Redox Cascades and Making of a C–C Bond: 1,2-Benzodiazinyl Radicals and a Copper Complex of a Benzodiazine

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 ( Py Cn • ). 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 Py Cn • is a breakthrough. The Cn • /Cn – and Py Cn • / Py Cn – redox couples are reversible, and the reaction of Cn • with [Cu I (PPh 3 ) 3 Cl] in the presence of hydrazine hydrate and Et 3 N affords a Cn – complex of copper(I), [(Cn – )Cu I (PPh 3 ) 2 ] ( 1 ). Similar to phenalenyl radical, Py Cn • exists in three redox states, Py Cn +, Py Cn •, and Py Cn –, 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. Py Cn • 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 •, Py Cn •, and 1 ·1/2MeOH were confirmed by single crystal X-ray crystallography, EPR spectroscopy, and DFT calculations.