A transition-metal-free fast track to flavones and 3-arylcoumarins

Article abstract of DOI:10.1039/c7cc02107k

A highly regioselective and transition-metal free one-pot arylation of chromenones with arylboronic acids has been achieved employing K₂S₂O₈. The procedure consists of a sequence of some reactions including an arylation/decarboxylation cascade and proceeds well in aqueous media to afford biologically interesting flavones and 3-arylcoumarins. This method exhibited excellent selectivity and functional group tolerance under mild conditions. The reaction also showed perfect efficacy for the preparation of styryl coumarins.

Full text of DOI:10.1039/c7cc02107k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
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DOI: 10.1039/C7CC02107K
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Transition-Metal-Free Fast Track to Flavones and 3-Arylcoumarins
a,b
a,c
b
b
Mostafa Golshani, Mehdi Khoobi, * Nafiseh Jalalimanesh, Farnaz Jafarpour * and Alireza
Ariafard
d,e
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Highly regioselective and transition-metal free one-pot arylation of also been reported utilizing excess of Ag salts in strong acids.3h
chromenones with arylboronic acids has been achieved employing Notwithstanding the advances attained, most of the developed
K S O . The procedure consists of a sequence of some reactions approaches require expensive metal catalysts which are
2 2 8
including arylation/decarboxylation cascade and proceeds well in sensitive to oxygen and moisture and require the use of more
aqueous media to afford biologically interesting flavones and 3-aryl expensive supporting ligands and costly additives. Also, most of
coumarins. This method exhibited excellent selectivity and TMs are toxic, and removal of trace amounts of their impurities
functional group tolerance under mild conditions. The reaction also from final products, especially in pharmaceutical industry, is
showed perfect efficacy for the preparation of styryl coumarin.
costly and challenging. These difficulties have prompted
chemists to employ alternative pathways to construct C−C
bonds under TM-free conditions. This field has attracted
increasing attention in recent years and a plethora of reactions
released have now been conducted under these conditions.4
Natural and synthetic compounds featuring a 2-arylchromone
(flavone) scaffold, are proved to have a positive effect on
diseases related to oxidative stress, inflammatory and
neurodegenerative disorders, diabetes mellitus, peptic ulcers,
As part of our continuing interest in functionalization of
1
cancer and so on (Scheme 1). They also exhibit anti-microbial,
5
heterocycles, and to establish a complementary route for
anti-HIV, anti-oxidant, anti-platelet and vasorelaxing
construction of flavone-based prominent lead compounds, we
envisioned TM-free arylation of chromone-2-carboxylic acids
with arylboronic acids. While a direct arylation of chromones
towards flavones excluding metals remains elusive, it would be
one of the greenest and most powerful procedures for quick
synthesis of a large library of natural and unnatural flavones.
This approach may exhibit interesting features as avoidance of
expensive transition-metals, mild conditions and generation of
environmentally friendly side products.
1
properties. The widespread use of flavones in pharmaceutical
as well as food industry have ensured a continuous demand for
direct and efficient synthesis of these structural motifs.
Although robust methods including chalcone and diketone
2
route have been established in this regard, significant effort
have been devoted toward more feasible transition metal (TM)-
catalyzed direct instalment of aryls onto an extant enone
nucleus as an alternative synthetic route with less functional
3
group limitations. Thus far, regioselective direct arylation of
We commenced our study using chromone-2-carboxylic aid (1a
and p-tolylboronic acid (2a) as model substrates to verify our
hypothesis. A brief solvent survey in presence of K as a
)
chromones via TM-catalyzed coupling reactions using
3c-d
3b,3f-g
haloarenes and boronic acids
are achieved. Pd-catalyzed
2 2 8
S O
approaches to functionalization of chromones with arenes have
a
Nanobiomaterials group, Pharmaceutical Sciences Research Center, Tehran
University of Medical Sciences, Tehran 141761411, Iran. E-mail:
mkhoobi@tums.ac.ir, Mehdi.khoobi@gmail.com
b
School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-
6
455, Tehran, Iran. E-mail: jafarpur@khayam.ut.ac.ir
c
Department of Pharmaceutical Biomaterials and Medical Biomaterials Research
Center, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran.
d
School of Physical Sciences, University of Tasmania, Private Bag 75, Hobart,
Tasmania 7001, Australia. E-mail: Alireza.Ariafard@utas.edu.au
e
Department of Chemistry, Faculty of Science, Central Tehran Branch, Islamic
Azad University, Shahrak Gharb, Tehran, Iran.
