Highly selective C3-H iodination of pyrrolo[1,2-a]quinoxalines

Article abstract of DOI:10.1039/d1ob00759a

We report a C3-H direct iodination of pyrrolo[1,2-a]quinoxalines with TBAI or I₂; a series of novel 3-iodopyrrolo[1,2-a]quinoxaline derivatives were obtained with excellent regioselectivity and broad substrate scope. Mechanism studies show that a catalytic amount ofp-toluenesulfonic acid significantly promotes the selectivity and conversion of the reaction. Notably, the reaction can be performed on a gram scale, and the iodinated products can be further transformed into potentially biologically active pyrrolo[1,2-a]quinoxaline derivatives by palladium-catalyzed coupling reactions.

Full text of DOI:10.1039/d1ob00759a

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Highly selective C3–H iodination of pyrrolo[1,2-a]
quinoxalines†
Cite this: Org. Biomol. Chem., 2021,
19, 5191
Yali Liu, Yu Wei,
Zhen Yang, Yang Li, Yan Liu* and Ping Liu
*
We report a C3–H direct iodination of pyrrolo[1,2-a]quinoxalines with TBAI or I2; a series of novel 3-iodo-
pyrrolo[1,2-a]quinoxaline derivatives were obtained with excellent regioselectivity and broad substrate
scope. Mechanism studies show that a catalytic amount of p-toluenesulfonic acid significantly promotes
the selectivity and conversion of the reaction. Notably, the reaction can be performed on a gram scale,
and the iodinated products can be further transformed into potentially biologically active pyrrolo[1,2-a]
quinoxaline derivatives by palladium-catalyzed coupling reactions.
Received 20th April 2021,
Accepted 20th May 2021
DOI: 10.1039/d1ob00759a
rsc.li/obc
solvent. This approach represents a feasible route for the
late-stage functionalization of pyrrolo[1,2-a]quinoxalines
Introduction
Pyrrolo[1,2-a]quinoxalines are common motifs in a variety of (Scheme 1a).15 Theoretically, the regioselectivity of the electro-
natural products and pharmaceutical compounds (Fig. 1)1–6 philic halogenation matches the higher electron cloud density
and are also useful synthetic intermediates.7–9 Accordingly, at the C1-position on the pyrrole ring. Therefore, it is challen-
numerous attempts have been made to synthesize these valu- ging to realize the regioselective C–H bond halogenation
able pyrrolo[1,2-a]quinoxalines through various cyclization of pyrrolo[1,2-a]quinoxalines other than C1–H. Herein we
reactions.10 However, most of the above reactions mainly con- reported a high regioselectivity C3–H iodination of pyrrolo[1,2-
struct 4-position substituted pyrrolo[1,2-a]quinoxalines, which a]quinoxalines to access various 3-iodopyrrolo[1,2-a]quinoxa-
greatly limits the development of diverse pyrrolo[1,2-a]quinoxa- line derivatives under the action of an acid (Scheme 1b).
line compounds.11 In contrast, pyrrolo[1,2-a]quinoxalines have
multiple reactive sites, which provides the possibility for their
further transformation. So direct C–H functionalization of
Results and discussion
arenes has become an alternative strategy for the preparation
of structurally diverse arene derivatives.12 As part of our
We first investigated the iodination reactions of pyrrolo[1,2-a]
ongoing efforts toward the regioselective functionalization of
pyrrolo[1,2-a]quinoxalines, we reported a KI/TBHP-catalyzed
quinoxaline 1a (0.3 mmol) with an iodine source (Table 1).
When 3 equiv. of TBAI were used, the desired reaction was
[3
+ 2] cycloaddition of pyrrolo[1,2-a]quinoxalines and
N-tosylhydrazones.13 The reaction provided a series of novel
fused [1,2,4]triazolo[3,4-c]quinoxalines. Subsequently, we
achieved a NCS-promoted direct C1–H thiocyanation and sele-
nocyanation of pyrrolo[1,2-a]quinoxalines.14 Recently, Nguyen
et al. developed a novel chlorination of C1–H bonds in 4-aryl
pyrrolo[1,2-a]quinoxalines in the presence of NCS as a chlori-
nating reagent, a catalytic amount of DMSO, and CHCl3 as the
solvent. In addition, the bromination of pyrrolo[1,2-a]quinoxa-
lines was also successful in the presence of CuBr2 as a bromi-
nating reagent, K2S2O8 as an oxidant, and toluene as the
Fig. 1 Pyrrolo[1,2-a]quinoxalines in biologically important targets.
