Possible intermediates of Cu(phen)-catalyzed C-O cross-coupling of phenol with an aryl bromide by in situ ESI-MS and EPR studies

Article abstract of DOI:10.1039/c4dt00582a

The C-O coupling reaction between 2,4-dimethylphenol and 4-bromotoluene catalyzed by the CuI/K₂CO₃/phen system can be inhibited by the radical scavenger cumene. Complexes [Cu(i)(phen)(1-(2,4-dimethylphenoxy)-4- methylbenzene)]⁺ (denoted as A), {H[Cu(i)(phen)(2,4-dimethylphenoxy)] }⁺ and [Cu(i)(2,4-dimethylphenoxy)₂]^- (denoted as B) were observed by in situ electrospray ionization mass spectrometry (ESI-MS) analysis of the copper(i)-catalyzed C-O coupling reaction under the catalytic reaction conditions indicating that they could be intermediates in the reaction. The in situ EPR study of the reaction solution detected the Cu(ii) species with a fitted g value of 2.188. A catalytic cycle with a single electron transfer (SET) step was proposed based on these observations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00582a

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Possible intermediates of Cu(phen)-catalyzed C–O
cross-coupling of phenol with an aryl bromide by
in situ ESI-MS and EPR studies†
Cite this: Dalton Trans., 2014, 43,
11410
Hong-Jie Chen,a,b I-Jui Hsu,c Mei-Chun Tsenga and Shin-Guang Shyu*a,b
The C–O coupling reaction between 2,4-dimethylphenol and 4-bromotoluene catalyzed by the CuI/
K2CO3/phen system can be inhibited by the radical scavenger cumene. Complexes [Cu(I)(phen)(1-(2,4-
dimethylphenoxy)-4-methylbenzene)]+ (denoted as A), {H[Cu(I)(phen)(2,4-dimethylphenoxy)]}+ and
[Cu(I)(2,4-dimethylphenoxy)2]− (denoted as B) were observed by in situ electrospray ionization mass
spectrometry (ESI-MS) analysis of the copper(I)-catalyzed C–O coupling reaction under the catalytic reac-
tion conditions indicating that they could be intermediates in the reaction. The in situ EPR study of the
reaction solution detected the Cu(II) species with a fitted g value of 2.188. A catalytic cycle with a single
electron transfer (SET) step was proposed based on these observations.
Received 25th February 2014,
Accepted 28th May 2014
DOI: 10.1039/c4dt00582a
www.rsc.org/dalton
and {K[Cu(SPh)2(Ph)]}+ were clearly observed by in situ ESI-MS
Introduction
studies, except for the phen-containing intermediate,6 indicat-
The synthesis of aryl esters by the Ullmann type reaction using ing that the ligand may not be involved in the catalytic center.
Cu(I) salt as the catalyst is prevalent in biological, pharma- In CuI/tBuONa/phen catalyzed C–N coupling reaction, the
ceutical, and materials science due to their economic attrac- copper(I) complex [Na(phen)3][Cu(NPh2)2] was isolated from
tiveness, low toxicity, and air and moisture stability.1 Workable the catalytic system, and intermediates Cu(NPh2)2 and {Na-
−
catalytic systems usually consist of a ligand, a base and a [Cu(NPh2)2(p-tolyl)]}+ were observed in the ESI-MS spectra.7
copper salt.1d,2,3 Better yields are often obtained when ligands Based on these observations, mechanisms involving 2e oxi-
are added, and different ligands may have different catalytic dative addition of aryl halides to the corresponding phen-free
−
−
activities.1d,1j,2c,3 Thus, a Cu(I) complex containing the added Cu(I) complexes Cu(NPh2)2 or Cu(SPh)2 were proposed for
ligand is generally proposed as the working intermediate of both reactions. It is therefore possible that Cu(I) catalyzed C–O
the catalytic reaction.4 Complexes with the corresponding coupling reaction may also have involved intermediates similar
ArO− ligand have been prepared, and their catalytic activities to that of the corresponding C–N and C–S coupling reactions.
