Synthesis of Quinolines via the Metal-free Visible-Light-Mediated Radical Azidation of Cyclopropenes

Article abstract of DOI:10.1021/acs.orglett.1c01775

We report the synthesis of quinolines using cyclopropenes and an azidobenziodazolone (ABZ) hypervalent iodine reagent as an azide radical source under visible-light irradiation. Multisubstituted quinoline products were obtained in 34-81% yield. The reaction was most efficient for 3-trifluoromethylcyclopropenes, affording valuable 4-trifluoromethylquinolines. The transformation probably proceeds through the cyclization of an iminyl radical formed by the addition of the azide radical on the cyclopropene double bond, followed by ring-opening and fragmentation.

Full text of DOI:10.1021/acs.orglett.1c01775

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Letter
Synthesis of Quinolines via the Metal-free Visible-Light-Mediated
Radical Azidation of Cyclopropenes
Vladyslav Smyrnov, Bastian Muriel, and Jerome Waser*
Cite This: Org. Lett. 2021, 23, 5435−5439
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ABSTRACT: We report the synthesis of quinolines using cyclopropenes and an
azidobenziodazolone (ABZ) hypervalent iodine reagent as an azide radical source under
visible-light irradiation. Multisubstituted quinoline products were obtained in 34−81%
yield. The reaction was most efficient for 3-trifluoromethylcyclopropenes, affording
valuable 4-trifluoromethylquinolines. The transformation probably proceeds through the
cyclization of an iminyl radical formed by the addition of the azide radical on the
cyclopropene double bond, followed by ring-opening and fragmentation.
Scheme 1. Radical-Mediated Transformations of
Cyclopropenes
s the smallest cyclic alkenes, cyclopropenes contain a
substantial ring strain (ca. 228 kJ mol−1).1 Nevertheless,
A
cyclopropenes bearing one or two substituents on the aliphatic
position are generally stable. Because of the presence of the
ring strain, cyclopropenes are useful intermediates in organic
synthesis;2 however, reports of reactions relying on the
addition of radicals to cyclopropenes remain scarce, despite
the fast growing use of radicals for alkene functionalization
(Scheme 1a). The first example of such a reaction was a radical
hydrostannylation reported by Nakamura in 1994.3 Different
research groups then reported the addition of carbon-centered
radicals to the strained double bond,4 resulting in hydro-
trichloromethylation (eq 1), carbocyanation (eq 2), and 3 + 2
annulation of cyclopropenes (eq 3) among other trans-
formations (Scheme 1a).
In particular, the addition of heteroatom-centered radicals
has been mostly neglected. Recently, our group reported the
radical azidation of cyclopropenes to give alkenylnitriles
(Scheme 1b).5 During the optimization of this work, small
amounts of quinolines were observed as side products for aryl-
substituted cyclopropenes, in the absence of CuCl2. Quinolines
have found numerous applications in medicine, industry,6 and
material sciences.7 This heterocycle is present in the structure
of many natural products8 and synthetic bioactive compounds,
including examples of approved drugs.9 For instance, the
structures of the antimalarial drugs quinine (1) and
chloroquine (2) and the acetylcholinesterase inhibitor tacrine
(3) are based on the quinoline core (Scheme 2a). Many
classical synthetic methods exist for the synthesis of quinolines,
such as the Skraup, Friedlander, Doebner−von Miller,
Conrad−Limpach, and Pfitzinger reactions.10 Most of these
methods require acidic or basic conditions, which are not
compatible with sensitive functionalities. Therefore, the use of
radical-based methods for quinoline synthesis is particularly
attractive.11 A special subclass of such methods are trans-
formations based on cyclizations of iminyl radicals onto the
aryl ring.
Received: May 27, 2021
Published: June 25, 2021
https://doi.org/10.1021/acs.orglett.1c01775
Org. Lett. 2021, 23, 5435−5439
© 2021 American Chemical Society
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Letter
Scheme 2. Quinolines: Bioactive Compounds and Radical-
based Synthetic Strategies
Table 1. Optimization of the Reaction Conditions with
a
Cyclopropene 4a
b
entry
reagents
yield (%)
1
2
3
4
5
6
7
8
4CzIPN (5 mol %)
4DPAIPN (5 mol %)
none
BIOAc (20 mol %)
28
26
0−32
34
34
40
42
44
c
BIOAc (1 equiv)
BIOAc (20 mol %), K2HPO4 (2 equiv)
BIOAc (20 mol %), 2,6-lutidine (2 equiv)
BIOAc (20 mol %), pyridine (2 equiv)
a
b
Reactions were performed on a 0.1 mmol scale. Yield was
1
determined by H NMR of the concentrated reaction mixture using
CH2Br2 as an internal standard. Reaction outcome was dependent on
c
the batch of ABZ.