School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing
of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi 832004,
China. E-mail: liuyan1979810@aliyun.com, liuping1979112@aliyun.com
†Electronic supplementary information (ESI) available. CCDC 2062350 and
2017290. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d1ob00759a
Scheme 1 Selective halogenations of pyrrolo[1,2-a]quinoxalines.
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Table 1 Optimization of the reaction conditionsa
1,4-dioxane as the optimal solvent provided the product 3a in
88% yield (entry 13). EtOH and toluene were also effective in
this reaction (entries 11 and 12). The use of MeOH only gave
50% yield (entry 10). Subsequently, KI and NIS as iodine
sources were tested again in 1,4-dioxane, but no better yield
was obtained (entries 14 and 15).
With the optimized conditions in hand, a variety of pyrrolo
[1,2-a]quinoxalines were evaluated (Table 2). Pyrrolo[1,2-a]qui-
noxaline derivatives bearing 8-Me and 8-F substituents at the
benzene ring position afforded the corresponding products 3b
and 3c in 71% and 68% yields, respectively. Surprisingly, when
H2O was used as the solvent, the iodination reaction of
7-methylpyrrolo[1,2-a]quinoxaline could proceed smoothly to
provide the desired product 3d in 62% yield with good regio-
selectivity. Subsequently, the iodinations of various 4-arylpyr-
rolo[1,2-a]quinoxalines were performed under the optimized
reaction conditions. The substrates bearing various substitu-
ents at the para-positions of the benzene ring were well toler-
ated, providing the desired products 3e–3l in 53–82% yields.
The results showed that the reactivity of substrates with elec-
tron-withdrawing groups (–F, –Cl, –Br, –CN, and –NO2) was
better than that of substrates with electron-donating groups
(–CH3 and –OCH3). 7-Methyl-4-arylpyrrolo[1,2-a]quinoxalines
as substrates also provided the desired products 3m and 3n in
60% and 81% yields, respectively. Furthermore, 4-(2,4-dichlor-
ophenyl)pyrrolo[1,2-a]quinoxaline afforded the product 3o in
86% yield. However, when 4-(furan-2-yl)pyrrolo[1,2-a]quinoxa-
line was used as the reactant, only 16% yield of the product 3p
was obtained.
[I] source
(equiv.)
T
(°C)
Yieldb
(%)
Entry
Additive
Solvent
1
2
3
4
5
6
7
8
TBAI (3)
TBAI (3)
NH4I (3)
KI (3)
TsNHNH2
—
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
MeOH
EtOH
Toluene
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
100
100
100
100
100
80
60
80
80
80
80
80
80
80
25
Nr
22
25
24
55
36
57
39
50
79
77
88
66
15
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
NIS (3)
TBAI (3)
TBAI (3)
TBAI (2)
TBAI (2)
TBAI (2)
TBAI (2)
TBAI (2)
TBAI (2)
KI (2)
9c
10
11
12
13
14
15
NIS (2)
80
a Reaction conditions: 1a (0.3 mmol), additive (0.9 mmol), solvent
(0.5 mL), TBHP (0.5 mL, 70% aqueous solution), 12 h, in air. b Isolated
yield. c Reaction time, 3 h.
achieved in the presence of 4-methylbenzenesulfonohydrazide
(TsNHNH2) and TBHP in H2O at 100 °C, and the target
product 3a was obtained in 25% yield (entry 1). The structure
of 3a was established by X-ray crystallographic analysis (Fig. 2).
The results of the control experiment showed that this reaction
was difficult to occur in the absence of TsNHNH2 (entry 2).
