Cyclohepta-1,3,5,7-tetraene

Scheme 6.108 Products ofthe interception of 1,2,4,6-cyclohepta-tetraene (5) with 1,3-diphenylisobenzofuran, anthracenes, 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dien-l-one, methyl 2-pyrone-5-carboxylate, tropones, 8,8-dicyanoheptafulvene and 1,3,5-cycloheptatrienes.

The rearrangement of phenyl carbene to cyclohepta-1,2,4,6-tetraene has been detected at 10 K (Westetal., 1982 cf. Fig. 3.3), but the relevance of this to photochemistry in solution at higher temperatures is not yet clear. Ortho-substituents that might react with photogenerated aryl carbenes should be avoided as they are with arylazides (Section 3.2.4 Fig. 3.4). 

With respect to the carbene, arylcyclopropane formation occurs with rather low stereospecificity. In most cases the ewt/o-isomer predominates, which is in accordance with the general observation made when arylmethylenes from other sources are added to alkenes. Thus, photolysis of phenyldiazomethane dissolved in cyclohexene gave 7-phenylbicyclo[4.1.0]heptane (2) in 34% yield with a endojexo ratio of 1.1. Preference for the e (/o-isomer is also observed when monoarylcarbenes are reacted with cyclohepta-l,3,5-triene, but when 1-naphthylcar-bene, 2-naphthylcarbene and 9-anthrylcarbene are reacted with cycloocta-l,3,5,7-tetraene the exo-isomer predominates. ... 

Unlike phenyldiazomethane which is converted on irradiation into cyclohepta-1,2,4,6-tetraene, the carbenes (52) generated by photodecomposition of the naphthyldiazomethanes (53) undergo rearrangement to the benzobicyclo[4,1.0]hepta-2,4,6-trienes (54). Hydrogen abstraction by triplet diphenylcarbene is observed on photodecomposition of diphenyldiazomethane in cyclohexane, whereas singlet-derived solvent insertion reactions compete with hydrogen... 

Further examples of the dimerization of allenes to give dimethylenecyclo butanes have been reported. The strained allenes cyclo-octa-l,2,4,6-tetraene and cyclohepta-1,2,5-triene have been generated and trapped. In the absence of trapping agents, the dimers (114) and (115) are formed. The relative reactivities of bicyclo[3,2,l]octa 2,3-diene and cyclohexa-1,2-diene with conjugated dienes and styrenes have been investigated. Both allenes give mainly [2 + 2] adducts. 

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