Ligand free copper(i)-catalyzed synthesis of diaryl ether with Cs2CO3via a free radical path

Article abstract of DOI:10.1039/c5dt00151j

Complexes [Cu(i)(2,4-dimethylphenoxy)₂]^- (A) and [Cu(ii)(2,4-dimethylphenoxy)₂(p-tolyl)]^- (B) were observed by in situ electrospray ionization mass spectrometry (ESI-MS) analysis of the ligand free copper(i)-catalyzed C-O coupling reaction using Cs₂CO₃ under the catalytic reaction conditions indicating that they could be intermediates in the reaction. The radical scavenger cumene retarded the reaction. Catalytic cycles involving a free radical path are proposed based on these observations.

Full text of DOI:10.1039/c5dt00151j

Dalton
Transactions
View Article Online
View Journal | View Issue
COMMUNICATION
Ligand free copper(I)-catalyzed synthesis of diaryl
ether with Cs CO via a free radical path†
2
3
Cite this: Dalton Trans., 2015, 44,
12086
a,b
a
c
a,c
Hong-Jie Chen, Mei-Chun Tseng, I-Jui Hsu, Wei-Ting Chen,
Received 13th January 2015,
Accepted 24th March 2015
b
a
Chien-Chung Han* and Shin-Guang Shyu*
DOI: 10.1039/c5dt00151j
www.rsc.org/dalton
Complexes [Cu(I)(2,4-dimethylphenoxy)
]− (A) and [Cu(II)(2,4-di- is identified as the key factor for improving the reaction con-
2
methylphenoxy)
spray ionization mass spectrometry (ESI-MS) analysis of the ligand similar good results.
free copper(I)-catalyzed C–O coupling reaction using Cs CO the nature of cesium because cesium phenoxide is relatively
under the catalytic reaction conditions indicating that they could soluble in organic solvents and may also enhance the solubi-
2
(p-tolyl)]− (B) were observed by in situ electro- ditions because other metal carbonates and bases do not give
6
a,c,7
The improvement is proposed due to
2
3
−
6a
be intermediates in the reaction. The radical scavenger cumene lity of the proposed intermediate [Cu(OAr) ] . Recently, we
2
retarded the reaction. Catalytic cycles involving a free radical path have reported the Ullmann type C–N cross coupling arylation
are proposed based on these observations.
having different reactivities when using different metal tert-
butoxides. The aryl halide activation step changes from a
non-free radical path to a free radical path when sodium tert-
8
Ullmann-type C–O cross coupling arylation in the synthesis of
aryl ethers usually consists of a ligand, a base and a copper
butoxides in the catalytic system switch to potassium tert-
1
salt. The addition of a ligand usually gives a better yield of
8a,b
butoxides.
These discoveries in both Ullmann type C–N and
the reaction, and different ligands may have different catalytic
C–O cross coupling reactions may imply that the activity
enhancement in the Ullmann type C–O cross coupling reaction
2
activities. Thus, a Cu(I) complex with an additive ligand is
generally proposed as the intermediate involved in the aryl
using cesium carbonate alone may have
a mechanism
3
,4
halide activation step of the catalytic cycle. For elucidating
the reaction mechanism, complexes LCu(I)(OAr) with the addi-
tive ligand L have been prepared, and their catalytic activities
different from reactions using other metal carbonates. We
9
herein report the in situ ESI-MS analysis of a ligand free
Ullmann type Cu(I)-catalyzed cross C–O coupling reaction
4
have been evaluated. Both the free radical path and con-
using Cs
2
CO
]
3
as the base. Complexes [Cu(I)(2,4-dimethyl-
cerned 2e oxidative addition path have been proposed for the
−
phenoxy)
2
(denoted as A) and [Cu(II)(2,4-dimethyl-
3
b,c,4
aryl halide activation step.
Experimental and theoretical
−
phenoxy) (p-tolyl)] (denoted as B) were observed in the
2
studies support both mechanisms indicating that various
reaction system indicating that they could be the intermediates
in the reaction. Addition of the radical scavenger cumene
retarded the reaction indicating the existence of a free radical
pathway in the catalytic cycle of the reaction. In addition, an
in situ EPR study of the reaction solution detected a Cu(II)
species with a fitted g value of 2.065. A catalytic cycle com-
posed of a free radical path is proposed based on these
observations.
factors (such as solvent and substrates used) affect the reaction
3
b,c,5
mechanism.
