Suzuki-Miyaura cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with carboxylic anhydrides: A new method to furans

Article abstract of DOI:10.1016/j.tetlet.2014.05.106

An efficient one-step method has been developed to construct furans via a Suzuki-Miyaura cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with carboxylic anhydrides. In the presence of Pd(OAc)₂/PCy₃, the multi-substituted alkenylboron compounds could couple with anhydrides to obtain furans in moderate-to-good yields. The addition of bases promoted the coupling reaction, and the plausible reaction mechanism was proposed.

Full text of DOI:10.1016/j.tetlet.2014.05.106

Accepted Manuscript
Suzuki-Miyaura cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with car-
boxylic anhydrides: a new method to furans
Tao Yu, Xin-Yan Wu, Jun Yang
PII:
S0040-4039(14)00937-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.05.106
TETL 44697
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 March 2014
18 May 2014
26 May 2014
Please cite this article as: Yu, T., Wu, X-Y., Yang, J., Suzuki-Miyaura cross-coupling reaction of 1,2-
oxaborol-2(5H)-ols with carboxylic anhydrides: a new method to furans, Tetrahedron Letters (2014), doi: http://
dx.doi.org/10.1016/j.tetlet.2014.05.106
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Suzuki-Miyaura cross-coupling reaction of
1,2-oxaborol-2(5H)-ols with carboxylic
anhydrides: a new method to furans
Tao Yu, Xin-Yan Wu, Jun Yang

1
Tetrahedron Letters
journal homepage: www.els evier.com
Suzuki-Miyaura cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with carboxylic
anhydrides: a new method to furans
Tao Yua, Xin-Yan Wua, ∗ and Jun Yangb, ∗
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai,
200237, China
bShanghai Institute of Organic Chemistry, Chinese Academy of Science, 345 Lingling Road, Shanghai, 200032, China
ARTICLE INFO
ABSTRACT
Article history:
Received
An efficient one-step method has been developed to construct furans via a Suzuki-Miyaura
cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with carboxylic anhydrides. In the presence of
Pd(OAc)2/PCy3, the multi-substituted alkenylboron compounds could couple with anhydrides to
obtain furans in moderate-to-good yields. The addition of bases promoted the coupling reaction,
and the plausible reaction mechanism was proposed.
Received in revised form
Accepted
Available online
2013 Elsevier Ltd. All rights reserved.
Keywords:
Furans
Palladium-catalyzed cross-coupling
1,2-Oxaborol-2(5H)-ols
Carboxylic anhydrides
Furans are an important class of five-membered heterocycles,
which have been found as key structural units in natural products
such as tournefolin C and ircinin 1 (Figure 1).1 They are not only
common in an extensive variety of pharmaceuticals such as FNO
analogs (Figure 1),2 but also used as building blocks in organic
synthesis.3
well established methods that achieve the synthesis of furans, it is
still challenging to develop new synthetic strategies with less
reaction steps, milder conditions, less steric hindrance impacts
and greater tolerance of functional groups.
Suzuki-Miyaura cross-coupling reaction is a powerful tool for
the formation of carbon-carbon bond, which is characteristic of
mild conditions, high selectivity and tolerance with a broad range
of functional groups.12 These advantages are based on the
properties of the air- and moisture-stable organoboron
nucleophiles, which can be easily obtained by a variety of
synthetic routes and have low toxicity.13 Cross-coupling reactions
of organoboron reagents with carboxylic derivatives such as
carboxylic anhydrides are reported to synthesize ketones.14
However, only a few examples of cross-coupling reactions
between alkenylboron compounds and carboxylic derivatives
have been reported.15 And the cross-coupling reactions of multi-
substituted alkenylboron compounds with carboxylic derivatives
have never been described.
Figure 1. Examples of natural products and pharmaceuticals
containing a furan core.
1,2-oxaborol-2(5H)-ols are
a class of multi-substituted
alkenylboron compounds, which were prepared via copper-
catalyzed carbomagnesiation of propargyl alcohol, followed by
transmetallation of magnesium to boron in a one-pot procedure.16
In our previous studies, these alkenylboron compounds were
shown high reactivity in Suzuki-Miyaura cross-coupling reaction
with benzyl bromide17 and Petasis borono-Mannich reaction with
salicylaldehydes.18
These advantageous attributes have prompted researchers to
find better strategies to synthesize furans. A number of synthetic
methods have been reported to construct furans, such as
cyclization of alkynyl ketones,4 alkynyl epoxides,5 alkynols,6
propargyl ethers,7 allenyl ketones,8 1,4-dicarbonyl compounds,9
γ-hydroxy-α,β-imines10 and γ-hydroxy-α,β-enones.11 Despite the
———
∗ Corresponding author. Tel.: +86-21-64252011; fax: +86-21-64252758; e-mail: xinyanwu@ecust.edu.cn
∗ Corresponding author. Tel.: +86-21-54925330; fax: +0-000-000-0000; e-mail: yangj@sioc.ac.cn

