Ligand-free Cu(ii)-catalyzed aerobic etherification of aryl halides with silanes: An experimental and theoretical approach

Article abstract of DOI:10.1039/c9nj01777a

Owing to their wide occurrence in nature and immense applications in various fields, the synthesis of aryl alkyl ethers has remained a focus of interest. In contrast to the conventional/traditional methods of etherification, herein, we have reported a more efficient method, which is better yielding and more general in application. The etherification of aryl halides by alkoxy/phenoxy silanes was catalyzed by copper acetate in the presence of cesium carbonate and oxygen in DMF at 145 °C. All the as-synthesized compounds were characterized via the ¹H-NMR and ¹³C-NMR spectroscopic techniques. Density functional theory calculations using the B3LYP functional were performed to elucidate the reaction mechanism. The C-O coupling reaction between 2-nitroiodobenzene and tetramethoxysilane was used as a model reaction. The activation energy barriers for the generation of catalytic species (31.6 kcal mol^-1) and the σ-bond metathesis (16.0 kcal mol^-1), oxidative addition/reductive elimination (20.3 kcal mol^-1), halogen atom transfer (19.2 kcal mol^-1) and single electron transfer (SET) (29.5 kcal mol^-1) mechanisms for the C-O coupling reaction were calculated. Calculations for the key reaction steps were repeated with the B3PW91, PBEH1PBE, wB97XD, CAM-B3LYP and mPW1LYP functionals. The formation of catalytic species via a single electron transfer reaction between tetramethoxysilane and copper acetate, formation of methoxy radicals and methoxylation of copper showed an overall energy barrier of 31.6 kcal mol^-1, and therefore is the rate determining step.

Full text of DOI:10.1039/c9nj01777a

NJC
View Article Online
View Journal
PAPER
Ligand-free Cu(II)-catalyzed aerobic etherification
of aryl halides with silanes: an experimental and
Cite this:DOI: 10.1039/c9nj01777a
theoretical approach†
b
Muhammad Naeem Ahmed, *a Khalil Ahmad,
Khawaja Ansar Yasin,a
Tayyaba Farooq,a Bilal Ahmad Khana and Soumendra K. Roy
*
c
Owing to their wide occurrence in nature and immense applications in various fields, the synthesis of
aryl alkyl ethers has remained a focus of interest. In contrast to the conventional/traditional methods of
etherification, herein, we have reported a more efficient method, which is better yielding and more
general in application. The etherification of aryl halides by alkoxy/phenoxy silanes was catalyzed
by copper acetate in the presence of cesium carbonate and oxygen in DMF at 145 1C. All the
1
as-synthesized compounds were characterized via the H-NMR and 13C-NMR spectroscopic techniques.
Density functional theory calculations using the B3LYP functional were performed to elucidate the
reaction mechanism. The C–O coupling reaction between 2-nitroiodobenzene and tetramethoxysilane
was used as a model reaction. The activation energy barriers for the generation of catalytic species
(31.6 kcal molÀ1) and the s-bond metathesis (16.0 kcal molÀ1), oxidative addition/reductive elimination
(20.3 kcal molÀ1), halogen atom transfer (19.2 kcal molÀ1) and single electron transfer (SET) (29.5 kcal molÀ1
)
mechanisms for the C–O coupling reaction were calculated. Calculations for the key reaction steps were
repeated with the B3PW91, PBEH1PBE, wB97XD, CAM-B3LYP and mPW1LYP functionals. The formation of
catalytic species via a single electron transfer reaction between tetramethoxysilane and copper acetate,
formation of methoxy radicals and methoxylation of copper showed an overall energy barrier of
31.6 kcal molÀ1, and therefore is the rate determining step.
Received 7th April 2019,
Accepted 17th June 2019
DOI: 10.1039/c9nj01777a
rsc.li/njc
haloarenes with phenols. Both reactions require harsh reaction
conditions and show limited substrate scope, making them
1. Introduction
The prime interest in the field of organic synthesis is the inappropriate for the functionalization of multifaceted organic
selective formation of the carbon–oxygen bond at a specific molecules at the advanced stage of a synthetic series.9 Thus,
position of an arene since the aryl ether functionality is extensive research has been conducted for the development of
abundantly found in agrochemicals, pharmaceuticals, materials new methods for the selective synthesis of ether linkages that
science, polymers and natural products.1–5 There are numerous can be used for the late-stage functionalization of complex
examples of aryl ether-containing compounds in pharmaceuticals organic molecules in a complex synthetic sequence.10–13
such as esomeprazole,6 pioglitazone7 and venlafaxine.8
Contemporary methods involve transition metal-catalyzed
The classical methods to synthesize aryl ethers include sp2 C–O bond formation by two main groups. Palladium-14–16
Williamson ether synthesis via aromatic nucleophilic substitution and copper-catalyzed17–19 Buchwald–Hartwig-type coupling
of aryl halides with phenoxides and Ullmann coupling of takes place between electrophilic aryl halides with nucleophilic
phenols and alcohols.20–24 Palladium catalysts are limited to
phenols and alcohols with no b-hydrogen atom, whereas
aliphatic alcohols bearing a b-hydrogen atom require a more
a Department of Chemistry, The University of Azad Jammu & Kashmir,
Muzaffarabad 13100, Pakistan. E-mail: drnaeem@ajku.edu.pk
b Department of Chemistry, Mirpur University of Science and Technology (MUST),
complex ligand system to attain good yields of the aryl/alkyl
ethers under palladium catalysis. Copper complexes and salts
are generally used to synthesize aryl ethers from aryl halides
and alcohols since they do not promote b-hydride elimination.
Copper-aided oxidative etherification of aryl nucleophiles
produced from aryl boronic acids25–29 and arenecarboxylic
acid-based nucleophiles30 represent the second category to
Mirpur 10250 (AJK), Pakistan
c Institute of Theoretical and Computational Chemistry, Shaanxi Key Laboratory of
Catalysis, School of Chemical & Environmental Science, Shaanxi University of
Technology, Hanzhong, Shaanxi 723001, P. R. China.
E-mail: soumya029@yahoo.in
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nj01777a
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
access aryl ethers. In both reactions, a stoichiometric quantity Alfa Aesar, while other reagents were supplied by Acros and
of copper acetate is used in the presence of an oxidant. Merck. The progress of the reactions was monitored by thin-
The former Chan–Evan–Lam coupling of aryl boronic acids is layer chromatography and purification was achieved through
1
limited to the synthesis of biaryl ethers, whereas the latter flash column chromatography (FCC). H-NMR (400 MHz) and
decarboxylative approach produces only aryl alkyl ethers by 13C-NMR (100 MHz) spectra were recorded on a JEOL JNM-ECA
using alkoxy silanes, while phenoxylation is executed by using instrument in CDCl3, while J values were calculated in Hz.
triphenoxyborane.
Melting points were determined using a Yanaco melting point
For copper-assisted etherification, the use of alkoxy silanes apparatus and reported as uncorrected.
is an attractive approach due to the wide availability of alkoxy-
lated and aryloxylated silanes either synthetically31 or commer-
cially. With the development of new approaches that involve
2.1 General procedure for the synthesis of alkyl aryl ethers
and biaryl ethers
the synthesis of both alkyl aryl ethers and aryl aryl ethers, the A mixture of aryl halide (1.0 mmol), silane (1.5 mmol) and
use of silicon-based etherification reagents will be valued by copper acetate (20 mol%) in 4 mL of DMF was stirred at 145 1C
the synthetic chemical community. We hypothesized that the for 18 h under ambient oxygen pressure. After cooling to room
use of silanes with aryl halides instead of arenecarboxylic acids temperature, the reaction mixture was diluted with 30 mL of
as etherification reagents can extend the scope to diaryl ethers dichloromethane. The resultant mixture was washed with
as well as aryl alkyl ethers.
water (2 Â 30 mL), brine (30 mL), and dried over anhydrous
The kinetics and thermodynamics of the copper-catalyzed magnesium sulfate. The solvent was removed under vacuum
C–N and C–O cross coupling reactions between aryl halides and the residue was purified by flash column chromatography
and N- and O-nucleophiles have been studied widely by (petroleum ether: ethyl acetate) to obtain the corresponding
experimentalists32–37 as well as theorists.38–45 Theoretical ethers (2a–2z).
studies/investigations have focused on oxidative addition/
reductive elimination, s-bond metathesis, single electron
1-Methoxy-2-nitrobenzene (2a)
1
transfer (SET) and halogen atom transfer mechanisms. The Yellow liquid, yield = 85%, H-NMR (400 MHz, CDCl3) d ppm
density functional theory (DFT) studies by Jones et al.38 7.84 (m, 1H), 7.56–7.54 (m, 1H), 7.08–7.02 (m, 2H), 3.95 (s, 3H);
favoured the single electron transfer mechanism for the 13C-NMR d ppm 152.8, 139.6, 134.1, 125.5, 120.2, 113.4, 56.3.
b-diketone promoted N-arylation of methylamine and halogen
1-Ethoxy-2-nitrobenzene (2b)
atom transfer mechanism for the 1,10-phenanthroline-promoted
O-arylation of methanol, disfavouring the oxidative addition/ Yellow liquid, yield = 81%, H-NMR (400 MHz, CDCl3) d ppm
1
reductive elimination and s-bond metathesis mechanisms. 7.99 (m, 1H), 7.50 (m, 1H), 7.08–6.99 (m, 2H), 4.18 (q, 2H, J =
However, subsequent theoretical studies by Yu et al. indicated 6.9 Hz), 1.45 (t, 3H, J = 6.9 Hz); 13C-NMR d ppm 152.1, 139.8,
the oxidative addition/reductive elimination mechanism for the 133.9, 125.2, 119.9, 114.3, 65.1, 14.3.