Cesium carbonate is a common base used in the Ullmann
type C–O cross coupling reaction since the first report of its
1
a,b,2f,6
application in the reaction.
The reaction can be carried
out in toluene, a relatively environmentally benign solvent,
and good yield is obtained under relatively mild reaction con-
ditions without the addition of a ligand. Cesium carbonate
6
a
We followed the general procedure reported in the literature
to investigate the Cu(I)-catalyzed C–O cross coupling arylation
between aryl bromide and 2,4-dimethylphenol using Cs CO
a
2
3
Institute of Chemistry, Academia Sinica, 128 Academia Road Sec. 2, Nankang,
without a ligand. A mixture of 2,4-dimethylphenol (1.2 equiv.),
aryl bromide (1 equiv.), Cs CO (3 equiv.) and CuI (2.5 mol%)
Taipei 115, Taiwan, Republic of China. E-mail: sgshyu@chem.sinica.edu.tw;
Fax: +886-2-2783-1237; Tel: +886-2-2789-8593
2
3
b
1b,6b
Department of Chemistry, National Tsing Hua University, 101, Sec 2, Kuang-Fu Rd,
was stirred in toluene at 120 °C for 8 h.
reactions are summarized in Table 1.
The results of the
Hsinchu 30013, Taiwan, Republic of China
c
Department of Molecular Science and Engineering, National Taipei University of
The possibility of a free radical path was evaluated by
Technology, 1, Sec. 3, Zhongxiao E Road, Taipei 10608, Taiwan, Republic of China
1
0
adding a free radical scavenger (cumene) in the reaction.
†
Electronic supplementary information (ESI) available: GC data and ESI spectra.
See DOI: 10.1039/c5dt00151j
Under the same reaction conditions, the addition of cumene
12086 | Dalton Trans., 2015, 44, 12086–12090
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Communication
Table 1 Ligand-free C–O cross coupling reactions between aryl halide
2 3
and 2,4-dimethylphenol using Cs CO
Scheme 1 Reaction between cumene and aryl radical.
1
1
c,d
ponding Ar–H (Scheme 1). The detection of prop-1-en-2-
Conv./GC yield
(%) with 50 mol% ylbenzene by GC-MS confirms the presence of [CH
C H ] . The
3 6 4
•
Aryl b
ether
CuI
Conv./GC
c,d
e
Entry ArX
(mol%) yield (%) cumene
other product toluene cannot be identified because toluene is
the reaction medium. In order to reconfirm the presence of Ar
•
1
2
3
4
5
2.5
2.5
—
76/68(90)
—
—
in the C–O cross coupling reaction, 2-bromo-1,4-dimethyl-
benzene and 1-bromonaphthalene were used instead, and
both prop-1-en-2-ylbenzene and the respective Ar–H products,
i.e. p-xylene and naphthalene, were observed (Table 1 entries 7
and 9).
The theoretical study suggests that [(ket)Cu(I)OPh] (ket =
β-diketonate) reacts with ArI to generate Ar and I through a
single electron transfer (SET) process in the C–O cross coup-
69/33(48)
—
9/4(44)
22/22(99)
—
—
—
−
—
51/6(12)
•
−
6
7
2.5
2.5
42/41(98)
—
2 3
ling reaction catalyzed by the CuI-β-diketone-Cs CO catalyst
system. Similar to [(ket)Cu(I)(2,4-dimethylphenoxy)] , complex
A generated by the reaction among 2,4-dimethylphenol,
−
f
—
31/24(77)
Cs
2 3
CO and CuI may react with 4-bromotoluene to produce
•
[
Cu(II)(2,4-dimethylphenoxy)
2
3 6 4
] (denoted as C), [CH C H ] and
8
9
2.5
2.5
25/23(92)
—
−
3b
Br through SET.
