2,3-Dihydro-1H-benzo[c]pyrazolo[1,2-a]cinnolines and derived radical cations

Abstract

A series of 2,3-dihydro-1H-benzo[c]pyrazolo[1,2-a]cinnolines (3a–f) has been prepared via alkylation of the benzo[c]cinnolines (1a–f) with 1,3-dibromopropane and subsequent reduction of the formed benzo[c]pyrazolo[1,2-a]cinnolinium bromides (2a–f) with sodium borohydride. The dark-red intermediate in the synthesis, reported to have a 2,3-dihydrobenzo[c]pyrazolo[1,2-a]cinnolinium structure (6a), is shown to be (2a) contaminated with some radical cation (3a)^+·. The radical cations (3a–f)^+·, readily generated in the oxidation of (3a–f), have been studied using e.s.r. and ENDOR spectroscopy. Relative signs of the ¹H-coupling constants have been obtained by general triple resonance. This has led to full assignments in all cases.