have been evaluated.5 Mechanisms involving either the 2e oxi-
Recently, [(phen)Cu(OAr)(HOAr2)2] was proposed as the
dative addition path or the free radical path for the activation intermediate in the C–O cross-coupling reaction between
of aryl halides in the copper-catalyzed C–O cross-coupling reac- (E)-bromostilbene and phenol catalyzed by the CuI/K3PO4/phen
tion have also been reported.4b,c,5 However, the mechanism catalytic system, and a 2e oxidative addition reaction path was
of the overall catalytic reaction so far has not been well proposed based on the observation of [LCu(OAr)(HOAr2)2]+
established.
(L is an ionically-tagged phen derivative) in the in situ ESI-MS
In recent studies on the Ullmann type C–S coupling reac- measurement. However, the oxidative addition intermediate
tion with the catalyst system CuI/tBuOK/phen (phen = 1,10- was not observed and the negative-ion mode of the ESI-MS has
phenanthroline), intermediates including Cu(SPh)2−, [Cu(SPh)I]− not been reported. The existence of [Cu(OAr)2]− in the reaction
system is still unclear.8
We herein report the in situ ESI-MS analysis9 for the
Ullmann type copper(I)-catalyzed C–O coupling reaction using
K2CO3 as the base and phen as the ligand under the reaction
conditions described. {H[Cu(I)(phen)(2,4-dimethylphenoxy)]}+
and [Cu(I)(phen)(1-(2,4-dimethylphenoxy)-4-methylbenzene)]+
(denoted as A) or [Cu(III)(phen)(2,4-dimethylphenoxy)(p-tolyl)]+
(denoted as A′) were observed in the CuI/K2CO3/phen reaction
system indicating that they are possible intermediates in the
aInstitute of Chemistry, Academia Sinica, Taipei 115, Taiwan, Republic of China.
E-mail: sgshyu@chem.sinica.edu.tw; Fax: (+886)2-2783-1237
bDepartment of Chemistry, National Central University, 300 Jhongda Rd,
Jhongli City, Taoyuan County 32001 Taiwan, Republic of China
cDepartment of Molecular Science and Engineering, National Taipei University of
Technology, Taipei 10608, Taiwan, Republic of China
†Electronic supplementary information (ESI) available: GC data and ESI and
EPR spectra. See DOI: 10.1039/c4dt00582a
11410 | Dalton Trans., 2014, 43, 11410–11417
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reaction. Addition of the radical scavenger cumene inhibits the
reaction indicating the existence of a free radical pathway in
the mechanism of the reaction. In addition, the in situ EPR
study of the reaction solution detected a Cu(II) species with a
fitted g value of 2.188. A catalytic cycle is proposed based on
these observations.
Results and discussion
In situ ESI-MS analysis
We followed the procedure reported in the literature to investi-
gate the copper-catalyzed C–O coupling reaction between an
aryl bromide and a phenol.2c,3g,h,10 The reason to choose
toluene as the solvent is that toluene is considered as a
greener solvent and easier to work up as compared to DMF or
DMSO which have been frequently used.11 A mixture of 2,4-
dimethylphenol (1.2 equiv.), 4-bromotoluene (1a, 1 equiv.),
K2CO3 (3 equiv.), phen (5.0 mol%) and CuI (2.5 mol%) was
allowed to stir in toluene at 120 °C for 8 h. The result of the
reaction is summarized in Scheme 1.
