Palladium-catalyzed suzuki coupling with terminal alkynes - Application to the synthesis of 2,3-disubstituted benzo[b]furans

Article abstract of DOI:10.1002/ejoc.200500166

The Suzuki coupling reaction between alkynylboronic esters (generated in situ from acetylenic derivatives) and aryl bromides, pyridyl bromides or vinyl bromides is reported. 5-endo-dig-iodocyclisation of o-alkynylanisoles, generated by this palladium-catalyzed Suzuki coupling reaction, was performed with N-iodosuccinimide (NIS) in the presence of BCl₃. 2-Substituted 3-iodobenzo[b]furans were synthesized and transformed into 2,3-disubstituted benzo[b]furans by Suzuki coupling reactions in high yields. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejoc.200500166

FULL PAPER
Palladium-Catalyzed Suzuki Coupling with Terminal Alkynes – Application to
the Synthesis of 2,3-Disubstituted Benzo[b]furans
Françoise Colobert,*[a] Anne-Sophie Castanet,[a] and Olivier Abillard[a]
Keywords: Suzuki reaction / Alkynes / Iodocyclisation / Benzofuran / Palladium
The Suzuki coupling reaction between alkynylboronic esters
(generated in situ from acetylenic derivatives) and aryl bro-
mides, pyridyl bromides or vinyl bromides is reported. 5-
endo-dig-iodocyclisation of o-alkynylanisoles, generated by
this palladium-catalyzed Suzuki coupling reaction, was per-
formed with N-iodosuccinimide (NIS) in the presence of
BCl3. 2-Substituted 3-iodobenzo[b]furans were synthesized
and transformed into 2,3-disubstituted benzo[b]furans by Su-
zuki coupling reactions in high yields.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
The benzo[b]furan ring can be found as a key structural
unit in many biologically active compounds which have ap-
plications not only as pharmaceuticals but also as flavoring
and fragrance compounds.[1] Recently there has been a
growing interest in developing general and versatile syn-
thetic methods for the synthesis of benzofuran derivatives
because of their activities as anti-tumor agents,[2] as ligands
of the adenosine A1 receptor,[3] as inhibitors of 5-lipoxygen-
ase, as antagonists of angiotensin II receptors[4] or as blood
coagulation factor Xa inhibitors.[5] Some 2-arylbenzofuran
derivatives are potent calcium blockers[6] while others show
very good fungicidal activity in vitro and in vivo.[7]
Although various methods for the preparation of
benzo[b]furans[8] are known, recent research has focused on
the utilization of palladium-catalyzed systems. Most of
these strategies employ suitably functionalized alkynes as
starting materials.[9] The palladium-catalyzed coupling/cyc-
lization of alkynes with o-hydroxy(hetero)aryl halides repre-
sents one of the most straightforward methods for the prep-
aration of 2-substituted benzofurans.[10]
Scheme 1. Suzuki coupling of acetylenic boronic esters.
sp-sp2 Suzuki coupling reaction, giving access to function-
alized benzofuran derivatives. o-Alkynylanisoles, generated
by the palladium-catalyzed Suzuki coupling reaction, were
submitted to 5-endo-dig-iodocyclisation using NIS in the
presence of BCl3. Further Suzuki coupling reactions af-
forded 2,3-disubstituted benzo[b]furans in high yields
(Scheme 2).
Recently, we reported on the palladium-catalyzed coup-
ling reaction between aryl bromides and in situ generated
acetylenic boronic esters[11] (Scheme 1).
As part of a program to develop direct synthesis of biolo-
gically active compounds, we report herein a full account
of our results as well as a new synthetic application of this
[a] Laboratoire de stéréochimie associé au CNRS, Université Louis
Pasteur (ECPM),
25 Rue Becquerel, 67087 Strasbourg, France
Fax: +33-3-90242742
E-mail: fcolober@chimie.u-strasbg.fr
Scheme 2. Synthesis of 2,3-disubstituted benzo[b]furans.
3334
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200500166
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Pd-Catalyzed Suzuki Coupling with Terminal Alkynes
FULL PAPER
to generate nucleophilic acetylides in situ, avoiding the ne-
cessity of preparing and storing large quantities of poten-
tially unstable organometallic reagents.
Results and Discussion
Suzuki Coupling Reaction of Alkynes with Various
Halogenobenzene Derivatives[11]
Additionally, the coupling reaction was attempted with
B(OMe)3 instead of B(OiPr)3. In this case, the addition of
THF was not necessary to dissolve the “boronate complex”
and yields were similar to those obtained with B(OiPr)3.
Thus we attempted to perform the sp-sp2 Suzuki coup-
ling reaction sub-stoichiometrically with respect to boron.
The coupling reaction between 1-octyne and p-bromotol-
uene in the presence of 0.25 equiv. of trimethylborate in
THF was investigated. A clean reaction was observed, af-
fording the coupling product in 70% yield.
Treatment of alkynyllithium reagents (1.3 equiv.) with tri-
isopropylborate (1.3 equiv.) in DME/THF (10:1) at –78 °C
afforded the “lithiated boronate complex”. The latter un-
dergoes transmetallation with the Pd complexes obtained
by insertion of palladium [0.03 equiv. Pd(PPh3)4] into the
C–Br bond of aryl or vinylbromides (1 equiv.) followed by
reductive elimination to give the coupling products 2a–g
(Table 1).
Excellent to good yields were obtained especially with
Actually, trimethylborate generates in situ the alkynyl-
ortho-substituted and unactivated arylbromides without boronate complex. The latter undergoes then the transme-
homocoupling of terminal alkynes.
tallation step setting free again trimethylborate. This be-
Uncharacterized decomposition products were observed came obvious by studying the course of the reaction with
for the Suzuki coupling between bromostyrene and 1-oc- 11B NMR spectroscopy. The trimethylborate signal disap-
tyne (Table 1, entry 6).
pears first in favor of the corresponding alkynylboronate
The boronate complex can be in situ generated from the complex, then reappears and gains intensity as the reaction
lithiated acetylenic substrate with triisopropylborate in proceeds (Figure 1).