b-diketone-promoted N-arylation and the 1,10-phenanthroline-
promoted O-arylation of amino alcohols.40 Several other theo-
1-n-Propoxy-2-nitrobenzene (2c)
1
retical studies on copper-catalyzed C–N and C–O coupling Yellow liquid, yield = 77%, H-NMR (400 MHz, CDCl3) d ppm
reactions have supported the oxidative addition/reductive 7.72 (dd, 1H, J1 = 7.8 Hz, J2 = 1.8 Hz), 7.44–7.41 (m, 1H), 6.99
elimination mechanism.40–45
(m, 1H), 6.91 (m, 1H), 3.97 (t, 2H, J = 6.4 Hz), 1.78 (s, 2H, J =
In the present study, we established an efficient method for 6.4 Hz), 0.98 (t, 3H, J = 7.8 Hz); 13C-NMR d ppm 152.2, 139.8,
the synthesis of diaryl and aryl alkyl ethers via the coupling 133.9, 125.3, 119.9, 114.3, 70.9, 22.3, 10.34.
reaction of aryl halides with alkoxy and aryloxy silanes in the
1-n-Butoxy-2-nitrobenzene (2d)
presence of copper acetate. We also explored the oxidative
addition/reductive elimination, s-bond metathesis, single Yellow liquid, yield = 80%, H-NMR (400 MHz, CDCl3) d ppm
1
electron transfer and halogen atom transfer mechanisms for 7.81 (m, 1H), 7.49 (m, 1H), 7.0 (m, 2H), 4.10 (t, 2H, J = 6.4 Hz),
this reaction. These theoretical studies will help to understand 1.81 (q, 2H, J = 5.9 Hz), 1.51 (s, 2H, J = 7.3 Hz), 0.97 (t, 3H, J =
the reaction mechanism and hence in the further improvement 7.6 Hz).13C-NMR d ppm 152.4, 140.0, 133.9, 125.4, 119.9, 114.3,
of catalytic systems.
69.2, 30.9, 19.0, 13.7.
Methyl 2-methoxybenzoate (2e)
Colorless liquid, yield = 86%, 1H-NMR (400 MHz, CDCl3) d ppm
2. Experimental
7.79 (m, 1H), 7.44 (m, 1H), 6.98–6.94 (m, 2H), 3.88 (s, 3H), 3.87
All catalytic reactions were performed in oven-dried 20 mL glass (s, 3H); 13C-NMR d ppm 166.5, 158.9, 133.3, 131.4, 119.8, 111.8,
vials equipped with a Teflon-coated stirring bar and a septum 55.7, 51.8.
cap at ambient oxygen pressure. Aryl halides and different
silanes were purchased from J&K Scientific Limited China.
Ethyl 2-ethoxybenzoate (2f)
tert-Butyldimethyl(phenoxy)silanes were prepared according to Colorless liquid, yield = 81%, 1H-NMR (400 MHz, CDCl3) d ppm
the reported procedure.46,47 Copper catalysts were provided by 7.77 (m, 1H), 7.43 (m, 1H), 6.96 (m, 2H), 4.34 (q, 2H, J = 7.3 Hz),
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
4.10 (q, 2H, J = 7.3 Hz), 1.44 (t, 3H, J = 7.3 Hz), 1.37 (t, 3H, J = 1-Methoxy-4-nitrobenzene (2o)
7.3 Hz); 13C-NMR d ppm 166.4, 158.3, 133.0, 131.3, 120.8, 119.9,
113.1, 64.4, 60.6, 14.6, 14.4.
1
White solid, m.p. = 51–53 1C, yield = 80%, H-NMR (400 MHz,
CDCl3) d ppm 8.23 (d, 2H, J = 7.3 Hz), 6.99 (d, 2H, J = 7.3 Hz),
3.94 (s, 3H); 13C-NMR d ppm 164.5, 141.4, 125.8, 113.9, 55.9.
Ethyl 4-ethoxybenzoate (2g)
Colorless liquid, yield = 79%, 1H-NMR (400 MHz, CDCl3) d ppm
7.98 (d, 2H, J = 5.4 Hz), 6.88 (d, 2H, J = 5.4 Hz), 4.34 (q, 2H,
J = 7.3 Hz), 4.08 (q, 2H, J = 6.9 Hz), 1.42 (t, 3H, J = 7.3 Hz), 1.37
(t, 3H, J = 6.9 Hz); 13C-NMR d ppm 166.3, 162.6, 131.4, 122.7,
113.9, 63.6, 14.6, 14.0.
1-Phenoxy-2-nitrobenzene (2p)
Pale yellow liquid, yield = 82%, 1H-NMR (400 MHz, CDCl3)
d ppm 7.95–7.38 (m, 4H), 7.18–7.03 (m, 5H); 13C-NMR d ppm
155.7, 150.7, 141.3, 134.0, 130.0, 125.6, 124.5, 123.0, 120.4, 119.2.
Methyl 2-phenoxybenzoate (2q)
Methyl 2-n-propoxybenzoate (2h)
Colorless liquid, yield = 75%, 1H-NMR (400 MHz, CDCl3) d ppm
7.91–7.41 (m, 2H), 7.30–6.94 (m, 7H), 3.78 (s, 3H); 13C-NMR
d ppm 166.0, 157.5, 156.0, 133.4, 131.7, 129.6, 123.3, 123.0,
120.7, 118.0, 52.0.
Colorless liquid, yield = 75%, 1H-NMR (400 MHz, CDCl3) d ppm
7.78–7.46 (m, 2H), 6.96–6.93 (m, 2H), 3.99 (t, 2H, J = 6.4 Hz),
3.88 (s, 3H), 1.84 (s, 2H, J = 7.3 Hz), 1.06 (t, 3H, J = 7.8 Hz);
13C-NMR d ppm 166.9, 158.5, 133.2, 131.4, 120.3, 119.9, 113.1,
70.3, 51.8, 22.5, 10.4.
Methyl 4-phenoxybenzoate (2r)
1
Methyl 2-n-butoxybenzoate (2i)
White solid, m.p. = 57–59 1C, yield = 77%, H-NMR (400 MHz,
Colorless liquid, yield = 70%, 1H-NMR (400 MHz, CDCl3) d ppm
7.77–7.41 (m, 2H), 6.95–6.93 (m, 2H), 4.02 (t, 2H, J = 6.4 Hz),
3.87 (s, 3H), 1.80 (s, 2H, J = 5.9 Hz), 1.53 (q, 2H, J = 7.3 Hz), 0.97
(t, 3H, J = 7.3 Hz); 13C-NMR d ppm 166.8, 158.5, 133.1, 131.4,
120.3, 119.8, 112.9, 68.4, 51.7, 31.1, 19.0, 13.6.
CDCl3) d ppm 8.00 (d, 2H, J = 6.9 Hz), 7.40–7.16 (m, 3H), 7.05
(d, 2H, J = 6.4 Hz), 6.99 (d, 2H, J = 8.7 Hz), 3.80 (s, 3H); 13C-NMR
d ppm 166.6, 161.7, 155.6, 131.6, 130.0, 124.4, 120.0, 117.2, 51.9.
1-(3-Methoxyphenoxy)-2-nitrobenzene (2s)
Pale yellow liquid, yield = 77%, 1H-NMR (400 MHz, CDCl3)
d ppm 7.94–7.25 (m, 4H), 7.06–6.61 (m, 4H), 3.78 (s, 3H);
13C-NMR d ppm 161.1, 156.8, 150.4, 141.3, 134.0, 130.4, 125.6,
123.2, 120.7, 111.0, 110.2, 105.2, 55.4.
2-Methoxybenzaldehyde (2j)
Light yellow solid, m.p. = 36–38 1C, yield = 60%, 1H-NMR
(400 MHz, CDCl3) d ppm 10.4 (s, 1H), 7.84 (m, 1H), 7.55
(m, 1H), 7.0 (m, 2H), 3.93 (s, 3H); 13C-NMR d ppm 189.7,
181.8, 135.8, 128.5, 124.8, 120.6, 111.6, 55.6.
Methyl 4-(4-methoxyphenoxy)benzoate (2t)
1
White solid, m.p. 99–101 1C, yield = 71%, H-NMR (400 MHz,
1-Methoxy-2-(trifluoromethyl)benzene (2k)
CDCl3) d ppm 7.99 (d, 2H, J = 6.9 Hz), 7.00 (d, 2H, J = 6.4 Hz),
6.93–6.90 (m, 4H), 3.88 (s, 3H), 3.82 (s, 3H); 13C-NMR d ppm
166.6, 162.7, 156.6, 148.6, 131.5, 123.8, 121.6, 116.2, 115.0,
55.6, 51.9.
