Pyrrole acylation for the synthesis of 2-bromo-6-(2-pyrrolyl)pyridine and subsequent cross-coupling reactions

Article abstract of DOI:10.1055/s-0030-1258971

A facile synthesis of 2-bromo-6-pyrrolylpyridine 4 was developed starting from acylation of pyrrole with glutaric acid and TFAA. Suzuki cross-couplings of 4 with arylboronic acids gave a variety of novel 2-aryl-6-pyrrolylpyridines in excellent yields. Geo

Full text of DOI:10.1055/s-0030-1258971

PAPER
45
Pyrrole Acylation for the Synthesis of 2-Bromo-6-(2-pyrrolyl)pyridine and
Subsequent Cross-Coupling Reactions
S
ynthesis of 2-A
r
h
yl-6-(2-pyrr
o
lyl)pyridine
a
s
njun Song,*a Yan Liu,a Peng Zhao,a Xuelian Sun,b Wenjia Li,a Hui Liu,a Junbiao Chang*a
a
Department of Chemistry, Zhengzhou University, 100 Science Avenue, Zhengzhou, Henan Province 450001, P. R. of China
E-mail: chjsong@zzu.edu.cn; E-mail: changjunbiao@zzu.edu.cn
b
School of International Education, Zhengzhou University, 100 Science Avenue, Zhengzhou, Henan Province 450001, P. R. of China
Received 21 September 2010; revised 18 October 2010
only in the case of DDQ as oxidant a small amount (<10%
yield) of the desired pyrrolylpyridinone 7 was isolated.
No product was produced under other conditions. Oxida-
tion of 3b under the above conditions also failed to deliver
the corresponding pyrrolylpyridinone. The alternative
route was then tried, by reacting 3a,b with POBr3 first,43
followed by oxidation (Scheme 1). To our delight, when
dihydropyridinone 3a was subjected to 15–20 equivalents
of POBr3, 2-bromo-6-pyrrolylpyridine 4 was isolated di-
rectly in an optimized 35% yield. The same product was
obtained when dihydropyridinone 3b was used as the
starting material under the same reaction conditions, but
in much higher yield (65%).
Abstract: A facile synthesis of 2-bromo-6-pyrrolylpyridine 4 was
developed starting from acylation of pyrrole with glutaric acid and
TFAA. Suzuki cross-couplings of 4 with arylboronic acids gave a
variety of novel 2-aryl-6-pyrrolylpyridines in excellent yields.
Key words: pyrrole acylation, 2-bromo-6-pyrrolylpyridine, 2-aryl-
6-pyrrolylpyridines, Suzuki cross-coupling, detosylation
2-(2¢-Pyrrolyl)pyridines1,2 are a useful class of com-
pounds as potential semiconducting materials3 and metal-
binding ligands.4–14 They have also shown important bio-
logical activities.15–17 The synthesis of these molecules
generally falls into two categories, (1) coupling of 2-ha-
lopyridines with pyrroles or their organometallic deriva-
tives,18–26 and (2) formation of the pyrrole ring from a
pyridine derivative.2,3,9,11,14c,d,27–34 To our surprise, strate-
gies using a pyrrole derivative and formation of a pyridine
ring are rare,35–37 and either require usage of special sub-
strates and catalyst, or result in the formation of the de-
sired product in very low yield.
i
ii
iii
NHR
N
N
Ts
N
O
Ts
Ts
O
O
O
1
2a R = H
2b R = Bn
Previously, we developed a new pyrrole acylation method
using carboxylic acids and TFAA,38 and successfully ap-
plied this methodology to the synthesis of pyrrolylpy-
ridazines.39 We now report the application of this
methodology for the synthesis of 2-bromo-6-pyrrolylpy-
ridines and the subsequent cross-coupling reactions.
N
N
Ts
Ar
Ar
5c–f
v
iv
+
N
N
RN
N
Ts
Ts
O
Br
N
3a R = H
3b R = Bn
4
N
As shown in Scheme 1, treatment of N-tosylpyrrole40 with
glutaric acid and TFAA resulted in the formation of dihy-
dropyrone 1 through monoacylation, intramolecular cy-
clization of the remaining carboxylic acid group onto the
ketone function, and dehydration. Reaction of 1 with am-
monium acetate or benzylamine gave the ring-opening
products 2a,b, which on treatment with PTSA, furnished
dihydropyridinone 3a,b in excellent overall yields. Next,
transformation of 3a into 2-bromo-6-pyrrolylpyridine 4
was examined, first following the sequence as shown in
Scheme 2. A range of reagents under a variety of condi-
tions were employed for the oxidation of 3a into pyrro-
lylpyridinone 7, including MnO2 in refluxing CH2Cl2,41
SeO2 in dioxane at ambient temperature or under reflux,39
DDQ in toluene at –20 °C, ambient temperature or under
reflux,38 and 10% Pd/C in refluxing toluene.42 However,
H
6a–f
Scheme 1 Reagents and conditions: (i) glutaric acid, TFAA,
CH2Cl2, 40 °C, 40 h, 35%; (ii) for the synthesis of 2a: NH4OAc,
EtOH, reflux, 3 h, 98%; for the synthesis of 2b: BnNH2, toluene,
reflux, 1 h, 88%; (iii) PTSA, toluene, reflux, 84% for 2a, 90% for 2b;
(iv) POBr3, toluene, reflux, 35% for 3a, 65% for 3b; (v) ArB(OH)2,
Pd(PPh3)4, K2CO3, toluene–MeOH, reflux.
