Ligand-free palladium-catalysed oxidative Heck reaction of 4-vinylpyridine with arylboronic acids: Selective synthesis of (E)-4-styrylpyridines

Article abstract of DOI:10.3184/174751912X13352864811920

A ligand-free palladium-catalysed oxidative Heck reaction of various arylboronic acids with 4-vinylpyridine has been developed, achieving (E)-4-styrylpyridines with high selectively in moderate to good yields. A plausible mechanism for the formation of (E)-4-styrylpyridines has been proposed.

Full text of DOI:10.3184/174751912X13352864811920

322 RESEARCH PAPER
JUNE, 322–325
JOURNAL OF CHEMICAL RESEARCH 2012
Ligand-free palladium-catalysed oxidative Heck reaction of
4-vinylpyridine with arylboronic acids: selective synthesis of
(E)-4-styrylpyridines
Chuanli Shia, Jinchang Dinga,b, Jun Jianga, Jiuxi Chena, Huayue Wua and Miaochang Liua*
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China
bWenzhou Vocational andTechnical College, Wenzhou 325035, P. R. China
A ligand-free palladium-catalysed oxidative Heck reaction of various arylboronic acids with 4-vinylpyridine has been
developed, achieving (E)-4-styrylpyridines with high selectively in moderate to good yields. A plausible mechanism
for the formation of (E)-4-styrylpyridines has been proposed.
Keywords: oxidative Heck reaction, palladium, ligand-free, (E)-4-styrylpyridines
Table 1 Identification of the optimum conditionsa
(E)-4-Styrylpyridine and its derivatives are of importance
because they have found numerous applications in materials
chemistry1,2 and medicinal chemistry.3 (E)-4-Styrylpyridines
as monodentate ligands also play an important role in coor-
dination chemistry4–5 and photochemistry.6–8 Consequently
considerable effort has been made to develop efficient strate-
gies for their synthesis. Common synthetic routes for the
preparation of these compounds include a palladium-catalysed
Heck reaction of 4-vinylpyridine with organic halides in the
presence of a ligand and base (Scheme 1a).9–17 Compared to
organic halides, organoboronic acids are attractive due to their
ease of handling, stability, and low toxicity.18 Although the
palladium-catalysed oxidative Heck reaction of organoboronic
acids and olefins has been well-studied,19–20 The Pd-catalysed
coupling of organoboronic acids with 4-vinylpyridine has
never been achieved. This may be because the 4-vinylpyridine
containing nitrogen can coordinate to a transition metal thus
deactivating the catalyst.
Entry
Pd source
Base
Solvent
Yield/%b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
PdCl2(PPh3)2 NaHCO3
PdCl2 NaHCO3
PdCl2(dppf) NaHCO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
PhCl
11
76
42
75
Pd(OAc)2
Pd(dba)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
NaHCO3
NaHCO3
NaHCO3
NaOtBu
NaF
NaOAc
Na2CO3
LiOH·H2O
DABCO
Et3N
NaHCO3
NaHCO3 1,4-dioxane
NaHCO3 ClCH2CH2Cl
NaHCO3
NaHCO3
43
91, 92,c 67,d 43e
Trace
60
75
48
23
67
13
70
35
As part of our continuing interest in the development of
palladium-catalysed organoboron reactions,21–26 we now report
a new method for the synthesis of (E)-4-styrylpyridines by
the Pd-catalysed oxidative Heck reaction of 4-vinylpyridine
with arylboronic acids (Scheme 1b).
11
58
33
CH3CN
toluene
As shown in Table 1, the model coupling reaction between
4-vinylpyridine 1 and phenylboronic acid 2a was conducted to
identify the optimal reaction conditions, including catalysts,
bases and solvents. Initially, the model reaction was performed
using PdCl2(PPh3)2 as the catalyst, NaHCO3 as the base and
DMF as the solvent. We found that the desired product, (E)-
4-styrylpyridine 3a, was obtained in 11% yield (Table 1,
entry 1).
Encouraged by this promising result, the reaction conditions
were further modified including variation of the palladium
catalysts, bases and solvents. First, we examined other palla-
dium catalysts such as PdCl2, Pd(OAc)2, PdCl2(dppf), Pd(dba)2
a Reaction conditions: 4-vinylpyridine 1 (0.5 mmol), phenylbo-
ronic acid 2a (1.0 mmol), base (1.5 mmol), Pd source (5 mol%),
solvent (1 mL), 90 °C, 24 h, O2. b Isolated yields. c 7.5 mol%
Pd(acac)2. d 2.5 mol% Pd(acac)2. e 1.0 mol% Pd(acac)2.
and Pd(acac)2 (Table 1, entries 2–6). We were pleased to
discover that only when the palladium source changed to the
Pd(acac)2 did the yield dramatically increase to 91% yield
in DMF under an O2 atmosphere (Table 1, entry 6). Next, both
base and solvent was tested. Screening revealed that the use of
NaHCO3 as base achieved the best result (Table 1, entry 6).