THF/DME (THF is necessary to dissolve the “ate com-
Therefore, we propose a catalytic cycle for this reaction,
plex”). This Suzuki type methodology presents the ability catalytic both in palladium and boron (Scheme 3).
Table 1. Palladium-catalyzed arylation or vinylation of alkynyllithium reagents mediated by B(OiPr)3. [Reaction conditions: alkyne (1.3
equiv.), nBuLi (1.4 equiv.), B(OiPr)3 (1.3 equiv.), bromide (1 equiv.), Pd(PPh3)4 (0.03 equiv.), DME/THF: 10:1, reflux.].
[a] DME/THF: 2.5:1. [b] Formation of ditolylacetylene (40%). [c] Uncharacterized decomposition products were observed.
Eur. J. Org. Chem. 2005, 3334–3341
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F. Colobert, A.-S. Castanet, O. Abillard
FULL PAPER
Figure 1. 11B NMR signals during the Suzuki coupling of lithiated octynylboronate with p-bromotoluene.
Scheme 3. sp-sp2 Suzuki coupling reaction sub-stoichiometric in B(OMe)3.
Suzuki Coupling Reaction of Alkynes with Functionalized
o-Hydroxyhalogenobenzenes
Table 2. Suzuki coupling of alkynyllithium reagents 1 with o-hy-
droxyhalogenobenzene derivatives [reaction conditions: alkyne (1.3
equiv.), nBuLi (1.4 equiv.), B(ORЈ)3 (1.3 equiv.), aryl bromide or
aryl iodide (1 equiv.), Pd(PPh3)4 (0.03 equiv.) or Pd(OAc)2 (0.03
equiv.)/PPh3 (0.09 equiv.), reflux.].
With the intention of preparing o-hydroxyalkynylben-
zene derivatives as suitable precursors for benzo[b]furans,
the Suzuki coupling reaction of alkynes with functionalized
o-hydroxyhalogenobenzenes was next investigated. To de-
termine the most suitable o-hydroxyhalogenobenzene deriv-
ative, the sp-sp2 Suzuki coupling reaction was performed
between 1-octyne or phenylacetylene and either o-iodo-
phenol, o-(trimethylsilyloxy)iodobenzene, o-(tert-butyldi-
methylsilyloxy)iodobenzene, and o-bromo- and iodoanisole
(Table 2).
No reaction occurred between nonprotected o-iodo-
phenol and 1-octyne under the previously described[11] con-
ditions (Table 2, entry 1).
The reaction of o-(trimethylsilyloxy)iodobenzene and 1-
octyne gave the deprotected o-bromophenol and traces of
2-hexylbenzo[b]furan. Thus, the trimethylsilyl group was de-
protected under our reaction conditions and deprotection
Entry
R
RЈ
RЈЈ
X
Solvent
Yield [%]
1
2
3
4
5
6
C6H13 iPr
C6H13 iPr
C6H13 Me
C6H13 iPr
C6H13 Me
H
I
I
I
Br
Br
I
DME/THF[a]
DME/THF[a]
DME
0
SiMe3
TBDMS
Me
Me
Me
0[b]
2h, 74
2i, 75
2i, 79
2j, 89
DME/THF[a]
DME
Ph
Me
DME
[a] DME/THF: 10:1. [b] Traces of hexylbenzo[b]furan.
of the coupling product and cyclization occurred at once benzene in DME. The coupling product 2h was obtained in
(Table 2, entry 2).
74% yield (Table 2, entry 3).[12]
Therefore, we tried the coupling reaction with the ex-
When the sp-sp2 coupling reaction was performed with
pected more stable o-(tert-butyldimethylsilyloxy)iodo- 2-bromo- or 2-iodoanisole, and either 1-octyne or phenyl-
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Pd-Catalyzed Suzuki Coupling with Terminal Alkynes
FULL PAPER
acetylene, the coupling products were obtained in excellent ter gentle heating for 4 h in the formation of the corre-
yields (Table 2, entries 4, 5, 6).
sponding coupling product. For the deprotection/iodocy-
clization step, it was necessary to distil off the coupling sol-
vent (DME or CH3CN) before the addition of BCl3 (1.4
equiv.) and NIS (1.7 equiv.) in CH2Cl2. In this way, 2-sub-
stituted 3-iodobenzo[b]furans were obtained in moderate
yields (Table 4).
Formation of 2-Substituted Benzo[b]furans and 2-
Substituted 3-Iodobenzo[b]furans via Deprotection/(Iodo)-
cyclization
The Suzuki coupling products 2h–j were converted in a
“one pot” deprotection/cyclization step into benzo[b]furans
(Table 3).
Table 4. Tandem sp-sp2 Suzuki coupling/5-endo-dig-iodocyclization
starting from o-iodoanisole and terminal acetylenes.
Table 3. “One pot” deprotection/cyclization of 2h–j [reaction condi-
tions: alkyne 2 (1 equiv.)].
Entry
R
Solvent
Yield [%]
Entry
Starting
products
R
RЈЈ
R1 Reagents
Yield [%]
1
2
3
Ph
Ph
C6H13
DME
CH3CN
DME
3c, 52
3c, 37
3d, 58
1
2
3
4
5
6
7
8
2h
2j
2j
2i
2j
2j
2j
2i
C6H13 TBDMS
Ph
Ph
H
H
H
H
H
I
TBAF[a]
LiI[b]
3a, 75
3b, 35
3b, 61
3a, 46
3b, 59
3c, 63
3c, 80
3d, 78
Me
Me
[c]
BCl3
BCl3
[c]
C6H13 Me
As side products, varying ratios (10 to 20%) of 2-substi-
tuted benzo[b]furans, as well as small amounts (about 7%)
of diiodinated products with one iodine in position 3 and
another iodine on the aromatic cycle, were obtained.
[d]
Ph
Ph
Ph
Me
Me
Me
BCl3/AgNO3
[e]
BCl3/I2
I
I
BCl3/NIS[f]
BCl3/NIS[f]
C6H13 Me
[a] TBAF (1.3 equiv.), THF, room temp. [b] LiI (2.4 equiv.), collid-
ine, reflux. [c] BCl3 1 m in heptane (1.6 equiv.), CH2Cl2, room temp.
