S EAr–S NAr couplings of indolizines and related pyrrole derivatives with superelectrophilic nitrobenzoxadiazoles

Abstract

The nucleophilic aromatic substitutions of 7-chloro-4,6-dinitrobenzofurazan (DNBZ-Cl) and 7-chloro-4,6-dinitrobenzofuroxan (DNBF-Cl) with a series of differently substituted indolizines ( 5a– f) and a series of dihyropyrroloisoquinolines ( 11a– f) have been investigated. In accord with previous reports emphasizing the superelectrophilic character of these compounds in σ-complexation processes, DNBZ-Cl and DNBF-Cl react very readily and quantitatively with the weak carbon nucleophiles 5a– f and 11a– f at room temperature in acetonitrile. In the case of DNBZ-Cl, the resulting products ( 7Z, a– f and 12Z, a– f) are those expected from the displacement of the chlorine atom through a S EAr–S NAr mechanism. A significant result is that these compounds, despite the lack of coplanarity of the two rings, are characterized by an intense intramolecular charge transfer between the donor pyrrole-type moiety and the electron-deficient acceptor DNBZ moiety. Contrasting with this behaviour, the DNBF-Cl reactions show a totally different pattern, proceeding with loss of the N-oxide functionality and expansion of the pyrrole moiety to afford stable zwitterionic spiro adducts ( 8F, a– f and 13F, a– f) of a so far unknown type. Rapid NMR recordings have revealed that the formation of these adducts occurs after initial formation of the expected substitution products 7F, a– f and 12F, a– f. A mechanism accounting for the overall rearrangement leading to the spirobenzofurazan adducts is suggested. It is based on an initial nucleophilic attack of the oxygen atom of the N-oxide functionality at the electron-deficient and strongly olefinic C–C coupling bond generated by the aforementioned intramolecular charge transfer. This results in the formation of an unstable five-membered isoxazole ring whose decomposition goes along with loss of the N-oxide functionality and enlargement of the pyrrole moiety into a pyridine one. Also discussed are the factors accounting for the high thermodynamic stability of the spiro adducts, and their relevance to the stability of previously reported spiro adducts.