Copper-Iodide- and Diorganyl-Diselenide-Promoted Cyclization of 2-Alkynylphenols: Alternative Approach to 3-Organoselanylbenzo[b]furans

Article abstract of DOI:10.1002/ejoc.201701205

This paper describes an alternative method for the synthesis of 3-organoselanylbenzo[b]furans through the intramolecular cyclization of 2-alkynylphenols promoted by copper iodide and diorganyl diselenides. The cyclization reactions were carried out at roo

Full text of DOI:10.1002/ejoc.201701205

A Journal of
Accepted Article
Title: Copper Iodide and Diorganyl Diselenides-Promoted Cyclization
of 2-Alkynyl-phenols: Alternative Approach to 3-Organoselanyl-
benzo[b]furans
Authors: Jean Carlo Kazmierczak, Ana Maria Spohr Recchi, Fabiane
Gritzenco, Everton Berwanger Balbom, Thiago Barcellos,
Adriane Sperança, and Benhur Godoi
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201701205
Link to VoR: http://dx.doi.org/10.1002/ejoc.201701205
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10.1002/ejoc.201701205
European Journal of Organic Chemistry
FULL PAPER
Copper Iodide and Diorganyl Diselenides-Promoted Cyclization of
2-Alkynyl-phenols: Alternative Approach to 3-Organoselanyl-
benzo[b]furans
Jean C. Kazmierczak,[a] Ana M. S. Recchi,[b] Fabiane Gritzenco,[a] Everton B. Balbom,[a] Thiago
Barcellos,[c] Adriane Sperança,[d] and Benhur Godoi*, [a]
Abstract: This manuscript describes an alternative method for
the synthesis of 3-organoselanyl-benzo[b]furans through
Figure 1. Inhibitors of cyclin-dependent kinases.
intramolecular cyclization of 2-alkynyl-phenols promoted by copper
iodide and diorganyl diselenides. The cyclization reactions were
carried out at room temperature in the absence of base and under
ambient atmosphere (open flask). This synthetic methodology
proved to be efficient to both diorganyl diselenides and 2-alkynyl-
phenols bearing neutral, electron-donating and electron-withdrawing
substituents. The 2-phenyl-3-phenylselanyl-benzo[b]furan was
Furthermore, benzo[b]furan derivatives have also found
application on the materials chemistry being useful to organic
field effect transistors[8] and for electro-optical materials.[9] Such
applicability become benzo[b]furan rings important synthetic
targets and a range of strategies for the construction of these
heterocyclic derivatives have been developed from distinct
precursors by using different conditions.[10] Among them,
synthetic approaches based on electrophilic cyclization and
transition-metals mediated cyclization reactions of acyclic
substrates consist in a very useful tool to this aim.[11] In this way,
2-alkynyl-phenols and 2-alkynyl-anisoles are the most employed
precursors.[12]
applied as
a synthetic precursor in organochalcogen-lithium
exchange and bromination reactions, allowing the synthesis of
functionalized benzo[b]furans in good yields.
In the last years, reaction systems combining stoichiometric or
catalytic amounts of transition-metal salts and diorganyl
dichalcogenides have emerged as an alternative tool to promote
intramolecular cyclization of acyclic alkynes. Palladium, copper
and iron salts have being successfully used in these
transformations affording the synthesis of various heterocycle
scaffolds.[13] For a long period, organochalcogen derivatives, in
special organoselenium compounds, had a bad reputation
wrongfully receipt relating to their toxicity, instability and odor.
However, recent advances in the studies regarding their
pharmacological and toxicological properties highlight these
substances as promising therapeutic agents due to their singular
features.[14] In addition, the presence of an organochalcogen
moiety in the structure of different substances have drawn
attention of the scientific community because of their influence in
pharmacological activities[15] and large applicability as reactive
sites in different organic compound classes.[16] Considering that
there is no protocol describing the use of copper iodide and
dichalcogenide as the cyclization promoters of 2-alkynyl-phenols
for the preparation of benzo[b]furan nucleus in the scientific
literature, in this study we reported the synthesis of 3-
organoselanyl-benzo[b]furans 3 by intramolecular cyclization of
2-alkynyl-phenols 1 combining copper iodide and diorganyl
diselenides 2 to promote these transformations (Scheme 1).
Introduction
Organic compounds bearing benzo[b]furan nucleus are known
to have a range of biological properties.[1] Diverse benzo[b]furan
scaffold
containing
substances
such
have
as
demonstrated
anticancer and
pharmacological
activities,
antiangiogenic,[2] antidepressant,[3] anticonvulsant and anti-
inflammatory.[4] Potentially, small molecules derived from
benzo[b]furans were active as inhibitors of human protein
kinases, which are able to modify other proteins via
phosphorylation processes, being directly related with viral
infections and cancer disease.[5] For example, the
benzo[b]furan-2-carboxamide
A[6]
and
benzo[b]furan-2-
carbaldehyde B[7] (Figure 1) have been reported to have
potential as antiproliferative agents against tumor cells through
the inhibition of cyclin-dependent kinases.