[O]
N
N
N
HN
3a
HN
N
Ts
Ts
Ts
O
O
Br
7
4
Scheme 2
Next, the Suzuki cross-coupling reactions44 of bromopyri-
dine 4 with a variety of arylboronic acids were investigat-
ed. Initial attempt was carried out with phenylboronic
acid. Under the classic conditions with Pd(PPh3)4 as cata-
SYNTHESIS 2011, No. 1, pp 0045–0050
x
x
.
x
x
.2
0
1
0
Advanced online publication: 09.11.2010
DOI: 10.1055/s-0030-1258971; Art ID: F16710SS
© Georg Thieme Verlag Stuttgart · New York

46
C. Song et al.
PAPER
lyst, K2CO3 as base, cross-coupling reaction between bro- were obtained when KOH was used as the base. However,
mopyridine 4 and phenylboronic acid gave, with in situ if large excess (>10 equiv) of base was applied, com-
detosylation, 2-phenyl-6-pyrrolylpyridine (6a) in 98% pounds 5d–f could be transformed completely into 6d–f
isolated yield. Slightly lower, but still excellent yield during the cross-coupling reactions, with much shorter
(89%) was obtained when K3PO4 was used as the base. time required for KOH (<10 h) than for K2CO3 (>30 h).
Thus, the first mentioned reaction conditions were em- As an example, 6e could also be obtained in excellent
ployed for the rest of our study without further optimiza- yield by detosylation of 5e with NaOH in methanol.
tion. The results of cross-coupling reactions with other
boronic acids are collected in Table 1.
Finally, nucleophilic aromatic substitution of the bromide
in 4 by sodium methoxide was attempted. However, only
the detosylated product 8 was obtained, and the bromide
Table 1 Results of Suzuki Cross-Coupling Reactions of Bromide 4
was unaffected (Scheme 3). Suzuki cross-coupling reac-
tion of 8 with phenylboronic acid was also possible and
led to the formation of 6a in 89% isolated yield.
with Arylboronic Acids a
Entry Boronic acids
Products
Yield
(%) b
B(OH)2
i
1
6a (Ar = Ph)
6b (Ar = 4-FC 6 H 4
98
92
N
N
Br
Br
NH
NTs
4
8
B(OH)2
2
)
ii
F
N
B(OH)2
5c (Ar = 4-MeOC 6 H 4
6c (Ar = 4-MeOC 6 H 4
)
)
81
NH
3
4
trace c
6a
MeO
Scheme 3 Reagents and conditions : (i) NaOH, MeOH, reflux, 3.5
h, 63%; (ii) Ph B(OH)2, Pd(PPh3)4, K2CO3, toluene–MeOH, reflux,
20 h, 89%.
B(OH)2
5d (Ar = 4- t -BuC 6 H 4
6d (Ar = 4- t -BuC 6 H 4
)
)
66
27 d
In summary, an efficient synthesis of 2-bromo-6-pyrro-
lylpyridine 4 based on pyrrole acylation has been devel-
oped. Suzuki cross-coupling reactions of compound 4
with arylboronic acids give a variety of novel 2-aryl-6-
pyrrolylpyridines in excellent yields. Since acylations of
2-substituted pyrroles with glutaric acid and TFAA are
also successful, 38a this method could potentially be adapt-
ed for the synthesis of substituted pyrrolylpyridine deriv-
atives. These are currently under investigation and the
results will be published in due course.
B(OH)2
5e (Ar = 2-MeC 6 H 4
6e (Ar = 2-MeC 6 H 4
)
)
44
5
6
49 d
O2N
B(OH)2
5f (Ar = 3-O 2 NC 6 H 4
6f (Ar = 3-O 2 NC 6 H 4
)
)
57
32 d
a Bromide 4 /boronic acid/K 2 CO 3 = 1:1.25:2.5.
b Yields obtained after the bromide 4 had completely reacted; typical
reaction time 11–15 h, except for entry 2, in which case 23 h was
needed for completion of the reaction.
c Compound 6c could be isolated in 71% yield as the sole product
when the reaction time was extended to 35 h.
Solvents were dried according to standard procedures where need-
ed. Petroleum ether (PE) used refers to the fraction boiling at 60–90
°C. Melting points were measured on a XT4A melting point appa-
ratus and are not corrected. IR spectra were obtained using an IFS25
d Compounds 6d – f could be isolated as the sole products if large
excess (>10 equiv) of base was applied.
