Captodative olefins cycloaddition

The cycloaddition reactions of captodative olefins all are considered to proceed through the intermediacy of a 1,4-diradical, due to the captodative stabilization of the terminal radicals. In cross-cycloadditions captodative olefins easily give cyclobutanes when heated with fluoroolefins [141]. They also react with allenes to give methylenecyclobutanes [142], and with methylenecyclopropane to give spiro[2.3]hexanes [143]. 

Some captodative olefins homodimerize near room temperature [141,144-146] and an intramolecular example is also known (Scheme 4) [147]. These [2 + 2]-cycloadditions are often reversible. The unfavorable entropic contribution and the strain in the cyclobutane derivative can operate in the same direction so that the cycloaddition becomes thermodynamically impossible. Even when substituted with relatively small groups, a cyclobutane can cleave back into the starting... 

Thus cycloadditions involving captodative olefins proceed via a 1,4-diradical intermediate in accordance with the Woodward-Hoffmann rules. The ease with which an olefin can generate a 1,4-diradical depends both on the ability of its... 

We now propose that the diradical intermediates in the cycloaddition reactions of captodative olefins are also the initiators in the observed spontaneous polymerizations of these olefins. Tail-to-tail combination of captodative olefins are expected to provide a low but constant concentration of diradicals, which are capable of initiation. Whether or not polymerization ensues must depend on the experimental conditions, propagation equilibria and rates. 

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). 

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