2
Tetrahedron Letters
The cross-coupling reaction of 1,2-oxaborol-2(5H)-ols with
be Na2CO3, with which the yield increased to 45% (entry 6).
Next, different palladium sources were surveyed. When
PdCl2(PPh3)2, PdCl2 or Pd2(dba)3 was used instead of Pd(OAc)2,
the yield was decreased (entries 7-9). Then different ligands were
screened in the presence of Pd(OAc)2 and Na2CO3, and the
results indicated that the phosphine ligands played an important
role in the cross-coupling reaction between 4-phenyl-1,2-
oxaborol-2(5H)-ol and benzoic anhydride. Without PPh3 ligand,
the corresponding furan product could not be detected (entry 10).
When dppf was used as ligand, poor yield was obtained (entry
11), whereas alkyl phosphines were proved to promote this cross-
coupling reaction obviously. When PtBu3 and PCy3 were used as
ligands, the products were obtained in 65% and 73% yield,
respectively (entries 12 and 13).
carboxylic anhydrides could provide γ-hydroxy-α,β-enones,
which are known to undergo dehydrative cyclization to form
furans11 (Scheme 1).
Scheme 1. Retro-synthesis of furans.
To the best of our knowledge, direct formation of a furan motif
from alkenylboron compounds has never been reported. Herein,
we wish to report a one-step synthesis of furan compounds via
Suzuki-Miyaura cross-coupling reaction of 1,2-oxaborol-2(5H)-
ols with carboxylic anhydrides.
The solvent effect was next investigated. The yield decreased
when using toluene without the addition of water (Table 1, entry
14). Poor yields were obtained when polar solvents such as THF
and EtOH were chosen (entries 15 and 16). When the
temperature was increased from 80 oC to 100 oC, the yield was
similar (entry 17 vs 13). On the other hand, when the reaction
was carried out at room temperature, the yield was decreased to
38% (entry 18). When reaction time was reduced to 8 h or 4h,
73% NMR yield was provided, and 76% isolated yield was
obtained for 4 h (entry 20). Therefore, the optimal reaction
condition was determined to be: Pd(OAc)2/PCy3 as catalyst,
Na2CO3 as base with 1 equiv water in toluene at 80oC for 4 h.
To begin with, 4-phenyl-1,2-oxaborol-2(5H)-ol and benzoic
anhydride were chosen as model cross-coupling partners to
optimize the reaction conditions (Table 1). The initial conditions
were as follows: Pd(OAc)2 as palladium source, PPh3 as ligand, 1
o
equiv water19 in toluene as solvent at 80 C for 24 h (entry 1).
The resulting product was determined to be 2,4-diphenylfuran,
and the isolated yield was 9% resulted from the poor conversion.
To enhance the conversion of the cross-coupling reaction,
alkaline additives were examined to promote the transmetallition
of 4-phenyl-1,2-oxaborol-2(5H)-ol, which resulted in obvious
increase of yields (entries 2-6). The best base was determined to
Table 1
Optimization of the reaction conditionsa
Entry
Palladium source
Ligand
Base
Solvent
Time (h)
Yield (%)b
Temperature (℃)
Toluene (1eq H2Oc)
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
80
80
80
80
24
24
24
24
11(9d)
34
-
1
2
3
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PPh3
PPh3
PPh3
PPh3
K3PO4·3H2O
KF
17
K2CO3
21
NaHCO3
5
6
7
Pd(OAc)2
Pd(OAc)2
PdCl2(PPh3)2
PPh3
PPh3
PPh3
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
80
80
80
24
24
24
40
45
37
Na2CO3
Na2CO3
PdCl2
8
PPh3
PPh3
Na2CO3
Toluene (1eq H2O)
80
24
31
Pd2(dba)3
9
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
80
80
80
80
80
24
24
24
24
24
38
0
10
11
12
13
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
-
dppf
PtBu3
PCy3
8
65
73
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
14
15
16
17
18
19
20
21
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Toluene
THF
80
reflux
reflux
100
r.t.
24
24
24
24
24
8
56
24
EtOH
33
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
Toluene (1eq H2O)
71
38
80
73
73(76d)
80
4
80
2
69
aAll the reactions were carried out using a mixture of 4-phenyl-1,2-oxaborol-2(5H)-ol (0.6 mmol), benzoic anhydride (0.5 mmol), palladium source (0.015
mmol, 3 mol%), ligand (0.03 mmol, 6 mol%), base (1.5 mmol, 3 equiv) in 4 mL solvent under nitrogen atmosphere.
b 1H NMR yield.
c The amount of water was based on the amount of benzoic anhydride.
d Isolated yield.
and para-positions were able to react with 4-aryl-1,2-oxaborol-
With the optimal reaction conditions in hand, the cross-
coupling of various 1,2-oxaborol-2(5H)-ols and carboxylic
2(5H)-ols, giving the corresponding furans in 63-80% isolated
yields (entries 1-10). Likewise, aryl anhydrides chlorinated at the
ortho- or para-position reacted smoothly with 4-aryl-1,2-
oxaborol-2(5H)-ols, providing furans in good yields (entries 3, 4
anhydrides was investigated (Table 2). The results indicated that
aryl carboxylic anhydrides with substituents in the ortho-, meta-