1
FT-IR spectrometer. H and 13C NMR spectra were obtained on a
Similar to phenylboronic acid (entry 1), cross-coupling re-
action of 4-fluorophenylboronic acid with bromide 4 re-
sulted in the formation of detosylated pyrrolylpyridine 6b
as the sole product (entry 2). In all other cases a mixture
of compounds 5c–f and 6c–f were obtained (entries 3–6),
which could be isolated by column chromatography.
Compound 5c was obtained as the major product, together
with a small amount (<5%) of 6c, in the cross-coupling re-
action of 4-methoxyphenylboronic acid (entry 3) with
bromide 4 after a reaction time of 12 hours. However, 6c
could be isolated in 71% yield as the sole product after
prolonged reflux (35 h). For entries 4–6, remarkable
amounts of the detosylated products 6d–f were formed
when bromide 4 had completely reacted as indicated by
TLC (11–13 h), but the ratio of 5d–f to 6d–f did not vary
too much after prolonged reflux (35 h). Similar results
Bruker AV300 instrument. Mass spectra were recorded on a Micro-
mass Q-TOF mass spectrometer.
3,4-Dihydro-6-(1¢-tosylpyrrol-2¢-yl)pyran-2-one (1)
A mixture of N-tosylpyrrole (2.2 g, 10.0 mmol), glutaric acid (2.6 g,
20.0 mmol), and TFAA (16.8 g, 80.0 mmol) in anhyd CH2Cl2 (40
mL) was heated to reflux for 40 h and cooled. H2O (50 mL) was
added to quench the reaction. The separated organic layer was
washed successively with H2O (30 mL), sat. aq NaHCO3 (30 mL)
and H2O (30 mL). The organic layer was dried (MgSO4), filtered,
and evaporated in vacuo. The residue was purified by column chro-
matography on silica gel using EtOAc–PE (1:10, then 1: 3) as eluent
to give dihydropyrone 1 (1.1 g, 35%) as a pale yellow solid; mp
134–137 °C.
IR (KBr): 1764, 1370, 1169, 1145, 1048 cm–1.
1H NMR (300 MHz, DMSO-d6): d = 2.38 (3 H, s, CH3), 2.42 (2 H,
td, J = 7.4, 4.6 Hz, 4-CH2), 2.65 (2 H, t, J = 7.4 Hz, 3-CH2), 5.55 (1
Synthesis 2011, No. 1, 45–50 © Thieme Stuttgart · New York

PAPER
Synthesis of 2-Aryl-6-(2-pyrrolyl)pyridines
47
H, t, J = 4.6 Hz, 5-H), 6.35 (1 H, t, J = 3.3 Hz, 4¢-H), 6.45 (1 H, dd,
J = 3.3, 1.8 Hz, 3¢-H), 7.44 (2 H, d, J = 8.4 Hz, ArH), 7.54 (1 H, dd,
J = 3.3, 1.8 Hz, 5¢-H), 7.85 (2 H, d, J = 8.4 Hz, ArH).
13C NMR (75 MHz, DMSO-d6): d = 18.5, 21.1, 27.5, 108.8, 112.4,
117.4, 124.7, 127.2, 127.3, 130.0, 135.1, 142.6, 145.5, 168.3.
MS (ESI): m/z (%) = 340 (100, [M+ + Na]+), 318 (55, [M + H]+).
HRMS-ESI: m/z [M + Na]+ calcd for C16H15NO4S + Na: 340.0619;
1H NMR (300 MHz, DMSO-d6): d = 2.25–2.33 (4 H, m, 3- and 4-
CH2), 2.37 (3 H, s, CH3), 4.81 (1 H, t, J = 4.6 Hz, 5-H), 6.27–6.32
(2 H, m, 3¢ and 4¢-H), 7.39–7.44 (3 H, m, ArH), 7.69 (2 H, d, J = 8.3
Hz, ArH), 9.23 (1 H, s, NH).
13C NMR (75 MHz, DMSO-d6): d = 20.1, 21.1, 29.9, 107.4, 112.6,
116.6, 124.3, 126.9, 129.6, 129.9, 130.0, 135.2, 145.2, 169.6.
MS (ESI): m/z (%) = 339 (40, [M + Na]+), 317 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C16H17N2O3S: 317.0960; found:
found: 340.0626.
317.0949.
5-Oxo-5-(1¢-tosylpyrrol-2¢-yl)pentanamide (2a)
A mixture of dihydropyranone 1 (0.79 g, 2.5 mmol), NH4OAc (1.93
g, 25.0 mmol), and EtOH (15 mL) was heated to reflux for 3 h and
then evaporated in vacuo. The residue was partitioned between
CH2Cl2 (30 mL) and H2O (50 mL). The separated organic layer was
dried (Na2SO4), filtered, and evaporated in vacuo. The residue was
purified by column chromatography on silica gel using EtOAc–PE
(5:1) as eluent to give keto amide 2a (0.82 g, 98%) as a colorless
solid; mp 71–73 °C.