Because quinoline 5a was the only product isolable in a
substantial amount, we speculate that polymerization of
cyclopropene 1a was occurring as the main side reaction.
Despite the moderate yield obtained for the synthesis of 5a,
we turned to explore the scope of the reaction, as we expected
that the reaction efficiency would be highly dependent on the
structure of the cyclopropene (Scheme 3). Starting materials
were prepared by the metal-catalyzed cyclopropenation of
alkynes with diazo compounds using a Rh catalyst for terminal
alkynes18 and a Ag catalyst for internal alkynes.19 Cyclo-
propenes 4e−g were prepared by 1,2-elimination of the
corresponding cyclopropyl bromides.20 We started by
evaluating the influence of the substituent at the aliphatic
position of the cyclopropene ring. Different ester-substituted
cyclopropenes 4a−d were converted to the corresponding
quinoline products 5a−d in 38−43% yield. For the
monosubstituted cyclopropene 4d, an increased amount of
ABZ (6) and a prolonged reaction time were required for full
conversion. 3-Aryl- (4e, 4f) and 3-alkyl-substituted (4g)
cyclopropenes were also found to be suitable substrates for
the transformation. Aryl-substituted quinolines 5e and 5f could
be obtained in higher yields (68 and 81%, respectively). To our
delight, 3-trifluoromethyl cyclopropene 4h was converted to
quinoline 5h in 59% yield. The trifluoromethyl group is very
popular in medicinal chemistry.21 Despite the attractiveness of
such heterocycles, to the best of our knowledge, there are only
two reported examples of the synthesis of 3-aryl, 4-
trifluoromethylquinolines without the substituent at position
2 of the heterocyclic ring.22 Therefore, we focused on the
synthesis of trifluoromethyl-substituted quinolines for further
exploring the scope of the transformation. Different sub-
stituents on the aryl groups in the 1 and 3 positions of the
cyclopropenes were tolerated (products 5i−5m), including
electron-rich, electron-poor, and halogen substituents. 1-Alkyl-
substituted cyclopropene 4n gave quinoline 5n in 34% yield.
Interestingly, tetrasubstituted cyclopropenes 4o and 4p gave a
single regioisomer of quinoline 5o and 5p. This method can
These radicals can be accessed via homolysis of the N−O
bond in oxime derivatives12 or by fragmentation of α-
azidoradical species (Scheme 2b).13 Methods to generate the
desired iminyl radical remain limited, and new approaches are
highly desirable to give access to different substitution patterns.
Therefore, we decided to optimize the formation of the
quinoline product resulting from the radical azidation of
cyclopropenes (Scheme 1b) and report herein a new synthesis
of quinolines from cyclopropenes, which is particularly efficient
for the synthesis of trifluoromethylated derivatives (Scheme
2c).
Using cyclopropene (4a) as the model substrate, we were
pleased to find that the use of the safe hypervalent iodine
reagent azidobenziodazolone (ABZ, 6)14 in the presence of the
organic dyes 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanoben-
zene (4CzIPN)15 and 1,3-dicyano-2,4,5,6-tetrakis-
(diphenylamino)-benzene (4DPAIPN)16 in 1,2-dichloroethane
(DCE) gave the desired quinoline (5a) as the single product,
albeit in low yield (Table 1, entries 1 and 2). A control
experiment revealed that in the absence of a photocatalyst, the
reaction proceeded even slightly better (entry 3). No product
was observed in the absence of light; however, we found that
the reaction outcome was strongly dependent on the batch of
ABZ we used, resulting in no product formation in the worst
case. We thought that traces of iodine(III) impurities could act
as an initiator for the reaction. Indeed, the use of 20 mol % of
acetoxybenziodoxolone (BIOAc, 7)17 as an additive made the
reaction reproducible, giving the product in 34% yield (entry
4). No improvement was seen when using 1 equiv of BIOAc
(entry 5). The addition of bases to the reaction mixture was
examined (entries 6−8), resulting in an improved yield, with
pyridine performing the best (entry 8). Despite numerous
attempts to increase the reaction yield by fine-tuning the
reaction conditions, no improvement could be achieved.
