in catalyzed condition, the fact that in this case this difficult type of reaction (Csp3-Csp2 coupling) can be performed without catalyst, is very promising and encourages us to continue with future studies in order to find the best conditions. [1] Ocampo, R.A, Koll, L.C, Mandolesi, S.D. Ultrasonics Sonochemistry, Article in press. [2] a) Larhed, M. , Moberg, C., Hallberg, A. Acc. Chem. Res. 2002, 35, 717. b) Olofsson, K., Larhed, M. (Eds.: P. Lidstrum, J.P. Tierney), Blackwell, Oxford, 2004, Chap. 2 [3] a) Larhed, M., Lindeberg, G., Hallberg, A., Tetraheron Lett, 1996, 37, 8219. b) Berthault A., Berteina-Raboin, S., Finaru A., Guillaumet, G., QSAR Comb Sci, 2004, 23, 850 [4] Leadbeater, N.E., Marco, M. J. Org. Chem. 2003, 68, 5660 [5] Leadbeater, N.E., Marco, M., Tominack, B. Org. Lett. 2003, 5, 3919
Keywords: diarylmethanes, Barbier, Stille couplings, microwave