Benzimidazolium sulfonate ligand precursors and application in ruthenium-catalyzed aromatic amine alkylation with alcohols

Article abstract of DOI:10.1016/j.catcom.2015.10.028

New benzimidazolium sulfonate salts have been prepared and fully characterized. They have been associated in situ with [RuCl₂(p-cymene)]₂ to generate efficient catalytic systems operating at 120 °C under neat conditions in the presence of potassium tert-butylate for selective N-alkylation of primary aromatic amines into secondary amines.

Full text of DOI:10.1016/j.catcom.2015.10.028

Catalysis Communications 74 (2016) 33–38
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Benzimidazolium sulfonate ligand precursors and application in
ruthenium-catalyzed aromatic amine alkylation with alcohols
Nazan Kaloglu a, Ismail Özdemir a, Nevin Gürbüz a, Mathieu Achard b, Christian Bruneau b,
⁎
a
Inönü University, Catalysis Research and Application Center, 44280 Malatya, Turkey
b
Institut des Sciences Chimiques de Rennes, UMR 6226, Organometallics: Materials and Catalysis, Centre for Catalysis and Green Chemistry, Campus de Beaulieu, 35042 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 August 2015
Received in revised form 26 October 2015
Accepted 27 October 2015
Available online 1 November 2015
New benzimidazolium sulfonate salts have been prepared and fully characterized. They have been associated in situ
with [RuCl2(p-cymene)]2 to generate efficient catalytic systems operating at 120 °C under neat conditions in the
presence of potassium tert-butylate for selective N-alkylation of primary aromatic amines into secondary amines.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Benzimidazolium sulfonates
Ruthenium catalysis
N-alkylation
Hydrogen borrowing
1. Introduction
Then, new catalysts were designed in order to introduce milder condi-
tions, especially lower temperature conditions or lower amounts of
Amines constitute a class of organic compounds with a wide range of
applications in chemical industry ranging from ammonia, produced in
million ton scale, to fine chemicals with biological properties [1]. In
this context, the creation of C–N bonds from primary and secondary
amines has been investigated for a long time and more recently using
the hydrogen borrowing process starting from alcohol as alkylating
agent [2–6]. This methodology offers advantages in terms of atom econ-
omy and clean reaction over other methods based on organic or organ-
ometallic coupling reactions, as only water is formed as byproduct.
N-alkylation of aromatic amines has been mostly carried out with iridi-
um catalyst precursors such as [Cp*IrCl2]2 in the presence of a carbonate
or hydrogenocarbonate as mineral base in refluxing toluene. Intermo-
lecular reaction with alcohols [7] and cyclization with diols [8] have
thus been successfully achieved. N-alkylation of anilines with iridium
catalysts has also been carried out without base additive in water in
the presence of [Cp*IrI2]2 [9] or in neat conditions with dicationic cata-
lysts [10,11]. Catalytic systems based on ruthenium precursors have
proved to be also efficient for aniline N-alkylation. Initial results were
obtained using RuCl2(PPh3)3 or RuCl3·xH2O in the presence of phos-
phine without additional base but at high temperature (180 °C) with
or without a solvent, leading to substituted secondary and tertiary ani-
lines [12], N-aryl-substituted cyclic amines [13], and heterocycles [14].
base. In the iridium series, this was done by implementation of bidentate
(C, N), (P, N) and tridentate (P, N, P) ligands, which allowed alkylating
aniline below 100 °C at low catalyst loading with BuOK or CsOH as
t
base [15,16]. Phosphine-sulfonate ligands were also introduced on
Cp*Ir(III) complexes to achieve the double N-alkylation of primary ani-
line derivatives with pentanediols [17]. N-heterocyclic carbene ligands
including chelating ones [18–20] were also used with success. The
same strategy was applied to ruthenium catalysts and tridentate nitro-
gen ligands, (N,N,N) and (P,N,P) pincer ligands were introduced to
ruthenium(II) centers but the catalytic N-alkylation of aniline still re-
quired high temperature [21]. Tridentate (N,N,C) [22] and tetradentate
(N,N,N,N) [23] ligands provided efficient catalysts able to operate at
100–110 °C in the presence of a base. More simple ruthenium precursors
such as [Ru(Cl2(cod)]n/1,3,5-triaza-7-phosphaadamantane (PTA), [24]
[RuCl(PPh3)(MeCN)3][BPh4] [25] or the in situ generated system based
on [RuCl2(p-cymene)]2 and diphosphine ligand were found efficient for
the N-alkylation of aniline with catalytic amounts of base [26]. However,
only scarce examples of ruthenium complexes bearing a N-heterocyclic
carbene ligand have been investigated in N-alkylation of aromatic
amines by alcohols [27]. To our knowledge, RuX2(NHC)(p-cymene)
(X = Cl, I) complexes have revealed good catalytic activity in the reaction
of primary aliphatic amines with primary alcohols to form amides but no
N-alkylation products were formed [28]. Based on the ability of the basic
sulfonate group to transfer protons and generate water-soluble species
[29], we decided to prepare new N-heterocyclic carbene sulfonate li-
gands constructed on the benzimidazole core and evaluate their activity
⁎
Corresponding author.
