Benzocyclobutenes fulvenes

The influence of intramolecularity on the benzocyclobutene-fulvene cycloaddition was illustrated in dramatic fashion when the two reactive units were tethered together by a three-carbon chain. Heating nitrile (58) gave only the [6 -1- 4] cycloadduct (59) as an inseparable mixture of cyclopentadiene isomers in... 

While the electrocyclic ring opening to o-quinodimethanes is the major reaction pathway in the irradiation of substituted benzocyclobutenes (cf. Example 6.14), the irradiation of unsubstituted benzocyclobutene yields 1,2-dihydropentalene (119) and 1,5-dihydropentalene (120) as major products. The mechanism shown with prebenzvalene (118) as primary photochemical intermediate has been proposed to explain the formation of the isomeric dihydropentalenes (Turro et al.. 1988). Supporting calculations that yield the same mechanism for the benzene-to-fulvene transformation have been published (Dreyerand Klessinger, 1995). 

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