Electrochemical oxidative iodination of imidazo[1,2-a]pyridines using NaI as iodine source

Article abstract of DOI:10.1080/00397911.2020.1717539

An electrochemical oxidative iodination of imidazo[1,2-a]pyridines is achieved in this study. This reaction utilizes NaI as an iodine source and a supporting electrolyte under metal-free and exogenous chemical oxidant-free reaction conditions. The present

Full text of DOI:10.1080/00397911.2020.1717539

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Electrochemical oxidative iodination of
imidazo[1,2-a]pyridines using NaI as iodine source
Jin Wha Park, Yong Hwan Kim & Dae Young Kim
To cite this article: Jin Wha Park, Yong Hwan Kim & Dae Young Kim (2020): Electrochemical
oxidative iodination of imidazo[1,2-a]pyridines using NaI as iodine source, Synthetic
Communications, DOI: 10.1080/00397911.2020.1717539
To link to this article: https://doi.org/10.1080/00397911.2020.1717539
View supplementary material
Published online: 28 Jan 2020.
Submit your article to this journal
Article views: 13
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=lsyc20

R
SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1717539
Electrochemical oxidative iodination of
imidazo[1,2-a]pyridines using NaI as iodine source
Jin Wha Park, Yong Hwan Kim, and Dae Young Kim
Department of Chemistry, Soonchunhyang University, Asan, Republic of Korea
ABSTRACT
ARTICLE HISTORY
Received 23 October 2019
An electrochemical oxidative iodination of imidazo[1,2-a]pyridines is
achieved in this study. This reaction utilizes NaI as an iodine source
and a supporting electrolyte under metal-free and exogenous chem-
ical oxidant-free reaction conditions. The present protocol
provides an efficient route to prepare 3-iodoimidazo[1,2-a]pyridines
derivatives.
KEYWORDS
Electrochemical oxidation;
imidazo[12-a]pyridines; iod-
ination; 3-
iodoimidazo[1,2-a]pyridines
GRAPHICAL ABSTRACT
Introduction
The imidazo[1,2-a]pyridine scaffold is found as a ubiquitous motif in a wide variety of
natural products and biologically active compounds.[1] These derivatives exhibit a broad
range of biological and pharmacological activities, such as antibacterial,[2] antiulcer,[3]
antiviral,[4] and antitumor.[5] Among C-3 substituted imidazo[1,2-a]pyridine derivatives,
3-halogenated imidazo[1,2-a]pyridines have attracted substantial attention for use as
common intermediates enabling the displacement of halogen atoms at C-3 position.[6]
Although a number of bromination reactions of imidazo[1,2-a]pyridines with various
halogen sources have been reported,[7] the iodination reaction of imidazo[1,2-a]pyri-
dines has been rarely studied. There have been few reported examples of iodination of
imidazo[1,2-a]pyridines using iodine or N-iodosuccinimide (NIS) in the presence of
ionic solvent or additives (Scheme 1a).[8] In recent years, Adimurthy, Alla, and Katrun
groups described the oxidant-mediated iodination of imidazo[1,2-a]pyridines using
sodium iodide with K2S2O8 or polyvalent iodine reagent as oxidants (Scheme 1b).[9,7d,7f]
Despite the above advancements, more efficient and environmentally benign approaches
for the iodination of imidazo[1,2-a]pyridines are still desired. The electrochemical
method has emerged as an attractive approach in organic synthesis owing to its
environmental-friendly features such as cost-effectiveness, scalability, and toxic and
CONTACT Dae Young Kim
dyoung@sch.ac.kr
Department of Chemistry, Soonchunhyang University, Asan 31538,
Chungnam, Republic of Korea.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
J. W. PARK ET AL.
Scheme 1. (a–c) Iodination of imidazo[1,2-a]pyridines.
costly chemical reagent-free redox transformations.[10] To our knowledge, no studies
have reported on the electrochemical oxidative iodination of imidazo[1,2-a]pyridines.
Thus, we envisioned the iodination of imidazo[1,2-a]pyridines to 3-iodinated imi-
dazo[1,2-a]pyridines by electrochemical oxidative iodination with metal iodide as the
iodine source (Scheme 1c).
Results and discussion
In connection with our ongoing research program investigating the C–H bond function-
alization via redox reactions of alkene derivatives,[11] we recently reported on the elec-
trochemical oxidative functionalization of alkenes and aromatics.[12] Herein, we present
the electrochemical oxidative iodination of imidazo[1,2-a]pyridines using metal iodide.
