Rapid regio- and multi-coupling reactivity of 2,3-dibromobenzofurans with atom-economic triarylbismuths under palladium catalysis

Article abstract of DOI:10.3762/bjoc.12.195

A regio- and chemoselective cross-coupling study using 2,3-dibromobenzofurans and 2,3,5-tribromobenzofuran was achieved with sub-stoichiometric loadings of triarylbismuths as atom-economic reagents under Pd-catalyzed conditions. As part of this study, various 2,3-diaryl- and 2,3,5-triarylbenzofuran products were obtained in high yields, involving one-pot operations and short reaction times.

Full text of DOI:10.3762/bjoc.12.195

Rapid regio- and multi-coupling reactivity of
2,3-dibromobenzofurans with atom-economic
triarylbismuths under palladium catalysis
Maddali L. N. Rao*, Jalindar B. Talode and Venneti N. Murty
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2065–2076.
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016, U.P., India, Tel/ Fax: +91-512-2597532
doi:10.3762/bjoc.12.195
Received: 17 June 2016
Email:
Accepted: 25 August 2016
Published: 22 September 2016
Maddali L. N. Rao* - maddali@iitk.ac.in
* Corresponding author
Associate Editor: I. Marek
Keywords:
© 2016 Rao et al.; licensee Beilstein-Institut.
License and terms: see end of document.
bromobenzofuran coupling; cross-coupling; palladium; regio-selective;
triarylbismuth
Abstract
A regio- and chemoselective cross-coupling study using 2,3-dibromobenzofurans and 2,3,5-tribromobenzofuran was achieved with
sub-stoichiometric loadings of triarylbismuths as atom-economic reagents under Pd-catalyzed conditions. As part of this study,
various 2,3-diaryl- and 2,3,5-triarylbenzofuran products were obtained in high yields, involving one-pot operations and short reac-
tion times.
Introduction
The benzofuran scaffold is present in various biologically active site-selective studies involving the Sonogashira, Negishi,
molecules [1-8], natural products [9-15] and also part of various Kumada cross-couplings employing 2,3-dibromobenzofuran
functional materials [16]. Importantly, 2,3-disubstituted benzo- and 2,3,5-tribromobenzofuran substrates [31-33]. Additionally,
furans are biologically important (Figure 1a–d) and a few Langer et al. reported the synthesis of 2,3-dialkenylbenzofurans
reports about their isolation and synthetic methods are avail- and functionalized dibenzofurans with domino “twofold
able [17-19]. Substituted benzofurans serve as antitumor agents Heck/6π-electrocyclization” of 2,3-di- and 2,3,5-tribromo-
[20], protein tyrosine phosphatase-1B inhibitors [21], antimy- benzofuran substrates [34].
cobacterial agents [22] and as ambipolar materials (CZBDF,
Figure 1c) [16]. To note, synthetic functionalization under tran- In this regard, the cross-coupling studies of triarylbismuth
sition-metal-catalyzed conditions allows the preparation of reagents in regioselective studies with functionalized bromo-
multi-substituted benzofurans in a facile manner [23-28]. benzofurans were not reported so far (Scheme 1) [35]. Given
Langer et al. reported the site-selective Suzuki–Miyaura reac- the importance of threefold couplings’ reactivity realized with
tion of 2,3-dibromobenzofuran with arylboronic acids under the sub-stoichiometric loading of triarylbismuths in the cross-
palladium catalyzed conditions [29,30]. Bach et al. reported coupling reactions [35-42], we report herein, a novel regio- and
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Figure 1: Important benzofuran skeletons.
Scheme 1: Bis- and tris-couplings.
multi-coupling of bromobenzofurans with triarylbismuth 2-pyrrolidone (NMP) at 90 °C for 1 h as protocol conditions
reagents under palladium coupling conditions.
