Efficient P, O chelate palladium(II)/AgNO3cocatalyzed homocoupling of aromatic terminal alkynes in aqueous media under ambient atmosphere

Article abstract of DOI:10.1080/10426507.2016.1239199

A new and efficient protocol for the P, O chelate Pd(II)/AgNO₃cocatalyzed oxidative homocoupling reaction of aromatic terminal alkynes in the synthesis of symmetrical 1,4-disubstituted-1,3-diynes was described in aqueous media under ambient atmosphere. The results showed that both NEt₃and THF/H₂O (in 4:1 proportion) played crucial roles in the reaction. In contrast, this protocol employs a low palladium(II) complex loading and AgNO₃as cocatalyst to obtain the homocoupled products in moderate to good yields.

Full text of DOI:10.1080/10426507.2016.1239199

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EFFICIENT P, O CHELATE PALLADIUM(II)/AgNO3 COCATALYZED
HOMOCOUPLING OFAROMATIC TERMINALALKYNES IN AQUEOUS MEDIA
UNDER AMBIENT ATMOSPHERE
Bo Chen, Mengping Guo*, Yongju Wen, Xiuli Shen, Xiuling Zhou, Meiyun Lv
Institute of Coordination Catalysis, College of Chemistry and Bio-engineering, Yichun
University, Yichun 336000,People’sRepublic of China
*Correspondence to: Mengping Guo, Institute of Coordination Catalysis, College of Chemistry
and Bio-engineering, Yichun University, Yichun 336000,People’sRepublic of China. E-mail:
guomengping65@163.com
Abstract
A new and efficient protocol for the P, O chelate Pd(II)/AgNO3 cocatalyzed oxidative
homocoupling reaction of aromatic terminal alkynes in the synthesis of symmetrical
1,4-disubstituted-1,3-diynes was described in aqueous media under ambient atmosphere. The
results showed that both NEt3 and THF/H2O (in 4:1 proportion) played crucial roles in the
reaction. In contrast, this protocol employs a low palladium(II) complex loading and AgNO3 as
cocatalyst to obtain the homocoupled products in moderate to good yields.
Pd(II) (0.5 mol%)
AgNO3 (0.5 mmol)
2
THF:H2O (V/V=4:1)
NEt3 (1 mmol), 60 oC, 24 h
R
R
R
Keywords
Aromatic terminal alkynes; P, O chelate Pd(II) complex/AgNO3 cocatalyst; homocoupling
reaction; 1,4-disubstituted-1,3-diynes
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INTRODUCTION
The Glaser oxidative homocoupling reaction, involving the homocoupling of terminal
alkynes, has proven to be an extremely useful synthetic method for preparation of
1,4-disubtituted -1,3-diynes,[1-4] which play an important role as building blocks in natural
products,[5-6] polymers,[7-8] liquid crystals,[9]supramolecular materials[10] and pharmaceuticals
with anti-inflammatory, antibacterial, antitumor and antifungal activities.[11-16] The reaction is
normally promoted by palladium and copper complex catalyst systems.[17-19]For the
Pd(II)-catalyzed homocoupling reactions of terminal alkynes, triaryl phosphines are traditionally
employed as ligands for the reaction.[20-24]Moreover, some cocatalysis of Pd(II)/CuI systems,
such as cyclopalladated ferrocenylimines,[25] Pd(NH3)2Cl2,[26] NHC-Pd(II)[27] and
(dipyridin-2-ylmethyl)amine-derived palladium chloride[28] have been reported. We have recently
reported that an air-stable hemilabile P-O coordinated cyclopalladated complex 1, which is
characterized by single-crystal X-ray crystallography, catalyzed the Suzuki-Miyaura reaction[29]
and cyanation of aryl halide reaction.[30] Here we would like to report this P-O coordinated
cyclopalladated complex 1 (Figure 1) in the presence of AgNO3 as an efficient and air stable
catalytic system for the synthesis of aromatic 1,4-disubtituted -1,3-diynes via dimerization of
aromatic terminal alkynes. To the best of our knowledge, Pd(II)/AgNO3 cocatalyzed system for
homocoupling reaction of aromatic terminal alkyne has not been reported in the literature.
RESULTS AND DISCUSSION
At the outset of this investigation, our goal was to evaluate the catalytic activity of the P-O
coordinated cyclopalladated complex 1 on the homocoupling of aromatic terminal alkynes. We
performed the homocoupling of phenylacetylene (1 mmol, 1a) in the presence of 1 (0.5 mol%)
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and NEt3(1 mmol) in THF at 60 0C, only a small amount of the desired homocoupling product
1,4-disubstituted-1,3-diyne (2a) (21%) was obtained after10 h under air. When AgNO3 (0.5
mmol) was added to this mixture, a dramatic effect has been showed and 55% yield of the
desired product was isolated (Table 1, entry 1), whereas 93% yield of 2a was obtained when a
co-solvent THF:H2O (in 4:1 proportion) was added (Table 1, entry 2). Among the co-solvent
screened (Table 1, entries 16-23), ethanol and n-butyl alcohol gave good result (Table 1, entries
22-23). Other solvents such as PEG400, acetone, N,N-dimethylacetylamide, 1,4-dioxane，
methanol and DMSO were less effective than THF (Table 1, entries 16-21).
