A novel regiospecific intramolecular cycloaddition of an aromatic ring. Molecular orbital study of conjugated ketenes thermally generated from six-membered α-monocarbonyl azo-compounds

Abstract

When a methylene dichloride solution containing 4,4-diarylphthalazin-1(4H)-one(3) was warmed above –50 °C, nitrogen extrusion occurred to result in the formation of the quinonoid ketene (19). The quinonoid ketene thus generated underwent a facile and regiospecific intramolecular [_π4_a+_π2_a] cycloaddition with NN-dimethylaniline to afford a norcaradiene (22), followed by ring opening to an azulenone derivative (14). It was found, however, that this process takes place only with an aromatic ring bearing strong electron-donating substituents such as dialkylamino groups. FMO analyses based on the model system calculated by the CNDO/2 method showed that the dominant HOMO–LUMO interaction in the transition state should involve the HOMO of NN-dialkylaniline and LUMO of the quinonoid ketene. Thus, the in-plane π*-orbital induces the orthogonal approach of the two components favouring an antarafacial pathway and the cyclisation is initiated by preferential bond formation between the central carbon of the ketene moiety and the para-carbon of NN-dialkylaniline to result in the formation of (22).