Pd(II)-Catalyzed Oxidative Annulation via Double C-H Activations: Synthesis and Photophysical Properties of Bis-Coumarins

Article abstract of DOI:10.1021/acs.orglett.9b03932

A Pd(II)-catalyzed oxidative annulation reaction of 4-hydroxycoumarin and arylcarboxylic acid via double C-H bond activations has been accomplished for the synthesis of bis-coumarins. This synthetic strategy provides a wide range of structurally diversified bis-coumarins in moderate to good yields with a variety of functional group compatibility. Moreover, photophysical properties of synthesized bis-coumarins have been evaluated, which reveals their interesting fluorescent properties.

Full text of DOI:10.1021/acs.orglett.9b03932

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd(II)-Catalyzed Oxidative Annulation via Double C−H Activations:
Synthesis and Photophysical Properties of Bis-Coumarins
Kumud Sharma,†,‡ Kashmiri Neog,†,‡ and Pranjal Gogoi*,†,‡
†Applied Organic Chemistry Group, Chemical Science and Technology Division, CSIR-North East Institute of Science and
Technology, Jorhat, Assam 785006, India
‡Academy of Scientific and Innovative Research (AcSIR), CSIR-NEIST Campus, Jorhat, Assam 785006, India
S
* Supporting Information
ABSTRACT: A Pd(II)-catalyzed oxidative annulation reaction of 4-hydroxycoumarin and arylcarboxylic acid via double C−H
bond activations has been accomplished for the synthesis of bis-coumarins. This synthetic strategy provides a wide range of
structurally diversified bis-coumarins in moderate to good yields with a variety of functional group compatibility. Moreover,
photophysical properties of synthesized bis-coumarins have been evaluated, which reveals their interesting fluorescent
properties.
of coumarins for photonic applications. With regard to the
synthesis of coumarin-fused heterocycles, several such
compounds have been synthesized and their photophysical
properties were evaluated. Among coumarin-fused hetero-
cycles, bis-coumarins are highly targeted and have been
synthesized using diverse synthetic methods. Taniguchi and
co-workers synthesized some S-shaped bis-coumarins using
double Pechmann reactions.9a Kim and co-workers developed
some high-planarity bis-coumarins using BBr3-promoted
lactonization of p-terphenyldicarboxylic acid derivatives.9b
Some novel types of bis-coumarins were synthesized by
Gryko and co-workers using modified Knoevenagel bis-
condensation followed by oxidative aromatic coupling
reactions.9c,d Another interesting bis-coumarin fused in the
pyran-2-one ring, i.e., chromeno[3,4-c]chromene-6,7-diones,
could be synthesized using two different ways by heating 3-
methoxyphenol with diethyl ethoxymethylenemalonate or 2-
oxo-2H-chromene-3-carboxylate derivatives in the presence of
a Lewis acid.10 A similar bis-coumarin, isochromeno[4,3-
c]chromene-6,11-dione, is also known; however, a few studies
have been devoted to their synthesis. Pechmann condensation
of 4-hydroxycoumarin with ethyl 2-oxocyclohexane-carboxylate
followed by dehydrogenation,11a reaction of 4-hydroxycoumar-
oumarin is one of the most widely found skeletons in
1
Cnature.
It constitutes the central core unit of many
natural products and exhibits interesting biological properties,
including antioxidant, anti-inflammatory, anticancer, and
antimicrobial activities.2 Due to its wide availability and
unique properties, the π-extended version of it has become an
area of interest and several such compounds have been
developed for photonic applications.3 Coumarin compounds
exhibit interesting photophysical properties, including strong
light absorption,4a,b high fluorescence quantum yields,4c and
large Stokes shifts.5 As a result, they have been widely used in
fluorescent probes,6a,b two-photon fluorescence microsco-
pes,6c,d OLEDs,7a,b dye-sensitized solar cells,7c photolabile
materials,7d,e and other electronic devices. With regard to π-
extended coumarins, benzocoumarins are common and were
initially synthesized by Pechmann in 1984.8a Later, various
synthetic protocols such as Knoevenagel condensation
followed by intramolecular cyclization,6c,8b electrophilic
substitution of naphthols with β-keto esters followed by
cyclization,8c,d and metal-catalyzed reactions8e,f have been
developed for the synthesis of benzocoumarins.
On the other hand, heterocycle-fused coumarin derivatives
make up another important and emerging subarea of π-
extended coumarins, where various heterocycle units are
annulated to coumarin to enhance photophysical properties.
