Operando X-ray absorption and EPR evidence for a single electron redox process in copper catalysis

Article abstract of DOI:10.1039/c5sc00807g

An unprecedented single electron redox process in copper catalysis is confirmed using operando X-ray absorption and EPR spectroscopies. The oxidation state of the copper species in the interaction between Cu(ii) and a sulfinic acid at room temperature, and the accurate characterization of the formed Cu(i) are clearly shown using operando X-ray absorption and EPR evidence. Further investigation of anion effects on Cu(ii) discloses that bromine ions can dramatically increase the rate of the redox process. Moreover, it is proven that the sulfinic acids are converted into sulfonyl radicals, which can be trapped by 2-arylacrylic acids and various valuable β-keto sulfones are synthesized with good to excellent yields under mild conditions.

Full text of DOI:10.1039/c5sc00807g

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Operando X-ray absorption and EPR evidence for a
single electron redox process in copper catalysis†
Cite this: Chem. Sci., 2015, 6, 4851
Qingquan Lu,‡a Jian Zhang,‡a Pan Peng,a Guanghui Zhang,ac Zhiliang Huang,ac
abc
Hong Yi,a Jeffrey T. Millercd and Aiwen Lei
*
An unprecedented single electron redox process in copper catalysis is confirmed using operando X-ray
absorption and EPR spectroscopies. The oxidation state of the copper species in the interaction between
Cu(II) and a sulfinic acid at room temperature, and the accurate characterization of the formed Cu(I) are
clearly shown using operando X-ray absorption and EPR evidence. Further investigation of anion effects
on Cu(II) discloses that bromine ions can dramatically increase the rate of the redox process. Moreover, it
is proven that the sulfinic acids are converted into sulfonyl radicals, which can be trapped by 2-
arylacrylic acids and various valuable b-keto sulfones are synthesized with good to excellent yields under
mild conditions.
Received 5th March 2015
Accepted 26th May 2015
DOI: 10.1039/c5sc00807g
www.rsc.org/chemicalscience
Copper is an essential trace element for nearly all aerobic example, oxidation of copper(I) to copper(II), has been exten-
organisms. Multi-copper oxidases (MCOs), for example, are sively investigated for more than one century, particularly the
encoded in genomes and play a critical role in the physiology of oxidation of copper(I) by molecular oxygen due to its impor-
essentially all aerobes, including metal metabolism and O2- tance in chemistry and biology.7 While Cu(I) oxidation has been
reduction.1 Copper has also been extensively used as a catalyst well studied, the reductive half reaction and factors that inu-
for the synthesis of organic and inorganic compounds,2 poly- ence the Cu(II) reducibility are less well understood and remain
mers,3 biomaterials4 etc. Although copper-assisted chemistry rudimentary,1,8 which has been the main barrier for the devel-
has many important applications and is widely researched, the opment of new catalytic processes. Herein, we communicate a
majority of studies are devoted to methodology development, direct observation of the reduction of Cu(II) by a sulnic acid at
rather than a detailed mechanistic understanding of the cata- room temperature. A single electron transfer from a sulnic
lyst structure, the oxidation states and the effect of ligands on acid to Cu(II) and the formation of Cu(I) are demonstrated using
rate and selectivity. The lack of fundamental understanding has X-ray absorption and electron paramagnetic resonance
limited further developments.
spectroscopies.
Catalysis is the key technology for organic synthesis, and
The reaction between benzenesulnic acid and Cu(II) under
insight into the nature of the catalytic species has become an inert atmosphere was initially monitored by electron para-
particularly important in chemical manufacture.5 Until now, magnetic resonance (EPR). As shown in Fig. 1, upon addition of
unambiguous identication of the oxidation states and struc- excess benzenesulnic acid to a DMF solution of CuBr2, the EPR
tures of the catalytic species under realistic conditions was a signal of Cu(II) disappears rapidly at room temperature, sug-
major obstacle for mechanistic studies.6 As a result, redox states gesting that diamagnetic copper(I) has formed.9 This interesting
of +1/+3, +1/+2 and 0/+2, have been proposed for Cu catalyzed phenomenon spurred us to investigate what really happened in
reactions, oen with limited experimental evidence.2 As is well the process.
