Palladium-catalyzed cross-coupling reaction of alkenyl aluminums with 2-bromobenzo[b]furans

Article abstract of DOI:10.1039/d0ra02984j

Highly efficient and simple cross-coupling reactions of 2-bromobenzo[b]furans with alkenylaluminum reagents for the synthesis of 2-alkenylbenzo[b]furan derivatives using PdCl₂(3 mol%)/XantPhos (6 mol%) as catalyst are reported. Excellent yields (up to 97%) were obtained for a wide range of substrates at 80 °C for 4 h in DCE.

Full text of DOI:10.1039/d0ra02984j

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Palladium-catalyzed cross-coupling reaction of
alkenyl aluminums with 2-bromobenzo[b]furans†
Cite this: RSC Adv., 2020, 10, 19610
*
Chang Wen,‡ Xin Jiang,‡ Kun Wu, Ruiqiang Luo and Qinghan Li
Received 2nd April 2020
Accepted 16th May 2020
Highly efficient and simple cross-coupling reactions of 2-bromobenzo[b]furans with alkenylaluminum
reagents for the synthesis of 2-alkenylbenzo[b]furan derivatives using PdCl2 (3 mol%)/XantPhos (6 mol%)
as catalyst are reported. Excellent yields (up to 97%) were obtained for a wide range of substrates at
80 ꢀC for 4 h in DCE.
DOI: 10.1039/d0ra02984j
rsc.li/rsc-advances
2-Substituted benzo[b]furans are important structural scaffolds benzo[b]furans. The process was simple and easily performed,
found in many natural products and pharmaceutical products.1 and it provides an efficient method for the synthesis of 2-alkenyl
Some of these compounds have been known to exhibit anti- benzo[b]furans derivatives. Notably, in our procedure palla-
inammatory,2 antitumor,3 anticancer,4 and anti-fungal,5 anti- dium is used as the single catalyst and base free.
plasmodial,6 antioxidant,7 anti-HIV,8 and estrogenic activities.9
Our initial studies used 2-bromo-6-methoxybenzo[b]furan
In addition, they serve as building blocks for many organic (2e) with diethyl(oct-1-enyl)aluminum (1a) as model substrates.
transformations.10 Thus, their synthesis and applications have Treatment of compound 2e with the alkenylaluminum (1a)
attracted considerable attention in the chemical and pharma- using PdCl2 (3 mol%)/XantPhos (6 mol%) as catalyst in toluene
ceutical industries over the past decades.11 Until now numerous at 60 C for 4 h, the coupled product 6-methoxy-2-(oct-1-enyl)
ꢀ
effective synthetic methodologies of synthesis 2-substituted benzo[b]furan (3ae) was obtained in 46% isolated yield (Table 1,
benzo[b]furans have been reported.12,13 Among these methods entry 1). However, when using other palladium catalysts, such
hitherto developed, the metal-catalyzed 2-halobenzo[b]furans as Pd(OAc)2 and Pd(acac)2 the yield is lower than that of palla-
coupling with organometallic nucleophiles is one of the most dium dichloride(Table 1, entries 2 and 3). Some bases were
effective methods (Scheme 1).13 However, in most cases gener- investigated to further improve the yield of coupled products
ally suffer from one or more drawbacks such as requirement co- (3ae). When Et3N was used as a base, the reaction of compound
catalyst like Cu salts, limited substrate scope, high catalyst 2e with alkenylaluminum (1a) produced the coupled product
loading, high temperature and poor chemoselectivity etc. (3ae) with a 27% isolated yield only (Table 1, entry 4). While, the
Therefore, the development of more efficient and atom coupled product (3ae) could not obtain when K2CO3 and
economical approaches for the preparation of 2-substituted TMEDA were used as base (Table 1, entries 5 and 6). To further
benzo[b]furans remains as desirable work. Previous studies understand the nature of this catalysis, we tested the cross-
show that organoaluminum reagents are highly efficient coupling reaction of 1a with 2e under various solvents and the
nucleophiles for cross-coupling reactions with aromatic results revealed that DCE was the solvent of choice (Table 1,
halides14 or benzylic halides,15 and the investigations have entry 9). In hexane or THF, the isolated yield of the coupled
demonstrated that palladium is a good catalytic metal.16
product (3ae) was low efficient (Table 1, entries 7 and 8). To our
At present, a variety of methods have been developed to delighted, the isolated yield of the coupled product (3ae)
prepare compounds containing olen functional groups increased from 74% to 85% when the reaction temperature was
through hydrocarbon activation of olens.17 To continue our increased from 60 ꢀC to 80 ꢀC (Table 1, entries 9 and 10).
