Rapid Access of Alkynyl and Alkenyl Coumarins via a Dipyridinium Methylide and Propargylamine Cascade Reaction

Article abstract of DOI:10.1021/acs.orglett.0c02674

A DMAP-catalyzed cascade approach allowing facile assembly of alkynyl coumarins is reported. By virtue of reactive o-AQM (in situ generated from modular propargylamine) and a new synthetic equivalent of acyl carbene (from pyridinium ylide), the reaction proceeds smoothly to afford a variety of alkynyl coumarins in good-to-excellent yields. This transition-metal-free and oxidant-free process features moderate functional group tolerance, particularly the-Br group; thus, this protocol circumvents the inherent shortcomings of the existing Sonogashira coupling of coumarin triflates. This versatile method is also found to be applicable to the preparation of β-alkenyl coumarins, resembling the outcomes of the current Heck-type coupling reaction.

Full text of DOI:10.1021/acs.orglett.0c02674

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Letter
Rapid Access of Alkynyl and Alkenyl Coumarins via a Dipyridinium
Methylide and Propargylamine Cascade Reaction
Xinwei He,* Ruxue Li, Pui Ying Choy, Tianyi Liu, On Ying Yuen, Man Pan Leung, Yongjia Shang,
and Fuk Yee Kwong*
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ABSTRACT: A DMAP-catalyzed cascade approach allowing facile
assembly of alkynyl coumarins is reported. By virtue of reactive o-
AQM (in situ generated from modular propargylamine) and a new
synthetic equivalent of acyl carbene (from pyridinium ylide), the
reaction proceeds smoothly to afford a variety of alkynyl coumarins
in good-to-excellent yields. This transition-metal-free and oxidant-
free process features moderate functional group tolerance,
particularly the −Br group; thus, this protocol circumvents the
inherent shortcomings of the existing Sonogashira coupling of
coumarin triflates. This versatile method is also found to be
applicable to the preparation of β-alkenyl coumarins, resembling
the outcomes of the current Heck-type coupling reaction.
oumarin scaffolds are abundantly found in many natural
products and biologically active intermediates, and
coupling between two different nucleophiles (Scheme 1d).22
Nonetheless, the aforementioned catalytic approaches usually
require expensive transition metal catalyst systems and perform
under inert atmospheric conditions, as well as often require
prefunctionalization of coumarin to the corresponding electro-
philic components using either Tf2O or POBr3. To circumvent
the inherent difficulties, it is desirable to explore a new
organocatalytic pathway, especially if it is complementary to
the existing methods for assembling of the above pharmaceuti-
cally useful and materially valuable coumarin frameworks.
Propargylamine is highly versatile, as this structure can be
simply attained by a three-component modular reaction of
aromatic aldehyde, terminal alkyne, and amine.23−25 Indeed, it
exhibits rich entities of structural diversity. Inspired by this
alkyne-containing precursor, we are intrigued by the possibility
of it being favorably employed for rapid assembly of the alkynyl
coumarin skeleton. Herein we report a DMAP-catalyzed
cascade reaction between propargylamine and pyridinium
ylide26 (Scheme 1e). This process proceeds via the in situ
generated ortho-alkynyl quinone methide (o-AQM)27−30
intermediate from propargylamine under basic conditions
and then continues the 1,4-conjugate addition and subsequent
annulation process.
C
constitute one of the most important structural motifs for
developing new pharmaceuticals.1−3 Coumarins containing an
unsaturated lactone skeleton are classically synthesized by aldol
or Knoevenagel condensations.4,5 A recent convergent
synthetic tool, i.e., cross-coupling technology,6 has emerged
to allow rapid assembly of diversified coumarin structures.
Particularly the alkynyl coumarins, which not only are found to
be useful in pharmaceuticals7 but also prevail in unique
applications in functional organic materials, for instance laser
dyes,8,9 dye-sensitized solar cells (DSSCs),10,11 organic light-
emitting diodes (OLEDs),12 semiconductors,13 and photo-
voltaics,14 are often synthesized by a Pd-catalyzed cross-
coupling strategy.
