Abstract
Much recent attention has been focused on the structure and reactivity of transition-metal superoxide complexes, among which mononuclear copper(II)-superoxide complexes are recognized as key reactive intermediates in many biological and abiological dioxygen-activation processes. So far, several types of copper(II)-superoxide complexes have been developed and their electrophilic reactivity has been explored in C–H and O–H bond activation reactions. Here we demonstrate that a mononuclear copper(II)-(end-on)superoxide complex supported by a N-[(2-pyridyl)methyl]-1,5-diazacyclooctane tridentate ligand can induce catalytic C–C bond formation reaction between carbonyl compounds (substrate) and the solvent molecule (acetone), giving β-hydroxy-ketones (aldol). Kinetic and spectroscopic studies at low temperature as well as DFT calculation studies support a nucleophilic reactivity of the superoxide species toward the carbonyl compounds, providing new insights into the reactivity of transition-metal superoxide species not only in biological oxidation reactions but also in synthetic organic chemistry.
Highlights
Much recent attention has been focused on the structure and reactivity of transition-metal superoxide complexes, among which mononuclear copper(II)-superoxide complexes are recognized as key reactive intermediates in many biological and abiological dioxygenactivation processes
We examine the reactivity of another mononuclearsuperoxide complex 1 generated by using a similar N3-tridentate ligand LPym, which has a shorter pyridylmethyl sidearm (–CH2Py) instead of the pyridylethyl one (–CH2CH2Py) in LPye (Fig. 1)
Neither copper(I) complex supported by LPye nor the copper(II) complex of LPym was sufficient catalyst (Entries 6 and 7). These results unambiguously demonstrate that superoxide complex 1 is essential for the catalytic C–C bond formation reaction, and that simple Lewis acid catalysis by the copper(I/II) complexes can be ruled out
Summary
Much recent attention has been focused on the structure and reactivity of transition-metal superoxide complexes, among which mononuclear copper(II)-superoxide complexes are recognized as key reactive intermediates in many biological and abiological dioxygenactivation processes. We demonstrate that a mononuclear copper(II)-(end-on)superoxide complex supported by a N-[(2-pyridyl)methyl]-1,5-diazacyclooctane tridentate ligand can induce catalytic C–C bond formation reaction between carbonyl compounds (substrate) and the solvent molecule (acetone), giving β-hydroxy-ketones (aldol). Kinetic and spectroscopic studies at low temperature as well as DFT calculation studies support a nucleophilic reactivity of the superoxide species toward the carbonyl compounds, providing new insights into the reactivity of transition-metal superoxide species in biological oxidation reactions and in synthetic organic chemistry. We examine the reactivity of another mononuclear (end-on)superoxide complex 1 generated by using a similar N3-tridentate ligand LPym, which has a shorter pyridylmethyl sidearm (–CH2Py) instead of the pyridylethyl one (–CH2CH2Py) in LPye (Fig. 1) In this case, superoxide complex 1 exhibits completely different reactivity from that of 2 to induce catalytic C–C bond formation between carbonyl compounds (substrates) and the solvent molecule (acetone), giving βhydroxy-ketones (aldol). The present study demonstrates interesting synergistic effects of the superoxide ligand and Lewis acidic metal center for the catalytic C–C bond formation reaction, providing a new insight into the role of copper catalyst in synthetic organic chemistry
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