Colorless liquid, yield = 51%, 1H-NMR (400 MHz, CDCl3) d ppm
7.57 (m, 2H), 7.01 (m, 2H), 3.90 (s, 3H); 13C-NMR d ppm 157.7
(q, JC–F = 1.9 Hz), 133.2, 127.0 (q, JC–F = 5.6 Hz), 125.0 (q, JC–F
272.8 Hz), 119.9, 111.9 (q, JC–F = 31.4 Hz), 55.8.
=
Methyl 4-(4-nitrophenoxy)benzoate (2u)
2-Methoxybenzaldehyde (2l)
Solid, m.p. 108–110 1C, yield = 74%, 1H-NMR (400 MHz, CDCl3)
d ppm 8.26 (d, 2H, J = 9.1 Hz), 8.11 (d, 2H, J = 9.1 Hz), 7.26–7.08
(m, 4H), 3.93 (s, 3H); 13C-NMR d ppm 166.1, 161.8, 158.9, 143.4,
132.0, 126.8, 119.4, 118.3, 52.2.
Colorless liquid, yield = 86%, 1H-NMR (400 MHz, CDCl3) d ppm
7.79 (m, 1H), 7.44 (m, 1H), 6.98–6.94 (m, 2H), 3.88 (s, 3H), 3.87
(s, 3H); 13C-NMR d ppm 166.5, 158.9, 133.3, 131.4, 119.8, 111.8,
55.7, 51.8.
Methyl-4-(4-(tert-butyl)phenoxy)benzoate (2v)
Methyl 3-methoxybenzoate (2m)
Colorless liquid, yield = 69%, 1H-NMR (400 MHz, CDCl3) d ppm
7.99 (d, 2H, J = 6.8 Hz), 7.39 (d, 2H, J = 6.8 Hz), 6.99–6.96
(m, 4H), 3.88 (s, 3H), 1.33 (s, 9H); 13C-NMR d ppm 166.6, 162.1,
153.0, 147.4, 131.5, 126.7, 124.1, 119.5, 117.0, 51.9, 34.4, 31.4.
Colorless liquid, yield = 56%, 1H-NMR (400 MHz, CDCl3) d ppm
7.62 (m, 1H), 7.56 (m, 1H), 7.35 (m, 1H), 7.10 (m, 1H), 3.91
(s, 3H), 3.85 (s, 3H); 13C-NMR d ppm 166.8, 159.4, 131.3, 129.2,
121.8, 119.3, 113.8, 55.3, 52.0.
Methyl 4-methoxybenzoate (2n)
Methyl 4-(p-tolyloxy)benzoate (2w)
1
White solid, m.p. = 46–48 1C, yield = 80%, H-NMR (400 MHz, White solid, m.p. 60–62 1C, yield = 73%, 1H-NMR (400 MHz,
CDCl3) d ppm 7.98 (d, 2H, J = 6.9 Hz), 6.91 (d, 2H, J = 6.9 Hz), CDCl3), d ppm 8.00–7.46 (m, 2H), 7.25–7.16 (m, 2H), 7.01–6.93
3.88 (s, 3H), 3.85 (s, 3H); 13C-NMR d ppm 166.8, 163.2, 131.5, (m, 4H), 3.88 (s, 3H), 2.35 (s, 3H); 13C-NMR d ppm 166.6, 162.2,
122.5, 113.5, 55.3, 51.7.
153.1, 134.2, 131.5, 130.4, 124.0, 120.1, 116.8, 51.9, 20.7.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
Table 1 Optimization of the reaction conditionsa
Methyl 4-(3-methoxyphenoxy)benzoate (2x)
Colorless liquid, yield = 68%, 1H-NMR (400 MHz, CDCl3) d ppm 7.99
(d, 2H, J = 6.9 Hz), 7.25 (t, 1H, J = 8.2 Hz), 6.99 (d, 2H, J = 6.9 Hz),
7.00–6.61 (m, 3H), 3.88 (s, 3H), 3.76 (s, 3H); 13C-NMR d ppm 166.6,
161.5, 156.6,131.5, 130.3, 124.4, 117.3, 111.9, 110.0, 105.9, 55.2, 51.8.
Methyl 4-(2-methoxyphenoxy)benzoate (2y)
#
Catalyst
Base
Additive
Solvent
Yield
Colorless liquid, yield = 70%, 1H-NMR (400 MHz, CDCl3) d ppm
7.96 (d, 2H, J = 6.8 Hz), 7.30–6.98 (m, 6H), 3.87 (s, 3H), 3.78
(s, 3H); 13C-NMR d ppm 166.6, 162.2, 151.6, 143.3, 131.4, 126.0,
123.8, 122.2, 121.2, 115.7, 112.9, 55.8, 51.8.
1
2
3
4
5
6
7
8
9
—
—
—
—
—
—
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
—
—
45
84
30
Trace
20
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cu(OAc)2
Cu(OAc)2
CuF2Á2H2O
CuSO4
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
Methyl 2-(3-methoxyphenoxy)benzoate (2z)
Cu(II)formateÁ4H2O Cs2CO3
1
Cu(OBz)2Á2H2O
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
NaOH
K2CO3
Ag2CO3
Ag2O
56
Colorless liquid, yield = 69%, H-NMR (400 MHz, CDCl3) d ppm
Cu3(PO)4
Trace
25b
17c
40d
50e
—
7.91–7.46 (m, 2H), 7.20–7.00 (m, 2H), 6.56–6.50 (m, 4H), 3.80 (s, 3H),
3.75 (s, 3H); 13C-NMR d ppm 166.0, 160.8, 158.7, 155.8, 133.4, 131.7,
130.0, 123.5, 123.1, 120.9, 110.1, 108.6, 104.1, 55.2, 55.0.
10 Cu(OAc)2
11 Cu(OAc)2
12 Cu(OAc)2
13 Cu(OAc)2
14 Cu(OAc)2
15 Cu(OAc)2
16 Cu(OAc)2
17 Cu(OAc)2
18 Cu(OAc)2
19 Cu(OAc)2
20 Cu(OAc)2
21 Cu(OAc)2
22 Cu(OAc)2
23 Cu(OAc)2
24 Cu(OAc)2
25 Cu(OAc)2
49
3. Results and discussion
Trace
Trace
41
60
47
—
—
—
10
DMF
DMF
For the synthesis of alkyl aryl ethers and diaryl ethers using
silanes as etherification reagents, we started our investigations
with the highly reactive 2-nitroiodobenzene and tetramethoxy-
silane in DMF at 145 1C. After 18 h, 2-nitroiodobenzene was
fully recovered even in the absence and presence of a base,
indicating that the aromatic nucleophilic substitution mecha-
nism was absent (entries 1 and 2). When the reaction was
conducted in the presence of 20 mol% of copper acetate,
2-nitroiodobenzene was converted into the 2-nitroanisole in
45% yield, indicating the role of the copper catalyst (entry 3).
The presence of oxygen dramatically increased the yield to 84%,
confirming the mechanistic proposition of Stahl48 (entry 4).
(C2H5)3N O2
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
O2
O2
O2
O2
BQ
DMSO
NMP
Toluene f
Acetonitrileg
DMF
1,10-Phenan. DMF
PPh3 DMF
33
a
Reaction conditions: 2-nitroiodobenzene (1 mmol), Si(OMe)4 (1.5 mmol),
Cat. (20 mol%), Cs2CO3 (2 eq.), 4 mL Solv., 145 1C, 18 h. Isolated yields.
b
c
Cu(OAc)2 (15 and 10 mol% respectively). Cu(OAc)2 (15 and 10 mol%
d
e
f
g
respectively). 100 1C. 12 h. 105 1C and 95 1C. 105 1C and 95 1C.
The screening of different quantities of copper sources and The reaction conditions are tolerant to various substituents
bases showed that 20 mol% of the copper acetate catalyst including ether, nitro and tert-butyl on the phenyl ring of the
provided the optimum yield in the presence of 2 equivalents O-nucleophiles.
of cesium carbonate (entries 5–16). Testing of various ligands
After successful transfer of alkoxy and phenoxy groups from
and oxidants resulted in a decrease in the yield of the reaction. silane partner, we next investigated the scope of the transfor-
The solvent screening showed that the polar aprotic DMF is the mation with respect to aryl halides. The results showed that the
solvent of choice for the etherification reaction. The tempera- reaction methodology is not restricted to aryl iodides (2e–2j),
ture and time optimization showed that the reaction is best and aryl bromides and even aryl chlorides produced above 40%
performed at 145 1C for 18 h, and lower than that resulted a of the corresponding ethers (2l, 2n, 2t, 2x, 2y, 2z and 2a). In case
decrease in the yield (Table 1).
of aryl bromides and chlorides, the presence of an electron
Having established the optimized reaction protocol for the withdrawing group is necessary, whereas aryl iodides produce
methoxylation of 2-nitroiodobenzene with tetramethoxysilane aryl ethers even in the absence of electron withdrawing groups.
in the presence of 20 mol% of copper acetate and ambient
oxygen pressure at 145 1C for 18 h, we next tested the scope of
the transformation with regard to the O-nucleophile and aryl
halides (Table 2).
