Synthesis of methylene- and methine-bridged oligopyridines

Article abstract of DOI:10.1002/ejoc.200400435

A variety of methylene- and methine-bridged oligopyridines are conveniently accessible through stepwise nucleophilic aromatic substitution with fluoro-substituted pyridines. The yields achieved are regularly above 90%. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejoc.200400435

SHORT COMMUNICATION
Synthesis of Methylene- and Methine-Bridged Oligopyridines
Gerald Dyker*[a] and Oliver Muth[b]
Keywords: Multidentate ligands / Lithiation / Nucleophilic substitution / Nitrogen heterocycles
A variety of methylene- and methine-bridged oligopyridines
are conveniently accessible through stepwise nucleophilic
aromatic substitution with fluoro-substituted pyridines. The
yields achieved are regularly above 90%.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
Oligo(azaheterocyclic) compounds are excellent ligands
for building up large polymetallic complexes.[1] The struc-
tures 1Ϫ3 are rather rigid because of the direct connection
of the hetarene units and have been used to study self-
organization to gridlike complexes[2,3] and even to heli-
cates.[4]
However, structural rigidity is not at all a prerequisite
to a rich supramolecular chemistry with the formation of
oligonuclear complexes featuring fascinating architectures.
Far more flexible oligopyridines,[5] with the pyridine units
linked by methylene groups or even longer alkanediyl
chains, can also take part in such self-organization pro-
cesses as Lehn et al.[6,7] impressively demonstrated with the
synthesis of heteroduplex and circular helicates. Moreover,
the structural diversity of these flexible oligopyridines also
includes branched structures such as the pentadentate li-
gand 4, which has been proven to form stable complexes
with a variety of metal ions.[8] This ligand is capable of sta-
bilizing FeII and MnII centers,[9] thus modeling intermediates
in the oxygenation cycle with lipoxygenase[10,11] and promis-
ing the development of novel biomimetic catalysts. The sim-
plest representative of this type of flexible oligopyridines, 2-
(2-pyridylmethyl)pyridine (11), is commonly synthesized by
nucleophilic aromatic substitution with lithiated 2-methyl-
pyridine as nucleophile and either pyridine or 2-bromopyri-
dine as electrophilic component.[12] Vedernikov et al. devel-
oped a synthesis for substituted 2,6-bis(2-pyridylmethyl)py-
ridine with 2,6-dimethylpyridine-dilithium as bis(nucleo-
phile) and pyridine as electrophile.[13] According to our own
results, nucleophilic substitutions of this type with pyridine
as well as with 2-bromopyridine as reactants lead to rather
Scheme 1
[a]
Fakultät für Chemie, Ruhr-Universität Bochum,
impure crude products, which necessitate tedious separation
procedures and result in at best moderate yields of the final
products. In order to develop an improved general method
for the synthesis of methylene- and methine-linked oligopy-
Universitätsstrasse 150, 44780 Bochum, Germany
Fax: (internat.) ϩ 49-(0)234-3214353
E-mail: Gerald.Dyker@rub.de
[b]
Fakultät für Naturwissenschaften, Universität Duisburg-Essen,
Lotharstrasse 1, 47048 Duisburg, Germany
Eur. J. Org. Chem. 2004, 4319Ϫ4322
DOI: 10.1002/ejoc.200400435
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4319

G. Dyker, O. Muth
SHORT COMMUNICATION
ridines, we tested 2-fluoropyridine (5) and 2,6-difluoropyri- of 2,6-difluoropyridine, it is necessary to use at least four
dine (6) as reactants.
equivalents (actually up to eight equivalents) of lithiated 8,
leading to an excellent yield of the tris(pyridine) product 9.
After deprotonation with n-butyllithium, the methylene-
linked bis(pyridine) 11 is a suitable nucleophile for the reac-
tion with 6, forming the monofluoro-functionalized tris(py-
ridine) compound 10. For a double nucleophilic substi-
tution at 6, six equivalents of lithiated 11 have to be applied,
again because of the increased acidity of the final product.
In this case a remarkable yield of 96% of the pentakis(pyri-
dine) compound 12 is obtained.
The versatility of fluoropyridines as building blocks in
nucleophilic substitution reactions was further demon-
strated by the successful synthesis of the chiral tetradentate
ligand 14 from 7 and binaphthol 13.