[d] BCl3 1 m in heptane (1.6 equiv.), AgNO3 (0.1 equiv.). [e] BCl3
1 m in heptane (1.6 equiv.), I2 (3.2 equiv.), CH2Cl2, room temp. [f]
BCl3 1 m in heptane (1.6 equiv.), NIS (2 equiv.), CH2Cl2, room
temp.
Formation of 2,3-Disubstituted Benzo[b]furans via Suzuki
Coupling
The 2-substituted 3-iodobenzo[b]furans 3c–d were then
submitted to a further Suzuki coupling reaction, affording
2,3-disubstituted benzo[b]furans 4 (Table 5).
Deprotection of the tert-butyldimethylsilyloxy derivative
2h in the presence of TBAF and molecular sieves afforded
the cyclization product 3a in 75% yield (Table 3, entry 1).
Furthermore, the deprotection/cyclization of the meth-
oxy derivatives 2i and 2j was performed under different con-
ditions (LiI, BCl3, BCl3/AgNO3) as illustrated in Table 3
(entries 2–5). The best yield (61%) was obtained starting
from the anisole 2j in the presence of BCl3 (entry 3). With
these reaction conditions, we noticed that the deprotection
of the methoxy with BCl3 was complete. Therefore, in order
to enhance the reactivity of the triple bond towards cycliza-
tion, AgNO3 was added but the yield was not improved
(Table 3, entry 5).
Table 5. Suzuki coupling reaction of 3c–d with various boronic ac-
ids [reaction conditions: benzofuran 3 (1 equiv.), Pd(OAc)2 (5.2%),
PPh3 (14.6%), boronic acid (1.5 equiv.), CsF (4.7 equiv.), DME,
75 °C].
Entry
R
R2
Yield [%]
We next attempted a “one pot” deprotection/iodocycliz-
ation of anisole 2j in the presence of iodine.[13] The concom-
itant treatment of 2j with BCl3 and I2 gave rise to iodoben-
zofuran 3c in 63% yield. Replacement of I2 by NIS led to
an improvement of the yield and compounds 3c and 3d
were isolated respectively in 80% and 78% yield.
1
2
3
4
C6H13
Ph
Ph
Ph
4a, 90
4b, 91
4c, 91
4d, 64
Naphthyl
o-MeO–Ph
o-Me–Ph
Ph
We next investigated the tandem sp-sp2 Suzuki coupling/
Good to excellent yields were obtained using cesium
5-endo-dig-iodocyclization from o-iodoanisole and terminal fluoride as base. The coupling reaction with sterically more
alkynes. Addition of B(OMe)3 (0.85 equiv.) to the lithiated hindered ortho-substituted boronic acids, as for example o-
alkyne at room temperature, followed by a solution of tolylboronic acid, gave the 2,3-disubstituted benzo[b]furan
Pd(OAc)2 (5%), PPh3 (15%), and o-iodoanisole, resulted af- 4d in lower yield (Table 5, entry 4).
Eur. J. Org. Chem. 2005, 3334–3341
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F. Colobert, A.-S. Castanet, O. Abillard
FULL PAPER
mixture was heated under reflux for 16 h (or until the complete
disappearance of the aryl halide) and allowed to cool to room tem-
perature. After addition of water and extraction with ethyl acetate,
the combined organic phases were dried with magnesium sulfate,
filtered and concentrated under reduced pressure. The residue was
purified by flash chromatography.
1-Methyl-4-(oct-1-ynyl)benzene (2a): Yield 98%, colorless oil. 1H
NMR (200 MHz, CDCl3): δ = 0.91 (t, J = 6 Hz, 3 H), 1.27–1.61
(m, 8 H), 2.33 (s, 3 H), 2.40 (t, J = 7.0 Hz, 2 H), 7.19 (AB, J =
8.1 Hz, Δν = 40.6 Hz, 4 H) ppm. 13C NMR (50 MHz, CDCl3): δ =
14.2, 19.5, 21.5, 22.7, 28.7, 28.9, 31.5, 80.7, 89.7, 121.1, 129.0,
Conclusions
We have shown that Suzuki aryl-alkynyl or vinyl-alkynyl
coupling is very efficient in the synthetic chemistry of enyne
derivatives. Suzuki coupling of acetylenic boronic esters
with o-halogenoanisole or o-silyloxy-iodobenzene followed
by deprotection/intramolecular cyclization afforded 2-sub-
stituted benzo[b]furans in good yield. Furthermore “one
pot” deprotection/5-endo-dig-iodocyclisation of o-alkynyl-
anisoles represents a good way to obtain 2-substituted 3-
iodobenzo[b]furans. These compounds can be transformed
into 2,3-disubstituted benzo[b]furans by further palladium-
catalyzed Suzuki coupling reactions.
131.5, 137.5 ppm. IR (neat): ν = 2956 (m), 2930 (s), 2858 (m), 2199
˜
(w), 1510 (m), 1456 (w), 1106 (w), 816 (m), 757 (w) cm–1. This com-
pound was previously described.[14]
1-Methyl-4-(phenylethynyl)benzene (2b): Yield 96%, White solid,
m.p. 70 °C. 1H NMR (200 MHz, CDCl3): δ = 2.37 (s, 3 H), 7.30
(AB, J = 8 Hz, Δν = 53.9 Hz, 4 H), 7.31–7.36 (m, 3 H), 7.50–7.55
(m, 2 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 21.6, 88.9, 89.7,
120.3, 123.6, 128.2, 128.4, 129.3, 131.6, 131.7, 138.5 ppm. IR
Experimental Section
General Remarks: All reactions were carried out under argon.
NMR spectra were recorded with a Bruker AC-200. Chemical
shifts (δ) are given in ppm, coupling constants (J) in Hz. 1H NMR
spectra are relative to tetramethylsilane (δ = 0.00 ppm). 13C NMR
spectra are relative to CDCl3 (δ = 77.0 ppm). IR spectra were re-
corded with a Perkin–Elmer Spectrum One. Melting points were
obtained with a Büchi 535 apparatus and were not corrected. Flash
Chromatography was carried out with silica gel 70–230 Mesh
(Merck) using hexane as eluent. Thin layer chromatography was
performed with silica gel 60 F254 (Merck). DME was dried with
sodium/benzophenone and degassed prior to use. B(OMe)3 and
B(OiPr)3 were freshly distilled over sodium.
