Trifluoroethylene reaction with butadiene

As part of his elegant and comprehensive examination ot the competition between [2+2] and [2+4] reactions of fluorinated ethylenes, Bartlett found that trifluoroethylene s [2+4] reaction with butadiene competed somewhat with its [2+2] cycloaddirion [66] (equation 62). 

Conversely, cases are known where % cycloadditions take place by the ERj mechanism in preference to an alternative allowed Diels-Alder process. A good example is the reaction of butadiene with trifluoroethylene to give the adducts (236)-(239). The fact that (236) and (239) are formed in equal amounts shows that the reaction is not a pericyclic cycloaddition but must take place via the biradical (240). In this the F2C—CFH bond is single, so rotation is possible, the configuration of the original reactants being lost. Even the 13% of normal Diels-Alder product may be, and probably is, formed by cyclization of the same intermediate biradical, either (240) or the precursor (241) of (238). 

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