1-Benzyl-3,4-dihydro-6-(1¢-tosylpyrrol-2¢-yl)pyridinone (3b)
A solution of keto amide 2b (0.80 g, 1.88 mmol) and PTSA (0.19 g,
1.13 mmol) in toluene (25 mL) was heated to reflux using a Dean–
Stark apparatus for 24 h and cooled. The solution was washed with
H2O (30 mL), then dried (Na2SO4), filtered, and evaporated in vac-
uo. The residue was purified by column chromatography on silica
gel using EtOAc–PE (1:3) as eluent to give dihydropyridinone 3b
(0.68 g, 90%) as a colorless solid; mp 124–126 °C.
IR (KBr): 3495, 3392, 1661, 1610, 1435, 1368, 1178, 1145, 1067
cm–1.
IR (KBr): 1679, 1385, 1365, 1173, 1152, 1087, 1052 cm–1.
1H NMR (300 MHz, DMSO-d6): d = 2.32 (2 H, br, 4-H2), 2.39 (3 H,
s, CH3), 2.60 (2 H, br, 3-CH2), 3.55 (1 H, br, CHHPh), 5.06 (1 H, t,
J = 4.8 Hz, 5-H), 5.07 (1 H, br, CHHPh), 5.89 (1 H, dd, J = 3.3, 1.7
Hz, 3¢-H), 6.25 (1 H, t, J = 3.3 Hz, 4¢ -H), 6.95 (2 H, d, J = 6.8 Hz,
ArH), 7.17–7.23 (3 H, m, ArH), 7.44 (2 H, d, J = 8.3 Hz, ArH), 7.51
(1 H, dd, J = 3.3, 1.7 Hz, 5¢-H), 7.73 (2 H, d, J = 8.3 Hz, ArH).
13C NMR (75 MHz, DMSO-d6): d = 19.2, 21.1, 30.8, 45.2, 112.7,
113.7, 118.1, 124.4, 126.1, 126.7, 126.9, 128.2, 130.0, 132.5, 134.9,
138.4, 145.5, 169.4.
1H NMR (300 MHz, CDCl3): d = 1.86 (2 H, quint, J = 7.2 Hz, 3-
CH2), 2.15 (2 H, t, J = 7.2 Hz, 2-CH2), 2.36 (3 H, s, CH3), 2.71 (2
H, t, J = 7.2 Hz, 4-CH2), 6.01 (1 H, br s, NH), 6.10 (1 H, br s, NH),
6.28 (1 H, t, J = 3.5 Hz, 4¢-H), 7.04 (1 H, dd, J = 3.8, 1.7 Hz, 3¢-H),
7.26 (2 H, d, J = 8.4 Hz, ArH), 7.73 (1 H, dd, J = 3.1, 1.7 Hz, 5¢-H),
7.84 (2 H, d, J = 8.4 Hz, ArH).
13C NMR (75 MHz, CDCl3): d = 20.3, 21.5, 34.2, 37.9, 110.3,
123.7, 128.0, 129.2, 130.0, 132.8, 135.5, 144.7, 175.2, 188.2.
MS (ESI): m/z (%) = 357 (100, [M + Na]+), 335 (18, [M + H]+).
MS (ESI): m/z (%) = 429 (100, [M + Na]+), 407 (75, [M + H]+).
HRMS-ESI: m/z [M + Na]+ calcd for C16H18N2O4S + Na: 357.0885;
HRMS-ESI: m/z [M + Na]+ calcd for C23H22N2O3S + Na: 429.1249;
found: 357.0885.
found: 429.1240.
N-Benzyl-5-oxo-5-(1¢-tosylpyrrol-2¢-yl)pentanamide (2b)
A solution of dihydropyranone 1 (0.51 g, 1.6 mmol) and benzyl-
amine (1.36 g, 12.7) in anhyd toluene (20 mL) was heated to reflux
for 1 h. The cooled solution was washed with aq 1 M HCl (20 mL)
and brine (20 mL), then dried (Na2SO4), filtered, and evaporated in
vacuo to give keto amide 2b (0.59 g, 88%) as a sticky oil.
2-Bromo-6-(1¢-tosylpyrrol-2¢-yl)pyridine (4)
From 3a: A solution of dihydropyridinone 3a (50 mg, 0.16 mmol)
and POBr3 (900 mg, 3.14 mmol) in anhyd toluene (20 mL) was
heated to reflux for 1.5 h, then cooled and diluted with EtOAc (50
mL). Ice-water was added carefully until no more bubbling was ob-
served. The separated organic phase was washed successively with
H2O (30 mL), sat. aq NaHCO3 (20 mL) and brine (20 mL), then
dried (Na2SO4), filtered, and evaporated in vacuo. The residue was
purified by column chromatography on silica gel using EtOAc–PE
(1:10) as eluent to give bromopyridine 4 (21 mg, 35%) as a colorless
solid; mp 135–138 °C.
IR (film): 3513, 1665, 1636, 1440, 1367, 1174, 1147, 1070 cm–1.