Switching TBAI to NH4I, KI or NIS provided comparable yields
of 3a (entries 3–5). The examination of the reaction tempera-
ture suggested that 80 °C was the optimal choice, allowing the
formation of 3a in 55% yield (entries 6 and 7). When the
amount of TBAI was reduced to 2 equivalents, 57% yield could
be obtained (entry 8). Further investigation showed that short-
ening the reaction time was detrimental to the reaction (entry
9). Then, with 2 equiv. of TBAI as the iodine source at 80 °C,
solvents were next screened (entries 10–13). To our delight,
Table 2 Scope of pyrrolo[1,2-a]quinoxalines with TBAIa
a Reaction conditions: 1 (0.3 mmol), TBAI (2 equiv.), TsNHNH2 (3
equiv.), 1,4-dioxane (0.5 mL), TBHP (0.5 mL, 70% aqueous solution),
80 °C, in air, isolated yield. b H2O as the solvent.
Fig. 2 The crystal structure of 3a (CCDC: 2017290†).
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We speculate that p-toluenesulfonic acid (PTSA) formed (0.2 mmol), I2 (1 equiv.), and PTSA·H2O (15 mol%) at 100 °C in
during the redox process of p-toluenesulfonyl hydrazide and DMSO (2 mL) (entry 7).
the iodide anion (TBAI) should have a certain promoting
Next, we investigated the reactions of pyrrolo[1,2-a]quinoxa-
effect on the iodination of pyrroloquinoxaline. To verify the lines with I2 (Table 4). 8-Fluoro or 8-methylpyrrolo[1,2-a]qui-
postulation, a series of experiments were performed. First, we noxalines as substrates gave the desired products 3b and 3c in
screened various iodine sources (1 equiv.) in the presence of good to excellent yields. However, 7-methylpyrrolo[1,2-a]qui-
PTSA·H2O (10 mol%) and DMSO as the solvent at 100 °C for noxaline as a partner only provided the product 3d in 59%
12 h (Table 3). It turned out that TBAI and KI were unsuccess- yield. 4-Arylpyrrolo[1,2-a]quinoxalines also worked well to
ful in the absence of a reducing agent (entries 1 and 2). In con- afford the products 3e–3k in 52–75% yields. The fluoro,
trast, I2 and NIS as iodine sources afforded the desired chloro, bromo, methoxy, and nitro groups were well tolerated,
product 3a in 72% and 77% yields, respectively (entries 3 and and the electronic effect of the substituents on aryl rings had
4). In addition, the reaction could also proceed in the absence little impact. Furthermore, the 4,7-disubstituted pyrrolo[1,2-a]
of p-toluenesulfonic acid, but the product 3a was obtained in a quinoxaline substrates were also explored for the reactions,
low yield (entry 5). As the amount of p-toluenesulfonic acid affording the desired products 3m and 3n in 71% and 65%
was increased to 15 mol%, the reaction yield significantly yields, respectively. 4-(2,4-Dichlorophenyl)pyrrolo[1,2-a]qui-
improved (entries 6 and 7). When 1.5 equiv. of I2 were used, noxaline reacted well and gave the desired product 3o in 86%
the yield was not significantly improved (entry 8). Lowering the yield. In addition, the iodination of 4-(furan-2-yl)pyrrolo[1,2-a]
reaction temperature leads to a decrease in yield (entry 9). But quinoxaline gave the product 3p in 24% yield.
increasing the reaction temperature did not improve the yield
The extension of the above reaction conditions to indolo
(entry 10). To enhance the yield of 3a, we then examined the [1,2-a]quinoxalines was also explored (Scheme 2). The iodina-
effect of solvents, such as DMF, 1,4-dioxane, EtOH, DMA, tion of indolo[1,2-a]quinoxaline with I2 under the action
CH3CN, and THF; however, no superior results were achieved of PTSA·H2O could proceed smoothly, providing the desired
(entries 11–16). Shortening the reaction time to 6 hours led to product 7-iodoindolo[1,2-a]quinoxaline 3q in 76% yield
a decrease in the yield (entry 17). Moreover, the yield was not (Scheme 2a). Importantly, the reaction of indolo[1,2-a]quinoxa-
significantly improved by further extending the reaction time line with TBAI in the presence of TsNHNH2 gave the sulfeny-
and increasing the amount of PTSA·H2O (entries 18 and 19). lated product 7-tosylindolo[1,2-a]quinoxaline 3q′ in 91% yield,
Eventually, the optimal reaction conditions emerged as 1a and an iodinated product was not obtained (Scheme 2b). The
scale-up reaction was easily achieved using 6 mmol pyrrolo
Table 3 C3–H iodination of pyrrolo[1,2-a]quinoxalines under the
action of PTSAa
a
Table 4 The reactions of pyrrolo[1,2-a]quinoxalines with I2
[I] source
(equiv.)
