Visible light-induced photocatalytic C-H ethoxycarbonylmethylation of imidazoheterocycles with ethyl diazoacetate

Article abstract of DOI:10.1039/d0ra05795a

A visible light-mediated regioselective C3-ethoxycarbonylmethylation of imidazopyridines with ethyl diazoacetate (EDA) was achieved under mild reaction conditions. In contrast to the carbene precursors from α-diazoester a first C3-ethoxycarbonylmethylatio

Full text of DOI:10.1039/d0ra05795a

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Visible light-induced photocatalytic C–H
ethoxycarbonylmethylation of
Cite this: RSC Adv., 2020, 10, 27984
imidazoheterocycles with ethyl diazoacetate†
Suvam Bhattacharjee, Sudip Laru, Sadhanendu Samanta,
Mukta Singsardar
*
and Alakananda Hajra
A visible light-mediated regioselective C3-ethoxycarbonylmethylation of imidazopyridines with ethyl
diazoacetate (EDA) was achieved under mild reaction conditions. In contrast to the carbene precursors
from a-diazoester a first C3-ethoxycarbonylmethylation of imidazopyridines via a radical intermediate
has been established. The present methodology provides a concise route to access pharmacologically
useful esters with wide functional group tolerance in high yields.
Received 10th February 2020
Accepted 21st July 2020
DOI: 10.1039/d0ra05795a
rsc.li/rsc-advances
Ester groups are a most powerful and versatile synthetic building phenomena (ESIPT).10b In particular, ethoxycarbonylmethylated
block in natural products as well as in organic synthesis since imidazopyridines are the core structure of several marketed
they can be derivatized to diverse functional groups such as drugs such as alpidem, zolpidem, saripidem, etc.11 Therefore,
carbonyl, hydroxymethyl, and amide, etc.1 Beside the traditional practical methodologies for the synthesis of alkylated ester
methods of using xanthates for ethoxycarbonylmethylation,2 the substituted imidazoheterocycles are highly desirable and will be
introduction of diazoesters represents a very important strategy of great interest to the synthetic chemists.11 However, to the best
for the synthesis of acetate derivatives.3 Generally, thermal,4a of our knowledge there is no such method for the direct photo-
photoinduced,4b or metal catalyzed4c reactions of diazo initiated ethoxycarbonylmethylation reaction of imidazohe-
compounds produce reactive carbene precursors which further terocycles with diazo compounds through distinct radical
undergo an addition reaction, insertion reaction, cyclo- pathway.11–13 As part of our ongoing research on photoredox-
propanation, Wolff rearrangement, etc. (Scheme 1a).4 In contrast catalyzed C–H functionalization reactions,13 herein we
to the carbene precursors from a-diazoester, a new methodology describe an environmentally benign visible light-promoted
is always highly desirable especially with a signicantly different ethoxycarbonylmethylation for the synthesis of alkyl
mode of activation under a distinct mechanism.5 In this context, substituted imidazopyridines by the coupling between imida-
a new strategy for installation of alkyl radicals from diazo zopyridines and ethyl diazoacetate using Ru(bpy)3Cl2 as
compounds under photoredox-catalysis is very important.6 photoredox-catalyst under argon atmosphere at room temper-
Recently, visible light-mediated photoredox catalyzed direct C–H ature (Scheme 1b).
bond functionalization reactions have come to the forefront in
To begin, the reaction was carried out using 2-phenylimidazo
synthetic organic chemistry7 where Ru(II)-polypyridyl complexes [1,2-a]pyridine (1a) and ethyl diazoacetate (EDA) (2) (2.0 equiv.)
have been extensively used as photoredox-catalysts due to their
unique photo-physical properties.8
Imidazo[1,2-a]pyridines, an important class of nitrogen-
based privileged structural motifs widely found in a variety of
natural products and alkaloids.9 In pharmaceutical chemistry, it
possesses large number of biological activities like antitumor,
antibacterial, antifungal, antipyretic, analgesic, antiin-
ammatory, etc.10a It is also important in material sciences due
to its unique excited state intramolecular proton transfer
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan, 731235,
West Bengal, India. E-mail: alakananda.hajra@visva-bharati.ac.in; Web: http://www.
visvabharati.ac.in/AlakanandaHajra.html
Scheme
1 Visible light-mediated ethoxycarbonylmethylation. (a)
† Electronic supplementary information (ESI) available. CCDC 1976157. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0ra05795a
Previous reports through carbene pathway. (b) This work: visible-light
mediated radical reaction of diazoacetate with imidazopyridine.
