Visible Light-Induced, Metal-Free Denitrative [3+2] Cycloaddition for Trisubstituted Pyrrole Synthesis

Article abstract of DOI:10.1002/asia.201901068

A metal-free, regioselective synthesis of trisubstituted pyrroles has been developed through a formal [3+2] cycloaddition reaction between 2H-azirines and nitroalkenes under visible light/photoredox-catalyzed conditions. The reaction proceeds through 2H-a

Full text of DOI:10.1002/asia.201901068

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Visible Light Induced, Metal-free Denitrative Formal [3+2]
Cycloaddition for Trisubstituted-Pyrrole Synthesis
Authors: Namrata Rastogi, Bhupal S. Karki, Lalita Devi, Ayushi
Pokhriyal, and Ruchir Kant
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201901068
Link to VoR: http://dx.doi.org/10.1002/asia.201901068
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

10.1002/asia.201901068
Chemistry - An Asian Journal
FULL PAPER
Visible Light Induced, Metal-free Denitrative Formal [3+2]
Cycloaddition for Trisubstituted-Pyrrole Synthesis
Bhupal S. Karki,a,b Lalita Devi,a,b Ayushi Pokhriyal,a Ruchir Kant,c and Namrata Rastogia,b*
Abstract: A metal free regioselective synthesis of trisubstituted
pyrroles has been developed through a formal [3+2] cycloaddition
reaction between 2H-azirines and nitroalkenes under visible light
photoredox catalyzed conditions. The reaction proceeds through 2H-
azaallenyl radical addition on β-nitrostyrenes in a Michael fashion
followed by base-mediated denitration reaction. The directive group
influence of the nitro group controls the regiochemistry of the
reaction.
In 2014, Lu and Xiao reported pyrrole synthesis through visible
light irradiation of 2H-azirines and alkynes in the presence of an
organic photosensitizer.6 The reaction involves azirine C-C bond
cleavage resulting into the generation of 2-azaallenyl radical
cation which undergo formal [3+2] cycoaddition with alkynes.
Thereafter, a handful of other reports followed employing
azirines for the construction of heterocyclic scaffolds such as
oxazoles,7 pyrrolidines,8 triazolines and triazoles9 and recently
dihydro-oxadiazoles10 via 2-azaallenyl radical cation formed by
the cleavage of azirine C-C bond under visible light irradiation.
However, Li and Wang documented a visible light photoredox
catalyzed synthesis of oxazoles from azirines that involved
azirine C-N bond cleavage.11 Notably, azirines usually undergo
C-N bond cleavage under metal catalyzed conditions generating
vinyl nitrenoid intermediates and in few cases simultaneous
cleavage of both C-C and C-N bonds results into the formation
of carbenoids and nitriles. Such behavior of azirines has been
well exploited for assembling diverse acyclic as well as
heterocyclic scaffolds under metal catalysis.12
Nitroalkenes are versatile building blocks for several target
scaffolds including various carbocycles and hetrocycles.13 The
strong electron withdrawing nature of the nitro group makes
them highly electron deficient alkenes, a suitable substrate for
numerous organic reactions such as Michael reaction, Mannich
reaction, Baylis-Hillman reaction, Rauhut-Currier reaction,
cycloadditions and metal-catalyzed coupling reactions.14
Recently, nitroalkenes have been exploited as the source of
alkenyl function in visible light mediated denitrative arylation,15
trifluoromethylation,16 carboxylation17 and alkylation reactions.18
However, to the best of our knowledge, nitroalkene have never
been used as dipolarophiles in visible light mediated cycloadition
reactions.