4. Mechanistic studies
4.1 Computational details
Among the O-nucleophiles tested, methoxy, ethoxy,
n-propoxy and n-butoxy groups were smoothly transferred from DFT calculations were performed using the Gaussian09 suite of
silanes to aromatic rings (2a–d). In addition to alkoxy groups, programs49 using the unrestricted UB3LYP functional.50–52 All
phenoxy and substituted phenoxy groups were also transferred geometries were optimized using the 6-31G(d) basis set for H,
from silane to the substituted iodobenzenes to produce the C, N and O atoms. Copper atom was described with the
respective diaryl ethers in good yields (2p–z). In earlier reports, Stuttgart RSC 1997 ECP basis set with 10 core electrons. For
boron reagents were employed to access diaryl ethers.30 the iodine atom, the LANL2DZ effective core potential of Hay
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
Table 2 Scope of the transformation
Entry
Aryl halides
Silanes
Product
Yields (%)
85
1
Si(OMe)4
40
81
2
3
4
5
Si(OEt)4
Si(OPr)4
77
80
86
Si(OBu)4
Si(OMe)3Me
6
7
Si(OEt)4
Si(OEt)4
81
79
8
Si(OPr)4
Si(OBu)4
Si(OMe)4
75
70
60
9
10
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
Table 2 (continued)
Entry
Aryl halides
Silanes
Product
Yields (%)
51
11
12
Si(OMe)4
Si(OMe)(CH3)3
86
56
13
14
Si(OMe)4
Si(OMe)4
80
80
82
15
16
17
Si(OMe)4
75
77
77
18
19
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
Table 2 (continued)
Entry
Aryl halides
Silanes
Product
Yields (%)
20
71
21
74
22
69
23
73
24
68
25
26
68
69
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
and Wadt augmented with additional p and d polarization cleavage to deliver the alkoxy radicals. In the reaction system
functions (exponents 0.0308 and 0.2940, respectively) was under investigation, the tetramethoxysilane could undergo
used.53 The Berny algorithm was used to optimize the transi- radical reaction by transferring a single electron to copper(II)
tion states.54 All ground and transition states were verified by acetate. The Gibbs free energy of the electron transfer reaction
vibrational frequency analysis. Intrinsic reaction coordinate was calculated from the energies of the separated reactants and
calculations were performed to confirm the transition states. products of the reaction.38 The activation energy for the elec-
The CPCM solvent model was employed to consider the solvent tron transfer was calculated from the Gibbs free energy of the
effects.55–57 Single point energy calculations in the solution reaction using the Marcus–Hush model.94–101 The calculated
phase were erformed with the unrestricted UB3LYP, UB3PW91, activation energy barrier for the electron transfer reaction between
UPBEH1PBE, UwB97XD, UCAM-B3LYP and UmPW1LYP func- tetramethoxysilane and copper(II) acetate is 105.7 kcal molÀ1
tionals using the 6-311+G(d,p) basis set for H, C, N and O (Scheme 1). Due to the very high activation energy barrier, the
atoms. For copper and iodine atoms, the cc-pVTZ-PP basis sets transfer of electrons from tetramethoxysilane to copper(II) acetate
were used. The solvent parameters for DMF solvent (e = 36.71) was ruled out.
are not available in Gaussian09; therefore, the solvent para-
It has been observed that in the presence of anions, the
meters for acetonitrile (e = 36.64) were employed. A similar tetra-coordinated silicon compounds form penta- and hexa-
approach has been adopted elsewhere in the literature.38
coordinated species.102 Hence, the carbonate anion could react
with the tetramethoxysilane to give tetramethoxysilicon carbo-
nate, which could undergo single electron transfer reaction,
and the Gibbs free energy for the formation of tetramethoxysi-
licon carbonate is +2.3 kcal molÀ1. The single electron transfer
(SET) reaction of tetramethoxysilicon carbonate and further
reaction steps describing the formation of methoxy radicals,
methoxylation of copper by the methoxy radicals and activation
of methoxysilane compounds by electron transfer reactions
with Cu(II) are represented by reactions (a) to (h) in Scheme 2.
In reaction step (a), a single electron is transferred from
tetramethoxysilicon carbonate to copper(II) acetate, and the
calculated Gibbs free energy and the activation energy barrier
4.2 Generation of the catalytic species
Copper-catalyzed C–N and C–O cross-coupling reactions are
catalyzed by the three common oxidation states of copper, i.e.
Cu(0), Cu(I) and Cu(II).58–72 The Cu(I)-catalyzed reactions are
slightly faster than Cu(0) and Cu(II). It is believed that Cu(I) is
the actual catalyst for these reactions and both Cu(0) and Cu(II)
are converted to Cu(I) prior to the initiation of the catalytic
reactions.73–78 Experimental studies also support the conver-
sion of Cu(0) and Cu(II) to Cu(I) in their catalytic reactions.79–88
For CuH-based catalytic systems, the catalytically active Cu(I)
species ‘‘LCuH’’ is formed by the reaction of Cu(II) acetate with
a stoichiometric amount of trialkoxyhydrosilanes ((RO)3SiH).89–92
In our reaction, tetramethoxysilane is used instead of trialkoxy-
hydrosilane, and we propose that tetramethoxysilane reacts with
copper(II) acetate to give LCu1OMe species, where L can be a
methoxy (Int1; [Cu(OCH3)2]À), acetate group (Int2; [Cu(OCH3)-
(CH3COO)]À) or a DMF molecule (Int3; [Cu(OCH3)(DMF)]).
The optimized geometries and relative energies of Int1, Int2
and Int3 are given in Fig. 1. Among the three plausible inter-
mediates, intermediate Int1 is 16.8 and 27.6 kcal more stable
than intermediates Int2 and Int3, respectively.
for the single electron transfer reaction are 4.4 and 6.8 kcal molÀ1
,
respectively. In reaction step (b), the tetramethoxysilicon carbo-
nate radical (compound 3) disintegrates to produce trimethoxy-
silicon carbonate (compound 5) and a methoxy radical via the
transition state TS(b). In step (c), the methoxy radical displaces an
acetate ligand from complex 4 to yield Cu(II) complex 6. In step (d),
an acetate group adds to compound 5 to produce compound 7,
the formation of the compound 7 is endothermic by 12.2 kcal molÀ1
.
In step (e), the compound 7 transfers single electron to the Cu(II)
complex 6 to afford a radical compound 8 and Cu(I) complex 9, and
the Gibbs free energy and activation energy for the single electron
transfer reaction are 8.4 and 9.8 kcal molÀ1, respectively. In step (f),
the radical compound 8 disintegrates to provide a methoxy radical
via the transition state TS(f), and the activation energy of TS(f) is
15.5 kcal molÀ1. In reaction step (g), the methoxy radical created
The mechanistic studies performed by Selvakumar et al. for
the copper-mediated etherification of heteroarenes with alkoxy-
silanes supported the radical mechanism for the reaction.93
The transfer of single electron from alkoxysilanes to copper(II)
acetate affords alkoxysilane radicals, which undergo Si–O bond
Fig. 1 Optimized geometries of Int1, Int2 and Int3 (values within parentheses represent relative Gibbs free energies in kcal molÀ1).
Scheme 1 Gibbs free energy and activation energy barrier for the single electron transfer reaction between tetramethoxysilane and copper(II) acetate.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
Scheme 2 Calculated Gibbs free energies for reaction steps (a–h) and activation energies for steps (a), (b), (e), (f) and (h).
in step (f) reacts with Cu(I) complex 9 to afford Cu(II) complex 11. 4.3 Activation of C–I bond of aryl iodide and C–O coupling
In step (h), the tetramethoxysilicon carbonate reduces the Cu(II) reaction
complex 11 to Cu(I) complex (Int1). The overall reaction com-
prises three single electron transfer reactions, whereas each
Four mechanisms, i.e. oxidative addition/reductive elimination
(OA/RE), s-bond metathesis, single electron transfer (SET) and
tetramethoxysilicon carbonate provides two electrons, and thus
halogen atom transfer (HAT) reaction, were studied for the
the use of 1.5 equivalents of tetramethoxysilane in the reaction
mechanistic investigations of copper-catalyzed C–N and C–O
is justified.
cross coupling reactions between aryl halides and N- and
O-nucleophiles.38–45 A general scheme representing the methox-
The energy level diagram depicting the Gibbs free energies
of reaction steps (a) to (h), activation energies of steps (a), (b),
(e), (f) and (h), and the overall activation energy barrier is
ylation of 2-nitroiodobenzene with tetramethoxysilane via differ-
ent mechanisms is depicted in Scheme 3.
presented in Fig. 2. The overall barrier for reactions (a) to (h)
In the oxidative addition/reductive elimination mechanism,
is 31.6 kcal molÀ1
.
the aryl halide adds to the Cu(I) complex to yield a Cu(III)
Fig. 2 Energy level diagram for the generation of catalytic species through reaction steps (a) to (h).
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
Scheme 3 General scheme representing: (a) oxidative addition/reductive elimination (OA/RE), (b) s-bond metathesis, (c) single electron transfer (SET)
and (d) halogen atom transfer (HAT) mechanisms for the methoxylation of 2-nitroiodobenzene with tetramethoxysilane.
intermediate. The Cu(III) intermediate undergoes reductive presented in Fig. 5. The calculated activation energies for
elimination to produce the coupling product and Cu(I) species, the s-bond metathesis reaction of Int1 and Int2 are 16.0
which is available for another catalytic cycle. The energy level and 16.5 kcal molÀ1 and the overall barriers are 16.0 and
diagram for the oxidative addition/reductive elimination reac- 33.3 kcal molÀ1, respectively.
tions of Int1 and Int2 is displayed in Fig. 3. The intermediate
Copper complexes Int1 and Int2 can transfer a single
Int1 undergoes oxidative addition reaction with 2-nitroiodo- electron to aryl iodide to activate the C–I bond. The transfer
benzene via a three-centered transition state Int1-TSOA to yield of the electron can take place either in a stepwise or a concerted
the Cu(III) intermediate Int1-M1. The calculated activation manner. In the stepwise electron transfer, the activation energy
energy for the oxidative addition step is 20.3 kcal molÀ1 and was estimated using the outer-sphere Marcus–Hush model.94–101
the free energy for the formation of Int1-M1 is À7.2 kcal molÀ1
.
