In nature, some enzymes have adopted the metal and oxygen to activate their catalytic cycle. The adopted metal and oxygen can be the metal-reactive oxygen species such as metal-superoxo, -hydroperoxo, -peroxo, and –oxo intermediates. The metal-reactive oxygen species have conducted the various functions depending on their own character. Among the metalloenzymes, peptidylglycine-α-hydroxylating monooxygenase (PHM) and dopamine-β monooxygenase (DβM) have been studied as representative copper-containing metalloenzymes. The mechanism of catalytic activation in PHM and DβM has been exhibited with the function of copper-superoxo, -hydroperoxo, and -oxyl radical species. Recently, lytic polysaccharide monooxygenases (LPMOs) have received much attention as enzymes, which directly oxidize the polysaccharide to the potential biomass. Although the accurate mechanism of LPMOs is still ambiguous, the study of mechanism and reactivity of LPMOs is in progress. Among the copper-oxygen species, especially, Cu(II)-hydroperoxo species has been believed in a crucial intermediate (Figure 1). It has been proposed that Cu(II)-hydroperoxo adducts were converted to Cu(II)-oxyl radical and water (in PHM and DβM) by an electron and proton transfer. In LPMOs, Cu(II)-hydroperoxo species also has been proposed that it reacts directly with the polysaccharide.
In synthetic chemistry, there are a lot of efforts to investigate the reactivity of copper(II)-hydroperoxo adducts. Numerous examples of electrophilic reactivity in Cu(II)-OOH complexes have been shown to occur via O-O bond cleaved intermediates. There is no report of direct reactions of Cu(II)-OOH species with external substrates. In this study, we observed nucleophilic reactivity of Cu(II)-hydroperoxo complex. The mononuclear end-on copper(II)-hydroperoxo complex, which has been successfully characterized by various physicochemical methods including UV-vis, rRaman, CSI-MS and EPR, is a very reactive oxidant in nucleophilic reactions. A positive Hammett ρ value (2.0(3)) is observed in the reaction of copper(II)-hydroperoxo complex with para-substituted acyl chlorides, which clearly indicates the nucleophilic character for the copper(II)-hydroperoxo complex. The copper(II)-hydroperoxo complex is an especially reactive oxidant in aldehyde deformylation with 2-PPA and CCA relative to the other metal-bound reactive oxygen species reported so far. The observation of nucleophilic reactivity for a copper(II)-hydroperoxo species significantly expands the known chemistry of metal-reactive oxygen species.
As described above, we succeed in verification of the nucleophilic reactivity of Cu(II)-OOH species clearly. However, we were faced with some obstructions. The first problem is stability of Cu(II)-hydroperoxo complex. The exposure of the moment at higher temperature causes the decomposition of Cu(II)-OOH species. The control of thermally unstable Cu(II)-hydroperoxo adduct is no easy matter. So we adopted cryo-system to prevent the thermal decomposition of Cu(II)-hydroperoxo complex. By using cold spray ionization-mass and low temperature rRaman with cold bath, which is filled with ethanol, we successfully characterized the copper(II)-hydroperoxo species. Second thing is that why only this Cu(II)-hydroperoxo complex shows nucleophilic reactivity? Actually, we couldn’t suggest the reason. In the future, we will investigate the computational study to give the answer about nucleophilic reactivity of Cu(II)-hydroperoxo complex.
- Nucleophilic reactivity of a copper(II)-hydroperoxo complex. Kim, B., Jeong, D., Ohta, T., Cho, J. Commun. Chem. 2019, 81. https://doi.org/10.1038/s42004-019-0187-3