A visible-light-induced intermolecular [3 + 2] alkenylation-cyclization strategy: Metal-free construction of pyrrolo[2,1,5-cd] indolizine rings

Article abstract of DOI:10.1039/c9gc01766f

A simple and efficient visible-light-induced intermolecular [3 + 2] alkenylation-cyclization process has been developed. This reaction provided an unprecedented metal-free double C(sp²)-H bond oxidation coupling of indolizines with electron-deficient alkenes. Through this cascade reaction, a series of pyrrolo[2,1,5-cd]indolizine derivatives with a large π-system were synthesized. Furthermore, this approach features easily available starting materials, good functional group tolerance, step-economy, high efficiency and mild conditions.

Full text of DOI:10.1039/c9gc01766f

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DOI: 10.1039/C9GC01766F
COMMUNICATION
Visible-Light-Induced Intermolecular [3+2] Alkenylation–
Cyclization Strategy: Metal-Free Construction of Pyrrolo[2,1,5-
cd]indolizine Rings
Received 00th January 20xx,
Accepted 00th January 20xx
Yaofeng Liang,a Lili Teng,a Yajun Wang,a Qiuxing He*a and Hua Cao*a
DOI: 10.1039/x0xx00000x
Abstract:
A
simple and efficient visible-light-induced mild conditions.12 They provide uniquely direct access to
intermolecular [3+2] alkenylation–cyclization process has been molecular structures that are often difficult to synthesize using
developed. This reaction provided an unprecedented metal-free other reaction types. The development of photocatalytic
double C(sp2)−H bond oxidation coupling of indolizines with reaction is an attractive synthetic strategy in synthetic
electron-deficient alkenes. Through this cascade reaction, a series chemistry. Compared with transition metal catalysts, organic
of pyrrolo[2,1,5-cd]indolizine derivatives with a large π-system dyes are more excellent due to their low cost, versatility in
were synthesized. Furthermore, this approach features easily synthesis, non-toxicity and environmental friendliness.13
available starting materials, good functional group tolerance, step-
economy, high efficiency and mild conditions.
Recently, we have developed efficient methods to construct
heterocycles.14 Herein, in continuation of our current focus in
synthesizing fused indolizines via photocatalyzed alkenylation-
cyclization, which were typically challenging to achieve
straightforward using facile and simple methods.
The importance of developing convenient and straightforward
strategies for the synthesis of fused indolizines1 is well
demonstrated by the extensive application2 of these
compounds as pharmaceuticals or agrochemicals in the
chemical fields. In addition, they can be used as fluorescent
molecular probes in cell functional studies,3 as well as
important fluorescent sensor and fluorescent materials in
materials chemistry.4 The synthesis of fused indolizines has
been of intense interest for scientists over the past few
decades.5 Among these methods, transition-metal-catalyzed
reactions are considered to be one of the most valuable and
powerful tools for producing higher-value products in both
fundamental and applied chemistry.6 Many elegant and
successful C(sp2)-H activations and transformations have been
developed by using Pd,7 Cu,8 Ru9 salts as the catalysis10.
Although the tremendous strategies have been developed that
allow for highly selective reactions, these reactions are unable
to achieve secondary dehydrogenation coupling to synthesize
fused indolizines (Scheme 1a). 11 The straightforward routes to
improve their activity and widen the range of feedstocks
remain a longstanding challenge for new practical applications.
Photocatalytic transformations are considered to be a
promising alternative for the activation of C-H bonds under
previous work
EWG
GWE
H
[Pd]
(a)
N
N
N
EWG
GWE
H
this work
H
cat.
N
N
N
?
