A one-pot domino C-H, C-C activation in coumarins: A fast track to 2,3-diaryl benzo[b]furans

Article abstract of DOI:10.1039/c5cc01427a

An approach to synthesize 2,3-diaryl benzo[b]furans using coumarins and aryl bromides is developed. This state-of-the-art strategy capitalizes on a palladium-catalyzed one-pot decarbonylative diarylation of coumarins, paving the way to achieve biologicall

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A One-Pot Domino C-H, C-C Activation in Coumarins:
Fast Track to 2,3-diaryl benzo[b]furans†
Cite this: DOI: 10.1039/x0xx00000x
Mehdi Khoobi, a Fatemeh Molaverdi, b Farnaz Jafarpour, b Masoumeh Abbasnia, b
Maciej Kubickic and Abbas Shafieea*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
high yields. It is noteworthy that 2,3ꢀdiayl benzo[b]furans was
previously reported to be prepared directly from benzofuran which
was prepared either through stepwise arylation14aꢀb or in just low13c
to moderate yields (the latter is conducted only with the aid of
cataCXium A as the ligand).14d
An approach to 2,3-diaryl benzo[b]furans using coumarins
and aryl bromides is developed. This state-of-the-art strategy
capitalizes on a palladium-catalyzed one-pot decarbonylative
diarylation of coumarins, paving the way to reach to
biologically interesting 2,3-diaryl benzo[b]furans.
By this protocol, one can eliminate the need to utilize benzofuran as
starting material to be diarylated later. Needless to say that coumarin
and its derivatives, compared to benzofurans, are both more
accessible and economical.
In the past decade, direct CꢀH activation bloomed as a ubiquitous
method to functionalize organic molecules.1 On the contrary, CꢀC
activation has only recently risen from obscurity to fame.2
Decarbonylation is among those reactions that perfectly exemplifies
this area of study. Thus far, some cases have been revealed in which
transition metalꢀcatalyzed decarbonylation is the center of attention.3
To begin with, an appropriate base along with an efficient ligand
seemed to be necessary for this reaction as was typical of Pdꢀ
catalyzed direct C–H arylation reactions using aryl bromide.15
Therefore, choosing coumarin (1a) and bromobenzene as the model
substrates, various conditions were explored. Our preliminary
experiments employing Pd(OAc)2, Cs2CO3 as the base and PPh3 as
the ligand led to only 30% of the desired product 3a (Table 1, entry
1). Several phosphines as well as bidentate nitrogen ligands were
screened to improve the yield (entries 2ꢀ5) among which 1,10ꢀ
phenanthroline was the best. Further optimizing reactions proved
Cs2CO3 and mesitylene to be the superior base and solvent,
respectively (entry 5). Also, among different palladium sources
(entries 12ꢀ15), Pd(OAc)2 seemed to be the most effective catalyst.
Furthermore, it turned out that no product was observed in the
absence of the catalyst. Finally our attempts to lower the reaction
time and temperature led to lower yields of the desired product.
To date, much interest has been focused on the efficient synthesis of
2,3ꢀdiaryl benzo[b]furans as privileged scaffolds of natural4 and
pharmaceutical products5 including Heck oxyarylation6 and
annulations of phenols with alkynes7 along with other methods
reported.5bꢀc,8 In this paper, the synthesis of 2,3ꢀdiaryl benzo[b]furans
through domino decarbonylative direct diarylation of coumarins
employing aryl bromides is demonstrated. Further to our previous
reports on arylation of coumarins9 as well as other papers disclosed
in this area,10 we embarked on a project to diarylate coumarin. To
this end, coumarin was exposed to excess aryl bromide under
appropriate conditions to achieve 3,4ꢀdiarylcoumarin. But much to
our surprise, these efforts led to a serendipitous discovery, i.e. an
extraordinary conversion of coumarin to diaryl benzo[b]furans
rendering this reaction alluring. The coumarin–benzofuran ring
contraction was previously reported to take place only via
irradiation11 or through a Perkin rearrangement via exposure of a 3ꢀ
halosubstituted coumarin to a strong base.12 Also, the conversion of
3ꢀphenylcoumarin to 2ꢀarylbenzofuran was reported through metal
catalysis.13 Here, in the procedure presented none of the foregoing
harsh conditions is needed. The most attractive feature of this
protocol is an unprecedented combined coumarin–benzofuran ring
contraction/direct benzofuran arylation which leads to construction
of privileged highly substituted benzofuran scaffolds in one process.