E-mail address: christian.bruneau@univ-rennes1.fr (C. Bruneau).
http://dx.doi.org/10.1016/j.catcom.2015.10.028
1566-7367/© 2015 Elsevier B.V. All rights reserved.

34
N. Kaloglu et al. / Catalysis Communications 74 (2016) 33–38
Scheme 1. Synthesis of benzimidazolium sulfonate salts L1–L10.
in the N-alkylation of aniline and 2-aminopyridine in the presence of
ruthenium catalysts.
column chromatography (petroleum ether/Et2O or EtOAc) to afford
the pure secondary amine.
2. Experimental
3. Results and discussion
2.1. Preparation of benzimidazolium sulfonates
3.1. Preparation of benzimidazolium sulfonate salts
The zwitterionic carbene precursors L1–L10 were obtained in a three
step procedure according to Scheme 1. The N-alkyl or N-benzyl benzimid-
azole was first prepared by deprotonation of benzimidazole by NaH in
THF at room temperature for 1 h. The resulting sodium benzimidazolate
was reacted with the appropriate alkyl or benzyl halide in refluxing THF
during 24 h. The resulting N-alkyl or N-benzyl-benzimidazole was isolat-
ed and purified as a white solid. 1-Substituted benzimidazole and 1,3-
propanesultone were then dissolved in acetonitrile and refluxed during
72 h. The benzimidazole sulfonate salts L1–L10 were isolated as solids
in good to excellent yields.
A library of 10 benzimidazolium sulfonate salts L1–L10 has been pre-
pared (Scheme 2). Each of them is equipped with a 1-n-propylsulfonate
group linked to one nitrogen atom. The other nitrogen atom is substitut-
ed either by an aliphatic ether or an acetal functionality, or a benzylic
group diversely substituted on the phenyl ring. The acetal groups are in
principle stable in basic conditions and the oxygen atoms might be useful
for hydrogen bonding or transfer. As far as the phenyl groups are con-
cerned it has been previously shown that they can easily substitute a co-
ordinated p-cymene ligand on a ruthenium(II) center upon thermal
treatment [30].
L1–L10 are soluble in water and protic solvents; they were charac-
terized by 1H and 13C NMR, and gave satisfactory elemental analysis.
The benzimidazolium fragment showed characteristic C(2)–H proton
chemical shift at 8.86–9.64 ppm associated to the 13C NMR chemical
shift of the C(2) carbon atom deshielded at 140–150 ppm (Table A.1).
These chemical shifts are typical of all types of benzimidazolium salts
including less functionalized ones. The 1H NMR spectra of the linear
n-propylsulfonate group are clearly identified by a triplet at 4.5–
4.8 ppm for NCH2, another triplet at 2.8–3.0 ppm for CH2SO3, and a
2.2. Catalytic reactions
The alcohol derivative (1.6 mmol) was added to a stirred solution
of aromatic amine (1 mmol) in a Schlenk tube. Subsequently, BuOK
(1 mmol), the preligand (L) (1 mol%) and [RuCl2(p-cymene)]2
(0.5 mol%) were added and the sealed Schlenk tube was stirred at
120 °C for 24 h. The crude mixture was collected by the addition of
CH2Cl2 (2 ml) for GC analysis, and the product was then purified by
t
Scheme 2. Library of benzimidazolium sulfonates.

