Copper-promoted methylene C-H oxidation to a ketone derivative by O₂

The methylene group of the ligand 1,2-di(pyridin-2-yl)-ethanone oxime (dpeo) is slowly oxygenated by the O₂ in air under ambient conditions when [Cu(dpeo)₂](ClO₄)₂ is dissolved in ethanol or acetonitrile. An initial transient ketone product, 2-(hydroxyimino)-1,2-di(pyridine-2-yl)ethanone, (hidpe) was characterized in the heteroleptic copper(ii) complex [Cu(bpca)(hidpe)](ClO₄). The co-ligand in this complex, N-(2'-pyridylcarbonyl)pyridine-2-carboximidate (bpca^-), is derived from a copper-promoted Beckmann rearrangement of hidpe. In the presence of bromide only [Cu(bpca)Br] is isolated. When significant water is present in reaction mixtures copper complexes of dpeo, hidpe and bpca^- are not recovered and [Cu(pic)₂H₂O] is isolated. This occurs since two equivalents of picolinate are ultimately generated from one equivalent of oxidized and hydrolysed dpeo. The copper-dependent O₂ activation and consequent stoichiometric dpeo C-H oxidation is reminiscent of the previously observed catalysis of dpeo oxidation by Mn(ii) [C. Deville, S. K. Padamati, J. Sundberg, V. McKee, W. R. Browne, C. J. McKenzie, Angew. Chem., Int. Ed., 2016, 55, 545-549]. By contrast dpeo oxidation is not observed during complexation reactions with other late transition metal(ii) ions (M = Fe, Co, Ni, Zn) under aerobic conditions. In these cases bis and tris complexes of bidentate dpeo are isolated in good yields. It is interesting to note that dpeo is not oxidised by H₂O₂ in the absence of Cu or Mn, suggesting that metal-based oxidants capable of C-H activation are produced from the dpeo-Cu/Mn systems and specifically O₂. The metastable copper complexes [Cu(dpeo)₂](ClO₄)₂ and [Cu(bpca)(hidpe)](ClO₄), along with [NiX₂(dpeo)₂] (X = Cl, Br), [Ni(dpeo)₃](ClO₄)₂, [Co(dpeo)₃](ClO₄)₃ and the mixed valence complex [Fe^IIIFe(dpeo-H)₃(dpeo)₃](PF₆)₄, have been structurally characterized.