Benzofuran synthesis via copper-mediated oxidative annulation of phenols and unactivated internal alkynes

Article abstract of DOI:10.1039/c3sc51489g

The first example of copper-mediated oxidative annulation of phenols and unactivated internal alkynes to afford benzofuran derivatives was reported. Various phenols and unactivated internal alkynes were successfully employed. Mechanistic studies disclosed a new strategy on annulations of alkynes with phenols through reversible electrophilic carbocupration of phenol followed by alkyne insertion and cyclization.

Full text of DOI:10.1039/c3sc51489g

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Benzofuran synthesis via copper-mediated oxidative
annulation of phenols and unactivated internal
alkynes†
Cite this: DOI: 10.1039/c3sc51489g
a
a
ab
*
Ruyi Zhu, Jiangbo Wei and Zhangjie Shi
The first example of copper-mediated oxidative annulation of phenols and unactivated internal alkynes to
afford benzofuran derivatives was reported. Various phenols and unactivated internal alkynes were
successfully employed. Mechanistic studies disclosed a new strategy on annulations of alkynes with
phenols through reversible electrophilic carbocupration of phenol followed by alkyne insertion and
cyclization.
Received 27th May 2013
Accepted 3rd June 2013
DOI: 10.1039/c3sc51489g
www.rsc.org/chemicalscience
be named as a heteroatom- or carbanion-directed oxidative
annulation reaction of alkynes with aryl or alkenyl substrates. In
Introduction
Benzofurans are a class of important structural motifs owing to
their wide existence in natural products, pharmaceuticals and
biologically active compounds (Scheme 1).1 The synthesis
towards this core backbone always draws chemists' attention,
and so far many efficient synthetic methods have been devel-
oped.2 Among these methods, the annulation reaction repre-
sents one of the most powerful and straightforward ways to
afford the target molecules. Traditional annulations which are
accomplished through cross coupling of ortho-halo phenol and
alkyne or a tandem Sonagashira coupling/5-endo-dig cyclization
usually require prefunctionalized substrates, such as ortho-halo
or -alkynyl phenol.3 To the best of our knowledge, phenols are
rarely utilized as substrates to synthesize the benzofuran scaf-
folds directly.4,9b,c
Since the boom of C–H bond activation and functionaliza-
tion,5 we planned to take this advantage to develop a straight-
forward synthetic method to approach benzofuran derivatives
through the oxidative annulation of phenols and unactivated
internal alkynes. Based on pioneers' contributions to the metal-
catalyzed oxidative annulations of alkynes with aryl or alkenyl
substrates bearing various heteroatom-containing functional
groups, a range of valuable heterocyclic compounds have been
synthesized efficiently.6 Recently, two examples of carbanion
directed oxidative annulations of internal alkynes with aryl
substrates have been reported.7 This strategy (strategy I) could
this strategy, a ve- or six-membered metallacycle was formed
as the key intermediate,8 followed by subsequent insertion of
the alkyne and reductive elimination to generate the product.
However, substrates such as anilines and phenols cannot form
such a key intermediate through directed ortho C–H activation.
In 2009, Jiao and co-workers achieved the direct synthesis of
indole derivatives via Pd-catalyzed oxidative annulation of
anilines with activated internal alkynes. In their work, the
authors hypothesized that the reaction was initiated by the
nucleophilic attack of the amino group to the electron-decient
alkyne to form an aryl alkenyl amine intermediate. This strategy
(strategy II) faced the difficulty that unactivated alkynes were
inefficient partners due to their low reactivity.9 Up to date, either
directing groups or activated internal alkynes are necessary
despite the signicant advances shown. Only recently, Sahoo
and co-workers reported the palladium-catalyzed oxidative
annulation of phenols and unactivated internal alkynes to form
benzofurans during the preparation of this manuscript.10
Simultaneously, we also achieved the direct synthesis of
aBeijing National Laboratory of Molecular Sciences (BNLMS), Key Laboratory of
Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College
of Chemistry and PKU Green Chemistry Center, Peking University, Beijing 100871,
China. E-mail: zshi@pku.edu.cn; Fax: +86 10-6276-0890; Tel: +86 10-6276-0890
bState Key Laboratory of Organometallic Chemistry Chinese Academy of Sciences,
Shanghai 200032, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3sc51489g
Scheme 1 Selected natural products, pharmaceuticals and biologically active
compounds containing benzofuran structural motifs.
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Table 1 Optimization of reaction conditionsa
benzofuran via copper-mediated oxidative annulation of unac-
tivated internal alkynes with phenols.