(neat): ν = 3080 (w), 3051 (w), 2923 (m), 1655 (w), 1594 (w), 1510
˜
(m), 1441 (s), 1110 (m), 1070 (m), 915 (m), 818 (s), 755 (s), 690
(s) cm–1. This compound was previously described.[14]
1
Trimethyl(p-tolylethynyl)silane (2c): Yield 55%, pale yellow oil. H
NMR (200 MHz, CDCl3): δ = 0.25 (s, 9 H), 2.35 (s, 3 H), 7.24 (AB,
J = 8 Hz, Δν = 51.9 Hz, 4 H) ppm. 13C NMR (50 MHz, CDCl3): δ
= 0.1, 21.6, 93.3, 105.4, 120.1, 129.0, 132.0, 138.7 ppm. IR (neat):
ν = 3015 cm–1 (w), 2961 (w), 2925 (w), 2156 (m), 1507 (w), 1251
˜
(m), 1216 (m), 865 (m), 843 (m), 818 (w), 758 (s), 699 (w) cm–1.
This compound was previously described.[15]
1
Representative Procedure for the Preparation of Lithiated 1-Alkynyl-
(triisopropyl)borates and Their Cross-Coupling with Aryl and Vinyl
Bromides: A solution of n-butyllithium in hexanes (1.35 equiv.) was
slowly added to a cooled solution (–78 °C) of 1-alkyne (1.85 mmol,
1.30 equiv.) in dimethoxyethane (10 mL). After 1 h at –78 °C, trii-
sopropylborate (1.35 equiv.) was slowly added, and, 2 h later, the
temperature was raised during 30 min to 20 °C. In parallel, tetrakis-
(triphenylphosphane)palladium(0) (0.03 equiv.) and the o-hydroxy-
halogenobenzene derivative (1.00 equiv.) were dissolved in dime-
thoxyethane (10 mL) and stirred for 10 min at room temperature.
To the lithiated alkynylborate were successively added anhydrous
tetrahydrofuran (3 mL) and the solution of Pd(PPh3)4 and aryl ha-
lide via cannula. The cannula and the flask were rinsed with dime-
thoxyethane (2×5 mL). The reaction mixture was heated under re-
flux for 16 h (or until the complete disappearance of the aryl ha-
lide) and allowed to cool to room temperature. After addition of
water and extraction with ethyl acetate, the combined organic
phases were dried with magnesium sulfate, filtered and concen-
trated under reduced pressure. The residue was purified by flash
chromatography.
4-(Oct-1-ynyl)benzonitrile (2d): Yield 79%, colorless oil. H NMR
3
(200 MHz, CDCl3): δ = 0.91 (t, J = 6.7 Hz, 3 H), 1.26–1.65 (m, 8
3
H), 2.43 (t, J = 7.0 Hz, 2 H), 7.51 (AB, J = 8.6 Hz, Δν = 21.7 Hz,
4 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 14.1, 19.6, 22.6, 28.5,
28.7, 31.4, 79.5, 95.8, 110.8, 118.8, 129.2, 132.0, 132.2 ppm. IR
(neat): ν = 3020 (w), 2956 (m), 2931 (s), 2227 (s), 1604 (m), 1500
˜
(m), 1466 (m), 1406 (w), 1379 (w), 1330 (w), 1271 (w), 1216 (w),
1177 (w), 1105 (w), 840 (s), 758 (s), 668 (w), 555 (m) cm–1. This
compound was previously described.[16]
1-Methyl-2-(oct-1-ynyl)benzene (2e): Yield 75%, colorless oil. 1H
NMR (200 MHz, CDCl3): δ = 0.94 (t, 3J = 6.5 Hz, 3 H), 1.30–1.70
3
(m, 8 H), 2.44 (s, 3 H), 2,47 (t, J = 6.7 Hz, 2 H), 7.09–7.20 (m, 3
H), 7.39 (d, J = 6.5 Hz, 1 H) ppm. 13C NMR (50 MHz, CDCl3):
3
δ = 14.2, 19.7, 20.8, 22.7, 28.7, 29.0, 31.5, 79.6, 94.5, 124.0, 125.5,
127.5, 129.4, 131.9, 140.0 ppm. IR (neat): ν = 3062 (m), 3022 (m),
2930 (s), 2858 (s), 2227 (w), 1600 (w), 1486 (m), 1457 (m), 1378
(m), 1330 (w), 1115 (w), 1044 (w), 755 (s), 716 (m) cm–1. This com-
pound was previously described.[14]
(Dec-1-en-3-ynyl)benzene (2f): Yield 60%, pale yellow oil. 1H NMR
˜
3
(200 MHz, CDCl3): δ = 0.92 (t, J = 6.7 Hz, 3 H), 1.26–1.62 (m, 8
H), 2.38 (td, J = 6.8, J = 2.2 Hz, 2 H), 6.16 (dt, J = 16.2, J =
Representative Procedure for the Preparation of Lithiated 1-Alkynyl-
(trimethyl)borates and Their Cross-Coupling with o-Hydroxyhalo-
genobenzenes: A solution of n-butyllithium in hexanes (1.35 equiv.)