To determine the optimized reaction conditions for the iodination of imidazo[1,2-
a]pyridines, we examined the electrochemical iodination of 2-phenylimidazo[1,2-a]pyri-
dine (1a) with NaI (2a) as an iodine source and supporting electrolyte. Encouragingly,
using DMF as the solvent, 3-iodo-2-phenylimidazo[1,2-a]pyridine (3a) was obtained in
64% isolated yield with platinum plate anode and platinum plate cathode in the undiv-
ided cell (Table 1, entry 1). To improve the yield, we then surveyed the common sol-
vents, such as methanol, water, acetonitrile, tetrahydrofuran (THF), and co-solvents of
acetonitrile:water (9:1) (Table 1, entries 1–6), the yield increased to 97% when aceto-
nitrile was used (Table 1, entry 4). Various metal iodides such as NaI, LiI, KI, and MgI2
were screened (Table 1, entries 4 and 7–9). NaI was found to be the most efficient iod-
ine source and supporting electrolyte for this reaction (Table 1, entry 4). Reducing the
amount of NaI to 1.0 equiv. gave a modest decrease in the yield of 3a (Table 1, entries
4 and 10–11). Furthermore, changing the current intensity (7 or 3 mA) led to slightly
decreased yields. (Table 1, entries 4 and 12–13). Using graphite rod or nickel plates to
replace platinum plates was less efficient (Table 1, entries 14–16).
With the optimal reaction conditions determined, we investigated the substrate scope
of imidazopyridines (Table 2). We initially examined the iodination of

R
SYNTHETIC COMMUNICATIONSV
3
Table 1. Reaction condition optimization.a
Entry
Solvent
Electrolyte
Yieldb (%)
1g
2
3
DMF
MeOH
H2O
MeCN
MeCN:H2O
THF
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
NaI
NaI
NaI
NaI
NaI
NaI
LiI
64
53
65
92
70
72
65
73
81
92
80
90
81
82
82
87
0
4
5c
6
7
8
9
KI
MgI2
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
10d
11e
12f
13g
14h
15i
16j
17k
18l
trace
aReaction conditions: platinum plate anode, platinum plate cathode, 2-phenylimidazo[1,2-a]pyridine (1a, 0.1 mmol),
metal iodide (2a, 0.2 mmol), solvent (3.0 mL) at room temperature for 4 h using DC regulated power supply.
bIsolated yield.
c9:1 mixture of CH3CN:H2O.
d0.3 mmol of 2a loading.
e0.1 mmol of 2a loading.
fThe current was 7 mA.
gThe current was 3 mA for 5 h.
hC(þ)–C(ꢀ).
iC(þ)–Pt(ꢀ).
jC(þ)–Ni(þ).
kNo electricity.
lThree equiv. of BHT was added.
2-arylimidazo[1,2-a]pyridines 1 bearing various substituents on the C-2 aryl group with
NaI (2a) under the optimized conditions. 2-Arylimidazo[1,2-a]pyridines 1 bearing vari-
ous substituents such as p-methylphenyl, p-methoxyphenyl, p-fluorophenyl, p-bromo-
phenyl, m-methylphenyl, 2-naphthyl, 1-naphthyl, and 2-thienyl groups of the C-2 aryl
ring underwent a facile transformation to give the corresponding 3-iodinated products
3a–3i in moderate to excellent yields (72–98%). Moreover, 2-phenylimidazo[1,2-a]pyri-
dines 1 with electron-donating groups such as 6-methyl, 7-methyl, and 8-methyl sub-
stituents on the pyridine ring provided the corresponding 3-iodinated products 3j–3l in
moderate yields (61–70%). The iodination of 2-phenylimidazo[1,2-a]pyridines 1 having
electron-withdrawing groups such as 6-bromo, 6-fluoro, and 8-bromo substituents on
the pyridine ring also, respectively, delivered the desired 3-iodinated products 3m–3o in
moderate-to-high yields (72–81%). Fortunately, the iodination of 2-alkyl substituted imi-
dazo[1,2-a]pyridine, 2-methylimidazo[1,2-a]pyridine, gave the corresponding 3-iodo-2-
methylimidazo[1,2-a]pyridine (3p) with relatively low yield (51%). Even unsubstituted
imidazopyridine gave the corresponding iodinated products 3q in good yield under the
same conditions. Similarly, the bromination of 2-phenylimidazo[1,2-a]pyridine (1a)

4
J. W. PARK ET AL.
Table 2. Substrate scope.a,b
aReaction conditions: platinum plate anode, platinum plate cathode, imidazo[1,2-a]pyridine 1 (0.1 mmol), NaI (2a,
0.2 mmol), acetonitrile (3.0 mL) at room temperature for 4 h using DC regulated power supply.
bIsolated yield.
cNaBr (0.2 mmol) was used instead of NaI.
with NaBr as halogen source provided the corresponding C-3 brominated product 3r in
excellent yield.