[a]
Núcleo de Síntese, Aplicação e Análise de Compostos Orgânicos e
Inorgânicos, Universidade Federal da Fronteira Sul, Cerro Largo,
Rio Grande do Sul, 97900-000, Brazil
e-mail: benhur.godoi|@uffs.edu.br
[b]
[c]
[d]
Laboratório de Síntese, Reatividade, Avaliação Farmacológica e
Toxicológica de Organocalcogênios, CCNE, UFSM, Santa Maria,
Rio Grande do Sul, 97105-900, Brazil
Universidade de Caxias do Sul, Laboratório de Biotecnologia de
Produtos Naturais e Sintéticos, Caxias do Sul, RS, 95070-560,
Brazil
Scheme 1. CuI/R3SeSeR3-promoted cyclization of 2-alkynyl-phenols.
Instituto Federal Farroupilha, Santo Augusto, Rio Grande do Sul,
98590-000, Brazil
Results and Discussion
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201701205
European Journal of Organic Chemistry
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In order to determine the ideal reaction conditions, 2-
phenyl(ethynyl)phenol 1a, copper iodide and diphenyl diselenide
were chosen as standard substrates to screen the cyclization
reaction parameters such as temperature, base, solvent,
reaction stoichiometry, amount of copper iodide and reaction
atmosphere. The results of these experiments are summarized
in table 1. Firstly, 2-alkynyl-phenol 1a (0.25 mmol) was added to
a mixture of copper iodide (1 equiv) and PhSeSePh 2a (1.1
equiv) in DMF, at room temperature under ambient atmosphere
(open flask). By using these conditions, the 2-phenyl-3-
phenylselanyl-benzo[b]furan 3a was isolated in 48% yield (Table
1, entry 1). The evaluation of different solvents showed that both
DMF and DMSO afforded the desired product while MeCN and
DCM were quite inefficient (Table 1, entries 1-4). In order to
verify the influence of bases in the cyclization process, the
reactions were carried out using 1.5 equivalents of NaHCO3,
K2CO3 and NaOH. These tests showed that the presence of
bases did not significantly improve the cyclization yields (Table 1,
entries 5-7). Increasing the diphenyl diselenide amount from 0.5
to 1.5 equivalents did not improve the reaction yields (Table 1,
entries 8-10). However, when the amount of PhSeSePh 2a was
kept constant and increasing the amount of copper iodide from
1.0 to 1.5 equivalents a significant increment in the cyclization
efficiency was observed, giving the 3-organoselanyl-
benzo[b]furan 3a in 72% yield (Table 1, entry 13). Using more
than 1.5 equivalents of copper iodide, a slight decrease in the
reaction yield was observed even if higher temperature was
used (Table 1, entries 14-16). When copper iodide was
employed in catalytic quantities, the cyclization yields
dramatically decreased (Table 1, entries 11-12). Regarding the
influence of temperature into cyclization, it was clear that high
temperature had no positive effect, affording the product 3a in
lower yields (Table, entries 16-18). The cyclization proved to be
no sensitive to the oxygen present in the air because when the
reactions were carried out under argon atmosphere no
difference in the reaction behavior was observed (Table, entries
19-20). The fact that, this cyclization protocol was carried out at
room temperature, in the absence of bases and under an open
flask, represents economical and practical advantages
compared with other methodologies.
NaHCO3
(1.5)
5
1.0
1.1
DMSO
22
6
1.0
1.0
1.0
1.0
1.0
0.2
0.5
1.5
2.0
2.0
2.0
1.5
1.5
1.5
2.0
1.1
1.1
0.5
0.75
1.5
1.1
1.1
1.1
1.1
0.75
0.75
1.1
1.1
1.1
1.1
K2CO3 (1.5)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
25
52
7
NaOH (1.5)
8
-
-
-
-
-
-
-
-
-
-
-
-
-
44
40
9
10
11
12
13
14
15
16
17
18
19
20
44
4
6
72
65
58
58[b]
52[c]
68[d]
70[e]
72[e]
[a] The reaction was performed in the presence of 1a (0.25 mmol), solvent
(3 mL), in an open flask for 20 h. [b] Reaction was performed at 100 ºC.
[c] The Reaction was performed at 80 ºC. [d] Reaction was performed at
50 ºC. [e] The Reaction carried out under argon.
In addition, we carried out some experiments to evaluate the
reaction behavior by employing different copper salts and these
results are shown in table 2. When Cu(I) species such as CuCl
and CuBr were used, the expected benzo[b]furan 3a was
obtained in lower yield compared with that of copper iodide
(Table 2, entries 1-2). On the other hand, Cu(II) salts, such as
CuBr2, CuO and CuOnano, showed low or no activity to promote
the cyclization of 1a (Table 2, entries 3-5). These results could
suggest that the halogen atom present in the copper salt plays
an important role in the cyclization process. We also observed
that in the absence of copper salt the cyclization did not proceed
(Table 2, entry 6). Thus, the best reaction condition to provide
the cyclization of the 2-alkynyl-phenol 1a (0.25 mmol) consisted
in the use of copper iodide (1.5 equiv), diphenyl diselenide 2a
(1.1 equiv), employing DMSO (3 mL) as solvent in the absence
of bases, at room temperature and in an open flask.