Scheme 1 Palladium-catalysed synthesis of (E)-4-styrylpyridines.
* Correspondent. E-mail: mcl@wzu.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2012 323
Other bases, including NaOtBu, NaF, NaOAc, Na2CO3,
LiOH·H2O, DABCO and Et3N, were less efficient for this
transformation (Table 1, entries 7–13). The choice of solvent
was also vital to the success of the catalytic reaction. DMF
appeared to be the best choice among the common solvents
such as PhCl, 1,4-dioxan, ClCH2CH2Cl, CH3CN, and toluene
(Table 1, entries 6, 14–18). In addition, the reaction failed to
deliver the desired product when the procedure was carried out
under a N2 atmosphere. In the light of these results, subsequent
studies were carried out under the following optimized
condition using 5 mol% Pd(acac)2 and NaHCO3 (3 equiv.) as
the base in DMF at 90 °C under O2 atmosphere.
With the optimal conditions in hand, we explored the scope
and generality of this oxidative Heck reaction using various
arylboronic acids 2b–l and 4-vinylpyridine 1 (Table 2).
As shown in Table 2, the electronic properties of the
groups on the phenyl ring of arylboronic acids had obvious
effects on the yields of the reaction (Table 2, entries 1–8).
Electron-donating substituents were beneficial for the transfor-
mation, while electron-withdrawing groups slightly decreased
the efficiency. For example, p-tolylboronic acid 2b and p-
methoxyphenylboronic acid 2e afforded the desired products
3b and 3e in good yields, while the yield of 3h was decreased
to 35% yield. We also investigated steric effects on our
system. A monosubstitution group on the ortho-, meta-, or
para-position for arylboronic acids (Table 2, entries 2–7), had
little effect on the yields of the reaction. It was observed that
4-tert-butylphenylboronic acid 2i was also a good reaction
partner (Table 2, entry 9). Notably, the chloro, and bromo
moieties (commonly used for cross-coupling reactions) in
arylboronic acids 2j–l were all tolerated and afforded a novel
route to compounds 3j–l in moderate yields, making further
elaboration to the corresponding biaryl products possible
(Table 2, entries 10–12). Note that biarylboronic acids such as
2-naphthylboronic acid 2m also gave the corresponding 3m in
63% yield (Table 2, entry 13).
A tentative mechanism for the formation of (E)-4-styryl-
pyridines is proposed in Scheme 2 based on related reports
in the literature.27,28 The first step may involve the transmetal-
lation of Pd(II) with arylboronic acid 2 to afford an organopal-
ladium intermediate A. Next step, the intermediate A was
shown to give the intermediate B by the addition reaction of
4-vinylpyridine 1. A β-H elimination from the intermediate
B occurred to give the desired products 3 along with Pd (0)
species. Finally, Pd(0) species was oxidised to regenerate the
starting Pd(II) in the presence of O2.
Table 2 Palladium-catalysed synthesis of (E)-4-styrylpyridinesa
Entry Arylboronic acid 2
Product 3
Yield/%b
91
1
2
2a
3a
Conclusion
2b
3b
73
75
In summary, we have developed a palladium-catalysed
oxidative Heck reaction of arylboronic acid with 4-vinylpyri-
dine, achieving (E)-4-styrylpyridines in fair to good yields.
Applications of this palladium-catalysed oxidative Heck
coupling transformation in organic synthesis are ongoing in
our laboratory.
3
2c
3c
4
5
6
7
2d
3d
76
80
74
81
Experimental
Chemicals were either purchased or purified by standard techniques
without special measures. 1H NMR and 13C NMR spectra were mea-
sured on a 500 MHz Bruker spectrometer (1H 500 MHz and 13C 125
MHz), using CDCl3 or DMSO-d6 as the solvent with tetramethylsilane
(TMS) as the internal standard at room temperature. Chemical shifts
are given in δ relative to TMS, the coupling constants J are given
in Hz. Mass spectra (MS) were measured with a Thermo Finnigan
LCQ-Advantage. Column chromatography was performed using
EM Silica gel 60 (300–400 mesh). All compounds are known.