After gaining insight into the mechanism, some controlled experiments were con-
ducted. The absence of an electric current suppressed this reaction (Table 1, entry 17).

R
SYNTHETIC COMMUNICATIONSV
5
Figure 1. Plausible reaction pathway.
In the presence of 3 equiv. of 2,6-di-tert-butyl-4-methylphenol (BHT), a trace amount
of desired product 3a was obtained (Table 1, entry 18). Thus, the possibility of a radical
pathway was involved in this reaction. Additionally, the cyclic voltammetry (CV) experi-
ments were conducted to study the redox potential of the substrates. An oxidation
potential of NaI (2a) was lower than that of 2-phenylimidazo[1,2-a]pyridine (1a) (1.36
and 1.45 V vs. Ag/AgCl). These results indicate that the oxidation of iodide should
occur first in this electrochemical system. Based on the preliminary experimental results
obtained here, as well as the related literature,[13] we propose the plausible reaction
pathway shown in Figure 1. The reaction begins with the anodic oxidation of iodide ion
to generate the iodine radical and molecular iodine. 2-Phenylimidazo[1,2-a]pyridine
(1a) react with molecular iodine to generate cationic intermediate B. The radical inter-
mediate A could then be formed through a radical addition of iodine radical to 1a.
Finally, further single-electron oxidation led to cationic intermediate B which is depro-
tonated to afford the iodinated product 3a.
Conclusion
In conclusion, we have developed an efficient synthesis of 3-iodoimidazo[1,2-a]pyridines
via the electrochemical oxidative iodination under exogenous chemical oxidant-free
conditions. This protocol is environmentally friendly because it uses inexpensive, non-
toxic, and easily handled NaI as the molecular iodine source and electricity as the
oxidant. This reaction represents a convenient and mild method for the preparation of
3-iodoimidazo[1,2-a]pyridine derivatives.
Experimental
General procedure for the electrochemical iodination of imidazo[1,2-a]pyridines
An undivided cell was equipped with a platinum plate (1.0 ꢁ 0.7 cm2) as the anode and
a platinum plate (1.0 ꢁ 0.7 cm2) as the cathode and connected to a DC regulated power
supply. A mixture of imidazo[1,2-a]pyridine 1 (0.1 mmol) and NaI (2a, 0.2 mmol) in

6
J. W. PARK ET AL.
acetonitrile (3.0 mL) was added to an undivided cell. The reaction mixture was stirred
and electrolyzed at a constant current of 5 mA under room temperature for 4 h. When
the reaction was completed, electrodes were washed with ethanol (EtOH). The com-
bined organic solvent was removed in vacuo. The residue was purified by flash chroma-
tography (EtOAc:Hex, 1:5) to afford 3-iodo imidazo[1,2-a]pyridine 3.
3-Iodo-2-phenylimidazo[1,2-a]pyridine (3a): Yield: 92%; yellow solid; mp
1
140–142 ꢂC; H NMR (400 MHz, CDCl3) d 8.24 (d, J ¼ 6.8 Hz, 1H), 8.08–8.05 (m, 2H),
7.63 (d, J ¼ 8.4 Hz, 1H), 7.52–7.46 (m, 2H), 7.43–7.37 (m, 1H), 7.30–7.24 (m, 1H),
6.97–6.92 (m, 1H); 13C NMR (100 MHz, CDCl3) d 148.0, 147.9, 133.4, 129.0, 128.5,
128.4, 126.6, 125.7, 117.5, 113.3, 59.6; HRMS (ESI) calculated for C13H10IN2 [M þ H]þ
320.9889; found 320.9893.
1
Supplementary data (copies of H, 13C, and 19F NMR spectra of all the synthesized
compounds) associated with this article can be found through the “Supplementary
Content” section of this article’s webpage.
