O-Aryloxime ether analogues as novel and efficient ligands for palladium-catalyzed Suzuki-Miyaura coupling in water

Article abstract of DOI:10.1016/j.tetlet.2014.03.113

O-Aryloxime ether analogues L1-L3 were studied as ligands in palladium-catalyzed Suzuki-Miyaura cross-coupling reaction of aryl bromides and aryl boronic acids in water at room temperature. Reaction conditions for the cross-coupling were optimized using PdCl₂ and Pd(OAc)₂ under aerobic condition. From the three electronically diverse O-aryloxime ether ligands studied herein, the use of 1-phenyl-ethanone O-(4-chloro-phenyl)-oxime L2 exhibits the best catalytic system in the presence of K₂CO ₃ as the base and TBAB as the promoter.

Full text of DOI:10.1016/j.tetlet.2014.03.113

Tetrahedron Letters 55 (2014) 3038–3040
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
O-Aryloxime ether analogues as novel and efficient ligands
for palladium-catalyzed Suzuki–Miyaura coupling in water
Manoj Mondal a, Utpal Bora a,b,
⇑
a Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
b Department of Chemical Sciences, Tezpur University, Tezpur 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
O-Aryloxime ether analogues L1–L3 were studied as ligands in palladium-catalyzed Suzuki–Miyaura
cross-coupling reaction of aryl bromides and aryl boronic acids in water at room temperature. Reaction
conditions for the cross-coupling were optimized using PdCl2 and Pd(OAc)2 under aerobic condition.
From the three electronically diverse O-aryloxime ether ligands studied herein, the use of 1-phenyl-
ethanone O-(4-chloro-phenyl)-oxime L2 exhibits the best catalytic system in the presence of K2CO3 as
the base and TBAB as the promoter.
Received 2 January 2014
Revised 24 March 2014
Accepted 25 March 2014
Available online 2 April 2014
Keywords:
Ó 2014 Elsevier Ltd. All rights reserved.
Suzuki–Miyaura
Cross-coupling
O-Aryloxime ether
Water
Palladium chloride
The palladium-catalyzed Suzuki–Miyaura cross-coupling of
organic halides with boronic acids is one of the most widely used
methods for the synthesis of biaryls1 and alkene derivatives, that
are structural components of numerous agrochemicals, natural
products, pharmaceuticals, and polymers.2 Conventionally, the
reactions have been carried out with numerous phosphorus and
nitrogen based ligands, as they are known to increase the electron
density over palladium, which can accelerate the oxidative
addition step (Scheme 1).3–16 Usually, these reactions are
performed in organic solvents. However from the viewpoint of
green chemistry, the use of water as an economical and environ-
mentally benign alternative to organic solvent has received tre-
mendous interest.17 Water, compared to common organic solvent
is most abundant, non-toxic, non-corrosive, as well as non-flam-
mable.18 In addition, water shows excellent chemical reactivity,
and in many cases facilitates the solubility of base, salts, and polar
compounds, thus enhancing the rate of reaction.19 Moreover, stud-
ies of various natural processes occurring in aqueous environment
reveal that water could be an effective media for almost every
organic reaction.20 Accordingly, numerous catalytic protocols have
been developed for the Suzuki–Miyaura cross-coupling in water.21
However in majority of cases either an elevated reaction tempera-
ture or the use of co-solvent is required to maximize catalytic
performances.22
1b,2a
PCy33/ PtBu33/ SPhos4/ PPh3
PPh2(p-C6H4NMe2)5
(Phosphorus-based ligands)
R1
X
B(OH)2
R2
in Organic Solvent
NHC6/ guanidine7/ amine8/ bisamide9/
Pd-Source
or r.t.,
base
+
Schiff-base10/ acetanilide11
Δ
R1
(Nitrogen-based ligands)
X = I, Br, Cl
R2
Biaryl
imine12/ triazole13/ NHC14/ amine15
Stilbazo16/ Schiff-base10a,b
in Water
Scheme 1.