Electronic Supplementary Information (ESI) available: Experimental procedures and
characterization data. See DOI: 10.1039/x0xx00000x
Scheme 1. Biologically interesting natural and synthetic Flavones
.
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J. Name., 2013, 00, 1-3 | 1
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Table 1 Optimization of reaction conditions for arylation of chromone-2-carboxylic acida
Table 2 Scope of the regioselective arylation of chromones towards flavonesa
View Article Online
Entry Solvent
Oxidant (equiv)
T (°C)
100
100
100
100
100
100
100
100
120
Yield (%)b
DOI: 10.1039/C7CC02107K
1
2
3
4
5
6
7
8
9
DMSO
K
K
K
K
K
2
2
2
2
2
S
S
S
S
S
2
2
2
2
2
O
O
O
O
O
8
8
8
8
8
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
15
0
68
10
75
82
trace
61
c
DCE
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O
O/DMSO 1:1
O/DCE (1:1)
O/DCE (4:1)
O/DCE (8:2)
O/DCE (8:2)
O/DCE (4:1)
c
K
2
S
2
O
8
H
2
O
2
(2.0)
d
BP (2.0)
K
2
S
2
O
8
(2.0)
81
a
Reaction conditions: 1a (0.5 mmol), 2a (1 equiv), oxidant (2 equiv), solvent (3 mL),
00 °C, 2 h; Isolated yield. DCE = 1,2-Dichloroethane. BP = Benzoyl peroxide.
b
c
d
1
low-priced, commercially available and nontoxic radical
initiator, approved H O-DCE interface as the most effective one
Table 1, entries 1-5). Importantly, a confident relationship
2
(
between the solvent volume ratio and the reaction earning was
observed where increasing the water content markedly
enhanced the yield (entry 6). An increase or decrease in amount
of the initiator had no beneficial effect. Moreover, screening the
2 2 8
reaction conditions with respect to oxidants proved K S O as
the most effective one (entries 7-8). To our delight, under the
optimal conditions chromone 1a participated well in a TM-free
arylation/decarboxylation cascade to afford flavone 3a in 82%
yield. This unprecedented protocol may afford a viable
alternative route to flavones with an emphasis on avoiding toxic
metals and harsh conditions.
Next we sought to explore the scope of the reaction for
construction of various structurally diverse flavones employing
chromones and arylboronic acids with different electronic and
steric circumstances. As it is exposed in Table 2, various
phenylboronic acids comprising alkyl, methoxy and halo groups
aAll reactions were run under the optimized reaction conditions.
frameworks such as coumarins. The omnipresence of
arylcoumarin scaffolds in pharmaceutical and materials
9
sciences ensures a constant demand for their efficient
synthesis. Compared to classical strategies, TM-catalyzed
approaches for direct instalment of aryls on coumarins are the
most prevalent and desired ones conveyed for construction of
10
privileged arylcoumarin scaffolds. Thus far, we also have
settled some progresses in TM-catalyzed C-3 arylation of
were compatible with this reaction (3a-3e). Furthermore,
sterically demanding naphthylboronic acid with an ortho
substitution pattern proved as a suitable coupling partner for
arylation of chromone (3f). A substrate scope with respect to
chromones was considered next where, different alkyl, hydroxyl
and halo groups at different positions were well tolerated. It is
noteworthy that methyl substitution at different positions of
1
0e
10d
coumarins with arenes,
arylsulfonyl chlorides
and halo
1
0h
arenes in recent years. Furthermore, we also have conveyed
Pd-catalyzed arylation of coumarins by means of arylboronic
acids where, the regioselectivity was nonetheless in bias of β–
3f, 10g
arylation.
However, when we employed the present
methodology for arylation of coumarins avoiding any TMs, a
fascinating switch in regioselectivity occurred and aryls were
remarkably installed at α-position of the enone ring (Scheme 2).
With this interesting outcome, we envisioned expanding the
scope of the reaction to include more arylboronic acids as well
as coumarins with different electronic natures (Table 3).