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in presence of 0.2 mol% of Ru(bpy)3Cl2 as a photoredox-catalyst and 17) which conrm that photo-catalyst and light source are
under the irradiation of 34 W blue LED lamp in methanol at essential for this transformation. We also performed the reac-
room temperature. To our delight, 49% yield of the ethyl 2-(2- tion in thermal condition at 60 ꢀC (Table 1, entry 18). The
phenylimidazo[1,2-a]pyridin-3-yl)acetate (3a) was obtained aer desired product (3a) was not formed which indicated that the
36 h under argon atmosphere. However, no desired product was reaction probably go through the charge transfer/EDA-type
formed in aerobic conditions (Table 1, entry 1). Next, we complex not via carbene pathway. The yield of the reaction
screened the effect of different solvents such as EtOH, DCM, decreased when the amount of ethyl diazoacetate was either
MeCN, and toluene but these were not suitable like MeOH lowered or increased (Table 1, entry 19). In addition, 56% yield
(Table 1, entries 2–5). Interestingly, the binary solvent mixture of the desired product was obtained under the irradiation of
(MeOH and H2O, 1 : 1) was found to be effective for the 20 W blue LED and no alkylated product was observed using
formation of alkylated product in 76% yield (Table 1, entry 6). 30 W white LED (Table 1, entry 20). From this experiment, we
The yield of the reaction was increased using a binary mixture of conrmed that the probable reaction pathway via charge-
MeOH : H2O (2 : 1) (Table 1, entry 7). However, only a trace transfer/EDA-type complex absorbs wave length only from the
amount of product was detected with the mixture of (1 : 2) emitted blue LED light not from the emitted 30 W white LED.
MeOH : H2O and no product was observed in H2O (Table 1, Finally, optimized yield (89%) was achieved using 0.2 mol%
entries 8 and 9). Instead of MeOH : H2O (2 : 1) by using Ru(bpy)3Cl2 and 2.0 equiv. ethyl diazoacetate in 2.0 mL mixture
EtOH : H2O (2 : 1) desired product (3a) was obtained in 68% of MeOH : H2O (2 : 1) with the irradiation of 34 W blue LED for
yield aer 36 h (Table 1 entry 10). The reaction did not proceed 36 h at room temperature under argon atmosphere (Table 1,
at all in presence of other classical photoredox-catalysts like entry 7).
Ir(ppy)3, rose bengal, eosin Y, eosin B, and rhodamine B (Table
With the optimized reaction conditions in hand we explored
1, entries 11–15). Moreover, no product was formed in absence the substrate scope of this methodology with various
of photoredox-catalyst as well as light source (Table 1, entries 16 substituted imidazopyridines. At rst we checked the effect of
various substituents on the pyridine ring of imidazopyridine
derivatives and the results are summarized in Scheme 2. Imi-
dazopyridine containing various electron-donating groups (8-
Me and 7-OMe) efficiently reacted with ethyl diazoacetate to
provide the desired products in good yields (3b and 3c). It is
important to note that substrates bearing electron-withdrawing
groups did not afford alkylated products under the optimized
reaction condition. However, addition of 10 mol% N,N-
dimethyl-m-toluidine as a redox-active additive gave a satisfac-
tory yield of the desired products (see ESI†). Therefore in
modied reaction conditions, halogens containing imidazo-
pyridines like –F, –Br, –I and strong electron-withdrawing group
such as –CF3, –CN containing substrates were successfully
afforded the desired products in 67–90% yields (3d–3h). Next,
Table 1 Optimization of the reaction conditionsa
Photocatalyst
Entry
(0.2 mol%)
Solvent
Yield (%)
1
2
3
4
5
6
7
8
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ir(ppy)3
Rose bengal
Eosin Y
Eosin B
Rhodamine B
—
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
MeOH
EtOH
DCM
MeCN
49, NRb
21
43
47
NR
76
89
Trace
NR
68
Toluene
MeOH : H2O (1 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (1 : 2)
H2O
9
10
11
12
13
14
15
16
17
18
19
20
EtOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
MeOH : H2O (2 : 1)
Trace
NR
NR
NR
NR
NRc
NRd
NRe
45f, 84g
56h, NRi
a
Reaction conditions: all reactions were carried out with 0.25 mmol of
1a, 0.5 mmol of 2, and 0.2 mol% of photocatalyst in 2.0 mL of solvent for
36 h at room temperature under argon atmosphere and irradiation with
34 W blue LED. NR ¼ no reaction. b In aerobic condition. c Absence of
Scheme 2 Substrate scope of imidazopyridines.a aReaction condi-
tions: 0.25 mmol of 1, 0.5 mmol of 2 in presence of 0.2 mol% of
Ru(bpy)3Cl2 in 2.0 mL of MeOH : H2O (2 : 1) at room temperature
under 34 W blue LED and argon atmosphere for 36 h. b10 mol% N,N-
dimethyl-m-toluidine was added as an additive. c6.0 mmol scale.
