Cobalt(III)-Catalyzed Construction of Benzofurans, Benzofuranones and One-Pot Orthogonal C?H Functionalizations to Access Polysubstituted Benzofurans

Article abstract of DOI:10.1002/adsc.201800298

Benzofuran and benzofuranone derivatives have been synthesized through exclusive 5-exo-dig intramolecular hydroarylation using the amide-directed, cost-effective, high-valent Cp*Co^III-catalytic system. Challenging one-pot, orthogonal C?H functionalizations using two different electrophiles are also reported to afford polysubstituted benzofurans. Several valuable functional group interconversions along with removal of the amide directing group provide a route to access several diversely functionalized benzofurans. The mechanistic study suggests a reversible cobaltation step is operative here. (Figure presented.).

Full text of DOI:10.1002/adsc.201800298

Title: Cobalt(III)-Catalyzed Construction of Benzofurans,
Benzofuranones and One-Pot Orthogonal C−H Functionalizations
to Access Polysubstituted Benzofurans
Authors: Sourav Sekhar Bera, Suvankar Debbarma, Sripati Jana, and
Modhu Maji
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800298
Link to VoR: http://dx.doi.org/10.1002/adsc.201800298

10.1002/adsc.201800298
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Cobalt(III)-Catalyzed Construction of Benzofurans,
Benzofuranones and One-Pot Orthogonal C−H
Functionalizations to Access Polysubstituted Benzofurans
Sourav Sekhar Bera, Suvankar Debbarma, Sripati Jana and Modhu Sudan Maji*
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur – 721302, India
E-mail: msm@chem.iitkgp.ernet.in
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract: Benzofuran and benzofuranone derivatives
have been synthesized through exclusive 5-exo-dig
intramolecular hydroarylation using the amide-directed,
cost-effective, high-valent Cp*CoIII-catalytic system.
Challenging one-pot, orthogonal C−H functionalizations
using two different electrophiles are also reported to
afford polysubstituted benzofurans. Several valuable
functional group interconversions along with removal of
the amide directing group provide a route to access
several diversely functionalized benzofurans. The
mechanistic study suggests a reversible cobaltation step
is operative here.
Keywords: benzofuran • benzofuranone • cobalt •
intramolecular • orthogonal C−H activation
Figure 1. Bioactive benzofuran and benzofuranone
derivatives.
However, intramolecular C−H activation is not
Benzofuran and benzofuranone are privileged
always sterically facile. The possibility of an
heterocycle motifs present in many natural products,
intermolecular reaction renders the reaction profile
biologically active compounds, pharmaceuticals and
more complicated. Furthermore, in one-pot, two-fold,
drugs (Figure 1).[1,2] They are also a key component
orthogonal C−H functionalizations, due to electronic-,
found in organic materials possessing hole-
steric- or coordinating-effect of mono-functionalized
transferring[3a] and photosensitizing properties.[3b]
product, the situation becomes more challenging
along with the selectivity issue.[8] Generally, directing
group and catalytic systems are too specific to an
electrophile for selective bond formation, which
indicates another difficulty in two-fold orthogonal
C−H functionalization.
Owing to their diverse applications, the synthesis of
this heterocyclic-backbone always lures chemists’
attention. As a result, several synthetically feasible
strategies have been developed for the construction of
the benzofuran core such as transition metal-
catalyzed annulation of alkynyl-substituted phenols,[4]
direct functionalization of phenols with alkynes and
other substrates,[5,6] or one-pot cross-coupling and
subsequent annulation of ortho-halo phenols with
alkynes.[7] Though a number of synthetic routes have
been reported, the development of conceptually
different synthetic strategies enabling synthesis of
diversely substituted benzofurans are still in great
demand. In this context, transition metal-catalyzed,
directed intramolecular C−H bond functionalization
approach can be very much effective. Additionally, a
directing group-promoted, one-pot, orthogonal C−H
functionalizations can lead to the rapid access to
polysubstituted benzofuran derivatives.