Additionally, the introduction of a heterocycle to coumarin is a
powerful approach for controlling the photophysical properties
Received: November 4, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03932
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in with 2-bromobenzoic acid,11b and the use of an organolead
a
Table 1. Optimization Studies
reagent11c are known for their synthesis.
Although there are several ways to synthesize bis-coumarins,
they are limited in structural diversification. On the other hand,
the photophysical properties of most synthesized bis-
coumarins have not been thoroughly explored. Therefore, a
common synthetic protocol using readily available starting
materials for the synthesis of diversified bis-coumarins is highly
desirable and necessary. As a continuation of our research on
coumarin chemistry12a,b and synthesis of fused heterocycles for
photonic applications,12c we have developed a new synthetic
strategy for the direct synthesis of bis-coumarin via Pd(II)-
catalyzed reaction of 4-hydroxycoumarin with arylcarboxylic
acid (Scheme 1). Pd(II)-catalyzed ortho functionalization of
aryl carboxylic acids via weakly coordinating -COOK or
-COONa to direct C−H activations has been described.13
yield
b
entry
catalyst
Pd(OAc)2
oxidant
base
solvent
(%)
c
1
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Ag2O
air
O2
NaOAc CH3CN
32
42
53
ND
15
25
ND
63
c
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
3
4
5
6
7
8
CuCl2
Cu(OAc)2
1,4-
dioxane
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
K2CO3
K3PO4
KF
1,4-
ND
45
Scheme 1. Synthesis of Bis-Coumarin 3 Using 4-
Hydroxycoumarin 1 and Arylcarboxylic Acid 2
dioxane
10
11
1,4-
dioxane
1,4-
dioxane
ND
12
13
14
15
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PPh3)4
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
−
KOAc
KOAc
KOAc
KOAc
DMSO
DMF
toluene
1,4-
dioxane
ND
ND
trace
ND
We commenced our optimization studies by considering 4-
hydroxycoumarin 1a (1 mmol) and benzoic acid 2a (1.2
mmol) as model substrates for the synthesis of bis-coumarin
3aa (Table 1). Initially, the reaction of 4-hydroxycoumarin 1a
with benzoic acid 2a was performed by using our reported
oxidative annulation process12a of using Pd(OAc)2 (5 mol %),
Cu(OAc)2 (1.2 equiv), NaOAc (1.2 equiv), and CH3CN (5
mL) in an ace pressure tube at 120 °C for 24 h (Table 1, entry
1). Under these conditions, product 3aa was obtained in 32%
yield. To improve the yield of 3aa, several deviations from our
reported method have been made. In an initial modification,
we used KOAc instead of NaOAc as a base for the synthesis of
3aa. Under these reaction conditions, the desired product 3aa
was isolated in 42% yield (Table 1, entry 2). Interestingly,
when we increased the oxidant and base loading, the targeted
annulated product 3aa was obtained in 53% yield (Table 1,
entry 3). During our optimization studies, several other
oxidants such as Ag2O, air, oxygen, and CuCl2 were tested
instead of Cu(OAc)2; however, they were inefficient as
compared to Cu(OAc)2 (Table 1, entries 4−7, respectively).
In addition to those, various bases and solvents were also
screened for our cascade annulation reaction. After several
investigations, an improved yield of 63% was obtained when
our oxidative annulation reaction was performed using 2.2
equiv of Cu(OAc)2 as the oxidant and 4 equiv of KOAc as the
base in 1,4-dioxane for 24 h in an ace pressure tube at 120 °C,
which was considered as our optimized reaction conditions
(Table 1, entry 8). The synthesized bis-coumarin 3aa was
confirmed by 1H NMR, 13C NMR, and HRMS analysis, and its
data were compared with the reported data.11a,c The required
Pd(II) catalyst is vital for our oxidative annulation process as
Pd(0), Rh(III), and Ru(II) catalysts did not produce our
desired product (Table 1, entries 15−17, respectively). In
addition to those, several control experiments were also
performed in the absence of a catalyst as well as an oxidant,
and no desired product 3aa was observed (Table 1, entries 18
and 19). Furthermore, the imidazolium ligand and lithium salt
16
17
18
19
[Cp*RhCl2]2
Cu(OAc)2
Cu(OAc)2
−
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
1,4-
ND
ND
ND
ND
20
dioxane
[RuCl2(p-
cymene)]2
−
1,4-
dioxane
1,4-
dioxane
1,4-
dioxane
1,4-
dioxane
−
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
d
20
Pd(OAc)2
Pd(OAc)2
e
21
1,4-
dioxane
53
a
Conditions: 4-hydroxycoumarin (1.0 mmol), benzoic acid (1.2
mmol), and solvent (6 mL) stirred in ace pressure tube at 120 °C (oil
bath) for 24 h. ND: Not detected. Isolated yield. Oxidant (1.2
equiv) and base (1.2 equiv). 1,3-Bis(2,4,6-trimethylphenyl)-
imidazolium chloride (10 mol %) used as a ligand. LiCl (0.5
equiv) was used.
