PdCl2-promoted electrophilic annulation of 2-alkynylphenol derivatives with disulfides or diselenides in the presence of iodine

Article abstract of DOI:10.1021/jo9016309

(Chemical Equation Presented) An efficient synthesis of 3-chalcogen-benzo[b]furans via palladium-promoted annulation reactions of 2-alkynylphenol derivatives with disulfides or diselenides and iodide has been developed. In the presence of I₂ an

Full text of DOI:10.1021/jo9016309

pubs.acs.org/joc
PdCl2-Promoted Electrophilic Annulation of 2-Alkynylphenol Derivatives
with Disulfides or Diselenides in the Presence of Iodine
Hui-Ai Du, Xing-Guo Zhang,* Ri-Yuan Tang, and Jin-Heng Li*
College of Chemistry and Materials Science, Wenzhou University, Wenzhou 325035, China
jhli@hunnu.edu.cn; xgzhang99@yahoo.com.cn
Received July 27, 2009
An efficient synthesis of 3-chalcogen-benzo[b]furans via palladium-promoted annulation reactions
of 2-alkynylphenol derivatives with disulfides or diselenides and iodide has been developed. In the
presence of I2 and PdCl2, both 3-sulfenylbenzofurans and 3-selenenylbenzofurans were selectively
prepared from the cyclization of 2-alkynyanisoles with disulfides or diselenides in moderate to
excellent yields.
Introduction
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pounds, are widely presented in a great number of biologi-
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7844 J. Org. Chem. 2009, 74, 7844–7848
Published on Web 09/25/2009
DOI: 10.1021/jo9016309
r
2009 American Chemical Society

Du et al.
JOCArticle
SCHEME 1
TABLE 1. Screening Optimal Conditionsa
entry
R
[Pd]
additive (equiv)
T (°C) yield (%)
1b
2
3
4
H (1a) PdCl2
H (1a) PdCl2
H (1a)
80
0
80 trace
80 50
80 mixture
80 77
80 57
80 57
80 25
80 trace
80 trace
80 46
80 53
80 56
80 55
80 49
100 76
100 58
25 34
80 <5
80 10
80 44
80 49
80 92
80 55
I2 (2)
H (1a)
I2 (2)/NaHCO3 (2)
I2 (2)
I2 (1.1)
I2 (2)
NIS (2)
ICl (2)
PhI(OAc)2 (2)
I2 (2)
5
H (1a) PdCl2
H (1a) PdCl2
H (1a) PdCl2
H (1a) PdCl2
H (1a) PdCl2
H (1a) PdCl2
H (1a) Pd(OAc)2
6
7c
8
9
10
11
12
13
14
15
16
17d
18
19
20
21
22
23
24
H (1a) Pd(PPh3)2Cl2 I2 (2)
H (1a) Pd(PPh3)4
H (1a) CuI
H (1a) AgI
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
I2 (2)
H (1a) PdCl2
H (1a) PdCl2
H (1a) PdCl2
Ts (1b) PdCl2
Bz (1c) PdCl2
Bn (1d) PdCl2
Ac (1e) PdCl2
Me (1f) PdCl2
3-chalcogen-benzo[b]furans, such as sulfenylbenzo[b]furans
and selenenylbenzo[b]furans, has been scarcely explored,8
although these compounds are known to exhibit a broad
range of biological activities. Larock and co-workers have
reported three examples of 3-chalcogen-benzo[b]furans
by the electrophilic cyclization of 2-(phenylethynyl)anisole
with 4-NO2C6H4SCl or PhSeCl (eq 1 in Scheme 1).9 Re-
cently, Zeni and co-workers described a novel approach to
2-chalcogen-benzo[b]furans and 2,3-dichalcogenebenzo[b]-
furans via the electrophilic cyclization of 2-chalcogen-
alkynylanisoles (eq 2).10 However, the two methods were
restricted to the use of toxic and instable sulfenyl halides as
the reaction partners. As a result, the development of direct
and concise method for synthesizing 3-chalcogenebenzofur-
ans using the stable and readily available materials remains
a challenging area for exploration. Very recently, we re-
ported a novel protocol for the synthesis of 3-sulfenylindoles
from the reaction between 2-(1-alkynyl)benzenamines and
disulfides in moderate to good yields using the Pd/air
catalytic systems (eq 3).11 Unfortunately, the reported stan-
dard conditions (PdCl2/air/DMSO) are incompatible with
1-(2-(2-methoxyphenyl)ethynyl)benzene and PhSSPh (2a)
for the construction of 2-phenyl-3-(phenylthio)benzofuran
(3) (entry 1 in Table 1). After a series of trials, we found that
in the presence of PdCl2 and I2 2-alkynylphenol derivatives
could undergo the annulation reaction with disulfides or
diselenides to afford the target 3-chalcogen-benzo[b]furans
Me (1f)
;
aReaction conditions: 1 (0.2 mmol), 2a (0.1 mmol), [Pd] (10 mol %),
and MeCN (2 mL) under N2 atmosphere for 36 h. In DMSO (2 mL)
b
under air atmosphere. cPdCl2 (5 mol %). dFor 12 h. The conversion of 1a
is 80% as determined by GC-MS analysis.
