A practical synthesis of 2-aminobenzothiazoles via copper(I)-catalyzed tandem reaction of 2-iodoanilines with isothiocyanates in ionic liquids

Article abstract of DOI:10.1002/aoc.3453

A copper(I)-catalyzed tandem reaction of 2-iodoanilines with isothiocyanates was achieved in hydrophobic [bmim][PF₆] ionic liquid under mild conditions, generating a variety of 2-aminobenzothiazoles in good to excellent yields. The tandem reaction that was carried out in [bmim][PF₆] has some obvious advantages such as accelerated reaction rate and increased yield as compared with the reaction run in volatile solvents such as toluene. Furthermore, the CuI/1,10-phenanthroline catalytic system can be reused up to eight times without loss of activity and efficiency.

Full text of DOI:10.1002/aoc.3453

Full paper
Received: 4 December 2015
Revised: 3 January 2016
Accepted: 18 January 2016
Published online in Wiley Online Library: 10 March 2016
(wileyonlinelibrary.com) DOI 10.1002/aoc.3453
A practical synthesis of 2-aminobenzothiazoles
via copper(I)-catalyzed tandem reaction of
2-iodoanilines with isothiocyanates in
ionic liquids
Ling Chen, Bin Huang, Quan Nie and Mingzhong Cai*
A copper(I)-catalyzed tandem reaction of 2-iodoanilines with isothiocyanates was achieved in hydrophobic [bmim][PF6] ionic
liquid under mild conditions, generating a variety of 2-aminobenzothiazoles in good to excellent yields. The tandem reaction that
was carried out in [bmim][PF6] has some obvious advantages such as accelerated reaction rate and increased yield as compared
with the reaction run in volatile solvents such as toluene. Furthermore, the CuI/1,10-phenanthroline catalytic system can be reused
up to eight times without loss of activity and efficiency. Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: copper(I) catalysis; 2-aminobenzothiazole; tandem reaction; ionic liquid; 2-iodoaniline
challenges presented either by mounting the catalyst on a solid sup-
port or by preparing designer ligands for use in aqueous or fluorous
Introduction
2-Aminobenzothiazoles, one of the important classes of heterocy-
clic compounds, are broadly found in biological chemistry and
medicinal areas.[1] A number of 2-anilinobenzothiazole derivatives
are anticancer active and the 2-aminobenzothiazole moieties are
privileged pharmacophores as well as useful intermediates.[2]
Therefore, the development of new 2-aminobenzothiazole
structures and new methods for their constructions has attracted
considerable interest.[3] 2-Aminobenzothiazoles are usually pre-
pared via palladium- or copper-catalyzed intramolecular cyclization
of ortho-bromobenzothioureas.[4] However, the substrates are not
readily available and an additional step for the preparation of
ortho-bromobenzothioureas is necessary, starting from amines
and 2-bromophenyl isothiocyanates. Recently, the transition-
metal-catalyzed tandem reactions of 2-halobenzenamines and
isothiocyanates for the construction of 2-aminobenzothiazoles
have received much attention because of their efficiency and low
cost.[5] For example, Wu and co-workers reported a copper(I)-cata-
lyzed tandem reaction between 2-iodobenzenamines and isothio-
cyanates using the CuI/1,10-phenanthroline catalytic system to
prepare 2-aminobenzothiazoles;[5a] the groups of Li and Ding
described iron-catalyzed tandem reactions of 2-halobenzenamines
and isothiocyanates leading to 2-aminobenzothiazoles.[5d,e] Ma et al.
developed a new copper-catalyzed cascade three-component reac-
tion for the synthesis of 2-aminobenzothiazoles from 2-haloanilines,
carbon disulfide and amines.[6]
biphasic systems.