d
e
Ru(bpy)3Cl2. Without light source at rt. Reaction at 60 ꢀC without
f
g
h
light scource. 1.5 equiv. EDA. 2.5 equiv. EDA. Irradiation with
20 W blue LED. i 30 W white LED.
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Fig. 1 Modification of ester group via direct amidation process.
Scheme 3 Substrate scope.a aReaction conditions: 0.25 mmol of 4,
0.5 mmol of 2 in presence of 0.2 mol% of Ru(bpy)3Cl2 in 2.0 mL of
MeOH : H2O (2 : 1) at room temperature under 34 W blue LED and
argon atmosphere for 36 h. b10 mol% N,N-dimethyl-m-toluidine was
added as an additive.
we examined the substrate scope with various substitutions in
phenyl part of imidazo[1,2-a]pyridine. Imidazopyridine with
electron-releasing (4-Me, and 4-OMe), halogens (4-F, 4-Cl, and 3-
Br) and strong electron-withdrawing group (4-CF3, and 4-CN)
containing substrates produced the desired products with good
to excellent yields (3i–3p). 2-Hydroxy substituted imidazo[1,2-a]
pyridine was also worked well in this transformation (3k). In
addition, disubstituted imdazopyridines with different substit-
uents at both the phenyl and pyridine ring underwent the
reaction very smoothly (3q, and 3r). Heteroaryl and naphthyl
substituted imidazopyridines produced the desired products
without any difficulties (3s–3u). However, unsubstituted imi-
dazo[1,2-a]pyridine did not produce the desired product in this
protocol. To highlight the applicability of the present protocol
a gram-scale reaction was carried out taking 1a in 6.0 mmol
scale under normal laboratory setup. The yield of ethyl 2-(2-
phenylimidazo[1,2-a]pyridin-3-yl)acetate (3a) was slightly
diminished to 70% aer 36 h along with the recovery of excess
starting material.
Next, to show the generality of this present methodology, we
investigated the reaction taking other imidazoheterocycles like
imidazo[2,1-b]thiazole and benzo[d]imidazo[2,1-b]thiazoles
(Scheme 3). 6-Phenylimidazo-[2,1-b]thiazole reacted smoothly
to afford 5a in 66% yield. Benzo[d]imidazo[2,1-b]thiazole con-
taining –Me, –OMe, and –Cl substituted derivatives were well
tolerable under the present reaction conditions (5b–5e). The
structure of ethyl 2-(7-methoxy-2-phenylbenzo[d]imidazo[2,1-b]
thiazol-3-yl)acetate (5c) was conrmed by X-ray crystallog-
raphy.14 In addition, thiophene substituted benzo[d]imidazo
[2,1-b]thiazole furnished the corresponding alkylated product
in good yield (5f).
Scheme 4 Control experiments.
diphenylethylene (DPE), and p-benzoquinone (BQ) which
suggest the reaction probably proceeds through radical pathway
(Scheme 4, eqn (A)). Moreover, 3-phenylimidazo[1,2-a]pyridine
(1v) did not produce the C-2 alkylated product that signies the
regioselectivity of the present protocol (Scheme 4, eqn (B)).
Taking 2-(2-(vinyloxy)phenyl)imidazo[1,2-a]pyridine we also
performed this reaction with ethyl diazoacetate under this
standard reaction conditions. Desired C-3 alkylated product (8a)
was formed in 35% yield (Scheme 4, eqn (C)). But cyclo-
propanation product was not formed in the alkene part of the
allyl group via carbene pathway. From this experimental result
strongly imply that the present strategy only proceeds through
radical mechanistic pathway.
