A facile and practical process for the synthesis of polysubstituted furans from alkynes using atmospheric oxygen as a terminal oxidant

Article abstract of DOI:10.1002/cjoc.201400386

We present here a facile and practical procedure for the synthesis of tetrasubstituted furans from alkynes catalyzed by palladium acetate together with cupric acetate in acetic acid, using atmospheric oxygen as a terminal oxidant. Various internal aromati

Full text of DOI:10.1002/cjoc.201400386

COMMUNICATION
DOI: 10.1002/cjoc.201400386
A Facile and Practical Process for the Synthesis of Polysub-
stituted Furans from Alkynes Using Atmospheric
Oxygen as a Terminal Oxidant
Jingxiu Xu, Xingdong Song, and Jinwu Zhao*
School of Pharmacy, Guangdong Medical College, Dongguan, Guangdong 523808, China
We present here a facile and practical procedure for the synthesis of tetrasubstituted furans from alkynes cata-
lyzed by palladium acetate together with cupric acetate in acetic acid, using atmospheric oxygen as a terminal oxi-
dant. Various internal aromatic alkynes afforded the target furans in satisfactory yield.
Keywords furan, alkyne, palladium, copper, oxygen
Introduction
dant. Although these processes could provide an alter-
native approach to furan derivatives, they are less prac-
tical because of operability and cost. We present herein
that tetrasubstituted furan compounds could be synthe-
sized from alkynes under the catalysis of Pd(OAc)2 and
Cu(OAc)2 using atmospheric oxygen as a terminal oxi-
dant in acetic acid (Eq. 1). This transformation could be
performed in common solvent connected to an oxygen
balloon, which established a more practical procedure
for furan synthesis.
From the viewpoint of green and sustainable chem-
istry, molecular oxygen is reputed to be an ideal oxidant
due to its natural, inexpensive, and environmentally
friendly characters, and therefore offers attractive aca-
demic and industrial prospects.[1] On the other hand,
transition-metal catalyzed reactions are considered as
the most attractive tools to construct heterocycles be-
cause they can regioselectively produce highly func-
tionalized heterocyclic motifs from readily available
feedstock under mild conditions.[2] Therefore, transi-
tion-metal-catalyzed transformations with molecular
oxygen as a terminal oxidant are more attractive by
combining the advantages of both molecular oxygen as
an oxidant and the catalysis of transition-metal.
O2 balloon
R1
R1
Pd(OAc)2 (5 mol%)
R2
R2
R1
R1
R2
R2
Cu(OAc)2 (10 mol%)
(1)
+
AcOH
O
Furan derivatives, as featured moieties, occur in a
number of plants and marine organism, such as furo-
dysinin,[3] kallolide,[4] pinguisone,[5] and methyl voua-
capenate.[6] These heterocyclic molecules also have a
wide range of industrial applications in pharmaceuticals,
fragrances, insecticides, dyes and so on.[7] Moreover,
functionalized furans are widely employed as versatile
building blocks to fabricate complicated natural prod-
ucts, and as privileged scaffolds in drug discovery due
to their diverse biological activities.[8] Although a num-
ber of strategies have been developed for the synthesis
of furan derivatives,[9] the development of novel proce-
dures for the synthesis of highly functionalized furans
remains appealing to organic chemist.
Results and Discussion
Our studies began by optimizing the reaction condi-
tions for the synthesis of tetrasubstituted furans, choos-
ing diphenylethyne as model substrate. The results are
tabulated in Table 1. When this reaction was carried out
in N,N-dimethylacetamide (DMA) at 100 ℃ for 12 h,
using 5 mol% Pd(OAc)2 as the catalyst, in the presence
of 20 mol% ZnCl2, under 101 kPa of pressure of dioxy-
gen, no desired tetraphenyl furan was obtained (Table 1,
Entry 1). The investigation on the effect of acid sug-
gested that Brønsted acids could promote this reaction
against Lewis acids (Table 1, Entries 2－5). The possi-
ble reason was that the recycling of catalyst necessitated
proton under normal pressure. After screening common
polar solvents, AcOH was found to be the best medium,
which acted as proton source as well (Table 1, Entries
It is known that the oxidation of alkynes could afford
furans under the catalysis of palladium.[10] The reported
procedures could be conducted in an autoclave,[11] or in
fluorous media,[12] using molecular oxygen as an oxi-
*
E-mail: jwzhao@gdmc.edu.cn
Received June 18, 2014; accepted July 11, 2014; published online September 15, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400386 or from the author.
