Tetrafluoroethene cycloaddition

The mixed dimerization of polyhalogenated alkenes with both activated and nonactivated alkcncs has been well documented (see Houben-Weyl, Vol. 4/4, p 206) and continues to represent a convenient preparative method for polyhalogenated cyclobutanes. Of the polyhalogenated alkenes, the fluorinated ethenes are the most reactive towards [2 + 2] cycloadditions. The method is, however, limited by the nonstereoselectivity due largely to the formation of 1,4-diradical intermediates and the requirement of high temperatures. The observation of stereochemical equilibration is seen in the cycloaddition products of tetrafluoroethene (1) with (island (Z)-but-2-ene and (Z)-l,2-[2H2]ethene where mixtures of stereoisomeric cyclobutanes are obtained.19-20... 

The [2 + 2] cycloaddition of tetrafluoroethene (1) with silyl- and siloxy-substituted alkenes (9) can be a preparatively useful reaction.21... 

We have not given you much evidence to decide why it is that some thermal [2 + 2] cycloadditions occur but not others. What is special about fluoroalkenes, allenes, and ketenes in these reactions One possibility is that Mobius rather than the Hiickel transition states are involved, but the Mobius transition states are expected to suffer from steric hindrance (Section 21-10B). It is also possible that [2 + 2] cycloadditions, unlike the Diels-Alder additions, proceed by stepwise mechanisms. This possibility is strongly supported by the fact that these reactions generally are not stereospecific. Thus with tetrafluoroethene and trans,trans-2,5-bexadiene two products are formed, which differ in that the 1-propenyl group is trans to the methyl group in one adduct, 45, and cis in the other, 46 ... 

Using mercury derivatives, 1-bromo-l-chlorocyclopropanes are prepared from alkenes of low reaetivity, such as fluoro- and chloroethene, 1,1-difluoro-, tri- and tetrafluoroethene, tetrachloroethene, etc. In the case of some alkenes, C —H insertion of bromochlorocarbene competes effectively with cycloaddition, e.g. formation of 14 and 15. ... 

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