Cross C-S coupling reaction catalyzed by copper(i) N-heterocyclic carbene complexes

Article abstract of DOI:10.1039/c6ra27757h

Copper(i) N-heterocyclic carbene has a good activity towards aryl halides and was used as a model complex to study the catalytic cycle of Cu(i) to catalyze the cross C-S coupling reaction because the N-heterocyclic carbene has a strong electron donating property, and ligand dissociation can be avoided. Free radical scavenger cumene does not retard the yield of the reaction indicating that the catalytic reaction goes through a non free radical path. Switching the solvent from toluene to DMF lowered the yield of the reaction. DFT calculation shows that the activation of aryl halide is the rate determining step, and the activation energy is higher for the reaction in DMF than in toluene. A plausible catalytic cycle is proposed with the support of DFT calculation.

Full text of DOI:10.1039/c6ra27757h

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Cross C–S coupling reaction catalyzed by copper(I)
N-heterocyclic carbene complexes†
Cite this: RSC Adv., 2017, 7, 4912
ab
c
c
b
Wei-Kai Huang, Wei-Ting Chen, I.-Jui Hsu,* Chien-Chung Han*
and Shin-Guang Shyu*a
Copper(I) N-heterocyclic carbene has a good activity towards aryl halides and was used as a model complex
to study the catalytic cycle of Cu(I) to catalyze the cross C–S coupling reaction because the N-heterocyclic
carbene has a strong electron donating property, and ligand dissociation can be avoided. Free radical
scavenger cumene does not retard the yield of the reaction indicating that the catalytic reaction goes
through a non free radical path. Switching the solvent from toluene to DMF lowered the yield of the
reaction. DFT calculation shows that the activation of aryl halide is the rate determining step, and the
activation energy is higher for the reaction in DMF than in toluene. A plausible catalytic cycle is proposed
with the support of DFT calculation.
Received 5th December 2016
Accepted 30th December 2016
DOI: 10.1039/c6ra27757h
www.rsc.org/advances
N-Heterocyclic carbene has a strong electron donating
property and ligand dissociation can be avoided. Thus, LCu(-
Introduction
7
Ligands can enhance the Ullmann type Cu(I) catalyzed C–S cross SAr) (L ¼ N-heterocyclic carbene ligand) can maintain as
1
,2
8
coupling reaction, and phen-based (phen ¼ phenanthroline) a monomer in the solution. We conceive that this highly stable
3
ligands have been studied intensively in the last decade. A Cu(I) LCu(SAr) may serve as the dominant intermediate in the cata-
phen complex is generally considered as an intermediate in the lytic cycle, which could help simplify the elucidation on the
3
a,c,4
catalytic cycle,
and DFT calculation has been carried out to corresponding catalytic activity. Herein, we report the synthesis
5
support this hypothesis. Several studies indicate that the of Cu(I) N-heterocyclic carbenes, and the studies on their cata-
mechanism is more complicated than rst thought. Actually, lytic activities in the C–S cross coupling reaction between aryl
6
phen does not coordinate with the Cu(I) rmly. An equilibrium iodide and thiophenol. DFT studies were carried out and
+
1ꢀ
between (phen)CuSAr, [Cu(phen)
2
]
and [Cu(SAr)
2
]
was a catalytic cycle based on experimental results and calculation is
proposed and conrmed by the observations from in situ ESI- proposed.
6e
2
MS studies. [K(phen)][Cu(SAr) ] was also proposed as an
intermediate in the catalytic reaction based on in situ ESI-MS
studies. All these results echo the labile coordination property Experimental
1
+
6e
of the phen ligand towards K and Cu(I). In addition, aggre-
General procedures
6a–d,f,g
gation of CuSR can occur in the reaction.
In fact, [K(Me2-
2
ꢀ
All reagents were purchased from commercial sources and
used without further purication. Copper iodide(I), iodo-
benzene, 1-iodo-naphthalene, 2-iodotoluene, 3-iodotoluene, 4-
iodotoluene, 4-iodoanisole, 4-iodobenzonitrile, lithium tert-
butoxide, sodium tert-butoxide, potassium tert-butoxide, thi-
ophenol, and 2,2,6,6-tetramethyl-1-piperidinyloxy were
purchased from ACROS. 1,4-Di-tert-butylbenzene was
purchased from Alfa Aesar. Isopropylbenzene and 4-iodo-
benzotriuoride were purchased from Aldrich. p-Xylene,
toluene and DMF (dried) were purchased from Merck. All
phen)
3
]
2
,
[Cu
4
(SCH
2
Ph)
6
]
and [phenCuSAr] have been
2
synthesized (through the respective reaction between CuCl and
t
KSR, and that between CuO Bu and HSAr, in the presence of
6f
phen) and studied to elucidate the possible involvements of
2
ꢀ
[Cu
4
(SCH
2
Ph)
6
]
2
or [phenCuSAr] in the catalytic cycle. All
these diverse observations indicate that the mechanism of C–S
cross coupling reaction catalyzed by Cu(I)/ligand has not been
settled yet.