was slowly added to a cooled solution (–78 °C) of 1-alkyne
(10.4 mmol, 1.30 equiv.) in dimethoxyethane (50 mL). After 1 h at
–78 °C, trimethylborate (1.35 equiv.) was slowly added and, 2 h
later, the temperature was raised during 30 min to 20 °C. In paral-
lel, tetrakis(triphenylphosphane)palladium(0) (0.02 equiv.) and the
o-hydroxyhalogenobenzene derivative (1.00 equiv.) were dissolved
in dimethoxyethane (30 ml) and stirred for 10 min at room tem-
perature. To the lithiated alkynylborate was added the solution of
Pd(PPh3)4 and the aryl halide via cannula. The cannula and the
flask were rinsed with dimethoxyethane (2×5 mL). The reaction
3
4
3
5
3
2.2 Hz, 1 H), 6.87 (d, J = 16.2 Hz, 1 H), 7,24–7,39 (m, 5 H) ppm.
13C NMR (50 MHz, CDCl3): δ = 14.2, 19.8, 22.7; 28.7, 31.5, 79.8,
93.2, 109.0, 126.1, 128.3, 128.7, 136.7, 140.1 ppm. IR (neat): ν =
˜
3061 (w), 3028 (w), 2955 (m), 2930 (s), 2858 (m), 2210 (w), 1697
(br), 1635 (w), 1599 (w), 1492 (w), 1449 (m), 1271 (w), 952 (m),
748 (m), 692 (m) cm–1. This compound was previously described.[17]
(3-Oct-1-ynyl)pyridine (2g): Yield 75%, pale yellow oil. 1H NMR
3
(200 MHz, CDCl3): δ = 0.91 (t, J = 6.7 Hz, 3 H), 1.25–1.70 (m, 8
H), 2.42 (t, 3J = 6.9 Hz, 2 H), 7.17–7.24 (m, 1 H); 7.66 (dt, 3J =
4
3
4
7.9, J = 1.9 Hz, 1 H), 8.47 (dd, J = 4.8, J = 1.6 Hz, 1 H), 8.62
(d, J = 1.4 Hz, 1 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 14.1,
4
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Pd-Catalyzed Suzuki Coupling with Terminal Alkynes
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19.5, 22.6, 28.6, 28.7, 31.4, 77.4, 94.2, 121.3, 123.0; 138.5, 148.0,
centrated under reduced pressure. Flash column chromatography
152.5 ppm. IR (neat): ν = 3030 (w), 2931 (s), 2859 (m), 2230 (w),
afforded 3b as a white solid (57 mg, 35%), m.p. 120–123 °C. 1H
˜
1561 (w), 1476 (m), 1407 (m), 1186 (w), 1023 (w), 803 (m), 756 (s), NMR (200 MHz, CDCl3): δ = 7.04 (s, 1 H), 7.20–7.62 (m, 7 H),
706 (m) cm–1. C13H17N (187.28): calcd. C 83.37, H 9.15, N 7.48; 7.86–7.91 (m, 2 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 101.4,
found C 83.56, H 8.82, N 7.64.
111.3, 121.0, 123.0, 124.4, 125.0, 128.7, 128.9, 129.3, 130.6, 155.0,
156.0 ppm. IR (neat): ν = 3068 (w), 2924 (w), 1614 (w), 1563 (w),
˜
1-(tert-Butyldimethylsilyloxy)-2-(oct-1-ynyl)benzene (2h): Yield
1471 (w), 1456 (m), 1272 (w), 1208 (w), 1074 (w), 1038 (m), 1021
(m), 919 (m), 807 (m), 763 (m), 746 (s), 690 (m) cm–1. This com-
pound was previously described.[15]
1
74%, pale yellow oil. H NMR (200 MHz, CDCl3): δ = 0.24 (s, 6
H), 0.92 (t, 3J = 6.6 Hz, 3 H), 1.05 (s, 9 H), 1.28–1.64 (m, 8 H),
3
3
2.43 (t, J = 7.0 Hz, 2 H), 6.79–6.89 (m, 2 H), 7.15 (td, J = 7.7,
4J = 1.9 Hz, 1 H), 7.36 (dd, J = 7.7, J = 1.9 Hz, 1 H) ppm. 13C
NMR (50 MHz, CDCl3): δ = –4.2, 14.2, 18.4, 19.9, 22.7, 25.8, 28.8,
28.9, 31.5, 94.1, 116.7, 119.8, 121.2, 128.6, 133.6, 156.4 ppm. IR
3
4
Representative Procedure for the Synthesis of 2-Substituted Benzo-
[b]furans via Deprotection/Cyclization of o-Alkynylanisoles in the
Presence of BCl3
(neat): ν = 3014 (w), 2957 (m), 2931 (m), 2858 (m), 1596 (w), 1568
˜
Synthesis of 2-Phenylbenzo[b]furan (3b):[16] Under argon, boron tri-
chloride (1 m in heptanes, 1.3 mL, 1.3 mmol, 1.6 equiv.) was added
to a solution of 2b (165.7 mg, 0.795 mmol, 1 equiv.) in dry dichlo-
romethane (8 mL). The reaction mixture was stirred for 16 h at
room temperature before water (8 mL) was added. The two phases
were separated and the aqueous phase was extracted with dichloro-
methane. The combined organic phases were dried with magnesium
sulfate, filtered and concentrated under reduced pressure. Flash
chromatography gave 3b as a white solid (94 mg, 61%).
(w), 1488 (m), 1444 (m), 1284 (m), 1256 (m), 1216 (m), 1108 (w),
907 (m), 840 (m), 807 (w), 758 (s), 668 (w) cm–1.
1-Methoxy-2-(oct-1-ynyl)benzene (2i): Yield 75% and 79%, respec-
1
3
tively, colorless oil. H NMR (200 MHz, CDCl3): δ = 0.90 (t, J =
3
6.2 Hz, 3 H), 1.24–1.70 (m, 8 H), 2.47 (t, J = 6.85 Hz, 2 H), 3.88
3
4
(s, 3 H), 6.88 (m, 2 H), 7.24 (td, J = 7.8, J = 1.6 Hz, 1 H), 7.38
(dd, 3J = 7.5, 4J = 1.6 Hz, 1 H) ppm. 13C NMR (50 MHz, CDCl3):
δ = 14.5, 20.2, 23.0, 29.0, 29.2, 31.8, 56.2, 77.0, 95.1, 110.9, 113.6,
120.8, 129.2, 134.0, 160.2. IR (neat): ν = 3012 (w), 2956 (m), 2930
˜
2-Hexylbenzo[b]furan (3a)[16] was obtained analogously. The analyt-
ical data are given above.