b
c
d
e
as the additive were also tested during our optimization
studies; however, no encouraging results were observed (Table
1, entries 20 and 21, respectively).
After we had optimized the reaction conditions of our
cascade annulation process, we explored the substrate scope
for the synthesis of structurally diverse bis-coumarins 3.
This cascade annulation of 4-hydroxycoumarins with a range
of arylcarboxylic acids took place smoothly, affording good
yields of our desired bis-coumarins, and the results are
presented in Scheme 2. Initially, a variety of arylcarboxylic
acids bearing electron-donating and electron-withdrawing
groups were allowed to react with 4-hydroxycoumarin under
our optimized reaction conditions, and seven bis-coumarins
with various functionalities, including -OMe, F, CF3, and Bu,
were synthesized in good yields (Scheme 2, 3aa−3ag).
Specifically, arylcarboxylic acids having electron-donating
groups (OMe- and Bu-) gave our annulated products 3 in
high yields (Scheme 2, 3ab and 3ad). Interestingly, when 3-
methoxybenzoic acid was treated with 4-hydroxycoumarin 1a
t
t
B
DOI: 10.1021/acs.orglett.9b03932
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Organic Letters
Letter
a
Scheme 2. Synthesis of Bis-Coumarin
Scheme 3. Synthesis of Bis-Coumarin 4ah
Table 2. Photophysical Properties of Compounds 3
a
a,b
compound λmax (nm) [ε (M−1 cm−1)]
λem (nm)
ΦF
1
2
3
4
5
6
7
8
3aa
3ab
3ac
3ag
3ba
3bb
3bc
3bd
3dg
3ga
3gd
339 [16400]
339 [11600]
348 [21100]
364 [6300]
350 [10200]
348 [11900]
357 [14800]
349 [25500]
366 [8200]
346 [27100]
347 [24200]
382
388
402
413
401
400
415
400
415
393
393
0.12
0.32
0.16
0.26
0.28
0.60
0.16
0.35
0.50
0.15
0.17
9
10
11
a
b
Concentration of 1 × 10−5 M in DCM. Excited at 330 nm.
a
Conditions: 1a (1 mmol), 2a (1.2 mmol), Pd(OAc)2 (5 mol %),
Cu(OAc)2 (2.2 mmol), KOAc (4 mmol), and 1,4-dioxane (6 mL)
stirred in an ace pressure tube at 120 °C for 24 h.
under our optimized reaction conditions, the product 3ac was
obtained regioselectively. This regioselective oxidative annula-
tion could be due to steric reasons. Next, varieties of
substituted 4-hydroxycoumarins were also examined as
substrates for our oxidative annulation reaction and gave bis-
coumarins in good yields. With regard to the reactivity of 4-
hydroxycoumarins, both electron-donating and electron-with-
drawing substituents of 4-hydroxycoumarins equally partici-
pated in our cascade annulation process irrespective of their
electronic nature. The Pd(II)-catalyzed annulation reactions
are mild and notably compatible with alkyl, methoxy, fluoro,
and trifluoromethyl groups, and even some coupling prone
functionalities like chloro and bromo remained intact during
the process. During our generalization process, 2-naphthoic
acid was also applied for the synthesis of structurally diversified
bis-coumarins 3ag, 3cg, and 3dg, although in low yields, which
could be due to steric hindrance.
Interestingly, when 4-hydroxycoumarin 1a was treated with
4-acetoxybenzoic acid 2h under our optimized reaction
conditions, we obtained the hydrolyzed product 4ah in 18%
yield (Scheme 3).