a
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, Republic of China. reagents were transferred to the reaction vessel (Pyrex tube
E-mail: sgshyu@chem.sinica.edu.tw
1
13
with a Teon screw cap) in a glove box. H and C NMR
spectra were recorded in CDCl or C on Bruker AV 400 or
b
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan,
3
6 6
D
Republic of China. E-mail: cchan@mx.nthu.edu.tw
Bruker AMX 400 instruments. GC experiments were performed
c
Department of Molecular Science and Engineering, National Taipei University of
on Agilent 6890N gas chromatograph equipped with a 30 m ꢁ
Technology, Taipei 10608, Taiwan, Republic of China. E-mail: ijuihsu@ntut.edu.tw
0
.53 mm ꢁ 3.0 mm HP-1 or 30 m ꢁ 0.25 mm ꢁ 0.25 mm DB-
†
Electronic supplementary information (ESI) available: Experimental details,
DFT calculation data and spectral data. See DOI: 10.1039/c6ra27757h
5MS capillary columns and a FID detector.
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Preparation of N-heterocyclic carbene
mixture and the resultant suspension was ltered through
Celite. The ltrate was characterized by GC with 1,4-di-tert-
butylbenzene as the internal standard.
IMes-Cu-Cl.
(1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-yli-
dene)copper(I) chloride was prepared according to literature
9
method.
The mixture of 1,3-bis(2,4,6-trimethylphenyl)
O (0.8 g, 5.59
mmol) in THF (50.0 mL) in a 100 mL round-bottom ask was
Reaction of copper(I) catalyzed C–S coupling with different N-
heterocyclic carbenes
imidazolium chloride (2.0 g, 5.90 mmol), Cu
2
heated at reux for one day. The reaction was then allowed to The same procedure, amount of reagents and reaction condi-
cool to room temperature, and the remaining solids were tions as in the typical procedure stated above was applied in the
removed by gravity ltration. The volume of the ltrate was then reaction. For reaction using IMes-Cu-Cl as the copper catalyst,
reduced to 5.0 mL using a rotary evaporator, and the concen- 40.0 mg, 0.1 mmol of IMes-Cu-Cl was used. For reaction using
ꢂ
trated solution was set aside for 24 h at ꢀ25 C. The solid was IMes-Cu-SPh as the copper catalyst, 47.7 mg, 0.1 mmol of IMes-

ltered and washed with 5.0 mL of ice-cold diethyl ether, the Cu-SPh was used. For reaction using IPr-Cu-Cl as the copper
residue solvent was evaporated under vacuum, a white solid catalyst, 47.0 mg, 0.1 mmol of IPr-Cu-Cl was used.
(
1,3-bis(2,4,6-trimethylphenyl)-imidazole-2-yilidene)copper(I)
In toluene. Reaction between iodobenzene, thiophenol and
LiO Bu with IMes-Cu-Cl: GC yield: 81% (response factor for
t
chloride was obtained (0.95 g, 40% yield).
1
H NMR (CDCl ): d 2.09 (s, 12H), 2.33 (s, 6H), 6.98 (s, 4H), diphenyl sulde: 1.13).
3
1
1
3
t
7
.03 (s, 2H); C{ H} NMR (CDCl ): d 17.9, 21.3, 122.4, 129.6,
Reaction between iodobenzene, thiophenol and NaO Bu
3
134.7, 135.3, 139.7.
with IMes-Cu-Cl: GC yield: 88% (response factor for diphenyl
IPr-Cu-Cl. (1,3-Bis(2,6-diisopropyl phenyl)-imidazole-2-yili- sulde: 1.11).
dene)copper(I) chloride was prepared according to literature
Reaction between iodobenzene, thiophenol and KO Bu with
method. The mixture of 1,3-bis(2,6-diisopropyl phenyl)imida- IMes-Cu-Cl: GC yield: 84% (response factor for diphenyl sulde:
zolium chloride (1.24 g, 2.90 mmol), Cu
O (0.40 g, 2.795 mmol) 1.11).
in THF (50.0 mL) in a 100 mL round-bottom ask was heated at
Reaction between iodobenzene, thiophenol and LiO Bu with
t
9
2
t
reux for one day. The reaction was then allowed to cool to IMes-Cu-SPh: GC yield: 83% (response factor for diphenyl
room temperature, and the remaining solids were removed by sulde: 1.11).
t
gravity ltration. The ltrate was then reduced in volume to 5.0
Reaction between iodobenzene, thiophenol and LiO Bu with
mL using a rotary evaporator, and the concentrated solution set IPr-Cu-Cl: GC yield: 48% (response factor for diphenyl sulde:
ꢂ
aside for 24 h at ꢀ25 C. The solid was ltered and washed with 1.11).
