Cyclobutadienes, dimerization Cyclobutanation

Thermal cycloaddition, to form a cyclobutane ring, is not confined to fluorinated ethylenes. Cyclobutadiene derivatives are formed as intermediates and dimerize to yield a cyclobutane ring 48... 

Even the concerted pericyclic dimerization of an olefin to a cyclobutane [equation (5.290)] can occur if the two double bonds are twisted sufficiently. It may then be possible for the top lobes of the 2p AOs in one olefin moiety to overlap with a top and a bottom lobe of the 2p AOs of the other (165). The transition state is then of anti-Hiickel type and so is aromatic, being an anti-Hiickel analog of cyclobutadiene. The arrows in (165) indicate the relative directions in which the terminal atoms of the C=C bonds are twisted. 

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. 

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