1H NMR (300 MHz, DMSO-d6): d = 1.71 (2 H, quint, J = 7.4 Hz,
3-CH2), 2.12 (2 H, t, J = 7.4 Hz, 2-CH2), 2.38 (3 H, s, CH3), 2.74 (2
H, t, J = 7.4 Hz, 4-CH2), 4.24 (2 H, d, J = 5.8 Hz, CH2Ph), 6.48 (1
H, t, J = 3.4 Hz, 4¢-H), 7.22–7.30 (6 H, m, ArH), 7.43 (2 H, d,
J = 8.1 Hz, ArH), 7.86–7.89 (3 H, m, ArH), 8.31 (1 H, t, J = 5.8 Hz,
NH).
13C NMR (75 MHz, DMSO-d6): d = 20.4, 21.1, 34.2, 38.0, 42.0,
111.0, 124.2, 126.7, 127.2, 127.9, 128.3, 129.6, 130.4, 132.9, 135.5,
139.5, 139.7, 144.9, 171.6, 188.2.
IR (KBr): 1583, 1545, 1422, 1371, 1170, 1145 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.41 (3 H, s, CH3), 6.32 (1 H, t,
J = 3.3 Hz, 4¢-H), 6.45 (1 H, dd, J = 3.6, 1.7 Hz, 3¢-H), 7.29 (2 H,
d, J = 8.2 Hz, ArH), 7.41 (1 H, dd, J = 7.5, 0.9 Hz, ArH), 7.47–7.59
(3 H, m, ArH), 7.76 (2 H, d, J = 8.2 Hz, ArH).
13C NMR (75 MHz, CDCl3): d = 21.7, 111.9, 117.5, 123.7, 125.4,
126.9, 127.8, 128.7, 129.7, 136.0, 138.3, 140.6, 145.0, 151.6.
MS (ESI): m/z (%) = 447 (26, [M + Na]+), 425 (100, [M + H]+).
HRMS-ESI: m/z [M + Na]+ calcd for C23H24N2O4S + Na: 447.1354;
MS (ESI): m/z (%) = 401 (100, [M (81Br) + Na]+), 399 (100, [M
found: 447.1374.
(79Br) + Na]+), 379 (55, [M (81Br) + H]+), 377 (55, [M (79Br) + H]+).
3,4-Dihydro-6-(1¢-tosylpyrrol-2¢-yl)pyridinone (3a)
HRMS-ESI: m/z [M + Na]+ calcd for C16H1379BrN2O2S + Na:
398.9779; found: 398.9773.
A solution of keto amide 2a (0.82 g, 2.45 mmol) and PTSA (0.25 g,
1.47 mmol) in toluene (20 mL) was heated to reflux using a Dean–
Stark apparatus for 1 h and cooled. The solution was washed with
H2O (30 mL), then dried (Na2SO4), filtered, and evaporated in vac-
uo. The residue was purified by column chromatography on silica
gel using EtOAc–PE (1:1) as eluent to give dihydropyridinone 3a
(0.65 g, 84%) as a colorless solid; mp 130–132 °C.
From 3b: Similarly, the title compound could also be synthesized in
65% isolated yield by refluxing a mixture of dihydropyridinone 3b
and POBr3 in anhyd toluene for 5–8 h.
2-Bromo-6-(1¢-pyrrol-2¢-yl)pyridine (8)
A mixture of tosylpyrrolylpyridine 4 (80 mg, 0.21 mmol), NaOH
(252 mg, 6.30 mmol), and MeOH (25 mL) was heated to reflux for
3.5 h and cooled. The bulk of the solvent was evaporated in vacuo.
IR (KBr): 3173, 1671, 1367, 1167, 1152, 1088, 1052 cm–1.
Synthesis 2011, No. 1, 45–50 © Thieme Stuttgart · New York

48
C. Song et al.
PAPER
MS (ESI): m/z (%) = 261 (11, [M + Na]+), 239 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C15H12FN2: 239.0985; found:
239.0983.
The residue was partitioned between EtOAc (30 mL) and H2O (30
mL). The separated organic layer was dried (Na2SO4), filtered, and
evaporated in vacuo. The residue was purified by column chroma-
tography on silica gel using EtOAc–PE (1:10) as eluent to give pyr-
rolylpyridine 8 (30 mg, 63%) as a colorless solid; mp 71–73 °C.
2-(4¢-Methoxyphenyl)-6-(1¢¢-tosylpyrrol-2¢¢-yl)pyridine (5c) and
2-(4¢-Methoxyphenyl)-6-(1¢¢-pyrrol-2¢¢-yl)pyridine (6c)
Formation of 5c: The reaction mixture was heated to reflux for 12
h. The crude product was purified by column chromatography on
silica gel using EtOAc–PE (1:10) as eluent to give tosylpyrrolylpy-
ridine 5c (81%) as a colorless solid; mp 128–132 °C.
IR (KBr): 3380, 1585, 1542, 1437, 1159, 1118, 1101, 1034 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.20 (1 H, m, 4¢-H), 6.63 (1 H, m,
3¢-H), 6.82 (1 H, m, 5¢-H), 7.10 (1 H, t, J = 4.2 Hz, 4-H), 7.35 (2 H,
d, J = 4.2 Hz, 3- and 5-H), 9.45 (1 H, br s, NH).