Furthermore, the effect of bases on the Pd(II)/AgNO3-catalyzed homocoupling of
phenylacetylene (1a) was evaluated (Table 1, entries 2-14). As shown in Table 1, moderate to
good yields were observed when Na2CO3, K2CO3, NaHCO3, NaH2PO4, KHCO3, KH2PO4,
K3PO4 and Cs2CO3 were chosen separately as a base (Table 1, entries 5-12), In contrast, NaOH,
KOH, NaF, CH3COONa and ryridine gave slightly lower reactivities under the same reaction
conditions (Table 1, entries 3-4, 13-15). The results showed that use of THF:H2O (in 4:1
proportion) as a co-solvent and the use of NEt3 as a base gave the best results (Table 1, entry 2).
Based on the best base and solvent conditions, other reaction conditions such as reaction
temperature, time, the amount of the catalyst and AgNO3 were also examined. Results are shown
in Table 2. Among the amount of the catalyst and AgNO3 studied, only 21% yield of 2a was
obtained when complex 1 (0.5 mol%) as a catalyst was added to reaction system in the absence
of AgNO3 (Table 2, entry 1), in the next place, using AgNO3 (0.5 mmol) as a catalyst without
complex 1 was studied, but poor yield was observed (Table 2, entry 2) so that complex 1 (0.5
mol%) as a catalyst and AgNO3 (0.5 mmol) as a cocatalyst worked very well in THF:H2O (in 4:1
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0
proportion) at 60 C in air (Table 2, entry 5). Next, the temperature effect on this reaction was
also studied. Results are consistent with Shi’s report,[27] the homocoupling product 2a was
0
0
obtained in lower yield (60%) at 80 C after 10 h, but in higher yield at 60 C (93%) (Table 2,
0
entries 5, 20). With a lower temperature (40 C), 2a was obtained in poor yield (15%) (Table 2,
entry 17). In addition, prolonging the homocoupling reaction time from 4 h to 30 h, the yield of
2a increased from 54% to 93% (Table 2, entries 5, 13-17).
Under the optimized reaction conditions, the homocoupling reactions of a series of aromatic
terminal alkynes were carried out smoothly to afford the corresponding aromatic 1,4-
disubstituted-1,3-diyne derivatives in moderate to high yields, and the results are summarized in
Table 3. The homocoupling of aromatic terminal alkynes with alkyl group at the para-position
and meta-position gave almost the same high yields (84-89%) (Table 3, entries 2, 4-6). The lower
yield (70%) for the homocoupling of 2-ethynyltoluene was probably due to the steric effect of
the methyl group located at the ortho-position[26] (Table 3, entry 3). However, the aromatic
terminal alkynes bearing an electron-donating group were homocoupled to give the
corresponding aromatic 1,4- disubstituted-1,3-diyne derivatives in (51-64%) yields (Table 3,
entries 7-11).
CONCLUSIONS
In summary, a new, general and efficient methodology based on the Pd(II)/AgNO3 system
has been developed. The use of AgNO3 as cocatalyst for the homocoupling reaction of aromatic
terminal alkyne represents an interesting alternative to existing catalytic systems based on the
use of Pd(II)/CuI. The Pd(II)/AgNO3 as effective catalytic system for the aromatic terminal
alkyne homocoupling reaction in the presence of THF:H2O (in 4:1 proportion) at 60 0C in air, as
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well as NEt3 as base. The corresponding coupled products were obtained in moderate to good
yields. Currently, further efforts to study the catalytic mechanism are underway in our laboratory.
EXPERIMENTAL
The reagents and solvents were obtained from commercial sources and were generally used
without further purification. Thin layer chromatography was performed on silica gel 60 GF2541H
and 13C NMR spectra were measured on a Bruker Avance III (400 MHz) spectrometer using
tetramethylsilane as the internal standard and CDCl3 as the solvent.