Based on the previous literature reports of N-containing
ligands,6–15 we expect that the O-aryloxime ether bearing nitrogen
and oxygen atoms (Fig. 1) will be an interesting class of ligand in the
Suzuki–Miyaura reaction. However, to the best of our knowledge
the potential of oxime ether as ligand has never been investigated
in the Suzuki–Miyaura reaction. Through this communication, we
wish to report the use of simple catalytic system, composed of
palladium chloride and O-aryloxime ether as ligands, which pro-
motes Suzuki–Miyaura cross-coupling reactions of aryl halides
and arylboronic acids at room temperature in water.
Initially, we synthesized a series of the analogous aryloxime
⇑
Corresponding author. Tel.: +91 373 2370210; fax: +91 373 2370323.
E-mail addresses: utbora@yahoo.co.in, ubora@tezu.ernet.in (U. Bora).
ethers L1–L3,23 (Fig. 1) to study their efficiency as ligands in
http://dx.doi.org/10.1016/j.tetlet.2014.03.113
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

M. Mondal, U. Bora / Tetrahedron Letters 55 (2014) 3038–3040
3039
Table 2
O
O
O
N
N
N
Optimization of the Suzuki–Miyaura reaction of 4-bromonitrobenzene with phenyl-
boronic acid in the presence of L2a
Cl
Me
PdCl2/L2
O2N
Br
+
O2N
B(OH)2
L2
L3
L1
Base, Solvent,
TBAB, r.t.
Figure 1. Ligands screened for Suzuki–Miyaura reaction.
Entry Solvent Base
PdCl2/L2
Time (h) Yieldb (%)
(mol %)
palladium-catalyzed Suzuki–Miyaura cross-coupling reaction in
water. The reaction of 4-bromonitrobenzene (0.5 mmol) and phen-
ylboronic acid (0.55 mmol) was chosen as the prototype, and carried
out in the presence of 1 mol % of palladium salt, K2CO3 (2 equiv), and
ligands L1–L3 (2 mol %) at room temperature.24 It could be seen
from Table 1 that the reaction proceeds more efficiently with the
ligand L2 compared to L1 and L3, in the presence of PdCl2 and TBAB
(tert-butyl ammonium bromide) (Table 1, entry 4).
1
2
3
4
5
6
7
8
9
H2O
H2O
H2O
i-PrOH
H2O
H2O
H2O
H2O
H2O
H2O
H2O
K2CO3
K2CO3
K2CO3
K2CO3
1/2
1/1
0.5/1
1/2
2.5
5
6.5
4
6
6
99
96
96
94c
92
Na3PO4Á12H2O 1/2
Na2CO3
Cs2CO3
KOH
1/2
1/2
1/2
1/2
1/2
1/2
85
75
73
78
Trace
no
12
24
24
24
24
NaOH
Et3N
—
10
11
This is a significant result, as most of the reported ligand-based
palladium-catalyzed Suzuki–Miyaura reactions require high tem-
perature, long reaction time, and organic or biphasic media.
Encouraged by the highest yield in the presence of ligand L2, we
further optimized PdCl2/L2 (Table 2). As shown in entries 1 and 2
(Table 2), it was found that the time required for the formation
of 4-nitro-biphenyl increases to 5 h from 2.5 h when 1 mol % of
L2 was used, while less amount of PdCl2 (0.5 mol %) led to slightly
lower yield with increase in reaction time (Table 2, entry 3). Gen-
erally, presence of water increases solubility of the bases, which
are responsible for the activation of boronic acid resulting in
enhancing the rate of the reaction in an aqueous medium. This
may be the reason for low yield in isopropanol (Table 2, entry 4).
To further optimize the reaction conditions, different bases were
screened in the presence of 2 mol % of L2 and 1 mol % of PdCl2
(Table 2, entries 5–10). As previous work has revealed that phos-
phate base was able to perform as highly efficient base in Pd-cata-
lyzed Suzuki–Miyaura reaction,25 we employed Na3PO4Á12H2O as
the base to study if the rate of reaction could get enhanced, but
it turned out to be poorer base as compared to K2CO3 (Table 2,
entries 1 and 5). Similarly, Na2CO3 and Cs2CO3 also gave lower
yields of product (Table 2, entries 6 and 7). We further examined
the effect of metal hydroxides in our reaction conditions. KOH
and NaOH have provided 73% and 78% isolated yield after 24 h,
respectively, (Table 2, entries 8 and 9). Organic base such as tri-
ethyl-amine (Et3N) gave only a trace amount of product after
24 h (Table 1, entry 10). However, no cross-coupling product was
observed in the absence of base (Table 2, entry 11). Usually a
strong base stimulates side reactions lowering the yield, and a
reaction
a
Reaction conditions: 4-bromonitrobenzene (0.5 mmol), phenylboronic acid
(0.55 mmol), base (1 mmol), TBAB (0.5 mmol), solvent (4 mL), ca. 27 °C in air unless
otherwise noted.
b
Isolated yield.