Introduction
The nitrogen-containing heterocycles or aza-heterocycles are
ubiquitous in biologically active molecules of natural as well as
synthetic origin.1 Among these aza-heterocycles, the pyrrole
derivatives register
a remarkable presence due to their
importance as nature’s building blocks, drugs, dyes, insecticides,
optoelectronic materials etc (figure 1).2 Quite understandably,
literature is replete with synthetic strategies to access pyrrole
derivatives.3 Despite several elegant methods available, the
milder and more efficient synthetic routes are still desired for the
synthesis of pyrroles bearing divergent substitution patterns. In
this context, the visible light photoredox catalysis has emerged
lately as a greener alternative to the conventional synthetic
approaches.4 The potential of photoredox catalysis has been
realized immensely by its application for the construction of
various heterocyclic motifs including natural products.5
We envisaged that owing to the directive group influence of the
nitro group, the nitroalkenes can impart regioselectivity in the
dipolar cycloaddition with 2-azaallenyl radical cations generated
from 2H-azirines under visible light mediated conditions. Also,
the excellent leaving group ability of the nitro group can be
exploited for aromatization of the expected cycloaddition
products i.e. pyrrolidines. Further, the 2-H azirines usually
undergo C-C bond cleavage under visible light irradiation which
would install 2- and 3- substituents on azirines on the 2- and 5-
position of the pyrroles, which cannot be achieved under metal
catalysis due to C-N bond cleavage of azirines under metal
catalyzed conditions. We herein report our findings on the visible
light mediated denitrative formal [3+2] cycloaddition reaction
between 2H-azirines and aromatic nitroalkenes for the
construction of trisubstituted pyrroles (Scheme 1).
Figure 1. Pyrrole motif in natural bioactive molecules and synthetic drugs
[a]
B. S. Karki, L. Devi, A. Pokhriyal, Dr. N. Rastogi
Medicinal & Process Chemistry Division
CSIR-Central Drug Research Institute, Sec. 10, Jankipuram
Extension, Sitapur Road, P.O. Box 173, Lucknow-226031 (India)
E-mail: namrata.rastogi@cdri.res.in; namrataiit@gmail.com
Academy of Scientific and Innovative Research, New Delhi-110001
(India)
[b]
[c]
Dr. R. Kant
Molecular & Structural Biology Division
CSIR-Central Drug Research Institute, Sec. 10, Jankipuram
Extension, Sitapur Road, P.O. Box 173, Lucknow-226031 (India)
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201901068
Chemistry - An Asian Journal
FULL PAPER
12
13
14
Amount of PC-I = 3 mol%
Amount of PC-I = 5 mol%
96 (90)
79
12 %
+ 4a (40 %)e
without DBU
[a] Unless otherwise mentioned, 1a (0.4 mmol) with specified amounts of 2a
and photocatalyst was irradiated in solvent (4.0 mL) with a 450 nm blue LED
for 16 h followed by light switch-off and addition of DBU (2 equiv.) in the
reaction mixture under N2. [b] NMR yields. [c] Isolated yields in parentheses.
[d] Green light was used. [e] 4a: 3-Nitro-2,4,5-triphenyl-3,4-dihydro-2H-pyrrole.
After extensive optimization of reaction conditions, we set out to
examine the scope of nitroalkenes in the reaction. Several aryl,
heteroaryl and alkyl nitroalkenes were reacted with 2,3-diphenyl-
2H-azirine 1a under standard reaction conditions (Table 2).
Table 2. Scope of the reaction in terms of nitroalkenes.a
Scheme 1. Formal cycloadditions of 2H-azirines under visible light photoredox
catalyzed (VLPC) conditions
Results and Discussion
The initial experiment was carried out with 2,3-diphenyl-2H-
azirine 1a and β-nitrostyrene 2a in the presence of organic dye
- 19
9-mesityl-10-methylacridinium tetrafluoroborate (Mes-Acr+BF4 )
or PC-I in DCM under blue light irradiation (Table 1). After 16 h,
the light source was removed and 2 equivalents of DBU was
added in the reaction mixture and reaction was continued at
room temperature for 2 hours. We were delighted to isolate the
product 3a in 76% yield after total 18 h of reaction time (Table 1,
entry 1).
[a] Unless otherwise mentioned, 1a (0.4 mmol), 2 (0.2 mmol) and PC-I (0.006
mmol) were irradiated in DCM (4.0 mL) with a 450 nm blue LED for 16 h
followed by light switch-off and addition of DBU (0.4 mmol) in the reaction
mixture under N2.
Table 1. Optimization of reaction conditions.a
The results in Table 2 clearly indicate wide scope of the [3+2]
cyclization reaction with respect to nitroalkenes. The aryl
nitroalkenes bearing electron-releasing groups such as Me,
OMe and SMe reacted efficiently with 1a under optimized
reaction conditions allowing swift access to the corresponding
trisubstituted pyrroles 3a-3f in good yields. Similarly, the
electron-withdrawing groups including CN, Cl, CF3 and NO2 on
nitroalkenes too were well accommodated and the products 3g-
3k were isolated in excellent yields. In addition to aryl
nitroalkenes, the heteroaryl nitroalkenes also worked well in the
[3+2] cyclization with 1a affording heteroarene-substituted
pyrroles 3l-3m in high yields. However, a complex mixture was
obtained when aliphatic nitroalkenes were employed in the
reaction.
