Fluorinated ethylene, cycloaddition reactions

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

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). 

A number of cases are known where ethylene derivatives undergo n cycloaddition to form cyclobutanes. Tetracycanoethylene [TCNE (NC)2C=C(CN)2] and fluorinated ethylenes are particularly prone to behave in this way. These reactions were discovered at the laboratories of duPont de Nemours Co. and have been studied in great detail by Bartlett and his collaborators. As we have already seen, concerted cis dimerizations of this kind should be less favorable than an path via an intermediate 1,4-butadiyl biradical all these reactions in fact take place in this way. An interesting example is the reaction of TCNE with bismethylenecyclobutene (230). The normal Diels-Alder reaction to form (231) is inhibited because this would be an antiaromatic cyclobutadiene derivative and because the transition state leading to it would be isoconjugate with benzocyclobutadiene. The product is therefore the spiran (232), formed as indicated by an process. 

Fluorinated alkenes have been reported to give cyclobutane derivatives by thermal additions to other alkenes. Olefins in their ground electronic states have not been observed to give stereospecific cycloaddition one with another whereas a carbon-carbon double bond which is part of an allene or keten will undergo stereospecific concerted cycloadditions. The separate reactions of tetrafluoroethylene with cis- and trans-[l,2- H2]ethylene, which should not be subject to an adverse steric effect, have been studied to establish whether these reactions are stereospecific. Identical mixtures of products were obtained from both reactions, as expected if they proceed through the... 

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