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10.1002/adsc.201800298
Advanced Synthesis & Catalysis
Scheme 1. Benzofuran and benzofuranone synthesis and
one-pot, two-fold, orthogonal C−H functionalizations.
temperature, the yield of 3a increased slightly (entry
2). In the absence of second additive, the product 3a
formed preferentially (entries 3-4). The incorporation
of 10 mol% Cu(OAc)2·H2O, largely improved the
desired ratio, providing 58% of 3a as the major
product (entry 5). The best result was obtained in the
presence of 20 mol% of Cu(OAc)2·H2O (3a, 72%).
Surprisingly, with 40 mol% Cu(OAc)2·H2O, the
selectivity was completely reversed, yielding 3a′ as
Intramolecular C−H bond functionalization often
proves to be very advantageous for manufacturing
heterocyclic motifs with fused ring system by
stitching
a
molecular unit with desirable
regioselectivity.[9] In this context, various research
groups have reported the Rh, Ru, Ir, and Co-catalyzed, the major product (entry 6 vs 7). Though on changing
directed intramolecular hydroarylation of olefin-
tethered arenes to synthesize several carbo and
heterocycles.[10a-g] The Park and Glorius groups[11a,b]
also described a Rh and Co-catalyzed intramolecular
counter cation from Na to K alters the selectivity but
the complete selectivity towards 3a was not obtained
(entries 8-9). CsOAc was also not a suitable additive
for this reaction (entry 10).
hydroarylation
reaction
of
alkyne-tethered
hydroxamic esters. Recently, the Shibata group
demonstrated the C−H alkenylation on N-(pent-4-
ynyl)indole using a low valent Rh- and Ir-catalyst.[11c]
Herein, inspired by the recent development of high
valent cobalt catalysis[12] which was pioneered by
Kanai and Matsunaga et al. we report a novel
approach to construct benzofuran and benzofuranone
cores through cost-effective, high-valent, cobalt-
catalyzed, amide-directed, intramolecular C−H
activation. This method offers complete 5-exo-dig
cyclization followed by aromatization in a one-pot
fashion (Scheme 1). 5-Substituted benzofurans are
also synthesized through one-pot, two-fold,
unsymmetrical C−H functionalizations by employing
a second electrophile.
Table 1. Optimization of the reaction conditions.[a]
Temp Time
Entry Additive II (mol%)
Yield [%][b]
Scheme 2. Scope of 3-substituted benzofurans. 1a-1n (0.2
mmol) and 2.5 mL DCE were used. All yields are isolated
yields.
[oC]
[h]
3a 3a′
1
2
3
4
5
6
7
8
9
Cu(OAc)2·H2O (100)
Cu(OAc)2·H2O (100)
none
100
120
120
120
120
120
120
120
120
120
6
6
6
12
12
12
12
12
12
12
4
8
57
60
36
28
12
4
63
68
36
35
28
32
58
72
8
4
32
28
Having this optimized CoIII-catalytic system in
hand (entry 6, Table 1), we examined the scope of
this reaction by varying first the alkyne moiety
(Scheme 2). Electron-withdrawing functional groups
such as CO2Me, NO2, and Cl at the para-position of
the phenyl ring are tolerated and furnished 3b-3d in
74-82% yields. Alkynes bearing electron-donating
substituents at the phenyl ring are also suitable
substrates (3e-3f, 62-63%). Likewise, differently
meta- and ortho-substituted phenyl rings attached to
the alkyne also responded well under the reaction
conditions to provide products 3g-3j in 68-78%
yields. Sterically demanding 1-naphthyl alkyne also
proceeded in the reaction to provide 3k in 60% yield.
Gratifyingly, alkynes bearing n-hexyl and a benzyl-
protected ether also afforded benzofurans 3l-3m in
52-74% yields. Interestingly, in situ trimethylsilyl-
deprotected product 3n was isolated in 70% yield
from the corresponding TMS-substituted alkyne
substrate. However, our attempt to obtain 3n by using
none
Cu(OAc)2·H2O (10)
Cu(OAc)2·H2O (20)
Cu(OAc)2·H2O (40)
NaOAc (20)
KOAc (20)
10 CsOAc (20)
[a]1a (0.2 mmol) and 2.5 mL DCE were used. [b]Isolated
yields.
