Journal of Organic Chemistry (2019)
Update date:2022-08-11
Topics:
Mondal, Sandip
Bera, Sachinath
Ghosh, Prasanta
Two 1,2-benzodiazinyl radicals, cinnolinyl radicals by name, were successfully isolated by cascade routes using 1,4-naphthoquinone as a precursor. Reaction of 1,4-naphthoquinone with hydrazine hydrate promotes a (5e + 5H+) redox cascade affording benzo[g]naphtho[1,2-c]cinnolinyl-7,12,14-trione (Cn·) in 69% yields, while the similar reaction with 2-hydrazinopyridine is a (7e + 7H+) oxidative cascade and furnishes N-pyridinecinnolinyl radical (PyCn·). The cascades are composed of C-N and C-C bond making reactions. The neutral even alternate arenes are always diamagnetic; thus, the isolation of Cn· and PyCn· is a breakthrough. The Cn·/Cn- and PyCn·/PyCn- redox couples are reversible, and the reaction of Cn· with [CuI(PPh3)3Cl] in the presence of hydrazine hydrate and Et3N affords a Cn- complex of copper(I), [(Cn-)CuI(PPh3)2] (1). Similar to phenalenyl radical, PyCn· exists in three redox states, PyCn+PyCn·, and PyCn-, in a smaller potential range (-0.30 V to -0.60 V vs Fc+/Fc couple) and can be used as an oxidant as well as a reductant. PyCn· acts as a catalyst for the oxidative cleavages of benzil to benzoic and 2,2′-pyridil to picolinic acids in methanol in the presence of air. The molecular and electronic structures of Cn·PyCn·, and 1·1/2MeOH were confirmed by single crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.
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