[35]. This protocol furnished the preferential cross-coupling at
the more electrophilic 2-Br position of 2,3-dibromobenzofuran
(1.1) [29]. This reaction delivered 2-aryl-3-bromobenzofuran
Results and Discussion
This study was initiated with 2,3-dibromobenzofuran for the in- 2.1 in 46% yield (Table 1, entry 1) and the corresponding bis-
vestigation of the regio-selective coupling using a triarylbis- arylation product involoving both 2- and 3-Br positions was not
muth reagent in substoichiometric amounts under Pd-catalyzed formed. Under similar conditions but with Cs2CO3 (4 equiv) as
conditions (Table 1). A trial reaction was performed with 2,3- base, the cross-coupling yield was increased to 73% (Table 1,
dibromobenzofuran (1.1, 3.3 equiv) and tri(p-anisyl)bismuth entry 2). A further change in reaction time to 2 h raised the
(1 equiv) with Pd(OAc)2/PPh3, Cs2CO3 (3 equiv) in N-methyl- desired yield to 95% (Table 1, entry 3). An additional check
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Table 1: Screening for mono-arylation.a
Entry
Base (equiv)
Solvent
Temp. (°C)
Time (h)
Yield (%) (2.1)
1
2
Cs2CO3 (3)
Cs2CO3 (4)
Cs2CO3 (4)
K3PO4 (4)
KOAc (4)
NMP
NMP
NMP
NMP
NMP
DMF
DMA
NMP
NMP
NMP
NMP
NMP
90
90
90
90
90
90
90
110
60
90
90
90
1
1
2
2
2
2
2
2
2
2
2
2
46
73
3
95
4
79
5
61
6
Cs2CO3 (4)
Cs2CO3 (4)
Cs2CO3 (4)
Cs2CO3 (4)
Cs2CO3 (4)
None
71
7
80
8
84
9
47
10
11
12
86b
05
Cs2CO3 (4)
Nonec
aReaction conditions: 2,3-Dibromobenzofuran (1.1) 0.825 mmol, 3.3 equiv), Bi(p-anisyl)3 (0.25 mmol, 1 equiv), Pd(OAc)2 (0.025 mmol, 0.1 equiv),
PPh3 (0.1 mmol, 0.4 equiv), base (0.75–1 mmol, 3–4 equiv), and solvent (3 mL), temp., time. Isolated yields based on three aryl couplings from BiAr3.
Bianisyl formed in 5–15% yields. bWith 2,3-dibromobenzofuran (1.1) (0.75 mmol, 3 equiv). cWithout Pd-catalyst.
with bases K3PO4 or KOAc did not furnish high yields prompted us to extend our study to other functionalized 2,3-
(Table 1, entries 4 and 5). Investigations using solvents such as dibromobenzofuran substrates. For example, a few bismuth
N,N-dimethylformamide (DMF) and N,N-dimethylacetamide couplings carried out with 2,3-dibromo-5-nitrobenzofuran (1.2)
(DMA) furnished lowered yields (Table 1, entries 6 and 7) in furnished the corresponding 2-aryl-3-bromobenzofurans
comparison with NMP solvent. Carrying out the cross- 2.13–2.15 in 76–88% yields (Table 2, entries 13–15). Addition-
couplings at different temperatures also gave lower yields ally, we have also planned chemoselective couplings with dif-
(Table 1, entries 8 and 9). Additionally, the stoichiometric com- ferently functionalized 2,3-dibromobenzofurans. This study
bination of 3 equiv of 2,3-dibromobenzofuran (1.1) and 1 equiv using 2,3-dibromobenzofurans functionalized with 5-chloro,
of bismuth reagent gave 86% yield (Table 1, entry 10). A few 5,7-dichloro, 7-bromo-5-chloro and 5-bromo groups 1.3–1.6
control reactions without base or palladium catalyst showed furnished exclusive arylations at C-2 position.
inferior or no cross-coupling reactivity (Table 1, entries 11 and
12). This investigation results that the desired regio-selective In these cases, the corresponding 2-aryl-3-bromobenzofuran
cross-coupling reactivity could be obtained in excellent yield products 2.16–2.27 were obtained in high yields (Table 2,
with Pd(OAc)2/4 PPh3 (0.1 equiv) Cs2CO3 (4 equiv) in NMP at entries 16-27). It is to be mentioned that, despite the known
90 °C and 2 h reaction time (Table 1, entry 3) and it was consid- facile coupling nature of aryl bromide [42] and its presence as
ered as optimized protocol for our further study.