Without TBAB.
c
weak base remains unable to activate boronic acids. Metal carbon-
ates as compared to other bases offer clean and mild reaction con-
dition, high yield, and reaction rate, and simple work-up
procedure. As recognized from the literature, the presence of elec-
tron-rich and bulky ligands in palladium-catalyzed reaction pro-
vides extra stabilization to the rate determining transition
state.26 Similar to the ligands like aryl oximes27 and arylamines,28
the aryl ring of the aryloxime ether is expected to undergo ortho-
metalation through CH activation to form highly active
palladacycle.
After attaining the optimal reaction conditions, we then exam-
ined the applicability of the present catalytic system to the cross-
coupling of various electronically diverse aryl bromides and aryl
boronic acids. Generally, aryl bromides with electron-donating
groups at the para position are significantly less reactive than aryl
bromides bearing electron-withdrawing groups. However, as
shown in Table 3, most of the aryl bromides were found equally
reactive toward electronically different aryl boronic acids, yielding
corresponding biphenyl derivatives in good to excellent yields (90–
100%, Table 3). Similarly, 4-bromonitrobenzene reacts with elec-
tronically diverse arylboronic acids with almost comparable reac-
tivity (Table 3, entries 1–3). When 4-bromoanisole was used as
substrate, slight decrease in yield was observed with all types of
arylboronic acids (Table 3, entries 4–6). However, in case of 4-bro-
motoluene, higher yield was observed (Table 3, entry 7 and 8).
Conversely, for other aryl bromides, having electron-withdrawing
substituent at para position, such as for 4-bromobenzaldehyde
and 4-bromoacetophenone, the amount of biaryl products remains
relatively similar to that with 4-bromonitrobenzene (Table 3,
entries 1–3 and 9–14). In another case, bromobenzene was found
to react efficiently with all three types of aryl boronic acids yield-
ing biaryl products in excellent yields (Table 3, entries 15–17).
These results are quite significant as the desired biaryls could be
efficiently achieved at room temperature using water as a solvent
and with relatively lower palladium loading (1 mol %) in the pres-
ence of O-aryloxime ether (2 mol %).
Table 1
Effect of the O-aryloxime ether ligand (L1–L3) on palladium-catalyzed Suzuki–
Miyaura reactiona
Pd, Ligand
K2CO3, H2O,
r.t., 12 h
O2N
Br +
O2N
B(OH)2
Entry
Ligand
[Pd]-source
Additive
Yieldb (%)
1
2
3
4
5
6
7
8
9
—
PdCl2
PdCl2
PdCl2
PdCl2
—
—
10
23
91
99
87
9
87
94
81
L1
L1
L2
L3
—
L1
L2
L3
TBAB
TBAB
TBAB
—
TBAB
TBAB
TBAB
PdCl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
In conclusion, we have developed a simple and convenient
methodology based on PdCl2 and O-aryloxime ether analogous
for Suzuki–Miyaura coupling of aryl bromides and aryl boronic
acids. From the three electronically diverse ligands studied herein,
the use of 1-phenyl-ethanone O-(4-chloro-phenyl)-oxime L2 resulted
a
Reaction conditions: 4-bromonitrobenzene (0.5 mmol), phenylboronic acid
(0.55 mmol), K2CO3 (1 mmol), Pd-source (1 mol %), ligand (2 mol %), water (4 mL),
TBAB (0.5 mmol), 12 h, ca. 27 °C in air unless otherwise noted.
b
Isolated yield.

3040
M. Mondal, U. Bora / Tetrahedron Letters 55 (2014) 3038–3040
14. (a) Schaper, L.-A.; Hock, S. J.; Herrmann, W. A.; Kuhn, F. E. Angew. Chem., Int. Ed.