5
.0 mL of ice-cold diethyl ether, the residue solvent was evap-
In DMF. Reaction between iodobenzene, thiophenol and
t
orated under vacuum, a white solid (1,3-bis(2,6-diisopropyl LiO Bu with IPr-Cu-Cl: GC yield: 39% (response factor for
phenyl)-imidazole-2-yilidene)copper(I) chloride was obtained diphenyl sulde: 0.99).
(
0.51 g, 36% yield).
1
H NMR (CDCl ): d 1.34 (d, 12H, J ¼ 6.9 Hz), 1.41 (d, 12H, J ¼ Reaction of copper(I) catalyzed C–S coupling with CuI
3
6
7
1
.9 Hz), 2.65–2.70 (m, 4H), 7.37 (s, 2H), 7.41 (d, 4H, J ¼ 7.8 Hz),
t
In a glove box, CuI (19.0 mg, 0.1 mmol), LiO Bu (120.0 mg, 1.5
1
3
1
.60 (t, 2H, J ¼ 7.5 Hz); C{ H} NMR (CDCl
3
): d 24.1, 25.0, 28.9,
mmol), iodobenzene (114.0 mL, 1.0 mmol), and toluene (3.0 mL)
were transferred to a Pyrex tube tted with a Teon screw-cap.
Thiophenol (113.0 mL, 1.1 mmol) was added under nitrogen
outside the glove box. The mixture was then heated at 120 C in oil
bath for 6 h and then cooled to room temperature. Ethyl acetate
10.0 mL) was added to the reaction mixture and the resultant
suspension was ltered through Celite. The GC yield of diphenyl
sulde was 24% (response factor for diphenyl sulde: 1.11).
23.3, 124.4, 124.9, 130.8, 154.8.
8
IMes-Cu-SPh. A solution of IMes-Cu-Cl (0.33 g, 0.82 mmol),
sodium thiophenolate (0.136 g, 1.03 mmol) in 15.0 mL toluene in
a round bottom ask was stirred at room temperature under
nitrogen for one day and a green solution was obtained. Aer
ꢂ
(

ltration through Celite, the colorless ltrate was concentrated by
rotary evaporator. A white powder of IMes-Cu-SPh (0.207 g) was
obtained through recrystallization by adding hexanes. Yield: 51%.
1
6 6
H NMR (C D ): d 1.90 (s, 12H), 2.11 (s, 6H), 5.97 (s, 2H), 6.68
Reaction of copper(I) catalyzed C–S coupling with radical
(
s, 4H), 6.89–6.90 (overlapping multiplets, 3H), 7.34 (br m, 2H);
scavengers
1
3
1
C{ H} NMR (C
6 6
D ): d 18.1, 21.5, 121.5, 127.9, 129.9, 134.0,
t
In a glove box, IMes-Cu-Cl (40.0 mg, 0.1 mmol), LiO Bu (120.0 mg,
1
35.4, 136.2, 139.2, 147.3.
1
.5 mmol), iodobenzene (114.0 mL, 1.0 mmol), toluene (3.0 mL),
and radical scavengers (TEMPO, 156.0 mg, 1.0 mmol or cumene,
42.0 mL, 1.0 mmol) were transferred to a Pyrex tube tted with
In a glove box, copper catalyst (0.1 mmol, 10 mol%), ligand a Teon screw-cap. Thiophenol (113.0 mL, 1.1 mmol) was added
Typical procedure for copper(I)-catalyzed C–S coupling
1
(
0.1 mmol, 10 mol%), aryl iodide (1.0 mmol), base (1.5 mmol), under nitrogen outside the glove box. The mixture was then
ꢂ
and solvent (3.0 mL) were transferred to a Pyrex tube tted with heated at 120 C in oil bath for 6 h and then cooled to room
a Teon screw-cap. Thiophenol (113.0 mL, 1.1 mmol) was added temperature. Ethyl acetate (10.0 mL) was added to the reaction
under nitrogen outside the glove box. The mixture was then mixture and the resultant suspension was ltered through Celite.