13C NMR (75 MHz, CDCl3): d = 108.7, 110.7, 116.7, 121.0, 124.1,
129.6, 138.9, 140.9, 151.4.
IR (KBr): 1610, 1590, 1572, 1514, 1438, 1371, 1296, 1253, 1171,
1146, 1089, 1063, 1028 cm–1.
MS (ESI): m/z (%) = 225 (100, [M (81Br) + H]+), 223 (100, [M
1H NMR (300 MHz, DMSO-d6): d = 2.33 (3 H, s, CH3), 3.87 (3 H,
s, CH3O), 6.35 (1 H, t, J = 3.3 Hz, 4¢¢-H), 6.46 (1 H, dd, J = 3.3, 1.8
Hz, 3¢¢-H), 6.92 (2 H, d, J = 8.4 Hz, ArH), 7.07 (2 H, d, J = 8.0 Hz,
ArH), 7.36 (1 H, d, J = 7.5 Hz, ArH), 7.45 (1 H, dd, J = 3.3, 1.8 Hz,
5¢¢-H), 7.51 (2 H, d, J = 8.0 Hz, ArH), 7.60 (1 H, d, J = 7.8 Hz,
ArH), 7.62–7.77 (3 H, m, ArH).
(79Br) + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C9H879BrN2: 222.9871; found:
222.9865.
Suzuki Cross-Coupling Reaction; General Procedure
A mixture of bromopyridine 4 (100 mg, 0.27 mmol), arylboronic
acid (0.33 mmol), Pd(PPh3)4 (12 mg, 0.01 mmol), and K2CO3 (91
mg, 0.66 mmol) in toluene (25 mL)–MeOH (5 mL) was heated to
reflux under N2. The reaction was monitored by TLC. Upon com-
pletion of the reaction (reaction time as indicated below), the bulk
of the solvents were removed in vacuo. The residue was partitioned
between EtOAc (30 mL) and H2O (20 mL). The separated organic
layer was dried (Na2SO4), then evaporated in vacuo. The crude
product was purified by column chromatography.
13C NMR (75 MHz, CDCl3): d = 21.6, 55.4, 111.9, 113.8, 116.8,
119.0, 123.3, 124.8, 127.2, 128.7, 129.5, 131.3, 136.3, 136.7, 144.4,
150.3, 156.0, 160.5.
MS (ESI): m/z (%) = 427 (41, [M + Na]+), 405 (100, [M + H]+), 357
(15).
HRMS-ESI: m/z [M + H]+ calcd for C23H21N2O3S: 405.1273; found:
405.1265.
Formation of 6c: The reaction mixture was heated to reflux for 35
h. The crude product was purified by column chromatography on
silica gel using EtOAc–PE (1:10) as eluent to give pyrrolylpyridine
6c (71%) as a colorless solid; mp 150–155 °C.
2-Phenyl-6-(1¢-pyrrol-2¢-yl)pyridine (6a)
From 4: The reaction mixture was heated to reflux for 15 h. The
crude product was purified by column chromatography on silica gel
using EtOAc–PE (1:10) as eluent to give pyrrolylpyridine 6a (98%)
as a colorless solid; mp 96–101 °C.
IR (KBr): 3410, 1607, 1591, 1564, 1515, 1457, 1299, 1249, 1182,
1111, 1037, 1025 cm–1.
IR (KBr): 3399, 1592, 1568, 1457, 1116, 1102, 1083, 1035 cm–1.
1H NMR (300 MHz, CDCl3): d = 3.87 (3 H, s, CH3), 6.32 (1 H, m,
4¢¢-H), 6.73 (1 H, m, 3¢¢-H), 6.93 (1 H, m, 5¢¢-H), 7.01 (2 H, d,
J = 9.0 Hz, 3¢- and 5¢-H), 7.40–7.44 (2 H, m, 3- and 5-H), 7.65 (1 H,
t, J = 7.8 Hz, 4-H), 8.01 (2 H, d, J = 9.0 Hz, 2¢- and 6¢-H), 9.70 (1
H, br s, NH).
13C NMR (75 MHz, CDCl3): d = 55.4, 107.1, 110.3, 114.0, 115.9,
116.6, 119.6, 128.2, 128.5, 131.6, 137.3, 149.9, 155.8, 160.5.
1H NMR (300 MHz, CDCl3): d = 6.33 (1 H, m, 4¢-H), 6.75 (1 H, m,
3¢-H), 6.94 (1 H, m, 5¢-H), 7.43–7.52 (5 H, m, ArH), 7.70 (1 H, t,
J = 7.8 Hz, 4-H), 8.30–8.06 (2 H, m, 3- and 5-H), 9.72 (1 H, br s,
NH).
13C NMR (75 MHz, CDCl3): d = 107.2, 110.2, 116.5, 117.3, 119.7,
126.9, 128.6, 128.9, 131.6, 137.2, 139.4, 150.1, 156.3.