To obtain the best representative results, in situ ESI-MS ana-
lysis was also carried out at 120 °C in order to detect the Cu
containing species which may be possible intermediates. A
similar reaction mixture in toluene was stirred at 120 °C for
2 h. The solution was then transferred to a GC vial in a dry
box. The temperature of the solution was kept at 120 °C by
immersing the GC vial in a sand bed, and ESI-MS spectra of
the solution were recorded (Fig. 1). The ESI-MS spectrum in
negative-ion mode showed a peak at m/z = 305.06 which is
identified as [Cu(I)(2,4-dimethylphenoxy)2]− (denoted as B)
according to its measured accurate mass and isotopic distri-
bution pattern (Fig. 2).12 In the positive-ion mode, peaks at
m/z = 365.07, m/z = 455.13 and m/z = 423.07 are identified
Fig. 1 (a) ESI(−)-MS and (b) ESI(+)-MS from the solution taken during
the reaction of 2,4-dimethylphenol and 4-bromotoluene with K2CO3 in
the presence of CuI and phen in toluene at 120 °C.
(pyrr = pyrrolidinonate) correspondingly,14 and Cu(I)(phen)-
(2,4-dimethylphenoxy) (denoted as C) was observed as {H[Cu(I)-
(phen)(2,4-dimethylphenoxy)]}+ in the ESI-MS spectrum.
Peak at m/z = 455.13 observed in the ESI-MS spectrum may
be assigned as A and/or A′ since they are isomers. 4-Bromo-
toluene reacts with C, via 2e oxidative addition reaction
(Scheme 2), to form Cu(III)(phen)(2,4-dimethylphenoxy)-
(p-tolyl)Br (denoted as D), which may be fragmented into Br−
and A′ during the ESI-MS measurement.4c Formation of A is
through the single electron transfer (SET) path in which C
transfers an electron to 4-bromotoluene to form [Cu(II)(phen)-
(2,4-dimethylphenoxy)]+ and a 4-bromotoluene free radical
anion. Further recombination of these two species produces A
(or A′) and Br− as suggested by DFT calculations (Scheme 2).4b
Thus observation of a peak at m/z = 455.13 may indicate that
the reaction goes through the free radical path if the signal is
A, or the non-free-radical path if the signal is A′ as both
mechanisms have been proposed and supported by DFT calcu-
lations.4b,c MS/MS experiments were tried in order to obtain
additional information. No informative data were observed
due to the very low intensity of the signal.
as {H[Cu(I)(phen)(2,4-dimethylphenoxy)]}+,13
A or A′ and
[Cu(phen)2]+ correspondingly according to their respective
measured accurate mass and isotopic distribution pattern
(Fig. 3).12,13
The presence of [Cu(phen)2]+ and B suggests the formation
of [Cu(phen)2][Cu(I)(2,4-dimethylphenoxy)2] in the reaction. It
has been reported that in a polar solvent (DMF) at −25 °C,
Cu(phen)(phth) and Cu(phen)(pyrr) can form [Cu(phen)2][Cu-
(phth)2] (phth = phthalimidate) and [Cu(phen)2][Cu(pyrr)2]
Effect of a free radical scavenger
One effective way to elucidate the possibility of a free radical
Scheme 1 C–O coupling reaction between aryl bromide and phenol.
path is the addition of a free radical scavenger into the system.
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Fig. 2 Simulated and experimental isotopic distribution of [Cu(I)(2,4-dimethylphenoxy)2]− (denoted as B), (a) simulated, (b) with 5 mol% phen and
(c) without phen.
Fig. 3 Isotopic distributions of (a) H[Cu(I)(phen)(2,4-dimethylphenoxy)]+, (b) A or A’ and (c) [Cu(phen)2]+.
11412 | Dalton Trans., 2014, 43, 11410–11417
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Fig. 4 Experimental and simulated EPR spectra for the reaction of 2,4-
dimethylphenol and 4-bromotoluene with K2CO3 in the presence of CuI
and phen in toluene at 373 K.
