Evidence of CuI/CuII Redox Process by X-ray Absorption and EPR Spectroscopy: Direct Synthesis of Dihydrofurans from β-Ketocarbonyl Derivatives and Olefins

Article abstract of DOI:10.1002/chem.201503822

The Cu^I/Cu^II and Cu^I/Cu^III catalytic cycles have been subject to intense debate in the field of copper-catalyzed oxidative coupling reactions. A mechanistic study on the Cu^I/Cu^II redox proc

Full text of DOI:10.1002/chem.201503822

DOI: 10.1002/chem.201503822
Communication
&
Copper Catalysis |Hot Paper|
Evidence of CuI/CuII Redox Process by X-ray Absorption and EPR
Spectroscopy: Direct Synthesis of Dihydrofurans from
b-Ketocarbonyl Derivatives and Olefins
Hong Yi,[a] Zhixiong Liao,[a] Guanghui Zhang,[a] Guoting Zhang,[a] Chao Fan,[a] Xu Zhang,[a]
Emilio E. Bunel,[c] Chih-Wen Pao,[d] Jyh-Fu Lee,[d] and Aiwen Lei*[a, b]
This is partially due to the fact that copper precursors are used
Abstract: The CuI/CuII and CuI/CuIII catalytic cycles have
in various available oxidation states and the metal center of
copper complex can participate in either two-electron or
single-electron processes, sometimes both in the same catalyt-
ic cycle.[3] The CuI/CuIII catalytic cycle has been widely proposed
in many copper-mediated coupling reactions. Some progress
has been achieved in that several CuIII complexes have been
isolated, either with the help of very special ligands or under
harsh conditions.[4] Although there is no solid evidence to
show the presence of CuIII species under live and realistic cata-
lytic conditions, it provides for the possibility of this mecha-
nism in copper catalysis.
been subject to intense debate in the field of copper-cata-
lyzed oxidative coupling reactions. A mechanistic study on
the CuI/CuII redox process, by X-ray absorption (XAS) and
electron paramagnetic resonance (EPR) spectroscopies,
has elucidated the reduction mechanism of CuII to CuI by
1,3-diketone and detailed investigation revealed that the
halide ion is important for the reduction process. The oxi-
dative nature of the thereby-formed CuI has also been
studied by XAS and EPR spectroscopy. This mechanistic in-
formation is applicable to the copper-catalyzed oxidative
cyclization of b-ketocarbonyl derivatives to dihydrofurans.
This protocol provides an ideal route to highly substituted
dihydrofuran rings from easily available 1,3-dicarbonyls
and olefins.
Recently, copper-catalyzed oxidative coupling reactions have
been extensively developed, in which copper salts are often
proposed to serve as the one-electron oxidant to promote the
single-electron transfer process.[5] Although several methodolo-
gies have been reported, the single-electron redox between
the C/XÀH compound and the copper salts has less been ex-
plored. Jutand and co-workers investigated CuII precursor sup-
ported by phenanthroline ligands, which could be reduced by
alcohols or amines to generate the active CuI species, moni-
tored by UV/Vis and NMR spectroscopies.[6] Recently, we used
X-ray absorption spectroscopy (XAS) and electron paramagnet-
ic resonance (EPR) spectroscopy to obtain information on the
reduction of CuII to CuI species by alkynes in the presence of
tetramethylethylenediamine (TMEDA).[7] XAS can provide direct
information on the oxidation state and the coordination envi-
ronment of the metal in solution.[8] Therefore, XAS can serve as
a unique and powerful technique for probing the structures of
reaction intermediates in homogeneous catalysis. Herein, we
report a study on CuI/CuII redox mechanism by XAS and EPR
spectroscopies. In addition, we have also used this mechanistic
information to realize a copper-catalyzed oxidative cyclization
of b-ketocarbonyl derivatives to dihydrofurans. This protocol
provides an ideal route to highly substituted dihydrofuran
rings from easily available 1,3-dicarbonyls and olefins under
non-acidic conditions.
Copper has been widely used as the catalyst in chemical syn-
thesis. A wide variety of copper-catalyzed reactions, including
Glaser–Hay and Ullmann–Goldberg couplings, have been em-
ployed as powerful tools for the preparation of biological and
pharmaceutical active compounds.[1] This versatility in combi-
nation with low toxicity and cost make copper among the
most promising transition metal in homogeneous catalysis.