Results and Discussion
The methylene-linked bis(pyridine) 11 was synthesized in
96% yield by lithiation of 2-methylpyridine (8) in dry THF
and addition of 0.5 equivalents of 2-fluoropyridine (5) as
nucleophilic component. Using an excess of the lithiated 2-
methylpyridine (8) is crucial, since the product 11 contains Conclusion
an acidic methylene unit, leading to the consumption of one
equivalent of lithiated 8. As a result, the negative charge of
The family of methylene- and methine-bridged oligopyri-
lithiated 11 is stabilized by the two conjugated pyridyl dines is efficiently accessible through stepwise nucleophilic
units,[14] and the formation of branched byproducts is aromatic substitution with 2-fluoropyridine (5) or 2,6-di-
somewhat suppressed.
fluoropyridine (6) in yields of 90% and more. Especially bis-
The synthesis of 2-fluoro-6-(2-pyridylmethyl)pyridine (7, and tris-pyridine units with one remaining fluorine func-
90% yield) starts from lithiated 2-methylpyridine (8) and 0.5 tionality, such as 7 and 10, are valuable building blocks for
equivalents of 2,6-difluoropyridine (6). Again, excess nucle- further transformations. This straightforward entry to a
ophile is necessary because of the consumption by product broad variety of multidentate ligands is currently under
7 (see above). To replace both the 2- and the 6-position investigation.
Scheme 2. a: 1. nBuLi, dry THF at Ϫ78 °C; 2. addition of 5 or 6 at Ϫ20 °C, then reflux
Scheme 3. b: KOtBu, 18-crown-6, xylene, reflux, 3 days
4320
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 4319Ϫ4322

Synthesis of Methylene- and Methine-Bridged Oligopyridines
SHORT COMMUNICATION
ture was heated to reflux for 3 h. After cooling to room tempera-
ture overnight, water was added (25 mL), and after separation the
water layer was extracted three times with CH2Cl2 (25 mL). The
combined organic layers were dried with Na2SO4. The solvent was
evaporated, and the residue was distilled bulb-to-bulb (0.5 mbar,
190 °C). Yield: 2.53 g (9.7 mmol, 97%) of 9 as a light yellow oil,
which crystallizes after longer standing to give a slightly yellow
solid, m.p. 46 °C. IR (KBr): ν˜ ϭ 3086 (w), 3057 (w), 2998 (m), 2962
(w), 2931 (w), 1589 (s), 1473 (s), 1459 (s), 1439 (s), 1328 (m), 1309
(w), 1211 (m), 1097 (w), 1052 (w), 997 (m), 793 (m), 767 (m), 756
Experimental Section
General Remarks: Melting points (uncorrected values) were deter-
mined with a Reichert Thermovar. Infrared spectroscopy was per-
formed with a PerkinϪElmer 983 instrument. UV/Vis spectra were
recorded with a PerkinϪElmer Lambda 40 apparatus. H and 13C
1
NMR spectra were recorded with a Bruker DRX 500 by use of
CDCl3 as solvent and TMS as the internal standard. Assignments
of the NMR signals are based on CH COSY spectra and chemical
shift considerations. Mass spectroscopy was performed with a
Varian MAT 311A ITD (70 eV). For analytical TLC, POLYGRAM
SIL G/UV254 precoated plastic sheets from MachereyϪNagel were
used. Elemental analyses were performed with an Euro Elemental
Analyzer 3000.
(s), 608 (m), 586 (m), 573 (m) cmϪ1 1H NMR (500.1 MHz,
.
CDCl3): δ ϭ 4.34 (s, 4 H, CH2), 7.05 (d, J ϭ 7.7 Hz, 2 H, 3/ 5-H),
7.10 (ddd, J ϭ 7.5, 4.9, 1.0 Hz, 2 H, 5Ј/5ЈЈ-H), 7.23 (dm, J ϭ
7.8 Hz, 2 H, 3Ј/ 3ЈЈ-H), 7.49 (t, J ϭ 7.6 Hz, 1 H, 4-H), 7.56 (dt,
J ϭ 7.7, 1.8 Hz, 2 H, 4Ј/ 4ЈЈ-H), 8.53 (ddd, J ϭ 4.9, 1.9, 0.9 Hz, 2
H, 6Ј/ 6ЈЈ-H) ppm. 13C NMR (125.8 MHz, CDCl3): δ ϭ 47.1 (t,
CH2), 121.1 (d, C-3/ 5), 121.3 (d, C-5Ј/ 5ЈЈ), 123.6 (d, C-3Ј/ 3ЈЈ),
136.3 (d, C-4Ј/4ЈЈ), 137.0 (d, C-4), 149.2 (d, C-6Ј/ 6ЈЈ), 158.9 (s),
159.5 (s) ppm. MS (EI): m/z (%) ϭ 261 (43) [Mϩ], 260 (100), 183
(76), 169 (99), 168 (37), 129 (15), 123 (12), 106 (17), 93 (12), 79 (4),
78 (11). C17H15N3 (261.33): calcd. C 78.13, H 5.79, N 16.08; found
C 77.99, H 5.75, N 16.18.