Results and discussion
At the beginning of our study, 4-tert-butylphenol 1a and
diphenylacetylene 2a were chosen as model substrates (Table 1).
Initially, the classical Rh(III)/Cu(II) system which was widely
applied to strategy I was tested in our study.6a–z,7a Fortunately, a
trace amount of desired product was observed. The examination
of oxidants disclosed that Cu(OTf)2 was optimal. The combi-
nation of cationic rhodium complex and Cu(OTf)2 in decalin
gave a 35% yield (entry 1). The ratio of 1a to 2a was also
important, an excess amount of phenol gave better results
(Table S1†). The addition of stoichiometric silver salts generally
more or less increased the yield (Table S2†), AgPF6 showed
signicant acceleration of the reaction and promotion of the
yield (entry 2). To our surprise, the transformation could also
proceed in the absence of rhodium complex albeit in a slightly
lower yield (entry 3). This result implied a completely different
pathway and the rhodium complex might play a role as a Lewis
acid. Besides the rhodium complex, other metals were also
surveyed, while no better results were obtained (Table S4†).
Among the trials of N-containing ligands, acetanilide was
proven benecial (entry 5, Table S5†). Meanwhile, the rhodium
complex was also advantageous under the circumstances
(entry 4). Considering the presence of inorganic salts in the
nonpolar solvent, phase transfer catalysts (PTCs) were taken
into account to speed up the reaction. Aer screening we found
that DTAC was the best one (entry 6, Table S8†). Interestingly,
under this condition, the rhodium complex seemed to play a
more important role in facilitating the transformation (entry 7).
Notably, the product was not detected in the absence of
Cu(OTf)2 (entry 8). This result indicated that Cu(OTf)2 was
indispensable for this reaction. The type of solvent was also
crucial, and generally, non- or low polar solvents, such as dec-
alin, xylene, PhCl etc., were better.
Entry
Additives
Yieldh
1
2d
3
4
5
6
7
8
Rhc
35
45
41
48
57
66
45
n.d.
AgPF6e and Rh
AgPF6
Lf and AgPF6
L, AgPF6 and Rh
DTAC,g L, AgPF6 and Rh
DTAC, L and AgPF6
AgPF6 and Rh, but no Cu(OTf)2
a
Reaction conditions: 0.30 mmol of 1a, 0.10 mmol of 2a, 0.10 mmol
b
of Cu(OTf)2, 0.50 mL of solvent, 120 ꢀC, 18 h.
Decalin:
c
decahydronaphthalene.
Rh: [Cp*Rh(MeCN)3](SbF6)2 (5 mol%,
0.005 mmol). d Reaction time is 3 h for entries 2–8. AgPF6: 2.5 equiv.,
e
f
g
0.25 mmol. L: acetanilide (20 mol%, 0.02 mmol). DTAC: dodecyl
trimethyl ammonium chloride (0.5 equiv, 0.05 mmol). h Isolated yield.
Table 2 Substrate scope of phenolsa
With the optimized reaction conditions in hand, various
phenols were employed in the reaction (Table 2). Particularly,
the reaction of para-alkyl- and aryl-substituted phenols led to
the products in moderate to good yields (3aa, 3ca, 3da). The
chloro substituent was compatible albeit in a moderate yield
(3ea). Unfortunately, some electron-donating groups, such as
methoxy and amino groups, were not compatible, which may
arise from the preferential oxidation or other side processes.11
On the other hand, electron-withdrawing groups, like the ester
group, were well tolerated (3fa). However, a low yield was
obtained when simple phenol was utilized (3ba). Based on this
result, we speculated that the electrophilic carbocupration
preferred occurring at the relatively electron-rich ortho and para
positions.11,12 Since the para position of simple phenol was
vacant, the subsequent reaction occurring at the para position
would lead to low yield. A similar result was observed for meta-
substituted phenol (3ga), which might prove the hypothesis.
Moreover, these results may also properly rule out the possi-
bility that this transformation is similar to strategy I or strategy
II. Meanwhile, moderate regioselectivity was observed for 3ga,
a
Reaction conditions: 0.30 mmol of 1, 0.10 mmol of 2a, 0.005 mmol of
[Cp*Rh(MeCN)3](SbF6)2, 0.10 mmol of Cu(OTf)2, 0.25 mmol of AgPF6,
0.02 mmol of L, 0.05 mmol of DTAC, 0.50 mL of solvent, 120 ꢀC, 3 h.
b
c
L: acetanilide. Major isomer is shown, number in parenthesis is
the ratio of two regioisomers which was determined by 1H NMR. d 18 h.