effort to develop coupling reactions using reactive organo- Interestingly, the isolated yield of the coupled product (3ae) was
aluminum reagents,18 we herein report a palladium(II)-cata- almost unchanged when the alkenylaluminum loading was
lyzed, base free cross-coupling reactions of 2ꢀ-bromo benzo[b] decreased from 1.0 mmol to 0.8 mmol (Table 1, entries 10 and
furans with alkenylaluminum reagents at 80 C in short reac-
tion time with good to excellent isolated yields for 2-alkenyl
College of Chemistry and Environmental Protection Engineering, Southwest University
for Nationalities, Chengdu, 610041, China. E-mail: lqhchem@163.com; lqhchem@
swun.cn; Fax: +86-28-8552-4382; Tel: +86-28-8592-8015
† Electronic supplementary information (ESI) available. See DOI:
Scheme 1 Palladium-catalyzed cross-coupling reactions of 2-hal-
10.1039/d0ra02984j
obenzo[b]furans derivatives with organometallic nucleophiles.
‡ These authors contributed equally to this work.
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Table 1 The cross-coupling reaction of diethyl(oct-1-enyl) aluminum Table 2 The cross-coupling reaction of alkenylaluminums (1) with 2-
(1a) with 2-bromo-6-methoxybenzo[b]furan (2e) catalyzed by bromobenzo[b]furans derivatives (2) catalyzed by palladiuma
palladiuma
1a
Base
3aeb
Entry
Pd salt.
(equiv.)
(x equiv.)
Solvent
yield (%)
1
2
3
4
5
6
7
8
PdCl2
Pd(OAc)2
Pd(acac)2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.8
0.6
0.8
—
—
—
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Hexane
THF
DCE
DCE
DCE
DCE
46
19
10
27
NR
NR
47
51
74
85
84
53
49
Et3N (2.0)
K2CO3 (2.0)
TMEDA (2.0)
—
—
—
—
—
—
—
9
10c
11c
12c
13c,d
DCE
a
1a/2a/PdCl2/XantPhos ¼ 1.0/0.5/0.03/0.06 mmol, 60 ꢀC, 3 mL solvent,
b
c
d
4 h, Ar2. Isolated yield of 3ae. 80 ꢀC. 1a/2a/PdCl2/XantPhos ¼ 0.8/
0.5/0.02/0.04 mmol.
11). However, the isolated yield of the coupled product (3ae) is
53% only when the alkenylaluminum loading was decreased
from 0.8 mmol to 0.6 mmol (Table 1, entries 11 and 12). Further
studies indicated that the catalyst loading dramatically inu-
enced the isolated yield of the coupled product (3ae). It was
found that the most favorable catalyst loading is 3 mol% PdCl2/
6 mol% XantPhos (Table 1, entry 11). Extensive screening
showed that the optimized coupling conditions were 3 mol%
PdCl2/6 mol% XantPhos, 0.8 mmol 1a, 0.5 mmol 2e in DCE at
ꢀ
80 C for 4 h (Table 1, entry 11).
Under the optimized conditions, coupling reactions of
aliphatic alkenylaluminum reagents, such as di-sec-butyl(oct-
1-enyl)aluminum (1a) and di-sec-butyl(dec-1-enyl)aluminum
(1b), proceed with electron-neutral, electron rich and
electron-decient 2-bromo benzo[b]furans derivatives afford-
ing the products in good yields (Table 2, 3(ae–ak), 3(ba–bk)).
