Cobalt Ion Promoted Redox Cascade: A Route to Spiro Oxazine-Oxazepine Derivatives and a Dinuclear Cobalt(III) Complex of an N-(1,4-Naphthoquinone)-o-aminophenol Derivative.

The study discloses that the redox activity of N-(1,4-naphthoquinone)-o-aminophenol derivatives (LRH2) containing a (phenol)-NH-(1,4-naphthoquinone) fragment is notably different from that of a (phenol)-NH-(phenol) precursor. The former is a platform for a redox cascade. LRH2 is redox noninnocent and exists in Cat-N-(1,4-naphthoquinone)(2-) (LR 2-) and SQ-N-(1,4-naphthoquinone) (LR •-) states in the complexes. Reactions of LRH2 with cobalt(II) salts in MeOH in air promote a cascade affording spiro oxazine-oxazepine derivatives (OXLR) in good yields, when R = H, Me, tBu. Spiro oxazine-oxazepine derivatives are bioactive, and such a molecule has so far not been isolated by a schematic route. In this context this cascade is significant. Dimerization of LRH2 → OXLR in MeOH is a (6H+ + 6e) oxidation reaction and is composed of formations of four covalent bonds and 6-exo-trig and 7-endo-trig cyclization based on C-O coupling reactions, where MeOH is the source of a proton and the ester function. It was established that the active cascade precursor is [(LMe •-)CoIIICl2] (A). Notably, formation of a spiro derivative was not detected in CH3CN and the reaction ends up furnishing A. The route of the reaction is tunable by R, when R = NO2, it is a (2e + 4H+) oxidation reaction affording a dinuclear LR 2- complex of cobalt(III) of the type [(LNO2 2-)2CoIII2(OMe)2(H2O)2] (1) in good yields. No cascade occurs with zinc(II) ion even in MeOH and produces a LMe •- complex of type [(LMe •-)ZnIICl2] (2). The intermediate A and 2 exhibit strong EPR signals at g = 2.008 and 1.999, confrming the existence of LMe •- coordinated to low-spin cobalt(III) and zinc(II) ions. The intermediates of LRH2 → OXLR conversion were analyzed by ESI mass spectrometry. The molecular geometries of OXLR and 1 were confirmed by X-ray crystallography, and the spectral features were elucidated by TD DFT calculations.