In the concerted method, the activation energy was estimated
103–105
´
The intermediate Int1-M1 undergoes reductive elimination to using Saveant’s model,
which includes the effect of C–I
yield 1-methoxy-2-nitrobenzene and a Cu(I) species (Int1-M2). bond cleavage energy in the estimation of activation energy. The
The activation energy for the reductive elimination reaction is Gibbs free energy of the single electron transfer reactions was
10.0 kcal molÀ1 and the free energy for the overall reaction is calculated from the energies of the separated reactants and
À43.7 kcal molÀ1. In the overall reaction, the oxidative addition products of the reaction.38 The Gibbs free energies for the outer-
is the rate-limiting step with an activation energy of 20.3 kcal molÀ1
.
and inner-sphere single electron transfer reactions of Int1 and
For Int2, the activation energy for the oxidative addition and Int2 are presented in Scheme 4.
reductive elimination steps is 22.5 and 3.7 kcal molÀ1, respectively.
In the halogen atom transfer mechanism, the iodine atom of
The overall barrier for the oxidative addition/reductive elimination the aryl halide is transferred to the Cu(I) complex. The transition
mechanism of Int2 is 39.3 kcal molÀ1
.
state for the iodine atom transfer could not be located after many
The optimized geometries of the transition states and the attempts. Therefore, the activation energy for the iodine atom
different reaction intermediates are presented in Fig. 4. transfer was taken to be equal to the free energy of the iodine
In the s-bond metathesis mechanism, the copper atom atom transfer reaction, and a similar approach has been adopted
remains in the 1+ oxidation state throughout the catalytic cycle. in the literature.38,106 The Gibbs free energies for the iodine atom
The aryl halide bond is activated via a four-centered transition transfer reaction of Int1 and Int2 are described in Scheme 5.
state between the aryl halide and the copper catalyst. The
The calculated activation energy barriers for the single electron
energy level diagram for the activation of the C–I bond of aryl transfer and halogen atom transfer mechanisms of Int1 and Int2
iodide by s-bond metathesis reaction of Int1 and Int2 is are given in Table 3.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
Fig. 3 Energy profile for the oxidative addition/reductive elimination mechanism of Int1 and Int2 (values in parentheses represent the Gibbs free energy
in kcal molÀ1).
Fig. 4 Optimized geometries of the different species in the oxidative addition/reductive elimination reaction of Int1 (H atoms are removed for clarity
and bond lengths are given in Å).
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
s-bond metathesis mechanism showed the lowest activation
energy. Interestingly, this is in contrast to the copper-catalyzed
C–N coupling reactions, which proceeded mainly via the oxida-
tive addition/reductive elimination mechanism.39–45 The com-
puted activation energy for the s-bond metathesis mechanism
is very low under the given reaction conditions (145 1C, 18 h).
Further investigations were carried out by calculating the key
reaction steps of the s-bond metathesis, oxidative addition/
reductive elimination and iodine atom transfer mechanism of
Int1 with the B3PW91, PBEH1PBE, wB97XD, CAM-B3LYP and
mPW1LYP functionals, including a DMF molecule in the coor-
dination sphere of copper, and by calculating the activation
energy barriers for iodobenzene and comparing them with that
of 2-nitroiodobenzene (Table 4).
With all the DFT functionals used, the s-bond metathesis
mechanism displayed the lowest barrier for the activation of
the C–I bond of 2-nitroiodobenzene. Moreover, the oxidative
addition reaction showed the lowest barrier for the activation of
iodobenzene, which is much higher than that for 2-nitroiodo-
benzene, and furthermore the inclusion of a DMF molecule in
the coordination sphere of copper changed the activation
energies. For further insight into the reaction mechanism,
the Gibbs free energies for the dissociation of the C–I bond
of 2-nitroiodobenzene and iodobenzene were calculated. The
calculated Gibbs free energy for the dissociation of the C–I
Fig. 5 Energy profile for the s-bond metathesis mechanism of Int1 and
Int2 (values in parentheses represent the Gibbs free energy in kcal molÀ1).
The activation energy barrier for the activation of the C–I
bond of 2-nitroiodobenzene is lower for intermediate Int1 than
Int2 for all four mechanisms investigated theoretically, and the
Scheme 4 Gibbs free energy for the outer-sphere (a and c) and inner-sphere (b and d) single electron transfer reactions of Int1 and Int2.
Scheme 5 Gibbs free energies for the iodine atom transfer reaction of Int1 and Int2.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
Table 3 Gibbs free energies (DGr), activation energies (Ea) and overall barriers for single electron transfer (SET) and halogen atom transfer mechanisms of
Int1 and Int2
Intermediate
Mechanism
DGr (kcal molÀ1
)
Ea (kcal molÀ1
)
Overall barrier (kcal molÀ1
)
Int1
Int1
Int1
Int2
Int2
Int2
Outer sphere SETa
Inner sphere SETb
Halogen atom transfer
Outer sphere SETa
Inner sphere SETb
Halogen atom transfer
28.7
30.8
19.2
32.2
34.3
23.2
29.5
35.9
19.2
35.0
38.6
23.2
29.5
35.9
19.2
51.8
55.4
23.2
a
b
´
Stepwise electron transfer (Marcus–Hush model). Concerted electron transfer (Saveant’s model).
Table 4 The C–I bond activation energies of 2-nitroiodobenzene and bond of 2-nitroiodobenzene (8.5 kcal molÀ1) is significantly
iodobenzene calculated with different DFT functionals and by adding a
lower than the C–I bond dissociation energy of iodobenzene
(Scheme 6). The lower C–I bond dissociation energy of
DMF molecule to the coordination sphere of Cu
Activation energy (kcal molÀ1
)
2-nitroiodobenzene is plausibly responsible for its very facile
cleavage through the s-bond metathesis reaction.
2-Nitroiodobenzene
Iodobenzene
s-Bond
s-Bond
metathesis OA
DFT functional metathesis OA
IAT
IAT
4.4 Role of O2
B3LYP
16.0
14.4
11.6
10.7
16.1
15.4
16.6
20.3 19.2 26.8
19.2 19.3 25.6
18.3 18.6 23.1
23.1 23.2 23.3
28.2 21.0 28.0
21.4 17.2 26.5
24.2 24.4
22.7 24.2
21.3 23.4
23.7 27.8
29.0 25.9
24.8 22.4
During the optimization of the reaction conditions, it was
observed that the reaction gave a much higher yield under
aerobic conditions. Various experimental107–112 and theoretical
studies110,111 have shown that atmospheric oxygen (O2) can
oxidize Cu(I) species to Cu(II). In the current reaction, O2 could
facilitate the regeneration of the catalytic species by oxidizing
the copper(I) complex (Int1-M2) to copper(II), which could
undergo redox reactions with tetramethoxysilane to yield the
catalytic species (Scheme 2). The calculated Gibbs free energies
of the different steps involved in the oxidation of Cu(I) to Cu(II)
are described in Scheme 7. In step (I), the iodide ligand of
Int1-M2 could be replaced by an acetate ligand to yield complex
9, which could coordinate with an O2 molecule to provide the
O2-coordinated complex 9-O2. The reaction of the Cs+ ion with
complex 9-O2 could generate Cu(II) complex 6 together with the
formation of CsO2. The calculated Gibbs free energies for
the replacement of the iodide ligand with acetate, coordination
of O2 with complex 9 and formation of complex 6 are À5.0, À1.3
and +13.5 kcal molÀ1, respectively. Further reactions of
B3PW91
PBEH1PBE
wB97XD
CAM-B3LYP
mPW1LYP
B3LYPa
20.6 19.2
—
—
—
a
Activation energies calculated by adding DMF molecule to the
coordination sphere of Cu.
Scheme 6 Gibbs free energy for the dissociation of the C–I bond of
2-nitroiodobenzene and iodobenzene.
Scheme 7 Calculated Gibbs free energies for the aerobic oxidation of Cu(I) to Cu(II).
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
Fig. 6 Catalytic cycle A representing the etherification of 2-nitroiodobenzene by the s-bond metathesis mechanism and catalytic cycle B representing
the etherification of iodobenzene by the oxidative addition/reductive elimination mechanism.
complex 6 to generate catalytic species (Int1) are described ([Cu(OCH3)(CH3COO)]À), were selected for the mechanistic
in Scheme 2.
studies. Calculations were performed for the generation of
catalytic species as well as for the oxidative addition/reductive
elimination, s-bond metathesis, single electron transfer and
halogen atom transfer mechanisms for the C–O coupling step
in the methoxylation of 2-nitroiodobenzene and iodobenzene.
For the C–O coupling step, the s-bond metathesis mechanism
showed the lowest activation energy (16.0 kcal molÀ1) for the
methoxylation of 2-nitroiodobenzene, whereas the oxidative
addition/reductive elimination mechanism showed the lowest
barrier (24.2 kcal molÀ1) for the methoxylation of iodobenzene.
The calculated C–I bond dissociation energy of 2-nitroiodo-
benzene (45.4 kcal molÀ1) is significantly lower than the C–I
bond dissociation energy of iodobenzene (53.9 kcal molÀ1) and
is plausibly responsible for its facile C–O coupling reaction. The
overall activation energy barrier for the generation of catalytic
species (31.6 kcal molÀ1) through single electron transfer from
tetramethoxysilane to copper acetate, formation of methoxy
radicals and methoxylation of copper is the rate-determining
reaction step. Thus, the calculated activation energy barrier is
reasonable under the given experimental conditions (145 1C, 18 h).