Table 3
Suzuki–Miyaura reaction of arylbromides with different arylboronic acidsa
2012, 51, 2–22; (b) Fleckenstein, C.; Roy, S.; Leuthaußer, S.; Plenio, H. Chem.
Commun. 2007, 2870–2872; (c) Karimi, B.; Akhavan, P. F. Chem. Commun. 2011,
7686–7688.
B(OH)2
Br
+
PdCl2(1 mol%), L2(2mol%)
K2CO3, TBAB, H2O, r.t.
15. (a) Das, P.; Sarmah, C.; Tairai, A.; Bora, U. Appl. Organomet. Chem. 2011, 25, 283–
288; (b) Chahen, L.; Therrien, B.; Suss-Fink, G. Eur. J. Inorg. Chem. 2007, 32,
5045–5051.
16. Peng, Y.-Y.; Liu, J.; Lei, X.; Yin, Z. Green Chem. 2010, 12, 1072–1075.
17. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford
University Press: New York, 1998.
R1
R2
Yieldb (%)
R2
R1
Entry
R1
R2
Time (h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
NO2
NO2
NO2
OCH3
OCH3
OCH3
CH3
H
Cl
OCH3
H
Cl
OCH3
H
OCH3
H
OCH3
Cl
H
OCH3
Cl
H
OCH3
Cl
2.5
4.5
6
2.5
4
4.5
1.5
2
5
4
6
4
5.5
6
1
3
4.5
99
95
94
94
90
92
98
99
98
97
95
95
93
90
100
95
96
18. (a)Organic Reactions in Aqueous Media; Li, C.-J., Chan, T.-H., Eds., 2nd ed.; Wiley:
New York, 1997; (b)Organic Synthesis in Water; Grieco, P. A., Ed.; Academic
Press: Dordrecht, The Netherlands, 1997; (c)Aqueous-Phase Organometallic
Catalysis; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, 2004.
19. Cornils, B.; Herrmann, W. A.; Horvth, I. T.; Leitner, W.; Mecking, S.; Olivier-
Bourbigou, H.; Vogt, D. Multiphase Homogeneous Catalysis; Wiley-VCH:
Weinheim, 2008; pp 2–23.
20. (a) Dallinger, D.; Kappe, C. O. Chem. Rev. 2007, 107, 2563–2591; (b) Lipshutz, B.
H.; Ghorai, S. Aldrichimica Acta 2008, 41, 59–72; (c) Chanda, A.; Folkin, V. V.
Chem. Rev. 2009, 109, 725–748; (d) Manabe, K.; Kobayashi, S. Chem. Eur. J. 2002,
8, 4094–4101; (e) Li, C. J. Chem. Rev. 2005, 105, 3095–3166.
CH3
CHO
CHO
CHO
COCH3
COCH3
COCH3
H
21. (a) Lamblin, M.; Nassar-Hardy, L.; Hierso, J.-C.; Fouquet, E.; Felpin, F.-X. Adv.
Synth. Catal. 2010, 352, 33–79; (b) Franzen, R.; Xu, Y. Can. J. Chem. 2005, 83,
266–272; (c) Shaughnessy, K. H.; DeVasher, R. B. Curr. Org. Chem. 2005, 7, 585–
604; (d) Bai, L.; Wang, J.-X. Curr. Org. Chem. 2005, 9, 535–553; (e) Mondal, M.;
Bora, U. Green Chem. 2012, 14, 1873–1876; (f) Uozumi, Y.; Matsu-ura, Y.;
Arakawa, T.; Yamada, Y. M. A. Angew. Chem., Int. Ed. 2009, 48, 2708–2710.
22. (a) Liu, C.; Ni, Q.; Bao, F.; Qiu, J. Green Chem. 2011, 13, 1260–1266; (b) Liu, C.; Ni,
Q.; Hu, P.; Qiu, J. Org. Biomol. Chem. 2011, 9, 1054–1060; (c) Marziale, A. N.;
Faul, S. H.; Reiner, T.; Schneider, S.; Eppinger, J. Green Chem. 2010, 12, 35–38;
(d) Karimi, B.; Akhavan, P. F. Chem. Commun. 2009, 2750–2752; (e) Cohen, A.;
Crozet, M. D.; Rathelot, P.; Vanelle, P. Green Chem. 2009, 11, 1736–1742; (f)
Yamada, Y. M. A.; Sarkar, S. M.; Uozumi, Y. J. Am. Chem. Soc. 2012, 134, 3190–
3198.