PTSA
(mol%)
T
(°C)
Yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
TBAI (1)
KI (1)
I2 (1)
NIS (1)
I2 (1)
NIS (1)
I2 (1)
I2 (1.5)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
I2 (1)
10
10
10
10
—
15
15
15
15
15
15
15
15
15
15
15
15
15
20
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
1,4-Dioxane
EtOH
DMA
CH3CN
THF
DMSO
DMSO
DMSO
100
100
100
100
100
100
100
100
80
120
100
100
100
100
100
100
100
100
100
nr
nr
72
77
50
87
86
88
34
85
42
36
36
46
37
45
77
88
89
9
10
11
12
13
14
15
16
17c
18d
19
a Reaction conditions: 1a (0.2 mmol), DMSO (2 mL), 100 °C, 12 h, in a Reaction conditions:
1
(0.2 mmol), I2 (1 equiv.), PTSA·H2O
air. b Isolated yield. c 6 h. d 24 h.
(15 mol%), DMSO (2 mL), 100 °C, in air; isolated yield.
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Table 6 Pd-Catalyzed Sonogashira reactions of 1aa
a Reaction conditions: 3a (0.2 mmol), 4 (1.5 equiv.), Et3N (2 mL), 65 °C,
12 h, in air; isolated yield.
Scheme 2 The reactions involving indolo[1,2-a]quinoxalines.
yields. 1-Ethynyl-4-methylbenzene showed a higher reactivity
than other substituents of aryl acetylenes, providing the
product 5j in 96% yield. The structure of 5j was determined by
X-ray diffraction analysis (Fig. 3). 2-Methylbut-3-yn-2-ol was
also suitable for this reaction, affording the product 5n in 92%
yield.
To investigate the possible mechanism, control experiments
were performed (Scheme 4). In the presence of 2,2,6,6-tetra-
methylpiperidinooxy (TEMPO) as a free radical scavenger, the
reaction was obviously suppressed under the standard con-
ditions. We speculate that the reaction highlights the for-
mation of a free radical intermediate. Furthermore, the
[1,2-a]quinoxaline 1a in the presence of 15 mol% PTSA·H2O,
affording 1.306 g of 3a in 74% yield (Scheme 3).
To highlight the synthetic utility of 3-iodopyrrolo[1,2-a]qui-
noxaline products, the derivatization reactions that take advan-
tage of the Caryl–I bond were investigated (Table 5). First, we
explored the Suzuki–Miyaura reactions of 3a with arylboronic
acids with Pd(PPh3)4 as the catalyst and Na2CO3 as the base at
80 °C under a N2 atmosphere, and the desired products 5a–5h
were obtained in 52–99% yields with good functional group
tolerance.
Next, the Sonogashira reactions of 3a with various terminal
alkynes 4 were performed with PdCl2(PPh3)2 and CuI as
catalysts, and Et3N as the base at 65 °C under in air (Table 6).
The expected coupling products 5i–5n were isolated in 52–96%
Fig. 3 The crystal structure of 5j (CCDC: 2062350†).
Scheme 3 Gram-scale synthesis of 3a.
Table 5 Pd-Catalyzed Suzuki–Miyaura reactions of 1aa
a Reaction conditions: 3a (0.2 mmol), 4 (2 equiv.), Pd(PPh3)4 (5 mol%),
Na2CO3 (2 equiv.), 1,4-dioxane/H2O (4 : 1, 2.5 mL), 80 °C, 5 h, under a
N2 atmosphere; isolated yield.
Scheme 4 The control experiments.
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multidisciplinary fields for the preparation of potential bio-
logically active pyrrolo[1,2-a]quinoxaline derivatives.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the financial support of this work
by the National Natural Science Foundation of China (No.
21563025). We also thank Jixing Zhao and Leifang Wu from
the Analysis and Testing Center of Shihezi University for their
help with X-ray single-crystal analysis.