3
and 10). Aryl bromides are usually applied as highly active
electrophiles in Suzuki-Miyaura cross-oupling reactions.12 We
found that the coupling reaction between 1,2-oxaborol-2(5H)-ol
and carboxylic anhydride containing a bromo substituent also
achieved furan in good yield (entry 5). 4-Alkyl-1,2-oxaborol-
2(5H)-ols were also tolerable for reaction with aryl anhydrides,
and the corresponding 4-alkyl-2-arylfurans products were
obtained in moderate yields (entries 11 and 12). The cross-
coupling reaction of alkenylboron compounds with alkyl
anhydrides has never been reported. Under the optimal reaction
conditions, acetic anhydride could react with 4-(p-tolyl)-1,2-
oxaborol-2(5H)-ol, affording the corresponding product in 34%
yield (entry 13). However, cyclohexanecarboxylic anhydride
showed no reactivity at the same reaction conditions (entry 14).
The lower reactivity of alkyl anhydrides might result from the
electronic effect. Aliphatic carbon chains are electron donating
groups, so the aliphatic carboxylic anhydrides are weaker
electrophiles than the aromatic carboxylic anhydrides in the
Suzuki-Miyaura cross-coupling reactions.
Scheme 2. Proposed mechanism of cross-coupling reaction.
In summary, an effective one-step method for the synthesis of
furans was described, which undergoes an unprecedented
palladium-catalyzed cross-coupling reaction of the readily
available multisubstituted alkenylboron compounds with
carboxylic anhydrides. This reaction will be useful for the
construction of a number of natural products and bioactive
molecules of pharmaceutical value.
Table 2
Cross-coupling reactions of 1,2-oxaborol-2(5H)-ols with
carboxylic anhydridesa
Acknowledgments
Entry
1
R1
Ph
R2
Ph
Product
3a
3b
3c
Yield (%)b
76
73
73
74
78
75
63
71
77
80
61
69
34
0
2
Ph
3-MeC6H4
4-ClC6H4
2-ClC6H4
2-BrC6H4
2-furyl
Ph
We are grateful for the financial support from National
Science Foundation of China (Nos. 20802088, 910017006,
90917017) and the Fundamental Research Funds for the Central
Universities.
3
Ph
4
Ph
3d
3e
5
Ph
6
Ph
3f
7
4-MeOC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
n-Bu
3g
3h
3i
References and notes
8
Ph
9
Ph
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furans is shown in Scheme 2. Initially, Pd(0) is formed via
reduction of Pd(II) by Cy3P,20 and then oxidative addition of
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