For example, 2-bromobenzo[b]furans containing methyl and
methoxy affording the corresponding coupled products in 72–
97% isolated yields (Table 2, 3(ab–af), 3bf). 2-Bromobenzo[b]
furans containing chloro and bromo groups affording the
corresponding coupled products in 33–95% isolated yields
(Table 2, 3(ah–ak), 3(bh–bk)). Interestingly, 5,7-dichloro-2-
bromobenzo[b]furan affording the corresponding coupled
products (3ak) and (3bk) in 93% and 93% isolated yields,
respectively (Table 2). Furthermore, the 2-bromonaphtho[2,3-
b]furan was also produced the 2-(oct-1-enyl)naphtho[2,1-b]
furan (3am) with isolated yield of 83% (Table 2). Besides
aliphatic alkenylaluminums, aromatic alkenyl aluminums
such as di-sec-butyl(styryl)aluminum (1c) and di-sec-butyl(3-
a
1/2/PdCl2/XantPhos ¼ 0.8/0.5/0.03/0.06 mmol, 80 ꢀC, 4 h. Isolated
yield of 3, two runs.
phenylprop-1-enyl)aluminum (1d) also reacted smoothly to
afford satisfactory isolated yields (61–95%) (Table 2, 3(ca–ck),
3(da,df)). Importantly, the coupling reactions with 2,5-
dibromo benzo[b]furan, 2-bromo-5-chloro-benzo[b]furan and
2-bromo-5,7-dichlorobenzo[b]furan reacted regioselectivity at
2-position affording the corresponding 2-ynylbenzo[b]furans
derivatives in 33–95% isolated yields (Table 2, 3(ah–ak),
3(bh,bk), 3ck). At the same time, the dehalogenation was not
observed in the cross-coupling with 2-bromobenzo[b]furans
derivatives containing halogen-substituents (Table 2, 3(ah–
ak), 3(bh,bk), 3ck).
The reaction was also found to be effective in gram-scale
synthesis, which indicated its potential for practical
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and can be utilized as precursors for the synthesis of impor-
tant bioactive compounds. The methodology provides useful
procedure for the synthesis of 2-alkenylbenzo[b]furans deriv-
atives. The coupling reactions with 2-bromo-5,7-dichlor-
obenzo[b]furan reacted regioselectivity at 2-position
furnishing the corresponding 2-substituted benzo[b]furans
derivatives in good yields. Further studies on the application
of this catalytic system to synthesis of bioactive compounds
are currently under way.
Scheme 2 Preparative scale synthesis of compound 3ah.
Experimental
Melting points were determined with an XRC-1 micro melting
point apparatus and uncorrected. 1H and 13C NMR spectra were
recorded using a Varian 400 MHz spectrometer in CDCl3 with
tetramethylsilane as internal standard. HRMS were recorded on
a Bruker Micro TOF spectrometer equipped with an ESI ion
source. Analytical thin-layer chromatography (TLC) was per-
formed on silica 60F-254 plates. Flash column chromatography
was carried out on silica gel (200–400 mesh). All reactions were
carried out under an Argon gas atmosphere. The starting mate-
rial 2-bromo benzo[b]furans was prepared according to litera-
ture.19 Alkenylaluminum reagents were prepared according to
literature.15a Chemical reagents and solvents were purchased
from Adamas-beta, Aldrich and XPKchem, and were used without
further purication with the exception of these reagents: THF,
hexane and toluene were distilled from sodium in mitrogen, and
DCE was distilled from CaH2. Other reagents were commercially
available and used as received.
Scheme 3 The proposed mechanism for the formation of coupled
product 3.
application (Scheme 2). 2-Substituted benzo[b]furans deriva-
tives 3ah was synthesized in 1.31 gram using this methodology.
