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.