Funding
This research was supported by Soonchunhyang University Research Fund and Basic Science
Research Program through the National Research Foundation of Korea [NRF-
2019R1F1A1059821].
References
[1] (a) Enguehard-Gueiffier, C.; Gueiffier, A. Recent Progress in the Pharmacology of
Imidazo[1,2-a]Pyridines. Mini Rev. Med. Chem. 2007, 7, 888–899. DOI: 10.2174/
138955707781662645. (b) Bagdi, A. K.; Santra, S.; Monir, K.; Hajra, A. Synthesis of
Imidazo[1,2-a]Pyridines: A Decade Update. Chem. Commun. 2015, 51, 1555–1575. DOI:
10.1039/c4cc08495k. (c) Pericherla, K.; Kaswan, P.; Pandey, K.; Kumar, A. Recent
Developments in the Synthesis of Imidazo[1,2-a]pyridines. Synthesis 2015, 47, 887. DOI:
10.1055/s-0034-1380182.
[2] (a) Al-Tel, T. H.; Al-Qawasmeh, R. A.; Zaarour, R. Design, Synthesis and in Vitro
Antimicrobial Evaluation of Novel Imidazo[1,2-a]Pyridine and Imidazo[2,1-
b][1,3]Benzothiazole Motifs. Eur. J. Med. Chem. 2011, 46, 1874–1881. DOI: 10.1016/j.
ejmech.2011.02.051. (b) Shukla, N. M.; Salunke, D. B.; Yoo, E.; Mutz, C. A.; Balakrishna,
R.; David, S. A. Antibacterial Activities of Groebke-Blackburn-Bienaymu-Derived
Imidazo[1,2-a]Pyridin-3-Amines. Bioorg. Med. Chem. 2012, 20, 5850–5863. DOI: 10.1016/
j.bmc.2012.07.052.
[3] Kaminski, J. J.; Doweyko, A. M. J. Antiulcer Agents. 6. Analysis of the in Vitro
Biochemical and in Vivo Gastric Antisecretory Activity of Substituted Imidazo[1,2-
a]Pyridines and Related Analogues Using Comparative Molecular Field Analysis and
Hypothetical Active Site Lattice Methodologies. J. Med. Chem. 1997, 40, 427–436. DOI:
10.1021/jm950700s.
ꢀ
[4] (a) Veron, J.-B.; Allouchi, H.; Enguehard-Gueiffier, C.; Snoeck, R.; Andrei, G.; De Clercq,
E.; Gueiffier, A. Influence of 6- or 8-Substitution on the Antiviral Activity of 3-
Arylalkylthiomethylimidazo[1,2-a]Pyridine against Human Cytomegalovirus (CMV) and
Varicella-Zoster Virus (VZV): Part II. Bioorg. Med. Chem. 2008, 16, 9536–9545. DOI: 10.
1016/j.bmc.2008.09.027. (b) Feng, S.; Hong, D.; Wang, B.; Zheng, X.; Miao, K.; Wang, L.;
Yun, H.; Gao, L.; Zhao, S.; Shen, H. C. A. C. S. Discovery of Imidazopyridine Derivatives

R
SYNTHETIC COMMUNICATIONSV
7
as Highly Potent Respiratory Syncytial Virus Fusion Inhibitors. ACS Med. Chem. Lett.
2015, 6, 359–362. DOI: 10.1021/acsmedchemlett.5b00008.
[5] (a) Kim, O.; Jeong, Y.; Lee, H.; Hong, S.-S.; Hong, S. Design and Synthesis of
Imidazopyridine Analogues as Inhibitors of Phosphoinositide 3-Kinase Signaling and
Angiogenesis. J. Med. Chem. 2011, 54, 2455–2466. DOI: 10.1021/jm101582z. (b) El-Sayed,
W. M.; Hussin, W. A.; Al-Faiyz, Y. S.; Ismail, M. A. The Position of Imidazopyridine and
Metabolic Activation Are Pivotal Factors in the Antimutagenic Activity of Novel
Imidazo[1,2-a]Pyridine Derivatives. Eur. J. Pharmacol. 2013, 715, 212–218. DOI: 10.1016/j.
ejphar.2013.05.018. (c) Gladysz, R.; Adriaenssens, Y.; De Winter, H.; Joossens, J.; Lambeir,
A.-M.; Augustyns, K.; Van der Veken, P. Discovery and SAR of Novel and Selective
Inhibitors of Urokinase Plasminogen Activator (uPA) with an Imidazo[1,2-a]Pyridine
Scaffold. J. Med. Chem. 2015, 58, 9238–9257. DOI: 10.1021/acs.jmedchem.5b01171.