ꢂ
heated at 120 C in oil bath for 6 h and then cooled to room The GC yield of diphenyl sulde were 86% (TEMPO); 78%
temperature. Ethyl acetate (10.0 mL) was added to the reaction (cumene) (response factor for diphenyl sulde: 1.11).
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1
3
1
Reaction of IMes-Cu-Cl catalyzed C–S coupling with different
aryl iodide
(CDCl
NMR (CDCl
134.6, 145.8.
-Triuoromethylphenyl phenyl sulde (entry 7, Table 2). GC
3
): d 7.14 (d, 2H, J ¼ 6.8 Hz), 7.15–7.50 (m, 7H); C{ H}
), d 108.9, 118.9, 127.5, 129.5, 130.0, 131.0, 132.5,
3
t
In a glove box, IMes-Cu-Cl (40.0 mg, 0.1 mmol), LiO Bu
4
(120.0 mg, 1.5 mmol), aryl iodide (1.0 mmol), and solvent (3.0
yield: 99% (response factor for 4-triuoromethylphenyl phenyl
sulde: 1.18); a colourless liquid; isolated yield: 240.0 mg (94%).
mL) were transferred to a Pyrex tube tted with a Teon screw-
cap. Thiophenol (113.0 mL, 1.1 mmol) was added under nitrogen
1
ꢂ
HNMR (CDCl
3
): d 7.25 (d, 2H, J ¼ 8.4 Hz), 7.36–7.39 (m, 3H),
outside the glove box. The mixture was then heated at 120 C in
13 1
7
.45–7.47 (m, 4H); C{ H} NMR (CDCl
27.8, 128.5, 129.3, 129.9, 132.8, 133.7, 143.1.
-Naphtylphenyl sulde (entry 8, Table 2). GC yield: 90%
response factor for 1-naphtylphenyl sulde: 0.505); a colourless
3
): d 126.0, 126.1, 127.4,
oil bath for 6 h and then cooled to room temperature. Ethyl
acetate (10.0 mL) was added to the reaction mixture and the
resultant suspension was ltered through Celite. The GC yields
were obtained via quantitative GC analysis with 1,4-di-tert-
butylbenzene as the internal standard.
1
1
(
1
liquid; isolated yield: 215.0 mg (91%). H NMR (CDCl ): d 7.14–
.23 (m, 5H), 7.41–7.44 (m, 1H), 7.49–7.53 (m, 2H), 7.66 (dd, 2H,
J ¼ 6.8 Hz), 7.84–7.88 (m, 2H), 8.36–8.37 (m, 1H); C{ H} NMR
CDCl ): d 125.9, 126.0, 126.3, 126.6, 127.2, 128.8, 129.2, 129.3,
29.4, 131.5, 132.8, 133.8, 134.5, 137.
3
7
The same procedure, amount of reagents and reaction
conditions was applied for obtaining the isolated yields. The
products were separated by column chromatography on silica
gel with hexane as the eluent. Aryl suldes were characterized by
1
3
1
(
3
1
10
NMR and were consistent with the literature data.
Diphenyl sulde (entry 1, Table 2). GC yield in toluene: 81%
response factor for diphenyl sulde: 1.11); a colourless liquid; In a glove box, 1.0 mmol of copper catalyst (amount of catalyst
Kinetic experiments
(
1
t
isolated yield: 152.9 mg (82%). H NMR (CDCl
1
3
): d 7.21–7.35 (m, used: IMes-Cu-Cl, 400.0 mg; IMes-Cu-SPh, 477.0 mg), LiO Bu
): d 127.3, 129.4, 131.3, 136.0.
(1.2 g, 15.0 mmol), iodobenzene (1.14 mL, 10.0 mmol), and p-
GC yield in DMF: 55% (response factor for diphenyl sulde: xylene (30.0 mL) were transferred to a round-bottom ask.
1
3
1
0H); C{ H} NMR (CDCl
3
0.99).
Thiophenol (1.13 mL, 11.0 mmol) was added under nitrogen
2-Methylphenyl phenyl sulde (entry 2, Table 2). GC yield in outside the glove box. The mixture was heated at reux under
toluene: 94% (response factor for 2-methylphenyl phenyl nitrogen. We followed the reaction by quantitative GC analysis
sulde: 1.08); a colourless liquid; isolated yield: 192.5 mg (96%). (1,4-di-tert-butylbenzene as the internal standard) every ten-
1
13
1
H NMR (CDCl
3
): d 2.41 (s, 3H), 7.17–7.33 (m, 9H); C{ H} NMR minute for the rst two hours and then every 30 min for the
(
CDCl ): d 20.8, 126.6, 126.9, 128.1, 129.3, 129.9, 130.8, 133.2, next four hours.