N. Kaloglu et al. / Catalysis Communications 74 (2016) 33–38
35
pentuplet centered at 2.4 ppm for the internal CH2 group, all coupling
constants being close to 7 Hz.
of alcohol (1.6 equivalent) was required to achieve complete conversion
of aniline. Indeed, potassium benzoate was formed as an undesired side
product of this reaction resulting from Tishchenko or Cannizzaro dispro-
portionation reaction from the intermediate benzaldehyde in the pres-
ence of tBuOK and water arising from imine formation. It is noteworthy
that in the presence of the benzimidazolium salt and base but in the ab-
sence of ruthenium precursor, no reaction took place (entry 15). Only
24% conversion was obtained in the presence of the metallic precursor
and proligand showing the important role of tBuOK in this catalytic sys-
tem (entry 16). 72% conversion with an amine/imine ratio of 90/10 was
observed when the ruthenium dimer and the base were used in the ab-
sence of ligand (entry 14) confirming the preponderant role of the base
and the ligand for efficient alkylation of aromatic amines [32].
3.2. Catalytic application in the alkylation of aniline derivatives with alcohols
In order to evaluate the potential of carbene sulfonate ligands in
hydrogen borrowing processes, we associated these ligands with
[RuCl2(p-cymene)]2, a very established precursor of efficient catalytic
systems for this type of reaction [26], and studied their ability to achieve
benzylation of aniline with benzyl alcohol as a model reaction. Aniline
being a weak base (pKa = 4.6) and the benzimidazolium sulfonate
salts L1–L10 were deprotonated by tBuOK in the presence of the ruthe-
nium source to generate a catalytic system in situ.
The reaction was carried out at 120 °C for 24 h in neat conditions
with 0.5 mol% of ruthenium dimer and 1 mol% of benzimidazolium sul-
fonate in the presence of a slight excess of benzyl alcohol. The results
presented in Table A.1 show that all the ligand precursors led to high
conversion with the amine 3a as the major product, the imine 4a
being detected only as trace amount in some cases.
Based on this preliminary study, the scope of the reaction was
investigated with substituted aromatic amines and benzylic alcohols
applying our best experimental conditions (Table 2). Aniline reacted
with para-substituted benzylic alcohols 2b and 2c to give the corre-
sponding secondary benzylic anilines 3ab and 3ac in good yields
and high selectivity (entries 2, 3). 4-Methylaniline (entries 4, 5) re-
acted similarly and provided excellent conversions and good isolated
yields in 3ba and 3bc with benzyl alcohol 2a and isopropylbenzyl
alcohol 2c.
When 2,4-dimethylaniline 1c was used as substrate, the reaction be-
came more difficult and conversions of 77 and 63% only could be obtain-
ed with benzyl alcohol and 4-methoxybenzyl alcohol as respective
partners after 24 h (entries 6, 7). 2,3,4,5,6-Pentafluoroaniline 1d reacted
also with benzyl alcohol 2a to give the fluorinated aniline 3da in 67%
yield (entry 8). On the other hand, steric hindrance at the ortho-
position of the aniline or the benzylic alcohol substrate completely
inhibited the reaction. More precisely, the ortho-disubstituted anilines
1e and 1f (entries 10, 11) and ortho-monosubstituted hydroxy or halo-
genated benzylic alcohols (entry 9) were unreactive.