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Xu, Song & Zhao
COMMUNICATION
6－8). To our delight, the addition of cupric salt greatly
improved the oxidative cyclization reaction and we
found that Cu(OAc)2 was a superior co-catalyst (Table 1,
Entries 9－11). As shown in Table 1, the yield of target
tetraphenylfuran decreased when the transformation
proceeded under the catalysis of other palladium agent
such as PdCl2, Pd(NCMe)2Cl2, Pd2(dba)3 (Table 1, En-
tries 12－14).
homo-coupled one was obtained when bis(4-butylphe-
nyl)ethyne reacted with bis(4-(trifluoromethyl)phenyl)-
ethyne or diphenylethyne under the optimized reaction
conditions. The cross-coupled furan product was iso-
lated in the yields of 60% and 52% respectively (Table
2, Entries 11, 12).
Table 2 Scope of alkynes for the synthesis of furansa
O2 balloon
Pd(OAc)2 (5 mol%)
R1
R2
Table 1 Optimizing the reaction conditions for the synthesis of
R1
R1
R2
R2
furans from alkynesa
Cu(OAc)2 (10 mol%)
+
AcOH
O2 balloon
cat./acid
R1
O
R2
Ph
Ph
Ph
Ph
a
－
l
Ph
Ph
solvent
Entry
1
R1
C6H5
R2
Product Yieldb/%
O
Co-catalyst Acid Solvent Yieldb/%
C6H5
92
78
71
96
85
90
70
76
72
50
60
52
a
b
c
d
e
f
Entry
1
Catalyst
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
2
4-n-BuC6H4
4-MeOC6H4
4-(CF3)C6H4
4-FC6H4
4-n-BuC6H4
4-MeOC6H4
4-(CF3)C6H4
4-FC6H4
－
－
ZnCl2
Zn(OTf)2 DMA
HCl DMA
DMA
N.P.
N. P.
17
3
2
4
3
－
5
4
－
PTSAd DMA
25
6
4-BrC6H4
3-MeC6H4
3-ClC6H4
3-FC6H4
4-BrC6H4
3-MeC6H4
3-ClC6H4
3-FC6H4
5
－
AcOH DMA
26
7
g
h
i
6
－
－
AcOH
DMF
39
8
7
－
AcOH
25
9
8
－
AcOH DMSO
20
10
11
12
2-FC6H4
2-FC6H4
j
9c
10c
11c
12c
CuCl2
Cu(OAc)2
Cu(OTf)2
Cu(OAc)2
－
－
－
－
－
－
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
87
4-n-BuC6H4
C6H5
4-(CF3)C6H4
4-n-BuC6H4
k
l
95
95
a Reactions were carried out using alkyne (R1) (0.5 mmol), alkyne
(R2) (0.5 mmol), Pd(OAc)2 (5 mol%), Cu(OAc)2 (10 mol%),
AcOH (2 mL), 100 ℃, O2 balloon (101 kPa), 12 h. b Isolated
yields.
57
13c Pd(NCMe)2Cl2 Cu(OAc)2
14c
Pd2(dba)3 Cu(OAc)2
55
23
a Reaction conditions: 1,2-diphenylacetylene (1.0 mmol), catalyst
(5 mol%), acid (20 mol%), solvent (2 mL), 100 ℃, O2 balloon
(101 kPa), 12 h. b Determined by GC. c Co-catalyst (10 mol%).
d PTSA＝p-toluenesulfonic acid.