We started the optimization of the reaction
conditions by taking the substrate 1a in the presence
of 10 mol% Cp*Co(CO)I2 as a catalyst and 30 mol%
AgSbF6 as a silver additive (Table 1). Initially,
conducting the reaction by addition of 1.0 equiv of
Cu(OAc)2·H2O as
a second additive in 1,2-
o
dichloroethane (DCE) solvent at 100 C provided the
exocyclic product 3a′ as a major product in 57%
yield (Table 1, entry 1). On increasing the
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10.1002/adsc.201800298
Advanced Synthesis & Catalysis
the corresponding terminal alkyne did not provide the
desired product under the optimized conditions.
The synthesis of 2-substituted benzofurans 3o-3p
were achieved in 72-74% yields by incorporating
methyl and cyclohexyl substitutions at the
appropriate position of the alkyne moiety (Scheme 3).
We subsequently investigated the effect of different
substitution on the aryl ring attached to the amide
directing group. All the substitutions were well
tolerated to produce 3q-3s in good to excellent yields.
On increasing the alkyne chain length, the exo-attack
delivered a chroman moiety 3t in 56% yield. In the
case of amide 1u, one out of the four possible C−H
bonds reacted selectively to provide benzofuran
derivative 3u in 48% yield.
reaction provided similar results, albeit with longer
reaction time (8 h). Aryl or alkyl-substituted alkynes
are fully compatible and delivered benzofuranones
4a-4c in good to excellent yields (76-92%).
Subsequently, the incorporation of various functional
groups such as methoxy, nitro, ester, and bromide on
the aryl ring are well tolerated under the reaction
conditions
and
furnished
polysubstituted
benzofuranone derivatives 4d-4h in 66 to 86% yields.
The geometry of the hydroarylated products 3a′, 3t,
and 4a-4h were assigned in analogy to our previous
work.[14]
Scheme 3. Synthesis of polysubstituted benzofurans. 1o-
1u (0.2 mmol) and 2.5 mL DCE were used. All yields are
isolated yields.
Scheme 5. One-pot, orthogonal, two-fold C−H
functionalizations.
Although, single directing group promoted two-
fold C−H activations are well documented, one-pot,
orthogonal C−H functionalizations are scarce.[15] In
our continuous efforts to develop one-pot sequential
catalytic methods,[15a-b] herein amide-directed two-
fold orthogonal C−H functionalizations are reported
by employing two different electrophiles. The major
challenge to execute this method is to get the desired
regioselectivity in our product. In the presence of
starting material 1a and diphenyl acetylene 5, one-pot
unsymmetrical
intra-
and
intermolecular
hydroarylations were achieved, and C5-alkenylated
benzofuran scaffold 6 was isolated in 54% yield
(Scheme 5a).[14] The excellent regioselectivity of the
reaction was supported by the fact that no
intermolecular hydroarylated product at the 2-
position of 1a was detected. On introducing allylating
agent 7 as a coupling partner, intramolecular
Scheme 4. Scope of benzofuranones. 2a-2h (0.2 mmol)
and 2.5 mL DCE were used. All yields are isolated yields.
hydroarylation
and
allylation
occurred
simultaneously to afford C5-allylated benzofuran 8 in
42% yield (Scheme 5b).[16a-c] In the presence of an
α,β-unsaturated ketone 9, product 10 is obtained in
47% yield via simultaneous intramolecular
hydroarylation and C–H alkylation reactions (Scheme
Considering the immense importance of
benzofuranones,[13] several benzofuranone derivatives
were synthesized by extending our method (Scheme
4). This method afforded a rapid generation of
benzofuranone derivatives within 20 minutes.