There are very important economic and environmental reasons
for developing recyclable catalytic systems for organic transforma-
tions from both academic and industrial perspectives. In order to
satisfy both recyclability and environmental concerns, a more facile
and efficient approach is to immobilize the catalyst in a liquid phase
by dissolving it into a nonvolatile and nonmixing liquid, such as ionic
liquids[7] and poly(ethylene glycol)s.[8] Room temperature ionic
liquids, especially those based upon the 1,3-dialkylimidazolium
cation, have attracted much attention in recent years.[9] They offer
an alternative and ecologically sound medium compared to
conventional organic solvents because of their unique properties
such as nonvolatility, nonflammability, excellent chemical and
thermal stability, recyclability and dissolving ability for a wide range
of organic and inorganic materials. Furthermore, their good
compatibility with transition metal catalysts and limited miscibility
with common solvents enable easy product and catalyst separation
with the retention of the stabilized catalyst in the ionic phase.[7]
These and related ionic liquids have been used as green solvents
for hydrogenations,[10] alkene dimerizations,[11] Friedel–Crafts
reactions,[12] Diels–Alder reactions,[13] Heck reactions,[14] Bechmann
condensations,[15] Suzuki reactions,[16] Baylis–Hillman reactions,[17]
Stille reactions,[18] Sonogashira reactions[19] and carbonylation
Although these transition-metal-catalyzed tandem reactions of
2-halobenzenamines and isothiocyanates are highly efficient for
the construction of 2-aminobenzothiazoles, homogeneous catalysts
cannot be recycled, and are difficult to separate from the product
mixture, which is a particularly significant drawback for application
in the pharmaceutical industry. Ideally, one would like to be able to
recover and recycle the entire catalyst system but avoid the
*
Correspondence to: Mingzhong Cai, Key Laboratory of Functional Small
Organic Molecules, Ministry of Education and College of Chemistry and
Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR
China. E-mail: caimzhong@163.com
Key Laboratory of Functional Small Organic Molecules Ministry of Education and
College of Chemistry and Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, PR China
Appl. Organometal. Chem. 2016, 30, 446–450
Copyright © 2016 John Wiley & Sons, Ltd.

Cu(I)-catalyzed synthesis of 2-aminobenzothiazoles in ionic liquids
reactions.[20] However, copper-catalyzed organic transformations in
ionic liquids have received less attention.[21]
copper catalysts examined, Cu(OAc)2, CuSO4 and CuI are
efficient catalysts and afford the desired product in high yields
(Table 1, entries 1, 2 and 5), with CuI being the best choice (entry
5). While other copper catalysts such as CuCl2, CuBr2, CuBr and
CuCl give moderate to good yields (Table 1, entries 3, 4, 6 and
7). We next tested other ligands such as bipyridine, glycine and
proline in the reaction. These ligands usually exhibit high effi-
ciency in copper-catalyzed cross-coupling reactions.[22] How-
ever, the yield could not be improved (Table 1, entries 8–10).
Blank experiment indicates that 1,10-phenanthroline is necessary
as a ligand in the reaction in order to obtain high yield (entry 11).
Finally, the effect of bases on the model reaction was also
examined. Other inorganic bases such as Na2CO3 and Cs2CO3 also
give good yields, whilst K3PO4 and NaOAc are substantially less ef-
fective (Table 1, entries 12–15). Among organic bases tested,
1,4-diazabicyclo[2.2.2]octane (DABCO), 1,8-diazabicyclo[5.4.0]
undec-7-ene (DBU), Bu3N and Et3N give good to high yields and
pyridine is substantially less effective (Table 1, entries 16–20). Other
room temperature ionic liquids such as [bmim][BF4], [bmim][NTf2]
and [bmim][OTf] were also examined; however, no better results
can be observed (Table 1, entries 21–23). Therefore, the optimal
The increasing importance of imidazolium-based ionic liquids in
organic synthesis motivated us to examine the efficacy of ionic
liquids for copper(I)-catalyzed C–S coupling reactions. In continua-
tion of our efforts to develop economic and eco-friendly synthetic
pathways for organic transformations, we herein describe the
copper(I)-catalyzed one-pot tandem reaction of 2-iodoanilines with
isothiocyanates in room temperature ionic liquids.
