A biradical mechanism in the Diels-Alder reactions of 5-methylene-2(5H)-furanones: experimental evidence and theoretical rationalization

V.BRANCHADELL, J. FONT, A. G. MOGLIONI, C. OCHOA DE ECHAGÜEN, A. OLIVA, R. M. ORTUÑO, J. VECIANA, J. VIDAL-GANCEDO

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

AMER CHEMICAL SOC

Año: 1997 vol. 119 p. 992 - 10003

0002-7863

Reactions of protoanemonin, 1, and 3-bromo- and 3-methoxy-5-methylene-2(5H)-furanones, 2 and 3, withdienes have been performed under both thermal and photochemical activation conditions. Photochemical cycloadditionsof butadiene and trans-piperylene to this kind of furanones have afforded 1,2-adducts, 10-13, as stereoisomericmixtures, in 40-60% yields, and have shown that furanones 1-3 react specifically at the exocyclic double bond asthey do in thermal conditions. Furthermore, thermal cycloadditions have led to the production of the corresponding1,4-adducts, 4-7, in good yields, but 10-13 have also been detected at the initial stages of these reactions. Compounds10-13 have been proved to be intermediates in the production of adducts 4-7, isomerization taking place underheating and involving the formation of biradical species as intermediates. Moreover, cycloreversion from 1,4-adducts has not been observed, and only very small proportions (1-8%) of the corresponding furanones have beenfound when 1,2-adducts have been heated. In good agreement with these results, CASSCF and DFT ab initio theoretical calculations, performed on the reaction of 1 and butadiene to give 4, have shown the nonconcertedmechanism to be more favorable than the concerted process, and have allowed the location of all intermediates and transition states involved in the overall process. Since, in addition, the presence of radicals in the isomerization of 1,2-adducts to 1,4-adducts has been confirmed with ESR spin trapping techniques, we can conclude that the resultingDiels-Alder adducts are mainly produced Via a biradical mechanism. To our knowledge, this is the first time that a biradical mechanism operating in a Diels-Alder cycloaddition and accounting for the production of 1,4-adducts has been evidenced and rationalized.