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Cycloaddition. XIII. The Thermal and Photosensitized Cycloaddition of Trifluoroethylene to Butadiene. Competitive Concerted and Biradical Mechanisms Paul D. Bartlett," Barry M. Jacobson, and Leigh E. Walker
Contribution from the Converse Memorial Laboratory, Harvard University, Cambridge, Massachusetts 02138. Received April 20, 1972 Abstract: Unlike tetrafluoroethylene, trifiuoroethylene yields 13 % of the 1,4 adduct, 4,4,5-trifluorocyclohexene (5, Table I) on thermal addition to butadiene at 215", the remainder of the cross-cycloadducts being a mixture of four vinylcyclobutanes differing in orientation and configuration (1-4). The preferred orientation, by 2: 1, is that with the CFH group adjacent to the vinyl. The products of photosensitization show diminished regioselectivity among the vinylcyclobutanes. The proportion of trifluorocyclohexene in the product varies with the triplet excitation energy of the sensitizer, from less than 1 % for phenanthrene and acetophenone to 22% for fluorenone. It is concluded that not more than one-sixth of the cyclohexene in the thermal cycloadduct can have arisen oia the biradical mechanism, most of this product representing direct competition of the concerted (2 4) Diels-Alder mechanism with the stepwise biradical path.
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t is a peculiarity of tetrafluoroethylene and of 1,ldichloro-2,2-difluoroethylene (1 122) that their thermal cycloadditions to conjugated dienes do not follow the concerted Diels-Alder path to cyclohexenes, but instead yield cyclobutanes by way of biradicals. ' The formation of small amounts of 1,4-addition products by 1122 with butadiene2 and especially with 2-substituted butadienes3 and certain cis-fixed dienes4 has shown that this mode of addition is favored by increased s-cis conformation in the diene, but no single experiment has been able to establish whether fluorinated olefins perform 1,4 addition by a concerted mechanism or by a partial 1,6 ring closure of a cisoid biradical. It does appear,' from what rate measurements there are, that substitution of fluorine into ethylene greatly increases the rate of cyclobutane formation with relatively much less effect upon the rate of 1,4 addition. It is obvious that there must be a class of borderline reagents in which neither the concerted nor the biradical stepwise mode of cycloaddition will occur to the complete exclusion of the other. We have observed such behavior in vinylidene fluoride, trifluoroethylene, and alleneS5 We here report the behavior of trifluoroethylene in cycloaddition to butadiene.
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Results Structures of Cycloadducts. The thermal cycloaddition of trifluoroethylene and 1,3-butadiene at 21 5 O gave a product showing four fractions on analysis by glpc on a 6-ft column of diisodecyl phthalate. No systematic change in the relative peak heights was observed in four samples in which the ratio of diene to trifluoroethylene was varied from 1 :3 to 3 : 2. By catalytic hydrogenation the third fraction (after separation on a 3 / a in. X 20 ft column of diisodecyl phthalate, 20 on Chromosorb P, at 120") was converted into a mixture readily separable into two fractions by the same (1) P. D. Bartlett, Science, 159, 833 (1968), and references cited there. (2) J. S. Swenton and P. D. Bartlett, J . Amer. Chem. SOC.,90, 2056 (1968). (3) P. D. Bartlett, G. E. H. Wallbillich, A. S. Wingrove, J. S. Swenton, L. I
