Visible-Light-Induced Regioselective Dicarbonylation of Indolizines with Oxoaldehydes via Direct C-H Functionalization

Article abstract of DOI:10.1021/acs.orglett.0c01094

A metal-free system for regioselective dehydrogenative cross-couplings between indolizines and oxoaldehydes catalyzed by visible light under mild conditions has been described. As an atom economical and eco-friendly protocol, the reaction proceeds in good yields using inexpensive, readily available visible-light sources and the environmentally friendly oxidant oxygen. Various valuable 1,2-dicarbonyl derivatives attached to an indolizine core were easily accessed by the direct dicarbonylation of the sp² C-H bond.

Full text of DOI:10.1021/acs.orglett.0c01094

pubs.acs.org/OrgLett
Letter
Visible-Light-Induced Regioselective Dicarbonylation of Indolizines
with Oxoaldehydes via Direct C−H Functionalization
Lili Teng, Xiang Liu, Pengfeng Guo, Yue Yu,* and Hua Cao*
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ABSTRACT: A metal-free system for regioselective dehydrogenative
cross-couplings between indolizines and oxoaldehydes catalyzed by
visible light under mild conditions has been described. As an atom
economical and eco-friendly protocol, the reaction proceeds in good
yields using inexpensive, readily available visible-light sources and the
environmentally friendly oxidant oxygen. Various valuable 1,2-
dicarbonyl derivatives attached to an indolizine core were easily
accessed by the direct dicarbonylation of the sp2 C−H bond.
Scheme 1. Our Study of the Synthesis of 1,2-Diketones via
C−H Functionalization
ndolizine derivatives, important and unique single-nitrogen
heterocycles that frequently exist in nature, have been
I
extensively applied in biological chemistry, pharmaceuticals,
agrochemicals, and functional organic materials.1,2 Chemists
have spent a great deal of time synthesizing a range of
indolizine derivatives in the past two decades.3 Among these
synthetic routes, the transition metal-mediated dehydrogen-
ative cross-coupling reaction has exhibited tremendous
advantages in building the C−C bond directly from relatively
simple starting materials, such as a low catalyst load, ready
availability, atom economy, and the lack of a requirement that
it be prefunctionalized.4 In particular, a stupendous and
exquisite achievement has been realized in the metal-catalyzed
C−H bond dicarbonylation of a nitrogen-containing hetero-
cycle to synthesize 1,2-dicarbonyl compounds.5 1,2-Dicarbonyl
derivatives are applied as fluorescent materials in the chemical
industry and used in bioactive molecules and drug discovery.6
This key structural unit was generally synthesized by the
oxidation of 1,2-diols,7 alkyne,8 and alkene.9 In 2015, our
group10 disclosed a Cu(II)-catalyzed oxidative coupling of
imidazo[1,2-a]pyridines with N,N-disubstituted acetamide or
ketone in the presence of an acidic mixture and an oxygen
atmosphere that produced imidazo[1,2-a]pyridine-derived 1,2-
dicarbonyl compounds (Scheme 1a). Despite some elegant
surveys of catalysis by transition metals, the development of a
new, cleaner, more environmentally friendly, and greener
approach is still highly desirable.
between indole and tertiary amine by a combination of visible-
light catalysis utilizing eosin Y as the photosensitizer and G-
RuO2 catalysis.13 However, to the best of our knowledge,
visible-light-driven direct dehydrogenative cross-couplings
between indolizines and oxoaldehydes have not been studied.
As a continuation of our research on C−H bond
functionalization of nitrogen-containing heterocycles,14 herein
we report a straightforward protocol for visible-light-induced
regioselective C−H bond dicarbonylation of indolizines using
Rose Bengal (RB) as the photosensitizer under an air
atmosphere (Scheme 1b).
We choose easily prepared compound 2-phenylindolizine
(1a) and 2-oxo-2-phenylacetaldehyde (2a) as our starting
materials to investigate the visible-light-driven dicarbonylation
The past decade has witnessed a dramatic increase in the
amount of research in the area of organic photocatalysis. Also,
photocatalytic transformations are considered to be appealing
synthetic protocols for the activation of C−H bonds due to
their sustainable and green nature.11 Recently, organic dyes, as
efficient photocatalysts that are inexpensive and readily
available, have been widely used in diverse photocatalytic
C−H functionalizations.12 For example, Wu et al. reported the
first case of a cross-coupling hydrogen evolution reaction
Received: March 26, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01094
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
a
reaction; the screening conditions are summarized in Table 1.
