On the reaction of azulen-2-ylmethylene(triphenyl)phosphorane. Convenient preparation of azuleno[1,2-f]- and azuleno[1,2-a]-azulenes and their properties

Abstract

Novel azulen-2-ylmethyl(triphenyl)phosphonium bromide has been shown to be a synthon for a rapid new annulation leading to azulenoazulenes. The base treatment of the phosphonium bromide generating azulen-2-ylmethylenephosphorane 9 in situ and subsequent reaction with 5-(dimethylaminomethylene)cyclopenta-1,3-dienecarbaldehyde 10 and 2-chlorotropone 14 affords unsubstituted azuleno[1,2-f]- and azuleno[1,2-a]-azulenes 1 and 2, respectively. The reaction of 9 with 2-chloro-3,5,7-trideuteriotropone has also been carried out to elucidate the reaction paths leading to the formation of 2. The reaction paths involve the Michael-type addition of 9 onto 10 and 14, subsequent proton migration to regenerate the phosphorane moiety, intramolecular condensation of the formyl and carbonyl groups (Wittig reaction) and aromatization. In order to gain insight into the mechanism, PM3 calculations on compounds 9, 10 and 14 as well as on the related compound, (azulen-2-ylimino)tributylphosphorane, have been performed. The reactivity and site-selectivity of the annulation are discussed on the basis of frontier molecular orbital (FMO) theory. The electrophilic aromatic substitution of the azulenoazulenes as well as their spectroscopic and electrochemical properties have also been analyzed.