H
H
a
Reaction conditions: aryl bromide (0.5 mmol), arylboronic acid (0.55 mmol),
K2CO3 (1 mmol), PdCl2 (1 mol %), L2 (2 mol %), water (4 mL), TBAB (0.5 mmol), ca.
27 °C in air unless otherwise noted.
b
Isolated yield.
23. Preparation of ligands L1–L329a
in most efficient conversion in pure water at room temperature.
The present method, involving aerobic condition, is in accordance
with the concept of green chemistry, and offers mild and effective
alternative to the existing protocols. Further effort to identify the
exact role of the ligands in the catalytic system and more applica-
tion of this system are currently under investigation in our
laboratory.
Acetophenone oxime (1 mmol) and corresponding aryl boronic acids (2 mmol)
were stirred in
a round bottom flask with Cu(OAc)2 (0.5 mmol), Cs2CO3
(1 mmol), and 4 mL of DMSO under open atmosphere. The progress of the
reaction was monitored by TLC (Merck silica gel 60F254 plates). After
completion, the reaction mixture was quenched with dil. NH4Cl–H2O solution
(10 mL) and extracted with ethyl acetate (3 Â 20 mL). The combined extract
was washed with brine (2 Â 20 ml) and dried over anhydrous Na2SO4. After
evaporation of ethyl acetate under reduced pressure, the residue was
chromatographed {silica gel (60–120 mesh) ethyl acetate-hexane; 1.5:8.5} to
obtain the desired products. The analytically pure product was obtained in 82%
(L1), 80% (L2), and 79% (L3).
Acknowledgements
Ligand L1: Colorless oil.29
1H NMR: (CDCl3, 300 MHz, ppm) d: 7.63–7.60 (m, 2H), 7.39–7.37 (m, 3H), 7.22–
7.20 (m, 2H), 6.91 (s, 1H), 6.82–6.79 (m, 2H), 2.31 (s, 3H, –CH3). 13C NMR
(CDCl3, 75 MHz, ppm) d: 156.2, 155.6, 136.4, 129.6, 129.3, 128.6, 126.1, 120.5,
115.4, 12.5.
We gratefully thank UGC, New Delhi (India) for support (No. 41-
254/2012 (SR). M.M. thanks UGC, New Delhi for UGC-BSR (RFSMS)
fellowship.
24. Typical experimental procedure for Suzuki–Miyaura reaction of aryl bromides
with aryl boronic acids.
References and notes
All the reactions were carried out in open atmosphere. A mixture of aryl
bromide (0.5 mmol), aryl boronic acid (0.55 mmol), K2CO3 (1 mmol), PdCl2
(1 mol %), ligand (2 mol %), TBAB (0.5 mmol), and water (4 mL) was stirred at
room temperature for the indicated time in a 25 mL round bottom flask. The
progress of the reaction was monitored using TLC (Merck silica gel 60F254
plates) under UV light. After completion, the reaction mixture was diluted with
brine (10 mL) and extracted with ether (3 Â 10 mL). The combined extract was
dried over anhydrous Na2SO4. After evaporation of ether under vacuum, the
product was isolated by short-chromatography {silica gel (60–120 mesh), ethyl
acetate-hexane; 0.5:9.5}. The isolated products were confirmed by comparing
their 1H and 13C NMR and mass spectral data with reported samples.
25. (a) Maegawa, T.; Kitamura, Y.; Sako, S.; Udzu, T.; Sakurai, A.; Tanaka, A.;
Kobayashi, Y.; Endo, K.; Bora, U.; Kurita, T.; Kozaki, A.; Monguchi, Y.; Sajiki, H.
Chem. Eur. J. 2007, 13, 5937–5943; (b) Karami, K.; Rahimi, N.; Shehni, M. B.
Tetrahedron Lett. 2012, 53, 2428–2431; (c) Mao, S.-L.; Sun, Y.; Yu, G.-A.; Zhao,
C.; Han, Z.-J.; Yuan, J.; Zhu, X.; Yang, Q.; Liu, S.-H. Org. Biomol. Chem. 2012, 10,
9410–9417.
