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The thermal cycloisomerization of some alkylacetylenes was investigated in a tubular quartz reactor. At 570°C 1-hexyne (1) rearranges to 3-methyl-I-cyclopentene (5) with a selectivity of about 27 by a reaction sequence including an acetylene-vinylidene rearrangement and 1,5-C,H insertion of the intermediately formed alkylidenecarbene species. 5-methyl-1-hexyne (2) behaves analogously forming 3,3-dimethyl-l-cyclopentene (6), while 2-hexyne (3) provides 1-methyl-1-cyclopentene (7) indicating that the acetylene-vinylidene rearrangement is obviously not restricted to 1J- Very recently we have studied the gas-phase pyrolysis of alkylacetylenes 2 C s at 500 to 600°C'' (see also ref.*l3'). Whilst the cracked gases formed are obviously a result of radical chain processes the origin of essential parts of the cracked liquids is somewhat mysterious. Particularly surprising is the formation of hydrocarbons with cyclopentene structures, which in each case are isomeric to the corresponding parent alkynes. One might expect that the formation of cyclopentenes should happen by internal addition of alkenyl radicals known to give live-membered ring species (see, for example, ref.')). If radical pathways are involved appropriate product mixtures would have been expected, but the results described in ref.') illustrate that the products in question seem to consist essentially of one compound only.Quite recently we have observed that the formation of five-membered ring hydrocarbons from alkynes > Cs does not require bimolecular reaction steps as are necessary for radical cyclizations I). These results4.') suggested that the cycloisomeres originate by acetylene-vinylidene rearrangements@ followed by cyclization of the vinylidene species by 1,5-C,H insertion (Scheme 1).Within the scope of our investigations on the thermal cyclization and aromatization of highly

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