Benzocyclobutenones, formation

Flash-vacuum pyrolysis of 3//-indcne-l,2-dione at 600-100 C produced benzocyclopropene 2 in 1-8% yield. At low temperatures, benzocyclobutenone (1) was the main product, white at higher temperatures fulvencallene (3), which is the pyrolysis product of benzocyclopropene (see Section 4.1.), and ethynylcyclopentadiene predominated. Benzocyclopropene (2, 8-16%) and fulvencallene (3, 1 8%), together with a large number of other products, were obtained when isobenzofuran-l(3//)-one was reacted in a RF plasma.In both cases, benzocyclopropene formation was attributed to cyclization of intermediate diradicals. 

Attack of pyridine on the bis-dibromocarbene adduct (474) results in formation of a pyridinium benzocyclobutene (475). Krohnke reaction of the latter with p-nitro-sodimethylaniline affords the benzocyclobutenone (476), which is opened by sodium hydroxide to the arylacetic acid (477 R = H, n = 1). Direct oxidation of (475) with alkaline permanganate gives a mixture of the phthalate (478) and benzoate (477 R = Br, = 0) the latter probably being formed via (476). °... 

The chemistry of 2-hydroxy-2,4,6-cyclo-octatrienone (134) has also been investigated. Pyrolysis of (134) gave phenol, presumably by loss of ketene from the valence isomer (135). Reaction with iV-phenyltriazolinedione gave the adduct (136) and treatment with thionyl chloride gave benzocyclobutenone. The intermediacy of the bicyclic form can account for the formation of all the products. 

Ab initio calculations have been used to compare charge redistribution and structural changes upon enolate anion formation from benzocyclobutenone, cyclobutenone, and cyclobutanone there is little evidence of antiaromatic destabilization of benzocyclobutenone enolate, which has been found to behave as a carbon nucleophile. 

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