[6] (a) Enguehard, C.; Renou, J. L.; Collot, V.; Hervet, M.; Rault, S.; Gueiffier, A. Reactivity of
3-Iodoimidazo[1,2-a]Pyridines Using a Suzuki-Type Cross-Coupling Reaction. J. Org.
Chem. 2000, 65, 6572–6575. DOI: 10.1021/jo000698z. (b) Gudmundsson, K. S.; Williams,
J. D.; Drach, J. C.; Townsend, L. B. Synthesis and Antiviral Activity of Novel
Erythrofuranosyl Imidazo[1,2-a]Pyridine C-Nucleosides Constructed via Palladium
Coupling of Iodoimidazo[1,2-a]Pyridines and Dihydrofuran. J. Med. Chem. 2003, 46,
1449–1455. DOI: 10.1021/jm020339r. (c) Zhang, H.; Wei, Q.; Wei, S.; Qu, J.; Wang, B.
Highly Efficient and Practical Thiocyanation of Imidazopyridines Using an N-
Chlorosuccinimide/NaSCN Combination. Eur. J. Org. Chem. 2016, 2016, 3373–3379. DOI:
10.1002/ejoc.201600485. (d) Chen, Z.; Cao, G.; Zhang, F.; Li, H.; Xu, J.; Miao, M.; Ren, H.
Metal-Free Mediated C-3 Methylsulfanylation of Imidazo[1,2-a]-Pyridines with Dimethyl
Sulfoxide as a Methylsulfanylating Agent. Synlett 2017, 28, 1795. DOI: 10.1055/s-0036-
1588419. (e) Guo, Y.-J.; Lu, S.; Tian, L.-L.; Huang, E.-L.; Hao, X.-Q.; Zhu, X.; Tian, S.;
Song, M.-P. Iodine-Mediated Difunctionalization of Imidazopyridines with Sodium
Sulfinates: Synthesis of Sulfones and Sulfides. J. Org. Chem. 2018, 83, 338–349. DOI: 10.
1021/acs.joc.7b02734. (f) Tian, L.-L.; Lu, S.; Zhang, Z.-H.; Huang, E.-L.; Yan, H.-T.; Zhu,
X.; Hao, X.-Q.; Song, M.-P. Copper-Catalyzed Double Thiolation to Access Sulfur-Bridged
Imidazopyridines with Isothiocyanate. J. Org. Chem. 2019, 84, 5213–5221. DOI: 10.1021/
acs.joc.9b00186. (g) Yu, Y.; Su, Z.; Cao, H. Strategies for Synthesis of Imidazo[1,2-
a]Pyridine Derivatives: Carbene Transformations or C–H Functionalizations. Chem. Rec.
2019, 19, 2105–2118. DOI: 10.1002/tcr.201800168.
[7] (a) Marhadour, S.; Bazin, M. A.; Marchand, P. An Efficient Access to 2,3-
Diarylimidazo[1,2-a]Pyridines via Imidazo[1,2-a]Pyridin-2-yl Triflate through a Suzuki
Cross-Coupling Reaction-Direct Arylation Sequence. Tetrahedron Lett. 2012, 53, 297–300.
DOI: 10.1016/j.tetlet.2011.11.015. (b) Zhou, X.; Yan, H.; Ma, C.; He, Y.; Li, Y.; Cao, J.;
Yan, R.; Huang, G. Copper-Mediated Aerobic Oxidative Synthesis of 3-Bromo-Imidazo[1,
2-a]Pyridines with Pyridines and Enamides. J. Org. Chem. 2016, 81, 25–31. DOI: 10.1021/
acs.joc.5b02384. (c) Li, J.; Tang, J.; Wu, Y.; He, Q.; Yu, Y. Transition-Metal-Free
Regioselective CiosHalogenation of Imidazo[1,2- a ]Pyridines: Sodium Chlorite/Bromite as
the Halogen Source. RSC Adv. 2018, 8, 5058–5062. DOI: 10.1039/C7RA12100H. (d)
Indukuri, D. R.; Potuganti, G. R.; Alla, M. Hypervalent Iodine Mediated Efficient Solvent-
Free Regioselective Halogenation and Thiocyanation of Fused N-Heterocycles. Synlett
2019, 30, 1573. DOI: 10.1055/s-0037-1611856. (e) Katrun, P.; Kuhakarn, C. K2S2O8-
Mediated Halogenation of 2-Arylimidazo[1,2-a]Pyridines Using Sodium Halides as the
Halogen Sources. Tetrahedron Lett. 2019, 60, 989–993. DOI: 10.1016/j.tetlet.2019.03.008.