Typical experimental procedure for the homocoupling reactions of terminal alkynes
All homocoupling reactions of aromatic terminal alkynes were carried out under air. A
mixture of aromatic terminal alkyne (1.0 mmol), NEt3 (1.0 mmol) and P-O coordinated
cyclopalladated complex 1 (0.005 mmol)/AgNO3 (0.5 mmol) in THF:H2O (in 4:1 proportion, 2.5
0
mL) was allowed to react at 60 C. The reaction progress was analysed by GLC. The mixture
was added brine (4 mL) and extracted three times with ethyl acetate (3 × 15 mL), dried over
Na2SO4, concentrated in vacuo and purified by thin layer chromatography. The purified products
1
were identified by H-NMR and 13C-NMR spectroscopy. The Supplemental Materials presents
sample 1H and 13C NMR spectra of the diyne products (Figures S 1 – S 24).
o
o
1,4-diphenylbuta-1,3-diyne (Table 3, entry 1): White solid (m.p. = 86–87 C, lit[31] 85–86 C).
13
1H-NMR (400 MHz, CDCl3) δ 7.58 – 7.60 (m, 4H), 7.27 – 7.42 (m, 6H). C-NMR (101 MHz,
CDCl3) δ 132.59, 129.32, 128.55, 121.85, 81.71, 74.10.
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o
o
(Table 3, entry 2): White solid (m.p. = 182-183 C, lit[24] 183 C).
1,4-di-p-tolybuta-1,3-diyne
1H NMR (400 MHz, CDCl3) δ 7.43-7.45 (d, 4H), 7.15-7.17 (d, 4H), 2.38 (s, 6H). 13C NMR (101
MHz, CDCl3) δ 139.53, 132.42, 129.25, 118.80, 81.58, 73.48, 21.66.
o
o
(Table 3, entry 3): White solid (m.p. = 72–74 C, lit[32] 72–74 C).
1,4-di-o-tolybuta-1,3-diyne
1H NMR (400 MHz, CDCl3) δ 7.60-7.36 (d, 2H), 7.34-7.29 (m, 4H), 7.26-7.22 (t, 2H), 2.59 (s,
6H). 13C NMR (101 MHz, CDCl3) δ 141.70, 133.01, 129.68, 129.22, 125.77, 121.81, 81.29,
77.70, 20.72.
o
o
(Table 3, entry 4): White solid (m.p. = 69–71 C, lit[32] 68–70 C).
1,4-di-m-tolybuta-1,3-diyne
H-NMR (400 MHz, CDCl3) δ 7.40-7.30 (d, 4H), 7.29-7.22 (ddd, 4H), 2.39 (s, 6H). 13C-NMR(101
MHz,CDCl3) δ138.22, 133.03, 130.19, 129.67, 128.39, 121.68, 81.70, 73.75, 21.27.
(Table 3, entry 5): White solid (m.p. = 65–66 oC, lit[33] 67
1,4-bis(4-butylphenyl)buta-1,3-diyne
1
oC). H-NMR (400 MHz, CDCl3) δ 7.49-7.51 (m, 4H), 7.20-7.29 (d, 4H), 2.66-2.70 (t, 4H), 1.66
13
(m, 4H), 1.41-1.43 (dd, 4H), 1.00 (t, 6H). C-NMR(101 MHz, CDCl3) δ 144.50, 132.47, 128.63,
119.06, 81.66, 73.64, 35.76, 33.39, 22.40, 14.00.
1,4-bis(4-(tert-butyl)phenyl)buta-1,3-diyne (Table 3, entry 6): White solid (m.p. = 202–204 oC,
o
1
lit[34] 203–204 C). H NMR (400 MHz, CDCl3) δ 7.54-7.52 (d, 4H), 7.43-7.41 (d, 4H), 1.37 (s,
18H). 13C NMR (101 MHz, CDCl3) δ 152.60, 132.33, 125.54, 118.89, 81.58, 73.60, 34.96, 31.17.
o
1,4-bis(4-methoxyphenyl)buta-1,3-diyne (Table 3, entry 7): White solid (m.p. = 39–141 C,
o
1
lit[31] 138–139 C). H NMR (400 MHz, CDCl3) δ 7.51-7.48 (d, 4H), 6.90-6.87 (d, 4H), 3.84 (s,
6H). 13C NMR (101 MHz, CDCl3) δ 159.80, 133.95, 131.90, 129.11, 114.17, 113.93, 80.93,
73.13, 54.87.
1,4-bis(2-methoxyphenyl)buta-1,3-diyne (Table 3, entry 8): White solid( m.p. = 72–74 oC, lit[26]
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72–74 oC). 1H-NMR (400 MHz, CDCl3) δ7.52-7.50 (dd, 2H), 7.37 – 7.32 (m, 2H), 6.96-6.90 (dd,
4H), 3.91 (s, 6H).13C- NMR(101 MHz, CDCl3) δ 161.37, 134.40, 130.65, 120.55, 111.23, 110.72,
78.75, 76.83, 55.84.
1,4-bis(4-fluorophenyl)buta-1,3-diyne (Table 3, entry 9): White solid (m.p. = 190–192 oC, lit [31]
o
1
13
192–193 C). H NMR (400 MHz, CDCl3) δ 8.09 (dd, 4H), 7.62 (t, 4H). C NMR (101 MHz,
CDCl3) δ 161.82, 134.52, 117.84, 115.82, 80.44, 73.54.