Next we aimed to investigate the range of 2H-azirines
employable in the reaction with various nitroalkenes (Table 3).
Several azirines with diverse substitution patterns were prepared
following standard literature procedures. The azirines 1b and 1c
bearing electron-releasing groups Me and OMe, respectively
reacted well with electron-rich as well as electron-deficient
Entry
difference from
“initial conditions”
None
Yield (%)b, c
1
2d
3
4
5
6
7
8
9
87 (76)
Eosin Y, instead of PC-I
[Ru(bpy)3Cl2], instead of PC-I
fac-[Ir(ppy)3], instead of PC-I
DCE as solvent
0
0
0
80
82
0
traces
38
66
64
MeCN as solvent
DMF as solvent
1a:2a = 1:1
1a:2a = 1:1.5
10
11
1a:2a = 1.5:1
Amount of PC-I = 1 mol%
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201901068
Chemistry - An Asian Journal
FULL PAPER
nitroalkenes under optimized reaction conditions yielding
products 3n-3s in good yields. The Cl-substituted azirine 1d too
reacted efficiently with nitroalkenes possessing 4-OMe, 4-CN
and 4-NO2 substitution providing corresponding pyrroles in 50%,
87% and 81% yields, respectively. Also, azirine featuring 2-
thienyl-substitution 1e reacted smoothly with various aryl as well
as heteroaryl nitroalkenes affording products 3w-3y under the
standard reaction conditions. The alkyl-aryl azirine 1f could also
be successfully employed in the reaction, however yield of the
product 3z was low. It is especially noteworthy that the reaction
exhibited complete regioselectivity in favour of the products
obtained through 2-azaallenyl radical cation addition on the β-
position of nitroalkenes over the other regioisomer expected to
form through the α-addition of radical cation (please refer to the
plausible mechanism; Scheme 3). The correct structure for the
exclusive regioisomer was established by NMR spectroscopy
and single crystal X-ray analysis of 3r and the same
regiochemistry was assigned to other products on the basis of
analogy.20
data), confirming the involvement of 2-azaallenyl radical cation
in the reaction, as proposed in the literature6-10 (Scheme 2).
Scheme 2. Control Experiments
The mechanism of 2H-azirine - nitroalkene [3+2] cycloaddition
proposed on the basis of control experiments and previous
literature reports6-10 is depicted in Scheme 3, exemplified by the
reaction between substrates 1a and 2a.
Table 3. Scope of the reaction in terms of 2-H azirines.a
Scheme 3. Plausible mechanism
The excited photocatalyst PC-I* generated upon visible light
irradiation undergoes reductive quenching by single electron
transfer from strongly reducing 2H-azirine 1a.21 This result into
the generation of azirine radical cation A and the reduced
photocatalyst. The radical cation A forms 2-azaallenyl radical
cation B/B’ following ring opening through C-C bond cleavage.
The radical cation B’ adds on to the β-position of nitroalkene 2a
furnishing intermediate C, which oxidizes the reduced
photocatalyst and leads to the intermediate species D. The
intermediate D follows cyclization and base-mediated denitration
sequence to afford the trisubstituted pyrrole 3a. It is noteworthy
that radical addition on the nitroalkene takes place in Michael
fashion unlike usual reactions of nitroalkenes under visible light
irradiation15-18 owing to the charge stabilizing capacity of the nitro
[a] Unless otherwise mentioned, 1 (0.4 mmol), 2 (0.2 mmol) and PC-I (0.006
mmol) were irradiated in DCM (4.0 mL) with a 450 nm blue LED for 16 h
followed by light switch-off and addition of DBU (0.4 mmol) in the reaction
mixture under N2.
Further, the necessity of both visible light and photocatalyst was
established through key control experiments since no product
formation were noticed in absence of PC-I or visible light.
Additionally, a test reaction of 3-(4-methoxyphenyl)-2-phenyl-2H-
azirine 1c with the radical trapping agent allyltributyltin in the
presence of PC-I furnished N-benzyl-1-phenylbut-3-en-1-imine 5
under visible light irradiation (see the Supporting Information for
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201901068
Chemistry - An Asian Journal
FULL PAPER
Res. 2016, 25, 173-210; f) J. A. Joule, Adv. Het. Chem. 2016, 119, 81-
106; g) Z. Hosseinzadeh, A. Ramazani, N. Razzaghi-Asl, Curr. Org.