•
The other possible path generating Ar is the reaction
between aryl halide and Cs(2,4-dimethylphenoxy) generated in
the reaction. (phen)M(OtBu) (phen = 1,10-phenanthroline;
g
—
30/0(0)
h
i
M = Na, K) and (phen)K(NAr′
Ar which can further react with the solvent toluene to form
) can react with ArI to generate
1
1
1
1
0
1
2
3
2.5
2.5
2.5
2.5
67/64(95)
65/60(92)
2
•
8,11c
•
the C–H arylation product CH C H Ar′.
The Ar is proposed
)] to generate [phenCu(II)(NAr′ )-
Ar)] as an intermediate in the C–N cross coupling reaction
3
6
4
i
j
to react with [phenCu(I)NAr′
(
70/49(70)
2
2
k
72/67(90)
catalyzed by CuI-phen-(tBuONa/K CO ) mixed-base or CuI-
phen-tBuOK catalytic systems.
2
3
8
b,c
In order to evaluate the
a
8
h for reactions with 2.5 mol% of CuI, 2.5 mol% of CuCl and 48 h possibility that Cs(2,4-dimethylphenoxy)—similar to (phen)M-
2
b
c
for reactions without CuI. R = 2,4-dimethylphenyl. Yield calculation
(
OtBu) (M = Na, K) and (phen)K(NAr′ )—can react with aryl
2
based on the amount of ArX used; using 1,4-di-tert-butylbenzene as an
internal standard in GC analysis. Selectivity in the parenthesis.
Prop-1-en-2-ylbenzene was detected by GC-MS. p-Xylene was detected coupling reaction between 2,4-dimethylphenol and 4-iodoto-
by GC-MS. Naphthalene was detected by GC-MS. 2.5 mol% CuI and
mol% Cs(2,4-dimethylphenoxy) in toluene were stirred at 120 °C for
h. Then 2,4-dimethylphenol and 4-bromotoluene were added at RT,
•
d
2 3
halide to form Ar , only Cs CO was used in the C–O cross
e
f
g
h
luene (or 4-bromotoluene) under similar reaction conditions
5
8
(
Table 1, entries 3 and 4).
i
j
and the mixture was stirred at 120 °C for 8 h. 2.5 mol% CuCl
2
. Prop-
-en-2-ylbenzene was not detected by GC-MS. 2.5 mol% CuCl and
mol% Cs(2,4-dimethylphenoxy) in THF were stirred at 120 °C for 8 h.
Product yields of 22% and 4% were obtained with selectivity
k
1
5
2
of 99% and 44% for the reactions with 4-iodotoluene and with
-bromotoluene, respectively. Addition of the radical scavenger
4
After removing THF, 2,4-dimethylphenol, 4-bromotoluene and toluene
were added, and the mixture was stirred at 120 °C for 8 h.
cumene to the reaction reduced the product yield to 6% in the
case of 4-iodotoluene, and prop-1-en-2-ylbenzene was observed
by GC-MS (Table 1, entry 5).
In order to confirm the role of Cs(2,4-dimethylphenoxy) in
the electron transfer path, Cs(2,4-dimethylphenoxy), syn-
reduced the yield from 68% (without cumene) to 33% in the thesized through transmetalation reaction between K(2,4-di-
case of 4-bromotoluene indicating the presence of a radical methylphenoxy) and CsF, was allowed to react with
reaction path (Table 1, entries 1 and 2).
4-iodotoluene to produce 1-(2,4-dimethylphenoxy)-4-methyl-
•
Reaction between [CH C H ] and cumene should produce benzene. Again, the addition of cumene to the reaction
3 6 4
•
prop-1-en-2-ylbenzene and toluene because Ar reacts with reduced the product yield from 10% to 3% indicating that
cumene to produce prop-1-en-2-ylbenzene and the corres- Cs(2,4-dimethylphenoxy) is capable of transferring electrons to
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 12086–12090 | 12087

View Article Online
Communication
Dalton Transactions
Scheme 2 C–O cross coupling reaction between Cs(2,4-dimethyl-
phenoxy) and iodotoluene.
•
Scheme 3 The proposed mechanism of path I.
aryl halide to form Ar under the reaction conditions
(Scheme 2).