Scheme 2 Two possible reaction paths: 2e oxidative addition path and
single electron transfer (SET) path.
ture of 100 °C. A similar reaction mixture in toluene was
stirred at 120 °C for 2 h in a sealed tube. The upper portion of
the reaction solution was then transferred to an EPR tube in a
dry box. The EPR spectrum was recorded at 100 °C and a
signal around 3200 G was observed (Fig. 4). We are unable to
obtain for all temperatures. After fitting,18 the isotropic g value
is 2.188 which is in agreement with a Cu(II) signal.19 We tenta-
tively assign this signal to the complex E based on the result of
DFT calculations reported and our ESI-MS spectra.4b Although
the assignment cannot be conclusive, the presence of a Cu(II)
complex further supports the reaction which may go through a
SET mechanism (Scheme 2).
Scheme 3 C–O coupling reaction with 1-bromonaphthalene and
4-chlorobenzonitrile.
When 20 mol% of cumene was added into the reaction system,
the reaction yield dropped from 65% to 29%.15 When the
amount of cumene was increased to 50 mol%, the yield
dropped further to 8%. The radical scavenger inhibits the reac-
tion indicating the existence of a radical path (Scheme 1).16
It has been reported that 1-bromonaphthalene has a higher
reactivity than 4-chlorobenzonitrile in the C–O coupling reac-
tion if the non-free-radical path dominates.5 When the same
comparison study was performed based on our catalytic
system, 1-bromonaphthalene reacted to give 1-(2,4-dimethyl-
phenoxy)naphthalene (3a) in 41% yield; while 4-chlorobenzo-
nitrile gave 4-(2,4-dimethylphenoxy)benzonitrile (4a) in 84%
yield (Scheme 3). These observations together with the reaction
inhibition by cumene and the observation of prop-1-en-
2-ylbenzene (product of the reaction between cumene and the
tolyl radical) indicate that the free radical path is present or
even dominates in the reaction.17 Such a dual reaction path,
i.e. both the 2e oxidative addition path and free-radical path,
has been observed in the C–N coupling reaction in the mixed
base system.7b
Catalytic cycle for the Cu(I) catalyzed C–O coupling reaction
between 2,4-dimethylphenol and 4-bromotoluene in the
presence of phen
Two possible paths were proposed if the reaction goes through
the single electron transfer (SET) mechanism when phen was
used as the ligand (Scheme 2).
Based on all above observations and the reported DFT
studies, a catalytic cycle is proposed and shown in Scheme 4.
Complexes B and [Cu(phen)2]+ are generated by the reaction
among 2,4-dimethylphenol, potassium carbonate, phen and
CuI. Likewise, the ligand redistribution reaction between B
and [Cu(phen)2]+ produces the complex C in an equilibrium
fashion (C picks up a H+ and was observed as {H[Cu(I)(phen)-
(2,4-dimethylphenoxy)]}+ in the ESI-MS measurement).13,14
4-Bromotoluene then reacts with C to form A and Br− through
the SET oxidative addition process. Substitution of the ligand,
1-(2,4-dimethylphenoxy)-4-methylbenzene (2a), on A by Br−
produces Cu(I)(phen)Br, which further reacts with
a
2,4-dimethylphenoxy anion to produce C and completes the
catalytic cycle.4b
EPR measurements
In order to evaluate the existence of any Cu(II) intermediates,
The role of B in the reaction is still unclear. It may act as a
an in situ EPR study was also carried out on components of precursor of C by reacting with [Cu(phen)2]+, a species also
g-tensor because the spectrum was recorded at a high tempera- observed in the ESI-MS spectra, to generate C.5,14 It may also
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Scheme 4 The proposed catalytic cycle with phen.
act as a catalytically active species to react with an aryl
bromide forming an ether through the oxidative addition and
reductive elimination cycle.6,7 One way to evaluate the reactiv-
ity of B is to generate B in situ and evaluate its reactivity toward
aryl bromides in the absence of phen.6,7 In order to generate B
as the sole intermediate, only CuI/K2CO3 was used as the cata-
lyst to repeat the above reaction. The yield of the ligand-free
reaction reduced to zero under the same reaction conditions
and it was as low as 1.4% even when much longer reaction
time (48 h) was applied (Scheme 1).