known, the active metal undergoes oxidation and reduction half
To identify the variation of the valence states of copper in the
reactions in a catalytic cycle. Oxidation of low valent copper, for solution, this reaction was also studied using X-ray absorption
near-edge structure (XANES) spectroscopy. Initially, the XANES
spectrum of the DMF solution of CuBr2 gave a spectrum with a
small pre-edge energy peak at 8977.3 eV, which is typical of
Cu(II) compounds (Fig. 2, blue line).8c,10 However, when benze-
aCollege of Chemistry and Molecular Sciences, The Institute for Advanced Studies (IAS),
Wuhan University, Wuhan 430072, Hubei, P. R. China. E-mail: aiwenlei@whu.edu.cn
bNational Research Center for Carbohydrate Synthesis, Jiangxi Normal University,
nesulnic acid was added at room temperature, a new Cu
Nanchang 330022, Jiangxi, P. R. China
cChemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. species with an edge energy of 8981.3 eV was observed (Fig. 2,
Cass Ave, Argonne, IL 60439, USA
dDepartment of Chemical Engineering, Purdue University, W. Lafayette, IN 47907, USA
red line), conrming that Cu(II) was reduced to Cu(I) during the
reaction.8c,10 These results provided direct evidence that the
sulnic acid reduces Cu(II) through a single electron redox
process.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5sc00807g
‡ These authors contributed equally to this work.
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DMF, or low-polar solvents including MeCN and THF at room
temperature (Fig. 4A). While the solvent has little effect on the
reducibility, the type of anion is very important. For example,
Cu(OAc)2 shows little reduction at room temperature (Fig. 4B,
red line), and reduces only slightly at 80 ꢁC (Fig. 4B, green line).
Addition of a stoichiometric amount of LiBr, however, leads to
rapid reduction of Cu(OAc)2 with benzenesulnic acid even at
room temperature (Fig. 4B, blue line, edge energy is 8981.6
eV).8c,10
The inuence of Brꢀ ions on the reducibility of Cu(OAc)2 was
also investigated using EPR spectroscopy. As shown in Fig. 5,
Cu(OAc)2 exhibits a very weak EPR signal in DMF, which is
consistent with being present as an EPR-silent paddlewheel
dimer.9 However, once LiBr was added to the Cu(OAc)2 solution,
a strong EPR signal was observed, which increased in intensity
with increasing amounts of LiBr. The addition of LiBr appears
to dissociate Cu(OAc)2 into mononuclear Cu(II) species.9,11 More
importantly, upon addition of benzenesulnic acid to the
solution of Cu(OAc)2 and LiBr, the Cu(II) EPR signal disappeared
immediately and as shown by the XANES spectrum in Fig. 4B,
Cu(I) is formed concurrently. These results suggest that coor-
dinative saturation of Cu(OAc)2 and the inability for substrate
coordination may be the obstacle to Cu(II) reduction by benze-
nesulnic acid.
Fig. 1 Blue line: CuBr2 (0.4 mmol) in DMF (4.0 mL) at r.t. under N2; red
line: CuBr2 (0.4 mmol) and benzenesulfinic acid 2a (0.80 mmol) in
DMF (4.0 mL) kept at r.t. for 5 min under N2.
Based on the above results, it appears likely that the sulnic
acid serves as the one electron donor for reduction of Cu(II) to
Cu(I) with formation of oxidized sulfonyl radicals. If true, it is
Fig. 2 Blue line: CuBr2 (0.4 mmol) in DMF (4.0 mL) at r.t. under N2; red
line: CuBr2 (0.4 mmol) and benzenesulfinic acid (0.80 mmol) in DMF
(4.0 mL) kept at r.t. for 5 min under N2.
Furthermore, in order to determine the number and types of
ligands on Cu(I), an EXAFS spectrum of the Cu(I) species was
next tted. The k3-weighted R-space EXAFS spectrum shows that
the CuI center was coordinated to two Br ligands with an average
˚
distance of 2.24 A (Fig. 3). The Cu(I) species, therefore, was
assigned as diBr anion complex [CuIBr2]ꢀ1
.