The process involves domino activation of three types of CꢀH(C)
bonds including coumarin C3ꢀH and CꢀC bonds as well as
benzofuran C3ꢀH bond in oneꢀpot and involves an unexpected
formation of highly hindered 2,3ꢀdiaryl benzo[b]furans in good to
We found that under the optimized reaction conditions, coumarin
(0.1 mmol), bromobenzene (2.5 equiv), Pd(OAc)2 (20 mol %), Phen.
(20 mol %) in mesitylene at 160 °C for 18 h, 2,3ꢀdiphenylbenzofuran
3a was formed in 76% isolated yield.
To survey the scope of the reaction, a variety of substituted
bromoarenes were employed under optimized conditions (Scheme
1). As shown in Scheme 1, aryl bromides bearing either neutral or
electronꢀdonating groups, such as methyl, ethyl, and methoxy, gave
the corresponding products in high yields (3c-3e). The reaction also
revealed good tolerance towards chloro groups, which provides a
synthetically useful handle for further functionalizations. Also,
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DOI: 10.1039/C5CC01427A
Table 1. Screening of Reaction Conditions for Domino C-C and Scheme 1. Scope of Cross-Coupling reaction between Coumarins
C-H Activation of Coumarinsa
and Halo Arenesa
yield
(%)
entry
[Pd]
L
base
solvent
1
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
PPh3
PCy3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
KOtꢀBu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
1,4ꢀDioxane
Dimethylether
CH3CN
30
28
25
25
76
20
5
3
Neocuproine
bpy
4
5
Phen
6
Phen
7
Phen
8
Phen
15
10
25
69
30
40
14
58
9
Phen
10
11
12
13
14
15
Phen
Phen
DMF
Phen
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Pd(dba)2
Pd(acac)2
Pd(PPh3)4
Phen
Phen
Phen
aAll reactions were run under the following conditions: Coumarin 1a (0.1
mmol, 1.0 equiv), bromobenzene 2a (2.5 equiv), Pd catalyst (20 mol %), base
(2.5 equiv), ligand (20 mol %), solvent (0.2 M), 160 °C, 18 h. Phen. = 1,10ꢀ
Phenanthroline, Neocuproine = 2,9ꢀDimethyl phenanthroline, DMAP =
Dimethylaminopyridine.
different coumarin derivatives including 6ꢀ methylꢀ and 7ꢀ methoxyꢀ
coumarins were exposed to the reaction conditions which led to the
desirable products in good to excellent yields. As a specimen, the
configuration of the compound 3e was confirmed by Xꢀray
crystallography (see ESI).
aAll reactions were run under the following conditions: Coumarin 1 (0.1
mmol, 1.0 equiv), aryl bromide 2 (2.5 equiv), Pd(OAc)2 (20 mol %), Cs2CO3
(2.5 equiv) 1,10ꢀphenanthroline (20%), were heated in a sealed tube at 160
°C for 18 h. bIodobenzene was used.
Also, the method worked well even with sterically hindered aryl
bromide, i.e. 1ꢀbromonaphthalene, enabling one to accommodate
two congested groups on adjacent carbons of benzofuran.