in moderate to excellent yields (eq 4). Here, we report our
results in detail.12
Results and Discussion
We began our study on the reaction between 2-(phenyl-
ethynyl)phenol (1a) and 1,2-diphenyl disulfide (2a) to screen
the optimal reaction conditions, and the results are summari-
zed in Table 1. The results demonstrated that no desired
product 3 was observed from the reaction of substrate 1a
with disulfide 2a and PdCl2 using DMSO or MeCN solvents
(entries 1 and 2).11 After checking the results of Zeni and
Larock, we envisioned that the generation of PhSX in situ
might promote the reaction by adding electrophiles.9,10 After
a series of failures, we were pleased to find that I2 could
mediate the annulation reaction (entries 3 and 4). Treatment
of 2-(phenylethylnyl)phenol (1a) with 1,2-diphenyl disulfide
(2a) and 2 equiv of I2 afforded the target product 3 in 50%
yield (entry 3). However, a mixture of products were ob-
served in the presence of NaHCO3 (entry 4).7n To our
delight, the yield of 3 was enhanced to 77% at 10 mol %
loading of PdCl2 (entry 5). It is noteworthy that decreasing
amount of either I2 or PdCl2 reduces the yield (entries 6 and
7). Encouraged by these results, other iodide reagents, such
as NIS, ICl, or PhI(OAc)2, were investigated in the presence
of PdCl2, and they are less effective than I2 (entries 8-10).
Among the examination of palladium catalysts, other palla-
dium catalysts, including Pd(OAc)2, Pd(PPh3)2Cl2, and Pd-
(PPh3)4, were inferior to PdCl2 (entries 11-13). Other
transition-metal catalysts, CuI and AgI, were also tested,
(8) (a) Abramov, M. A.; Dehaen, W.; D’hooge, B.; Petrov, M. L.; Smeets,
S.; Toppeta, S.; Voetsa, M. Tetrahedron 2000, 56, 3933. (b) Matsugi, M.;
Murata, K.; Nambu, H.; Kita, Y. Tetrahedron Lett. 2001, 42, 1077. (c)
D’hooge, B.; Smeets, S.; Toppet, S.; Dehaen, W. Chem. Commun. 1997, 1753.
(9) Yue, D.; Yao, T.; Larock, R. C. J. Org. Chem. 2005, 70, 10292.
~
(10) Manarin, F.; Roehrs, J. A.; Gay, R. M.; Brandao, R.; Menezes, P.
H.; Nogueira, C. W.; Zeni, G. J. Org. Chem. 2009, 74, 2153.
(11) Guo, Y.-J.; Tang, R.-Y.; Li, J.-H.; Zhong, P.; Zhang, X.-G. Adv.
Catal. Synth. 2009, in press.
(12) For papers on the use of disulfides in our group, see: (a) Tang, R.-Y.;
Zhong, P.; Lin, Q.-L. J. Fluor. Chem. 2006, 127, 948. (b) Tang, R.-Y.; Zhong,
P.; Lin, Q.-L. Synlett 2007, 1, 85. (c) Wang, Z.-L.; Tang, R.-Y.; Luo, P.-S.;
Deng, C.-L.; Zhong, P.; Li, J.-H. Tetrahedron 2008, 64, 10670. (d) Luo, P.-S.;
Yu, M.; Tang, R.-Y.; Zhong, P.; Li, J.-H. Tetrahedron Lett. 2009, 50, 1066.
(e) Luo, P.-S.; Wang, F.; Li, J.-H.; Tang, R.-Y.; Zhong, P. Synthesis 2009, 6,
921.
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JOCArticle
TABLE 2. PdCl2-Protomoted Electrophilic Annulation Reactions of 2-Alkynylanisoles (1) with Disulfides (2)a
aReaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), PdCl2 (10 mol %), I2 (2 equiv), and MeCN (2 mL) at 80 °C under N2 atmosphere. bA mixture of
byproduct was isolated.
and the results showed that they were less active (entries 14
and 15). Screening effect of reaction temperature indicated
that results identical to those of 80 °C were observed at
100 °C for 36 h (entries 5 and 16) and at 100 °C shortening
reaction time to 12 h resulted in an unsatisfactory yield (entry
17). It was found that room temperature gave a low yield
(entry 18). Finally, several O-substituted 2-(phenylethylnyl)-
phenol derivatives were investigated (entries 14-18). We
found that three O-substituents, acetyl, benzyl, and methyl,
were tolerated well, but both Ts (4-methylbenzenesulfonoyl)
and Bz (benzoyloxy) displayed less activity (entries 19-23).