In order to further explore the reaction mechanism, control
experiments were carried out (see the ESI†). We performed the
reaction between 2-bromobenzo[b]furan (2a, 0.5 mmol) with di-
sec-butyl(oct-1-enyl)aluminum (1a, 0.8 mmol) in the presence of
PdCl2 (3 mol%)/XantPhos (6 mol%) in DCE at 80 ꢀC for 4 h. The
reaction mixture was analyzed by 31P NMR, it was found that the
characteristic peak of 31P NMR appeared around at 22.98 ppm
and 30.98 ppm. However, 31P NMR peak of pure XantPhos is
ꢁ18.03 ppm. The results show that XantPhos work as a ligand of
the palladium center. Thus, a proposed possible reaction mech-
anism for the cross-coupling reaction is shown in Scheme 3. The
rst step is the oxidative addition of 2-bromobenzo[b]furans (2) to
Pd(0) phosphine complex (4) (which in turn from PdCl2 and
RAlMe2 (1) reagents) to form the organopalladium(II) bromide
intermediate (5). Transmetalation of RAlMe2 (1) with complex 5
gives R'PdR(II) intermediate (6) and Me2AlBr. Finally, complex 6
under goes reductive elimination to afford the desired coupling
product of 2-alkenylbenzo[b]furans (3) and regenerate the active
Pd(0) species for the next catalytic cycle.
General producer for cross-coupling of 2-bromobenzo[b]
furans with alkenylaluminum reagents
Under an atmosphere of Argon gas, PdCl2 (2.6 mg, 0.015 mmol),
XantPhos (17.4 mg, 0.015 mmol), 2-bromobenzo[b]furans
(98.0 mg, 0.5 mmol) and DCE (3 mL) were mixed in a Schlenk
ask. Shortly aerwards, a solution of alkenylaluminums (0.8
mmol) was added with a syringe pump. At the end of the
addition, the reaction mixture stirring was continued for 4 h at
ꢀ
80 C. Aer completion the reaction, the mixture was diluted
with 1 N aqueous HCl solution (10 mL) and extracted with EA (3
ꢂ 15 mL). The combined organic layers were dried over anhy-
drous Na2SO4, ltered and evaporated in vacuum. The residue
was subjected to ash column chromatography on silica gel
(hexane gradient) to afford the corresponding products.
(E)-1-(Benzofuran-2-yl)oct-1-en-3-one(3aa).12e
Colourless
liquid; yield: 0.099 g (82%), 1H NMR (400 MHz, CDCl3, ppm) d:
7.47 (dd, J ¼ 7.9, 24.9 Hz, 2H), 7.27–7.17 (m, 2H), 6.56–6.46 (m,
2H), 6.34 (d, J ¼ 15.8 Hz, 1H), 2.27 (q, J ¼ 7.1 Hz, 2H), 1.57–1.49
(m, 2H), 1.41–1.28 (m, 6H), 0.96–0.89 (m, 3H). 13C NMR (101
MHz, CDCl3, ppm) d: 155.3, 154.6, 134.0, 129.2, 123.9, 122.6,
120.6, 118.6, 110.7, 102.6, 33.0, 31.7, 29.0, 28.9, 22.6, 14.1.
Conclusions
A palladium-catalyzed the cross-coupling reactions of 2-bro-
mobenzo[b]furans derivatives with alkenylaluminum reagents
is reported. The cross-coupling reactions of aliphatic and
aromatic alkenylaluminum reagents proceed with electron-
neutral, electron rich and electron-decient 2-bromobenzo[b]
furans derivatives affording the coupled products 2-alkenyl
benzo[b]furans in 33–97% isolated yields. More importantly,
Conflicts of interest
the reaction was found to be effective in gram-scale synthesis, The authors declare no competing nancial interest.
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carbene-palladium(II)-1-methylimidazole complex catalyzed
direct C-H bond arylation of benzo[b] furans with aryl
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Acknowledgements
The authors are grateful to the Fundamental Research Funds
for the Central Universities, Southwest Minzu University (No.
2018NZD06) and the Sichuan Provincial Department of Science
and Technology support program (No. 2015NZ0033) for nan-
cial support.
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