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

Abstract

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.