However, detailed mechanistic understanding of these homo-
geneous copper-catalyzed reactions is still incomplete.[1a,d,2]
[a] H. Yi, Z. Liao, G. Zhang, G. Zhang, C. Fan, X. Zhang, Prof. Dr. A. Lei
College of Chemistry and Molecular Sciences
the Institute for Advanced Studies (IAS), Wuhan University
Wuhan, Hubei 430072 (P. R. China)
E-mail: aiwenlei@whu.edu.cn
Homepage: http://aiwenlei.whu.edu.cn/Main_Website/
[b] Prof. Dr. A. Lei
National Research Center for Carbohydrate Synthesis
Jiangxi Normal University
Nanchang 330022, Jiangxi, (P. R. China)
b-Diketone derivatives have been widely used as ligands in
copper-mediated coupling reactions.[9] Revealing the mecha-
nism of the interaction between the Cu center and the b-dike-
tone ligand will help to better define the potential limits of
catalytic reactions.[10] Initially, we commenced our study by in-
vestigating the interaction between CuX2 (X=Br, Cl) and
acetylacetone under nitrogen atmosphere by XAS (Figure 1).
The X-ray absorption near edge structure (XANES) spectrum of
[c] Prof. Dr. E. E. Bunel
Chemical Sciences and Engineering Division
Argonne National Laboratory
9700 S. Cass Ave. Argonne, IL 60439 (USA)
[d] Dr. C.-W. Pao, Prof. Dr. J.-F. Lee
National Synchrotron Radiation Research Center
Hsinchu 30076 (Taiwan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503822.
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duced to the CuI species (for details, see the Supporting Infor-
mation), indicating that the CuII precursor can be reduced to
CuI species by acetylacetone in DMF. Extended X-ray absorp-
tion fine structure (EXAFS) spectra were also taken to probe
the local structures of the Cu species (Figure 1b). The EXAFS
spectrum of the reaction between CuCl2 and acetylacetone is
very different from that of the reaction between CuBr2 and
acetylacetone, which suggests coordination of the halide ion
to the CuI complex. Fitting of the R-space EXAFS spectrum re-
vealed that the obtained CuI species had two coordinating Cl
atoms (Figure 1c). The CuÀCl bond length was determined as
2.09 Š. The structure of the obtained CuI species was thus as-
signed as a [CuCl2]À ate complex.
X-ray absorption spectroscopy was also applied to explore
the effect of the halide on the reduction of CuII species.[11]
When mixing Cu(OAc)2 and acetylacetone under the standard
conditions, a CuII species was detected with a pre-edge at
8978.0 eV, indicating that the Cu(OAc)2 cannot be reduced to
CuI complex by acetylacetone (Figure 2, black line). After the
Figure 2. XANES spectra; black line: Cu(OAc)2 (0.5 mmol) and acetylacetone
(1.0 mmol) in DMF (5.0 mL) at 1008C; light gray line: Cu(OAc)2 (0.5 mmol),
acetylacetone (1.0 mmol) and LiCl (1.0 mmol) in DMF (5.0 mL) at 1008C; dark
gray line: Cu(OAc)2 (0.5 mmol) and LiCl (1.0 mmol) in DMF (5.0 mL) at 1008C.
addition of two equivalents of LiCl to the system, a CuI species
was formed with an edge energy of 8981.8 eV, which revealed
that ClÀ can promote the reduction of Cu(OAc)2 to CuI species
(Figure 2, light gray line).To further confirm this reduction pro-
cess, a blank experiment, just mixing Cu(OAc)2 and LiCl, was
carried out, resulting in a CuII species with a pre-edge energy
of 8976.6 eV. These results indicate that Cu(OAc)2 cannot be re-
duced by LiCl. Therefore, the halide anion plays an important
role in the reduction of CuII to CuI by acetylacetone.
Figure 1. X-ray absorption spectra of various Cu species: a) XANES spectra;
black line: CuCl2 (0.5 mmol) and acetylacetone (1.0 mmol) in DMF (5.0 mL) at
1008C; light gray line: CuBr2 (0.5 mmol) and acetylacetone (1.0 mmol) in
DMF (5.0 mL) at 1008C; dark gray line: CuCl2 (0.5 mmol) in DMF (5.0 mL) at
RT. b) EXAFS spectra; black line: CuCl2 (0.5 mmol) and acetylacetone
(1.0 mmol) in DMF (5.0 mL) at 1008C; light gray line: CuBr2 (0.5 mmol) and
acetylacetone (1.0 mmol) in DMF (5.0 mL) at 1008C; dark gray line: CuCl2
(0.5 mmol) in DMF (5.0 mL) at RT. c) k2-weighted Fourier transform magni-
tudes of the R-space EXAFS spectrum. FT=Fourier transform; CN=coordina-
tion number; d=bond length; Ds2 =Debye–Waller factor; E0 =threshold
energy (2.96 ŠÀ1 <k<14.11 ŠÀ1; 1.12 Š<R<1.99 Š).