2-(2-Pyridylmethyl)pyridine (11): A solution of 2-methylpyridine
(7.46 g, 80.0 mmol) in dry THF (80 mL) was cooled to Ϫ78 °C,
and n-butyllithium (40 mL, 80.0 mmol, 2.0  in pentane) was ad-
ded within 15 min. The reaction mixture was stirred for 45 min at
Ϫ78 °C and then warmed up to Ϫ20 °C. 2-Fluoropyridine (3.88 g,
40.0 mmol) was added within 5 min, and the reaction mixture was
heated to reflux for 25 min and hydrolyzed with ice (75 g). The
water layer was extracted three times with CH2Cl2 (50 mL), and
the combined organic layers were dried with Na2SO4. The solvent
was evaporated, and the residue was distilled bulb-to-bulb
(0.8 mbar, 150 °C). Yield: 6.51 g (38.2 mmol, 96%) of 11 as a
slightly yellow oil. 1H NMR (300.1 MHz, CDCl3): δ ϭ 4.34 (s, 2
H, CH2), 7.12 (m, 2 H, 5/5Ј-H), 7.26 (d, J ϭ 7.8 Hz, 2 H, 3/3Ј-H),
7.60 (td, J ϭ 7.7, 1.9 Hz, 2 H, 4/4Ј-H), 8.55 (ddd, J ϭ 4.9, 1.8,
0.8 Hz, 2 H, 6/6Ј-H) ppm.
2-Fluoro-6-[bis(2-pyridyl)methyl]pyridine (10): A solution of 2-(2-
pyridylmethyl)pyridine (11) (1.70 g, 10.0 mmol) in dry THF
(50 mL) was cooled to Ϫ78 °C, and n-butyllithium (5 mL,
10.0 mmol, 2.0  in pentane) was added within 2 min. The reaction
mixture was stirred at Ϫ78 °C for 30 min and then warmed up to
Ϫ20 °C. 2,6-Difluoropyridine (0.58 g, 5.00 mmol) was added within
5 min, and the reaction mixture was heated to reflux for 25 min
and hydrolyzed with ice (20 g). The water layer was extracted three
times with CH2Cl2 (25 mL), and the combined organic layers were
dried with Na2SO4. The solvent was evaporated, and the residue
was distilled bulb-to-bulb (0.5 mbar, 200 °C). Yield: 1.06 g
(4.0 mmol, 80%) of 10 as a light orange sticky oil which crystallizes
after longer standing to give a slightly yellow solid, m.p. 81 °C. IR
(KBr): ν˜ ϭ 3049 (w), 3000 (w), 1603 (s), 1590 (s), 1469 (s), 1449
(s), 1273 (w), 1238 (m), 1221 (m), 1152 (w), 995 (m), 971 (w), 800
2-Fluoro-6-(2-pyridylmethyl)pyridine (7): A solution of 2-methyl-
pyridine (1.86 g, 20.0 mmol) in dry THF (20 mL) was cooled to
Ϫ78 °C, and n-butyllithium (10 mL, 20.0 mmol, 2.0  in pentane)
was added within 5 min. The reaction mixture was stirred at Ϫ78
°C for 45 min and then warmed up to Ϫ20 °C. 2,6-Difluoropyri-
dine (1.15 g, 10.0 mmol) was added within 5 min, and the reaction
mixture was heated to reflux for 25 min and hydrolyzed with ice
(20 g). The water layer was extracted three times with CH2Cl2
(25 mL), and the combined organic layers were dried with Na2SO4.
