On the reactions of (vinylimino)phosphoranes and related compounds. Part 28. Synthesis and chemical properties of dicyclohepta[b,d]pyrrole ring system

Abstract

A new synthesis of 6,7-dihydro-5H-dicyclohepta[b,d]pyrrole 10 consists of the reaction of cyclohepta-1,3,5-trienylimino(tributyl)phosphorane with tropone in an enamine alkylation process and subsequent condensation of the iminophosphorane moiety with a carbonyl function (aza-Wittig reaction). Hydride abstraction from 10 with Ph₃CBF₄ gives 5H-dicyclohepta[b,d]pyrrolium tetrafluoroborate 14, which cannot be converted into 5H-dicyclohepta[b,d]pyrrole 15. On the other hand, upon treatment with MnO₂, compound 10 is converted into 1H-dicyclohepta [b,d] pyrrole 16, 3H-dicyclohepta[b,d]pyrrol-3-one 17 and 1H-dicyclohepta[b,d]pyrrol-1-one 18. The chemical and spectral properties of 16, 17 and 18, together with their conversion into protonated or methylated compounds have been studied. Hydrogen abstraction from 16 with Ph₃CBF₄ gives the novel but unstable dicyclohepta[b,d]pyrrolium ion 4. It is shown that compound 4 absorbs at shorter wavelength than the cycloheptazulenium ion 1, its hydrocarbon analogue, because of the presence of an electronegative nitrogen atom; the spectrum exhibited fine structure, however, which resembled that of 1.