In order to identify the intermediates, in situ ESI-MS analy-
sis was carried out at 120 °C—the same temperature as the
parent catalytic reaction. A similar reaction mixture in toluene
was stirred at 120 °C for 2 h in a dry box. The solution was
then transferred to a GC vial. The temperature of the solution
was maintained at 120 °C by immersing the GC vial in a sand
bed, and ESI-MS spectra of the solution were then recorded.
Two peaks at m/z = 305.64 and m/z = 397.12 were observed
individually in the negative-ion mode of the ESI-MS in four
different measurements (Fig. 1). They are identified as A and B
•
deduced. In path I, a free radical [CH
3
C
6
H
4
] , generated
through the electron transfer process between Cs(2,4-dimethyl-
phenoxy) and 4-bromotoluene, reacts with [Cs(2,4-dimethyl-
phenoxy)] to produce ether (Scheme 3). We consider this
path as insignificant because the yield of the reaction with
only Cs CO is relatively low (Table 1, entry 3). In path II, an
•+
2
3
•
3 6 4
aryl free radical [CH C H ] , generated through the aryl halide
SET activation process between A and 4-bromotoluene, reacts
with C to form [Cu(I)(2,4-dimethylphenoxy)(1-(2,4-dimethyl-
phenoxy)-4-methylbenzene)] (denoted as D). Substitution of
the ligand 1-(2,4-dimethylphenoxy)-4-methylbenzene on D by
Br produces ether and [Cu(I)(2,4-dimethylphenoxy)Br] ,
which further reacts with the 2,4-dimethylphenoxy anion to
(or
[Cu(0)(2,4-dimethylphenoxy)(1-(2,4-dimethylphenoxy)-4-
−
methylbenzene)] , denoted as B′) corresponding to their
measured accurate mass and isotope distributions (Fig. 2).
Based on all the above observations, three possible reaction
paths in the Ullmann type C–O cross coupling arylation can be
−
−
3b
regenerate A to complete the catalytic cycle (Scheme 4). In
path III, [CH C H ] (generated through path I or path II)
•
3
6
4
reacts with A to form B (or B′) followed by reductive elimin-
ation (in the case of B) or substitution of 1-(2,4-dimethyl-
Fig. 1 In situ ESI(−)-MS from the solution taken during the reaction of
2 3
2,4-dimethylphenol and 4-bromotoluene with Cs CO in the presence
of CuI in toluene at 120 °C from two individual experiments.
Fig. 2 Simulated and experimental isotopic distributions of (a) A and (b)
B or B’.
Scheme 4 The proposed mechanism of path II.
12088 | Dalton Trans., 2015, 44, 12086–12090
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Communication
isotopic g value is 2.065 which is in agreement with a Cu(II)
1
3
signal. The spectrum was taken at a relatively high tempera-
ture (298 K), and all components of g-tensor could not be
resolved and hence further structural details of the Cu(II)
complex could not be obtained.
If path II dominates in the reaction, Cu(II) halide should be
1
4
able to catalyse the C–O coupling reaction because Cu(II)
halide can react with Cs(2,4-dimethylphenoxy) to form C
which is proposed as an intermediate in path II. Replacing CuI
2
with CuCl in the catalytic system produced ether with com-
parable yield (60%, Table 1, entry 11) supporting that C is a
possible intermediate in path II. In situ preparation of C by the
reaction of CuCl with two equivalents of Cs(2,4-dimethyl-
2
phenoxy) (attempt to synthesize pure C failed) followed by the
addition of other reagents obtained ether with 67% yield
Scheme 5 The proposed mechanism of path III.
(
Table 1, entry 13) further supporting the above argument.
•
We propose that the source of [CH C H ] is mainly from
3
6
4
−
the SET reaction between [Cu(I)(2,4-dimethylphenoxy)
-bromotoluene because the yield of the ether without the
addition of Cu(I) salt is very low (Table 1, entry 3) indicating
2
] and
phenoxy)-4-methylbenzene by Br− (in the case of B′) to gene-
rate the product 1-(2,4-dimethylphenoxy)-4-methylbenzene and
Cu(2,4-dimethylphenoxy) after oxidation of [Cu(2,4-dimethyl-
4
•
3 6 4
the insignificant contribution of [CH C H ] generated from
−
•+
phenoxy)] by [Cs(2,4-dimethylphenoxy)] or C (Scheme 5).