Interestingly, in the absence of acetate source, the
5c).[16d]
This
one-pot
orthogonal
C−H
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10.1002/adsc.201800298
Advanced Synthesis & Catalysis
functionalizations strategy was also applied to the
intramolecular hydroarylation and C−H amidation
reaction to furnish C5-aminated benzofuran 12 in
38% yield (Scheme 5d).[12i] Here the amidation
reaction was conducted after completing the
intramolecular-hydroarylation reaction in a one-pot
sequential fashion as the amidating agent 11 couldn’t
survive for sufficient time under the reaction
o
conditions in the presence of Cu(OAc)2 at 120 C.
Our attempt to conduct both reactions together
resulted in a complex reaction mixture.
Scheme 7. Mechanistic study.
To examine the potential utility and practicality of
our method, a gram scale synthesis of benzofuran
derivative 3a was performed. Under the standard
conditions, 0.95 g of 3a was successfully synthesized
from 1.53 g of starting material (Scheme 6a). Simple
amide 13 is synthesized by deprotection of the
tertiary amide of 3a using scandium triflate as a
Lewis acid catalyst,[17a] which lays a platform for
On performing the competition experiment using
electronically different benzamides tethered alkyne 2f
and 2g revealed that electron-rich benzamides are
preferentially converted to 4g with almost a 1.3:1
ratio, which indicated a base-assisted intermolecular
electrophilic substitution reaction (BIES) type
mechanism (Scheme 7a).[19] Incorporation of
deuterium in the presence of CD3CO2D at the C5-
position of benzofuran suggests the reversible cobalt
incorporation at the ortho-position (Scheme 7b). As
the presence of TEMPO did not inhibit the formation
of the product, thus excluding the involvement of any
stable radical intermediate (Scheme 7c).
accessing
several
4-substituted
benzofuran
derivatives. Dihydrobenzofurans are very important
structural motif found in many biologically active and
pharmaceutically important molecules such as
ramelteon, lithospermic acid etc.[10a,17b] In the
presence of palladium-charcoal and hydrogen,
dihydrobenzofuran derivative 14 was readily
synthesized from 3a′ (Scheme 6b). On treatment of
3a′ with meta-chloroperbenzoic acid, benzofuryl
alcohol 15 was prepared, which can be used as a
dienophile for [4+3] cycloaddition to afford fused
5,7,6-tricyclic skeleton present in the various natural
products.[17c] The synthesis of benzene derivatives
bearing several unique substituents are of immense
importance and is a challenging synthetic task.[18a]
Simple hydrolysis of compound 5a provided α,β-
unsaturated carboxylic acid 16, a potent structural
analogue
for
various
biologically
active
route to
molecules.[18b] This also provides
a
synthesize polysubstituted benzene derivatives
having unique substitution pattern. Compound 4a is
also a very effective Michael acceptor and reacted
with sodium diethyl malonate to produce 17 in 73%
yield (Scheme 6c).
Scheme 8. Proposed mechanism.
On the basis of the above observations and the
previous literature precedents,[14]
a
plausible
mechanism is proposed (Scheme 8). Initially,
treatment of a cobalt catalyst with AgSbF6 and
.
Cu(OAc)2 H2O led to the formation of reactive
cobalt(III)-species A, which upon reaction with 1a
affords the cyclic cobalt species B through acetate
assisted
directed
C−H
functionalization.
Subsequently, the cobalt in the complex
B
experiences chelation from the alkyne present in the
molecule and generates an intermediate C.
Subsequent migratory insertion of intermediate C,
delivers a seven-membered metallocycle D, which
upon protonolysis regenerates the active catalyst A
along with the product 3aʹ. Aromatisation of 3aʹ
Scheme 6. Gram scale synthesis of 3a and application of
the hydroarylated products.
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10.1002/adsc.201800298
Advanced Synthesis & Catalysis
5-Allyl-3-benzyl-N-(tert-butyl)benzofuran-4-carboxamide
furnishes 3a which can undergo a second C−H
functionalization. In another possibility, in presence
of the second electrophile, the intermediate E
proceeds for the second C−H functionalization.