With the electrophilic coumarin enol triflates made with
expensive Tf2O, the Sonogashira coupling of terminal alkynes
proceeded well to give the alkynyl coumarins (Scheme
1a).15−17 Wu also recently reported an improved procedure
for the conversion of the coumarin −OH group to the −OTs
moiety and subsequent coupling of terminal alkynes in a one-
pot fashion.18 Apart from pseudo halides, alkenyl bromides
were found to be capable substrates for alkyne coupling as
reported by Fairlamb (Scheme 1b).19,20 These coumarin
bromides were made by transforming the coumarin −OH
moiety to the −Br group using POBr3. In 2019, Knochel
disclosed the application of alkynyl zinc pivalate nucleophiles
for the construction of the C(sp)−C(sp2) bond under the Ni
catalyst system (Scheme 1c).21 Despite the traditional cross-
coupling between electrophilic and nucleophilic fragments,
Hong recently showed a successful Pd-catalyzed oxidative
Received: August 11, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02674
© XXXX American Chemical Society
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A

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a
Scheme 1. Modern Strategies for Accessing Alkynyl
Coumarins
Table 1. Evaluation of Reaction Parameters
b
entry
catalyst
solvent
yield (%)
1
2
3
4
5
6
7
8
Et3N
DMAP
DBU
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCE
83
92
trace
37
32
25
16
trace
33
18
34
78
28
52
8
DABCO
Sparteine
Na2CO3
NaHCO3
NaOH
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
c
9
10
11
12
13
14
15
16
17
DMF
EtOH
DMSO
toluene
MeCN
MeCN
MeCN
d
e
89
61
f
a
Reaction conditions: 4-methyl-2-(3-phenyl-1-(pyrrolidin-1-yl)prop-
2-yn-1-yl)phenol (1b) (0.2 mmol), 1,1′-(2-oxopropane-1,3-diyl)bis-
(pyridin-1-ium) bromide (2) (0.24 mmol), and catalyst (20 mol %)
b
in solvent (3 mL) at 80 °C for 5 h. Isolated yields were reported.
c
d
e
f
The catalyst loading was 10 mol %. At 50 °C. At 100 °C. For 2 h.
We initially explored the new reagent, dipyridinium ylide 2,
as a two-carbon feedstock for assembling the coumarin
scaffold. Thus, we employed propargylic amine 1b and 2 as
model substrates for this organocatalytic reaction (Table 1). In
the presence of 20 mol % organocatalysts, for instance Et3N
and DMAP, the reaction proceeded smoothly to give an up to
92% product yield (entries 1−2). Nevertheless, DBU,
DABCO, and sparteine were found to be less successful
(entries 3−5). Apart from the organic bases, commonly used
Na2CO3, NaHCO3, and NaOH did not facilitate this
transformation (entries 6−8). MeCN was the best solvent of
choice among other common organic solvents screened, e.g.,
DCE, DMF, EtOH, DMSO, and toluene (entries 10−14 vs 2).
There was no extra benefit of product yield at elevated reaction
temperature, e.g., 100 °C (entry 16 vs 2).
Having the optimized reaction conditions in hand, we next
explored the substrate scope (Scheme 2). In general, the
coumarin products were obtained in good-to-excellent yield.
Particularly noteworthy is that the −Br and −Cl groups
remained intact during the course of the reaction. This
outcome allows these products to be further modified by
established cross-coupling strategies at a later stage. There was
no significant electronic effect of the substituents displayed at
the phenolic ring (products 3a−3d and 3e−3h). Likewise, the
electronic property of the alkynyl arenes was also insignificant
(products 3i−3l vs 3m−3p). It is important to show that the
sterically hindered phenolic fragment did not affect the
efficiency of the cyclization (products 3q−3s). Even the
highly sterically bulky tert-butyl group was well-tolerated
(product 3r). There were no steric influences at the alkynyl
arenes as well (products 3t and 3w). Dihalo-substituents at
either phenolic or alkynyl arene rings were also compatible
(products 3s−3u). The product 3t was unambiguously
characterized by single crystal X-ray crystallography. The
highly electron-withdrawing nitro group did not affect the yield
of coumarin scaffold assembly (products 3v and 3x). The
possible gram-scale synthesis showed the potential practic-
ability for large-scale preparation of substituted coumarins.