Table 1. Effects of Different Reaction Parameters on the CuI/PhSeSePh-
Promoted Cyclization of 1a.[a]
Entry
CuI
(equiv)
(PhSe)2
(equiv)
Base
(equiv)
Solvent
(3 mL)
Yield
(%)
1
2
3
4
1.0
1.0
1.0
1.0
1.1
1.1
1.1
1.1
-
-
-
-
DMF
MeCN
DCM
48
2
Table 2. Effects of Different Copper Salts on the Cyclization of 1a.[a]
-
DMSO
48
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10.1002/ejoc.201701205
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Table 3. Scope and Limitations of the CuI/R3SeSeR3-Promoted Cyclization of 2-
Alkynyl-phenols 1.[a]
Entry
[Cu] (1.5 equiv)
Yield (%)
1
2
3
4
5
6
CuCl
CuBr
CuBr2
CuO
30
53
9
Entry
1
2-alkynyl-
phenols 1
(R3Se)2 2
Product 3
Yield
(%)/(h)
-
CuOnano
-
-
72/(20)
-
2a
2b
3a
[a] Reaction was performed using 1a (0.25 mmol), 2a (1.1 equiv) in
DMSO (3 mL) at 25 ºC, in an open flask for 20h.
1a
1a
2
3
57/(22)
61/(21)
Once determined the best cyclization parameters, we focused
on the study of scope and limitations of the methodology (Table
3). The cyclization approach proved to be tolerant to electron-
donating and electron-withdrawing groups into the aromatic
rings bonded to the selenium atom of the diaryl diselenides 2
given the corresponding 3-organoselanyl-benzo[b]furans 3 in
good yields (Table 3, entries 2-4). These results suggest that the
cyclization was not sensitive to electronic effects of the groups
bonded to the diaryl diselenides. When dialkyl diselenides were
used, a decrease in the cyclization yield was observed and the
3-alkylselanyl-benzo[b]furans 3e and 3f were isolated in 30 and
32% yields, respectively (Table 3, entries 5-6). In order to
evaluate the influence of electronic effects of the aromatic rings
directly bonded to the triple bond in the 2-alkynyl-phenols 1, the
substrates 1b-d were submitted to the cyclization conditions and
the results indicate that the expected product was obtained in a
moderate yield, suggesting a negative influence of the electron-
withdrawing fluorine atom (Table 3, entry 7). The methyl group
did not significantly affect the reaction behavior leading to the
benzo[b]furan 3h in good yield (Table 3, entry 8). The cyclization
approach showed toleration to the presence of the bulky 1-
naphthyl group bonded to the Csp on the substrate 1e furnishing
the benzo[b]furan derivative 3j in 57% yield (Table 3, entry 9).
The reaction showed to be sensitive to electronic effects from
substituents bonded into the phenol ring (Table 3, entries 10-12).
In fact, the electron-withdrawing fluorine atom in the substrates
1e and 1f did not negatively affect the reaction efficiency and the
corresponding benzo[b]furans 3j and 3k were obtained in good
yields (Table 3, entries 10 and 11). However, the presence of an
electron-donating methyl group led to a decrease in the
cyclization yield (Table 3, entry 12). In addition, the reaction
proved to be tolerant to the presence of an alkyl group directly
bonded to the carbon-carbon triple bond and the reaction of the
2-alkynyl-phenol 1h and dyphenyl diselenide 2a under the
optimized conditions furnished the desired 2-butyl-3-
(phenylselanyl)benzo[b]furan 3m in 55% yield (Table 1, entry
13).
3b
3c
2c
2d
1a
4
5
1a
1a
63/(20)
30/(24)
3d
3e
2e
6
7
1a
32/(24)
47/(22)
2f
3f
2a
3g
1b
1c
8
2a
57/(22)
3h
9
2a
2a
57/(22)
63/(22)
1d
1e
3i
3j
10
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10.1002/ejoc.201701205
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the selenium electrophile PhSeBr was added to the reaction
mixture (Scheme 3, Eq. 2). Using this protocol, the
benzo[b]furan 3a was not detected by GC-MS analysis. In fact,
this experiment became clear the importance of the
simultaneous presence of copper iodide and dichalcogenide to
the effectiveness of the cyclization system, becoming plausible
the formation of the Cu(III)-complex A shown in Scheme 4.
11
12
13
2a
2a
2a
61/(22)
24/(24)
55/(23)
1f
1g
1h
3k
3l
3m
[a] The reaction was performed using 2-alkynyl-phenols
1
(0.25 mmol),
R3SeSeR3 2 (1.1 equiv), CuI (1.5 equiv) in DMSO (3 mL) at 25 ºC, in an open
flask.
With the aim of studying the generality of the cyclization
method for the synthesis of organosulfur and organotellurium
benzo[b]furan derivatives, the substrate 1a was submitted to the
cyclization reaction in the presence of diphenyl disulfide and
diphenyl ditelluride (Scheme 2). Unfortunately, these
experiments demonstrated a limitation of the synthetic protocol
because the expected products 3p and 3q were not formed,
under the optimized conditions.
Scheme 3. Experiments regarding the mechanistic studies.
