Synthesis and stability of 2-methyl-2,4-diaza- and 2-methyl-2,5-diaza-indene (2-methyl-pyrrolo[3,4-b]pyridine and -pyrrolo[3,4-c]pyridine)

Abstract

2-Methyl-2,4-diaza-(2) and 2-methyl-2,5-diaza-(3) indenes have been prepared by oxidation of 2,3-dihydro-2-methyl-1H-2,4-diaza-(10) and 2,3-dihydro-2-methyl-1 H-2,5- diaza-(14) indene, respectively with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone., They are more stable than 2-methyl-2-azaindene (1) in dilute acid, as neutral species in aqueous solution, and to aerial oxidation, and form stable picrates. The two diazaindenes (2) and (3) are relatively strong bases; and although protonation occurs predominantly on N-4 and N-5, respectively, to form resonance-stabilised cations, the ease of exchange of H-1 and H-3 (the former being greater) by deuterium in dilute acid, suggests that some protonation also takes place on C-1 and C-3, separately, as in 2-methyl-2-azaindene. The n.m.r. chemical shifts of H-1 and H-3 in the three azaindenes (1)–(3) indicate that they are aromatic systems.