(s), 2857 (m), 2198 (w), 1729 (w), 1664 (w), 1597 (w), 1493 (m),
1465 (m), 1434 (m), 1260 (m), 1217 (w), 1117 (w), 1048 (w), 1026
(w), 754 (s) cm–1. This compound was previously described.[14]
Synthesis of 3-Iodo-2-phenylbenzo[b]furan (3c) via Deprotection/5-
endo-dig-Iodocyclization of 2c in the Presence of BCl3/I2: To a solu-
tion of 2c (142 mg, 0.682 mmol, 1 equiv.) in dry CH2Cl2 iodine
(550 mg, 2.167 mmol, 3.2 equiv.) and boron trichloride (1 m in hep-
tanes, 1.1 mL, 1.1 mmol, 1.6 equiv.) were added. The reaction mix-
ture was stirred for 16 h at room temperature, before being diluted
with water/ethyl acetate. The organic phase was washed with a satu-
rated aqueous solution of sodium sulfite, dried with magnesium
sulfate, filtered and concentrated under reduced pressure. Flash
chromatography afforded 3c as a pale yellow oil (137 mg, 63%
yield). 1H NMR (200 MHz, CDCl3): δ = 7.26–7.56 (m, 7 H), 8.16–
8.21 (m, 2 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 61.2, 111.3,
121.9, 123.6, 125.8, 127.6, 128.6, 129.3, 130.1, 132.6, 153.2,
1-Methoxy-2(phenylethynyl)benzene (2j): Yield 89%, colorless oil.
1H NMR (200 MHz, CDCl3): δ = 3.93 (s, 3 H), 6.90–7.00 (m, 2 H),
7.28–7.37 (m, 4 H), 7.50–7.62 (m, 3 H) ppm. 13C NMR (50 MHz,
CDCl3): δ = 56.0, 85.9, 93.6, 110.8, 112.6, 120.6, 123.7, 128.2,
128.4, 129.9, 131.8, 133.7, 160.0 ppm. IR (neat): ν = 3063 (w), 3014
˜
(w), 2941 (w), 2837 (w), 1594 (w), 1574 (w), 1498 (m), 1484 (m),
1462 (m), 1434 (m), 1276 (m), 1247 (m), 1217 (m), 1182 (w), 1162
(w), 1107 (m), 1047 (w), 1025 (m), 753 (s), 691 (m), 668 (w) cm–1.
This compound was previously described.[14]
Synthesis of 2-Hexylbenzo[b]furan (3a) via Deprotection/Cyclization
of 2a in the Presence of TBAF: A mixture of tetrabutylammonium
fluoride (TBAF, 1 m in THF, 10 mL) and molecular sieves (4 Å,
5 g) in tetrahydrofuran (17 mL) was stirred for 1 h at room tem-
perature. 0.75 mL of this solution (0.28 mmol of TBAF, 1.3 equiv.)
was then added to a solution of 2a (70.0 mg, 0.221 mmol, 1.0
equiv.) in dry tetrahydrofuran (1.5 mL) and the reaction mixture
was stirred at 55 °C. The evolution of the reaction was checked by
TLC and, after the complete disappearance of 2a, water was added.
After extraction with ethyl acetate, the combined organic phases
were dried with magnesium sulfate, filtered and concentrated under
reduced pressure. Flash chromatography afforded 3a (33.5 mg,
75%). 1H NMR (200 MHz, CDCl3): δ = 0.91 (t, 3J = 6.5 Hz, 3 H),
1.27–1.5 (m, 6 H), 1.67–1.8 (m, 2 H), 2.77 (t, 3J = 7.5 Hz, 2 H),
6.38 (s, 1 H), 7.15–7.26 (m, 2 H), 7.40–7.53 (m, 2 H) ppm. 13C
NMR (50 MHz, CDCl3): δ = 14.2, 22.7, 27.7, 28.5, 29.0, 31.7,
101.8, 110.8, 120.2, 122.4, 123.1, 129.1, 154.7, 159.9 ppm. IR
154.0 ppm. IR (neat): ν = 3059 (w), 2923 (w), 1590 (w), 1488 (m),
˜
1452 (m), 1442 (m), 1341 (w), 1253 (m), 1202 (m), 1062 (m), 1029
(m), 1008 (w), 971 (m), 888 (w), 818 (w), 764 (m), 742 (s), 689
(m) cm–1. This compound was previously described.[10c]
Representative Procedure for the Synthesis of 2-Substituted 3-Iodo-
benzo[b]furans by Using BCl3 and NIS
Synthesis of 3-Iodo-2-phenylbenzo[b]furan (3c):[10c] To a solution of
1-methoxy-2-(2-phenylethynyl)benzene (2c: 156.0 mg, 0.749 mmol,
1.0 equiv.) in dry dichloromethane (20 mL) was added successively
N-iodosuccinimide (335 mg, 1.49 mmol, 2.0 equiv.) and boron tri-
chloride (1 m in heptanes, 1.1 mL, 1.1 mmol, 1.5 equiv.). The reac-
tion mixture was stirred at room temperature for 2 h (monitored
by TLC) before water was added. After extraction with dichloro-
methane, the combined organic phases were dried with magnesium
sulfate, filtered and concentrated under reduced pressure. Flash
chromatography gave 3-iodo-2-phenylbenzo[b]furan (3c) as a pale
yellow oil (192 mg, 80% yield). The analytical data are mentioned
above.
(neat): ν = 3017 (w), 2930 (m), 2859 (w), 1602 (w), 1456 (m), 1254
˜
(w), 1216 (m), 1104 (w), 948 (w), 759 (s), 669 (w) cm–1. This com-
pound was previously described.[16]
Synthesis of 2-Phenylbenzo[b]furan (3b) via Deprotection/Cyclization
of 2b in the Presence of LiI: Lithium iodide (268 mg, 2.00 mmol,
2.4 equiv.) was added to a solution of 2b (176 mg, 0.845 mmol, 1
equiv.) in collidine (5 mL) and the resulting suspension was heated
at 150 °C for 18 h. The reaction mixture was cooled to room tem-
perature before the addition of a 10% aqueous solution of hydro-
gen chloride. After extraction with diethyl ether, the combined or-
ganic phases were dried with magnesium sulfate, filtered and con-
2-Hexyl-3-iodobenzo[b]furan (3d): Was obtained analogously as a
1
pale yellow oil (192 mg, 78% yield). H NMR (200 MHz, CDCl3):
3
δ = 0.90 (t, J = 6.7 Hz, 3 H), 1.27–1.46 (m, 6 H), 1.68–1.82 (m, 2
3
H), 2.86 (t, J = 7.4 Hz, 2 H), 7.25–7.42 (m, 4 H) ppm. 13C NMR
(50 MHz, CDCl3): δ = 14.2, 22.6, 27.9, 28.1, 28.8, 31.6, 62.6, 111.0,
120.8, 123.2, 124.5, 131.2, 154.3, 159.3 ppm. IR (neat): ν = 2955
˜
(m), 2928 (m), 2858 (m), 1587 (w), 1450 (s), 1256 (w), 1216 (w),
Eur. J. Org. Chem. 2005, 3334–3341
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3339

F. Colobert, A.-S. Castanet, O. Abillard
FULL PAPER
1190 (w), 1170 (w), 1106 (w), 1010 (w), 983 (w), 759 (s), 744
7.62 (m, 13 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 20.3, 111.4,
117.3, 120.6, 123.3, 125.1, 126.4, 126.8, 128.5, 128.6, 128.8, 128.9,
131, 131.2, 131.4, 132.7, 137.9, 150.7, 154.3 ppm. C21H16O
(284.35): calcd. C 88.7, H 5.67; found C 88.46, H 5.893.