In order to justify the practicability of this method, a
gram-scale synthesis was carried out. 1.0712 g of di-
phenylethyne was allowed to react for 12 h in a 50 mL
round bottom flask charged with 0.0683 g Pd(OAc)2,
0.1252 g Cu(OAc)2 and 20 mL AcOH connected to an
oxygen balloon, under magnetic stirring at 100 ℃. Af-
ter the reaction was completed, the reaction solution
was transferred into a separatory funnel and then 60 mL
water was added. The aqueous solution was extracted
with ethyl acetate (30 mL×3) and the combined extract
was dried with anhydrous MgSO4. The solvent was
vacuumed and the crude product was isolated by silica
gel column chromatography with light petroleum
ether/CH2Cl2 as eluent to give the pure product. The
product 2,3,4,5-tetraphenylfuran (a), in the amount of
0.9850 g, was obtained in 88% yield.
The scope of the method was then examined after
establishing the optimal reaction conditions, and the
results are listed in Table 2. The homo-coupling reac-
tions of alkynes were firstly tested. Symmetrical
4-substituted diphenylethynes could all go through the
reaction efficiently and provided the target furans in
good to excellent yield (Table 2, Entries 1－5). In com-
parison, electron-withdrawing groups favored the reac-
tion more than electron-donating groups. It is notewor-
thy that this procedure for the preparation of polysub-
stituted furan tolerates aryl bromides which allow fur-
ther functionalization (Table 2, Entry 6). Symmetrical
3-substituted diphenylethynes produced the corre-
sponding furans in a lower yield than 4-substituted ones
(Table 2, Entries 7－9), but in a higher yield than
2-substituted one (Table 2, Entry 10). The cross-cou-
pling reaction of diverse alkynes was then explored and
we found that a mixture of cross-coupled product and
Conclusions
In conclusion, we have developed a procedure for
the synthesis of highly substituted furans from alkynes
catalyzed by palladium using atmospheric oxygen as a
terminal oxidant. This protocol could be carried out in
1100
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Chin. J. Chem. 2014, 32, 1099—1102

A Facile and Practical Process for the Synthesis of Polysubstituted Furans from Alkynes
common solvent connecting to an oxygen balloon,
which was more facile and practical.
134.7, 134.5, 133.9, 131.6, 130.1, 130.0, 129.8, 128.4,
128.1, 128.0, 125.8, 124.5, 124.0; MS (EI, 70 eV) m/z
(%): 444 (M＋, 100), 321 (73), 123 (69), 95 (55).
2,3,4,5-Tetrakis(4-bromophenyl)furan (f):[11b,12] 1H
NMR (400 MHz, acetone-d6) δ: 7.57－7.50 (m, 8H),
7.48－7.42 (m, 4H), 7.21－7.13 (m, 4H); 13C NMR
(101 MHz, acetone-d6) δ: 148.0, 133.0, 132.9, 132.7,
132.6, 132.4, 130.0, 128.2, 125.4, 122.5, 122.3, 120.4;
MS (EI, 70 eV) m/z (%): 484 (100), 441 (32), 161 (14),
105 (29).
Experimental
General
1H and 13C NMR spectra were recorded on a
BRUKER DRX-400 spectrometer using indicated
deuterated solvents and TMS as an internal standard.
Mass spectra were obtained with a SHIMADZU model
GCMS-QP5000 spectrometer. TLC was performed by
using commercially prepared 100－400 mesh silica gel
plates (GF254) and visualization was effected at 254 nm.
2,3,4,5-Tetram-tolylfuran (g):[11,12] 1H NMR (400
MHz, CDCl3) δ: 7.41 (s, 2H), 7.24 (s, 2H), 7.12 (t, J＝
7.6 Hz, 4H), 7.06－6.92 (m, 8H), 2.30 (s, 6H), 2.23 (s,
6H); 13C NMR (101 MHz, CDCl3) δ: 147.6, 137.9,
137.7, 133.2, 131.1, 131.0, 128.1, 128.1, 128.0, 127.7,
127.5, 126.4, 125.1, 123.0, 21.6, 21.3; MS (EI, 70 eV)
m/z (%): 428 (M＋, 100), 309 (21), 119 (65), 91 (18).