Analytical spectra (1H and 13C NMR) of all compounds are given in
the supporting information.
2e
3e
3f
2f
2g
3g
8
9
2h
3h
3i
35
68
2i
2j
10
3j
79
11
12
2k
3k
66
72
2l
3l
13
2m
3m
63
a All reactions were run with 4-vinylpyridine 1 (0.5 mmol),
arylboronic acid 2 (1.0 mmol), NaHCO3 (1.5 mmol), Pd(acac)2
(5 mol%)in DMF (1 mL) for 24 h at 90 °C under O2 atmosphere.
b Isolated yields.
Scheme 2 A tentative mechanism for the formation of
(E)-4-styrylpyridines.

324 JOURNAL OF CHEMICAL RESEARCH 2012
Synthesis of (E)-4-styrylpyridines; general prodedure
(E)-4-(4-Tert-butylstyryl)pyridine (3j): Solid, m.p. 117–120 °C,
(lit.32 no report).1H NMR (500 MHz, CDCl3) δ 8.56 (d, J = 4.1 Hz, 2H,
pyridyl), 7.48–7.26 (m, 7H), 6.98 (d, J = 16.3 Hz, 1H, C=CH), 1.34 (s,
9H, CH3); 13C NMR (125 MHz, CDCl3) δ 152.1, 150.1, 144.9, 133.4,
133.1, 126.8, 125.8, 125.2, 120.8, 34.8, 31.3. MS (EI, 70 eV): m/z
(%) = 237 (M+, 47), 222 (100), 194 (6), 180 (6), 97 (10).
The arylboronic acids 2 (1.0 mmol), Pd(acac)2 (5 mol%), NaHCO3
(1.5 mmol) and dry DMF (1 mL) were added to a Schlenk tube . Then
the tube was charged with O2 (1 atm) followed by the addition of
4-vinylpyridine 1 (0.5 mmol). The mixture was kept stirring at 90 °C
(oil bath temperature) for 24 h. After completion of the reaction, as
monitored by TLC and GC-MS analysis, the reaction mixture was
cooled to room temperature, diluted with ethyl acetate, and washed
with brine. The aqueous phase was re-extracted with ethyl acetate.
The combined organic extracts were dried over Na2SO4 and concen-
trated in vacud, and the resulting residue was purified by silica gel
column chromatography (petroleum ether/ethyl acetate) to afford the
desired product 3. The physical and spectral data of compounds 3a–m
are as follows.
(E)-4-(4-Chlorostyryl)pyridine (3k)33: Solid, m.p. 97–100 °C, (lit.34
88–89 °C).1H NMR (500 MHz, CDCl3) δ 8.58 (d, J = 5.6 Hz, 2H,
pyridyl), 7.45–7.20 (m, 7H), 6.96 (d, J = 16.3 Hz, 1H, C=CH); 13C
NMR (125 MHz, CDCl3) δ 150.1, 144.4, 134.6, 134.5, 132.0, 129.1,
128.2, 126.5, 120.9. MS (EI, 70 eV): m/z (%) = 217 ([M+2]+, 28), 215
(M+, 83), 180 (100), 152 (45).
1
(E)-4-(2-Chlorostyryl)pyridine (3l): Oil, (lit.35 oil). H NMR (500
MHz, CDCl3) δ 8.60 (d, J = 5.1 Hz, 2H, pyridyl), 7.73–7.68 (m, 2H),
7.42–7.26 (m, 5H), 6.99 (d, J = 16.3 Hz, 1H, C=CH); 13C NMR (125
MHz, CDCl3) δ 150.2, 144.3, 134.3, 134.0, 130.0, 129.6, 129.3, 128.6,
127.1, 126.8, 121.1.
(E)-4-Styrylpyridine (3a): Solid, m.p. 122–124 °C, (lit.29 122–
123 °C). 1H NMR (500 MHz, CDCl3) δ 8.58 (d, J = 6.0 Hz, 2H, pyri-
dyl), 7.54 (d, J = 7.5 Hz, 2H), 7.42–7.28 (m, 6H), 7.02 (d, J = 16.3
Hz, 1H, C=CH); 13C NMR (125 MHz, CDCl3) δ 150.2, 144.7, 136.2,
133.2, 128.9, 128.8, 127.0, 126.0, 120.9. MS (EI, 70 eV): m/z
(%) = 181 (M+, 80), 166 (7), 152 (33), 76 (15).
