Aromatic azapentalenes: 1H- and (mesoionic) 2H-pyrrolotetrazoles. Part 2. Reaction with electrophiles

Abstract

Protonation, acetylation, benzoylation, carbamoylation, formylation, bromination, azo coupling, nitrosation and addition to DMAD were studied. Monosubstitution occurred as a rule (bromination excepted), the preferred site of attack being C(5) if both the 5 and 7 positions were free. A number of observations point to a slightly higher reactivity of the mesoionic isomers 2; this is consistent with AM1 calculations. 1,3-Dipolar cycloaddition behaviour of 2 towards DMAD, a conceivable process, could not be detected; only linear addition was observed. Nitroso derivatives of the series 3, 4 and 8 were not isolated as such but as the ring-opened nitrile oxides 11 and 12; at elevated temperature analogous valence isomers arise also from the nitroso derivative 7e and the azo compounds 3e and 4e.