(f) Semwal, R.; Ravi, C.; Kumar, R.; Meena, R.; Adimurthy, S. J. Sodium Salts (NaI/NaBr/
NaCl) for the Halogenation of Imidazo-Fused Heterocycles. J. Org. Chem. 2019, 84,
792–805. DOI: 10.1021/acs.joc.8b02637. (g) Yuan, Y.; Yao, A.; Zheng, Y.; Gao, M.; Zhou,
Z.; Qiao, J.; Hu, J.; Ye, B.; Zhao, J.; Wen, H.; et al. Electrochemical Oxidative Clean
Halogenation Using HX/NaX with Hydrogen Evolution. Science 2019, 12, 293–303. DOI:
10.1016/j.isci.2019.01.017. (h) Zhou, Z.; Yuan, Y.; Cao, Y.; Qiao, J.; Yao, A.; Zhao, J.; Zuo,
W.; Chen, W.; Lei, A. Synergy of Anodic Oxidation and Cathodic Reduction Leads to

8
J. W. PARK ET AL.
Electrochemical Cemical Cction L. Chin. J. Chem. 2019, 37, 611–615. DOI: 10.1002/cjoc.
201900091.
[8] (a) Gudmundsson, K. S.; Drach, J. C.; Townsend, L. B. Palladium Catalyzed Coupling of
2,6-Dichloro-3-Iodoimidazo[1,2-a]Pyridine and 2,3-Dihydrofuran as an Approach to
Novel Imidazo[1,2-a]Pyridine C-Nucleosides. Tetrahedron Lett. 1996, 37, 6275–6278. DOI:
10.1016/0040-4039(96)01375-5. (b) Kazzouli, S. E.; Berteina-Raboin, S.; Mouaddib, A.;
Guillaunet, G. Synthesis and Functionalization of Imidazo[1,2-a]Pyridines and Imidazo[1,
2-a]Pyrimidines on Solid Phase Using Suzuki-Miyaura Cross-Coupling Reactions. Lett.
Org. Chem. 2012, 9, 118. DOI: 10.2174/157017812800221753. (c) Mondal, S.; Samanta, S.;
Singsardar, M.; Mishra, S.; Mitra, S.; Hajra, A. Zwitterionic-Type Molten Salt Catalyzed
Iodination in Water: Synthesis of Iodoimidazoheterocycles. Synthesis 2016, 48, 4009. DOI:
10.1055/s-0035-1562492. (d) Ghosh, P.; Ganguly, B.; Kar, B.; Dwivedi, S.; Das, S. Green
Procedure for Highly Efficient, Rapid Synthesis of Imidazo[1,2- a]Pyridine and Its Late
Stage Functionalization. Synth. Commun. 2018, 48, 1076–1084. DOI: 10.1080/00397911.
2018.1434893.
[9] Katrun, P.; Kuhakarn, C. K2S2O8-Mediated Halogenation of 2-Arylimidazo[1,2-
a]Pyridines Using Sodium Halides as the Halogen Sources. C. Tetrahedron Lett. 2019, 60,
989–993. DOI: 10.1016/j.tetlet.2019.03.008.
[10] (a) Yan, M.; Kawamata, Y.; Baran, P. S. Synthetic Organic Electrochemical Methods since
2000: On the Verge of a Renaissance. Chem. Rev. 2017, 117, 13230–13319. DOI: 10.1021/
acs.chemrev.7b00397. (b) Liu, K.; Song, C.; Lei, A. Recent Advances in Iodine Mediated
Electrochemical Oxidative Cross-Coupling. Org. Biomol. Chem. 2018, 16, 2375–2387. DOI:
€
10.1039/C8OB00063H. (c) Mohle, S.; Zirbes, M.; Rodrigo, E.; Gieshoff, T.; Wiebe, A.;
Waldvogel, S. R. Modern Electrochemical Aspects for the Synthesis of Value-Added
Organic Products. Angew. Chem. Int. Ed. 2018, 57, 6018. DOI: 10.1002/anie.201712732.
