Exploring cyclopropane-heterocumulene [3 + 2] intramolecular cycloadditions on ortho-benzylydene scaffolds.

It is well known that heterocumulene functions are prone to participate in pericyclic processes, mainly in cycloaddition reactions. In a wide variety of such reactions, the heterocumulenic function usually acts as the two-atom component involving one of its two cumulated double bonds.   In the course of our recent research on the reactivity of heterocumulenes we have reported that ortho-(azidomethyl)phenyl carbodiimides undergo formal [3+2] intramolecular cycloaddition reactions by heating in solution to give tetrazolo[5,1-b]quinazolines. In these reactions the carbodiimide contributed with one N=C bond to the new five-membered tetrazole ring, whereas the remaining three nitrogen atoms came from the azide function. We reasoned that similar [3+2] cycloadditions could occur in related heterocumulenic compounds in which the three-atom azido component of those processes is replaced by a cyclopropane ring, thus forming a new pyrrolidine ring.  To this end we selected ortho-azidobenzylydenecyclopropane as the optimal starting material for opening the access to a variety of heterocumulenic functions via aza-Wittig reactions of its phosphazene derivative, which in turn could be prepared by the Staudinger imination reaction of triphenylphosphane with that azide.   In this communication we will disclose the successful building of the designed cyclopropane-heterocumulenes (isothiocyanates, ketenimines and carbodiimides) and the results obtained in our attempts of thermally inducing the respective intramolecular [3+2] cycloaddition reactions, that only in some instances occurred as planned.