On the reactions of (vinylimino)phosphoranes and related compounds. Part 30. Short new synthesis of 5-azaazulene derivatives. Some comments on reactivities of (vinylimino)phosphoranes

Abstract

A short new synthesis of phenyl-substituted and annulated 5-azaazulene (cyclopenta[c]azepine) derivatives 15–18 consists of the reaction of [(1-phenylvinyl)imino]- and benz-annulated [(cycloalkenyl)imino]-phosphoranes 8–11 with 5-(dimethylaminomethylene)cyclopenta-1,3-dienecarbaldehyde 1 in an enamine alkylation (Michael addition) process, subsequent proton migration–ketonization, and condensation of the formyl group with the iminophosphorane moiety (aza-Wittig reaction). On the other hand, reactions of aldehyde 1 with (vinylimino)phosphoranes12–14, which have no phenyl group at the α-position relative to the nitrogen atom, consist of a intramolecular aza-Wittig reaction or a substitution reaction of aldehyde 1 with phosphoranes 12–14 and subsequent hydrolysis to afford 5-(aminomethylene)cyclopenta-1,3-dienecarbaldehyde 19 and its derivatives 20 and 21, respectively. In the context of selectivity observed in the reaction of phosphoranes 8–11 and 12–14 with aldehyde 1, respectively, MNDO calculations on compounds 1 and 12A, 12B as well as on model compounds 8C–12C were performed to gain insight via a theoretical interpretation based upon frontier molecular orbital theory (FMO): the former reaction, giving 5-azaazulene derivatives, would be an FMO-controlIed reaction, while the latter is a charge-controlled reaction. Several spectral and chemical properties of heterocycles of 16–18 are analysed.