With these diverse bis-coumarin molecules in hand, we next
turned our attention to the photophysical properties of
synthesized bis-coumarins 3 by recording UV−vis and
emission spectra in DCM at a low concentration (1 × 10−5
M) as shown in Table 2 and Figure 1. They emit between 382
Figure 1. Fluorescence spectra of compounds 3 recorded in DCM.
and 415 nm in DCM with moderate to good quantum yields
(ranging from 0.12 to 0.60). The π-conjugation and nature of
substituents as well as their position on the peripheral ring of
bis-coumarin play an important role in their photophysical
properties.
Compounds 3ag and 3dg exhibit a bathochromic shift in
both their absorption and emission wavelengths due to the
presence of an extended π-conjugation system. The presence
of an electron-donating group at the C-2 or C-9 position of the
isochromeno[4,3-c]chromene-6,11-dione ring increases the
fluorescence quantum yield (see 3aa vs 3ab and 3aa vs 3ba)
due to enhanced intramolecular charge transfer (ICT)
throughout the molecule during excitation. Interestingly, with
an increasing number of electron donor methoxy groups at the
C-2 and C-9 positions of the isochromeno[4,3-c]chromene-
6,11-dione ring, the quantum yield increases (see 3aa vs 3ba vs
3bb). This is due to the larger resonance contribution to the
ICT by the increasing number of electron donor substituents
at the C-2 and C-9 positions of the core moiety (Figure 2). On
the other hand, the presence of this -OMe group at either C-2
or C-9 does not significantly change their quantum yields (see
3ab vs 3ba). Interestingly, chloro-substituted highly con-
jugated compound 3dg shows a dramatic increase in its
C
DOI: 10.1021/acs.orglett.9b03932
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Organic Letters
Letter
benzoate to form complex E. Afterward, complex F is formed
by the second C−H activation at the benzoate moiety.
Reductive elimination followed by lactonization of complex F
gave our desired product 3aa and Pd(0), which then was
oxidized to Pd(II) in the presence of Cu(OAc)2 to complete
the catalytic cycle.
In summary, we have developed a Pd(II)-catalyzed double
C−H bond activation strategy for the direct synthesis of
structurally diverse bis-coumarins via oxidative annulation
between arylcarboxylic acids and 4-hydroxycoumarins. Our
synthetic protocol consists of C−H activation as well as C−C
and C−O bond formation, which prevent the prefunctionaliza-
tion of both starting materials. This strategy provides a wide
range of structurally diverse bis-coumarins in good yields.
Additionally, the photophysical properties of synthesized
compounds were also studied, and some of them showed
good fluorescence quantum yields.
Figure 2. Resonance contribution of 3ba and 3bb to the ICT.
quantum yield versus that of nonsubstituted compound 3ag.
Such changes can be understood by evoking the enhanced
ICT. The photophysical properties are not affected by the
t
presence of alkyl groups (-Me and Bu) on the bis-coumarin
ring (compounds 3ga and 3gd). Some of our brightest
compound’s images under UV irradiation (365 nm) are shown
in Figure 3. Furthermore, a fluorosolvatochromic study was
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b03932.
■
S
Experimental details, characterization data, and copies of
NMR (1H and 13C) and HRMS spectra (PDF)
AUTHOR INFORMATION
■
Figure 3. Compounds 3 in DCM (1 × 10−5 M) under UV irradiation
(365 nm).
Corresponding Author
*E-mail: gogoipranj@yahoo.co.uk.
ORCID
also carried out on the brightest compound 3bb (see the
Supporting Information). Compound 3bb exhibits a good
fluorescence quantum yield from 0.39 in toluene to 0.60 in
DCM.
Kumud Sharma: 0000-0001-5093-095X
Pranjal Gogoi: 0000-0003-0711-0328
Notes
With regard to the reaction mechanism of our Pd(II)-
catalyzed oxidative annulation process, a proposed reaction
pathway is illustrated on the basis of our experiments and
reported literature12a,d for the synthesis of bis-coumarin 3aa
(Scheme 4).
At first, exchange of alkoxide anion between the potassium
salt of 4-hydroxycoumarin and palladium(II)acetate occurred
to form the Pd salt of 4-hydroxycoumarin B, which
subsequently forms species C. Concurrently, the first C−H
activation at the C-3 position of C occurred, generating C3−
Pd species D, which further undergoes ligand exchange with
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge Mr. Abhilash Sharma for
his assistance in image collection and discussion. The authors
acknowledge the DBT (File BT/PR16958/NER/95/371/
2015; Application BMB/2015/53), Government of India, for
research funding. K.N. acknowledges UGC, New Delhi, for the
fellowship.
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