1. (a) Suzuki, A. Angew. Chem., Int. Ed. 2011, 50, 6722–6737; (b) Miyaura, N.;
Suzuki, A. Chem. Rev. 1995, 95, 2457–2483; (c) Hassan, J.; Sevignon, M.; Gozzi,
C.; Schulz, E.; Lemaire, M. Chem. Rev. 2002, 102, 1359–1470.
2. (a) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176–4211; (b) Kotha,
S.; Lahiri, K.; Kashinath, D. Tetrahedron 2002, 58, 9633–9695.
3. Bellina, F.; Carpita, A.; Rossi, R. Synthesis 2004, 15, 2419–2440.
4. Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc.
2005, 127, 4685–4696.
5. Sarmah, C.; Borah, M.; Das, P. Appl. Organomet. Chem. 2011, 25, 552–558.
6. (a) Herrmann, W. A.; Reisinger, C.-P.; Spiegler, M. J. Organomet. Chem. 1998,
557, 93–96; (b) Navarro, O.; Kaur, H.; Mahjoor, P.; Nolan, S. P. J. Org. Chem.
2004, 69, 3173–3180; (c) Xiong, Z.; Wang, N.; Dai, M.; Li, A.; Chen, J.; Yang, Z.
Org. Lett. 2004, 6, 3337–3340.
7. (a) Li, S.; Lin, Y.; Cao, J.; Zhang, S. J. Org. Chem. 2007, 72, 4067–4072; (b) Ye, C.;
Chen, Z.; Wang, H.; Wu, J. Tetrahedron 2012, 68, 5197–5202.
8. (a) Tao, B.; Boykin, D. W. J. Org. Chem. 2004, 69, 4330–4335; (b) Li, J.-H.; Liu, W.-
J. Org. Lett. 2004, 6, 2809–2811.
9. Costa, D. P.; Nobre, S. M. Tetrahedron Lett. 2013, 54, 4582–4584.
10. (a) Banik, B.; Tairai, A.; Shahnaz, N.; Das, P. Tetrahedron Lett. 2012, 53, 5627–
5630; (b) Dewan, A.; Bora, U.; Borah, G. Tetrahedron Lett. 2014, 55, 1689–1692;
(c) Shahnaz, N.; Banik, B.; Das, P. Tetrahedron Lett. 2013, 54, 2886–2889.
11. Dewan, A.; Buragohain, Z.; Mondal, M.; Sarmah, G.; Borah, G.; Bora, U. Appl.
Organomet. Chem. 2014, 28, 230–233.
12. Zhou, J.; Guo, X.; Tu, C.; Li, X.; Sun, H. J. Organomet. Chem. 2009, 694, 697–702.
13. Amadio, E.; Scrivanti, A.; Beghetto, V.; Bertoldini, M.; Alam, M. M.; Matteoli, U.
RSC Adv. 2013, 3, 21636–21640.
26. Szilvási, T.; Veszprémi, T. ACS Catal. 2013, 3, 1984–1991.
27. (a) Dupont, J.; Pfeffer, M.; Spencer, J. Eur. J. Inorg. Chem. 2001, 1917–1927; (b)
Onoue, H.; Minami, K.; Nakagawa, K. Bull. Chem. Soc. Jpn. 1970, 43, 3480–3485;
(c) Constable, A. G.; McDonald, W. S.; Sawkins, L. C.; Shaw, B. L. J. Chem. Soc.,
Dalton Trans. 1980, 1992–2000.
28. Bedford, R. B.; Cazin, C. S. J. Chem. Commun. 2001, 1540–1541.
29. (a) Mondal, M.; Sarmah, G.; Gogoi, K.; Bora, U. Tetrahedron Lett. 2012, 53, 6219–
6222; (b) De, P.; Nonappa; Pandurangan, K.; Maitra, U.; Wailes, S. Org. Lett.
2007, 9, 2767–2770; (c) Feng, X.-H.; Zhang, G.-Z.; Chen, C.-Q.; Yang, M.-Y.; Xu,
X.-Y.; Huang, G.-S. Synth. Commun. 2009, 39, 1768–1780.