Results and Discussion
Our choice of solvent was the commercially readily available and
hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate
([bmim][PF6]). In our screening experiments, the reaction of 2-
iodobenzenamine with phenyl isothiocyanate was chosen as a
model reaction to optimize the reaction conditions. The results
are summarized in Table 1. Firstly, the effect of various copper
catalysts on the model reaction was investigated using 1,10-
phenanthroline as a ligand and K2CO3 as a base. Among the
Table 1. Reaction condition screening for the reaction of 2-iodobenzenamine with phenyl isothiocyanatea
Entry
Cu catalyst
Ligand
Base
Time (h)
Yield (%)b
1
Cu(OAc)2
CuSO4
CuCl2
CuBr2
CuI
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
Bipyridine
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
K3PO4
NaOAc
DABCO
DBU
5
5
86
89
78
76
94
69
64
85
67
65
57
84
81
68
58
88
85
78
75
53
87
81
76
2
3
5
4
5
5
4
6
CuBr
CuCl
CuI
8
7
8
8
5
9
CuI
Glycine
8
10
11
12
13
14
15
16
17
18
19
20
21c
22d
23e
CuI
Proline
10
12
8
CuI
None
CuI
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
1,10-Phenanthroline
CuI
8
CuI
10
12
5
CuI
CuI
CuI
5
CuI
Bu3N
6
CuI
Et3N
6
CuI
Pyridine
K2CO3
K2CO3
K2CO3
10
8
CuI
CuI
8
CuI
8
aReaction was carried out using 2-iodobenzenamine (1.0 mmol), phenyl isothiocyanate (1.2 mmol), base (2.0 mmol) in the presence of copper catalyst (0.1
mmol) and ligand (0.2 mmol) in [bmim][PF6] (3.0 ml) at 60°C under argon.
bIsolated yield.
c[Bmim][BF4] (3.0 ml) was used.
d[Bmim][NTf2] (3.0 ml) was used.
e[Bmim][OTf] (3.0 ml) was used.
Appl. Organometal. Chem. 2016, 30, 446–450
Copyright © 2016 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

L. Chen et al.
catalytic system for this reaction involves CuI (10 mol%),
1,10-phenanthroline (20 mol%) and K2CO3 (2.0 equiv.) in
[bmim][PF6] at 60°C (Table 1, entry 5).
to afford the corresponding product 3g in 82% yield. We found
that the tandem reaction carried out in [bmim][PF6] has the
advantages of accelerated rate and increased yield as compared
with the reaction run in toluene. For example, the reaction of
2-iodobenzenamine with phenyl isothiocyanate in the presence
of CuI (10 mol%) and 1,10-phenanthroline (20 mol%) using
K2CO3 (2.0 equiv.) as base at 60°C in [bmim][PF6] affords 3a in
94% yield after 4 h, while the same reaction run in toluene in
the presence of CuI (10 mol%) and 1,10-phenanthroline
(20 mol%) using K2CO3 (2.0 equiv.) as base at 50°C for 8 h gives
3a in 88% yield.[5a] Similarly, the reaction of 2-iodobenzenamine
with cyclohexyl isothiocyanate in toluene in the presence of CuI
(10 mol%) and 1,10-phenanthroline (20 mol%) using K2CO3
(2.0 equiv.) as base at 50°C for 48 h gives 3g in only 41% yield.[5a]
In order to compare with the reactions run in toluene under the
same conditions, we also performed the reactions of
2-iodobenzenamine with phenyl isothiocyanate or cyclohexyl
isothiocyanate in the presence of CuI (10 mol%) and
1,10-phenanthroline (20 mol%) using K2CO3 (2.0 equiv.) as base
at 50°C in [bmim][PF6] for 6 h; the corresponding products 3a
and 3g are obtained in 93 and 78% yields, respectively.
Encouraged by the efficiency of the reaction protocol described
above, we started to investigate the scope of both 2-iodoben-
zenamines and isothiocyanates under the optimized conditions.