(s) cm–1. This compound was previously described.[18]
Representative Procedure for the Tandem sp-sp2 Suzuki Coupling/5-
endo-dig-Iodocyclization of o-Iodoanisole with Terminal Alkynes: A
solution of n-butyllithium in hexanes (1.35 equiv.) was slowly added
to a cooled solution (–78 °C) of 1-alkyne (10.4 mmol, 1.30 equiv.)
in dimethoxyethane (10 mL). After 1 h at –78 °C, trimethylborate
(0.85 equiv.) was slowly added and, 30 min later, the temperature
was raised during 15 min. to 20 °C. In parallel, tetrakis(triphenyl-
phosphane)palladium(0) (0.015 equiv.) and o-iodoanisole (1.00
equiv.) were dissolved in dimethoxyethane (10 ml) and stirred for
10 min at room temperature. To the lithiated alkynyl borate was
added the solution of Pd(PPh3)4 and the aryl halide via cannula.
The cannula and the flask were rinsed with dimethoxyethane
(2×5 mL). The reaction mixture was heated under reflux for 4 h
(or complete disappearance of the aryl halide) and allowed to cool
to room temperature.
[1] a) A. R. Katritzky, in Advances in Heterocyclic Chemistry, Aca-
demic Press, New York, 1982; b) G. Vernin, in The Chemistry
of Heterocyclic Flavouring and Aroma Compounds, Hellis Hor-
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prehensive Heterocyclic Chemistry, Pergamon, New York, 1984;
d) H. B. Lipshutz, Chem. Rev. 1986, 86, 795–819; e) C. C.
Felder, K. E. Joyce, E. M. Briley, M. Glass, K. P. Mackie, K. J.
Fahey, G. J. Cullinan, D. C. Hunden, D. W. Johnson, M. O.
Chaney, G. A. Koppel, M. Brownstein, J. Pharmacol. Exp.
Ther. 1998, 291–297.
[2] A. Gangjee, R. Devraj, J. J. Mc Guire, R. L. Kisliuk, J. Med.
Chem. 1995, 38, 3798–3805.
[3] Z. Yang, H. B. Liu, C. M. Lee, H. M. Chang, H. N. Wong, J.
Org. Chem. 1992, 57, 7248–7257.
[4] K. A. Ohemeng, M. A. Appolina, V. N. Nguyen, C. F.
Schwender, M. Singer, M. Steber, J. Ansell, D. Argentieri, W.
Hageman, J. Med. Chem. 1994, 37, 3663–3667.
[5] T. Nagahara, Y. Yokoyama, K. Inamura, S. Katakura, S. Ko-
moriya, H. Yamaguchi, T. Hara, M. Iwamoto, J. Med. Chem.
1994, 37, 1200–1207.
[6] A. P. Kozikowsky, D. Ma, L. Du, N. E. Lewin, P. M. Blumberg,
J. Am. Chem. Soc. 1995, 117, 6666–6672.
[7] G. A. Carter, K. Chamberlain, R. L. Wain, Ann. Appl. Biol.
1978, 88, 57–64.
[8] a) A. Fürstner, D. N. Jumbam, Tetrahedron 1992, 48, 5991–
6010; b) R. S. Givens, P. S. Athey, B. Matuszewski, L. W.
Kuepper, J. Xue, T. Fister, J. Am. Chem. Soc. 1993, 115, 6001–
6012; c) D. Hellwinkel, K. Göke, Synthesis 1995, 1135–1141;
d) X. Du, R. W. Armstrong, J. Org. Chem. 1997, 62, 5678–
5679; e) Y. Ito, T. Aoyama, T. Shioiri, Synlett 1997, 1163–1164;
f) C. Fuganti, S. Serra, Tetrahedron Lett. 1998, 39, 5609–5610;
g) D. D. Hennings, S. Iwasa, V. H. Rawal, Tetrahedron Lett.
1997, 38, 6379–6382.
[9] a) N. J. Kundo, M. Pal, J. S. Mahanty, M. J. De, J. Chem. Soc.
Perkin Trans. 1 1997, 2815–2820; b) Y. Kondo, F. Shiga, N.
Murata, T. Sakamoto, H. Yamanaka, Tetrahedron 1994, 50,
11803–11812; c) A. Arcadi, S. Cacchi, M. Del Rosario, G. Fab-
rizi, F. Marinelli, J. Org. Chem. 1996, 61, 9280–9288; d) N.
Monteiro, G. Balme, Synlett 1998, 746–747; e) N. Monteiro, A.
Arnold, G. Balme, Synlett 1998, 1111–1113.
Evaporation of the DME was then performed before adding
CH2Cl2 (30 mL), NIS (1.7 equiv.) and BCl3 M in hexane (1.4
equiv.). After stirring overnight at room temperature, addition of
water and extraction with CH2Cl2, the combined organic phases
were dried with magnesium sulfate, filtered and concentrated under
reduced pressure. The residue was purified by flash column
chromatography. The analytical data of 3c and 3d are given above.