3
1
40.2.
GC yield in DMF: 61% (response factor for 2-methylphenyl
phenyl sulde: 0.96).
-Methylphenyl phenyl sulde (entry 3, Table 2). GC yield in
Computation methods
11
All calculations were carried out by Gaussian 09 (ver. C.01).
3
Based on the previous DFT calculations of Ullmann-type reac-
toluene: 69% (response factor for 3-methylphenyl phenyl
5,12
5,12a,b,d,m,13
tions, the exchange functional of B3LYP
In order to nd the more efficient basis set for our studies, the
was used.
sulde: 1.08); a colourless liquid; isolated yield: 142.0 mg (71%).
1
13
1
H NMR (CDCl
3
): d 2.30 (s, 3H), 7.05–7.33 (m, 9H); C{ H} NMR
14
15
basis sets of LanL2DZ(f) and SDD for Cu in combination with
-31+G(d), 6-31G(d), 6-311G(d,p), and 6-311+G(d,p) for all other
(
CDCl ): d 21.5, 127.0, 128.2, 128.5, 129.3, 131.0, 132.0, 135.4,
3
6
136.3, 139.2.
atoms were used to optimize structures and compare the
experimental geometries. The detailed combinations of
different basis sets were listed in ESI.† We noticed that the basis
set of 6-31+G(d) for main group atoms provided the consistent
results as that of triple-z level basis sets. Thus, the basis sets of
SDD and 6-31+G(d) were employed for heavy atoms (Cu and I)
and main group atoms (H, C, N, O, S, Cl, Li and Na), respec-
tively. This basis set is labeled as BS-1. For those single-point
energy calculations, the basis set of SDD for heavy atoms (Cu
and I) and 6-311+G(d,p) for main group atoms (H, C, N, O, S, Cl,
Li and Na) were used and labeled as BS-2. The tighter integra-
tion grid is specied by “Int ¼ UltraFine” keyword. Frequency
calculations were performed to verify the real minima (no
imaginary frequency) of stationary points or transition state
(one imaginary frequency) and also to provide those free ener-
gies at 298.15 K and 1 atm. The intrinsic reaction coordinate
GC yield in DMF: 60% (response factor for 3-methylphenyl
phenyl sulde: 0.96).
-Methylphenyl phenyl sulde (entry 4, Table 2). GC yield in
4
toluene: 75% (response factor for 4-methylphenyl phenyl
sulde: 1.08); a colourless liquid; isolated yield: 146.2 mg (73%).
1
H NMR (CDCl
3
): d 2.32 (s, 3H), 7.12 (d, 2H, J ¼ 8 Hz), 7.17–7.25
13 1
(
1
m, 5H), 7.28 (d, 2H, J ¼ 8 Hz); C{ H} NMR (CDCl
26.6, 129.3, 130.3, 131.5, 132.5, 137.3, 137.8.
GC yield in DMF: 50% (response factor for 4-methylphenyl
phenyl sulde: 0.96).
-Methoxyphenyl phenyl sulde (entry 5, Table 2). GC yield:
3% (response factor for 4-methoxyphenyl phenyl sulde: 1.47);
3
): d 21.3,
4
9
1
a colourless liquid; isolated yield: 192.5 mg (89%). H NMR
CDCl
(
5
1
3
): d 3.85 (s, 3H), 6.92 (d, 2H, J ¼ 8.8 Hz), 7.14–7.28 (m,
13 1
H), 7.44 (d, 2H, J ¼ 8.8 Hz); C{ H} NMR (CDCl
24.6, 125.9, 128.4, 129.1, 135.5, 138.8, 160.0.
-Cyanophenyl phenyl sulde (entry 6, Table 2). GC yield:
9% (response factor for 4-cyanophenyl phenyl sulde: 1.09);
3
): d 55.5, 115.2,
16
(IRC) was used to conrm that the transition state of Cu-
4
carbene complex reaction system is connected to correct reac-
9
1
tant and product. Solvent effect (toluene, 3 ¼ 2.37; DMF, 3 ¼
a colourless liquid; isolated yield: 204.1 mg (96%). H NMR
37.22) was calculated by self-consistent reaction led with
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7
2
CPCM solvation model, and these calculations were carried same reaction condition (Table 2), as expected. The results are
out on the optimized gas-phase geometries. In addition, the consistent with the general phenomena that ligand can
1–3,4b–g
energy barriers were also calculated by PBE0, MPWB1K, B3LYP- enhance the Ullmann-type C–S coupling reaction.