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Table 3 Substrate scope of alkynesa
alkyne containing fused ring such as the naphthyl ring could
also be utilized (3pi). In order to get more information about
the electronic and steric effect of alkynes on the regioselectivity
of this reaction, some representative unsymmetrical diary-
lalkynes were tested (3pj–pm). Good yields with low to
moderate regioselectivities were observed. The elucidation for
these regioselectivities is still in progress. Other phenol
substrates such as 1a could also be coupled with different
a
Reaction conditions: 0.30 mmol of 1, 0.10 mmol of 2, 0.005 mmol of
[Cp*Rh(MeCN)3](SbF6)2, 0.10 mmol of Cu(OTf)2, 0.25 mmol of AgPF6,
0.02 mmol of L, 0.05 mmol of DTAC, 0.50 mL of solvent, 120 ꢀC, 3 h.
b
Major isomer is shown, number in parenthesis is the ratio of two
regioisomers which was determined by 1H NMR. The ratio was
c
determined by isolated yield.
presumably due to the steric hindrance of the methyl group. In
addition, 3,4-disubstituted phenols would give products in
moderate to good yields as the para positions were blocked
(3ha–ka), and it was noteworthy that steric hindrance domi-
nated the regioselectivities for 3ha, 3ja and 3ka, while the
regioselectivity of 3ia might be controlled by the electronic
effect. A phenolic hydroxyl group was tolerated though in
moderate yield (3la). For 3,5-disubstitued substrates, moderate
yield was obtained though the para position was available
(3ma). Perhaps the steric hindrance of two adjacent methyl
groups restrained the side reaction. When 3,4,5-trisubstitued
phenols were examined, the yields were good which was in
accordance with expectations (3na–pa). Not only can the phenyl
substrates be used in this reaction, but the naphthyl substrates
also could be successfully employed in good yields with single
selectivity (3qa, 3ra). In addition, various functional groups
were compatible. The reaction of 4-hydroxycoumarin would
selectively result in the corresponding benzofuran product
which was endowed with many interesting biologically proper-
ties in good yield (3sa).13
Next the substrate scope for a variety of alkynes was
investigated (Table 3). Symmetrical diarylalkynes bearing
electron-donating or electron-withdrawing groups reacted
smoothly to give high yields (3pb–pi). Moreover, the reaction
was not inhibited by the steric hindrance of ortho-substituted
diphenylacetylene (3pb). Several functional groups such as
methoxy and halide groups were intact. Furthermore, an Scheme 2 Mechanistic studies.
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alkynes with moderate to good yields (3af, 3ah). To further test 5. Finally, the product is formed through a reductive elimina-
the utility of this method, heteroatom-containing alkynes were tion or single electron transfer (SET) process.
employed and resulted in corresponding products with good
yields and single regioselectivities (3pn, 3po). However, dia-
lkylalkynes or alkylarylalkynes were inefficient partners
Conclusions
presumably due to their instability under this oxidative In conclusion, we have developed the rst example of copper-
condition (3ap, 3aq). An activated internal alkyne, such as mediated oxidative annulation of phenols and unactivated
dimethyl acetylenedicarboxylate, was not an ideal coupling internal alkynes to afford benzofuran derivatives. Starting from
partner, which might evidence that the process of this reaction commercially available phenols and alkynes, the direct one-
was not analogous to strategy II.
step/pot synthesis of benzofuran derivatives has been achieved
To gain more insight into the reaction mechanism, with high efficiency. The mechanistic study gives us a hint that
competitive experiments were conducted between electronically the process is likely to initiate with the reversible electrophilic
different alkynes [Scheme 2 eqn (1) and (2)], showing remark- carbocupration, followed by alkyne insertion and cyclization to
able preference for electron-rich alkynes. However, there was afford the desired product. The study of the detailed mecha-
almost no distinction between electron-neutral and electron- nism of this reaction is in progress.
decient alkynes. The results probably excluded the likelihood
that the process was nucleophilic addition of phenol to alkyne
through strategy II or a Friedel–Cras-type reaction. The
Acknowledgements
competitive experiments between electronically different This work was supported by the “973” Project from the MOST of
phenols were also performed [Scheme 2 eqn (3)], and the results China (2009CB825300) and NSFC (Nos. 20925207 and
revealed that the reaction favored electron-rich phenols, 21002001).
agreeing with our hypothesis about electrophilic carbocupra-
tion.12 In addition, TEMPO was added to the reaction as a
radical scavenger, and the yield decreased signicantly to 29%
Notes and references
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Scheme 3 Proposed mechanism.
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