Oxygen activation by copper camphor complexes

Abstract

Cleavage of the carboximide CN bond promoted by reaction of camphor carboximide hydrochlorides (OC₁₀H₁₅CONHCH₂COOLi(H₂O)₂·2HCl (2) or OC₁₀H₁₅CONHCH(CH₂Ph)COOLi(H₂O)·HCl (3)) with CuCl₂ leads to the corresponding amino acid complexes ([Cu(H₂NCH₂COO)₂] (I) and [Cu{H₂NCH(CH₂Ph)COO}₂]·(H₂O) (II)), and the camphor carboxylate residue (OC₁₀H₁₅COO⁻) which is catalytically oxidized to camphorquinone by oxygen from air. The process is mediated by coordination of the camphor carboxylate to copper. The reaction of the camphor carboximide hydrochloride (OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)·HCl) (4) with CuCl₂ follows a different trend. In this case the CN camphor carboximide bond remains intact and the complexes [CuCl{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (III) and [{CuCl₂}₂{OC₁₀H₁₅CONH(CH₂)₂COOLi(H₂O)}] (IV) form. However, a redox process also occurs, since formation of III requires Cu(II) → Cu(I) reduction as confirmed by X-ray photoelectron spectroscopy and cyclic voltammetry. A related reduction process was identified in the formation of [CuCl(OC₁₀H₁₅COOLi)] (V) from CuCl₂ upon reaction with lithium camphor carboxylate (OC₁₀H₁₅COOLi) under nitrogen. The above results show that electron transfer is highly facilitated in the copper camphor carboximide/carboxylate system. Such an ability was used to activate oxygen from air and promote the oxidation of ethylacetoacetate to pyruvate using V as the catalyst.