R
(b)
R
Scheme 1 Regioselective Oxidative Alkenylation of Different Heterocycles
In the preliminary study, we employed 2-phenylindolizine 1a
and methyl acrylate 2a as the substrates to optimize this
reaction. The results are indicated in Table 1. We were pleased
to find that the desired product 3a was formed in the presence
of RB as a photosensitizer and 1 equiv. of TFA as an additive in
MeCN at room temperature for 10 h under 20 W blue LED
(entry 1). After that, different photocatalysts (entries 2-5),
such as eosin Y, eosin B, rhodamine 6G and fluorescein were
also investigated, and the results indicated that the use of
other photocatalysts were also capable of promoting the
reaction but with lower yields. The results encouraged us to
improve the yield, we then tested other solvents (DMSO, DMF,
CH2Cl2, Toluene, EtOH, Dioxane). It was found that only CH2Cl2
could provide target product 3a in good yield (entry 8), and
other solvents led to no or trace product. Subsequently, TsOH,
benzoic acid, PivOH, and AcOH was respectively added in the
reaction in place of TFA. However, those showed lower
a. School of Chemistry and Chemical Engineering and Guangdong Cosmetics
Engineering
& Technology Research Center, Guangdong Pharmaceutical
University, Zhongshan 528458, China .
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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reactivity, and the cyclized product was obtained in poor yield
(entries 12-15). The use of other stoichiometric oxidants such
as TBHP and K2S2O8 results in lower yields (entries 16-17).
When the reaction was carried out in air, only a trace product
3a was detected (entry 18). No product was formed without
any photocatalyst (entry 19). In addition, this reaction does
not occur in the dark as well as under an argon atmosphere
(entries 20 and 21). Thus, these results led us to establish the
optimized reaction conditions as follows: 3 mol% of RB and 1
View Article On2line
R
1
DOI: 10.1039/C9GC01766F
R
O
R2
N
RB, TFA
R1
+
N
O
CH2Cl2, O2, 10 h
20 W blue LED
O
3
2a
1
O
N
N
N
N
O
3d
O
O
70%
O
3b
3c
3a
78%
77%
80%
O
O
O
O
Table 1 Optimization of the reaction conditions a
F
Br
O
N
N
N
N
O
photocatalyst, additive
solvent, oxidant
N
Ph +
O
O
O
O
O
O
3g
3k
74%
70%
3e
3i
3f
3h
75%
75%
78%
66%
71%
71%
66%
N
O
O
O
O
O
O
2a
3a
1a
O
N
N
N
N
Br
Cl
Cl
F
O
O
3j
3l
O
O
entry
photocatalyst
oxidant
additive
TFA
solvent
MeCN
MeCN
MeCN
MeCN
MeCN
DMSO
DMF
yield b(%)
50
O
O
O
O
1
2
RB
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
Cl
O
N
N
N
N
O
F
O
eosin Y
TFA
43
O
3p
3o
O
70%
3m
3n
65%
68%
S
O
O
O
O
3
eosin B
TFA
39
N
4
rhodamine 6G
TFA
48
O
3q
69%
O
5
fluorescein
TFA
45
a Isolated yield.
Scheme 2 Substrate Scope of Indolizine a
6
RB
RB
RB
RB
RB
RB
RB
TFA
56
7
TFA
54
equiv. of TFA, 2.0 mL of CH2Cl2 as solvent under O2 atmosphere
in the presence of 20 W blue LED for 10 hours.
8
TFA
TFA
CH2Cl2
Toluene
EtOH
80
With the optimal reaction conditions in hand, a series of
different indolizines coupled with methyl acrylate were
employed to examine the generality of the current reaction
(Scheme 2). For the most part, the reactions were worked very
well under the optimized conditions and provided the
corresponding compound (3a-3q) in moderate to good yields.
A substituent on the indolizine ring, were well-tolerated and
afforded the desired products (3b-3d) in 70-78% yields.
Interestingly, electron-donating substituents (methyl or
methoxy) and electron-withdrawing substituents (chloro,
fluorine, bromine) substituted on the benzene ring did not
affect the reactivity or regioselectivity and were effective to
furnish the desired products (3e-3p) under the optimized
system. Additionally, the reaction of 2-(thiophene-2-
yl)indolizine with 2a was also performed to give the
corresponding product 3q in 69% yield. The molecular
structure of product 3g was determined by X-ray
crystallography (Figure 1).
