1,4-Benzenediyl diradical

Upon heating the enediyne la rearranges reversibly to the 1,4-benzenediyl diradical 2a, which in its turn can rearrange to the enediyne lb or—in the presence of a hydrogen donor (e.g. cyclohexa-1,4-diene)—react to the aromatic compound 3a. 

Benzenediyl diradical formation from enediyne via electrocyclization. 

The cyclization is induced thermally or photochemically. Most cyclizations have a high activation energy barrier and therefore temperatures around 200 °C are needed for the cycloaromatization. The Bergman Cyclizatic n forms a 1,4-benzenediyl diradical - a highly reactive species, that reacts with a H donor to give the corresponding arenes. 

The conversion of an ene-diyne 2.77 into a 1,4-benzenediyl diradical 2.78 on heating is known as the Bergmann reaction . The 1,4-aromatic radical 2.78 maybe converted into benzene or may react with CCI4 to givep-dichlorobenzene (2.79). Bergmann etal converted deuterium-labelled hexa-3-ene-l,5-diyne 2.80 on heating at 200°C into deuterium-labelled hexa-3-ene-l,5-diyne 2.81 in which both deuterium atoms were shifted from the terminal acylene positions to vinyl positions at the interior of the chain. 

Formation of a substituted benzene through 1,4-benzenediyl diradical formation from enediyne via electrocychzation. 

Bergman proposed that the reaction mechanism of the cyclization under thermal conditions (200 °C) involved the initial generation of a 1,4-benzenediyl diradical species known as para-benzyne (2). Bergman reported that when the reaction was carried out in a hydrocarbon solvent, such as 2,6,10,14-tetramethylpentadecane, benzene was formed as the final product. This suggests that the hydrocarbon solvent (RH) acts as a hydrogen atom donor to quench the diradical intermediate 2. This result hints at the radical nature of the mechanism operative in the Bergman cyclization. 

T,K-diradical intermediate 35 (and hence the transition state leading to it) over the 1,4-benzenediyl diradical species 2 involved in the Bergman cyclization ensured that the reaction proceeded under much milder conditions (typically 37-100 °C) than are usually required for Bergman cyclizations. Mild thermolysis of 34 in 1,4-cyclohexadiene afforded 36 in 60% yield. It is interesting that methyl substitution at the allene terminus of 34 caused acceleration of the cyclization (approximately six times faster than 34) and this was presumably due to formation of a more stable diradical intermediate. ... 

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