According to the scientific literature, electrophilic agents, such
as PhSeI, could be formed from the corresponding diorganyl
dichalcogenide in the presence of molecular iodine and
palladium salts.[13f] Through this system it is possible to promote
the cyclization of 2-alkynyl-anisoles and a plausible route for this
transformation
involves
the
formation
of
an
Scheme 2. Cyclization of 2-alkynyl-phenols using disulfide and ditelluride.
organochalcogenonium intermediate, which is responsible to
activate the alkyne for the electrophilic cyclization. In this context,
we performed some experiments to examine if an
organochalcogenyl electrophilic species could be in situ formed,
becoming plausible an electrophilic cyclization pathway for this
transformation (Scheme 3, Eq. 3-4). Herein, when PhSeSePh 2a
and copper iodide reacted under the cyclization conditions,
without any substrate 1, the formation of the electrophilic PhSeI
was not observed by GCMS analysis of the crude reaction
mixture (Scheme 3, Eq. 3). To further support this idea, we
submitted the substrate 1a to the electrophilic cyclization
reaction, in which the electrophilic source was PhSeI, prepared
by reaction of PhSeSePh 2a and molecular iodine. Under the
standard reaction parameters, this cyclization system proved to
be active to promote the cyclization of 1a affording the cyclized
product 3a in 15% yield (Scheme 3, Eq. 4). By analyzing these
results, an electrophilic cyclization pathway involving the R3SeI
as cyclizing agent cannot be discarded. This mechanistic
hypothesis could involve the in situ formation of R3SeI from
decomposition of the Cu(III)-tetracoordinated complex A by the
displacement of (R3Se)2Cu and [(R3Se)CuI]. The mechanistic
sequence should follow a typical electrophilic cyclization route
via activation of the alkyne by the electrophile (R3SeI) and
Regarding the mechanisms involved in the cyclization process,
we are not able to infer an undoubted proposal. Herein, based
on previous reports[17] and considering that the cyclization
system seems to be catalytically inactive (Table 1, entries 11-12)
we carried out some experiments in order to become the
cyclization pathway as clear as possible aiming to obtain
relevant data that could support
a plausible mechanism
(Scheme 3). First, the substrate 1a was submitted to the
cyclization conditions in the absence of diorganyl
dichalcogenides 2 (Scheme 3, Eq. 1). Analysis by GC-MS
showed the formation of the benzo[b]furan 4a, which contains a
hydrogen atom at C-3 position. As expected, when the reaction
was performed in the absence of any cooper salt no cyclized
product was detected and both substrates 1a and 2a were
recovered (Table 2, entry 6). These results confirm that copper
iodide plays an important role in the cyclization reaction by
activating the C-C triple bond. In order to investigate if
dichalcogenides could be acting as electrophilic sources by
replacing a Csp2-Cu bond for the organochalcogenyl moiety, the
2-alkynyl-phenol 1a was stirred in the presence of copper iodide
under inert atmosphere, until its complete consumption and then
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10.1002/ejoc.201701205
European Journal of Organic Chemistry
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subsequent attack of oxygen to the activated Csp; a further
deprotonation step could provide the expected product
3
(Scheme 4). The supposed generation of the (R3Se)2Cu(II)
species could justify the necessity of stoichiometric amounts of
copper iodide and diorganyl diselenide, because Cu(II) salts
proved to be inefficient to promote cyclization process (Table 2,
entries 3-5). Furthermore, the benzo[b]furan 4a was submitted to
the standard cyclization conditions using CuI and diphenyl
diselenide 2a (Scheme 3, Eq. 5). Herein, traces of the 3-
(phenylselanyl)benzo[b]furan 3a were observed by analysis of
the crude reaction mixture by GC-MS. This result suggest that a
C-H activation pathway cannot be totally discarded.[18]
Scheme 6. Bromination reaction of 3a.
Conclusions
In this study, we described an alternative method to access 3-
organoselanyl-benzo[b]furans by using 2-alkynyl-phenols 1 as
substrates to undergo intramolecular cyclization reactions
promoted by copper iodide and diorganyl diselenides, using
DMSO as solvent, at room temperature, under ambient
atmosphere (air) and in the absence of bases. Through this
protocol, the benzo[b]furan derivatives 3 could be obtained in
moderate to good yields, under mild reaction conditions. In fact,
this is the first report of copper iodide/R3SeSeR3-promoted
cyclization of 2-alkynyl-phenols. Furthermore, the 2-phenyl-3-
(phenylselanyl)benzo[b]furan 3a proved to be
a versatile
substrate to different synthetic transformations such as
organochalcogen-lithium exchange and bromination reactions,
being useful to the preparation of multi-functionalized
benzo[b]furan units.
Scheme 4. Electrophilic cyclization hypothesis.
Aiming to explore the synthetic applicability of the
organoselanyl groups as potentially reactive molecular sites for
the preparation of different functionalized substances, the 2-
phenyl-3-phenylselanyl-benzo[b]furan 3a was employed as
precursor to organochalcogen-lithium exchange reactions
(Scheme 5). Butyllithium was used to promote the formation of
the organometallic intermediate C, which was subsequently
trapped with different electrophiles, such as aqueous ammonium
chloride and benzoyl chloride. Through these reactions, the
benzo[b]furans 4a and 4b were obtained in 65 and 73% yield,
respectively.