In order to confirm the role of A in the proposed reaction
paths, we tried to evaluate the catalytic activity of Cs[Cu(2,4-di-
the reaction between Cs(2,4-dimethylphenoxy) and 4-bromoto-
luene (Scheme 3). Also, the addition of cumene in the CuCl2
catalytic system did not effectively retard the reaction (Table 1,
entry 12) implying that C can react with 4-bromotoluene
through a non-radical path. This echoes the partial reduction
of the yield of ether from 68% (without cumene) to 33%
methylphenoxy)
dimethylphenoxy) ], in situ prepared 2.5 mol% of Cs[Cu(2,4-di-
methylphenoxy) ]—by reacting 2.5 mol% CuI with 5.0 mol% of
2
Cs(2,4-dimethylphenoxy)—was allowed to react with 2,4-
dimethyl phenol (1.2 equiv.), aryl bromide (1 equiv.) and
2 3
Cs CO (3 equiv.) in toluene under similar reaction conditions,
and a comparable yield of ether (64%; Table 1, entry 10) was
obtained. In addition, stoichiometric reaction among CuI
2
]. Because we failed to obtain pure Cs[Cu(2,4-
2
(
addition of cumene) (Table 1, entries 1 and 2) which indicates
the presence of a nonradical path if cumene can 100% inhibit
the radical path in the reaction (Table 1, entries 1 and 2).
Based on the above considerations, recombination of
•
[
CH C H ] and C to form D in path II shown in Scheme 4 is
3
6
4
(1 equiv.), Cs(2,4-dimethylphenoxy) (2 equiv.) and 4-bromoto-
proposed as the major path of the reaction in spite of the
luene (2 equiv.) under similar reaction conditions producing
ether with 84% yield further supports the above argument.
In both paths II and III, a Cu(II) complex (C in path II, B in
path III) is involved as an intermediate. The in situ EPR study
was also carried out at 120 °C in order to evaluate the presence
of the Cu(II) moiety in the reaction. 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. We observed a signal around 3000G
detection of B (or B′) in path III which indicates that the com-
•
bination reaction between [CH C H ] and A exists. The
3
6
4
absence of D in the ESI-MS analysis does not confirm the non-
existence of D because neutral species is very difficult to be
detected by ESI-MS.
These results echo the findings in the studies of C–O cross
coupling using CuI and K
2
CO
the ligand. A similar intermediate A was detected in the
ESI-MS of the K CO –CuI catalytic system but the yield of the
3
without the addition of phen as
5
2
3
1
2
in the EPR spectrum taken at 25 °C (Fig. 3). After fitting, the
reaction was very low. We did not evaluate the possibility of
the radical path in the K CO –CuI system because of the low
2
3
−
2
yield. If [Cu(I)(2,4-dimethylphenoxy) ] can activate 4-bromoto-
luene via SET, then we should obtain similar yield in both
K CO –CuI and Cs CO –CuI catalytic systems. Different
2
3
2
3
counter cations should be the key factor for the difference in
yield because cesium phenoxide has a higher solubility than
6
a
potassium phenoxide.
Conclusions
Fig. 3 (a) Experimental and (b) simulated EPR spectra for the reaction
Catalytic cycles with a SET aryl bromide activation step are pro-
posed for the C–O cross coupling reactions catalyzed by the
between 2,4-dimethylphenol and 4-bromotoluene with Cs
in toluene at 298 K.