The influence of various parameters was then studied using the ligand
precursor L1 (Table 1). Up to 90% conversion was obtained with 15 mol%
of tBuOK (entries 1–4) and full conversion with high selectivity in favor of
the amine could be obtained only when the ligand was added and a stoi-
chiometric amount of tBuOK was used (entries 6, 7, 13), which was in ac-
cordance with many previous results starting from aniline, and indicated
that this strong base was not only useful for carbene generation but also
accelerated imine hydrogenation (entries 5, 6). It is noteworthy that in
the absence of base, a 2,6-bis(di-tert-butylphosphinomethyl)pyridine
pincer ruthenium complex led to the selective formation of imines from
alcohols and aliphatic amines [31]. Decreasing the reaction temperature
or the catalyst loading had a detrimental effect on both conversion and
selectivity towards amine formation. It is also worth mentioning that
the benzylation tolerated water as solvent (entry 12) and was also effi-
ciently performed in toluene with high selectivity (entry 13). An excess
With the objective of preparing aniline derivatives of natural prod-
ucts, we showed that under similar reaction conditions, citronellol 2d
Table 1
Benzylation of aniline 1a with benzyl alcohol 2a.a
Entry
Ruthenium precursor (mol%)
Preligand (mol%)
tBuOK (mol%)
Temp. (°C)
Conv. (%)
3aa/4aa ratio
1
2
3
4
5
6
7
8
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.25)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
[[RuCl2(p-cymene)]2 (0.25)
[RuCl2(p-cymene)]2 (0.0625)
[RuCl2(p-cymene)]2 (0.0625)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
[RuCl2(p-cymene)]2 (0.5)
No
L1 (1)
L1 (0.5)
L6 (1)
L8 (1)
L1 (1)
L1 (1)
L1 (1)
L1 (1)
L1 (0.5)
L1 (0.125)
L1 (0.125)
L1 (1)
L1 (1)
No
15
15
15
15
120
120
120
120
120
120
100
80
120
120
120
120
120
120
120
120
88b
89
91/9
79/21
84/16
79/21
85/15
100/0
95/5
87/10
95/5
90/10
92/8
98/2
96/4
90/10
–
90b
81b
89
50
100
100
100
100
100
100
100
100
100
100
0
100
100
76
97
85
9
10
11
12
13
14
15
16
98c
68d
100e
72
0
24
L1 (1)
No
[RuCl2(p-cymene)]2 (0.5)
96/4
a
Aniline (1 mmol, 1 equiv.), benzyl alcohol (1.6 equiv.), tBuOK as a base, no solvent, GC conversion based on the amine, 3/4 GC ratio, reaction time: 15 h.
Reaction time: 20 h.
Reaction time: 24 h.
Water as solvent (2 ml).
Toluene as solvent (1 ml).
b
c
d
e

36
N. Kaloglu et al. / Catalysis Communications 74 (2016) 33–38
Table 2
Scope of the benzylation of anilines with benzylic alcohols.
Entry
1
Amine 1
Alcohol 2
Conv.
100
3/4 ratio
100/0
3 yield (%)
3aa (53%)
2
3
4
100
89
100/0
98/2
3ab (69%)
3ac (72%)
3ba (59%)
100
100/0
5
6
99
77
100/0
87/13
3bc (77%)
3ca (45%)
7
8
63
67
100/0
94/6
3cb (55%)
3da (54%)
9
0
0
10
11
0
Aniline (1 mmol, 1 equiv.), benzyl alcohol (1.6 equiv.), tBuOK (1 mmol), [RuCl2(p-cymene)]2 (0.5 mol%), L1 (1 mol%), no solvent, 120 °C, 24 h, GC conversion based on the amine, 3/4 GC
ratio, isolated yield.
reacted with aniline 1a and 4-methylaniline 1b to give the anilines 3ad
and 3bd derived from terpene in 65 and 51% yield, respectively
(Eq. ((1)).
Alkylation of heteroaromatic amines such as aminoquinazolines,
aminopyrimidines, aminoazoles or aminopyridine by alcohols has
not been studied extensively in homogeneous catalysis and most of
the successful results have been obtained with iridium catalysts [6,
33,34]. With ruthenium catalysts, only a few examples of alkylation
of 2-aminopyridine with alcohols have been recently reported with
ruthenium complexes featuring tridentate (C,N,N) [24] ligand and
cationic ruthenium(arene) complex equipped with a chelating
pyridine–carbene ligand [27].
In the presence of our optimized catalytic system based on [RuCl2(p-
cymene)]2 and the carbene sulfonate ligand derived from L1, the
primary heteroaromatic 2-aminopyridine reacted readily with various
alcohols to selectively give the N-monoalkylated secondary amines
Eq. (1). Terpenylation of anilines with citronellol.

N. Kaloglu et al. / Catalysis Communications 74 (2016) 33–38
37
Table 3
N-Alkylation of 2-aminopyridine 1g.
Entry
1
Alcohol 2
Conv. (%)
97
3 yield (%)
3ga (80%)
2
3
100
94
3gc (75%)
3ge (60%)
4
6
91
3gf (55%)
3gg (50%)
75a
a
30 equiv. of isopropanol.