(8):
N-(tert-butyl)-3-((3-phenylprop-2-yn-1-yl)oxy)
benzamide 1a (61.4 mg, 0.2 mmol, 1.0 equiv) and allyl
carbonate 7 (46.4 mg, 2 equiv) were taken in a 12.0 mL
screw-capped sealed tube and 2.5 mL of 1,2-
dichloroethane was added to the mixture. Then catalyst
Cp*Co(CO)I2 (19 mg, 0.04 mmol, 20 mol%),
Cu(OAc)2·H2O (10.0 mg, 0.05 mmol, 25 mol%) and
AgSbF6 (34.3 mg, 0.1 mmol, 50 mol%) were added
successively to the reaction mixture. The resultant reaction
mixture was allowed to stir at 120 °C for 12 h. After
completion of the reaction as indicated by TLC, the crude
product was directly purified by silica gel column
chromatography using petroleum ether/ ethyl acetate as
eluent to give 8 as a white solid (29.3 mg, 42%).
Compared
to
the
intermolecular
C−H
functionalization, intramolecular funtionalization was
much faster, and we observed that aromatization of
3a′ to furnish 3a required longer reaction time.
In summary, we have developed a novel strategy
for the synthesis of benzofuran and benzofuranone
derivatives through high-valent Co(III)-catalyzed
directed C−H bond functionalization. The reaction
exhibits broad scope as differently substituted
benzofurans and benzofuranones were synthesized.
The single directing group promoted, one-pot,
3-Benzyl-N-(tert-butyl)-5-(3-oxo-3-
phenylpropyl)benzofuran-4-carboxamide (10): N-(tert-
butyl)-3-((3-phenylprop-2-yn-1-yl)oxy)benzamide 1a (61.4
mg, 0.2 mmol, 1.0 equiv) and phenyl vinyl ketone 9 (52.8
mg, 2 equiv) were taken in a 12.0 mL screw-capped sealed
tube and 2.5 mL of 1,2-dichloroethane was added to the
mixture. Then catalyst Cp*Co(CO)I2 (19 mg, 0.04 mmol,
20 mol%), Cu(OAc)2·H2O (8.0 mg, 0.04 mmol, 20 mol%)
and AgSbF6 (34.3 mg, 0.1 mmol, 50.0 mol%) were added
successively to the reaction mixture. The resultant reaction
mixture was allowed to stir at 120 °C for 12 h. After
completion of the reaction as indicated by TLC, the crude
product was directly purified by silica gel column
chromatography using petroleum ether/ ethyl acetate as
eluent to afford 10 as a major product (40.8 mg, 47%).
sequential,
two-fold
orthogonal
C−H
functionalization have also been demonstrated. This
efficient method can easily be scaled up to grams,
and is very much pragmatic for the preparation of
various synthetically valuable intermediates.
Experimental Section
Procedure for benzofuran synthesis: N-(tert-Butyl)-3-
((3-phenylprop-2-yn-1-yl)oxy)benzamide 1 (0.2 mmol, 1.0
equiv) was taken in a 12.0 mL screw-capped sealed tube
and 2.5 mL of 1,2-dichloroethane was added. Then catalyst
Cp*Co(CO)I2 (9.5 mg, 0.02 mmol, 10.0 mol%),
Cu(OAc)2·H2O (8.0 mg, 0.04 mmol, 20.0 mol%) and
AgSbF6 (20.6 mg, 0.06 mmol, 30.0 mol%) were added
successively to the reaction mixture. The resultant reaction
mixture was allowed to stir at 120 °C for 12 h. After
completion of the reaction as indicated by TLC, the crude
product was directly purified by silica gel column
chromatography using petroleum ether/ ethyl acetate as
eluent to obtain products 3-4.