Chem. 2018, 22, 2256-2279; h) M. Badolato, F. Aiello, N. Neamati,
RSC Adv. 2018, 8, 20894-20921; i) K. Walayat, N.-A. Mohsin, S. Aslam,
M. Ahmad, Turk. J. Chem. 2019, 43, 1-23;
group. This in effect accords high level of regioselectivity to the
reaction, affording products as a single regioisomer.
Conclusions
[2]
a) V. Bhardwaj, D. Gumber, V. Abbot, S. Dhiman, P. Sharma, RSC Adv.
2015, 5, 15233-15266; b) Nisha, K. Kumar, V. Kumar, RSC Adv. 2015,
5, 10899-10920; c) R. Khajuria, S. Dham, K. K. Kapoor, RSC Adv. 2016,
6, 37039-37066; d) M. Krzeszewski, D. Gryko, D. T. Gryko, Acc. Chem.
Res. 2017, 50, 2334-2345; e) G. I. Vargas-Zuniga, J. L. Sessler, Coord.
Chem. Rev. 2017, 345, 281-296; f) M. J. F. Calvete, S. M. A. Pinto, M.
M. Pereira, C. F. G. C. Geraldes, Coord. Chem. Rev. 2017, 333, 82-
107; g) M. J. F. Calvete, S. M. Pinto, Curr. Org. Syn. 2017, 14, 704-
714; h) S. Ahmad, O. Alam, M. J. Naim, M. Shaquiquzzaman, M. M.
Alam, M. Iqbal, Eur. J. Med. Chem. 2018, 157, 527-561; i) C. Wu, W.
Wang, L. Fang, W. Su, Chin. Chem. Lett. 2018, 29, 1105-1112; j) J.
Hatai, C. Schmuck, Acc. Chem. Res. 2019, 52, 1709-1720; k) F. Hu, J.
P. L. Ng, P. Chiu, Synthesis 2019, 51, 1073-1086; l) Z. Yu, G. N.
Pandian, T. Hidaka, H. Sugiyama, Adv. Drug Del. Rev. 2019, AOP.
a) F. J. Leeper, J. M. Kelly, Org. Prep. Proced. Int. 2013, 45, 171-210;
b) V. Estevez, M. Villacampa, J. C. Menendez, Chem. Soc. Rev. 2014,
43, 4633-4657; c) B. Quiclet-Sire, S. Z. Zard, Synlett 2017, 28, 2685-
2696; d) T. Fujita, J. Ichikawa, Heterocycles 2017, 95, 694-714; e) A.
Sharma, P. Piplani, J. Het. Chem. 2017, 54, 27-34; f) Z. Ma, Z. Ma, D.
Zhang, Molecules 2018, 23, 2666/1-2666/19; g) I. Azad, F. Hassan, M.
Saquib, N. Ahmad, R. K. Abdul, A.G. Al-Sehemi, M. Nasibullah, Orient.
J. Chem. 2018, 34, 1670-1700; h) M. Leonardi, V. Estevez, M.
Villacampa, J. C. Menendez, Synthesis 2019, 51, 816-828;
In conclusion, an efficient regioselective synthesis of
trisubstituted pyrroles has been developed using simple
substrates and organic dye as photocatalyst under visible light
irradiation. The reaction is reasonably general in nature in terms
of 2H-azirines and nitroalkenes employed in the reaction.
Further, the complete regioselectivity of the reaction and high
yields of product in most of the cases are noteworthy attributes
of the reaction.
Experimental Section
General procedure for visible light mediated trisubstituted
pyrrole 3 synthesis
[3]
In a 5 mL snap vial equipped with magnetic stirring bar, the 2H-
azirine 1 (0.4 mmol), nitroalkene 2 (0.2 mmol) and photocatalyst
-
(Mes-Acr+BF4 ) PC-I (3 mg, 0.006 mmol, 3 mol% w.r.t. 2) were
dissolved in anhydrous dichloromethane (4 mL). The resulting
reaction mixture was degassed by three “freeze-pump-thaw”
cycles via a syringe needle. The vial was irradiated using 450
nm blue LED with a cooling device maintaining a temperature
around 25 °C. After 16 h of irradiation (TLC monitoring), the light
source was removed and DBU (0.06 mL, 0.4 mmol) was added
in the reaction mixture which was stirred at room temperature for
additional 2 h (TLC monitoring). The reaction mixture was
washed with water (2 x 5 mL) and brine (2 x 5 mL). The
combined organic layer was dried (Na2SO4) and concentrated
under reduced pressure. Purification of the crude product was
achieved by column chromatograpy on silica gel using
hexane/ethyl acetate as eluent to afford the pure product 3.