When RB was used as the photosensitizer, under 20 W blue
Scheme 2. Substrate Scope of Indolizines
a
Table 1. Optimization of Reaction Conditions
b
entry
photocatalyst
oxidant
solvent
yield (%)
1
2
3
4
5
6
7
8
RB
eosin Y
eosin B
rhodamine 6G
fluorescein
RB
RB
RB
RB
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
air
TBHP
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
DMSO
DMF
toluene
acetone
DMSO
DMSO
75
63
65
73
70
73
88
78
69
56
91
25
9
10
11
12
RB
RB
RB
a
Conditions: 1a (0.3 mmol), 2a (0.6 mmol), photocatalyst (3 mol
%), solvent (2 mL), 20 W blue LED, room temperature, air
b
atmosphere, 12 h. Determined by GC analysis.
a
Conditions: 1a (0.5 mmol), 2a (2.0 equiv), RB (3 mol %), DMSO
(3 mL), 20 W blue LED, room temperature, air atmosphere, 12 h.
Isolated yields.
light-emitting diode (LED) irradiation, raw materials 1a and 2a
could react in an air atmosphere at room temperature and be
converted to dicarbonylated indolizine in 75% yield (entry 1).
To further improve the efficiency of the transformation, we
tried to screen different photosensitizers, such as eosin Y, eosin
B, rhodamine 6G, and fluorescein. The experimental results
showed that although these photosensitizers could also
promote the reaction, the yield has not been further improved
(Table 1, entries 2−5). As we know, organic solvents have an
effect on photochemical reaction, so we studied visible-light-
induced dicarbonylation under various solvents, such as
CH2Cl2, DMSO, DMF, toluene, and acetone (entries 6−10,
respectively). Inspiringly, the product yield in DMSO was as
high as 88%. When the reaction atmosphere was changed from
oxygen to open air, the yield of the target product was
increased to 91% (entry 11). In addition, some traditional
oxidants such as TBHP and K2S2O4 have been added to the
reaction mixture, which has led to a dramatic decrease in the
yield of dicarbonyl compounds (entry 12).
Under the optimal photocatalytic conditions, we explored
the functional group tolerance and substrate applicability of
coupling reactions using numbers of substituted indolizines
and 2-oxo-2-phenylacetaldehyde (2a). As shown in Scheme 2,
various valuable 1,2-dicarbonyl derivatives attached to the
indolizine core were easily produced (3a−3v). A substituent,
such as -Me and -OMe, on the indolizine ring has little effect
on the efficiency of the reaction and afforded the coupling
products (3b−3d) in 73−87% yields. We further explored the
effects of aryl substituents at position 2 of the indolizine (1a)
on the reaction. These different aryl-substituted indolizines
efficiently reacted with 2-oxo-2-phenylacetaldehyde to generate
the respective C-3 dicarbonylated indolizine derivatives (3e−
3n). In particular, the bromo-substituted 1,2-dicarbonyl
indolizine derivative is a promising candidate for further
transformations by a visible-light-induced intermolecular
coupling reaction. To our delight, the reaction of 2-(furan-2-
yl)indolizine with 2-oxo-2-phenylacetaldehyde (2a) was
performed smoothly to produce product 3r in 53% yield.
Alkyl ketone at position 2 of the indolizine could efficiently
undergo photocatalytic transformation to give the dicarbony-
lated indolizine product (e.g., 3t, 74%). Additionally, cyano
and esteryl indolizine produced molecules 3u and 3v in good
yields (89% and 75%, respectively). On the basis of the results
presented above, this work provided an atom economical and
eco-friendly method for the preparation of functionalized
indolizines under metal-free conditions.
To extend the substrate scope and limitations, we explored
the use of 2-oxo-2-phenylacetaldehyde to synthesize C-3
dicarbonylated indolizine derivatives under optimized reaction
conditions. As shown in Scheme 3, oxoaldehydes with
substituents such as methyl, bromo, and fluoro groups at
different positions of the aromatic ring also performed well
with 2-phenylindolizine to give the coupling products (4a−4d)
in 83−88% yields. Products 4e and 4f, in which the parent
oxoaldehydes contained two chlorine and alkoxy groups, could
be further derivatized in 81% and 78% yields, respectively.
Notably, the use of 2-(furan-2-yl)-2-oxoacetaldehyde resulted
in a moderate yield (4g, 63%). An alkylglyoxal such as
methylglyoxal under the photocatalytic conditions gave
product 4h in a relatively low yield (55%).
As mentioned above, many indolizine derivatives exhibit
good fluorescence properties and could be used as valuable
fluorescent sensors in the field of luminescent materials.
Therefore, we further investigated the photophysical property
of dicarbonylated indolizine derivatives prepared by the
method presented here. The relevant data are listed in Table
2 and shown in Figures 1 and 2. For example, compound 3a
displayed strong absorption (λabs) at a wavelength of ∼380 nm
in DMSO, MeCN, MeOH, DCM, DMF, and DCE and
B
https://dx.doi.org/10.1021/acs.orglett.0c01094
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Scheme 3. Substrate Scope of Oxoaldehydes
Figure 2. Emission spectra of compound 3a.