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a
Scheme 3. Substrate Scope
Scheme 4. Control Experiments
Scheme 5. Mechanism Proposal
a
Reaction conditions: 0.2 mmol of cyclopropene, 1.4 equiv of ABZ
(6), 2 equiv of pyridine, 20 mol % BIOAc (7), DCE (0.1 M), room
b
temperature, 14 h. Reaction conditions: 0.2 mmol of cyclopropene, 2
equiv of ABZ (6), 2 equiv of pyridine, 20 mol % BIOAc (7), DCE
c
d
(0.1 M), room temperature, 48 h. 1.5 mmol scale. 0.1 mmol scale.
also be used for the synthesis of tetra- and pentasubstituted
quinolines 5q and 5r. 1,2-Dialkylcyclopropenes were found to
be inert to reaction conditions, representing a limitation of our
methodology. Scale-up of the transformation was straightfor-
ward: 5h was obtained in 63% yield on a 1.5 mmol scale.
Several experiments were then performed to get insight into
the reaction mechanism (Scheme 4). It was found that the
reaction does not proceed in the dark or under air (eq 1).
Performing the reaction in the presence of TEMPO and
butylated hydroxytoluene (BHT) as radical scavengers fully
inhibits the formation of the product 5b (eq 2). It was found
that ABZ (6) slowly degrades under blue LED irradiation (eq
3), whereas the presence of BIOAc significantly accelerates this
process (eq 4). Other control experiments showed that
cyclopropene 2a as well as BIOAc (7) are stable when
irradiated by blue light-emitting diodes (LEDs) as a solution in
DCE.
excited form, prone to homolytic cleavage of the weak I−N3
bond, forming an iodanyl radical I and an azidyl radical. Once
formed, the azidyl radical would add to the cyclopropene
double bond, forming the reactive cyclopropyl radical II, which
would undergo an electrocyclic ring-opening to give the α-
azidoallyl radical III. α-Azido radicals are known to quickly
lose N2, forming the corresponding iminyl radicals IV,23 which
could then cyclize to the adjacent arene ring, giving rise to the
intermediate V. Subsequent oxidation−deprotonation of IV
with either iodanyl radical I or ABZ (6) would result in the
formation of product 2 and tosylamide 8. In the latter case, an
azido radical would also be generated, leading to a chain
process. The small but reproducible improvement in yield
observed in the presence of a base may be due to the
acceleration of the deprotonation step or the quenching of the
formed acidic species (imide 8 or acetic acid).
On the basis of these results and our previous inves-
tigations,5 we can suggest the following mechanism for the
transformation (Scheme 5). Irradiation of ABZ (6) in the
presence of BIOAc (7) would result in the formation of an
In summary, a protocol for the metal-free radical azidation of
cyclopropenes leading to the formation of quinolines was
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quinolines can be obtained, especially valuable trifluoromethy-
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ASSOCIATED CONTENT
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Preparation of starting materials, optimization of
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AUTHOR INFORMATION
Corresponding Author
■
Jerome Waser − Laboratory of Catalysis and Organic
Synthesis, Institut des Sciences et Ingénierie Chimique, Ecole
Polytechnique Fédérale de Lausanne, 1015 Lausanne,
Switzerland; orcid.org/0000-0002-4570-914X;
Email: jerome.waser@epfl.ch
Authors
Vladyslav Smyrnov − Laboratory of Catalysis and Organic
Synthesis, Institut des Sciences et Ingénierie Chimique, Ecole
Polytechnique Fédérale de Lausanne, 1015 Lausanne,
Switzerland; orcid.org/0000-0001-7180-0337
Bastian Muriel − Laboratory of Catalysis and Organic
Synthesis, Institut des Sciences et Ingénierie Chimique, Ecole
Polytechnique Fédérale de Lausanne, 1015 Lausanne,
Switzerland
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W.; Gan, Z.; Huang, W.; Li, J.; Zeng, H.; Shi, J. Electron Transport
Properties of an Ethanol-Soluble AlQ 3-Based Coordination Polymer
and Its Applications in OLED Devices. J. Mater. Chem. 2009, 19,
4551−4556.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01775
Notes
The authors declare no competing financial interest.
Raw data for NMR, IR, and MS are available at zenodo.org: 10.
5281/zenodo.4963847.
(8) Michael, J. P. Quinoline, Quinazoline and Acridone Alkaloids.
Nat. Prod. Rep. 2008, 25, 166−187.
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ACKNOWLEDGMENTS
We thank the Swiss National Science Foundation (grant no.
200020_182798) for financial support.
■
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