[4]
a) C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev. 2013, 113,
5322-5363; b) D. A. Nicewicz, T. M. Nguyen, ACS Catal. 2014, 4, 355-
360; c) R. A. Angnes, Z. Li, C. R. D. Correia, G. B. Hammond, Org.
Biomol. Chem. 2015, 13, 9152-9167; d) N. A. Romero, D. A. Nicewicz,
Chem. Rev. 2016, 116, 10075-10166; e) D. C. Fabry, M. Rueping, Acc.
Chem. Res. 2016, 49, 1969-1979; f) D. Staveness, I. Bosque, C. R. J.
Stephenson, Acc. Chem. Res. 2016, 49, 2295-2306; g) L. Revathi, L.
Ravindar, W.-Y. Fang, K. P. Rakesh, H.-L. Qina, Adv. Syn. Catal. 2018,
360, 4652-4698; h) F. Strieth-Kalthoff, M.l J. James, M. Teders, L.
Pitzer, F. Glorius, Chem. Soc. Rev. 2018, 47, 7190-7202; i) M. K.
Bogdos, E. Pinard, J. A. Murphy, Beil. J. Org. Chem. 2018, 14, 2035-
2064; j) A. Savateev, M. Antonietti, ACS Catal. 2018, 8, 9790-9808; k)
S. Angerani, N. Winssinger, Chem. Eur. J. 2019, 25, 6661-6672;
a) R. A. Rossi, J. I. Bardagi, M. E. Buden, Curr. Org. Synth. 2017, 14,
398-429; b) T. Glasnov, Top. Heterocycl. Chem. 2018, 56, 103-132; c)
M. Borjesson, A. Tortajada, R. Martin, Chem. 2019, 5, 254-256; d) B. M.
Hockin, C. Li, N. Robertson, E. Zysman-Colman, Catal. Sci. Tech. 2019,
9, 889-915; e) A. A. Festa, L. G. Voskressensky, E. V. Van der Eycken,
Chem. Soc. Rev. 2019, AOP.
[5]
Acknowledgements
BSK and LD thank UGC, New Delhi and CSIR, New Delhi,
respectively for the Ph. D. fellowships. AP thank DST, New Delhi
for project fellowship. We thank the SAIF division of CSIR-CDRI
for the analytical support. We gratefully acknowledge the X-ray
diffraction facility of Indian Institute of Technology, Kanpur for X-
ray data collection of 3r. We also acknowledge Department of
Science & Technology (DST), New Delhi for the financial
support (Project Ref. No.: EMR/2016/006975). CDRI
Communication No: 9896
[6]
[7]
[8]
[9]
J. Xuan, X.-D. Xia, T.-T. Zeng, Z.-J. Feng, J.-R. Chen, L.-Q. Lu, W.-J.
Xiao, Angew. Chem. Int. Ed. 2014, 53, 5653-5656.
T.-T. Zeng, J. Xuan, W. Ding, K. Wang, L.-Q. Lu, W.-J. Xiao, Org. Lett. 2015, 17,
4070-4073.
J. O. Mueller, F. G. Schmidt, J. P. Blinco, C. Barner-Kowollik, Angew. Chem. Int.
Ed. 2015, 54, 10284-10288.
H. Wang, Y. Ren, K. Wang, Y. Man, Y. Xiang, N. Li, B. Tang, Chem. Commun.
2017, 53, 9644-9647.
[10] B.-G. Cai, Z.-L. Chen, G.-Y. Xu, J. Xuan, W.-J. Xiao, Org. Lett. 2019, 21, 4234-
4238.
Keywords: 2H-azirines • nitroalkenes • [3+2]cycloaddition •
visible-light • trisubstituted-pyrroles
[11] L. Chen, H. Li, P. Li, L. Wang, Org. Lett. 2016, 18, 3646−3649.
