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 (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.