Experimental Section
General Procedure for the CuI/R3SeSeR3-Promoted Cyclization of 2-
Alkynyl-phenols 1 for Preparation of Benzo[b]furans 3: In a reaction
tube, under ambient atmosphere (air), containing the appropriated
diorganyl diselenide 2 (1.1 equiv) and CuI (1.5 equiv) at 25 ºC was added
DCM (3 mL). To the stirring reaction mixture was added the proper 2-
alkynyl-phenol 1 (0.25 mmol) diluted in 2 mL of DCM, in one portion. The
reaction mixture was stirred for the required time at 25 ºC. After that, the
reaction mixture was quenched with saturated aqueous NH4Cl solution (5
mL) and the aqueous layer was extracted with AcOEt (3 x 5 mL) and
dried over MgSO4. After filtering, the organic solution was concentrated
using a rotary evaporator under reduced pressure. The product was
purified by flash chromatography in silica gel using hexane/ethyl acetate
as eluent.
2-phenyl-3-(phenylselanyl)benzo[b]furan (3a):[13e] Yield: 0.063g (72%).
NMR 1H: CDCl3, 400 MHz, (ppm): 8.24-8.16 (m, 2H), 7.58-7.49 (m, 2H),
7.48-7.41 (m, 2H), 7.40-7.32 (m, 1H), 7.31-7.26 (m, 2H), 7.25-7.19 (m,
1H), 7.18-7.06 (m, 4H). NMR 13C: CDCl3, 100 MHz, (ppm): 157.2, 154.1,
131.9, 131.4, 130.1, 129.3, 129.2, 129.1, 128.4, 127.8, 126.2, 125.2,
123.4, 121.2, 111.1, 99.7. MS (EI, 70 eV) m/z (relative intensity): 350
(27), 348 (14), 270 (100), 255 (10), 241 (21), 165 (62), 139 (14), 77 (9).
3-((4-fluorophenyl)selanyl)-2-phenylbenzo[b]furan (3b): Yield: 0.052g
(57%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.20 (d, J = 7.7 Hz, 2H), 7.54-
7.20 (m, 9H), 6.85 (t, J = 8.8 Hz, 2H). NMR 13C: CDCl3, 100 MHz,
Scheme 5. Organochalcogen-lithium exchange reaction.
The introduction of halogen atoms in the structure of organic
compounds consist in a versatile synthetic tool, because the C-
halogen bond can be easily replaced under several protocols,
being especially useful on transition-metal catalyzed cross-
coupling reactions.[19] Thus, we tested the behavior of the
benzo[b]furan 3a as substrate under bromination reaction
conditions (Scheme 6). By this protocol, the benzo[b]furan 5a
bearing two bromine atoms into the benzofuran ring was
obtained in 63% yield. This result must be highlighted because it
becomes possible the high functionalization of the heterocycles
by a range of methodologies.
1
3
(ppm): 161.9 (d, JCF = 246.0 Hz), 157.0, 154.0, 131.7, 131.5 (d, JCF
=
7.7 Hz), 130.1, 129.3, 128.5, 127.8, 125.6 (d, 4JCF = 3.3 Hz), 125.2, 123.4,
2
121.0, 116.4 (d, JCF = 21.8 Hz), 111.2, 100.2. MS (EI, 70 eV) m/z
(relative intensity): 368 (26), 367 (3), 288 (100), 259 (14), 165 (33), 139
(7), 115 (4), 77 (2). HRMS calcd for C20H13FOSe: [M]+ 368.0116. Found
[M]+ 368.0112.
3-((4-chlorophenyl)selanyl)-2-phenylbenzo[b]furan (3c):[13e] Yield:
0.058g (61%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.17 (d, J = 7.0 Hz,
2H), 7.55 (d, J = 8.2 Hz, 1H), 7.49-7.37 (m, 4H), 7.36-7.31 (m, 1H), 7.26-
7.18 (m, 3H), 7.11 (d, J = 8.7 Hz, 2H). NMR 13C: CDCl3, 100 MHz,
(ppm): 157.4, 154.1, 132.4, 131.6, 130.5, 130.0, 129.6, 129.5, 129.4,
128.5, 127.8, 125.3, 123.5, 121.0, 111.2, 99.4. MS (EI, 70 eV) m/z
(relative intensity): 386 (14), 384 (29), 383 (4), 306 (34), 304 (100), 268
(22), 241 (19), 165 (56), 115 (8), 105 (2), 77 (4).
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3-((4-methylphenyl)selanyl)-2-phenylbenzo[b]furan (3d):[13e] Yield:
0.057g (63%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.22-8.19 (m, 2H),
7.55-7.50 (m, 2H), 7.47-7.42 (m, 2H), 7.41-7.36 (m, 1H), 7.34-7.29 (m,
1H), 7.24-7.19 (m, 3H), 6.97 (d, J = 7.9 Hz, 2H), 2.24 (s, 3H). NMR 13C:
CDCl3, 100 MHz, (ppm): 156.9, 154.1, 136.2, 132.0, 130.2, 130.1, 129.6,
129.2, 128.4, 127.8, 127.4, 125.1, 123.3, 121.2, 111.1, 100.1, 20.9. MS
(EI, 70 eV) m/z (relative intensity): 364 (24), 363 (3), 284 (100), 269 (13),
255 (10), 241 (14), 165 (27), 115 (4), 77 (2).