Scheme 4. Possible Mechanism of the Photocatalytic
Reaction
a
Conditions: 1a (0.5 mmol), 2a (2.0 equiv), RB (3 mol %), DMSO
(3 mL), 20 W blue LED, room temperature, air atmosphere, 12 h.
Isolated yields.
Table 2. Spectroscopic Data of Compound 3a in Different
a
Solvents
DMSO
MeCN
MeOH
DCM
DMF
DCE
b
λabs (nm)
385
435
382
430
383
434
384
434
384
430
384
478
c
λem (nm)
a
b
The extinction coefficient ε = 4.23 × 103 L mol−1 cm−1. Longest
c
wavelength absorption maximum. Excited at the longest wavelength
absorption maximum.
radical anion (O2•−) in the photoredox cycle. Intermediate A
was quickly transformed to its radical resonance B. Also, 2a
was oxidized by the anion radical of O2 to produce radical
intermediate C. Subsequently, indolizine radical intermediate
B could couple with carbonyl radical C to afford dicarbony-
lated indolizine cation D. Finally, cation D underwent electron
transfer to give product 3a along with H2O2.
In conclusion, we have developed an efficient and
straightforward visible-light-induced approach to synthesize a
number of dicarbonylated indolizine derivatives from indoli-
zines and oxoaldehydes via dehydrogenative cross-couplings
under metal-free conditions. This catalytic reaction proceeded
with some unique advantages, such as good functional group
tolerance, atom economy, good regioselectivity, and clean
operation. In addition, the photophysical property of
dicarbonylated indolizine derivatives has been documented.
This transformation represents the first photocatalyzed
dicarbonylation of indolizines.
ASSOCIATED CONTENT
* Supporting Information
Figure 1. Absorption spectra of compound 3a.
■
sı
emission (λem) in the purple region (430−435 nm) in DMSO,
MeCN, and DMF. It is worth noting that 3a had the highest
absorption intensity in DCM and the highest fluorescence
emission intensity when using DMSO as the solvent.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01094.
Experimental details, characterization data, and NMR
spectra of new compounds (PDF)
A possible mechanism based on our previous work14d and
the literature reports15 is depicted in Scheme 4. First, the
photosensitizer RB was excited to its excited state Rose
Bengal* under visible-light irradiation. Then, 2-phenylindoli-
zine 1a interacted with active Rose Bengal* in an oxygen
atmosphere to generate radical intermediate A and superoxide
AUTHOR INFORMATION
Corresponding Authors
■
Yue Yu − School of Chemistry and Chemical Engineering and
Guangdong Cosmetics Engineering & Technology Research
C
https://dx.doi.org/10.1021/acs.orglett.0c01094
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Center, Guangdong Pharmaceutical University, Zhongshan
528458, P. R. China; Email: yuyue@gdpu.edu.cn
Hua Cao − School of Chemistry and Chemical Engineering and
Guangdong Cosmetics Engineering & Technology Research
Center, Guangdong Pharmaceutical University, Zhongshan
528458, P. R. China; orcid.org/0000-0001-8825-0175;
Email: caohua@gdpu.edu.cn
pubs.acs.org/OrgLett
Letter
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Authors
Lili Teng − School of Chemistry and Chemical Engineering and
Guangdong Cosmetics Engineering & Technology Research
Center, Guangdong Pharmaceutical University, Zhongshan
528458, P. R. China
Xiang Liu − School of Chemistry and Chemical Engineering and
Guangdong Cosmetics Engineering & Technology Research
Center, Guangdong Pharmaceutical University, Zhongshan
528458, P. R. China; orcid.org/0000-0001-6973-3809
Pengfeng Guo − School of Chemistry and Chemical Engineering
and Guangdong Cosmetics Engineering & Technology Research
Center, Guangdong Pharmaceutical University, Zhongshan
528458, P. R. China
́
129, 7744. (d) Barluenga, J.; Lonzi, G.; Riesgo, L.; Lopez, L. A.;
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01094
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Innovation and Strong School
Project of Guangdong Pharmaceutical University
(2015cxqx212 and 2018KQNCX131), the Science and
Technology Planning Project of Guangdong Province
(201806040009 and 201804010349), the National Key
Research and Development Project (2017YFD0201600), the
Guangdong Cosmetics Engineering & Technology Research
Center, the Provincial Experimental Teaching Demonstration
Center of Chemistry & Chemical Engineering, the Special
Funds of Key Disciplines Construction from Guangdong, and
Zhongshan Cooperating.
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