Based on the above mechanistic studies and previous
literature reports,7,8,13 a possible radical mechanism was
proposed for the formation of ethyl 2-(2-phenylimidazo[1,2-
a]pyridin-3-yl)acetate (3a) as presented in Scheme 5. Initially,
2+
photo-catalyst Ru(bpy)3 is transformed to its excited-state
2+
*Ru(bpy)3 under blue LED irradiation. In the presence of
MeOH/H2O, diazo compounds are in equilibrium with their
protonated form (A). Then cationic diazoester involves
single-electron transfer (SET) process with excited photo-
2+
catalyst *Ru(bpy)3 leading to the formation of alkyl radical
3+
6b,c
intermediate (B) and Ru(bpy)3
.
Next, alkyl radical inter-
mediate (B) effectively reacts with imidazopyridine (1a) to
produce imidazole radical intermediate (C) which is succes-
sively transformed to imidazolium radical cation interme-
diate (D) along with the generation of ground state
photocatalyst. Finally, C-3 alkylated imidazopyridine (3a) is
obtained through deprotonation of intermediate D. It is
worthy to mention that Meggers et al.7h did not observe
Aer that, we have directly modied the synthesized ethox-
ycarbonylmethylated product (3q) to amide derivative (6q) in
56% yield using benzylamine in the presence of La(OTf)3 as
ꢀ
a catalyst under ambient air at rt to 70 C as shown in Fig. 1.
A few control experiments were performed to understand the
mechanistic insights of the present protocol (Scheme 4). The
alkylation reaction did not proceed in the presence of 3.0 equiv.
radical scavengers like 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO), 2,6-di-tert-butyl-4-methyl phenol (BHT), 1,1-
2+
quenching of *Ru(bpy)3 whereas Gryko et al.6c did observe
2+
the quenching of *Ru(bpy)3 by ethyl diazoacetate that was
accelerated in the presence of protic sources.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
A. H. acknowledges the nancial support from CSIR, New Delhi
(Grant no. 02(0307)/17/EMR-II). S. B. and S. L. thanks CSIR, S. S.
and M. S. thanks UGC-New Delhi for their fellowship.
Notes and references
Scheme 5 Proposed mechanistic pathway.
1 (a) R. Brown, J. Quirk and P. Kirkpatrick, Nat. Rev. Drug
˜
Discovery, 2003, 2, 177; (b) E. Ravina, The Evolution of Drug
Discovery: From Traditional Medicines to Modern Drugs,
Wiley, NewYork, 2011, pp. 254-272.
2 (a) Q. Huang and S. Z. Zard, Org. Lett., 2018, 20, 1413; (b)
S. Wang, X. Huang, Z. Ge, X. Wang and R. Li, RSC Adv.,
2016, 6, 63532; (c) A. Kaga, X. Wu, J. Y. J. Lim, H. Hayashi,
Y. Lu, E. K. L. Yeow and S. Chiba, Beilstein J. Org. Chem.,
2018, 14, 3047.
3 (a) A. Ford, H. Miel, A. Ring, C. N. Slattery, A. R. Maguire and
M. A. McKervey, Chem. Rev., 2015, 115, 9981; (b) X. Zhao,
Y. Zhang and J. Wang, Chem. Commun., 2012, 48, 10162; (c)
N. R. Candeias, R. Paterna and P. M. P. Gois, Chem. Rev.,
2016, 116, 2937; (d) I. D. Jurberg and H. M. L. Davies,
Chem. Sci., 2018, 9, 5112.
Scheme 6 Mechanistic pathway for electon-deficient substrates.
˜
´
4 (a) M. Pena-Lopez and M. Beller, Angew. Chem., Int. Ed., 2017,
´
56, 46; (b) Ł. W. Ciszewski, K. Rybicka-Jasinska and D. Gryko,
In case of halogen or electron-withdrawing substituted imi-
dazoheterocycles a possible mechanism for the radical alkyl-
ation reaction is illustrated in Scheme 6.6b,c Reductive
Org. Biomol. Chem., 2019, 17, 432; (c) Y. Xia, D. Qiu and
J. Wang, Chem. Rev., 2017, 117, 13810; (d) F. He and
R. M. Koenigs, Chem. Commun., 2019, 55, 4881; (e)
Q.-Q. Cheng, Y. Deng, M. Lankelma and M. P. Doyle,
Chem. Soc. Rev., 2017, 46, 5425; (f) Q. Huang, H. Dong,
B. Li, W. Hu and Y. Wang, Tetrahedron, 2020, 76, 130998.
5 (a) C. Grotenhuis, N. Heuvel, J. I. Vlugt and B. Bruin, Angew.