2,3,4,5-Tetrakis(3-chlorophenyl)furan (h):[12] 1H
NMR (400 MHz, acetone-d6) δ: 7.58－7.53 (m, 2H),
7.45－7.28 (m, 12H), 7.26－7.20 (m, 2H); 13C NMR
(101 MHz, acetone-d6) δ: 147.8, 135.1, 135.0, 134.9,
132.6, 131.3, 131.3, 130.8, 129.7, 129.0, 128.8, 126.1,
125.8, 124.9, 124.0, 122.1; MS (EI, 70 eV) m/z (%): 510
(M＋, 100), 183 (40), 169 (56), 139 (80), 111 (65), 57
(48).
Typical procedure
Alkyne (1 mmol), Pd(OAc)2 (5 mol%), Cu(OAc)2
(10 mol%) and AcOH (2 mL) were added into a test
tube attached to an oxygen balloon (101 kPa). The sys-
tem was stirred magnetically and heated at 100 ℃ with
an oil bath for 12 h. And then the reaction was quenched
by the addition of 10 mL water. The aqueous solution
was extracted with ethyl acetate (10 mL×3) and the
combined extract was dried with anhydrous MgSO4.
The solvent was vacuumed and the crude product was
isolated by TLC with light petroleum ether/CH2Cl2 as
eluent to give the pure product.
2,3,4,5-Tetrakis(3-fluorophenyl)furan (i):[12] 1H
NMR (400 MHz, acetone-d6) δ: 7.47－7.24 (m, 8H),
7.18－6.99 (m, 8H); 13C NMR (101 MHz, acetone-d6) δ:
164.8, 162.4, 147.7, 135.5, 135.4, 133.0, 132.9, 131.6,
131.5, 131.4, 127.2, 125.9, 122.3, 117.9, 117.7 115.8,
115.6, 115.6, 115.4, 113.0, 112.8; MS (EI, 70 eV) m/z
(%): 444 (M＋, 100), 321 (70), 201 (28), 123 (60), 95
(29).
2,3,4,5-Tetraphenylfuran (a):[11,12] 1H NMR (400
MHz, CDCl3) δ: 7.53－7.46 (m, 4H), 7.28－7.18 (m,
12H), 7.17－7.11 (m, 4H); 13C NMR (101 MHz, CDCl3)
δ: 147.7, 133.2, 130.9, 130.4, 128.4, 128.4, 127.3, 127.2,
125.9, 125.1; MS (EI, 70 eV) m/z (%): 372 (M＋, 100),
267 (34), 165 (19), 77 (11).
2,3,4,5-Tetrakis(4-butylphenyl)furan (b):[11b,12] 1H
NMR (400 MHz, acetone-d6) δ: 7.45－7.35 (m, 4H),
7.14－7.01 (m, 12H), 2.57 (td, J＝7.8, 2.8 Hz, 8H),
1.64－1.48 (m, 8H), 1.33 (hd, J＝7.3, 3.3 Hz, 8H), 0.90
(td, J＝7.4, 1.8 Hz, 12H); 13C NMR (101 MHz, ace-
tone-d6) δ: 148.1, 142.9, 142.5, 131.5, 131.0, 129.3,
129.2, 126.2, 125.6, 35.8, 34.2, 22.9, 14.2; MS (EI, 70
eV) m/z (%): 596 (M＋, 100), 553 (64), 539 (11), 213
(23), 161 (55), 133 (28), 57 (10), 44 (15).
2,3,4,5-Tetrakis(2-fluorophenyl)furan (j):[12] 1H
NMR (400 MHz, CDCl3) δ: 7.56－7.37 (m, 4H), 7.30－
7.25 (m, 4H), 7.23－7.20 (m, 4H), 7.14－7.10 (m, 2H),
7.03－6.98 (m, 2H); 13C NMR (101 MHz, acetone-d6) δ:
162.2, 161.3, 159.7, 158.8, 146.7, 132.7, 131.6, 131.6,
130.8, 130.7, 130.7, 125.3, 125.2, 124.9, 121.7, 121.2,
121.1, 119.3, 119.1, 117.1, 116.9, 116.3, 116.1; MS (EI,
70 eV) m/z (%): 444 (M＋, 100), 321 (54), 201 (14), 123
(59), 95 (27).