5. Catalytic cycle
The catalytic cycle for the reaction is given in Fig. 6. In catalytic
cycle A, copper acetate reacts with tetramethoxysilane to generate
the Cu(I) complex Int1. The s-bond metathesis reaction of Int1
and 2-nitroiodobenzene yields 1-methoxy-2-nitrobenzene and
Cu(I) complex Int1-M2. The iodide ligand in Int1-M2 is replaced
with an acetate ligand to yield complex 9. Atmospheric oxygen
oxidizes the Cu(I) complex 9 to Cu(II) complex 6, which on reaction
with tetramethoxysilane yields the catalytic species Int1. The
catalytic cycle B represents the etherification of iodobenzene
through the oxidative addition/reductive elimination mechanism.
The oxidative addition of iodobenzene to Int1 affords the Cu(III)
intermediate Int1-M1, which undergoes reductive elimination to
yield methoxy benzene and Cu(I) intermediate Int1-M2. The
replacement of the iodide ligand of Int1-M2 with acetate,
oxidation with O2 and methoxylation with tetramethoxysilane
regenerate the catalytic species Int1.
Conflicts of interest
6. Conclusion
There are no conflicts to declare.
We developed an efficient route for the synthesis of diaryl and
aryl alkyl ethers by the coupling reaction of aryl halides with
alkoxy and aryloxy silanes in the presence of 20 mol% of copper
acetate under oxidative conditions in DMF at 145 1C. The
Acknowledgements
reaction setup was easy to perform and utilized aryl iodides, We are thankful to Higher Education Commission (HEC, PIN:
bromides and chlorides. A broad variety of functional groups IRSIP 22 Ps 29) Pakistan for financial support. We are also thankful
was tolerated under the reaction conditions. A high degree of to Shaanxi University of Technology for financial support to S. K. R.
chemo-selectivity was also achieved in 2j. DFT studies were The DFT calculations were performed using the Shaanxi Key
carried out to study the mechanism of the reaction. Two Laboratory of Catalysis, School of Chemical and Environmental
plausible Cu(I) intermediates, Int1 ([Cu(OCH3)2]À) and Int2 Science, Shaanxi University of Technology, China.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
for CO Cross Coupling of Phenols with Aryl Halides,
ChemCatChem, 2011, 3, 146.
19 A. B. Naidu, E. Jaseer and G. Sekar, General, Mild, and
Intermolecular Ullmann-Type Synthesis of Diaryl and Alkyl
Aryl Ethers Catalyzed by Diol-Copper(I) Complex, J. Org.
Chem., 2009, 74, 3675.
20 J. F. Marcoux, S. Doye and S. L. Buchwald, A General
Copper-Catalyzed Synthesis of Diaryl Ethers, J. Am. Chem.
Soc., 1997, 119, 10539–10540.
21 S. L. Buchwald and C. Bolm, On the Role of Metal Con-
taminants in Catalyses with FeCl3, Angew. Chem., Int. Ed.,
2009, 48, 5586–5587.
22 P. F. Larsson, A. Correa, M. Carril, P. O. Norrby and C. Bolm,
Copper-Catalyzed Cross-Couplings with Part-per-Million
Catalyst Loadings, Angew. Chem., Int. Ed., 2009, 48, 5691–5693.
23 O. Bistri, A. Correa and C. Bolm, Iron-Catalyzed C–O Cross-
Couplings of Phenols with Aryl Iodides, Angew. Chem., Int.
Ed., 2008, 47, 586–588.
24 D. Saberi, M. Sheykhan, K. Niknam and A. Heydari, Pre-
paration of carbon nanotube-supported a-Fe2O3@CuO
nanocomposite: a highly efficient and magnetically separ-
able catalyst in cross-coupling of aryl halides with phenols,
Catal. Sci. Technol., 2013, 3, 2025–2031.
25 P. S. Herradura, K. A. Pendola and R. K. Guy, Copper-
Mediated Cross-Coupling of Aryl Boronic Acids and Alkyl
Thiols, Org. Lett., 2000, 2, 2019.
26 C. Savarin, J. Srogl, L. S. Liebeskind and A. Mild, Nonbasic
Synthesis of Thioethers. The Copper-Catalyzed Coupling of
Boronic Acids with N-Thio(alkyl, aryl, heteroaryl)imides,
Org. Lett., 2002, 4, 4309.
27 J. Simon, S. Salzbrunn, G. Surya Prakash, N. A. Petasis and
G. A. Olah, Regioselective Conversion of Arylboronic Acids
to Phenols and Subsequent Coupling to Symmetrical Dia-
ryl Ethers, J. Org. Chem., 2001, 66, 633.
28 P. Y. Lam, C. G. Clark, S. Saubern, J. Adams, M. P. Winters,
D. M. Chan and A. Combs, New Aryl/Heteroaryl C-N Bond
Cross-coupling Reactions via Arylboronic Acid/Cupric
Acetate Arylation, Tetrahedron Lett., 1998, 39, 2941.
29 K. S. Rao and T.-S. Wu, Chan–Lam coupling reactions:
synthesis of heterocycles, Tetrahedron, 2012, 68, 7735.
30 S. Bhadra, W. I. Dzik and L. J. Goossen, Decarboxylative
Etherification of Aromatic Carboxylic Acids, J. Am. Chem.
Soc., 2012, 134, 9938.
31 G. L. Witucki, A Silane Primer: Chemistry and Applications
of AIkoxy Silanes, J. Coat. Technol., 1993, 65, 57.
32 E. R. Strieter, D. G. Blackmond and S. L. Buchwald,
The role of chelating diamine ligands in the Goldberg
reaction: a kinetic study on the copper-catalyzed amidation
of aryl iodides, J. Am. Chem. Soc., 2005, 127, 4120–4121.
33 E. R. Strieter, B. Bhayana and S. L. Buchwald, Mechanistic
studies on the copper-catalyzed N-arylation of amides,
J. Am. Chem. Soc., 2009, 131, 78–88.
References
1 S. Enthaler, Palladium-catalysed hydroxylation and alkox-
ylation, Chem. Soc. Rev., 2011, 40, 4912.
2 J. J. Li, D. S. Johnson, D. R. Sliskovic and B. D. Roth,
Contemporary drug synthesis, John Wiley & Sons, 2004.
3 J. J. Li and D. S. Johnson, Modern drug synthesis, John Wiley
& Sons, 2013.
4 H. G. Elias, An introduction to polymer science, VCH, 1997.
5 J. H. Montgomery, Agrochemicals desk reference, CRC Press,
1997.
6 J. P. Koilpillai, P. B. Kulkami, A. D. Dabe and M. A. Khan,
Process for the preparation of esomeprazole magnesium
dihydrate, EP2186807 A1, 2010.
7 A. Les, W. Pucko and W. Szelejewski, Optimization of the
Reduction of a 5-Benzylidenethiazolidine-2,4-dione Deriva-
tive Supported by the Reaction Response Surface Analysis:
Synthesis of Pioglitazone Hydrochloride, Org. Process Res.
Dev., 2004, 8, 157.
8 S. P. Chavan, D. A. Khobragade, S. K. Kamat, L. Sivadasan,
K. Balakrishnan, T. Ravindranathan, M. K. Gurjar and
U. R. Kalkote, An efficient and green protocol for the
preparation of cycloalkanols: a practical synthesis of ven-
lafaxine, Tetrahedron Lett., 2004, 45, 7291.
9 M. B. Smith and J. March, March’s advanced organic chem-
istry: reactions, mechanisms, and structure, John Wiley &
Sons, 2007.
10 S. V. Ley and A. W. Thomas, Modern Synthetic Methods for
Copper-Mediated C(aryl)[BOND]O, C(aryl)[BOND]N, and
C(aryl)[BOND]S Bond Formation, Angew. Chem., Int. Ed.,
2003, 42, 5400.
11 J. F. Hartwig, Carbon-heteroatom bond formation cata-
lysed by organometallic complexes, Nature, 2008, 455, 314.
12 J. F. Hartwig, Transition Metal Catalyzed Synthesis of Aryla-
mines and Aryl Ethers from Aryl Halides and Triflates: Scope
and Mechanism, Angew. Chem., Int. Ed., 1998, 37, 2046.
13 F. Monnier and M. Taillefer, Catalytic CC, CN, and CO
Ullmann-Type Coupling Reactions, Angew. Chem., Int. Ed.,
2009, 48, 6954.
14 K. E. Torraca, X. Huang, C. A. Parrish and S. L. Buchwald,
An Efficient Intermolecular Palladium-Catalyzed Synthesis
of Aryl Ethers, J. Am. Chem. Soc., 2001, 123, 10770.
15 S. Gowrisankar, A. G. Sergeev, P. Anbarasan, A. Spannenberg,
H. Neumann and M. Beller, A General and Efficient Catalyst for
Palladium-Catalyzed C–O Coupling Reactions of Aryl Halides
with Primary Alcohols, J. Am. Chem. Soc., 2010, 132, 11592.
16 S. Gowrisankar, H. Neumann and M. Beller, A Convenient
and Practical Synthesis of Anisoles and Deuterated Ani-
soles by Palladium-Catalyzed Coupling Reactions of Aryl
Bromides and Chlorides, Chem. – Eur. J., 2012, 18, 2498.
17 Q. Zhang, D. Wang, X. Wang and K. Ding, (2-Pyridyl)-
acetone-Promoted Cu-Catalyzed O-Arylation of Phenols
with Aryl Iodides, Bromides, and Chlorides, J. Org. Chem.,
2009, 74, 7187.