Chem., Int. Ed., 2018, 57, 140; (b) J. Zhao, P. Li, Y. Xu, Y. Shi
and F. Li, Org. Lett., 2019, 21, 9386; (c) H. -X. Wang,
Y. Richard, Q. Wan, C. -Y. Zhou and C. -M. Che, Angew.
Chem., Int. Ed., 2020, 59, 1845.
2+
quenching of *Ru(bpy)3 by SET oxidation of aniline deriva-
tives affords radical cation E and Ru(bpy)3+. Then reduced
+
photocatalyst Ru(bpy)3 comes back to ground state with the
generation of radical intermediate B via the elimination of N2.
Subsequently, radical intermediate B reacts with imidazopyr-
idine 1 to produce radical intermediate C0. Finally, the desired
product 30 is produced through the hydrogen atom transfer
(HAT) process between radical cation E and radical interme-
diate C0.
6 (a) P. Li, J. Zhao, L. Shi, J. Wang, X. Shi and F. Li, Nat.
´
Commun., 2018, 9, 1972; (b) K. Rybicka-Jasinska,
Ł. W. Ciszewski and D. Gryko, Adv. Synth. Catal., 2016, 358,
1671; (c) Ł. W. Ciszewski, J. Durka and D. Gryko, Org. Lett.,
2019, 21, 7028.
7 (a) M. H. Shaw, J. Twilton and D. W. C. MacMillan, J. Org.
Chem., 2016, 81, 6898; (b) N. A. Romero and D. A. Nicewicz,
Chem. Rev., 2016, 116, 10075; (c) L. Marzo, S. K. Pagire,
Conclusions
In summary, we have developed an efficient photoredox-
catalyzed ethoxycarbonylmethylation of imidazoheterocycles
with ethyl diazoester as an alkylating reagent under the irradi-
ation of blue LED at room temperature. A wide variety of
functionalized imidazoheterocycles were well tolerated under
the mild reaction conditions. A gram-scale synthesis was also
performed to demonstrate the potential synthetic application of
this new method. In contrast to the carbene precursors from a-
diazoester a rst C3-ethoxycarbonylmethylation of imidazopyr-
idines via radical intermediate has been established. We believe
the present protocol to be of signicant applicability for the
diversication of heterocyclic moieties in a multitude of
medicinal applications.
¨
O. Reiser and B. Konig, Angew. Chem., Int. Ed., 2018, 57,
10034; (d) J. R. Chen, X. Q. Hu, L. Q. Lu and W. J. Xiao,
Acc. Chem. Res., 2016, 49, 1911; (e) D. P. Tiwari, S. Dabral,
J. Wen, J. Wiesenthal, S. Terhorst and C. Bolm, Org. Lett.,
2017, 19, 4295; (f) K. L. Skubi, T. R. Blum and T. P. Yoon,
Chem. Rev., 2016, 116, 10035; (g) J. C. Tellis, C. B. Kelly,
D. N. Primer, M. Jouffroy, N. R. Patel and G. A. Molander,
Acc. Chem. Res., 2016, 49, 1429; (h) X. Huang,
R. D. Webster, K. Harms and E. Meggers, J. Am. Chem. Soc.,
This journal is © The Royal Society of Chemistry 2020
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Paper
2016, 138, 12636; (i) P. Li, J. Zhao, L. Shi, J. Wang, X. Shi and 12 (a) K. Pericherla, P. Kaswan, K. Pandey and A. Kumar,