MS (ESI): m/z (%) = 243 (11, [M + Na]+), 221 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C15H13N2: 221.1079; found:
MS (ESI): m/z (%) = 273 (9, [M + Na]+), 251 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C16H15N2O: 251.1184; found:
251.1172.
221.1079.
From 8: The title compound could also be synthesized in 89% iso-
lated yield by refluxing a mixture of bromide 8 (50 mg, 0.22 mmol),
phenylboronic acid (34 mg, 0.28 mmol), Pd(PPh3)4 (cat. amount),
and K2CO3 (77 mg, 0.56 mmol) in toluene (20 mL)–MeOH (5 mL)
for 20 h.
2-(4¢-tert-Butylphenyl)-6-(1¢¢-tosylpyrrol-2¢¢-yl)pyridine (5d)
and 2-(4¢-tert-Butylphenyl)-6-(1¢¢-pyrrol-2¢¢-yl)pyridine (6d)
The reaction mixture was heated to reflux for 11 h. The crude prod-
uct was purified by column chromatography on silica gel using
EtOAc–PE (1:10) as eluent to give tosylpyrrolylpyridine 5d (66%)
and pyrrolylpyridine 6d (27%) both as colorless solids; mp 153–
156 °C.
2-(4¢-Fluorophenyl)-6-(1¢¢-pyrrol-2¢¢-yl)pyridine (6b)
The reaction mixture was heated to reflux for 23 h. The crude prod-
uct was purified by column chromatography on silica gel using
EtOAc–PE (1:10) as eluent to give pyrrolylpyridine 6b (92%) as a
colorless solid; mp 97–99 °C.
5d
IR (KBr): 1591, 1573, 1441, 1360, 1170, 1149, 1088 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.38 [9 H, s, C(CH3)3], 2.32 (3 H,
s, CH3), 6.36 (1 H, t, J = 3.2 Hz, 4¢¢-H), 6.48 (1 H, m, 3¢¢-H), 7.05 (2
H, d, J = 8.1 Hz, ArH), 7.41–7.78 (10 H, m ArH).
13C NMR (75 MHz, CDCl3): d = 21.6, 31.3, 34.6, 111.9, 116.5,
119.4, 123.5, 124.7, 125.4, 127.0, 127.2, 129.5, 136.2, 136.3, 136.5,
144.3, 150.5, 151.9, 156.5.
MS (ESI): m/z (%) = 453 (41, [M + Na]+), 431 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C26H27N2O2S: 431.1793; found:
IR (KBr): 3397, 1601, 1566, 1510, 1456, 1230, 1164, 1106 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.33 (1 H, m, 4¢-H), 6.77 (1 H, m,
3¢-H), 6.95 (1 H, m, 5¢-H), 7.17 (2 H, t, J = 8.6 Hz, 3¢- and 5¢-H),
7.43 (1 H, d, J = 7.8 Hz, ArH), 7.49 (1 H, d, J = 7.8 Hz, ArH), 7.69
(1 H, t, J = 7.8 Hz, 4-H), 8.03 (2 H, dd, J = 8.6, 5.5 Hz, 2¢- and 6¢-
H), 9.70 (1 H, br s, NH).
13C NMR (75 MHz, CDCl3): d = 107.4, 110.3, 115.5 (d, JF,C = 21.5
Hz), 116.4, 116.9, 119.8, 128.7 (d, JF,C = 8.3 Hz), 133.5 (d,
JF,C = 302.9 Hz), 137.3, 150.2, 155.3, 161.8, 165.1.
431.1771.
Synthesis 2011, No. 1, 45–50 © Thieme Stuttgart · New York

PAPER
Synthesis of 2-Aryl-6-(2-pyrrolyl)pyridines
49
6d
1H NMR (300 MHz, CDCl3): d = 2.31 (3 H, s, CH3), 6.39 (1 H, t,
J = 3.3 Hz, 4¢¢-H), 6.51 (1 H, dd, J = 3.3, 1.5 Hz, 3¢¢-H), 7.07 (2 H,
d, J = 8.1 Hz, ArH), 7.47–7.55 (4 H, m, ArH), 7.59 (1 H, t, J = 8.1
Hz, 4-H), 7.73 (1 H, dd, J = 8.1, 0.7 Hz, ArH), 7.84 (1 H, t, J = 7.8
Hz, 5¢-H), 8.15–8.25 (2 H, m, 4¢- and 6 ¢-H), 8.65 (1 H, t, J = 1.8 Hz,
2¢-H).
13C NMR (75 MHz, CDCl3): d = 21.5, 112.1, 116.9, 119.6, 122.1,
123.4, 124.9, 125.1, 127.0, 129.5, 133.0, 134.9, 136.3, 137.0, 140.8,
144.7, 148.6, 151.2, 153.8.
Mp 109–110 °C.