(Table 3). Primary benzylic alcohols gave excellent yields of N-alkylated
2-aminopyridines and the products 3ga, 3gc, and 3ge were isolated in
80, 75 and 60% yield, respectively. Alkylation with the purely aliphatic
1-hexanol was also possible in satisfactory yield but with the secondary
alcohols 2g the conversion was lower and required 30 equiv. of
isopropanol to reach 75% yield.
The catalytic systems generated in situ from these salts and [RuCl2(p-
cymene)]2 in the presence of tBuOK are efficient in hydrogen borrowing
processes and make possible the selective monoalkylation of primary ar-
omatic amines such as substituted anilines and 2-aminopyridine with a
variety of alcohols under neat conditions at 120 °C with low catalyst
loading. The scope and limitations mainly due to steric effects have
been established.
The ruthenium(carbene) complex RuCl2(p-cymene)(N,N′-bis
(mesityl)imidazolylidene) featuring aryl substituents directly connect-
ed to the nitrogen atoms of the carbene is known to catalyze the
formation of amides from alcohols and primary aliphatic amines
including aniline at a lower extend, in the presence of 15 mol% of
tBuOK [28]. Recently, ruthenium complexes such as (N,N-bis(benzyl)
benzimidazolylidene)- and (N,N-bis(isopropyl)benzimidazolylidene)-
RuCl2(p-cymene) equipped with N-alkyl or/and N-benzyl NHC ligands
with no chelating or hemilabile group on the NHC have revealed good
catalytic properties for the alkylation of anilines with various alcohols
under neat conditions at 130 °C with 5 mol% catalyst loading without
additional base [35]. The presence of the sulfonate group in our NHC
precursors does not seem to be essential in terms of reactivity, but
contributes to promote the formation of amines rather than imines or
amides. All these results show that the nature of the NHC carbene ligand
is crucial for the ruthenium-catalyzed selective alkylation of anilines by
alcohols with hydrogen borrowing processes.
However, at the moment, the most efficient catalytic systems for al-
kylation of aromatic amines operating under really mild conditions are
based on iridium catalysts. They include reactions carried out in diglyme
at 70 °C in the presence of tBuOK [16], a few examples at 50 °C in toluene/
dichloromethane as solvent in the absence of an extra base [10], and even
some scarce examples at room temperature in the presence of tBuOK
under solvent-free conditions with 1 mol% catalyst loading [20].
Attempts to generate ruthenium complexes with a chelating
carbene sulfonate ligand from [RuCl2(p-cymene)]2 as metal source
and a silver carbene prepared from L1 and Ag2O have failed but further
studies are currently under investigation.
Acknowledgment
The authors thank CNRS (France), the University of Rennes1 (France),
and the University of Malatya (Turkey) for support. They also acknowl-
edge the financial support of the Technological and Scientific Research
Council of Turkey TUBİTAK-BOSPHORUS (France) [113Z605] for a grant
to N.K.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.10.028.
References
[1] S.A. Lawrence (Ed.), Amines: Synthesis, Properties and Applications, Cambridge
University Press, Cambridge, 2004.
[2] Q. Yang, Q. Wang, Z. Yu, Chem. Soc. Rev. 44 (2015) 2305–2329.
[3] R. Yamaguchi, K.-i. Fujita, M. Zhu, Heterocycles 81 (2010) 1093–1140.
[4] G. Guillena, D.J. Ramon, M. Yus, Chem. Rev. 110 (2010) 1611–1641.
[5] G.E. Dobereiner, R.H. Crabtree, Chem. Rev. 110 (2010) 681–703.
[6] T.D. Nixon, M.K. Whittlesey, J.M.J. Williams, Dalton Trans. (2009) 753–762.
[7] K.-i. Fujita, Y. Enoki, R. Yamaguchi, Tetrahedron 64 (2008) 1943–1954.
[8] H. Aramoto, Y. Obara, Y. Ishii, J. Org. Chem. 74 (2009) 628–633.
[9] R. Kawahara, K.-i. Fujita, R. Yamaguchi, Adv. Synth. Catal. 353 (2011) 1161–1168.
