Trifluoroethylene cycloaddition

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] cycloaddition [66] (equation 62). 

Fluorine and hydrogen appear comparable as radical-stabilizing groups, for both 1,1-difluoroethylene and trifluoroethylene react with butadiene to give mixtures of [2 + 2] regioisomers, as well as some [2 + 4] products. " Captodative substituted alkenes, not surprisingly, are well suited to [2 + 2] cycloadditions. ... 

In agreement with the captodative concept, 2+2 cycloaddition of cd-olefins to trifluoroethylene via biradical intermediates is more facile than to captor-substituted olefines, or to the captor-substituted acetylenes. Phenylthioacetylene reacts smoothly and permits S-oxidation of the adducts (ref. 21, Scheme 25). 

Hydrocarbon and partially fluorinated olefins also undergo cycloaddition with fluorinated ketones. Thus, UV-induced addition of HFA to ethylene, vinyl fluoride, vinylidene fluoride, 1,2-difluoroethylene," trifluoroethylene, and 3,3,3-trifluor-opropylene was reported to afford corresponding oxetanes in the yield >70%. In all cases where it is possible, the formation of both regioisomers was observed. For example, irradiation of the equimolar mixture of HFA and CF2=CH2 resulted in high-yield formation of regioisomers 5 and 6 (Scheme 2.5). 

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). 

Biradical intermediates are more probable in the cycloaddition of chloro-trifluoroethylene and 1,1-dichlorodifluoroethylene to allene to give the... 

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