A Redox-Active Cascade Precursor: Isolation of a Zwitterionic Triphenylphosphonio-Hydrazyl Radical and an Indazolo-Indazole Derivative.

Abstract

A redox-active [ML] unit (M = CoII and MnII; LH2 = N'-(1,4-dioxo-1,4-dihydronaphthalen-2-yl)benzohydrazide) defined as a cascade precursor that undergoes a multicomponent redox reaction comprising of a C-N bond formation, tautomerization, oxidation, C-C coupling, demetalation, and affording 6,14-dibenzoylbenzo[f]benzo[5,6]indazolo[3a,3-c]indazole-5,8,13,16-tetraone (IndL2) is reported. Conversion of LH2 → IndL2 in air is overall a (6H++6e) oxidation reaction, and it opens a route for the syntheses of bioactive diarylindazolo[3a,3-c]indazole derivatives. The reaction occurs via a radical coupling reaction, and the radical intermediate was isolated as a triphenylphosphonio adduct. In presence of PPh3 the [ML] unit promotes a reaction that involves a C-P bond formation, tautomerization, and oxidation to yield a stable zwitterionic triphenylphosphonio-hydrazyl radical (PPh3L±•). Conversion of LH2 → PPh3L±• is a (3H++3e) oxidation reaction. To authenticate the [ML] unit, in addition to the IndL2, a zinc(II) complex, [(L3)ZnII(H2O)Cl]·2MeOH (1·2MeOH), was successfully isolated (L3H = a pyridazine derivative of 1,4 naphthoquinone) from a reaction of LH2 with hydrated ZnCl2. Conversion of 3LH2 → 1 is also a multicomponent (6H++6e) oxidation reaction promoted by zinc(II) ion via a radical intermediate. Facile oxidation of [L2-] to [L•-] that was considered as an intermediate of these conversions was confirmed by isolating a 1,4 naphthoquinone-benzhydrazyl radical (LH•) complex, [(LH•)ZnII(H2O)Cl2] (2H•). The intermediates of LH2 → IndL2, LH2 → PPh3L±•, and 3LH2 → 1 conversions were analyzed by electrospray ionization mass spectroscopy. The molecular and electronic structures of PPh3L±•, IndL2, 1·2MeOH, and 2H• were confirmed by single-crystal X-ray crystallography, electron paramagnetic resonance spectroscopy, and density functional theory calculations.