IR (KBr): 3451, 1593, 1566, 1458, 1092, 1079 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.38 [9 H, s, C(CH3)3], 6.32 (1 H,
m, 4¢¢-H), 6.74 (1 H, m, 3¢¢-H), 6.93 (1 H, m, 5¢¢-H), 7.46 (2 H, d,
J = 7.8 Hz, 3- and 5-H), 7.51 (2 H, d, J = 8.4 Hz, 3¢- and 5¢-H), 7.67
(1 H, t, J = 7.8 Hz, 4-H), 7.97 (2 H, d, J = 8.4 Hz, 2¢- and 6¢-H), 9.71
(1 H, br s, NH).
13C NMR (75 MHz, CDCl3): d = 31.3, 34.7, 107.0, 110.3, 116.2,
117.2, 119.5, 125.6, 126.6, 131.8, 137.2, 150.0, 152.1, 156.3.
MS (ESI): m/z (%) = 299 (13, [M + Na]+), 277 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C19H21N2: 277.1705; found:
MS (ESI): m/z (%) = 442 (70, [M + Na]+), 420 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C22H18N3O4S: 420.1018; found:
420.0992.
277.1700.
6f
IR (KBr): 3434, 1594, 1566, 1524, 1455, 1344, 1274, 1170, 1103,
1040 cm–1.
2-(2¢-Methylphenyl)-6-(1¢¢-tosylpyrrol-2¢¢-yl)pyridine (5e) and
2-(2¢-Methylphenyl)-6-(1¢¢-pyrrol-2¢¢-yl)pyridine (6e)
The reaction mixture was heated to reflux for 11 h. The crude prod-
uct was purified by column chromatography on silica gel using
EtOAc–PE (1:5) as eluent to give tosylpyrrolylpyridine 5e (44%) as
a colorless solid and pyrrolylpyridine 6e (49%) as an orange oil.
1H NMR (300 MHz, CDCl3): d = 6.26 (1 H, m, 4¢¢-H), 6.70 (1 H, m,
3¢¢-H), 6.90 (1 H, m, 5¢¢-H), 7.45 (1 H, d, J = 7.8 Hz, ArH), 7.47 (1
H, d, J = 7.8 Hz, ArH), 7.56 (1 H, t, J = 8.1 Hz, 5¢-H), 7.66 (1 H, t,
J = 7.8 Hz, 4-H), 8.16–8.29 (2 H, m, 4¢- and 6¢-H), 8.83 (1 H, t,
J = 2.1 Hz, 2¢-H), 9.62 (1 H, br s, NH).
5e
13C NMR (75 MHz, CDCl3): d = 108.2, 110.6, 117.4, 117.8, 120.5,
121.9, 123.6, 129.6, 130.9, 132.7, 137.8, 140.8, 148.8, 150.5, 153.5.
Mp 111–113 °C.
IR (KBr): 1572, 1447, 1360, 1170, 1145, 1083, 1060 cm–1.
MS (ESI): m/z (%) = 288 (71, [M + Na]+), 266 (100, [M + H]+).
1H NMR (300 MHz, CDCl3): d = 2.08 (3 H, s, CH3), 2.18 (3 H, s,
CH3), 6.25 (1 H, t, J = 3.6 Hz, 4¢¢-H), 6.41 (1 H, dd, J = 3.3, 1.8 Hz,
3¢¢-H), 6.80 (2 H, d, J = 8.1 Hz, ArH), 7.14–7.24 (5 H, m, ArH),
7.38–7.42 (4 H, m, ArH), 7.65 (1 H, t, J = 7.8 Hz, 4-H).
13C NMR (75 MHz, CDCl3): d = 20.2, 21.5, 111.8, 116.9, 122.8,
125.2, 125.7, 127.2, 128.2, 129.2, 129.6, 130.3, 135.0, 136.2, 136.3,
140.0, 144.1, 149.9, 158.8.
HRMS-ESI: m/z [M + H]+ calcd for C15H12N3O2: 266.0930; found:
266.0957.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis. Included
are 1H and 13C NMR spectra for all new compounds
MS (ESI): m/z (%) = 411 (39, [M + Na]+), 389 (100, [M + H]+).
HRMS-ESI: m/z [M + H]+ calcd for C23H21N2O2S: 389.1324; found:
Acknowledgment
389.1311.
We are grateful to the National Natural Science Foundation of Chi-
na (Young Scholarship to C. Song, #20902085; Outstanding Young
Scholarship to J. Chang, #30825043) for financial support .
6e
IR (film): 3424, 1589, 1567, 1458, 1160, 1103, 1031 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.30 (3 H, s, CH3), 6.20 (1 H, m,
4¢¢-H), 6.64 (1 H, m, 3¢¢-H), 6.76 (1 H, m, 5¢¢-H), 7.02 (1 H, d,
J = 7.8 Hz, ArH), 7.17–7.22 (3 H, m, ArH), 7.34 (1 H, m, ArH),
7.38 (1 H, d, J = 7.8 Hz, ArH), 7.56 (1 H, t, J = 7.8 Hz, 4-H), 9.53
(1 H, br s, NH).
13C NMR (75 MHz, CDCl3): d = 20.5, 107.1, 110.2, 116.0, 119.7,
120.7, 125.8, 128.3, 129.5, 130.7, 131.5, 135.9, 136.8, 140.4, 149.6,
158.9.
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