Fig. 6 Isotopic distributions of [Cu(I)(2,4-dimethylphenoxy)Br]−.
C–O coupling reaction between 2,4-dimethylphenol and
4-bromotoluene without phen
In situ ESI-MS analysis was carried out at 120 °C in order to
confirm the presence of B in the reaction (Fig. 5). Similar
experimental procedures were carried out with similar
amounts of reagents except that the reaction time was 14 h.
The ESI-MS spectrum in negative-ion mode showed two peaks
at m/z = 305.06 and m/z = 262.91, which are identified as B and
[Cu(I)(2,4-dimethylphenoxy)Br]− respectively, according to their
Scheme 5 The proposed reaction path of the stoichiometric C–O
coupling reaction.
measured accurate mass and isotopic distribution pattern
(Fig. 2c and 6).12
Based on these observed Cu species of the stoichiometric
reaction (the turnover number of the reaction is less than one)
and the reported catalytic cycles in C–S and C–N coupling reac-
tion with similar intermediates, [Cu(NPh2)2]− and [Cu-
(SPh)2]−,6,7 a 2e oxidative addition path is proposed as shown
in Scheme 5. Although B can go through oxidative addition fol-
lowed by reductive elimination to generate ether, the contri-
bution of this reaction path in the phen added catalytic system
should be very limited because the yield of the reaction was
zero in the ligand free reaction system under similar reaction
conditions (reaction time 8 h).
Fig. 5 ESI (−)-MS from the solution taken during the reaction of 2,4-
dimethylphenol and 4-bromotoluene with K2CO3 in the presence of CuI
in toluene at 120 °C.
11414 | Dalton Trans., 2014, 43, 11410–11417
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was used as the internal standard in the quantitative GC
analyses.
Typical procedure of copper-catalyzed C–O coupling reaction
2,4-Dimethylphenol (0.147 ml, 1.2 mmol) and 4-bromotoluene
(0.123 ml, 1.0 mmol) were added into a Pyrex tube with a
septum. The tube was evacuated and backfilled with nitrogen
through a needle for 3 cycles, and then capped with a parafilm
before it was put into a dry box. CuI (4.8 mg, 0.025 mmol,
2.5 mol%), 1,10-phenanthroline (9.0 mg, 0.050 mmol, 5.0 mol
%), 1,4-di-tert-butylbenzene (19.0 mg, 0.1 mmol), K2CO3
(414.0 mg, 3.0 mmol) and toluene (2 ml) were added into the
tube in a dry box at RT, and capped with a Teflon screwcap.
The reaction mixture was stirred at 120 °C for 8 h. The GC
yield of 2a: 63% (1,4-di-tert-butylbenzene was used as the
internal standard, response factor for 2a: 1.1325).
Scheme 6 Comparison of selectivities in the C–O coupling reactions.
In addition, when a mixture of 1a (2 equiv.), 2-bromo-
toluene (1b, 2 equiv.), 2,4-dimethylphenol (1 equiv.), K2CO3
(3 equiv.), phen (5 mol%) and CuI (2.5 mol%) was allowed to
react in toluene at 120 °C for 4 h (Scheme 6), the yields of 2a
and 1-(2,4-dimethylbenzyl)-2-methylbenzene (2b) were 42%
and 8% respectively, with the 2a/2b ratio of 5.25. But, when a
similar reaction was carried out without phen for 48 h (cf.