(m, 3H), 7.39-7.34 (m, 1H), 7.32-7.30 (m, 1H), 7.29-7.25 (m, 2H), 7.18-
7.10 (m, 4H), 2.40 (s, 3H). NMR 13C: CDCl3, 100 MHz, (ppm): 157.4,
152.5, 133.0, 131.9, 131.6, 130.2, 129.3, 129.2, 128.8, 128.4, 127.7,
126.5, 126.0, 120.8, 110.7, 99.1, 21.4. MS (EI, 70 eV) m/z (relative
intensity): 364 (31), 362 (16), 284 (100), 255 (7), 178 (35), 152 (8), 105
(1), 77 (8). HRMS calcd for C21H16OSe: [M]+ 364.0366. Found [M]+
364.0372.
2-butyl-3-(phenylselanyl)benzo[b]furan (3m): Yield: 0.045g (55%).
NMR 1H: CDCl3, 400 MHz, (ppm): 7.46-7.41 (m, 2H), 7.27-7.08 (m, 7H),
2.97 (t, J = 7.4 Hz, 2H), 1.71 (quint, J = 7.4 Hz, 2H), 1.35 (sext, J = 7.4
Hz, 2H), 0.89 (t, J = 7.4 Hz, 3H). NMR 13C: CDCl3, 100 MHz, (ppm):
163.7, 154.4, 131.8, 130.6, 129.1, 129.0, 126.0, 124.0, 123.0, 120.3,
110.9, 100.0, 30.3, 27.0, 22.2, 13.7. MS (EI, 70 eV) m/z (relative
intensity): 330 (57), 328 (30), 287 (34), 250 (46), 207 (100), 178 (36), 131
(48), 115 (13), 105 (1), 77 (14). HRMS calcd for C18H18OSe: [M]+
330.0523. Found [M]+ 330.0525.
3-(butylselanyl)-2-phenylbenzo[b]furan (3e):[13e] Yield: 0.025g (30%).
NMR 1H: CDCl3, 400 MHz, (ppm): 8.32-8.28 (m, 2H), 7.69-7.67 (m, 1H),
7.53-7.45 (m, 3H), 7.41-7.37 (m, 1H), 7.35-7.28 (m, 2H), 2.79 (t, J = 7.3
Hz, 2H), 1.56 (quint, J = 7.3 Hz, 2H), 1.33 (sex, J = 7.3 Hz, 2H), 0.79 (t, J
= 7.3 Hz, 3H). NMR 13C: CDCl3, 100 MHz, (ppm): 155.8, 153.8, 132.7,
130.6, 128.8, 128.3, 127.6, 124.9, 123.1, 121.0, 111.0, 100.3, 32.3, 28.2,
22.7, 13.4. MS (EI, 70 eV) m/z (relative intensity): 330 (24), 329 (3), 274
(9), 245 (12), 194 (100), 165 (43), 115 (3), 77 (3), 57 (3).
General Procedure for the Organochalcogen-lithium Exchange
Reaction of 3a for the Preparation of the Benzo[b]furan 4a: To a two-
necked round-bottomed flask, containing a solution of 3a (0.5 mmol) in
THF (5 mL) at -10 °C, under argon, was added dropwise BuLi (0.6 mmol,
of a 2.5 M solution in hexane). The reaction mixture has stirred for 30
minutes at -10 ºC and then saturated aqueous NH4Cl solution (2 mL) was
gradually added. The reaction mixture was warmed to 25 °C and stirred
for 1h. After this time, the mixture was diluted in ethyl acetate (10 mL)
and washed with a saturated aqueous NaCl solution (3 × 10 mL). The
organic layer was dried over MgSO4 and concentrated under reduced
pressure using a rotary evaporator. The product was purified by flash
chromatography in silica gel using hexane as eluent.
2-phenyl-3-(propylselanyl)benzo[b]furan (3f): Yield: 0.025g (32%).
NMR 1H: CDCl3, 400 MHz, (ppm): 8.30 (d, J = 7.4 Hz, 2H), 7.69-7.67 (m,
1H), 7.51-7.44 (m, 3H), 7.40-7.36 (m, 1H), 7.34-7.26 (m, 2H), 2.77 (t, J =
7.3 Hz, 2H), 1.60 (sex, J = 7.3 Hz, 2H), 0.90 (t, J = 7.3 Hz, 3H). NMR 13C:
CDCl3, 100 MHz, (ppm): 155.9, 153.9, 132.8, 130.7, 128.8, 128.3, 127.7,
124.9, 123.1, 121.0, 111.1, 100.4, 30.6, 23.7, 14.2. MS (EI, 70 eV) m/z
(relative intensity): 316 (30), 315 (3), 273 (10), 245 (18), 194 (100), 165
(58), 115 (4), 77 (4). HRMS calcd for C17H16OSe: [M]+ 316.0366. Found
[M]+ 316.0362.
2-(4-fluorophenyl)-3-(phenylselanyl)benzo[b]furan (3g): Yield: 0.043g
(47%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.21-8.17 (m, 2H), 7.54-7.50
(m, 2H), 7.34-7.21 (m, 5H), 7.18-7.10 (m, 4H). NMR 13C: CDCl3, 100
2-phenyl-benzo[b]furan (4a): Yield: 0.032g (65%). NMR 1H: CDCl3, 400
MHz, (ppm): 7.55-7.51 (m, 2H), 7.42-7.44 (m, 4H), 7.29-7.22 (m, 1H),
6.98 (d, J = 7.6 Hz, 1H), 6.89 (td, J = 7.6 Hz; J = 1.1 Hz, 1H), 5.86 (s, 1H).
