Studies of Some Strained Organic Molecules: Cyclopropa[ℓ]Phenanthrenes

Abstract

The present study has been directed towards the synthesis of 1H- cyclopropa[ℓ]phenanthrene (18) and its derivatives. Since the established procedures for cycloproparene synthesis have not proved readily adaptable to the formation of a cyclopropa[ℓ]phenanthrene, new approaches to the formation of this unknown ring system have been investigated. One approach required the syntheses of 1-oxocyclobuta[ℓ]phenanthrene derivatives (90) but the attempted 'Thorpe-type' condensation resulted instead in the formation of the first isoquinolinone derivative of phenanthrene (104). The second approach utilised substituted 1a, 9b-dihydro derivatives as precursors. Dehydrochlorination of 1,1-dichloro-and 1-chloro-1a, 9b-dihydrocyclopropa[ℓ]phenanthrene (50 and 115) provided excellent evidence for the formation of the dibenzbicycloheptatrienes (113 and 119) with a 1,1a-double bond. Aromatization of these species occurs only with opening of the three-membered ring. The Δ1,1a-olefin has been trapped by nucleophilic addition. Attempts to produce 1a-substituted 1a, 9b-dihydrocyclopropa[ℓ]phenanthrenes by direct bromination of the 1a, 9b-dihydro analogues (50 and 116) afforded ring opened products only. The ready availability of diarylcyclopropanes prompted an investigation of cyclodehydrogenation reactions as a route to 1a-substituted 1a, 9b-dihydrocyclopropa[ℓ]phenanthrenes. Using vanadium oxytrifluoride and boron trifluoride etherate a new and mild aryl-aryl coupling reaction for stilbenes bearing methoxyl substitution in only one of the aromatic rings has been elucidated. Applied to methyl 1, 2-bis (3, 4-dimethoxyphenyl) cyclopropane-l-carboxylate (155c), a dibenz[a,c]cycloheptatriene ester (171) is obtained which is presumed to arise by way of the desired dihydrocyclopropa[ℓ]phenanthrene (156c). A third potential route to cyclopropa[ℓ]phenanthrene derivatives would require aromatization of octahydrocyclopropa[ℓ]phenanthrenes (60) which are themselves unknown. Investigations into the synthesis of these latter cycloproparenes involving the double dehydrohalogenation of 1,14,15,15-tetrahalotetracyclo-[12,1,0,0,2,708,13]pentadec-7-enes (179) have been undertaken. Dehydrobromination of the 1, 14-dibromo-15, 15-difluoropentadecene (179d) afforded the corresponding benzaldehyde derivative (186) which is rationalized on the basis of an unusual debromination to give a difluorodecahydrocyclopropa[ℓ]phenanthrene (191). Attempts to synthesize this latter species provide further evidence for its intermediacy.