2,3,4,5-Tetrakis(4-methoxyphenyl)furan (c):[11,12] 1H
NMR (400 MHz, CDCl3) δ: 7.40－7.27 (m, 4H), 7.13－
7.01 (m, 4H), 6.92－6.77 (m, 8H), 3.72 (s, 6H), 3.71 (s,
6H); 13C NMR (101 MHz, CDCl3) δ: 132.6, 132.5,
132.4, 132.3, 132.3, 129.9, 114.2, 113.9, 113.8, 113.6,
55.4, 55.2, 29.7; MS (EI, 70 eV) m/z (%): 492 (M＋, 28),
318 (100), 275 (32), 135 (45).
2,3-Bis(4-butylphenyl)-4,5-bis(4-(trifluoromethyl)-
phenyl)furan (k):[12] 1H NMR (400 MHz, acetone-d6) δ:
7.74－7.61 (m, 1H), 7.51－7.37 (m, 1H), 7.19－7.03
(m, 1H), 2.60 (t, J＝7.7 Hz, 1H), 1.67－1.49 (m, 1H),
1.38－1.28 (m, 1H), 0.90 (td, J＝7.4, 0.8 Hz, 1H);13C
NMR (101 MHz, acetone-d6) δ: 145.0, 146.7, 143.8,
143.2, 138.0, 134.8, 131.8, 131.0, 130.4, 129.7, 129.5,
129.4, 129.3, 128.5, 126.7, 126.5, 126.4, 126.3, 35.9,
35.8, 34.2, 34.2, 22.9; MS (EI, 70 eV) m/z (%): 620 (M＋,
100), 577 (31), 173 (66), 161 (96), 145 (23), 91 (26), 57
(27), 43 (44).
2,3,4,5-Tetrakis(4-(trifluoromethyl)phenyl)furan
(d):[11a,12] 1H NMR (400 MHz, CDCl3) δ: 7.60－7.54 (m,
12H), 7.30－7.23 (m, 4H); 13C NMR (101 MHz, CDCl3)
δ: 148.0, 135.6, 133.0, 131.2, 130.5, 130.1, 129.8, 129.5,
128.0, 126.2, 125.9, 125.9, 125.7, 125.7, 125.3, 125.1,
122.6; MS (EI, 70 eV) m/z (%): 644 (M＋, 46), 471 (13),
173 (100), 145 (33).
2,3-Bis(4-butylphenyl)-4,5-diphenylfuran (l):[12] 1H
NMR (400 MHz, acetone-d6) δ: 7.53－7.38 (m, 4H),
7.33－7.16 (m, 8H), 7.15－7.06 (m, 6H), 2.69－2.47
(m, 4H), 1.66－1.46 (m, 4H), 1.40－1.23 (m, 4H), 0.90
(td, J＝7.3, 1.4 Hz, 6H); 13C NMR (101 MHz, ace-
2,3,4,5-Tetrakis(4-fluorophenyl)furan (e):[11,12] 1H
NMR (400 MHz, CDCl3) δ: 7.52－7.40 (m, 4H), 7.27－
7.05 (m, 12H); 13C NMR (101 MHz, CDCl3) δ: 147.2,
Chin. J. Chem. 2014, 32, 1099—1102
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Xu, Song & Zhao
COMMUNICATION
[5] Corbella, A.; Gariboldi, P.; Jommi, G.; Orsini, F.; DeMarco, A.;
Immirzi, A. J. Chem. Soc., Perkin Trans. 1 1974, 1875.
tone-d6) δ: 148.6, 147.9, 143.1, 142.7, 134.2, 131.7,
131.1, 131.0, 129.2, 128.2, 126.3, 125.6, 35.9, 34.2,
22.9, 14.2; MS (EI, 70 eV) m/z (%): 484 (M＋, 100), 441
(31), 161 (24), 105 (46), 59 (28).
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Acknowledgement
We thank Scientific and Technological Project of
Zhanjiang City (No. 2012C3106024) and Guangdong
Medical College (No. M2010006) for financial support.
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