Cope Rearrangements versus Ring Closure in the Flash Vacuum Pyrolysis of Acyclic Dienes and Enynes

ELIZABETH L. MOYANO; CEBALLOS NOELIA M.; ISAO IWATAKI; GLORIA I. YRANZO

Berkeley, California, USA

Simposio; Ninth Tetrahedron Symposium; 2008

Elsevier

The study of thermal reaction of dienes, enynes and diynes compounds has been developed into a useful method for the high temperature synthesis of polycyclic aromatics such as biphenyls, naphthalenes, corannulene and semibuckminsterfullerene.1 Cycloisomerizations, rearrangements via alkenylidene carbenes and vinyl-type radicals, Cope rearrangement and other process are possible reaction pathways in the thermolysis of unsaturared alkenes and acetylenes.2 In this work the thermal reaction of the 1,5-hexadiene 1 and the 1-hexen-5-ynes 2 and 3 has been studied at temperatures between 300-500ºC, contact times of 0.01 s and at pressures of 0.01 Torr (Flash Vacuum Pyrolysis). The results show that malonate derivatives 1 and 2 undergo the Cope rearrangement under reaction conditions. While in the case of 1 the initial Cope product 4 (37% yield at 350ºC) could be obtained, substrate 2 suffer a Cope rearrangement followed by a 1,2-acyl migration to give the triene 5 (44% yield at 350ºC). A different behavior was observed when the enyne 3 is submitted under FVP conditions. In this case, the unsaturated acetylene undergoes a cyclization reaction (via an alkenylidene carbene intermediate) followed by aromatization to give the biaryl 6 in 72% yield at 350ºC (Scheme 1). The results reported in this work reveal that there is a strong influence of the substituents of the C=C bond in the course of the thermal reaction. Also, the Cope rearrangement was observed with both ester substrates, enyne and diene derivatives.    References: 1a) Nüchter, U.; Zimmermann, G., Francke, V.; Hopo, H. Liebigs Ann./Recueil 1997, 1505; b) Scoot, L.T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. W. J. Am. Chem. Soc. 1991, 113, 7082. 2a)Viola, A.; Collins, J. J.; Filipp, N. Tetrahedron, 1981, 37, 3765; b) Scmittel, M.; Steffen, J-P.; Wencesla, A.; Engels, B.; Lennartz, C.; Hanrath, M. Angew. Chem. Int. Ed. 1998, 37, 1562.