[12] a) S. Jana, M. D. Clements, B. K. Sharp, N. Zheng, Org. Lett. 2010, 12, 3736-
3739; b) A. F. Khlebnikov, M. S. Novikov, Tetrahedron 2013, 69, 3363-3401; c)
P. W. Davies, M. Garzón, Asian J. Org. Chem. 2015, 4, 694-708; d) J. O.
Ruvinskaya, N. V. Rostovskii, I. P. Filippov, A. F. Khlebnikov, M. S. Novikov,
Org. Biomol. Chem. 2018, 16, 38-42; e) P. A. Sakharov, M. S. Novikov, A. F.
Khlebnikov, J. Org. Chem. 2018, 83, 8304−8314; f) Y. Baek, J. Kim, H. Kim, S. J.
[1]
a) C. T. Walsh, Tetrahedron Lett. 2015, 56, 3075-3081; b) J. F.
Gonzalez, I. Ortin, E. de la Cuesta, J. C. Menendez, Chem. Soc. Rev.
2012, 41, 6902-6915; c) I. Khan, A. Ibrar, N. Abbas, A. Saeed, Eur. J.
Med. Chem. 2014, 76, 193-244; d) A. E. Goetz, T. K. Shah, N. K. Garg,
Chem. Commun. 2015, 51, 34-45; e) M. Gaba, C. Mohan, Med. Chem.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201901068
Chemistry - An Asian Journal
FULL PAPER
Jung, H. Ryu, S. Kim, J.-Y. Son, K. Um, S. H. Han, H. J. Seo, J. Heo, K. Lee, M.-H.
Baik, P. H. Lee, Chem. Sci. 2019, 2019, 2678-2686.
[13] a) R. S. Varma, G. W. Kabalka, Heterocycles 1986, 24, 2645-2677; b) A. Z.
Halimehjani, I. N. N. Namboothiri, S. E. Hooshmand, RSC Adv. 2014, 4, 31261-
31299; c) A. Z. Halimehjani, I. N. N. Namboothiri, S. E. Hooshmand, RSC Adv.
2014, 4, 48022-48084; d) A. Z. Halimehjani, I. N. N. Namboothiri, S. E.
Hooshmand, RSC Adv. 2014, 4, 51794-51829;
[14] a) O. M. Berner, L. Tedeschi, D. Enders, Eur. J. Org. Chem. 2002, 1877-1894; b)
K. Kaur, I. N. N. Namboothiri, Chimia 2012, 66, 913-920; c) C. Nejera, J. M.
Sansano, Curr. Top. Med. Chem. 2014, 14, 1271-1282; d) D. K. Nair, T. Kumar,
I. N. N. Namboothiri, Synlett 2016, 27, 2425-2442; e) D. A. Alonso, A. Baeza, R.
Chinchilla, C. Gomez, G. Guillena, I. M. Pastor, D. J. Ramon, Molecules 2017,
22, 895/1-895/51. f) B. Maji, Asian J. Org. Chem. 2018, 7, 70-84; g) S. Gabrielli,
E. Chiurchiu, A. Palmieri, Adv. Synth. Catal. 2019, 361, 630-653.
[15] N. Zhang, Z.-J. Quan, Z. Zhang, Y.-X. Da, X.-C. Wang, Chem. Commun. 2016, 52,
14234-14237.
[16] S. P. Midya, J. Rana, T. Abraham, B. Aswina, E. Balaraman, Chem. Commun.
2017, 53, 6760-6763.
[17] S. Tripathi, L. D. S. Yadav, New J. Chem. 2018, 42, 3765-3769.
[18] a) S. Yadav, M. Srivastava, P. Rai, B. P. Tripathi, A. Mishra, J. Singh, J. Singh,
New J. Chem. 2016, 40, 9694-9701; b) M. Zhang, L. Yang, H. Yang, G. An, G. Li,
ChemCatChem 2019, 11, 1606-1609.
[19] For photoredox behaviour study of PC-I, please see: N. A. Romero, D. A.
Nicewicz, J. Am. Chem. Soc. 2014, 136, 17024-17035.
[20] Crystal structure of compound 3r has been deposited at the Cambridge
Crystallographic Data Center and allocated the reference no.CCDC 1944465.
[21] E. Albrecht, J. Averdung, E. W. Bischof, A. Heidbreder, T. Kirschberg, F. Miiller,
J. Mattay, J. Photochem. Photobiol. A: Chem. 1994, 82, 219-232.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201901068
Chemistry - An Asian Journal
FULL PAPER
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.