The results are summarized in Table 2. A variety of isothiocyanates
(2) were investigated by reacting with 2-iodobenzenamine in the
presence of CuI (10 mol%) and 1,10-phenanthroline (20 mol%) with
K2CO3 (2.0 equiv.) as base in [Bmim][PF6] at 60°C under argon. The
results demonstrate that several functional groups such as methyl,
methoxy, fluoro, chloro and nitro on the benzene ring are well
tolerated, and both electron-rich and electron-deficient aryl isothio-
cyanates undergo the tandem reaction effectively to afford the
desired products 3a–3f in good to excellent yields. 4-Methoxyphenyl
isothiocyanate, for instance, reacts with 2-iodobenzenamine to give
the target product 3c in 91% yield, and the reaction of 4-nitrophenyl
isothiocyanate with 2-iodobenzenamine affords the desired product
3f in 85% yield. To our delight, alkyl isothiocyanates are also suitable
substrates for the tandem reaction. For example, cyclohexyl isothio-
cyanate reacts with 2-iodobenzenamine under the same conditions
Table 2. Synthesis of a variety of 2-aminobenzothiazoles via CuI-catalyzed tandem reaction of 2-iodoanilines and isothiocyanates in [bmim][PF6]a
Entry
R
R1
Product
Yield (%)b
1
H
C6H5
3a
3b
3c
3d
3e
3f
94
93
91
92
91
85
82
92
90
92
89
91
95
92
93
91
85
96
91
90
81
84
83
0
2
H
4-CH3C6H4
4-CH3OC6H4
4-FC6H4
4-ClC6H4
4-NO2C6H4
Cyclohexyl
C6H5
3
H
4
H
5
H
6
H
7
H
3g
3h
3i
8
Cl
9
Cl
4-CH3C6H4
4-CH3OC6H4
4-FC6H4
4-ClC6H4
C6H5
10
11
12
13
14
15
16
17
18
19
20
21
22
23c
24d
25e
Cl
3j
Cl
3k
3l
Cl
CF3
CF3
CF3
CF3
CF3
CH3
CH3
CH3
CH3
CH3
H
3m
3n
3o
3p
3q
3r
4-CH3OC6H4
4-FC6H4
4-ClC6H4
4-O2NC6H4
C6H5
4-CH3OC6H4
4-ClC6H4
Cyclohexyl
4-O2NC6H4
C6H5
3s
3t
3u
3v
3a
3a
3a
H
C6H5
H
C6H5
87
aReaction was carried out using 2-iodoaniline (1.0 mmol), isothiocyanate (1.2 mmol), K2CO3 (2.0 mmol) in the presence of CuI (0.1 mmol) and 1,10-
phenanthroline (0.2 mmol) in [bmim][PF6] (3.0 ml) at 60°C under argon for 4 h.
bIsolated yield.
c2-Bromobenzenamine was used for 12 h.
d2-Chlorobenzenamine was used at 100°C for 24 h.
eCuI (0.01 mmol) and 1,10-phenanthroline (0.02 mmol) were used for 24 h.
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 446–450

Cu(I)-catalyzed synthesis of 2-aminobenzothiazoles in ionic liquids
100
80
60
40
20
0
With respect to the electronic nature of the substituents on the
2-iodobenzenamine, substituted 2-iodobenzenamines were subse-
quently investigated to react with a variety of isothiocyanates under
the optimized reaction conditions. The results are also summarized in
Table 2. It is evident that the optimized conditions are compatible
with various 2-iodobenzenamines bearing chloro, trifluoromethyl
and methyl groups. For example, 4-chloro-2-iodobenzenamine
undergoes the tandem reaction with aryl isothiocyanates smoothly,
affording the corresponding products 3h–3l in excellent yields.
Electron-deficient 2-iodo-4-(trifluoromethyl)benzenamine is also a
suitable substrate under the same conditions to give the desired
products 3m–3q in 85–95% yields. In addition, the tandem reactions
of electron-rich 2-iodo-4-methylbenzenamine with aryl or alkyl
isothiocyanates are also successful to afford the corresponding
products 3r–3v in good to excellent yields. Interestingly, a less active
substrate, 2-bromobenzenamine, also displays good reactivity for the
tandem reaction with phenyl isothiocyanate and the desired product
3a is isolated in 83% yield after a longer reaction time (Table 2, entry
23). We also attempted to perform the reaction of 2-chloroben-
zenamine with phenyl isothiocyanate, but no desired product 3a
could be detected even at 100°C for 24 h (Table 2, entry 24). To our
delight, even if 1 mol% of catalyst loading was utilized, the
reaction of 2-iodobenzenamine with phenyl isothiocyanate also
works well to afford the desired product 3a in 87% yield after 24
h (Table 2, entry 25).
A plausible mechanism, which accounts for the formation of
2-aminobenzothiazoles from 2-iodoanilines and isothiocyanates, is
outlined in Scheme 1. The nucleophilic N atom of 2-iodoanilines 1
would attack the carbon atom on NCS moiety of isothiocyanates 2
and intermediate A could be formed in the presence of a base.
Oxidative addition of intermediate A to the CuI/1,10-phenanthroline
complex gives intermediate B. The latter undergoes reductive
elimination to give the desired product 3 with concomitant regener-
ation of the catalyst.
