Transition metal-free cross-dehydrogenative coupling acylation of coumarins by the K2S2O8/Aliquat 336 catalytic system: a versatile strategy towards 4-aroylcoumarin derivatives

Article abstract of DOI:10.1039/c6ra26278c

A new and efficient transition metal-free oxidative cross-dehydrogenative coupling (CDC) reaction is described for the preparation of 4-aroylcoumarin derivatives. In this protocol aldehydes have been used as acylating agents for acylation of 3-substituted coumarins through C(sp²)-C(sp²) bond formation using the K₂S₂O₈/Aliquat 336 system as an inexpensive and environmentally friendly reagent. The reactions proceeded in acetonitrile at 80 °C for 2-3 h to afford the corresponding 4-aroylcoumarin derivatives in high yields.

Full text of DOI:10.1039/c6ra26278c

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Transition metal-free cross-dehydrogenative
coupling acylation of coumarins by the K2S2O8/
Aliquat 336 catalytic system: a versatile strategy
towards 4-aroylcoumarin derivatives†
a
Cite this: RSC Adv., 2016, 6, 110656
Mehdi Adib, Rahim Pashazadeh,a Saideh Rajai-Daryasarei,a Roya Kabirib
*
and Mehdi Jahania
A new and efficient transition metal-free oxidative cross-dehydrogenative coupling (CDC) reaction is
described for the preparation of 4-aroylcoumarin derivatives. In this protocol aldehydes have been used
as acylating agents for acylation of 3-substituted coumarins through C(sp2)–C(sp2) bond formation using
the K2S2O8/Aliquat 336 system as an inexpensive and environmentally friendly reagent. The reactions
proceeded in acetonitrile at 80 ꢀC for 2–3 h to afford the corresponding 4-aroylcoumarin derivatives in
high yields.
Received 28th September 2016
Accepted 15th November 2016
DOI: 10.1039/c6ra26278c
www.rsc.org/advances
The coumarin moiety is a heterocyclic scaffold that exists widely heteroatom bonds by use of CDC technique in the absence of
in a variety of biologically active synthetic and natural prod- transition metals has been considered signicantly.14
ucts.1 Notably, coumarin derivatives are well documented as
Transition metal-free synthetic protocols from simplicity
therapeutic agents and possess a variety of biological properties point of view as well as reducing adverse environmental
such as monoamine oxidase (MAO)2 as well as lipoxygenase consequences are elegant and robust approaches.15 On the
inhibitory activity and accordingly antioxidant,3 anti-inam- other hand, tetraalkylammonium halides/oxidants as catalytic
matory,4 antitumour,5 antimicrobial,6 antimalarial,7 anticoagu- systems have been found to be highly effective to replace the
lant8 and anti-HIV properties9 (Fig. 1). Moreover, coumarins toxic heavy metal catalysts and have a signicant role in devel-
have been used as sensitizers in older photovoltaic technolo- opment of C–C, C–N, C–O and C–S bonds formation in organic
gies10 and also as dyes in laser technology,11 as well as in the synthesis.16
preparation of soaps, sunscreen, cosmetics, perfumes and
avorings.12
Over the past decade, acylation of heterocyclic scaffolds
using acylating agents such as aldehydes and arylmethanes via
Due to formation of C–C bonds from two C–H bonds, CDC oxidative CDC reactions under transition metal-free conditions
reactions are applied as a high atom-efficient and step- has attracted considerable interest.17
economic approach for the construction of complex molecules
Recently, acylation of coumarins via CDC approaches using
in organic synthesis.13 Recently the formation of C–C and C– aldehydes as acylating agents has been investigated by our
group,18a Qu18b and Zhou.18c In addition, Duan and co-workers
reported an Ag-catalyzed diacylation of coumarins using a-
oxocarboxylic acids as acyl sources.18d
As part of our continuing effort to develop efficient methods
for the preparation of biologically active heterocyclic
Fig. 1 Examples of pharmacologically active coumarin derivatives.
aSchool of Chemistry, College of Science, University of Tehran, PO Box 14155-6455,
Tehran, Iran. E-mail: madib@khayam.ut.ac.ir; Fax: +98 21 66495291; Tel: +98 21
66495291
bNMR Lab, Faculty of Chemistry, Tabriz University, Tabriz, Iran
† Electronic supplementary information (ESI) available: Copy of 1H NMR and 13
C
Scheme 1 Model CDC reaction between 4-methylbenzaldehyde 1a
and 3-acetylcoumarin 2a.
NMR spectra of new compounds. See DOI: 10.1039/c6ra26278c
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Table 1 Condition screening for the CDC reaction of 4-methylbenzaldehyde 1a with 3-acetylcoumarin 2aa
Entry
Oxidantb (equiv.)
Additiveb (equiv.)
Solvent
t (ꢀC)
Yieldc (%)
1
2
3
4
5
6
7
8
K2S2O8 (1)
K2S2O8 (1)
K2S2O8 (1)
K2S2O8 (1)
Aliquat 336 (15)
Aliquat 336 (15)
Aliquat 336 (15)
Aliquat 336 (30)
Aliquat 336 (40)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
TBABe (30)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Toluene
Chlorobenzene
1,4-Dioxane
DCE
DMSO : H2O (1 : 1)
DMSO
r.t.