In addition to aromatic alkyne substituents, the applicability
of other alkenyl-, thienyl-, and alkyl-containing substrates were
examined (Scheme 3). It is worth noting that the conjugated
enyne moiety was also well-suited in this catalyst system
(product 3y). There was no deleterious effect of a heterocyclic
substrate under these reaction conditions (product 3z).
Not only the alkynyl coumarin scaffold has rich application
in material sciences, the corresponding alkenyl coumarin
skeleton also displays unique photophysical properties.31
Common modular assembly of these alkenyl coumarin units
relies on the palladium-catalyzed Suzuki−Miyaura and Heck
couplings of coumarin sulfonates/bromides with either
alkenylboronic acids32 or alkenes,33 respectively. The inherent
limitation of these existing protocols would be the incompat-
ibility of −Br or −Cl groups. Indeed, it would be highly
attractive if we can develop an organocatalytic method
particularly fit for moderate functional group tolerance. Our
further attempts of using dipyridinium ylide 2 as the acyl
carbene surrogate led us to have a variety of halo-containing
alkenyl coumarins (Scheme 4). There was no significant
substrate electronic effect with regard to the desired product
yields (products 5b−5f). The steric effect, where the −Br
group at the ortho-position to the phenolic group, was
insignificant (product 5g). It is noteworthy to show that this
catalyst system displayed entire compatibility with −Br and
−Cl groups. Thus it exhibits rich potential for subsequent
functionalization using cross-coupling technology.
B
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Scheme 2. Scope of Propargylamine Towards Coumarin
Scaffold Assembly
Scheme 3. Scope of Alkenyl, Thienyl, and Alkyl Substituted
Propargylic Amine for Accessing Coumarin Scaffolds
a
a
a
Reaction conditions: propargylamines 1 (0.3 mmol), 1,1′-(2-
oxopropane-1,3-diyl)bis(pyridin-1-ium) bromide (2) (0.36 mmol),
and DMAP (20 mol %) in MeCN (3 mL) at 80 °C for 5 h. Isolated
yields were reported.
Scheme 4. Attempted Experiments for Resembling the
a
Outcome of Heck Coupling Reaction
a
Reaction conditions: propargylamines 1 (0.3 mmol), 1,1′-(2-
oxopropane-1,3-diyl)bis(pyridin-1-ium) bromide (2) (0.36 mmol),
and DMAP (20 mol %) in MeCN (3 mL) at 80 °C for 5 h. Isolated
yields were reported.
a
Reaction conditions: (E)-2-(3-phenyl-1-(pyrrolidin-1-yl)allyl)-
phenols 4 (0.2 mmol), 1,1′-(2-oxopropane-1,3-diyl)bis(pyridin-1-
ium) bromide (2) (0.24 mmol), and DMAP (20 mol %) in MeCN (3
mL) at 80 °C under air atmosphere for 5 h. Isolated yields were
reported.
In order to gain insight into the reaction mechanism, we
performed the deuterium-labeling experiment (Scheme 5a).
The H/D exchange of pyridinium methylide 2 was carried out
1
in the presence of D2O. H NMR analysis revealed that all
intramolecular nucleophilic addition of intermediate B affords
the species C, which then converts to intermediate D upon
release of alkylpyridinium salt by a mechanism similar to that
involving the fission of β-carbonyl compound. Finally,
intermediate D undergoes β-H elimination and C−N bond
cleavage to deliver the product 3a.