XANES spectra was also applied to explore the inuence of
different factors which affect the reduction of Cu(II). It was
found that the choice of solvent has only a slight inuence, and
that the reduction of CuBr2 proceeds in polar solvents, such as
Fig. 4 (A) CuBr2 (0.4 mmol) and benzenesulfinic acid (0.80 mmol) in
different solvents (4.0 mL) kept at r.t. for 5 min under N2; green line:
MeCN; red line: THF; blue line: DMF. (B) Cu(OAc)2 (0.4 mmol) and
Fig. 3 k3-Weighted R-space EXAFS spectrum of the solution of the benzenesulfinic acid (0.80 mmol) in DMF (4.0 mL) reacted under
reaction of CuBr2 (0.4 mmol) and benzenesulfinic acid (0.80 mmol) in different conditions; red line: r.t.; green line: 80 ꢁC (the precipitate of
DMF (k-range: 2.93–13.67 A˚ꢀ1; R-range: 1.40–2.50 A˚).
the reaction); blue line: LiBr (1.2 mmol) was added at r.t.
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smoothly with 2-phenylarcylic acid, affording the correspond-
ing b-keto sulfones 3ab–3ah in good to excellent yields. Alkyl
sulnic acids, as exemplied by methyl sulnic acid, also effi-
ciently reacted with 1a and the product 3ai was obtained in 61%
yield. Besides, the arylacrylic acids, with either electron-rich or
electron-poor substituents on the aromatic ring, were all
compatible with this transformation, leading to the corre-
sponding decarboxylative products in good yields (3ba–3ha).
Non-terminal arylacrylic acid derivatives, such as a-ethylidene
benzeneacetic acid, exhibited relatively low reactivity and the
product 3ia was afforded in moderate yield, presumably due to
the steric hindrance from the pyramidal structure of the
sulfonyl radical.12 Arylacrylic acids containing chloro, bromo
and methyl groups also reacted effectively with p-toluene-
sulnic acid, offering the expected products 3cb–3eb in high
yields (81–88%). Furthermore, 18O labeling experiments indi-
cated that the carbonyl oxygen in the b-keto sulfones comes
from dioxygen (for details, see ESI†). To the best of our knowl-
edge, this is the rst example of the combination of S–H bond
alkylation, C–C s bond cleavage, and aerobic oxygenation in a
single step.13 These results not only serve as indirect evidence to
support the formation of the sulfonyl radical in situ, but also
provide a useful insight into the mechanism of Cu(II) reduction
and further synthetic applications.
Fig. 5 LiBr (0.4 mmol) ꢂ 3 and PhSO2H (0.80 mmol) were succes-
sively added to the DMF (4.0 mL) solution of Cu(OAc)2 (0.4 mmol) at r.t.
under N2.
reasonable to expect that a catalytic redox process could be used
for the synthesis of ne chemicals with suitable radical accep-
tors. When 2-arylacrylic acid was reacted with CuBr2/sulnic
acid, the product b-keto sulfones 3 could be isolated in good to
excellent yields at room temperature under air (Scheme 1).
As shown in Scheme 1, a variety of aryl sulnic acids, bearing
either electron-donating groups (R ¼ OMe, Me) or electron-
withdrawing groups (R ¼ F, Cl, Br) on the aryl ring, reacted
Conclusions
In conclusion, X-ray absorption and electron paramagnetic
resonance spectroscopies provide direct evidence for the single
electron redox process between sulnic acids and Cu(II), form-
ing Cu(I) and a sulfonyl radical at room temperature. Addition
of bromide ions increases the rate of Cu(II) reduction. Based on
these observations, catalytic oxysulfonylation of arylacrylic
acids under simple and mild conditions was developed for the
rst time. Ongoing research including further mechanistic
details and expanding the substrate scope is currently
underway.
Acknowledgements
This work was supported by the 973 Program (2012CB725302),
the National Natural Science Foundation of China (21390400,
21025206, 21272180, and 21302148), the Research Fund for the
Doctoral Program of Higher Education of China
(20120141130002) and the Ministry of Science and Technology
of China (2012YQ120060). The Program of Introducing Talents
of Discipline to Universities of China (111 Program) is also
appreciated. Use of the Advanced Photon Source was supported
by the U. S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under contract no. DE-AC02-06CH11357.
MRCAT operations are supported by the Department of Energy
and the MRCAT member institutions.
Scheme 1 Unless otherwise specified, all reactions were carried out
using 1 (0.20 mmol), 2 (0.60 mmol), and CuBr2 (0.02 mmol) in DMF
(2.0 mL) under air for 2 h at room temperature. Isolated yields shown.
aMethanesulfinic acid sodium salt (0.6 mmol) and TsOH (0.6 mmol)
were employed.
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3525.
4854 | Chem. Sci., 2015, 6, 4851–4854
This journal is © The Royal Society of Chemistry 2015