Methylthioꢀsubstituted 2,3ꢀdiaryl benzo[b]furans such as 3j and 3l
were also constructed in excellent yields. Unfortunately, no
satisfactory results were observed when either very electron deficient
aryl bromides or coumarins were employed (3o, 3p). It deserves also
to be noted that when iodobenzene was used in place of
bromobenzene, compound 3a was formed in a higher yield (86%).
Scheme 2. Bromopyridine Coupling with Coumarin
To expand the scope of the reaction, coumarin was exposed to a
bromo heteroarene, i.e. 2ꢀbromopyridine in which the reaction
proceeded smoothly and gave 2,3ꢀdi(2ꢀpyridyl)benzo[b]furan in 72%
yield (Scheme 2).
arylcoumarin (II), oxidative addition of Pd(0) to lactonic OꢀCO bond
of coumarin leads to the intermediate III.17 Then, the latter provides
2ꢀaryl benzofuran (V) via CO migration and reductive elimination.16
Finally, the desired 2,3ꢀdiaryl benzo[b]furan 3 is obtained via a
second arylation reaction.
Although the exact mechanism of this reaction is unclear, a plausible
mechanism is outlined in Scheme 3. As has been reported in
previous papers,16 palladium has the aptitude to be inserted into Oꢀ
CO bond of esters. Hence, it can be presumed that soon after the first
regioselective Cꢀ3 arylation of coumarin and formation of 3ꢀ
To corroborate the intervention of 3ꢀarylcoumarin prior to
benzofuran ring formation, the reaction of coumarin and
2 | J. Name., 2012, 00, 1-3
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Scheme 3. A Plausible Mechanism for Construction of 2,3-Diaryl
Benzo[b]furans.
ACKNOWLEDGMENT
This work was financially supported by Grants from the
Research Council of Tehran University of Medical Sciences.
Br
PdL2Br
H
PdL2Br
Notes and references
Department of Medicinal Chemistry, Faculty of Pharmacy and
O
O
a
Pharmaceutical Sciences Research Center, Tehran University of Medical
Sciences, 14176, Tehran, Iran.
b School of Chemistry, College of Science, University of Tehran, P.O. Box
PdL2Ar
Ar
H
PdL2Ar
O
O
14155ꢀ6455, Tehran, Iran
.
Ar
(VI)
O
c
O
Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b,
(I)
(V)
61ꢀ614 Poznań, Poland
.
Pd(0)
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/c000000x/
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CO
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bromobenzene was monitored and it was observed that 3ꢀ
phenylcoumarin was built up during the first 4 hours of the reaction.
h
2
i
In another effort, 3ꢀphenylcoumarin was reacted with bromobenzene
under the standard reaction conditions which resulted in 2,3ꢀ
diphenyl[b]benzofuran (Scheme 4). In this respect, some
unsymmetrical derivatives were synthesized via this route (3r, 3s, 3t,
3u). But when 4ꢀphenylcoumarin underwent the same reaction
conditions no product was formed.
j
k
l
m
.
,
,
Scheme 4. Utility of 3-Arylcoumarin as Substrate
n
a
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, 567 and references therein;
, 1062 and
1
(b
4
c
d
e
,
,
Furthermore, benzofuran itself was reacted with bromobenzene
under standard reaction conditions which furnished 3a in 86% yield.
But when coumarin was exposed under corresponding condition in
the absence of bromobenzene, no product was formed.
In conclusion, we have introduced a simple route to 2,3ꢀdiaryl
benzo[b]furans via a palladiumꢀcatalyzed domino coumarin Cꢀ
g
(h
,
i
j
H/CꢀC
and
benzofuran
CꢀH
activation.
Ordered
,
arylation/decarbonylation/arylation reactions render coumarins
as convenient surrogates for the synthesis of diaryl
benzo[b]furans. What makes this reaction stand out is that this
threeꢀbondꢀforming reaction takes place oneꢀpot. Also,
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coumarins is viable not through irradiation but a simple Pdꢀ
catalyzed CꢀH activation reaction.
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