Scheme 5 Plausible reaction mechanism.
addition of 2,6-di-tert-butyl-4-methylphenol (BHT) significantly
inhibited the reaction. All these indicated that the reaction is
likely to proceed via a radical pathway (Scheme 4[a]). When
hydrazine hydrate was replaced with p-toluenesulfonyl hydra-
zine, the iodination of pyrrolo[1,2-a]quinoxaline 1a did not
occur. This fact shows that p-toluenesulfonyl hydrazide is
essential (Scheme 4[b]). In addition, the control reaction of 1a
with I2 in the absence of TsOH only gives 51% yield of 3a. This
result means that p-toluenesulfonic acid has an obvious pro-
motion effect on the iodination. When p-toluenesulfinic acid
was used instead of TsOH to catalyze the iodination, the yield
of 3a was not improved (Scheme 4[c]). This experiment shows
that p-toluenesulfinic acid does not promote the reaction.
Next, we propose a plausible mechanism of iodination on
the basis of our experimental results (Scheme 5). First, the
iodine anion is oxidized into a tert-butoxy radical and elemen-
tal iodine. With the participation of tert-butoxy radicals, a sul-
fonyl radical is generated by the reaction of sulfonyl hydrazide
with TBHP.16 Subsequently, the sulfonyl radical interacts with
TBHP to produce p-toluenesulfonic acid (TsOH). Next, the iodi-
nation of 1a at the C3-position is achieved to form the desired
product 3a under the action of TsOH (route 1). On the other
hand, when indolo[1,2-a]quinoxaline 1q is used as the reac-
tant, the sulfonyl radical reacts with 1q to afford the arylsulfo-
nylated product 3q′ (route 2).17
Notes and references
1 A. Huang and C. Ma, Mini-Rev. Med. Chem., 2013, 13, 607.
2 (a) L. Ronga, M. Del Favero, A. Cohen, C. Soum, P. Le Pape,
S. Savrimoutou, N. Pinaud, C. Mullie, S. Daulouede,
P. Vincendeau, N. Farvacques, P. Agnamey, F. Pagniez,
S. Hutter, N. Azas, P. Sonnet and J. Guillon, Eur. J. Med.
Chem., 2014, 81, 378; (b) A. Jonet, J. Guillon, C. Mullie,
A. Cohen, G. Bentzinger, J. Schneider, N. Taudon,
S. Hutter, N. Azas, S. Moreau, S. Savrimoutou, P. Agnamey,
A. Dassonville-Klimpt and P. Sonnet, Med. Chem., 2018, 14,
293; (c) J. Guillon, A. Cohen, N. M. Gueddouda, R. N. Das,
S. Moreau, L. Ronga, S. Savrimoutou, L. Basmaciyan,
A. Monnier, M. Monget, S. Rubio, T. Garnerin, N. Azas,
J. L. Mergny, C. Mullie and P. Sonnet, J. Enzyme Inhib. Med.
Chem., 2017, 32, 547.