D2, and uB97XD to evaluate the effects of exchange functionals
and dispersion corrections in this study.
Both L-Cu-Cl and L-Cu-SPh (L ¼ IMes, IPr) remain as
a monomeric structure as shown by their single crystal X-ray
8,9b,18
diffraction studies.
In order to ensure the carbene
complex is monomeric in solution as in the solid state,
Results and discussion
Reactivity of Cu(I) N-heterocyclic carbenes
diffusion-ordered NMR spectroscopy (DOSY) of IMes-Cu-SPh in
8
d -toluene was carried out. The estimated hydrodynamic radius
˚
of IMes-Cu-SPh is 5.04 A – comparable to the corresponding
Cu(I) N-heterocyclic carbenes IMes-Cu-Cl and IPr-Cu-Cl were
synthesized according to literature method, and their struc-
˚
19
computed radii of 5.23 A. Addition of cumene or TEMPO as
radical scavenger did not actually reduce the reaction yield,
indicating that the contribution of free radical reaction path is
limited if there is any (Table 1, entry 2).
9
tures are shown in Scheme 1. We follow the reaction procedure
reported in the literature to investigate the inuence of N-
heterocyclic carbene ligand in the copper(I)-catalyzed C–S
coupling reaction by using LCu-Cl as the copper source.
A mixture of LCuCl (0.1 mmol), iodobenzene (1.0 mmol),
1
a
Proposed catalytic cycle and LCu-SPh kinetically competent
t
thiophenol (1.1 mmol) and 1.5 equiv. (1.5 mmol) of LiO Bu in As generally suggested, the reaction between LCu-Cl and MSPh
ꢂ
toluene was stirred at 120 C for 6 h. Reactions using only CuI, (M ¼ Li, Na or K) – produced from the deprotonation of HSPh by
t
4
were also carried out under the same reaction condition for MO Bu – generates LCu-SPh. Oxidative addition of ArI to Lcu-
comparison. All the results are listed in Table 1.
SPh produces LCu(SPh)(Ar)I which further produce PhSAr and
Using both Cu(I) carbene complexes resulted in much better LCu-I through reductive elimination. LCu-I can react with MSPh
yields than using CuI alone (entries 1 to 3). The fact that IPr-Cu- to regenerate LCu-SPh to complete the catalytic cycle (Fig. 1).
Cl catalyst gave a lower yield than that of IMes-Cu-Cl (entries 2
to 3) may be caused by the higher steric hindrance of the bulky
Table 2 Reactions of aryl iodides with thiophenol
IPr ligand. Thus, IMes-Cu-Cl was used as the catalyst for other
iodo-substrates, which all resulted in good yields under the
a
b
Entry
1
Halide
Product
Yield (%) GC /isolated
c
81(55) /82
Scheme 1 The structures of N-heterocyclic carbene.
c
2
3
94(61) /96
Table 1 Cu(I)-catalyzed cross coupling reaction of iodobenzene and
thiophenol under various conditions
c
69(60) /71
c
4
5
75(50) /73
93/89
99/96
99/94
a
Entry
Catalyst
Base
Yield (%)
6
7
t
1
2
3
4
5
6
7
CuI
LiO Bu
24
t
b
c
IMes-Cu-Cl
IPr-Cu-Cl
IMes-Cu-Cl
IMes-Cu-Cl
IMes-Cu-Cl
IMes-Cu-SPh
LiO Bu
81 (86) , (78)
48 (39)
14
89
84
83
t
d
LiO Bu
—
t
NaO Bu
t
KO Bu
8
90/91
t
LiO Bu
a
b
GC yield with 1,4-di-tert-butylbenzene as internal standard. Reaction
c
a
b
run using 100 mol% TEMPO as radical scavenger. 100 mol% cumene
was used as radical scavenger. GC yield with DMF (3.0 mL) as the was used as the eluent in silica gel column chromatography. GC
GC yield with 1,4-di-tert-butylbenzene as internal standard. Hexane
d
c
solvent.
yield with DMF (3.0 mL) as the solvent.
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Scheme 2 The possible non free radical mechanisms.
Moreover, in Cu-catalyzed C–X (X ¼ C, O, N, S, P) cross
coupling reactions, whether the reactions go through non-free
5b,d,e,g,6e,g,h,12b,d,e,20
radical or free radical paths still debatable.
Adding radical scavenger experiment in this study indicates that
the contribution of free radical reaction path is limited if there
Fig. 1 The proposed catalytic cycle by oxidative addition/reductive is any. Therefore, our DFT study focus on non-free radical paths,
elimination reaction.
and possible non free radical mechanisms are proposed
(Scheme 2).