Isolation of products from the [bmim][PF6] reaction mixtures can
be conveniently achieved by extraction with diethyl ether three
times. To evaluate the possibility of recycling the CuI/1,10-
phenanthroline/[bmim][PF6] catalytic system used in the reaction,
2-iodobenzenamine (1.0 mmol) and phenyl isothiocyanate
(1.2 mmol) were allowed to react in [bmim][PF6] (3.0 ml) under
the catalysis of CuI (0.1 mmol) and 1,10-phenanthroline (0.2 mmol)
with K2CO3 (2.0 mmol) as base for 4 h. Subsequently the product 3a
was extracted with diethyl ether three times to afford the cleaned,
ionic liquid, catalytic solution. After the recovered ionic liquid
1
2
3
4
5
6
7
8
Reused times
Figure 1. Recycling of the CuI/1,10-phenanthroline/[bmim][PF6] system.
containing copper catalyst and the ligand was concentrated in
vacuo (5.0 torr/room temperature for 1 h), a second amount of
reactants (2-iodobenzenamine, phenyl isothiocyanate and
K2CO3) were added and the process was repeated up to four
times. After the fourth cycle, the ionic liquid was washed with
distilled water (2 × 10 ml), dried in vacuo (5.0 torr/80°C for 1 h)
and recycled for another four times without addition of CuI and
1,10-phenanthroline. The results are presented in Fig. 1. It seems
that there is no obvious effect on the rate and yield of the
reaction during those eight cycles. For every reutilization of the
catalyst, we also determined the amount of CuI using inductively
coupled plasma atomic emission after the extraction with ether.
The results show that the amount of CuI in [bmim][PF6] for the
first to the eighth cycles is 0.098, 0.096, 0.092, 0.089, 0.087,
0.084, 0.082 and 0.079 mmol, respectively, indicating that the loss
of CuI is not significant for each cycle. Additionally, this ionic
liquid layer can be stored for several weeks with no special
precautions to exclude air or moisture and still affords results
comparable to those of the fresh ionic liquid/catalyst system. This
result is important from a practical point of view.
Conclusions
We have developed an environmentally benign, simple and highly
efficient method for the synthesis of 2-aminobenzothiazoles via a
copper(I)-catalyzed tandem reaction of 2-iodobenzenamines
with isothiocyanates using CuI (10 mol%) as catalyst and
1,10-phenanthroline (20 mol%) as ligand in [bmim][PF6]. The proce-
dure is suitable for various 2-iodobenzenamines and isothiocya-
nates, and a variety of 2-aminobenzothiazoles can be synthesized
conveniently with good to excellent yields. Easy product isolation,
ionic liquid and catalyst system recycling, enhanced reaction
efficiency as well as avoiding the use of easily volatile and toxic
toluene as solvent are important advantages of this developed
methodology.
Experimental
All chemicals and [bmim][PF6] were of reagent grade and used as
purchased. The products were purified by flash chromatography
on silica gel and a mixture of EtOAc and petroleum ether was
generally used as eluent. Copper content was determined with an
inductively coupled plasma atom emission Atomscan16 (TJA
Corporation). Infrared (IR) spectra were obtained with a PerkinElmer
683 instrument. 1H NMR spectra (400 MHz) were recorded using a
Bruker Avance 400 MHz spectrometer with tetramethylsilane as
Scheme 1. Plausible mechanism.
Appl. Organometal. Chem. 2016, 30, 446–450
Copyright © 2016 John Wiley & Sons, Ltd.
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K. R. Seddon, Org. Lett. 1999, 1, 997; c) J.-C. Xiao, B. Twamley,
J. M. Shreeve, Org. Lett. 2004, 6, 3845.
General Procedure for Copper(I)-Catalyzed Tandem Reaction of
2-Iodobenzenamines with Isothiocyanates in [bmim][PF6]
In a two-necked flask equipped with a magnetic stirring bar were
placed K2CO3 (2.0 mmol), CuI (0.1 mmol), 1,10-phenanthroline (0.2
mmol), 2-iodobenzenamine (1.0 mmol) and [bmim][PF6] (3.0 ml)
under an argon atmosphere. The mixture was pre-stirred at 30°C
for 30 min. Then isothiocyanate (1.2 mmol) was added and the
mixture was allowed to stir at 60°C. After completion of the reaction
as monitored by TLC, the reaction mixture was cooled to 25°C and
extracted with diethyl ether (3 × 10 ml). The recovered ionic liquid
phase containing CuI/1,10-phenanthroline complex was concen-
trated in vacuo (5.0 torr/room temperature for 1 h) and reused in
the next run. The combined ether solution was concentrated under
vacuum, and the residue was purified by flash chromatography on
silica gel to give the desired product.