3 H. Xu and L. L. Fan, Eur. J. Med. Chem., 2011, 46, 1919.
4 E. Morelli, S. Gemma, R. Budriesi, G. Campiani,
E. Novellino, C. Fattorusso, B. Catalanotti, S. S. Coccone,
S. Ros, G. Borrelli, V. Kumar, M. Persico, I. Fiorini,
V. Nacci, P. Ioan, A. Chiarini, M. Hamon, A. Cagnotto,
T. Mennini, C. Fracasso, M. Colovic, S. Caccia and
S. Butini, J. Med. Chem., 2009, 52, 3548.
5 (a) J. Guillon, M. Le Borgne, C. Rimbault, S. Moreau,
S. Savrimoutou, N. Pinaud, S. Baratin, M. Marchivie,
S. Roche, A. Bollacke, A. Pecci, L. Alvarez, V. Desplat and
J. Jose, Eur. J. Med. Chem., 2013, 65, 205; (b) V. Desplat,
S. Moreau, A. Gay, S. B. Fabre, D. Thiolat, S. Massip,
G. Macky, F. Godde, D. Mossalayi, C. Jarry and J. Guillon,
J. Enzyme Inhib. Med. Chem., 2010, 25, 204.
6 (a) V. Desplat, M. Vincenzi, R. Lucas, S. Moreau,
S. Savrimoutou, N. Pinaud, J. Lesbordes, E. Peyrilles,
M. Marchivie, S. Routier, P. Sonnet, F. Rossi, L. Ronga and
J. Guillon, Eur. J. Med. Chem., 2016, 113, 214; (b) V. Desplat,
M. Vincenzi, R. Lucas, S. Moreau, S. Savrimoutou, S. Rubio,
N. Pinaud, D. Bigat, E. Enriquez, M. Marchivie, S. Routier,
P. Sonnet, F. Rossi, L. Ronga and J. Guillon,
ChemMedChem, 2017, 12, 940; (c) T. Wang, Y. Tang,
Conclusions
In summary, we have developed a regioselective C3–H iodina-
tion of pyrrolo[1,2-a]quinoxalines. Using TBAI as an iodine
source, pyrrolo[1,2-a]quinoxalines are iodinated at the C3-posi-
tion in the presence of TsNHNH2 as a reducing agent, yielding
3-iodopyrrolo[1,2-a]quinoxaline derivatives. Using I2 as an
iodine source, 3-iodopyrrolo[1,2-a]quinoxalines are also
formed under the action of a catalytic amount of PTSA·H2O.
The two methods represent novel strategies for selective C3–H
iodination and feature excellent regioselectivity, broad sub-
strate scope, gram-scale synthesis, and diverse transformations
of the products. The reaction should gain much attention in
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Y. Yang, Q. An, Z. Sang, T. Yang, P. Liu, T. Zhang, Y. Deng 13 Z. Yang, J. He, Y. Wei, W. Li and P. Liu, Org. Biomol. Chem.,
and Y. Luo, Bioorg. Med. Chem. Lett., 2018, 28, 2084. 2020, 18, 3360.
7 Z. P. Chen, M. W. Chen, R. N. Guo and Y. G. Zhou, Org. 14 Z. Yang, J. He, Y. Wei, W. Li, P. Liu, J. Zhao and Y. Wei, Org.
Lett., 2014, 16, 1406. Biomol. Chem., 2020, 18, 9088.
8 S. B. Hu, X. Y. Zhai, H. Q. Shen and Y. G. Zhou, Adv. Synth. 15 H. Le, T. Hoang, T. Tran, C. Nguyen, L. Chiem,
Catal., 2018, 360, 1334.
N. Phan and T. Nguyen, Tetrahedron Lett., 2021, 67,
9 X. Liu, T. Liu, W. Meng and H. Du, Org. Lett., 2018, 20, 5653.
152879.
10 A. A. Kalinin, L. N. Islamova and G. M. Fazleeva, Chem. 16 (a) X. Li, X. Xu and C. Zhou, Chem. Commun., 2012, 48,
Heterocycl. Compd., 2019, 55, 584.
11 W. Cong, L. Wang, F. Yu and J. Li, Chin. J. Org. Chem.,
2018, 38, 2866.
12 (a) Y. Qin, L. Zhu and S. Luo, Chem. Rev., 2017, 117, 9433;
(b) L. Ping, D. S. Chung, J. Bouffard and S. G. Lee, Chem.
Soc. Rev., 2017, 46, 4299; (c) C. X. Liu, Q. Gu and S. L. You,
12240; (b) X. Li, X. Xu and Y. Tang, Org. Biomol. Chem.,
2013, 11, 1739; (c) X. Li, X. Xu, P. Hu, X. Xiao and C. Zhou,
J. Org. Chem., 2013, 78, 7343; (d) X. Li, X. Xu, P. Hu, X. Xiao
and C. Zhou, Tetrahedron Lett., 2013, 54, 3071;
(e) S. Vaishali and K. Bharti, New J. Chem., 2018, 42,
12796.
Trends Chem., 2020, 2, 737; (d) J. F. Hartwig and 17 S. Zhao, K. Chen, L. Zhang, W. Yang and D. Huang, Adv.
M. A. Larsen, ACS Cent. Sci., 2017, 46, 4299. Synth. Catal., 2020, 362, 3516.
5196 | Org. Biomol. Chem., 2021, 19, 5191–5196
This journal is © The Royal Society of Chemistry 2021