NMR 13C: CDCl3, 100 MHz, (ppm): 156.5, 131.6, 131.5, 130.4, 128.8,
128.4, 122.3, 120.4, 114.7, 109.5, 96.3, 83.0. MS (EI, 70 eV) m/z
(relative intensity): 195 (16), 194 (100, 165 (83), 139 (9), 115 (4), 82 (10),
63 (5).
1
MHz, (ppm): 163.9 (d, JCF = 249.0 Hz), 156.4, 154.0, 131.9, 131.6,
3
4
131.2, 129.8 (d, JCF = 8.2 Hz), 129.3, 129.1, 126.4 (d, JCF = 3.3 Hz),
2
126.3, 125.3, 123.5, 121.2, 115.5 (d, JCF = 21.7 Hz), 111.1. MS (EI, 70
eV) m/z (relative intensity): 368 (26), 367 (3), 288 (100), 259 (16), 183
(43), 165 (3), 157 (11), 77 (9). HRMS calcd for C20H13FOSe: [M]+
368.0116. Found [M]+ 368.0111.
2-(4-methylphenyl)-3-(phenylselanyl)benzo[b]furan (3h):[13e] Yield:
0.052g (57%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.10 (d, J = 8.2 Hz,
2H), 7.54-7.48 (m, 2H), 7.32-7.19 (m, 6H), 7.16-7.09 (m, 3H), 2.38 (s,
3H). NMR 13C: CDCl3, 100 MHz, (ppm): 157.6, 154.1, 139.5, 132.1,
131.6, 129.3, 129.2, 129.1, 127.7, 127.4, 126.2, 125.0, 123.3, 121.1,
111.1, 99.1, 21.4. MS (EI, 70 eV) m/z (relative intensity): 364 (31), 363
(4), 284 (100), 269 (11), 255 (9), 241 (13), 178 (33), 165 (4), 115 (1), 77
(22).
General Procedure for the Organochalcogen-lithium Exchange
Reaction of 3a for the Preparation of the Benzo[b]furan 4b: To a two-
necked round-bottomed flask, containing a solution of 3a (0.5 mmol) in
THF (5 mL) at -10 °C, under argon, was added dropwise BuLi (0.6 mmol,
of a 2.5 M solution in hexane). The reaction mixture has stirred for 30
minutes at -10 ºC and then benzoyl chloride (1.1 equiv) diluted in THF (2
mL) was gradually added. The reaction mixture was warmed to 25 °C
and stirred for 1h at this temperature. After that, the reaction mixture was
quenched with saturated aqueous solution of NH4Cl (10 mL) and
extracted with AcOEt (3 × 10 mL). The organic layer was dried over
2-(1-naphthyl)-3-(phenylselanyl)benzo[b]furan (3i): Yield: 0.057g
(57%). NMR 1H: CDCl3, 400 MHz, (ppm): 7.97-7.88 (m, 3H), 7.67 (dd, J
= 7.1 Hz; J = 1.1 Hz, 1H), 7.60-7.44 (m, 5H), 7.39-7.35 (m, 1H), 7.30-
7.23 (m, 3H), 7.13-7.08 (m, 3H). NMR 13C: CDCl3, 100 MHz, (ppm):
158.9, 154.9, 133.7, 132.2, 131.3, 130.9, 130.4, 129.9, 129.8, 129.1,
128.4, 127.3, 126.7, 126.3, 126.1, 125.9, 125.1, 124.8, 123.5, 121.3,
111.5, 103.8. MS (EI, 70 eV) m/z (relative intensity): 400 (63), 399 (8),
320 (100), 291 (23), 243 (27), 215 (41), 189 (13), 159 (9), 77 (8). HRMS
calcd for C24H16OSe: [M]+ 400.0366. Found [M]+ 400.0364.
MgSO4 and concentrated under reduced pressure using
a rotary
evaporator. The product was purified by flash chromatography in silica
gel using hexane/AcOEt (99:1) as eluent.
2-phenyl-benzo[b]furan (4b): Yield: 0.054g (73%). NMR 1H: CDCl3, 400
MHz, (ppm): 8.31-8.28 (m, 2H), 7.65-7.59 (m, 2H), 7.52-7.48 (m, 2H),
7.42-7.37 (m, 1H), 7.31-7.15 (m, 7H). NMR 13C: CDCl3, 100 MHz,
(ppm): 164.7, 151.9, 133.6, 132.8, 131.4, 130.3, 129.5, 129.4, 128.5,
128.3, 128.1, 125.9, 122.8, 122.4, 117.5, 94.5, 84.4. MS (EI, 70 eV) m/z
(relative intensity): 299 (7), 298 (29), 221 (2), 165 (7), 139 (3), 115 (2),
105 (100), 77 (34).