[10] A. Bartoszewicz, R. Marcos, S. Sahoo, A.K. Inge, X. Zou, B. Martin-Matute, Chem. Eur.
J. 18 (2012) 14510–14519.
[11] J. Norinder, A. Börner, ChemCatChem 3 (2011) 1407–1409.
[12] K.-T. Huh, Y. Tsuji, M. Kobayashi, F. Okuda, Y. Watanabe, Chem. Lett. (1988)
449–452.
In summary, we have prepared a library of new benzimidazolium sul-
fonate salts from N-substituted benzimidazoles and 1,3-propanesultone.
[13] S.G. Shim, C.H. Doh, B.W. Woo, H.S. Kim, K.T. Huh, W.H. Park, H. Lee, J. Mol. Catal. 62
(1990) L11–L15.

38
N. Kaloglu et al. / Catalysis Communications 74 (2016) 33–38
[14] C.S. Cho, B.T. Kim, H.J. Choi, T.J. Kim, S.C. Shim, Tetrahedron 59 (2003) 7997–8002.
[15] D. Wang, K. Zhao, C. Xu, H. Miao, Y. Ding, ACS Catal. 4 (2014) 3910–3918.
[16] S. Ruch, T. Irrgang, R. Kempe, Chem. Eur. J. 20 (2014) 13279–13285.
[17] K.D. Yuan, F. Jiang, Z. Sahli, M. Achard, T. Roisnel, C. Bruneau, Angew. Chem. Int. Ed.
51 (2012) 8876–8880.
[26] M.H.S.A. Hamid, C.L. Allen, G.W. Lamb, A.C. Maxwell, H.C. Maytum, A.J.A. Watson,
J.M.J. Williams, J. Am. Chem. Soc. 131 (2009) 1766–1774.
[27] F.E. Fernandez, M.C. Puerta, P. Valerga, Organometallics 31 (2012) 6868–6879.
[28] Y. Zhang, C. Chen, S.C. Ghosh, Y. Li, S.H. Hong, Organometallics 29 (2010)
1374–1378.
[18] D. Gnanamgari, E.L.O. Sauer, N.D. Schley, C. Butler, C.D. Incarvito, R.H. Crabtree,
[29] F. Jiang, M. Achard, C. Bruneau, Adv. Organomet. Chem. 62 (2014) 159–218.
[30] B. Cetinkaya, S. Demir, I. Özdemir, L. Toupet, D. Sémeril, C. Bruneau, P.H. Dixneuf,
Chem. Eur. J. 9 (2003) 2323–2330.
Organometallics 28 (2009) 321–325.
[19] A. Pontes da Costa, M. Viciano, M. Sanau, S. Merino, J. Tejeda, E. Peris, B. Royo,
Organometallics 27 (2008) 1305–1309.
[31] B. Gnanaprakasam, J. Zhang, D. Milstein, Angew. Chem. Int. Ed. 49 (2010)
[20] J.-Q. Li, P.G. Andersson, Chem. Commun. 49 (2013) 6131–6133.
[21] R.A.T.M. Abbenhuis, J. Boersma, G. van Koten, J. Org. Chem. 63 (1998) 4282–4290.
[22] S. Agrawal, M. Lenormand, B. Martin-Matute, Org. Lett. 14 (2012) 1456–1459.
[23] D. Weickmann, W. Frey, B. Plietker, Chem. Eur. J. 19 (2013) 2741–2748.
[24] V.R. Jumde, L. Gonsalvi, A. Guerriero, M. Peruzzini, M. Taddei, Eur. J. Org. Chem.
(2015) 1829–1833.
1468–1471.
[32] A.B. Enyong, B. Moasser, J. Org. Chem. 79 (2014) 7553–7563.
[33] F. Li, L. Chen, Q. Kang, J. Cai, G. Zhu, New J. Chem. 37 (2013) 624–631.
[34] B. Blank, S. Michlik, R. Kempe, Adv. Synth. Catal. 351 (2009) 2903–2911.
[35] S.P. Shan, X. Xiaoke, B. Gnanaprakasam, T.T. Dang, B. Ramalingam, H.V. Huynh, A.M.
Seayad, RSC Adv. 5 (2015) 4434–4442.
[25] S. Naskar, M. Bhattacharjee, Tetrahedron Lett. 48 (2007) 3367–3370.