While 2-(2-phenylethynyl)phenyl 4-methylbenzenesulfonate
(1b), for instance, underwent the reaction with disulfide 2a,
PdCl2 and I2 to afford the desired product 3 in less than 5%
yield (entry 19), 1-(2-(2-methoxyphenyl)ethynyl)benzene (1f)
gave 92% yield (entry 23). Noteworthy is that without Pd
catalysts only 55% yield is isolated from the reaction of
substrate 1f with I2 (entry 24).
with 2-(2-methoxyphenyl)alkynylanisole (1f) (entries 1-9).
The results demonstrated that the electronic effect of sub-
stituents on the aryl moiety affected the reaction, with the
order of the activity being electron-rich disulfides > elec-
tron-deficient disulfides (entries 1-7). Disulfide 2b bearing a
p-methyl group, for instance, reacted with substrate 1f,
PdCl2 and I2 to afford the corresponding product 4 in 92%
yield (entry 1), but disulfides 2c and 2h, having an o-methyl
group or a p-nitro group, has less activity (entries 2 and 7).
Gratifyingly, 1,2-dimethyl disulfide (2i), an aliphatic disul-
fide, was also suitable for the reaction with substrate 1f under
the standard conditions (entry 8). However, the reactions of
1,2-dimethyl disulfide (2j) or 1,2-dibenzyl disulfide (2k) with
alkyne 1f, PdCl2, and I2 were unsuccessful, and a mixture of
byproduct, 3-iodo-2-phenylbenzofuran, and 2-phenylbenzo-
furan was obtained in 70% total yields (entries 9 and 10).
Subsequently, a variety of 2-alkynylanisoles 1g-o were
examined (entries 11-23). We found that the standard
conditions were compatible with substrates with aromatic
groups at the terminal of 1-ethynyl-2-methoxybenzene but
inconsistent with alkyl group. For example, treatment of
substrate 1g bearing a p-CF3C6H4 group with disulfide 2a,
With the optimal reaction conditions in hand, the scope of
both 2-alkynylanisoles and disulfides as the reaction partners
was explored for the annulation reaction (Table 2). Initially,
a number of disulfides 2b-j were investigated by reacting
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Du et al.
JOCArticle
SCHEME 2. Selective Annulation Reaction
TABLE 3. Annulation Reactions of 2-Alkynylanisoles (1) with Disele-
nides (2)a
SCHEME 3. Possible Mechanism
89% yields, respectively (entries 1 and 2). Good yield was still
achieved from the reaction of substrate 1f with aliphatic
diselenide 2n (entry 3). Gratifyingly, substrate 1i, bearing a
4-MeC6H4 group at the terminal 1-ethynyl-2-methoxyben-
zene, smoothly underwent the reaction in satisfactory yield
(entry 4). Treatment of substrates 1n or 1o, bearing a group
on the anisole moiety, with 1,2-diphenyldiselenide (2l) was
also successful in good yields (entries 5 and 6).
To elucidate the mechanism, a controlled reaction be-
tween 3-iodo-2-phenylbenzofuran and disulfide 2a was car-
ried out in the presence of PdCl2 and I2 (eq 5). The results
showed that no target product 3 was observed by GC-MS
analysis, which suggested that the reaction proceeded not via
an iodocyclization process. The results of entries 9 and 10 in
Table 2 also supported it.
aReaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), PdCl2 (10 mol %),
I2 (2 equiv), and MeCN (2 mL) at 80 °C under N2 atmosphere.
PdCl2, and I2 afforded the corresponding product 13 in 65%
yield (entry 11). Substrate 1j having a bulky o-methyl group
was also successful in 44% yield (entry 14). We were pleased
to observe that 2-(2-(2-methoxyphenyl)ethynyl)thiophene
(1l) was a suitable substrate in moderate yield (entry 18).
However, annulation of 1-methoxy-2-(oct-1-ynyl)benzene
(1m) failed under the same conditions (entry 19). To our
delight, methyl and chloro substituents on the anisole moiety
were perfectly tolerated (entries 20-23). The reaction of
1-methoxy-4-methyl-2-(2-phenylethynyl)benzene (1n), for
instance, with disulfides 2a, 2d, or 2e, PdCl2, and I2
was conducted successfully in 90%, 94%, and 75% yields,
respectively (entries 20-22). It was noted that the reaction of
4-methyl-2-(phenylethynyl)phenol (1p) with disulfide 2a was
also conducted smoothly in 75% yield under the standard
conditions (entry 24).