`
34 G. Franc, Q. Cacciuttolo, G. Lefevre, C. Adamo, I. Ciofini and
A. Jutand, Mechanistic Insights into C–N Coupling Catalyzed
by 1,3-Diketonate-Ligated Copper: Unprecedented Activation of
Aryl Iodide, ChemCatChem, 2011, 3, 305–309.
18 R. Zhang, J. Liu, S. Wang, J. Niu, C. Xia and W. Sun,
Magnetic CuFe2O4 Nanoparticles as an Efficient Catalyst
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
`
35 G. Lefevre, G. Franc, A. Tlili, C. Adamo, M. Taillefer,
I. Ciofini and A. Jutand, Contribution to the Mechanism
of Copper-Catalyzed C–N and C–O Bond Formation, Organo-
metallics, 2012, 31, 7694–7707.
electron density, Phys. Rev. B: Condens. Matter Mater. Phys.,
1988, 37, 785.
52 P. J. Stephens, F. J. Devlin, C. F. Chabalowski and
M. J. Frisch, Ab Initio Calculation of Vibrational Absorp-
tion and Circular Dichroism Spectra Using Density Func-
tional Force Fields, J. Phys. Chem., 1994, 98, 11623.
53 P. J. Hay and W. R. Wadt, Ab Initio effective core potentials
for molecular calculations. Potentials for the transition
metal atoms Sc to Hg, J. Chem. Phys., 1985, 82, 270–283.
54 H. B. Schlegel, Optimization of equilibrium geometries
and transition structures, J. Comput. Chem., 1982, 3,
214–218.
55 M. Cossi, N. Rega, G. Scalmani and V. Barone, Energies,
structures, and electronic properties of molecules in
solution with the C-PCM solvation mode, J. Comput. Chem.,
2003, 24, 669–681.
56 V. Barone and M. Cossi, Quantum Calculation of Mole-
cular Energies and Energy Gradients in Solution by a
Conductor Solvent Model, J. Phys. Chem. A, 1998, 102,
1995–2001.
36 K. K. Gurjar and R. K. Sharma, Mechanistic Studies of
Ullmann Type C–N Coupling Reactions: Carbonate Ligated
Cu(III) Intermediate, ChemCatChem, 2017, 9, 862–869.
`
37 G. Lefevre, A. Tlili, M. Taillefer, C. Adamo, I. Ciofini and
A. Jutand, Discriminating role of bases in diketonate
copper(I)-catalyzed C–O couplings: phenol versus diaryl-
ether, Dalton Trans., 2013, 42, 5348–5354.
38 G. O. Jones, P. Liu, K. N. Houk and S. L. Buchwald,
Computational Explorations of Mechanisms and Ligand-
Directed Selectivities of Copper-Catalyzed Ullmann-Type
Reactions, J. Am. Chem. Soc., 2010, 132, 6205–6213.
39 S. Zhang and Y. Ding, Theoretical study on mechanism of
copper(I)-catalyzed cross-coupling between aryl halides
and alkylamines, Organometallics, 2011, 30, 633–641.
40 H.-Z. Yu, Y.-Y. Jiang, Y. Fu and L. Liu, Alternative Mecha-
nistic Explanation for Ligand-Dependent Selectivities in
Copper-Catalyzed N- and O-Arylation Reactions, J. Am.
Chem. Soc., 2010, 132, 18078–18091.
41 S. L. Zhang, L. Liu, Y. Fu and Q.-X. Guo, Theoretical Study
on Copper(I)-Catalyzed Cross-Coupling between Aryl
Halides and Amides, Organometallics, 2007, 26, 4546–4554.
42 K. Ahmad, B. A. Khan, B. Akram, J. Khan, R. Mahmood and
S. K. Roy, Theoretical investigations on copper catalyzed
C-N cross-coupling reaction between aryl chlorides and
amines, Comput. Theor. Chem., 2018, 1134, 1–7.
43 T. Chen, M. Yan, C. Zheng, J. Yuan, S. Xu, R. Chen and
W. Huang, A Theoretical Insight into the Mechanism of
Cu(I) Catalyzed C–N Coupling between Aryl Halides and
Aqueous Ammonia, Chin. J. Chem., 2015, 33, 961–966.
44 H. Guo and Y. Xue, Theoretical Study on CuI-catalyzed
Ligand-free N-arylation of Imidazole with Bromobenzene,
J. Theor. Comput. Chem., 2012, 11, 1135–1148.
45 K. Ahmad, C.-R. Chang and J. Li, Theoretical studies on
copper-catalyzed arylation of nitrogen heterocycles from
benzenediazonium acetate under ligand-free conditions,
J. Organomet. Chem., 2018, 864, 50–57.
¨
57 J. Andzelm, C. Kolmel and A. Klamt, Incorporation of
solvent effects into density functional calculations of
molecular energies and geometries, J. Chem. Phys., 1995,
103, 9312–9320.
58 F. Ullmann, Ueber Synthesen in der Biphenylreihe, Chem.
Ber., 1901, 34, 2174–2185.
59 F. Ullmann, Ueber eine neue Bildungsweise von Diphenyl-
aminderivaten, Chem. Ber., 1903, 36, 2382–2384.
60 F. Ullmann and P. Sponagel, Ueber die Phenylirung von
Phenolen, Chem. Ber., 1905, 38, 2211–2212.
61 I. Goldberg, Ueber Phenylirungen bei Gegenwart von
Kupfer als Katalysator, Chem. Ber., 1906, 39, 1691–1692.
62 Q. Shen, S. Shekhar, J. P. Stambuli and J. F. Hartwig,
Highly Reactive, General, and Long-Lived Catalysts for
Coupling Heteroaryl and Aryl Chlorides with Primary
Nitrogen Nucleophiles, Angew. Chem., Int. Ed., 2005, 44,
1371–1375.
63 X. Huang, K. W. Anderson, D. Zim, L. Jiang, A. Klapars and
S. L. Buchwald, Expanding Pd-Catalyzed C–N Bond-Forming
Processes: The First Amidation of Aryl Sulfonates, Aqueous
Amination, and Complementarity with Cu-Catalyzed Reac-
tions, J. Am. Chem. Soc., 2003, 125, 6653–6655.
46 C. R. David, Recent advances in silyl protection of alcohols,
Synth. Commun., 2013, 43, 2265–2279.
47 M. N. Ahmed, A novel route to aromatic ethers and synthesis
of biologically active heterocyclic esters/thioether, PhD thesis,
University of Azad Jammu and Kashmir, Muzaffarabad,
Pakistan, 2015, pp. 16–17.
48 A. E. King, T. C. Brunold and S. S. Stahl, Mechanistic Study
of Copper-Catalyzed Aerobic Oxidative Coupling of Aryl-
boronic Esters and Methanol: Insights into an Organo-
metallic Oxidase Reaction, J. Am. Chem. Soc., 2009, 131, 5044.
49 M. J. Frisch, et al., Gaussian 09, Revision D.01, Gaussian,
Inc., Wallingford CT, 2009.
64 A. Kiyomori, J.-F. Marcoux and S. L. Buchwald, An efficient
copper-catalyzed coupling of aryl halides with imidazoles,
Tetrahedron Lett., 1999, 40, 2657–2660.
65 H. B. Goodbrand and N.-X. Hu, Ligand-Accelerated Catalysis
of the Ullmann Condensation: Application to Hole
Conducting Triarylamines, J. Org. Chem., 1999, 64, 670–674.
66 R. K. Gujadhur, C. G. Bates and D. Venkataraman,
Formation of aryl-nitrogen, aryl-oxygen, and aryl-carbon
bonds using well-defined copper(I)-based catalysts, Org.
Lett., 2001, 3, 4315–4317.
50 A. D. Becke, Density-functional thermochemistry. III. The
role of exact exchange, J. Chem. Phys., 1993, 98, 5648.
51 C. Lee, W. Yang and R. G. Parr, Development of the Colle-
Salvetti correlation-energy formula into a functional of the
67 F. Y. Kwong, A. Klapars and S. L. Buchwald, Copper-
Catalyzed Coupling of Alkylamines and Aryl Iodides: An
Efficient System Even in an Air Atmosphere, Org. Lett.,
2002, 4, 581–584.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
68 D. Ma, Q. Cai and H. Zhang, Mild Method for Ullmann
Coupling Reaction of Amines and Aryl Halides, Org. Lett.,
2003, 5, 2453–2455.
69 F. Y. Kwong and S. L. Buchwald, Mild and efficient copper-
catalyzed amination of aryl bromides with primary alkyl-
amines, Org. Lett., 2003, 5, 793–796.
70 J. C. Antilla, J. M. Baskin, T. E. Barder and S. L. Buchwald,
Copper-Diamine-Catalyzed N-Arylation of Pyrroles, Pyra-
zoles, Indazoles, Imidazoles, and Triazoles, J. Org. Chem.,
2004, 69, 5578–5587.
71 J. Zanon, A. Klapars and S. L. Buchwald, Copper-Catalyzed
Domino Halide Exchange-Cyanation of Aryl Bromides,
J. Am. Chem. Soc., 2003, 125, 2890–2891.
72 L. Jiang, G. E. Job, A. Klapars and S. L. Buchwald, Copper-
Catalyzed Coupling of Amides and Carbamates with Vinyl
Halides, Org. Lett., 2003, 5, 3667–3669.