longer reaction time was needed to obtain a measurable yield),
the yields of 2a and 2b were 1.1% and 0.7% respectively, with
the 2a/2b ratio of 1.5. The higher selectivity (higher 2a/2b
ratio) in the CuI/K2CO3/phen system is consistent with the
observations in ESI-MS analysis that the reaction mainly goes
through C, which would show a greater steric effect because of
the presence of the bulky phen ligand.5
Typical procedure of copper-catalyzed C–O coupling reaction
with a radical scavenger
2,4-Dimethylphenol (0.147 ml, 1.2 mmol) and 4-bromotoluene
(0.123 ml, 1.0 mmol) were added into a Pyrex tube with a
septum. The tube was evacuated and backfilled with nitrogen
through a needle for 3 cycles, and then capped with a parafilm
before it was put into a dry box. CuI (4.8 mg, 0.025 mmol,
2.5 mol%), 1,10-phenanthroline (9.0 mg, 0.050 mmol, 5.0 mol
%), 1,4-di-tert-butylbenzene (19.0 mg, 0.1 mmol), K2CO3
(414.0 mg, 3.0 mmol), cumene (5 mol%, 0.069 ml; 20 mol%,
0.276 ml; 50 mol% 0.690 ml) and toluene (2 ml) were added
into the tube in a dry box at RT, and capped with a Teflon
screwcap. The reaction mixture was stirred at 120 °C for 8 h.
The GC yield of 2a: 64%; 29%; 8% (1,4-di-tert-butylbenzene
was used as the internal standard, response factor for 2a:
1.1325).
Conclusions
Phen can enhance the reactivity of the Cu(I)/K2CO3 catalyzed
C–O coupling reaction by forming a possible intermediate C
which reacts with 1a to form A as the intermediate via a free
radical path, and steric selectivity can be introduced because
of the bulky phen ligand in C. It is worth noting that in the
reaction between [L2Cu][Cu(OPh)2] (L = trans-N,N′-dimethyl-
1,2-cyclohexanediamine) and a mixture of 1-bromonaphthal-
ene and 4-chlorobenzonitrile in DMSO, 1-bromonaphthalene
has higher reactivity than 4-chlorobenzonitrile, and a non-free-
radical path was proposed.5 This report together with our
present study indicates that the reaction mechanism depends
on the substrate, the base and even on the solvent used in the
reaction.
Copper-catalyzed C–O coupling reaction of 2,4-dimethylphenol
with 1-bromonaphthalene
2,4-Dimethylphenol (0.147 ml, 1.2 mmol) and 1-bromo-
naphthalene (0.140 ml, 1.0 mmol) were added into a Pyrex
tube with a septum. The tube was evacuated and backfilled
with nitrogen through a needle for 3 cycles, and then capped
with a parafilm before it was put into a dry box. CuI (4.8 mg,
0.025 mmol, 2.5 mol%), 1,10-phenanthroline (9.0 mg,
0.050 mmol, 5.0 mol%), 1,4-di-tert-butylbenzene (19.0 mg,
0.1 mmol), K2CO3 (414.0 mg, 3.0 mmol) and toluene (2 ml)
were added into the tube in a dry box at RT, and capped with a
Experimental section
All reagents were purchased from commercial sources and Teflon screwcap. The reaction mixture was stirred at 120 °C for
used without further purification. Copper(I) iodide (fine grey 8 h. The GC yield of 3a: 41% (1,4-di-tert-butylbenzene was used
powder), 2,4-dimethylphenol, 4-bromotoluene, 1,10-phen- as the internal standard, correction factor for 3a: 1.0758).
anthroline, 1,4-di-tert-butylbenzene were purchased from ACROS.