Compounds 3a,[5a] 3b,[5b] 3c,[5a] 3d,[23] 3e,[5d] 3f,[5a] 3g,[5a] 3h,[5d]
3i,[5d] 3j,[24] 3k,[24] 3l,[24] 3m,[5a] 3n,[5f] 3o,[24] 3p,[5d] 3q,[5a] 3r,[5a]
3s,[5a] 3t,[5b] 3u[24] and 3v[5a] are known compounds and were
characterized by comparing their m.p. and 1H NMR, 13C NMR and
IR spectra with those found in the literature.
Detailed Experimental Procedure for Recycling of CuI/1,10-
Phenanthroline/[Bmim][PF6] System
After completion of the reaction of 2-iodobenzenamine with
phenyl isothiocyanate as monitored by TLC, the reaction mixture
was cooled to 25°C and extracted with diethyl ether (3 × 10 ml)
affording the cleaned, ionic liquid, catalytic solution. After the
recovered ionic liquid containing copper catalyst and ligand was
concentrated in vacuo (5.0 torr/room temperature for 1 h), a second
amount of reactants (2-iodobenzenamine, phenyl isothiocyanate
and K2CO3) were added and the process was repeated up to four
times without addition of CuI and 1,10-phenanthroline. After the
fourth cycle, the CuI/1,10-phenanthroline/[bmim][PF6] system was
washed with distilled water (2 × 10 ml), dried in vacuo (5.0
torr/80°C for 1 h) and recycled for another four times by adding
fresh reactants.
[15] X. R. Rex, D. Z. Larisa, O. Wei, Tetrahedron Lett. 2001, 42, 8441.
[16] a) C. J. Mathews, P. J. Smith, T. Welton, Chem. Commun. 2000, 1249; b)
R. Wang, B. Twamley, J. M. Shreeve, J. Org. Chem. 2006, 71, 426.
[17] a) J.-C. Hsu, Y.-H. Yen, Y.-H. Chu, Tetrahedron Lett. 2004, 45, 4673; b)
M. Y. Machado, R. Dorta, Synthesis 2005, 2473.
Acknowledgements
We thank the National Natural Science Foundation of China (no.
21462021) and Key Laboratory of Functional Small Organic Molecules,
Ministry of Education (no. KLFS-KF-201213) for financial support.
[18] a) S. T. Handy, X. Zhang, Org. Lett. 2001, 3, 233; b) C. Chiappe,
G. Imperato, E. Napolitano, D. Pieraccini, Green Chem. 2004, 6, 33; c)
H. Zhao, W. Hao, Z. Xi, M. Cai, New J. Chem. 2011, 35, 2661.
[19] T. Fukuyama, M. Shinmen, S. Nishitani, M. Sato, I. Ryu, Org. Lett. 2002, 4,
1691.
[20] a) E. Mizushima, T. Hayashi, M. Tanaka, Green Chem. 2001, 3, 76; b)
Y. Li, H. Alper, Z. K. Yu, Org. Lett. 2006, 8, 5199; c) Y. Li, Z. Yu,
H. Alper, Org. Lett. 2007, 9, 1647; d) H. Cao, H. Alper, J. Org.
Chem. 2007, 72, 8562; e) H. Cao, L. McNamee, H. Alper, J. Org.
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Tetrahedron Lett. 2003, 44, 6493; b) M. L. Kantam, B. Neelima,
C. V. Reddy, V. Neeraja, J. Mol. Catal. A 2006, 249, 201.
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F. Y. Kwong, S. L. Buchwald, Org. Lett. 2003, 5, 3517; c) M. Carril,
R. SanMartin, E. Dominguez, I. Tellitu, Chem. Eur. J. 2007, 13, 5100; d)
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Appl. Organometal. Chem. 2016, 30, 446–450