(Table 3, entry 10): White solid (m.p. = 73–74 oC, lit[35]
1,4-bis(3-chlorophenyl)buta-1,3-diyne
13
73 oC). 1H NMR (400 MHz, CDCl3) δ 7.86 (s, 2H), 7.74 (dd, 4H), 7.63 (dd, 2H). C NMR (101
MHz, CDCl3) δ 134.38, 132.29, 130.69, 129.75, 123.30, 80.59, 74.73.
o
(Table 3, entry 11): White solid (m.p. = 180–182 C,
1,4-bis(2-bromophenyl)buta-1,3-diyne
lit[36] 182 oC). 1H NMR (400 MHz, CDCl3) δ 7.68 – 7.55 (m, 4H), 7.38 – 7.20 (m, 4H).13C NMR
(101 MHz, CDCl3) δ 134.56, 132.63, 130.44, 127.15, 126.22, 124.07, 81.10, 76.74.
o
(Table 3, entry 12): White solid (m.p. = 96-97 C,
1,4-di([1,1’-biphenyl]-4-yl)buta-1,3-diyne
o
1
lit[37] 96 C). H NMR (400 MHz, CDCl3) δ 7.58-7.53 (m, 12H), 7.45-7.41 (t, 4H), 7.37-7.35 (t,
2H). 13C NMR (101 MHz, CDCl3) δ 141.62, 140.28, 132.62, 128.93, 127.79, 127.11, 127.06,
121.03, 83.62, 76.78.
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of China (No: 21363026) and
the Scientific and Technological Landing Project of Higher Education of Jiangxi Province (No:
KJLD13091).
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Table 1. Screening of solvents and bases for the homocoupling of phenylacetylenea
Entry
1
Base
NEt3
Solvent
THF
Yieldb (%)
55
2
NEt3
THF + H2O
93
3
4
NaOH
KOH
THF + H2O
THF + H2O
39
45
5
6
7
8
Na2CO3
K2CO3
NaHCO3
NaH2PO4
KHCO3
KH2PO4
K3PO4
Cs2CO3
NaF
CH3COONa
Pyridine
NEt3
NEt3
NEt3
THF + H2O
THF + H2O
THF + H2O
THF + H2O
THF + H2O
THF + H2O
THF + H2O
THF + H2O
74
70
85
72
65
78
67
61
41
53
52
25
29
20
38
41
9
10
11
12
13
14
15
16
17
18
19
20
21
22
THF + H2O
THF + H2O
THF + H2O
PEG400 + H2O
Acetone + H2O
N, N-dimethylacetylamide + H2O
1,4-dioxane + H2O
DMSO + H2O
Methanol + H2O
Ethanol + H2O
N-butyl alcohol + H2O
NEt3
NEt3
NEt3
NEt3
55
84
83
23
NEt3
a
The reaction was performed with phenylacetylene (1 mmol), Pd(II) complex catalyst (0.5
mol%), AgNO3 (0.5 mmol) and base (1 mmol) in solvent:H2O (2.5 mL, V/V=4:1) at 60 0Cunder
air for 10 h.
b Isolated yield.
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Table 2. Screening of other reaction conditions for the homocoupling of phenylacetylenea
Entry
Catalyst
(mol%)
0.5
AgNO3
(mmol)
0
Time (h)
Temperature Yield (%)
(0C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
10
10
10
10
10
10
10
10
10
10
10
10
4
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
80
21c
Tracec
67
0
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.25
0.75
1.0
1.5
0.5
0.5
0.5
0.5
0.5
0.125
0.25
0.5
1
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
73
93
81
78
66
73
86
75
30
54
85
88
91
90
65
15
6
8
16
24
30
10
10
0.5
0.5
0.5
0.5
60
a The reaction was performed with phenylacetylene (1 mmol) and NEt3 (1 mmol) in THF:H2O
(2.5 mL, V/V=4:1) under air.
b Isolated yield.
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Table 3. Pd(II)/AgNO3-catalyzed homocoupling reactions of aromatic terminal alkynes a
Entey
1
Alkyne
Product
Yield(%)b
93
Reference
[31]
2
3
88
70
[24]
[32]
4
5
84
84
[32]
[33]
6
7
89
61
[34]
[31]
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8
56
51
[26]
[31]
9
10
54
[35]
11
12
64
54
[36]
[37]
a
Carried out with aromatic terminal alkyne (1 mmol), Pd(II) complex catalyst (0.5 mol%),
AgNO3 (0.5 mmol) and NEt3 (1 mmol) at at 60 0C under air for 10 h.
b Isolated yield.
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Figure 1 P-O coordinated cyclopalladated complex 1
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