Representative Procedure for Suzuki Coupling Reaction of 2-Substi-
tuted 3-Iodobenzo[b]furans 3c–d with Various Boronic Acids
Synthesis of 2-Hexyl-3-phenylbenzo[b]furan (4a): To a solution of 2-
hexyl-3-iodobenzo[b]furan (3d: 182.5 mg, 0.556 mmol, 1 equiv.) in
dry dimethoxyethane (10 mL) were successively added palladi-
um(ii) acetate (6.5 mg, 0.029 mmol, 5.2%), triphenylphosphane
(21.3 mg, 0.081 mmol, 14.6%), phenylboronic acid (108.4 mg,
0.89 mmol, 1.5 equiv.) and cesium fluoride (407 mg, 2.65 mmol, 4.7
equiv.). The reaction mixture was heated for 16 h at 75 °C and al-
lowed to cool to room temperature, before water was added. The
mixture was extracted with dichloromethane, the combined organic
phases were dried with magnesium sulfate, filtered and concen-
trated under vacuum. The residue was purified by flash column
chromatography. (139 mg, 90%), colorless oil. 1H NMR (200 MHz,
3
CDCl3): δ = 0.89 (t, J = 6.5 Hz, 3 H), 1.29–1.44 (m, 6 H), 1.68–
3
1.86 (m, 2 H), 2.88 (t, J = 7.6 Hz, 2 H), 7.20–7,68 (m, 9 H) ppm.
13C NMR (50 MHz, CDCl3): δ = 14.2, 22.7, 26.9, 28.5, 29.1, 31.6,
[10] a) S. Cacchi, J. Organomet. Chem. 1999, 576, 42–64; b) A. Ar-
cadi, S. Cacchi, F. Marinelli, Synthesis 1986, 749–751; c) A.
Arcadi, S. Cacchi, G. Fabrizi, F. Marinelli, L. Moro, Synlett
1999, 9, 1432–1434; d) R. C. Larock, E. K. Yum, M. J. Doty,
K. K. C. Sham, J. Org. Chem. 1995, 60, 3270–3271; e) C. Ama-
tore, E. Blart, J. P. Genêt, A. Jutand, S. Lemaire-Audoire, M.
Savignac, J. Org. Chem. 1995, 60, 6829–6839.
110.9, 116.9, 119.5, 122.6, 123.6, 127.1, 128.8, 129.0, 129.2, 133.0,
154.1, 155.5 ppm. IR (neat): ν = 3058 (w), 3034 (w), 2955 (m), 2928
˜
(s), 2858 (m), 1611 (m), 1597 (w), 1497 (w), 1455 (s), 1377 (w), 1245
(w), 1207 (w), 1175 (w), 1106 (w), 1074 (w), 1012 (w), 969 (m), 859
(w), 771 (m), 749 (s), 700 (s) cm–1. C20H22O (278.38): calcd. C
86.28, H 7.96; found C 86.00, H 8.001.
3-(1-Naphthyl)-2-phenylbenzo[b]furan (4b): Yield 162 mg, 91%,
white solid. 1H NMR (200 MHz, CDCl3): δ = 7.17–7.81 (m, 14 H),
8.00 (d, 3J = 6.22 Hz, 2 H) ppm. 13C NMR (50 MHz, CDCl3): δ =
111.2, 115.8, 120.6, 123, 124.8, 126, 126.1, 126.3, 126.5, 126.7,
127.9, 128, 128.3, 128.5, 128.6, 130.6, 131.6, 132.3, 134.2, 151.4,
154 ppm. C24H16O (320.38): calcd. C 89.97, H 5.03, O 4.99; found
C 89.52, H 5.05, O 4.68.
[11] A.-S. Castanet, F. Colobert, T. Schlama, Org. Lett. 2000, 2,
3559–3561.
[12] Contrary, using o-(tert-butyldimethylsilyloxy)bromobenzene
the reaction was unreproducible, affording 2-bromophenol,
starting material, coupling product and 2-hexylbenzo[b]furan
in varying ratios.
[13] For other examples of iodine-promoted electrophilic cycliza-
tion of alkynes to give heterocycles, see: a) A. Arcadi, S. Cac-
chi, S. Di Giuseppe, G. Fabrizi, F. Marinelli, Org. Lett. 2002,
4, 2409–2412; b) T. Yao, M. A. Campo, R. C. Larock, Org.
Lett. 2004, 6, 2677–2680.
3-(o-Methoxyphenyl)-2-phenylbenzo[b]furan (4c):[19] Yield 152 mg,
1
91%, pale yellow oil. H NMR (200 MHz, CDCl3): δ = 3.66 (s, 3
H), 7.04–7.9 (m, 13 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 55.4,
111, 111.6, 113.9, 120.6, 121.1, 121.7, 122.7, 124.4, 126.5, 128.1,
128.3, 129.5, 130.7, 131.3, 131.9, 151, 154, 157.6 ppm. C21H16O2
(300.35): calcd. C 83.98, H 5.37; found C 83.69, H 5.476.
[14] Y. Ma, C. Song, W. Jiang, Q. Wu, Y. Wang, X. Liu, M. B.
Andrus, Org. Lett. 2003, 5, 3317–3319.
[15] G. W. Kabalka, L. Wang, R. M. Pagni, Tetrahedron 2001, 57,
8017–8028.
3-(o-Methylphenyl)-2-phenylbenzo[b]furan (4d): Yield 97 mg, 64%,
colorless oil. 1H NMR (200 MHz, CDCl3): δ = 2.17 (s, 3 H), 7.27–
[16] D. Gelman, S. L. Buchwald, Angew. Chem. Int. Ed. 2003, 42,
5993–5996.
3340
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 3334–3341

Pd-Catalyzed Suzuki Coupling with Terminal Alkynes
FULL PAPER
[17] H. Sashida, A. Kuroda, J. Chem. Soc. Perkin Trans. 1 2000,
[19] Y. Hu, K. J. Nawoschik, Y. Liao, J. Ma, R. Fathi, Z. Yang, J.
Org. Chem. 2004, 69, 2235–2239.
1965–1969.
[18] D. R. Buckle, C. J. M. Rockell, J. Chem. Soc. Perkin Trans. 1
1985, 2443–2446.
Received: March 5, 2005
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3341