The above results suggest that acetylacetone can serve as
the electron donor to transfer one electron to CuII, furnishing
the reduced CuI complex. To make this process catalytic, an
extra oxidant is needed to oxidize the CuI species. To prove
our hypothesis, we used di-tert-butylperoxide (DTBP) as the ox-
idant to reoxidize the CuI species and monitored the reaction
by XAFS and EPR spectroscopies (Figure 3). The XANES spectra
(Figure 3a) indicate that the copper salts from the CuCl and
CuCl2 precursors react with DTBP in the presence of acetylace-
tone to generate a mixture of CuI and CuII species. Based on
further EXAFS spectroscopic analysis (Figure 3b), the coordina-
tion environments of the generated Cu species from CuCl and
the DMF solution of CuCl2 gave a pre-edge at 8977.0 eV (Fig-
ure 1a, dark gray line). After the addition of acetylacetone at
1008C, a new Cu species was detected with an edge energy of
8981.6 eV (Figure 1a, black line). This edge energy is very close
to that of the CuCl standard sample (8982.0 eV), confirming
that a CuI species was generated during this process. When
mixing CuBr2 and acetylacetone in DMF at 1008C, a copper
species with the same edge energy was detected (Figure 1a,
light gray line). Interestingly, when performing the experiment
in DMF as solvent at 1008C, a very small amount of CuII was re-
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process, we have successfully realized a copper-catalyzed oxi-
dative cyclization of b-ketocarbonyl derivatives to dihydrofur-
ans, utilizing DTBP as the oxidant (Scheme 1). This protocol
provides an convenient route to highly substituted dihydrofur-
an rings from easily available 1,3-dicarbonyls and olefins.
Figure 3. a) XANES spectra; b) EXAFS spectra; c) EPR spectra (X band,
9.4 GHz, 160 K); black line: CuCl2 (0.5 mmol), DTBP (1.0 mmol) and acetylace-
tone (1.0 mmol) in DMF (5.0 mL) at 1008C; gray line: CuCl (0.5 mmol), DTBP
(1.0 mmol) and acetylacetone (1.0 mmol) in DMF (5.0 mL) at 1008C.
Scheme 1. Copper-catalyzed oxidative cyclization reactions of various 1,3-di-
carbonyl compounds 2 with 1. Reaction conditions (unless otherwise
stated): 1 (0.80 mmol), 2 (0.50 mmol), CuCl2 (0.05 mmol) and DTBP
(1.0 mmol) in CH3CN (2.0 mL) at 808C for 28 h. Yields refer to isolated prod-
ucts. [a] 1 (0.50 mmol), 2 (1.0 mmol), CuCl2 (0.05 mmol); [b] 1 (1.0 mmol), 2
(0.50 mmol), CuCl2 (0.10 mmol)
CuCl2 are similar. Furthermore, EPR spectroscopy was also used
to monitor this oxidation process (Figure 3c). The same CuII
species was formed from the reaction of CuCl or CuCl2 with
acetyl acetone and DTBP. Therefore, we have established that
the CuX2 could be reduced to the CuI species by acetylacetone.
The XAS and EPR results also indicate that the CuI species
could be oxidized by DTBP in the presence of acetylacetone.
During the last decade, radical addition of 1,3-dicarbonyl
compounds to alkenes to synthesize dihydrofurans, a useful or-
ganic building block,[12] mediated by metal salts (MnIII, CeIV, CoII,
and VV) has received considerable attention in organic synthe-
sis.[13] Although some remarkable progresses have been made,
the required loading, cost, and toxicity impact of these metal
salts, as well as the fact that most need more than fourfold
excess alkene in acid solvent, have hindered its further applica-
tion. Based on the mechanistic information on CuI/CuII redox
We started our evaluation of the reaction parameters with
1,1-diphenylethylene (1a) and ethyl acetoacetate (2a) as stan-
dard substrates (see the Supporting Information, Table S2).
After exploring a wide array of conditions, we found that the
oxidative cyclization product 3a was obtained in excellent
yield in the presence of 10 mol% CuCl2, 2 equivalents of DTBP,
and CH3CN as the solvent at 808C (see the Supporting Infor-
mation, Table S2, entry 13). With the optimal reaction condi-
tions established, we tested various substituted 1,3-dicarbonyl
compounds 2, combined with 1a (Scheme 1). Almost all of the
b-keto esters were effective under the standard conditions. As
shown in Scheme 1, ethyl, methyl, and n-butyl acetoacetates
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all reacted smoothly, affording the desired products 3a–c in
good to excellent yields. Ethyl benzoylacetate was also suitable
for this reaction (Scheme 1, 3d). In addition, b-diketones could
be readily introduced in the reaction, providing the corre-
sponding dihydrofurans 3e and 3 f in moderate yields. Further-
more, a 1,3-dicarbonyl compound bearing an amide group
was also tolerated in the reaction with 1a, affording the dihy-
drofuran product 3g in moderate yield. The b-ketosulfone was
also effective for this reaction and gave the desired product
3h in 79% yield.
