Subject 1,5-hexadiene

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

The unidentified peak in the chromatogram (Figure 3) gives a negative test for an acetylenic hydrogen (ammoniacal cuprous chloride solution) and does not disappear after the sample is subjected to free radical catalysis for several hours. Thus, the open chain acetylenic isomers of benzene, hexa-l,3-diene-5-yne, 1,4-hexadiyne, 1,5-hexadiyne, and 1,5-hexadien-3-yne do not appear to account for this peak. The possibility of valence isomers of benzene such as bicyclo (2.2.0) hexa-2, 5-diene,... 

Allyl radicals were tested as materials or substances that cause cyclization with olefins. 1,5-Hexadiene (diallyl) and diallyl oxalate (DAO) were used as the source materials for allyl radicals. These compounds were subjected to the thermal reaction in the presence of ethylene, which resulted in a rapid formation of five-membered cyclic compounds. The role of allyl radical for the formation of aromatics in pyrolysis reaction is however still... 

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