In summary, the manipulation of the physical properties of
coumarin by installing alkynyl or alkenyl moieties has been
found to be highly useful toward the advancement of material
science. Precedent methods for accessing these structural
motifs depend on the transition-metal-catalyzed cross-coupling
of prefunctionalized coumarin triflate/tosylate/bromide with
an alkyne or alkene under an inert atmosphere, or the oxidative
hydrogen atoms on the methylene group of 2 were completely
exchanged with deuterium within 10 min. Further reacting of
d-2 with 1g afforded the product d-3g in 85% yield. This
experiment suggested that the reaction involves a 1,4 conjugate
addition of pyridinium methylide to o-AQM, and subsequent
intramolecular annulation proceeds. A proposed mechanism is
shown in Scheme 5b. On the basis of precedent reports,27,34
we postulate that this one-pot cascade reaction involves the
initial deamination of the propargylamine 1a and subsequently
generates the o-AQM intermediate in the presence of the
DMAP catalyst. The 1,4-conjugate addition of dipyridinium
methylide to o-AQM proceeds to give intermediate A. The
C
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Scheme 5. Deuterium-Labeling Experiment and Proposed
Mechanism
AUTHOR INFORMATION
Corresponding Authors
■
Xinwei He − State Key Laboratory of Synthetic Chemistry and
Department of Chemistry, The Chinese University of Hong
Kong, New Territories, Hong Kong, China; Key Laboratory of
Functional Molecular Solids, Ministry of Education, Anhui
Laboratory of Molecule-Based Materials (State Key Laboratory
Cultivation Base), College of Chemistry and Materials Science,
Anhui Normal University, Wuhu 241002, P. R. China;
orcid.org/0000-0002-1974-2464; Email: xinweihe@
mail.ahnu.edu.cn
Fuk Yee Kwong − State Key Laboratory of Synthetic Chemistry
and Department of Chemistry, The Chinese University of Hong
Kong, New Territories, Hong Kong, China; orcid.org/0000-
0001-9105-1740; Email: fykwong@cuhk.edu.hk
Authors
Ruxue Li − Key Laboratory of Functional Molecular Solids,
Ministry of Education, Anhui Laboratory of Molecule-Based
Materials (State Key Laboratory Cultivation Base), College of
Chemistry and Materials Science, Anhui Normal University,
Wuhu 241002, P. R. China
Pui Ying Choy − State Key Laboratory of Synthetic Chemistry
and Department of Chemistry, The Chinese University of Hong
Kong, New Territories, Hong Kong, China
Tianyi Liu − State Key Laboratory of Synthetic Chemistry and
Department of Chemistry, The Chinese University of Hong
Kong, New Territories, Hong Kong, China
On Ying Yuen − State Key Laboratory of Synthetic Chemistry
and Department of Chemistry, The Chinese University of Hong
Kong, New Territories, Hong Kong, China
Man Pan Leung − State Key Laboratory of Synthetic Chemistry
and Department of Chemistry, The Chinese University of Hong
Kong, New Territories, Hong Kong, China
Yongjia Shang − Key Laboratory of Functional Molecular
Solids, Ministry of Education, Anhui Laboratory of Molecule-
Based Materials (State Key Laboratory Cultivation Base),
College of Chemistry and Materials Science, Anhui Normal
University, Wuhu 241002, P. R. China; orcid.org/0000-
0001-9873-9150
coupling between two nucleophilic fragments in the presence
of an oxidant. In fact, it would be highly attractive to develop
an organocatalytic method for tackling this issue, in which it
features transition metal-free and oxidant-free conditions, as
well as the reaction can be conveniently carried out under an
operationally simple air atmosphere. We have succeeded in
showing that the new acyl carbene surrogate, the dipyridinium
ylide, was able to react modular ortho-alkynyl quinone methide
(o-AQM) in generating a variety of alkynyl coumarins. This
metal-free process does not require inert atmosphere
protection and allows moderate functional group tolerance,
particularly the −Br and −Cl groups; thus, this protocol is
complementary to the inherent shortcomings of the existing
Sonogashira coupling of coumarin triflates. This versatile
method is also found to be applicable to the preparation of β-
alkenyl coumarins, resembling the outcomes of current Heck-
type coupling reactions. We believe this finding will be versatile
and offer a breadth of interest to organic material scientists
working in coumarin-related modifications and their corre-
sponding material advancements.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02674
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Research Grants Council of Hong Kong (GRF:
14307418-18P), the CUHK Direct Fund (4442353) and the
Anhui Provincial Natural Science Foundation (No.
1808085MB41) for financial support. We are thankful for
the financial support from the Innovation and Technology
Commission (HKSAR, China) to the State Key Laboratory of
Synthetic Chemistry.
ASSOCIATED CONTENT
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Experimental procedures and spectroscopic data for all
compounds (PDF)
Accession Codes
DEDICATION
■
CCDC 1996085 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Dedicated to Prof. Albert S. C. Chan on the occasion of his
70th birthday.
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