Copper-catalyzed C–O coupling reaction of 2,4-dimethylphenol
with 4-chlorobenzonitrile
K2CO3 was purchased from Alfa Aesar. Toluene (dried, secco-
Solv®) was purchased from Merck and purged with argon for
15 min before use. Reagents were transferred to the reaction 2,4-Dimethylphenol (0.147 ml, 1.2 mmol) and 4-chlorobenzo-
vessel (Pyrex tube with a Teflon screw cap) in a glove box. GC nitrile (0.138 g, 1.0 mmol) were added to a Pyrex tube with a
experiments were performed on an Agilent 6890N gas septum. The tube was evacuated and backfilled with nitrogen
chromatograph equipped with a 30 m × 0.53 mm × 3.0 m HP-1 through a needle for 3 cycles, and then capped with a parafilm
capillary column and a FID detector. 1,4-Di-tert-butylbenzene before it was put into a dry box. CuI (4.8 mg, 0.025 mmol,
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2.5 mol%), 1,10-phenanthroline (9.0 mg, 0.050 mmol, 5.0 mol%),
1,4-di-tert-butylbenzene (19.0 mg, 0.1 mmol), K2CO3
(414.0 mg, 3.0 mmol) and toluene (2 ml) were added into the
tube in a dry box at RT, and capped with a Teflon screwcap.
The reaction mixture was stirred at 120 °C for 8 h. The GC
yield of 4a: 84% (1,4-di-tert-butylbenzene was used as the
internal standard, response factor for 4a: 1.0714).
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Selectivity
2,4-Dimethylphenol (0.123 ml, 1.0 mmol), 4-bromotoluene
(1a) (0.246 ml, 2.0 mmol), and 2-bromotoluene (1b) (0.240 ml,
2.0 mmol) were added into a Pyrex tube with a septum. The
tube was evacuated and backfilled with nitrogen through a
needle for 3 cycles, and then capped with a parafilm before it
was put into a dry box. CuI (4.8 mg, 0.025 mmol, 2.5 mol%),
1,10-phenanthroline (9.0 mg, 0.050 mmol, 5 mol%), 1,4-di-tert-
butylbenzene (19.0 mg, 0.1 mmol), K2CO3 (414.0 mg,
3.0 mmol) and toluene (2 ml) were added into the tube in a
dry box at RT, and then capped with a Teflon screwcap. The
reaction mixture was stirred at 120 °C for 4 h. GC yield: 2a
42%, 2b 8% (1,4-di-tert-butylbenzene was used as the internal
standard, response factor for 2a and 2b: 1.1325).
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ESI-MS analysis
High-resolution ESI-MS were measured using a Waters LCT
Premier XE with a Z-spray atmospheric pressure ionization
source for ESI in the Mass Spectrometry Facility in the Insti-
tute of Chemistry, Academia Sinica. Leucine Enkephalin m/z
556.277 [M + H]+ was used as a reference standard. 10 μL of
samples were injected using a model Agilent 1100 autosampler
system with flow injection analysis (FIA). The mobile phase
was 100% acetonitrile at a flow rate of 50 μL min−1
.
EPR measurements. EPR measurements were performed at
the X-band using a Bruker E580 spectrometer equipped with a
Bruker ELEXSYS super-high-sensitivity cavity in National Tsing
Hua University. X-band EPR spectra of toluene reaction solu-
tion in a 4 mm EPR tube at 373 K were obtained with a micro-
wave power of 15.000 mW, a frequency at 9.6589 GHz, a ADC
conversion time of 20.39 ms, a receiver gain of 30, and a
modulation amplitude of 0.16 G at 100 kHz with a phase of 0.0
deg. EPR spectra were examined using the program WINEPR.
Simulations were carried out using the EasySpin toolbox in
Matlab.16
Acknowledgements
We are grateful to the National Science Council, Republic of
China, and Academia Sinica for financial support of this work.
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[Cu(NPh2)2]3 was isolated from the CuI/tBuONa/phen cata-
lyzed C–N coupling reaction C. K. Tseng, M. C. Tseng,
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11416 | Dalton Trans., 2014, 43, 11410–11417
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Adding cumene changes the volume of the reaction
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evaluate the % of free radical path in the system by com-
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and without cumene, 2.69 ml of toluene was used in the
reaction without cumene, and 2.00 of toluene and 0.690 ml
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This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 11410–11417 | 11417