The results of DOSY and kinetic experiment indicates that
In order to ensure that LCu-SPh is the catalyst or the domi-
nant intermediate in the catalytic cycle, IMes-Cu-SPh was
synthesized by the reaction between IMes-Cu-Cl and NaSPh.
Similar kinetic data were obtained for both reactions using
IMes-Cu-Cl and IMes-Cu-SPh indicating that IMes-Cu-SPh is
kinetically competent. (Fig. 2).
IMes-CuSPh dominates and is kinetically competent with IMes-
Cu-Cl in toluene. This observation is consistent with the re-
8
5e
ported theoretical study (1,10-phenanthrolines as ligand)
which states that L-CuSPh may transforms to other Cu species
through disproportionation, dimerization and ligand dissocia-
tion. These transformations are greatly endergonic with free
ꢀ1
energy differences 14–69 kcal mol (see ESI†) implying that L-
CuSPh dominates in toluene also. In addition, comparing the
binding capacities between phen and carbene ligands by the
DFT calculations
Four possible mechanisms of Ullmann type reaction – oxidative
addition/reductive elimination (OA/RE), s-bond metathesis
ꢀ1
dissociation of LCuSPh to form CuSPh (27.08 kcal mol for L ¼
ꢀ
1
ꢀ1
(
sBM), single electron transfer (SET) and halogen atom transfer
phen, 40.19 kcal mol for L ¼ IMes and 40.25 kcal mol for L
3
a,c,4,5,6e,g,h,12,20,21
(HAT) – have been proposed.
The rst two are
¼
IPr), the N-heterocyclic carbene ligands have stronger binding
usually non-free radical reactions. However, a singlet biradical
capacity than that of phen and explains the ligand labile issue.
The energy proles of L-CuSPh-catalyzed OA/RE and sBM
paths are shown in Fig. 3a. As expected, the rate-determining-
step is the oxidative addition of PhI to LCuSPh to produce
LCu(SAr)(Ph)I. The reaction with IMes-CuSPh catalyst has
transition state for sBM is also reported implying the possibility
12j
of free radical path of sBM. The others (SET and HAT) are free
radical paths.
ꢀ
1
a lower activation energy (40.61 kcal mol ) than that of IPr-Cu-
ꢀ
1
SPh (46.01 kcal mol ) implying that IMes-CuSPh gave a higher
reaction rate than that of IPr-Cu-SPh. In addition to the OA/RE
reaction path, the reaction may go through the hypothetical
sBM mechanism. DFT calculation with B3LYP functional
reveals that the path of sBM has a higher activation energy
ꢀ
1
(
43.18 kcal mol ) than that of the oxidative addition step (40.61
ꢀ1
kcal mol ) for L ¼ IMes (Fig. 3), but, for L ¼ IPr, both reaction
ꢀ
1
paths have similar activation energy (46.01 vs. 46.70 kcal mol ).
Base on such small energy differences, it is ambiguous to
ꢀ1
select the reaction path accordingly. However, ꢃ1 kcal mol
energy difference is used to explain the selection between C–N
12c
and C–O coupling, and both three-center and four-center
oxidative pathway must be envisaged due to the ꢃ1.19 kcal
ꢀ
1
12e
mol energy difference was also reported.
5b
In C–N coupling reaction, the energy barrier is 29 kcal
ꢀ
1
ꢂ
mol determined from the experimental kinetic data at 120 C.
However, the DFT calculation results indicate that the energy
Fig. 2 The kinetic experiments of C–S cross coupling with different
catalyst.
ꢀ1
barrier is 38 kcal mol (by using B3LYP functional). Similar
4916 | RSC Adv., 2017, 7, 4912–4920
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ꢀ
1
Fig. 3 Free energy profile (kcal mol ) for oxidative addition/reductive elimination paths and s-bond metathesis paths. (a) In toluene, (b) in DMF.
overestimation of energy barrier by using B3LYP functional was corrections in the energy barrier, PBE0 (hybrid GGA), MPWB1K
1
2d
also reported in the study of Liu et al.
Some theoretical (hybrid meta GGA) and dispersion-corrected functionals B3LYP-
ꢀ1
studies show that the energy barrier is about 28–35 kcal mol
for those experiments carried out around 90–120 C.
seems the overestimated energy barrier is a common phenom- kcal mol for L ¼ IPr) between OA/RE and sBM paths were
enon in theoretical studies. Moreover, PBE0 and MPWB1K were provided by PBE0, MPWB1K, B3LYP-D2 and uB97XD (see Table
also used to study Ullmann type reaction.
