J. Org. Chem. 1981, 46, 2948-2953
2948
Trapping Tetramethylene Biradical Intermediates in Cycloaddition Reactions of p-Methoxystyrene by Vinyl Copolymerization Reactions H. K. Hall, Jr.,* and M. Abdelkader T h e Chemistry Department, University of Arizona, Tucson, Arizona 85721 Received N o v e m b e r 26, 1980
Tetramethylene biradical intermediates in cycloaddition reactions of p-methoxystyrene (1) with trimethyl ethylenetricarboxylate (2) at 70-116 “C and dimethyl cyanofumarate (10) at 28 “C were detected through the formation of alternating 1:l copolymers 5 and 11, identical with those formed by deliberate free-radical-initiated copolymerizations of these monomers. The rates were insensitive to solvent polarity. Free-radical inhibitors suppressed copolymerization and led instead to cycloadducts, dihydropyrans 3 and 12, respectively, pointing to common intermediates in copolymerization and cycloaddition. Heating the kinetically favored dihydropyran 3 isomerized it to 1-butene 4. Vacuum distillation of 3 gave 1 and 2, while similar treatment of 12 gave copolymer by cycloreversion and spontaneous copolymerization. An ion-radical mechanism was excluded because deliberate generation of 1+. in presence of 2 gave cyclodimer 8 or homopolymer 7. Reaction of 1 with tetramethyl ethylenetetracarboxylate (14), which cannot copolymerize, led to cyclobutane 15 and traces of homopolymer 7 (some zwitterion character). The reactivity sequence 10 > 2 > 14 is dictated largely by resonance stabilization of and steric hindrance to tetramethylene formation. Consideration of the conformations available to tetramethylenes 16-18 provides a reasonable explanation of the results. Reactions of electron-rich olefins with electron-poor olefins continue to attract the attention of both organic and polymer chemists. A bewildering variety of products is known to form in such reactions. These include cyclobutane derivatives, both cycloadducts and cyclodimers, 1-butenes, cyclohexanes (from two molecules of one olefin and one of the other), homopolymers of either or both olefins, and 1:l alternating copolymers (for leading references, see ref 1-4). The current interpretation implicates a tetramethylene intermediate which is a resonance hybrid5s6of zwitterion and singlet biradical.
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