50
80
80
80
80
80
100
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
NRd
NR
65
78
78
87
83
82
NR
NR
NR
NR
NR
NR
50
50
30
NR
65
55
K2S2O8 (1)
K2S2O8 (1.2)
K2S2O8 (1.5)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
K2S2O8 (1.2)
(NH4)2S2O8 (1.2)
TBHPh (1.2)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
TBPBf (30)
NBSg (30)
I2 (30)
KI (30)
CuI (30)
Aliquat 336 (30)
Aliquat 336 (40)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
Aliquat 336 (30)
—
H2O
40
82
40
60
NR
NR
NR
NR
CH3CN : H2O (1 : 1)
CH3CN
CH3CN
CH3CN
CH3CN
i
H2O2 (1.2)
DDQ (1.2)
—
K2S2O8 (1.2)
CH3CN
CH3CN
a
b
c
d
Reaction conditions: 1a (1 mmol), 2a (0.5 mmol), solvent (2 mL) for 2 h. In respect to 2a. Isolated yield. NR ¼ no reaction.
Tetrabutylammonium bromide. f Tetrabutylphosphonium bromide. g N-Bromosuccinimide. h 70 wt% tBuOOH in H2O. i 30 wt% H2O2 in H2O.
e
compounds from readily available precursors,19 and in contin- yield (entry 3). Subsequently, different quantities of K2S2O8 and
uation of our recent studies on CDC reactions,20 herein, we Aliquat 336 were evaluated to improve the yields (entries 4–7).
describe an efficient approach for the intermolecular double Increasing the amount of additive to 30 mol%, led to the desired
2
C
sp –H functionalization between aromatic aldehydes and 3- product 3a in 78% yield (entry 4). However, increasing the
2
2
acetylcoumarins through Csp –Csp bond formation leading to 4- amount of Aliquat 336 to 40 mol% had no effect on the yield
aroylcoumarin derivatives. (entry 5). Then we employed 1.2 and 1.5 equiv. of K2S2O8 in the
To verify this theory, 4-methylbenzaldehyde 1a and 3-ace- presence of 30 mol% Aliquat 336. Thus the desired product 3a
tylcoumarin 2a were chosen as model substrates to optimize the was obtained in 87% and 83% yields, respectively (entries 6 and
CDC reaction conditions (Scheme 1 and Table 1). In this opti- 7). However, rising the temperature to 100 ꢀC decreased effi-
mization, the effect of several oxidants, solvents, additives, ciency of the reaction (entry 8). Some other additives including
reaction temperature and time as well as various equivalents of TBAB, TBPB, NBS (see footnotes in Table 1), I2, KI and CuI were
oxidants and additives were investigated. Initially, we tested this also screened in this study, but all were inert to this CDC
reaction in the presence of K2S2O8 (1.0 equiv.) as oxidant and reaction (entries 9–14). Next, the effect of different solvents such
Aliquat 336 (tricaprylmethylammonium chloride)21 (15 mol%) as toluene, chlorobenzene, 1,4-dioxane, DCE, DMSO/H2O (1 : 1,
as additive in acetonitrile at room temperature or 50 ꢀC for 2 h, v/v), DMSO and H2O on the efficiency of the reaction were
but no product was detected (entries 1 and 2). By rising the investigated. However, carrying out the reaction in these
temperature to 80 ꢀC, 4-aroylcoumarin 3a was obtained in 65% solvents 3a was obtained in 40–65% yields and in DCE 3a was
not detected (entries 15–21). Also, the reaction was performed
in CH3CN/H2O (1 : 1, v/v), but the yield decreased to 82% (entry
22). More experiments indicated that by use of (NH4)2S2O8 and
TBHP as oxidant the yield of 3a decreased to 60% and 40%,
respectively (entries 23 and 24). Furthermore, oxidants such as
H2O2 and DDQ were ineffective in this reaction (entries 25 and
26). Without K2S2O8 as oxidant and Aliquat 336 as additive, the
CDC reaction could not proceed in CH3CN at 80 ꢀC aer 2 h
(entries 27 and 28). Thus, the optimal conditions for the metal-
Scheme 2 CDC reaction between aldehydes 1 and coumarins 2.
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Table 2 Cross-dehydrogenative coupling of aldehydes 1 with 3-
substituted coumarins 2a
Entry
1
Ar
2
X
R
3
Yieldb (%)
1c
2
3
4
5
1a
1b
1c
1d
1e
1f
4-CH3C6H4
4-iPrC6H4
3-CH3OC6H4
4-CH3OC6H4
4-ClC6H4
C6H5
2a
2a
2a
2a
2a
2a
2a
2b
2b
2c
2c
2c
2c
2c
2d
2d
2d
2e
2e
2e
2e
2a
2f
H
H
H
H
H
H
H
8-OMe
8-OMe
6-Br
6-Br
6-Br
6-Br
6-Br
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
OEt
OEt
OEt
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
87
80
85
88
79
84
73
71
75
77
73
77
75
72
82
79
82
83
80
78
77
NR
NR
NR
NR
NR
Scheme 4 Investigation on the reaction mechanism in the presence
of TEMPO.