5-Benzamido-3-benzyl-N-(tert-butyl)benzofuran-4-
carboxamide (12): N-(tert-butyl)-3-((3-phenylprop-2-yn-1-
yl)oxy)benzamide 1a (30.7 mg, 0.1 mmol, 1.0 equiv) was
taken in a 12.0 mL screw-capped sealed tube and 1.5 mL
of 1,2-dichloroethane was added. Then catalyst
Cp*Co(CO)I2 (4.8 mg, 0.01 mmol, 10 mol%),
Cu(OAc)2·H2O (4.0 mg, 0.02 mmol, 20 mol%) and
AgSbF6 (10.3 mg, 0.03 mmol, 30 mol%) were added
successively to the reaction mixture. The resultant reaction
mixture was allowed to stir at 120 °C for 12 h. After that,
3-phenyl-1,4,2-dioxazol-5-one 11 (18 mg, 1.1 mmol, 1.1
equiv), [Cp*CoCl2]2 (1.0 mg, 0.002 mmol, 2.0 mol%),
sodium acetate (0.8 mg, 0.001 mmol, 10 mol%) were
added to the mixture and allowed to stir at 80 °C for 15 h.
After completion of the reaction, the crude product was
directly purified by silica gel column chromatography
using petroleum ether/ ethyl acetate as eluent to give 12 as
a white solid (16.0 mg, 38%).
Procedure for benzofuranone synthesis: 3-(tert-
Butylcarbamoyl)phenyl 3-phenylpropiolate 2 (0.2 mmol,
1.0 equiv) was taken in a 12.0 mL screw-capped sealed
tube and 2.5 mL of 1,2-dichloroethane was added. Then
catalyst Cp*Co(CO)I2 (9.5 mg, 0.02 mmol, 10.0 mol%),
Cu(OAc)2·H2O (4.0 mg, 0.02 mmol, 10.0 mol%) and
AgSbF6 (20.6 mg, 0.06 mmol, 30.0 mol%) were added
successively to the reaction mixture. The resultant reaction
mixture was allowed to stir at 100 °C for 20 min. After
completion of the reaction as indicated by TLC, the crude
product was directly purified by silica gel column
chromatography using petroleum ether/ ethyl acetate as
eluent to obtain pure products 5.
Acknowledgements
M.S.M. gratefully acknowledges SERB, Department of Science
and Technology (DST), New Delhi, India (Sanction No.
EMR/2015/ 000994), and IIT Kharagpur (ISIRD grant) for
funding. We thank DST (Sanction No. SR/FST/CSII-026/2013) for
500 MHz NMR facility at IIT Kharagpur. S.S.B. thanks CSIR
India and S.D. thanks IIT Kharagpur for fellowship.
Procedure for one-pot, orthogonal, two-fold C−H
functionalizations:
(E)-3-Benzyl-N-(tert-butyl)-5-(1,2
diphenylvinyl)benzofuran-4-carboxamide (6): N-(tert-
butyl)-3-((3-phenylprop-2-yn-1-yl)oxy)benzamide 1a (61.4 References
mg, 0.2 mmol, 1.0 equiv) and diphenyl acetylene 5 (46.3
mg, 1.3 equiv) were taken in a 12.0 mL screw-capped
[1] H. Khanam, S. Uzzaman, Eur. J. Med. Chem. 2015, 97,
sealed tube and 2.5 mL of 1,2-dichloroethane was added to
the mixture. Then catalyst Cp*Co(CO)I2 (19 mg, 0.04
mmol, 20 mol%), Cu(OAc)2·H2O (8.0 mg, 0.04 mmol, 20
mol%) and AgSbF6 (34.3 mg, 0.1 mmol, 50.0 mol%) were
added successively to the reaction mixture. The resultant
reaction mixture was allowed to stir at 120 °C for 12 h.
After completion of the reaction as indicated by TLC, the
crude product was directly purified by silica gel column
chromatography using petroleum ether/ ethyl acetate as
eluent to afford 6 as a major product (52.2 mg, 54%) along
with 3a (7.2 mg, 12%) as a minor product.
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Cobalt(III)-Catalyzed Construction of Benzofurans,
Benzofuranones and One-Pot Orthogonal C−H
Functionalizations to Access Polysubstituted
Benzofurans
Adv. Synth. Catal. Year, Volume, Page – Page
S. S. Bera, S. Debbarma, S. Jana, M. S. Maji*
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