part of the substrate, we obtained high regio-selective couplings
with polyhalogenated benzofurans. The corresponding cou-
To check the generality of this regio-selective coupling, various pling products 2.22–2.27 were obtained in high yields (Table 2,
2,3-dibromobenzofurans have been tested with differently func- entries 22–27). It is to be mentioned at this stage that in com-
tionalized triphenylbismuth reagents under the optimized condi- parison to similar cross-couplings carried out with aryl boronic
tions (Table 2). This study was performed with triphenylbis- acids [29], the present method with triarylbismuth reagents
muth reagents substituted with electronically activating and showed appreciable reactivity with threefold coupling advan-
deactivating groups. The cross-couplings performed with these tage and extended substrate scope. The overall resulted regio-
reagents demonstrated an excellent general reactivity (Table 2, and chemo-selective couplings encouraged us to investigate
entries 1–12). It was highly satisfying to note that the corre- further arylation studies with triarylbismuth reagents. It is to
sponding products 2.1–2.12 were obtained in 79–95% yields. It note that, under these mono-arylation coupling conditions com-
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Table 2: Cross-couplings of 2,3-dibromobenzofurans with BiAr3 reagents.a
Entry
1
2,3-Dibromobenzofurans
2-Aryl-3-bromobenzofurans
Yield (%)
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
95
88
81
85
89
86
82
93
90
79
92
2
3
4
5
6
7
8
9
10
11
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Table 2: Cross-couplings of 2,3-dibromobenzofurans with BiAr3 reagents.a (continued)
12
13
14
15
16
17
1.1
1.2
1.2
1.2
1.3
1.3
2.12
2.13
2.14
2.15
2.16
2.17
93
82
88
76
91
82
18
19
1.3
1.3
2.18
2.19
87
63
20
21
22
23
1.4
1.4
1.5
1.5
2.20
2.21
2.22
2.23
87
79
70
73
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Table 2: Cross-couplings of 2,3-dibromobenzofurans with BiAr3 reagents.a (continued)
24
1.5
2.24
68
25
26
27
1.6
1.6
1.6
2.25
2.26
2.27
92
87
77
aReaction conditions: 2,3-Dibromobenzofurans (0.825 mmol, 3.3 equiv), BiAr3 (0.25 mmol, 1 equiv), Cs2CO3 (1 mmol, 4 equiv), Pd(OAc)2
(0.025 mmol, 0.1 equiv), PPh3 (0.1 mmol, 0.4 equiv), NMP (3 mL), 90 °C, 2 h. Isolated yields based on three aryl couplings from BiAr3. Biaryl from
BiAr3 formed in minor amounts.
prising an 1:3 stoichiometric ratio of BiAr3:dibromide combina- diarylbenzofuran 3.1 in 54% yield along with mono-arylated 2.3
tion we have not obtained the formation of any bis-arylated in 31% yield (Table 3, entry 1). Importantly, this bis-coupling
product. Hence, it was of interest to explore towards a one-pot process was expected to go firstly through the formation of a
bis-coupling method to synthesize 2,3-diarylbenzofurans using mono-arylated product followed by its involvement in a second
2,3-dibromobenzofuran which involves couplings at both coupling with bismuth reagent. Hence to improve the bis-cou-
2-bromo and 3-bromo positions. With this aim, it was investi- pling process, a brief screening was performed under similar
gated with appropriate sub-stoichiometric amounts of triarylbis- conditions but with 4 h reaction time. This gave the desired bis-
muth reagents to obtain bis-coupling product (Table 3).
coupling in 72% yield along with minor amount of mono-
arylated product (Table 3, entry 2). It was further increased to
Our initial attempt with mono-arylation catalytic conditions but 77% at 110 °C in 2 h conditions (Table 3, entry 3) along with
with stoichiometric amount of bismuth reagent afforded 2,3- mono-arylated product 2.3 in minor amount. Hence, these
Table 3: Screening for bis-arylation.a
Entry
Catalyst
Temp. (°C)
Time (h)
Yield (%)b 3.1 (2.3)
1
2
3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
90
90
2
4
2
54 (31)
72 (4)
77 (2)
110
aReaction conditions: 2,3-Dibromobenzofuran (1.1) (0.375 mmol, 1.5 equiv), Bi(p-tolyl)3 (0.25 mmol, 1 equiv), Pd(OAc)2 (0.025 mmol, 0.1 equiv),
PPh3 (0.1 mmol, 0.4 equiv), Cs2CO3 (1 mmol, 4 equiv), NMP (3 mL), temp, time. bIsolated yield (3.1) based on three aryl couplings from BiAr3. Bitolyl
formed in 5–15% yields. Isolated yield of 2.3 given in parenthesis.
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conditions were adopted as optimized protocol for our bis-cou- furans. To accomplish this, we initially reacted 2-aryl-3-
pling study with triarylbismuth reagents (Table 4). Encourag- bromobenzofuran 2.1 with bismuth reagent using the palladium
ingly, these bis-couplings using the established conditions protocol conditions established for symmetrical bis-arylations
afforded symmetrically substituted 2,3-diarylbenzofurans (Table 5).