(d) Moeller, K. D. Using Physical Organic Chemistry to Shape the Course of
Electrochemical Reactions. Chem. Rev. 2018, 118, 4817–4833. DOI: 10.1021/acs.chemrev.
7b00656. (e) Yoshida, J.-I.; Shimizu, A.; Hayashi, R. Electrogenerated Cationic Reactive
Intermediates: The Pool Method and Further Advances. Chem. Rev. 2018, 118, 4702–4730.
DOI: 10.1021/acs.chemrev.7b00475. (f) Zhao, Y.; Xia, W. Recent Advances in Radical-
Based C–N Bond Formation via Photo-/Electrochemistry. Chem. Soc. Rev. 2018, 47,
2591–2608. DOI: 10.1039/c7cs00572e. (g) Nutting, J. E.; Rafiee, M.; Stahl, S. S.
Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-
Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions.
Chem. Rev. 2018, 118, 4834–4885. DOI: 10.1021/acs.chemrev.7b00763. (h) Waldvogel, S.
R.; Lips, S.; Selt, M.; Riehl, B.; Kamp, C. J. Electrochemical Arylation Reaction. Chem. Rev.
2018, 118, 6706–6765. DOI: 10.1021/acs.chemrev.8b00233. (i) Pletcher, D.; Green, R. A.;
Brown, R. C. D. Flow Electrolysis Cells for the Synthetic Organic Chemistry Laboratory.
€ €
Chem. Rev. 2018, 118, 4573–4591. DOI: 10.1021/acs.chemrev.7b00360. (j) Karkas, M. D.
Electrochemical Strategies for C-H Functionalization and C-N Bond Formation. Chem.
Soc. Rev. 2018, 47, 5786–5865. DOI: 10.1039/c7cs00619e. (k) Sauer, G. S.; Lin, S. An
Electrocatalytic Approach to the Radical Difunctionalization of Alkenes. ACS Catal. 2018,
8, 5175–5187. DOI: 10.1021/acscatal.8b01069.
[11] For a selection of our recent works on C-H activation, see: (a) Kang, Y. K.; Kim, S. M.;
Kim, D. Y. Enantioselective Organocatalytic Cganocatalytic CCrganocatalytic Cks on C-H
activatTransfer/Ring Closure: Asymmetric Synthesis of Tetrahydroquinolines. J. Am.
Chem. Soc. 2010, 132, 11847–11849. DOI: 10.1021/ja103786c. (b) Kang, Y. K.; Kim, D. Y.
Asymmetric Synthesis of Tetrahydroquinolines via 1,5-Hydride Transfer/Cyclization
Catalyzed by Chiral Primary Amine Catalysts. Adv. Synth. Catal. 2013, 355, 3131–3136.
DOI: 10.1002/adsc.201300398. (c) Suh, C. W.; Woo, S. B.; Kim, D. Y. Asymmetric
Synthesis of Tetrahydroquinolines via Saegusa-Type Oxidative Enamine Catalysis/1,5-
Hydride Transfer/Cyclization Sequences. Asian J. Org. Chem. 2014, 3, 399–402. DOI: 10.
1002/ajoc.201400022.;(d) Kang, Y. K.; Kim, D. Y. Enantioselective Organocatalytic
Oxidative Enamine Catalysis-1,5-Hydride Transfer-Cyclization Sequences: Asymmetric

R
SYNTHETIC COMMUNICATIONSV
9
Synthesis of Tetrahydroquinolines. Chem. Commun. 2014, 50, 222–224. DOI: 10.1039/
c3cc46710d. (e) Suh, C. W.; Kim, D. Y. Enantioselective One-Pot Synthesis of Ring-Fused
Tetrahydroquinolines via Aerobic Oxidation and 1,5-Hydride Transfer/Cyclization
Sequences. Org. Lett. 2014, 16, 5374–5377. DOI: 10.1021/ol502575f. (f) Kwon, S. J.; Kim,
D. Y. Organo- and Organometallic-Catalytic Intramolecular [1,5]-Hydride Transfer/
Cyclization Process through C(sp(3))-H Bond Activation. Chem. Rec. 2016, 16,
1191–1203. DOI: 10.1002/tcr.201600003. (g) Suh, C. W.; Kwon, S. J.; Kim, D. Y. Synthesis
of Ring-Fused 1-Benzazepines via [1,5]-Hydride Shift/7-Endo Cyclization Sequences. Org.