F. Li, Nat. Commun., 2018, 9, 1; (j) P. H. Given and
M. E. Peover, J. Chem. Soc., 1960, 385; (k) S. H. Cadle,
P. R. Tice and J. Q. Chambers, J. Phys. Chem., 1967, 71,
3517; (l) A. M. del Hoyo, A. G. Herraiz and M. G. Suero,
Angew. Chem., Int. Ed., 2017, 56, 1610; (m) H. E. Boneld,
Synthesis, 2015, 47, 887; (b) C. Ravi and S. Adimurthy,
Chem. Rec., 2017, 17, 1019; (c) H. Chen, H. Yi, Z. Tang,
C. Bian, H. Zhang and A. Lei, Adv. Synth. Catal., 2018, 360,
3220; (d) P. Sun, D. Yang, W. Wei, M. Jiang, Z. Wang,
L. Zhang, H. Zhang, Z. Zhang, Y. Wang and H. Wang,
Green Chem., 2017, 19, 4785; (e) C. Wang, S. Lei, H. Cao,
S. Qiu, J. Liu, H. Deng and C. Yan, J. Org. Chem., 2015, 80,
12725; (f) Y. J. Guo, S. Lu, L. L. Tian, E. L. Huang,
X. Q. Hao, X. Zhu, T. Shao and M. P. Song, J. Org. Chem.,
2018, 83, 338; (g) P. R. Nitha, M. M. Joseph, G. Gopalan,
K. K. Maiti, K. V. Radhakrishnan and P. Das, Org. Biomol.
´
T. Knauber, F. Levesque, E. G. Moschetta, F. Susanne and
L. J. Edwards, Nat. Commun., 2020, 11, 804.
8 (a) J. W. Tucker and C. R. J. Stephenson, J. Org. Chem., 2012,
77, 1617; (b) C. K. Prier, D. A. Rankic and D. W. C. MacMillan,
Chem. Rev., 2013, 113, 5322; (c) J.-Y. Shao, Z.-L. Gong and
Y.-W. Zhong, Dalton Trans., 2018, 47, 23.
´
9 (a) C. Enguehard-Gueiffier and A. Gueiffier, Mini-Rev. Med.
Chem., 2007, 7, 888; (b) A. T. Baviskar, S. M. Amrutkar,
N. Trivedi, V. Chaudhary, A. Nayak, S. K. Guchhait,
U. C. Banerjee, P. V. Bharatam and C. N. Kundu, ACS Med.
Chem. Lett., 2015, 6, 481.
10 (a) A. K. Bagdi, S. Santra, K. Monir and A. Hajra, Chem.
Commun., 2015, 51, 1555; (b) V. S. Padalkar and S. Seki,
Chem. Soc. Rev., 2016, 45, 169.
11 (a) N. Chernyak and V. Gevorgyan, Angew. Chem., Int. Ed.,
2010, 49, 2743; (b) Q. Chang, Z. Liu, P. Liu, L. Yu and
P. Sun, J. Org. Chem., 2017, 82, 5391; (c) D. K. Nair,
S. M. Mobin and I. N. N. Namboothiri, Org. Lett., 2012, 14,
Chem., 2018, 16, 6430; (h) Ł. Kielesinski, M. Tasior and
D. T. Gryko, Org. Chem. Front., 2015, 2, 21; (i) X. M. Zhao,
E. L. Huang, Y. S. Zhu, J. Li, B. Song, X. Zhu and
X. Q. Hao, Org. Biomol. Chem., 2019, 17, 4869.
13 (a) A. K. Bagdi and A. Hajra, Chem. Rec., 2016, 16, 1868; (b)
S. Mitra, M. Ghosh, S. Mishra and A. Hajra, J. Org. Chem.,
2015, 80, 8275; (c) M. Singsardar, S. Mondal, S. Laru and
A. Hajra, Org. Lett., 2019, 21, 5606; (d) G. Kibriya,
A. K. Bagdi and A. Hajra, J. Org. Chem., 2018, 83, 10619; (e)
G. Kibriya, S. Samanta, S. Jana, S. Mondal and A. Hajra, J.
Org. Chem., 2017, 82, 13722; (f) A. K. Bagdi and A. Hajra,
Org. Biomol. Chem., 2020, 18, 2611.
4580; (d) H. Kim, M. Byeon, E. Jeong, Y. Baek, S. J. Jeong, 14 Further information can be found in the CIF le. This crystal
K. Um, S. H. Han, G. U. Han, G. H. Ko, C. Maeng,
J. Y. Son, D. Kim, S. H. Kim, K. Lee and P. H. Lee, Adv.
Synth. Catal., 2019, 361, 2094.
was deposited in the Cambridge Crystallographic Data
Centre and assigned as CCDC 1976157†.
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