73 A. Klapars and S. L. Buchwald, Copper-Catalyzed Halogen
Exchange in Aryl Halides: An Aromatic Finkelstein Reac-
tion, J. Am. Chem. Soc., 2002, 124, 14844–14845.
74 J. C. Antilla, A. Klapars and S. L. Buchwald, The copper-
catalyzed N-arylation of indoles, J. Am. Chem. Soc., 2002,
124, 11684–11688.
Paper
85 H. Weingarten, Mechanism of the Ullmann Condensation,
J. Org. Chem., 1964, 29, 3624–3626.
86 A. J. Paine, Mechanisms and models for copper mediated
nucleophilic aromatic substitution. 2. Single catalytic spe-
cies from three different oxidation states of copper in an
Ullmann synthesis of triarylamines, J. Am. Chem. Soc.,
1987, 109, 1496–1502.
87 H. L. Aalten, G. van Koten, D. M. Grove, T. Kuilman,
O. G. Piekstra, L. A. Hulshof and R. A. Sheldon, The copper
catalysed reaction of sodium methoxide with aryl bro-
mides. A mechanistic study leading to a facile synthesis
of anisole derivatives, Tetrahedron, 1989, 45, 5565–5578.
88 R. Resnik, T. Cohen and Q. Fernando, The phenol oxida-
tion of a ligand by copper(II), J. Am. Chem. Soc., 1961, 83,
3344–3345.
89 S. Zhu, N. Niljianskul and S. L. Buchwald, Enantio- and
Regioselective CuH-Catalyzed Hydroamination of Alkenes,
J. Am. Chem. Soc., 2013, 135, 15746–15749.
90 S. Zhu and S. L. Buchwald, Enantioselective CuH-Catalyzed
Anti-Markovnikov Hydroamination of 1,1-Disubstituted
Alkenes, J. Am. Chem. Soc., 2014, 136, 15913–15916.
91 N. Niljianskul, S. Zhu and S. L. Buchwald, Enantioselective
Synthesis of a-Aminosilanes by Copper-Catalyzed Hydro-
amination of Vinylsilanes, Angew. Chem., Int. Ed., 2015, 54,
1638–1641.
75 A. Klapars, X. Huang and S. L. Buchwald, A general and
efficient copper catalyst for the amidation of aryl halides,
J. Am. Chem. Soc., 2002, 124, 7421–7428.
76 A. Klapars, J. C. Antilla, X. Huang and S. L. Buchwald, A
General and Efficient Copper Catalyst for the Amidation of
Aryl Halides and the N-Arylation of Nitrogen Heterocycles,
J. Am. Chem. Soc., 2001, 123, 7727–7729.
77 P. W. Weston and H. Adkins, Catalysis with copper in the
Ullmann reaction, J. Am. Chem. Soc., 1928, 50, 859–866.
78 E. Sperotto, G. P. M. van Klink, G. van Koten and J. G. de
Vries, The mechanism of the modified Ullmann reaction,
Dalton Trans., 2010, 39, 10338–10351.
79 Z. Tye, J. W. Weng, A. M. Johns, A. C. D. Incarvito and
J. F. Hartwig, Copper complexes of anionic nitrogen
ligands in the amidation and imidation of aryl halides,
J. Am. Chem. Soc., 2008, 130, 9971–9983.
92 E. Ascic and S. L. Buchwald, Highly Diastereo- and Enantio-
selective CuH-Catalyzed Synthesis of 2,3-Disubstituted Indo-
lines, J. Am. Chem. Soc., 2015, 137, 4666–4669.
93 J. Selvakumar, G. S. Grandhi, H. Sahoo and M. Baidya,
Copper-mediated etherification of arenes with alkoxysi-
lanes directed by an (2-aminophenyl)pyrazole group, RSC
Adv., 2016, 6, 79361–79365.
94 R. A. Marcus, On the Theory of Oxidation-Reduction Reac-
tions Involving Electron Transfer. I, J. Chem. Phys., 1956,
24, 966–978.
95 R. A. Marcus, Electrostatic Free Energy and Other Proper-
ties of States Having Nonequilibrium Polarization. I,
J. Chem. Phys., 1956, 24, 979–989.
80 R. Giri and L. F. Hartwig, Cu(I)–Amido Complexes in the
Ullmann Reaction: Reactions of Cu(I)–Amido Complexes
with Iodoarenes with and without Autocatalysis by CuI,
J. Am. Chem. Soc., 2010, 132, 15860–15863.
96 R. A. Marcus, On the Theory of Oxidation-Reduction Reac-
tions Involving Electron Transfer. III. Applications to Data
on the Rates of Organic Redox Reactions, J. Chem. Phys.,
1957, 26, 872–877.
81 J. W. Tye, Z. Weng and R. Giri, Copper(I) phenoxide
complexes in the etherification of aryl halides, J. F. Hart-
wig, Angew. Chem., Int. Ed., 2010, 49, 2185–2189.
97 N. S. Hush, Adiabatic Rate Processes at Electrodes. I. Energy-
Charge Relationships, J. Chem. Phys., 1958, 28, 962–972.
98 R. A. Marcus, On the Theory of Electrochemical and
Chemical Electron Transfer Processes, Can. J. Chem.,
1959, 37, 155–163.
`
82 G. Lefevre, G. Franc, C. Adamo, A. Jutand and I. Ciofini,
Influence of the Formation of the Halogen Bond ArXÁÁÁN on
the Mechanism of Diketonate Ligated Copper-Catalyzed Ami-
nation of Aromatic Halides, Organometallics, 2012, 31, 914–920.
99 N. S. Hush, Adiabatic theory of outer sphere electron-transfer
reactions in solution, Trans. Faraday Soc., 1961, 57, 557–580.
83 M. Mansour, R. Giacovazzi, A. Ouali, M. Taillefer and A. Jutand, 100 R. A. Marcus, On the Theory of Electron-Transfer Reac-
Activation of aryl halides by Cu(0)/1,10-phenanthroline: Cu(0)
as precursor of Cu(I) catalyst in cross-coupling reactions, Chem.
Commun., 2008, 6051–6053.
tions. VI. Unified Treatment of Homogeneous and Elec-
trode Reactions, J. Chem. Phys., 1965, 43, 679–701.
101 R. A. Marcus and N. Sutin, Electron Transfers in Chemistry
and Biology, Biochem. Biophys. Acta, 1985, 811, 265–322.
84 G. Franc and A. Jutand, On the origin of copper(I) catalysts
from copper(II) precursors in C–N and C–O cross- 102 C. Chuit, R. J. P. Corriu, C. Reye and J. C. Young, Reactivity
couplings, Dalton Trans., 2010, 39, 7873–7875. of Penta- and Hexacoordinate Silicon Compounds and
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019
New J. Chem.

View Article Online
Paper
NJC
Their Role as Reaction Intermediates, Chem. Rev., 1993, 93,
1371–1448.
and sputter deposited copper surfaces, J. Vac. Sci. Technol., A,
1998, 16, 1227.
´
103 J.-M. Saveant, A simple model for the kinetics of dissocia- 109 R. D. Gray, The Kinetics of Oxidation of Copper(I) by
tive electron transfer in polar solvents. Application to the
homogeneous and heterogeneous reduction of alkyl
halides, J. Am. Chem. Soc., 1987, 109, 6788–6795.
Molecular Oxygen in Perchloric Acid-Acetonitrile Solu-
tions, J. Am. Chem. Soc., 1969, 91, 56.
110 C. H. Kjaergaard, M. F. Qayyum, S. D. Wong, F. Xu,
G. R. Hemsworth, D. J. Walton, N. A. Young, G. J. Davies,
P. H. Walton, K. S. Johansen, K. O. Hodgson, B. Hedman
and E. I. Solomon, Spectroscopic and computational
insight into the activation of O2 by the mononuclear Cu
center in polysaccharide monooxygenases, Proc. Natl. Acad.
Sci. U. S. A., 2014, 111, 8797.
´
104 J.-M. Saveant, Dissociative electron transfer. New tests of
the theory in the electrochemical and homogeneous
reduction of alkyl halides, J. Am. Chem. Soc., 1992, 114,
10595–10602.
´
105 J.-M. Saveant, Electron transfer, bond breaking, and bond
formation, Acc. Chem. Res., 1993, 26, 455–461.
106 K. Ahmad, C.-R. Chang and J. Li, Mechanistic investigations 111 B. V. Popp, J. E. Wendlandt, C. R. Landis and S. S. Stahl,
of Co(II)-Catalyzed C-N coupling reactions, J. Organomet.
Chem., 2018, 868, 144–153.
107 P. Keil, D. Lu¨tzenkirchen-Hecht and R. Frahm, Investiga-
Reaction of Molecular Oxygen with an NHC-Coordinated
Pd0 Complex: Computational Insights and Experimental
Implications, Angew. Chem., Int. Ed., 2007, 46, 601–604.
tion of Room Temperature Oxidation of Cu in Air by 112 A. N. Campbell and S. S. Stahl, Overcoming the ‘‘Oxidant
Yoneda-XAFS, AIP Conf. Proc., 2007, 882, 490.
108 E. Apen, B. R. Rogers and J. A. Sellers, X-ray photoelectron
spectroscopy characterization of the oxidation of electroplated
Problem’’: Strategies to Use O2 as the Oxidant in Organo-
metallic C–H Oxidation Reactions Catalyzed by Pd
(and Cu), Acc. Chem. Res., 2012, 45, 851–863.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019