Lett. 2017, 19, 1334–1337. DOI: 10.1021/acs.orglett.7b00184. (h) Kim, Y. J.; Kim, Y.; Kim,
D. Y. Synthesis of Ring-Fused Tetrahydroquinoline Derivatives via [1,5]-Hydride Transfer/
Cyclization Sequences. Bull. Korean Chem. Soc. 2017, 38, 578–581. DOI: 10.1002/bkcs.
11126. (i) Yoon, T. H.; Kim, D. Y. Thiourea-catalyzed 1,5-hydride transfer and ring clos-
ure sequences of o-dialkylaminosubstituted nitrostyrenes: Synthesis of ring-fused tetrahy-
droquinoline derivatives. Synth. Commun. 2017, 47, 2109. DOI: 10.1080/00397911.2017.
1365908. (j) Kwon, S. J.; Gil, M. G.; Kim, D. Y. Visible Light Mediated Photocatalytic
Oxidative Coupling Reaction of N -Phenyl Tetrahydroisoquinoline with drKeto Acids.
Tetrahedron Lett. 2017, 58, 1592–1594. DOI: 10.1016/j.tetlet.2017.03.026. (k) Kim, J. H.;
Kim, D. Y. Synthesis of 2,4-Diaryl-4 H-Chromenes via Decarboxylative Alkylation of
AKeto Acids to Ortho -Quinone Methides as Reactive Intermediates. Bull. Korean Chem.
Soc. 2018, 39, 1007–1010. DOI: 10.1002/bkcs.11550. (l) Jeong, H. J.; Kim, D. Y.
Enantioselective Decarboxylative Alkylation of AKeto Acids to ortho-Quinone Methides as
Reactive Intermediates: Asymmetric Synthesis of 2,4-Diaryl-1-Benzopyrans. Org. Lett.
2018, 20, 2944–2947. DOI: 10.1021/acs.orglett.8b00993.
[12] For a selection of our recent works on electrochemical reactions, see: (a) Kim, Y. J.; Kim,
D. Y. Electrochemical Radical Selenylation/1,2-Carbon Migration and Dowdwdwdion and
Dowdarbon Migration and Dowd seAlkenylcyclobutanols. Org. Lett. 2019, 21, 1021–1025.
DOI: 10.1021/acs.orglett.8b04041. (b) Kim, Y. J.; Kim, D. Y. Electrochemical Oxidative
Selenylation of Imidazo[1,22zoPyridines with Diselenides. Tetrahedron Lett. 2019, 60,
739–742. DOI: 10.1016/j.tetlet.2019.02.001. (c) Jung, H. I.; Kim, Y.; Kim, D. Y.
Electrochemical Trifluoromethylation/Semipinacol Rearrangement Sequences of Alkenyl
Alcohols: Synthesis of latioSubstituted Ketones. Org. Biomol. Chem. 2019, 17, 3319–3323.
DOI: 10.1039/c9ob00373h. (d) Kim, Y. J.; Kim, D. Y. Electrochemical Radical
Arylsulfonylation/Semipinacol Rearrangement Sequences of Alkenylcyclobutanols:
Synthesis of noSulfonated Cyclic Ketones. Tetrahedron Lett. 2019, 60, 1287–1290. DOI:
10.1016/j.tetlet.2019.04.009.
[13] (a) Kataoka, K.; Hagiwara, Y.; Midorikawa, K.; Suga, S.; Yoshida, J. Practical
Electrochemical Iodination of Aromatic Compounds. Org. Process Res. Dev. 2008, 12,
1130–1136. DOI: 10.1021/op800155m. (b) Aiso, H.; Kochi, T.; Mutsutani, H.; Tanabe, T.;
Nishiyama, S.; Kakiuchi, F. Catalytic Electrochemical C-H Iodination and One-Pot
Arylation by on/off Switching of Electric Current. J. Org. Chem. 2012, 77, 7718–7724.
DOI: 10.1021/jo3012286. (c) Chen, C.; Niu, P.; Shen, Z.; Li, M. Electrochemical
Sulfenylation of Indoles with Disulfides Mediated by Potassium Iodide. J. Electrochem.
Soc. 2018, 165, G67–G74. DOI: 10.1149/2.0071807jes.