3.1–3.7 in 72–85% yields (Table 4, entries 1–7). In these cases,
the corresponding mono-arylated products (2.1–2.3, 2.8, 2.10) It was carried out at 110 °C for 2 h and this furnished unsym-
were also isolated in minor amounts. Evidently, this generaliza- metrically substituted 2,3-diarylbenzofuran 4.1 in 54% yield
tion of bis-coupling reactivity was proved to be operationally along with the recovery of starting 2-aryl-3-bromobenzofuran
simple with high reactivity and yields involving 2 h short reac- 2.1 in 42% yield (Table 5, entry 1). As the reaction was found
tion time and with sub-stoichiometric loadings of the bismuth to be incomplete, a further investigation was carried out with
reagent. Incidentally, we could also obtain the X-ray structure 3 h reaction time. In this case, the desired unsymmetrically
analysis for diarylbenzofuran 3.1 as given in Figure 2. The ease substituted bis-arylated product 4.1 was increased to 83% yield
of formation of symmetrically substituted bis-coupled products (Table 5, entry 2). Encouraged with this, a few more diaryla-
inspired us to additionally develop a viable procedure for tions were attempted with different bismuth reagents. These
the synthesis of unsymmetrically substituted bis-arylated benzo- reactions also furnished the corresponding unsymmetrically
Table 4: Bis-coupling of 2,3-dibromobenzofurans.a
Entry
2,3-Dibromobenzofurans
Bis- and mono-aryl benzofurans
1
1.1
2.3 (2%)
3.1 (77%)
2
3
1.1
1.1
2.2 (10%)
2.10 (9%)
3.2 (85%)
3.3 (72%)
4
1.1
3.4 (72%)b
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Table 4: Bis-coupling of 2,3-dibromobenzofurans.a (continued)
5
1.1
2.8 (11%)
3.5 (76%)
6
2.1 (10%)
1.1
3.6 (82%)
7
1.3
3.7 (85%)b
aReaction conditions: 2,3-Dibromobenzofurans (0.375 mmol, 1.5 equiv), BiAr3 (0.25 mmol, 1 equiv), Pd(OAc)2 (0.025 mmol, 0.1 equiv), PPh3
(0.1 mmol, 0.4 equiv), Cs2CO3 (1 mmol, 4 equiv), NMP (3 mL), 110 °C, 2 h. Isolated yields based on three aryl couplings from BiAr3. Biaryl from BiAr3
formed in minor amounts. bMono-arylated product 2 not found.
impressive. Hence, we were inquisitive to develop a pot-
economic protocol to directly access these products minimizing
the purification procedure after mono-arylation. Hence, it was
performed in a stepwise manner (step 1 and step 2) in a one-pot
operation without any intermediate isolation. These results are
given in Table 6. To elaborate, firstly we carried out the prepa-
ration of 2-aryl-3-bromobenzofuran in step 1 and it was fol-
lowed by a second arylation at the 3-position as part of step 2.
This pot-economic approach afforded mixed 2,3-diarylbenzofu-
rans 4.1–4.3 in 68–72% yields (Table 6, entries 1–3). In all
these reactions, we have also isolated the mono-aryl product
from step 1 (2-aryl-3-bromobenzofuran, 2.1) in minor amounts.
In fact, bis-arylated yields obtained in this one-pot operation are
on par with that obtained in Table 5. This reflects the efficient
nature of our established pot-economic protocol employing dif-
Figure 2: X-ray structure of bis-coupling product 3.1 (CCDC-1425338)
[43].
ferent triarylbismuth reagents. In literature, bis-aryl couplings
with arylboronic acids were reported with good reactivity under
different heating conditions for mono- and bis-arylations
substituted 2,3-diarylbenzofurans 4.2 and 4.3 in 81% and 73% [29,30]. As mentioned before, our couplings employing triaryl-
yields (Table 5, entries 3 and 4) with minor amount of unre- bismuths reacted on par with good reativity in addition to three-
acted starting material 2-aryl-3-bromobenzofuran 2.1.
fold coupling advantage with sub-stoichiometric loadings.