D2 and uB97XD are selected to calculate energy barriers. The
ꢂ
5d,e,12c
ꢀ1
It larger energy gap (5.37–9.37 kcal mol for L ¼ IMes; 3.22–8.67
ꢀ1
1
2c,e–g
. In order to 3). Using PBE0, MPWB1K, B3LYP-D2 and uB97XD can obtain
investigate the effects of exchange functionals and dispersion lower energy barrier than that of B3LYP, especially the results
obtained by dispersion-corrected DFT methods (lower 6.79–
ꢀ
1
2
0.66 kcal mol than that of B3LYP at BS-2 basis set). On the
Table 3 The energy barriers obtained by different DFT methods in
toluene
a
other hand, the basis set effect is limited, and especially the
results obtained by BS-1 and BS-2 are very close. These results
suggest that the catalytic cycle may predominantly go through
the OA/RE sequence. Moreover, all DFT methods report
a consistent result that IMes-CuSPh gave a higher reaction rate
than IPr-Cu-SPh (see Table 3). Thus, B3LYP can achieve quali-
tative analysis even it reports higher energy barriers.
OA/RE L ¼ IMes/
L ¼ IPr
sBM L ¼ IMes/
L ¼ IPr
b
B3LYP/BS-small
PBE0/BS-small
38.34/43.48
32.54/37.35
34.80/36.61
40.89/46.12
40.61/46.01
22.32/25.35
25.72/31.24
40.92/44.68
37.91/40.57
40.91/42.80
43.30/46.88
43.18/46.70
29.59/32.54
34.89/39.91
b
b
MPWB1K/BS-small
B3LYP/BS-1
c
B3LYP/BS-2
c
B3LYP-D2/BS-2
uB97XD/BS-2
The effect of aprotic solvent
c
The low solubility of base in toluene generates mass transfer
effects which affect the deprotonation rate so thus the overall
rate of the reaction. Using aprotic solvent can dissolve the base
and avoid these factors. We use DMF as the solvent to dissolve
a
ꢀ1
b
The free energy unit is kcal mol
.
The basis set in combination with
c
SDD (for Cu and I) and 6-31G(d) (for others) is noted as BS-small. BS-1
was used to carry out geometry optimization.
21
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Table 4 The energy barriers obtained by different DFT methods in further supports the observation. In addition, using smaller
a
DMF
basis set 6-31+G(d) for main group atoms (H, C, N, O, S, Cl, Li
and Na) obtains consistent results as that of triple-z basis sets.
OA/RE L ¼ IMes/L ¼ IPr
sBM L ¼ IMes/L ¼ IPr
Therefore, we suggest this basis set for Cu-catalysed C–S
coupling studies to reduce computational cost. In this study,
using dispersion-corrected functionals B3LYP-D2 and uB97XD
can reduce energy barrier effectively. This result indicates that
b
B3LYP/BS-2
42.93/48.50
24.36/27.59
27.43/33.42
44.97/48.86
30.64/35.53
35.83/41.13
b
B3LYP-D2/BS-2
uB97XD/BS-2
b
a
ꢀ1
b
The free energy unit is kcal mol
geometry optimization.
.
BS-1 was used to carry out dispersion correction is signicant for the C–S coupling reac-
tion catalysed by Cu(I) carbene complex.
Acknowledgements
the base to evaluate the above mentioned solvent effect. Both
reaction with IMes-Cu-Cl and IPr-Cu-Cl as the catalyst gave
better yield in toluene than in DMF (Table 1, entry 2; Table 2,
entries 1 to 4). These observations reveal that DMF retards
instead of increases the reaction rate even the mass transfer
effect is removed.
We are grateful to the Ministry of Science and Technology,
Republic of China and Academia Sinica for nancial support of
this work.
Notes and references
The presence of DMF may affect the rate determination step
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4
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Conclusions
N-Heterocyclic carbene complexes were used to catalyse Ull-
mann type C–S coupling reaction with good yields. The non-
labile carbene ligand enables the carbene complex to main-
tain its structure in the system and enter the catalytic cycle.
Kinetic study showed that IMes-Cu-SPh is kinetically competent
indicating IMes-Cu-SPh is an intermediate in the catalytic cycle.
The monomeric structure of carbene complex was supported by
single crystal X-ray structure in solid state and DOSY analysis in
solution. Radical scavenger did not retard the catalytic reaction
indicating non-free radical path dominates in the reaction
cycle. A catalytic cycle with the aryl iodide activation step as the
rate determining step is proposed based on DFT calculation. A
lower product yield in DMF indicates that DMF retard the
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4920 | RSC Adv., 2017, 7, 4912–4920
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