5-fluoro-2-phenyl-3-(phenylselanyl)benzo[b]furan (3j): Yield: 0.058g
(63%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.20-8.17 (m, 2H), 7.48-7.37
(m, 4H), 7.29-7.23 (m, 2H), 7.20-7.12 (m, 4H), 7.02 (td, J = 2.6 Hz; J =
1
9.0 Hz, 1H). NMR 13C: CDCl3, 100 MHz, (ppm): 159.6 (d, JCF = 239.6
General Procedure for the Bromination Reaction of 3a for the
3
Hz), 158.9, 150.2, 133.1 (d, JCF = 10.6 Hz), 130.9, 129.7, 129.5, 129.3,
Preparation of the Benzo[b]furan 5a: To
a solution of the 3-
129.1, 128.5, 127.7, 126.4, 112.9 (d, 2JCF4 = 26.5 Hz), 111.9 (d, 3’JCF = 9.5
phenylselanyl-benzo[b]furan 3a (0.25 mmol) in dried DCM (5 mL) under
argon, was slowly added the bromine (4 equiv) diluted in dried DCM (1
mL). Then, the reaction was warmed to reflux and stirred for 3h. After this
time, the mixture was diluted in DCM (10 mL) and washed with a
saturated aqueous NaCl solution (3 × 10 mL). The organic layer was
dried over MgSO4 and concentrated under reduced pressure using a
rotary evaporator. The product was purified by flash chromatography in
silica gel using hexane as eluent.
3,5-dibromo-2-phenylbenzo[b]furan (5a): Yield: 0.055g (63%). NMR
1H: CDCl3, 400 MHz, (ppm): 8.11 (d, J = 7.3 Hz, 2H), 7.65 (s, 1H), 7.49-
7.36 (m, 5H). NMR 13C: CDCl3, 100 MHz, (ppm): 153.3, 151.0, 131.9,
129.3, 128.8, 128.0, 126.9, 126.7, 120.8, 118.9, 114.7, 93.5. MS (EI, 70
eV) m/z (relative intensity): 354 (49), 352 (100), 350 (52), 245 (19), 243
(19), 192 (9), 163 (48), 138 (5), 122 (6), 82 (19), 63 (5). HRMS calcd for
C14H8Br2O: [M]+ 349.8942. Found [M]+ 349.8976.
2’
Hz), 106.7 (d,
J
= 25.6 Hz), 99.6 (d, JCF = 4.1 Hz) . MS (EI, 70 eV)
CF
m/z (relative intensity): 368 (35), 288 (100), 273 (8), 259 (17), 183 (42),
163 (7), 143 (6), 105 (2), 77 (6). HRMS calcd for C20H13FOSe: [M]+
368.0116. Found [M]+ 368.0122.
2-(4-chlorophenyl)-5-fluoro-3-(phenylselanyl)benzo[b]furan
(3k):
Yield: 0.061g (61%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.16-8.12 (m,
2H), 7.47-7.43 (m, 1H), 7.42-7.38 (m, 2H), 7.27-7.23 (m, 2H), 7.20-7.13
(m, 4H), 7.03 (td, J = 2.6 Hz; J = 9.0 Hz, 1H). NMR 13C: CDCl3, 100 MHz,
1
3
(ppm): 159.6 (d, JCF = 239.9 Hz), 157.7, 150.1, 135.5, 133.0 (d, JCF
=
2
10.6 Hz), 130.6, 129.4, 129.2, 129.9, 128.7, 128.2, 126.6, 113.2 (d, JCF
3’
2’
= 26.6 Hz), 111.9 (d,
J
= 9.5 Hz), 106.8 (d,
J
= 25.6 Hz), 100.2 (d,
CF
CF
4JCF = 4.1 Hz) . MS (EI, 70 eV) m/z (relative intensity): 404 (M+2; 16),
402 (M+; 40), 324 (34), 322 (100), 286 (19), 259 (15), 217 (24), 181 (26),
143 (12), 105 (1), 77 (7). HRMS calcd for C20H12ClFOSe: [M]+ 401.9726.
Found [M]+ 401.9727.
5-methyl-2-phenyl-3-(phenylselanyl)benzo[b]furan (3l): Yield: 0.022g
(24%). NMR 1H: CDCl3, 400 MHz, (ppm): 8.20-8.17 (m, 2H), 7.45-7.40
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201701205
European Journal of Organic Chemistry
FULL PAPER
Supporting Information (see footnote on the first page of this
manuscript): experimental procedures and characterization data,
including 1H and 13C NMR spectra.
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Acknowledgements
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The authors are grateful for fellowships and financial support
granted from the Fundação de Amparo à Pesquisa do Estado do
Rio Grande do Sul (FAPERGS/PQG-001/2013/1899-2551/13-0)
and Conselho Nacional de Desenvolvimento Científico
Tecnológico (CNPq/MCTI/Universal 14/2014/443080/2014-3).
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Keywords: Benzo[b]furans • Organoselanyl • Cyclization •
Copper • Heterocycles
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Synthesis of benzo[b]furans*
Jean C. Kazmierczak, Ana M. S. Recchi,
Fabiane Gritzenco, Everton B. Balbom,
Thiago Barcellos, Adriane Sperança
and Benhur Godoi*
Page No. – Page No.
Text for Table of Contents
*Heterocycles, Cyclization
Copper Iodide and Diorganyl
Diselenides-Promoted Cyclization of
2-Alkynyl-phenols: Alternative
Approach to 3-Organoselanyl-
benzo[b]furans
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