Coumarin-3-carboxylic acids with either electron-donating or -
the flavone nucleus retains the potency of the compound as
6
gastroprotective activity (3g
-
3i). Bromo-substituted chromone
joined the cascade reaction and resulted representative halo
substituted flavones (3j 3l) as good partners for further
-
functionalizations. Flavones with fluoro group at ring A are
proved to be selective MAO-B inhibitors which can be easily
7a
collected by the present procedure (3m). Gratifyingly,
hydroxy-substituted chromone participated well in this
arylation process to afford flavone 3n in 63% yield which can be
withdrawing groups furnished the desired products (5a
-5r) in
moderate to good yields with exquisite C3-selectivity. Nicely,
8
the drug of interest as novel antihypertensive agent.
1
Satisfyingly, chrysin 3o with antioxidant activities was also
constructed successfully in moderate yield via arylation of 5,7-
dihydroxy chromone applying the current protocol. Lastly,
thiophene-2-boronic acid participated well in this procedure, to
afford heteroarylated chromone 3p which is confirmed as a
selective inhibitor towards MAO-A.7
b
These results motivated us to expand the scope of this green Scheme 2. TM versus TM-free approaches toward arylation of coumarins.
approach for direct arylation of additional chromenone
2
| J. Name., 2012, 00, 1-3
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Table 3 Scope of the regioselective arylation of coumarins with aryl boronic acida
the arylation proceeded reasonably with electron deficient
coumarins containing nitro group which are hardly conveyed in
View Article Online
DOI: 10.1039/C7CC02107K
TM-catalyzed approaches (5g
reaction worked very well even with sensitive functionalities
such as a hydroxyl group (5j 5k). The significant feature that
-5i). It is noteworthy that the
-
eliminates the requirement of protection of hydroxyl groups
makes this procedure practicable for preparation of
hydroxylated 3-aryl coumarins having diverse biological
activities. Bromo-substituted substrates were satisfactorily
converted to 3-arylcoumarins in yields exceeding 75% which are
synthetically useful intermediates for subsequent modifications
(
5f
,
5k
-
5l). Arylation of benzocoumarins also proceeded
5q).
smoothly to afford the desired products in high yields (5p
-
The reaction also showed perfect efficacy for preparation of
styryl coumarin 5s which are valuable backbones with increased
1
1
spectroscopic bands intensity.
To get insight into the reaction mechanism, a plausible radical
reaction track (confirmed by reaction suppression in presence
of TEMPO) was explored by DFT calculations (B3LYP/6-31+G (d)
method, see SI). First, we investigated on the addition of aryl
radical (generated from arylboronic acid through thermal
decomposition of peroxydisulfate) on two possible C-2 and C-3
positions of chromone-2-carboxylic acid (Scheme 3).
Remarkably, the data showed that transition state related to
phenyl radical addition to chromone ring at C-3 (TS
energy than addition to C-2 (TS ) and later, intermediate
remains 7.6 kcal/mol more stable than . Subsequent
decarboxylation involves sulfate radical anion (SO •–) mediated
H-atom abstraction from carboxylic acid intermediate . Next a
single electron transfer from sulfate to C-2 occurs followed by
decarboxylation via TS . This process leaves one electron on C-
and generates a triplet carbene intermediate 10, which in turn
converts to singlet state 11, and subsequent phenyl group
migration from C-3 to C-2 via TS produces flavone 3b. Based on
2
) has a lower
1
7
6
4
aAll reactions were run under the optimized reaction conditions.
8
In conclusion, we have described an unprecedented transition-
metal-free synthesis of flavones and 3-aryl coumarins in one-
pot from readily available chromenones and boronic acids.
3
2
2 2 8
K S O in an aqueous media proved to be enough to ensue two
4
steps of the reaction cascade in a short reaction time. Providing
a practical and green condition, we hope this protocol may find
its way as an alternative approach for construction of flavones
and aryl coumarins in small and large-scales.
these results, DFT calculations point to an unprecedented
regioselective C-3 arylation of chromone-2-carboxylic acid and
subsequent C-3 to C-2 aryl migration mechanism. Similar result
was obtained for arylation of coumarins (see SI).
Scheme 3. Free energy (kcal/mol) profiles for the proposed mechanism of TM-free arylation of chromone-2-carboxylic acid. Calculated at the B3LYP/6-31+G (d) basis.
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