The selectivity toward O-nucleophile and N-nucleophile
was also investigated under the standard conditions, and the
results are shown in Scheme 2. We found that the selectivity
toward N-nucleophile, not O-nucleophile, was observed for
the reaction of substrate 1q with 1,2-diphenyl disulfide (2a),
PdCl2, and I2.11
As expected, the reactions of diselenides 2 with 2-alkynyl-
anisoles 1 proceeded successfully to give the desired products
in good yields (Table 3). In the presence of PdCl2 and I2,
substrate 1f was reacted with diaryl diselenides 2l or 2m
smoothly to give the target products 29 and 30 in 81% and
A possible mechanism was proposed as outlined in
Scheme 3 on the basis of the reported mechanism9,10 and
the present results. The reaction could be conducted in the
presence of I2 alone, which suggests that the reaction of
disulfide 2 with I2 yielded PhSI A in situ, followed by the
electrophilic addition of PhSI (A) with intermediate B to
afford intermediate C. Annulation of intermediate C gave
intermediate D. The Me group can be removed from inter-
mediate D with aid of an I- nucleophile to afford the target
product. However, we cannot rule out a sulfenylpalladation
process (1 f E) in the present reaction. We deduced that
roles of PdCl2 might be included: (1) complexation of the
J. Org. Chem. Vol. 74, No. 20, 2009 7847

Du et al.
JOCArticle
triple bond to active the reaction species; (2) as a Lewis acid
to stable and active the intermediates; and (3) as a catalyst for
the chalcogen-palladation process. Study of the detailed
mechanism is in progress.
mixture was poured into ethyl acetate, which was washed with
saturated NaS2O3 and extracted with diethyl ether. The organic
layers were dried over anhydrous Na2SO4 and evaporated under
vacuum. The residue was purified by flash column chromato-
graphy (hexane/ethyl acetate) to afford the products.
In summary, we have developed a practical PdCl2-pro-
moted electrophilic annulation method that allows the reac-
tions between 2-alkynylphenol derivatives and disulfides (or
diselenides) for the synthesis of 3-chalcogen-benzo[b]furans.
The key of the reaction is generation of RYI (Y = S, Se) in
situ from the reaction of RYYR with I2. It is noteworthy that
PdCl2 can improve the reaction. In the presence of PdCl2 and
I2, a variety of 2-alkynylphenol derivatives underwent the
annulation reactions with disulfides or diselenides success-
fully to afford the corresponding 3-sulfenylbenzofurans and
3-selenenylbenzofuran in moderate to excellent yields.
2-Phenyl-3-(phenylthio)benzofuran (3). White solid. Mp
65.0-66.8 °C (uncorrected). H NMR (300 MHz, CDCl3) δ:
1
8.23 (d, J = 8.4 Hz, 2H), 7.57(d, J = 8.1 Hz, 1H), 7.49-7.33 (m,
5H), 7.24-7.18 (m, 5H), 7.10 (m, 1H). 13C NMR (75 MHz,
CDCl3) δ: 157.5, 154.0, 136.1, 130.8, 129.8, 129.4, 129.0, 128.6,
127.4, 126.5, 125.5, 125.3, 123.5, 120.4, 111.3, 104.7. IR (KBr,
cm-1): 1478, 1454, 1439, 737, 686. LRMS (EI, 70 eV) m/z: 302
(Mþ, 100), 225 (33), 197 (30), 165 (28). HRMS (APCI): calcd for
C20H15OSþ ([M þ H]þ) 303.0838, found 303.0825.
Acknowledgment. We thank the National Natural Science
Foundation of China (No. 20872112), Zhejiang Provincial
Natural Science Foundation of China (Nos. Y407116 and
Y4080169), and Wenzhou University (No. 2007L004) for
financial support.
Experimental Section
Typical Experimental Procedure for the PdCl2-Promoted
Electrophilic Annulation Reaction in the Presence of I2. A
mixture of 2-alkynylphenol derivative 1 (0.2 mmol), disulfide
or diselenide 2 (0.1 mmol), I2 (2 equiv), and PdCl2 (10 mol %) in
MeCN (2 mL) was stirred at 80 °C for the indicated time until
complete consumption of starting material as monitored by
TLC and GC-MS analysis. After the reaction was finished, the
Supporting Information Available: Characterization data
for compounds 3, 4, 6-9, 11, 14-21, 23-27, and 29-34 and
copies of spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
7848 J. Org. Chem. Vol. 74, No. 20, 2009