Gem-Dihalogenobenzocyclopropenes

Abstract

The attempted dehydrochlorination of 1,6,7,7-tetrachlorobicyclo-[4,1,0]hept-3-ene to 7,7-dichlorobenzocyclopropene by the method used for the synthesis of the fluorine analogue was unsuccessful. Of a series of 2,5-disubstituted-1,6,7,7-tetrahalogenobicyclo[4,1,0]hept-3-enes synthesised with the intent of facilitating dehydrohalogenation, only 1,6,7,7-tetrachloro-2,5-diphenylbicyclo[4,1,0]hept-3-ene was successfully dehydrochlorinated - with hydroxide and alkoxide in alcohol solvents to 2,5-diphenylorthobenzoates, and with potassium t-butoxide in tetrahydrofuran to 7,7-dichloro-2,5-diphenylbenzocylcopropene which was isolated as a stable, crystalline solid in high yield. 1,6,7,7-Tetrabromo-2,5-diphenylbicyclo[4,1,0]hept-3-ene was similarly dehydrobrominated in protic solvents but 7,7-dibromo-2,5-diphenylbenzocyclopropene could not be synthesised under the same aprotic conditions. The gem-dichlorobenzocyclopropene rapidly solvolysed in the presence of water or alcohols to 2,5-diphenylbenzoic acid or its esters respectively, the latter reaction proceeding via intermediate 2,5-diphenylorthobenzoates. These latter species were isolated when the solvolysis was effected in the presence of base. Whilst heating 7,7-dichloro-2,5-diphenylbenzocyclopropene resulted in dimerisation to 9,9,10,10-tetrachloro-9,10-dihydro-1,4,5,8-tetraphenylanthracene and 9,9,10,10-tetrachloro-9,10-dihydro-1,4,5,8-tetraphenylphenanthrene, dissolution in polar aprotic solvent gave rise only to the former species. Reaction of the gem-dichlorobenzocyclopropene with phenyl magnesium bromide afforded 2,5,7,7-tetraphenylbenzocyclopropene, but this could not be isolated in a pure state due to its rapid rearrangement to 1,4,9-triphenylfluorene. Attempts to synthesise acetals of 2,5-diphenylbenzocyclopropenone and to generate and stabilise a benzocyclopropenyl cation from the gem-dichlorobenzocyclopropene were both unsuccessful. The existence of the cation could only be inferred from the mass spectrum of the proposed precursor.