9
trace
25
10
11
12
TFA
TFA
Dioxane
CH2Cl2
NR c
35
TsOH
Benzoic
Acid
13
RB
O2
CH2Cl2
39
14
15
RB
RB
RB
RB
RB
O2
O2
PivOH
AcOH
TFA
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
46
47
16
TBHP
K2S2O8
air
52
17
TFA
53
18
TFA
trace
NR
NR
NR
19
O2
TFA
20 d
21 e
RB
RB
TFA
O2
TFA
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), photocatalyst (3 mol %),
oxidant (0.4 mmol), additives (0.2 mmol) and solvent (2 mL), carried out in a
sealed tube (25 mL), and irradiation with a 20 W blue LED for 10 h.; b Determined
by GC analysis. c No reaction. d Under a N2 atmosphere. e In the dark.
2 | J. Name., 2012, 00, 1-3
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R2
DOI: 10.1039/C9GC01766F
R1
F
O
N
N
N
R2
RB, TFA
N
N
R1
+
R3
N
CH2Cl2, O2, 10 h
20 W blue LED
O
O
O
O
O
O
O
O
O
1
R3
2
4
5b
5c
5d 72%
5a
78%
69%
76%
a Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), RB (3 mol %), TFA (0.2 mmol),
MeOH (2 mL), irradiation with a 20 W blue LED for 10 h. Isolated yield.
Scheme 4 Synthesis of 3-substituted Indolizines a
N
O
N
N
O
HN
O
O
O
4a
HO
69%
4b
68%
4c
61%
N
N
N
N
N
N
4d
65%
4e
69%
4f
66%
a Isolated yield.
Scheme 3 Substrate Scope of other Electron-Deficient Alkene a
Next we further turned our attention to exploring the
substrate scope of this [3+2] alkenylation–cyclization. A variety
of electron-deficient alkene were also tested as the substrate
(Scheme 3). The substrates, such as butyl acrylate 2b, 2-
hydroxyethyl acrylate 2c, and N-(tert-butyl)acrylamide 2d were
employed and the desired products (4a-4c) were obtained in
61-69%. To our delight, 2-vinylpyridine 2e participated in the
reaction to give the products 4d in 65% yield, which provided a
new synthetic route to construct fused heterocyclic systems
with pyridine ring. Then, the reactions of indolizines with 2e
performed very well and produced the corresponding products
(4e, 4f) in 69 and 66% yields, respectively.
Figure 2 Absorption spectra of compound 4d
Table 2 Spectroscopic Data of Compound 4d in Different Solvent
solvent
CH2Cl2
MeCN
MeOH
DMSO
λ
λ
abs a (nm)
em b (nm)
452
477
446
475
442
472
450
480
a
Longest wavelength absorption maximum. b Excited at the longest wavelength
absorption maximum
Figure 1 Crystal Structure of Compound 3g
Interestingly, the C-3 alkylation product 5a was formed via
regioselective intramolecular addition process, when the
solvent replaced CH2Cl2 with methanol. The extent of the
reaction was also investigated as summarized in Scheme 4.
The presence of a substituent either on the indolizine or the
aromatic ring did not affect the reaction to give the desired
products (5a-d) in good yields.
Figure 3 Emission Spectra of Compound 4d
In case of 6-phenyl-8-(pyridin-2-yl)cyclopenta[hi]indolizine
4d, we examined the influence of solvents for its photophysical
property in air at 25 ⁰C. These data are summarized in Table 2,
Figure 2 and Figure 3. Here, the compound 4d equilibrated
CH2Cl2, MeCN, MeOH, and DMSO all have absorption (λabs) in
the deep blue region (442-452 nm), and have their emission
(λem) in the sky blue region (472-480 nm). It is also shown that
4d has the highest absorption intensity and fluorescence
emission intensity when using DMSO as solvent.
O
R1
RB, TFA
R1
+
N
R3
N
MeOH, O2, 10 h, rt
20 W blue LED
2
5
1
R3
O
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J. Name., 2013, 00, 1-3 | 3
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corresponding Stern-Volmer plots (Figure 4 and 5), it is shown
View Article Online
O
standard conditions
TEMPO (3 equiv.)
that the fluorescence emission of RBDOisI: 1q0u.1e0n39c/hCe9dGCb0y17616aF
obviously because the fluorescence emission intensity of RB
decreases with the increase of 1a. Therefore, we deduced that
1a was the energy receptor of the activated RB in this reaction.
