Thermal Aromatization of Alkylidenecyclopentenes and Related Hydrocarbons

Article abstract of DOI:10.1021/jo00253a031

3- and 4-alkylidenecyclopentenes (dihydrofulvenes) undergo thermal aromatization upon flash vacuum pyrolysis (FVP) at 700 deg C.For example, toluene and m-xylene are the principal products formed from 3- and 4-isopropylidenecyclopentene.Similar behavior is observed for other dihydrofulvenes that have alkyl groups on the exocyclic methylene carbon.Cyclohexadienes are believed to be the intermediates immediately preceding the aromatics, and the aromatization step is believed to occur by a free radical process involving successive loss of an alkyl group and a hydrogen atom.Small amounts of secondary aromatics are formed from the primary products by free radical aromatic substitution, dealkylation, etc.Upon FVP, 6,6-dialkylbicyclo<3.1.0>hexenes give mixtures of aromatics which are very similar to those obtained from the isomeric dihydrofulvenes, and it is proposed that these bicyclic derivatives may be intermediates in the formation of cyclohexadienes from dihydrofulvenes.Other possible pathways from dihydrofulvenes to cyclohexadienes are presented.