The good coupling reactivity of 2-aryl-3-bromobenzofurans to Further coupling study was carried out with 2,3,5-tribromo-
give unsymmetrically substituted 2,3-diarylbenzofurans was benzofuran (1.6) for the plausible threefold arylation
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Table 5: Cross-couplings of 2-aryl-3-bromobenzofurans.a
Entry
BiAr3
2,3-Diarylbenzofurans
Yield (%)b
1
4.1
4.1
4.2
4.3
54 (42)c
83 (11)
81(12)
2
3
4
73 (11)
aReaction conditions: 2-Aryl-3-bromobenzofuran (2.1, 0.375 mmol, 3 equiv), BiAr3 (0.125 mmol, 1 equiv), Pd(OAc)2 (0.0125 mmol, 0.1 equiv), PPh3
(0.05 mmol, 0.4 equiv), Cs2CO3 (0.5 mmol, 4 equiv), NMP (3 mL), 110 °C, 3 h. Isolated yields based on three aryl couplings from BiAr3. Biaryl from
BiAr3 formed in minor amounts. bRecovered 2.1 in parenthesis. cReaction time was 2 h.
under Pd-coupling conditions (Table 7). With some lar study reported with arylboronic acids [30]. Thus the present
experimentation, it was realized that threefold arylation of method of the preparation of 2,3,5-triarylbenzofurans is ex-
2,3,5-tribromobenzofuran (1.6) using triarylbismuth reagent is pected to serve as a useful protocol to access these skeletons in
possible to give 2,3,5-triarylbenzofuran in high yiled after a a facile manner.
reaction time of 4 h.
Conclusion
Hence, these couplings were conducted employing different We have established the couplings of 2,3-dibromobenzofurans
triarylbismuth reagents. These tris-couplings afforded 2,3,5- and 2,3,5-tribromobenzofuran with high yields and faster reac-
triarylated benzofurans 5.1–5.4 in 62–79% yields (Table 7, tivity using triarylbismuth reagents as atom-economic reagents.
entries 1–4). To note, all the three new aryl couplings were ob- The Pd-catalyzed couplings carried out with triarylbismuths in
tained in 4 h short duration of time in a one-pot operation and is sub-stoichiometric loadings allowed the synthesis of 2-aryl-3-
synthetically advantageous. This reactivity is on par with simi- bromobenzofurans and symmetrically/unsymmetrically substi-
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Table 6: Pot-economic synthesis of unsymmetrical 2,3-diarylbenzofurans.a
Entry
BiAr13
BiAr23
2,3-Diarylbenzofurans
1
2.1 (15%)
2.1 (16%)
2.1 (19%)
4.1 (72%)
4.2 (70%)
4.3 (68%)
2
3
aReaction conditions for Step 1: 2,3-Dibromobenzofuran (1.1, 0.375 mmol, 3 equiv), BiAr13 (0.125 mmol, 1 equiv), Cs2CO3 (0.5 mmol, 4 equiv),
Pd(OAc)2 (0.0125 mmol, 0.1 equiv), PPh3 (0.05 mmol, 0.4 equiv), NMP, 90 °C, 2 h; Reaction conditions for Step 2: BiAr23 (0.125 mmol, 1 equiv),
Cs2CO3 (0.25 mmol, 2 equiv), Pd(OAc)2 (0.0062 mmol, 0.05 equiv), PPh3 (0.025 mmol, 0.2 equiv), NMP, 110 °C, 2 h. Isolated yields based on three
aryl couplings from BiAr13 and BiAr23. Biaryl from BiAr3 formed in minor amounts.
Table 7: Tris-coupling of 2,3,5-tribromobenzofuran.a
Entry
BiAr3
2,3,5-Triarylbenzofuran
Yield (%)
1
5.1
79
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Table 7: Tris-coupling of 2,3,5-tribromobenzofuran.a (continued)
2
5.2
5.3
5.4
78
62
75
3
4
aReaction conditions: 2,3,5-Tribromobenzofuran (1.6, 0.25 mmol, 1 equiv), BiAr3 (0.25 mmol, 1 equiv), Pd(OAc)2 (0.025 mmol, 0.1 equiv), PPh3
(0.1 mmol, 0.4 equiv), Cs2CO3 (1.5 mmol, 6 equiv), NMP (3 mL), 110 °C, 4 h. Isolated yields based on three aryl couplings from BiAr3. Biaryl from
BiAr3 formed in minor amounts.
tuted 2,3-diarylbenzofurans in good to high yields. Additional Industrial Research (CSIR), New Delhi for research fellowship
threefold arylation of 2,3,5-tribromobenzofuran under Pd-cata- respectively.
lyzed conditions afforded 2,3,5-triarylbenzofurans in high
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