Based on the above observations and the literature
reports13, a plausible reaction mechanism has been proposed
in Scheme 6. Under visible light irradiation, the ground state of
RB is converted to its excited state RB*. Then, 1a could interact
with excited state to give the radical intermediate A and RB•−
which is combined with oxygen to produce both superoxide
radical anion (O2•−) and RB to complete the photoredox cycle.
The intermediate A is easily transferred to key radical
resonance structure B. The intermolecular addition of radical B
to 2a furnishes the radical intermediate C which undergoes
sequential dehydrogenation oxidation by the superoxide anion
radical to give intermediate E. Finally, electron transfer from
intermediate E affords product 3a along with H2O2.
N
(a)
+
N
O
O
3a
2a
1a
O
no product
O
standard conditions
BQ (3 equiv.)
N
+
(b)
N
O
O
3a
2a
1a
O
no product
Scheme 5 Control Experiments
To gain some insight into the mechanism for the metal-free
double C-H bonds oxidative coupling of functionalization of
indolizines with acrylate derivatives, several control
experiments were designed and conducted (Scheme 5). No
target product was observed and the substrate 1a was
recovered, when the reactions of 1a with 2a were carried out
in the presence of radical scavengeeither 2,2,6,6- tetramethyl-
1-piperidinyloxy (TEMPO) or benzoquinone (BQ). The results
indicated that a radial pathway might be involved in this
transformation.
hv
RB
standard conditions
O2
1a
3a
H2O2
O2
RB
H+
HOO
+
Ph
Ph
N
Ph
N
O
A
E
O
N
HO2
B
Ph
N
HOO
2a
H
Ph
H
N
O
O
O2
HO2
C
D
O
O
Scheme 6 Plausible Mechanistic Pathway
Figure 4 The quenching of RB fluorescence emission with 1a
Conclusions
In summary, we have developed a metal-free, visible-light
induced intermolecular [3+2] alkenylation–cyclization strategy.
A broad range of pyrrolo[2,1,5-cd]indolizine derivatives are
conveniently synthesized in moderate to good yields with good
functional group tolerance, thus providing
a
green
intermolecular [3+2] annulation reactions. Notably, synthesis
of challenging 6-phenyl-8-(pyridin-2-yl)cyclopenta[hi]indolizine
was also accomplished and its photophysical properties were
investigated. Further work expanding the utility of these
system to other types of coupling reaction is ongoing and will
be reported in the near future.
Figure 5 Stern-Volmer plot. I0 is the inherent fluorescence intensity of RB. I is the
fluorescence intensity of RB in the presence of 1a.
In order to further verify the process of energy transfer in
photocatalytic reaction, the fluorescence quenching
experiment of RB and 1a were investigated. In the blank
experiment, the emission spectra of 1×10-4 mol/L RB were
collected. Subsequently, the effect of 1a at different
concentrations on the fluorescence intensity of RB catalyst
were determined. According to the results and the
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Innovation and Strong School
Project of Guangdong Pharmaceutical University (2015cxqx212),
4 | J. Name., 2012, 00, 1-3
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J. Huang, H. Qiu, Q. Chen, S. Qiu and Y. Chen, RSC Adv., 2014,
the Science and Technology Planning Project of Guangdong
Province (201806040009, 201804010349), the National Special
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(201503112-8), the national key research and development project
(2017YFD0201600) and Provincial Experimental Teaching
Demonstration Center of Chemistry & Chemical Engineering,
Guangdong Cosmetics Engineering & Technology Research Center,
and Provincial Experimental Teaching Demonstration Center of
Chemistry & Chemical Engineering. Special funds of key disciplines
construction from Guangdong and Zhongshan cooperating.
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This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Green Chemistry
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Page 6 of 6
Journal Name
COMMUNICATION
View Article Online
DOI: 10.1039/C9GC01766F
TOC Graphic
Visible-Light-Induced Intermolecular [3+2] Alkenylation–Cyclization Strategy: Metal-
Free Construction of Pyrrolo[2,1,5-cd]indolizine Rings
Yaofeng Liang,a Lili Teng,a Yajun Wang,a Qiuxing He*a and Hua Cao*a
An intermolecular [3+2] alkenylation–cyclization of indolizines with electron-deficient alkenes was developed
using rose bengal as an organo photocatalyst.
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