Pyrolysis rearrangement, ring contraction

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

Ring contraction of cyclobutylidenes is known, but not so widely explored. Thus, pyrolysis (200°C) or irradiation of the lithium or sodium salt of cyclobutanone tosylhydrazone (262) gave the 2-vinylcyclobutylidene (263) which then rearranged mainly to ring contraction products (equation 181). 

In annelated triazoles the Wolfi-rearrangement leads to ring contraction. Thus gas-phase pyrolysis of benzotriazole (9) in a stream of argon gives a 99% yield of... 

Although arynes are reluctant to rearrange, examples of ring contraction of an aryne have been recorded. An example of an aryne rearrangement to a carbene has been recorded it is probable that the aryne and carbene are in equilibrium under the vigorous conditions (flash vacuum pyrolysis at 960 C) of the reaction . A similar ring contraction in a heterocyclic system probably proceeds via a similar mechanism. ... 

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