(E)-4-(4-Bromostyryl)pyridine (3m): Solid, m.p. 153–154 °C. (lit.1
1
160 °C). H NMR (500 MHz, CDCl3) δ 8.59 (s, 2H, pyridyl), 7.53–
7.21 (m, 7H), 7.01 (d, J = 16.3 Hz, 1H, C=CH); 13C NMR (125 MHz,
CDCl3) δ 150.2, 144.3, 135.1, 132.0, 131.9, 128.4, 126.7, 122.7,
120.9. MS (EI, 70 eV): m/z (%) = 261 ([M+2]+, 62), 259 (M+, 64), 180
(100), 152 (54), 90 (16), 76 (23).
(E)-4-(4-Methylstyryl)pyridine (3b): Solid, m.p. 139–143 °C, (lit.6
1
no report). H NMR (500 MHz, CDCl3) δ 8.56 (d, J = 5.8 Hz, 2H,
pyridyl), 7.44 (d, J = 8.0 Hz, 2H), 7.36–7.19 (m, 5H), 6.97 (d, J = 16.3
Hz, 1H, C=CH), 2.38 (s, 3H, CH3); 13C NMR (125 MHz, CDCl3) δ
150.1, 144.9, 138.9, 133.4, 133.2, 129.6, 127.0, 125.0, 120.8, 21.3.
MS (EI, 70 eV): m/z (%) = 195 (M+, 100), 180 (93), 165 (9), 152 (26),
115 (14).
(E)-4-(2-(Naphthalen-2-yl)vinyl)pyridine (3n): Solid, m.p. 156–
159 °C, (lit.36 no report).1H NMR (500 MHz, CDCl3) δ 8.61 (d, J = 4.3
Hz, 2H, pyridyl), 8.17 (d, J = 8.4 Hz, 1H), 8.08 (d, J = 16.0 Hz, 1H,
C=CH), 7.88–7.24 (m, 8H), 7.05 (d, J = 16.0 Hz, 1H, C=CH); 13C
NMR (125 MHz, CDCl3) δ 150.2, 144.9, 133.8, 133.8, 131.4, 130.4,
129.1, 129.0, 128.8, 126.5, 126.1, 125.6, 124.2, 123.5, 121.1. MS (EI,
70 eV): m/z (%) = 231 (M+, 36), 216 (6), 202 (23), 115 (13).
(E)-4-(3-methylstyryl)pyridine (3c): Solid, m.p. 69–70 °C, (lit.30
74–76 °C). 1H NMR (500 MHz, CDCl3) δ 8.58–8.53 (m, 2H, pyridyl),
7.35–7.24 (m, 6H), 7.00 (d, J = 16.3 Hz, 1H, C=CH), 2.39 (s, 3H,
CH3); 13C NMR (125 MHz, CDCl3) δ 150.2, 144.7, 138.5, 136.1,
133.3, 129.6, 128.7, 127.7, 125.8, 124.3, 120.8, 21.4. MS (EI, 70 eV):
m/z (%) = 195 (M+, 100), 180 (87), 165 (10), 152 (27), 115 (12).
(E)-4-(2-Methylstyryl)pyridine (3d): Solid, m.p. 74–76 °C, (lit.13 no
We thank the National Natural Science Foundation of China
(No. 21102105), Science and Technology Department of
Zhejiang Province (Nos. 2009R50002 and 2011R09002-03),
the Opening Foundation of Zhejiang Provincial Top Key
Discipline (No. 100061200103), and the Wenzhou Technology
Bureau Program (No.G20100065) for financial support.
1
report). H NMR (500 MHz, CDCl3) δ 8.58 (d, J = 5.5 Hz, 2H,
pyridyl), 7.62–7.57 (m, 1H, ArH), 7.55 (d, J = 16.2 Hz, 1H, C=CH),
7.37 (d, J = 5.9 Hz, 2H, pyridyl), 7.26–7.19 (m, 3H, ArH), 6.91 (d,
J = 16.2 Hz, 1H, C=CH), 2.45 (s, 3H, CH3); 13C NMR (125 MHz,
CDCl3) δ 150.2, 144.9, 136.3, 135.3, 131.0, 130.6, 128.6, 127.3,
126.4, 125.7, 120.9, 19.8. MS (EI, 70 eV): m/z (%) = 195 (M+, 100),
180 (86), 165 (12), 152 (27), 115 (19).
Received 10 February 2012; accepted 20 March 2012
Paper 1201158 doi: 10.3184/174751912X13352864811920
Published online: 4 June 2012
(E)-4-(4-Methoxystyryl)pyridine (3e): Solid, m.p. 122–125 °C,
1
(lit.12 133–135 °C). H NMR (500 MHz, CDCl3) δ 8.54 (m, 2H,
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