6
7
1g
1f
2-Thienyl
C6H5
8d
9
were coupled with 3-acetylcoumarin, affording the corre-
sponding products 3a–f in 84–88% yields (entries 1–6). Also,
with methoxy, chlorine or bromine substituents on acetylcou-
marin reaction time was increased and the desired products 3h–
m and 3r–u were obtained in 71–83% yields aer 3 h (entries 8–
13 and 18–21). Heteroaromatic aldehyde thiophene-2-
carbaldehyde 1g was also employed in this CDC reaction, and
gave the desired products 3g and 3n in 73 and 72% yields,
respectively (entries 7 and 14). To extend the scope of our
studies, ethyl-2-oxo-2H-chromen-3-carboxylate 2d was also
successfully reacted under the same conditions with aromatic
aldehydes to yield the corresponding products 3o–q aer 3 h in
79–82% yields (entries 15–17). Furthermore, the reaction was
tested using 6-nitro-3-acetylcoumarin (2f), 4-nitrobenzaldehyde
(1i) as well as aliphatic aldehydes including n-butyraldehyde (1j)
and phenylacetaldehyde (1k) under the optimum conditions,
but, the acylation reaction did not take place (entry 22–26).
Based on the above observations and previous reports on the
acylating agents and the heterocycle involved in the reac-
tion,17b–d we proposed and outlined a plausible mechanism, as
shown in Scheme 3. Initially, Aliquat 336 A may be converted to
methyltrioctylammonium persulfate B by potassium perox-
odisulfate (K2S2O8). Heating would lead to sulfate radical C
which could abstract “H” atom of aldehyde 1 to generate the
corresponding acyl radical E. Next, the acyl radical E reacts with
coumarin 2 at b-position to give radical F. Finally, a hydrogen
atom may be removed from radical F by another sulfate radical
C to afford 4-aroylcoumarin 3.
1c
1a
1b
1c
1e
1g
1f
1a
1e
1f
1h
1b
1d
1i
1f
1a
1j
3-CH3OC6H4
4-CH3C6H4
4-iPrC6H4
3-CH3OC6H4
4-ClC6H4
2-Thienyl
C6H5
4-CH3C6H4
4-ClC6H4
C6H5
3-CH3C6H4
4-iPrC6H4
4-CH3OC6H4
4-O2NC6H4
C6H5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
H
H
6-Cl
6-Cl
6-Cl
6-Cl
H
6-NO2
6-NO2
H
3s
3t
3u
3v
3w
3x
3y
3z
4-CH3C6H4
n-C4H9
C6H5CH2
2f
2a
2a
1k
H
a
Reaction conditions: aldehyde (1, 1 mmol), coumarin (2, 0.5 mmol),
b
K2S2O8 (1.2 mmol), Aliquat 336 (30 mol%), CH3CN (2 mL). Isolated
yields (in respect to 2). Reaction time for entries 1–7: 2 h. Reaction
time for entries 8–26: 3 h.
c
d
free CDC acylation of 2a by 1a were determined: K2S2O8 (1.2
equiv.), Aliquat 336 (30 mol%), in acetonitrile at 80 C for 2 h.
ꢀ
With the optimized conditions available, we investigated the
substrate scope (Scheme 2 and Table 2). Aromatic aldehydes
with electron-donating alkyl or methoxy groups 1a–d and 1h
those having electron-withdrawing chlorine group 1e regardless
of their position(s) on the aryl ring as well as benzaldehyde 1f
To gather further insight into our novel K2S2O8/Aliquat 336
promoted acylation, the reaction between benzaldehyde 1f and
3-acetylcoumarin 2a was performed in the presence of 2,2,6,6-
tetramethylpiperidin-1-yl-oxyl (TEMPO) as a radical scavenger
(Scheme 4). Under this condition, 2,2,6,6-tetramethylpiperidino
benzoate 4 was isolated in 96% yield and the desired acylated
product 3f was not detected. Thus an acyl radical is involved in
the catalytic cycle of the transformation.
Conclusions
In summary, we have successfully disclosed an expedient and
economical approach for 4-aroylcoumarin derivatives without
using expensive transition-metal catalysts. In this paper,
K2S2O8/Aliquat 336 system has been reported as a ubiquitous
and inexpensive catalytic combination at moderate temperature
for the C4–H acylation of C3-substituted coumarins with alde-
hydes as acylating agents through oxidative cross-
dehydrogenative coupling reaction. This new protocol
Scheme 3 A putative reaction mechanism for the synthesis of 4-
aroylcoumarins via CDC reactions mediated by K2S2O8/Aliquat 336.
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Acknowledgements
This research was supported by the Research Council of
University of Tehran.
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110660 | RSC Adv., 2016, 6, 110656–110660
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