2 3
CO and CuI
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 12086–12090 | 12089

View Article Online
Communication
Dalton Transactions
Cs
2
CO
3
–CuI system based on the detection of complexes A and
D. Venkataraman, Org. Lett., 2001, 3, 4315; (c) D. Ma and
Q. Cai, Org. Lett., 2003, 5, 3799; (d) Y.-J. Chen and
H.-H. Chen, Org. Lett., 2006, 8, 5609; (e) H. Rao, Y. Jin,
H. Fu, Y. Jiang and Y. Zhao, Chem. – Eur. J., 2006, 12, 3636;
(f) R. A. Altman, A. Shafir, A. Choi, P. A. Lichtor and
S. L. Buchwald, J. Org. Chem., 2008, 73, 284; (g) N. Xia and
M. Taillefer, Chem. – Eur. J., 2008, 14, 6037; (h) S. Benyahya,
F. Monnier, M. W. C. Man, C. Bied, F. Ouazzani and
M. Taillefer, Green Chem., 2009, 11, 1121.
B in the in situ ESI-MS study. The in situ EPR study confirms
the presence of a Cu(II) species in the reaction solution.
Addition of the free radical scavenger cumene reduced the
yield of the reaction, and the detection of the aryl compound
and prop-1-en-2-ylbenzene formed from the reaction between
the Ar and cumene further supports the existence of Ar and
thus the free radical path of the aryl halide activation step in
the catalytic cycle.
•
•
7
(a) B.-A. Wang, R.-S. Zeng, H.-Q. Wei, A.-Q. Jia and
J.-P. Zou, Chin. J. Chem., 2006, 24, 1062; (b) H. Fu, Y. Jin,
J. Liu, Y. Yin, Y. Jiang and Y. Zhao, Synlett, 2006, 1564;
Notes and references
(
c) Y. Zhao, Y. Wang, H. Sun, L. Li and H. Zhang, Chem.
1
(a) M. Wolter, G. Nordmann, G. E. Job and S. L. Buchwald,
Org. Lett., 2002, 4, 973; (b) H. Zhang, D. Ma and W. Cao,
Synlett, 2007, 243; (c) J. J. Niu, H. Zhou, Z. G. Li, J. W. Xu
and S. J. Hu, J. Org. Chem., 2008, 73, 7814; (d) Q. Zhang,
D. Wang, X. Wang and K. Ding, J. Org. Chem., 2009, 74,
Commun., 2007, 3186; (d) Q. Zhang, D. Wang, X. Wang and
K. Ding, J. Org. Chem., 2009, 74, 7187.
8 (a) C.-K. Tseng, C.-R. Lee, C.-C. Han and S.-G. Shyu, Chem.
– Eur. J., 2011, 17, 2716; (b) C.-K. Tseng, M.-C. Tseng,
C.-C. Han and S.-G. Shyu, Chem. Commun., 2011, 47, 6686;
(c) C.-K. Tseng, C.-R. Lee, M.-C. Tseng, C.-C. Han and
S.-G. Shyu, Dalton Trans., 2014, 43, 7020.
7
187; (e) J. Niu, P. Guo, J. Kang, Z. Li, J. Xu and S. Hu,
J. Org. Chem., 2009, 74, 5075.
2
(a) H. J. Cristau, P. P. Cellier, S. Hamada, J. F. Spindler and
M. Taillefer, Org. Lett., 2004, 6, 913; (b) W. L. Bao and X. Lv,
J. Org. Chem., 2007, 72, 3863; (c) A. B. Naidu and G. Sekar,
Tetrahedron Lett., 2008, 49, 3147; (d) S. Benyahya,
F. Monnier, M. Taillefer, M. W. C. Man, C. Bied and
F. Ouazzani, Adv. Synth. Catal., 2008, 350, 2205;
9 (a) P. Chen, Angew. Chem., Int. Ed., 2003, 42, 2832;
(b) P. Chen and X. Zhang, Chem. – Eur. J, 2003, 9, 1852;
(c) M. N. Eberlin, F. Coelho, L. S. Santos, C. H. Pavam and
W. P. Almeida, Angew. Chem., Int. Ed., 2004, 43, 4330;
(d) N. G. Tsierkezos, M. Buchta, P. Holy and D. Schroder,
Rapid Commun. Mass Spectrom., 2009, 23, 1550.
