The Non-innocent Role of Spin Traps in Monitoring Radical Formation in Copper-Catalyzed Reactions
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Applied Magnetic Resonance
ORIGINAL PAPER
The Non‑innocent Role of Spin Traps in Monitoring Radical Formation in Copper‑Catalyzed Reactions Mohammad Samanipour1 · H. Y. Vincent Ching1 · Hans Sterckx2 · Bert U. W. Maes3 · Sabine Van Doorslaer1 Received: 23 July 2020 / Revised: 29 September 2020 / Accepted: 6 October 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract Spin traps, like 5,5-dimethyl-1-pyrroline N-oxide (DMPO), are commonly used to identify radicals formed in numerous chemical and biological systems, many of which contain metal-ion complexes. In this study, continuous wave electron paramagnetic resonance and hyperfine spectroscopy are used to prove the equatorial ligation of DMPO(-derived) molecules to Cu(II), even in the presence of competing nitrogen bases. The experimental data are corroborated with density functional theory calculations. It is shown that 14N HYSCORE can be used as a fingerprint method to reveal the coordination of DMPO(-derived) molecules to Cu(II), an interaction that might influence the outcome of spin-trapping experiments and consequently the conclusion drawn on the mechanism under study.
1 Introduction Copper is one of the most abundant transition metals on earth with a relatively low toxicity [1]. Its rich redox chemistry is exploited both by nature and by synthetic chemists to functionalize organic molecules [2–5]. The efficient way in which Cu-containing enzymes can activate O2 to perform selective oxidase and oxygenase reactions has inspired chemists in their search of Cu/O2 systems for selective oxidation of organic molecules [2–4]. In many of the copper-catalyzed Electronic supplementary material The online version of this article (https://doi.org/10.1007/s0072 3-020-01284-0) contains supplementary material, which is available to authorized users. * Sabine Van Doorslaer [email protected] 1
Department of Chemistry, BIMEF, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium
2
Janssen Pharmaceutica, Turnhoutseweg 30, 2340 Beerse, Belgium
3
Department of Chemistry, ORSY, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
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synthesis pathways of organic molecules, the formation of highly reactive, shortliving radical intermediates, such as reactive oxygen species (ROS, e.g., O 2·−, · OH, …) and carbon-centered radicals, is hypothesized to play crucial roles [2–4]. Electron paramagnetic resonance (EPR) is by far the most used analytical technique to study (intermediate) radical formation and to investigate paramagnetic redox states of transition metals, such as Cu(II) [6]. The detection of very reactive radicals with EPR can in some cases be done directly after rapid freezing of the sample, but is mostly performed indirectly via spin-trap EPR [7–9]. Spin trapping exploits the rapid reaction of very short-living radicals with diamagnetic spintrap molecules to form a more stable radical that can be easily detected by EPR. In most cases, nitrones
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