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 (LH) 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. LH is redox noninnocent and exists in Cat-N-(1,4-naphthoquinone)(2-) (L) and SQ-N-(1,4-naphthoquinone) (L) states in the complexes. Reactions of LH with cobalt(II) salts in MeOH in air promote a cascade affording spiro oxazine-oxazepine derivatives (L) in good yields, when R = H, Me, Bu. 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 LH → L 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 [(L)CoCl] (A). Notably, formation of a spiro derivative was not detected in CHCN and the reaction ends up furnishing A. The route of the reaction is tunable by R, when R = NO, it is a (2e + 4H) oxidation reaction affording a dinuclear L complex of cobalt(III) of the type [(L)Co(OMe)(HO)] (1) in good yields. No cascade occurs with zinc(II) ion even in MeOH and produces a L complex of type [(L)ZnCl] (2). The intermediate A and 2 exhibit strong EPR signals at g = 2.008 and 1.999, confrming the existence of L coordinated to low-spin cobalt(III) and zinc(II) ions. The intermediates of LH → L conversion were analyzed by ESI mass spectrometry. The molecular geometries of L and 1 were confirmed by X-ray crystallography, and the spectral features were elucidated by TD DFT calculations.