(
e) N. Barbero, R. SanMartin and E. Domínguez, Green 10 H. L. Aalten, G. van Koten, D. M. Grove, T. Kuilman,
Chem., 2009, 11, 830; (f) A. B. Naidu, E. A. Jaseer and
G. Sekar, J. Org. Chem., 2009, 74, 3675; (g) Y.-H. Liu, G. Li
O. G. Piekstra, L. A. Hulshof and R. A. Sheldon, Tetra-
hedron, 1989, 45, 5565.
and L.-M. Yang, Tetrahedron Lett., 2009, 50, 343; 11 (a) C. J. C. Whitehouse, S. G. Bell and L.-L. Wong, Chem. –
(
4
2
h) L. M. Yang, Y. H. Liu and G. Li, Chin. J. Chem., 2009, 27,
23; (i) D. Maiti and S. L. Buchwald, J. Am. Chem. Soc.,
009, 131, 17423; ( j) P.-F. Larsson, A. Correa, M. Carril,
Eur. J., 2008, 14, 10905; (b) We did not detect 4-4′-bitolyl—a
C–H arylation product one might expect to be formed by
the reaction between the leaked product of 4-tolyl radical
(via the fragmentation of 4-tolylbromide radical anion)
with the solvent toluene. The 4-tolylbromide radical may
not be reactive enough to carry out the C–H arylation
reaction (E. Shirakawa, K. Itoh, T. Higashino and
T. Hayashi, J. Am. Chem. Soc., 2010, 132, 15537);
(c) W. Liu, H. Cao, H. Zhang, H. Zhang, K. H. Chung,
C. He, H. Wang, F. Y. Kwong and A. Lei, J. Am. Chem.
Soc., 2010, 132, 16737.
P.-O. Norrby and C. Bolm, Angew. Chem., Int. Ed., 2009, 48,
691.
5
3
(a) A. Shafir, P. A. Lichtor and S. L. Buchwald, J. Am. Chem.
Soc., 2007, 129, 3490; (b) G. O. Jones, P. Liu, K. N. Houk
and S. L. Buchwald, J. Am. Chem. Soc., 2010, 132, 6205;
(
c) H. Z. Yu, Y. Y. Jiang, Y. Fu and L. Liu, J. Am. Chem. Soc.,
010, 132, 18078; (d) E. Rezabal, L. Duchackova, P. Milko,
M. C. Holthausen and J. Roithova, Inorg. Chem., 2010, 49,
2
8
421; (e) G. Lefevre, G. Franc, A. Tlili, C. Adamo, 12 S. Stoll and A. Schweiger, J. Magn. Reson., 2006, 178, 42.
M. Taillefer, I. Ciofini and A. Jutand, Organometallics, 2012, 13 (a) E. Garribba and G. Micera, J. Chem. Educ., 2006, 83,
3
1, 7694.
1229; (b) P. Adão, S. Barroso, F. Avecilla, M. C. Oliveira and
J. C. Pessoa, J. Organomet. Chem., 2014, 760, 212;
(c) H. Gallardo, E. Meyer, A. J. Bortoluzzi, F. Molin and
A. S. Mangrich, Inorg. Chim. Acta, 2004, 357, 505.
4
5
J. W. Tye, Z. Q. Weng, R. Giri and J. F. Hartwig, Angew.
Chem., Int. Ed., 2010, 49, 2185.
(a) H.-J. Chen, I.-J. Hsu, M.-C. Tseng and S.-G. Shyu, Dalton
Trans., 2014, 43, 11410; (b) J. Limberger, B. C. Leal, 14 (a) A. Naidu, O. Raghunath, D. Prasad and G. Sekar, Tetra-
D. F. Back, J. Dupont and A. L. Monteiroa, Adv. Synth.
Catal., 2012, 354, 1429.
(a) J. F. Marcoux, S. Doye and S. L. Buchwald, J. Am. Chem.
Soc., 1997, 119, 10539; (b) R. K. Gujadur, C. G. Bates and
hedron Lett., 2008, 49, 1057; (b) K. Swapna, S. Murthy,
M. Jyothi and Y. Nageswar, Org. Biomol. Chem., 2011, 9,
5978; (c) S. Verma, N. Kumar and S. Jain, Tetrahedron Lett.,
2012, 53, 4665.
6
12090 | Dalton Trans., 2015, 44, 12086–12090
This journal is © The Royal Society of Chemistry 2015