The target tetrasubstituted dihydrofurans were obtained with
trans stereochemistry (confirmed by NMR spectroscopy), which
is a common structural moiety present in many natural prod-
ucts.[14] Interestingly, cyclic alkenes, such as indene and 1,2-di-
hydronaphthalene, could also be employed to give the target
dihydrofuran scaffolds 4 f and 4g without any difficulties
(Scheme 2).
According to previous reports on the oxidative coupling be-
tween 1,3-dicarbonyl compounds and alkenes[13] and the
above results,
a proposed mechanism is described in
Under the same reaction conditions, a-methylstyrene could
also react with ethyl acetoacetate (2a) yielding dihydrofuran
3i in 72% yield. The reaction was also readily extended to a va-
riety of aryl terminal alkenes. As shown in Scheme 1, p-methyl-,
p-methoxy-, and p-tert-butylstyrenes all reacted smoothly, af-
fording the desired products 3j–m in moderate yields.
Scheme 3. The acetylacetone is oxidized by the CuCl2/DTBP
Notably, four-substituted dihydrofurans could also be ob-
tained from this oxidative CÀH/OÀH cyclization (Scheme 2). (E)-
b-Methylstyrene and (E)-anethole were selected to react with
1,3-dicarbonyl compounds (2). Various b-keto esters and b-di-
ketones were effective for this transformation and the desired
dihydrofurans 4a–e were obtained in moderate to good yields.
Scheme 3. Proposed mechanism.
system to generate the acetylacetone radical. The acetylace-
tone radical coordinates with copper to generate the radical I,
which then attacks the alkene 1 to give another carbon radical
II. The adduct II undergoes a keto–enol tautomerization to
give intermediate III. Intermediate III undergoes intramolecular
combination of the acetylacetate radical with CuII to form the
CÀO bond, giving the desired dihydrofuran product.
In conclusion, we have reported a mechanistic study of the
CuI/CuII redox process by XAFS and EPR spectroscopy. The re-
duction of CuII to CuI by 1,3-diketone was evidenced by X-ray
absorption spectroscopy (XAS). Detailed investigation revealed
that the halide ion is very important for the reduction of CuII.
The oxidative nature of the CuI was also investigated by XAS
and EPR spectroscopy. The mechanistic findings were applica-
ble to the oxidative cyclization of b-ketocarbonyl derivatives to
give dihydrofurans with DTBP and copper catalyst. This process
provides a convenient and efficient route to highly substituted
dihydrofuran rings from simple chemical feedstocks.
Experimental Section
General procedure: A dried Schlenk tube equipped with a stir bar
was charged with 1a (0.80 mmol), 2a (0.50 mmol), CuCl2
(10 mol%), DTBP (1.0 mmol), and acetonitrile (2.0 mL) under an at-
mosphere of nitrogen. The mixture was then stirred at 808C for
28 h. After the completion of the reaction, it was quenched with
ethyl acetate (3”20 mL) and concentrated under reduced pressure.
The residue was then purified by flash chromatography on silica
Scheme 2. Copper catalyzed oxidative cyclization reactions of various 1,2-
disubstituted alkenes 1 with 2. Reaction conditions (unless otherwise
stated): 1 (1.0 mmol), 2 (0.50 mmol), CuCl2 (0.10 mmol) and DTBP (1.0 mmol)
in CH3CN (2.0 mL) at 808C for 28 h. Yields refer to isolated products. [a] 1
(1.0 mmol), 2 (0.5 mmol), CuCl2 (0.05 mmol); [b] 1 (0.8 mmol), 2 (0.5 mmol),
CuCl2 (0.05 mmol).
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Acknowledgements
This work was supported by the 973 Program (2012CB725302
and 2011CB808600), the National Natural Science Foundation
of China (21390400, 21272180, and 21302148), the Research
Fund for the Doctoral Program of Higher Education of China
(20120141130002), the Program for Changjiang Scholars and
Innovative Research Team in University (IRT1030), and the Min-
istry 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 (con-
tract no. DE-AC02-06CH11357). MRCAT operations are support-
ed by the Department of Energy and the MRCAT member insti-
tutions. This work was also funded by the Chemical Sciences
and Engineering Division at Argonne National Laboratory.
Some X-ray absorption spectroscopy studies were carried out
at beamline 17C1 of the National Synchrotron Radiation Re-
search Center (NSRRC) in Taiwan.
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Received: September 24, 2015
Published online on November 11, 2015
Chem. Eur. J. 2015, 21, 18925 – 18929
www.chemeurj.org
18929
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