The solvent was evaporated, and the residue was distilled bulb-to-
bulb (0.7 mbar, 150 °C). Yield: 1.70 g (9.0 mmol, 90%) of 7 as a
slightly yellow oil. IR (film): ν˜ ϭ 3078 (w), 3014 (w), 2927 (w), 1607
(s), 1592 (s), 1577 (s), 1475 (s), 1435 (s), 1264 (s), 1226 (s), 1149
(m), 998 (s), 974 (m), 797 (s), 751 (s), 723 (m), 668 (w), 552
(w) cmϪ1. UV/Vis (CH3CN): λmax. (lg ε) ϭ 204 (3.73), 262 (3.61),
(m), 760 (s), 668 (m), 613 (m) cmϪ1
. UV/Vis (CH3CN):
λmax. (lg ε) ϭ 207 (4.08), 262 (3.98), 268 (3.89) nm. 1H NMR
(500.1 MHz, CDCl3): δ ϭ 5.91 (s, 1 H, CH), 6.79 (ddd, J ϭ 8.2,
2.9, 0.4 Hz, 1 H, 3-H), 7.15 (m, 2 H, 5Ј/5ЈЈ-H), 7.21 (dd, J ϭ 7.4,
2.5 Hz, 1 H, 5-H), 7.33 (dm, J ϭ 7.9 Hz, 2 H, 3Ј/3ЈЈ-H), 7.63 (td,
J ϭ 7.7, 1.9 Hz, 2 H, 4Ј/ 4ЈЈ-H), 7.72 (dt, J ϭ 8.3, JH,F ϭ 7.5 Hz, 1
H, 4-H), 8.57 (ddd, J ϭ 4.9, 1.8, 0.9 Hz, 2 H, 6Ј/6ЈЈ-H) ppm. 13C
NMR (125.8 MHz, CDCl3): δ ϭ 63.3 (d, 1 H, CH), 107.5 (dd,
1
268 (3.49, sh), 330 (1.72) nm. H NMR (500.1 MHz, CDCl3): δ ϭ
JC,F ϭ 36.9 Hz, C-3), 121.4 (dd, JC,F ϭ 4.0 Hz, C-5), 121.9 (d, C-
4.27 (s, 2 H, CH2), 6.76 (ddd, J ϭ 8.2, 2.8, 0.5 Hz, 1 H, 3-H), 7.14
(m, 2 H, 5/5Ј-H), 7.29 (dm, J ϭ 7.8 Hz, 1 H, 3Ј-H), 7.62 (td, J ϭ
7.7, 1.9 Hz, 1 H, 4Ј-H), 7.69 (m, 1 H, 4-H), 8.54 (ddd, J ϭ 4.9,
1.8 , 0.9 Hz, 1 H, 6Ј-H) ppm. 13C NMR (125.8 MHz, CDCl3): δ ϭ
46.4 (t, CH2), 107.0 (dd, JC,F ϭ 36.9 Hz, C-3), 120.7 (dd, JC,F ϭ
4.0 Hz, C-5), 121.7 (d, C-5Ј), 123.7 (d, C-3Ј), 136.6 (d, C-4Ј), 141.4
(dd, JC,F ϭ 7.5 Hz, C-4), 149.4 (d, C-6Ј), 158.4 (d, JC,F ϭ 13.0 Hz,
C-6), 158.5 (s, C-2Ј), 163.0 (d, JC,F ϭ 238.9 Hz, C-2) ppm. MS (EI,
70 eV): m/z (%) ϭ 188 (29) [Mϩ], 187 (100), 186 (15). C11H9FN2
(188.20): calcd. C 70.20, H 4.82, N 14.88; found C 70.26, H 4.87,
N 15.01.
5Ј/5ЈЈ), 124.1 (d, C-3Ј/3ЈЈ), 136.6 (d, C-4Ј/4ЈЈ), 141.4 (dd, JC,F ϭ
7.5 Hz, C-4), 149.5 (d, C-6Ј/6ЈЈ), 159.9 (d, JC,F ϭ 13.0 Hz, C-6),
160.3 (s, C-2Ј/ 2ЈЈ), 162.9 (d, JC,F ϭ 239.4 Hz, C-2) ppm. MS (EI,
70 eV): m/z (%) ϭ 265 (68) [Mϩ], 264 (83), 187 (43), 186 (27),
170 (13), 169 (100), 168 (16), 78 (9). C16H12FN3 (265.29): calcd.
C 72.44, H 4.56, N 15.84; found C 72.08, H 4.63, N 15.97.
2,6-Bis[bis(2-pyridyl)methyl]pyridine (12): A solution of 2-(2-pyrid-
ylmethyl)pyridine (11) (1.02 g, 6.00 mmol) in dry THF (30 mL) was
cooled to Ϫ78 °C, and n-butyllithium (3 mL, 6.0 mmol, 2.0  in
pentane) was added within 3 min. The reaction mixture was stirred
at Ϫ78 °C for 30 min and then warmed up to Ϫ20 °C. 2,6-Difluoro-
pyridine (115 mg, 1.00 mmol) was added within 5 min, and the re-
action mixture was heated to reflux for 24 h and cooled to room
temperature. Water (20 mL) was added, and after separation the
2,6-Bis(2-pyridylmethyl)pyridine (9): A solution of 2-methylpyridine
(7.45 g, 80.0 mmol) in dry THF (40 mL) was cooled to Ϫ78 °C,
and n-butyllithium (40 mL, 80.0 mmol, 2.0  in pentane) was ad-
ded within 15 min. The reaction mixture was stirred at Ϫ78 °C for
45 min and then warmed up to Ϫ20 °C. 2,6-Difluoropyridine water layer was extracted three times with CH2Cl2 (25 mL). The
(1.15 g, 10.0 mmol) was added within 15 min, and the reaction mix- combined organic layers were dried with Na2SO4. The solvent was
Eur. J. Org. Chem. 2004, 4319Ϫ4322
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4321

G. Dyker, O. Muth
SHORT COMMUNICATION
[1c]
2000, 100, 3483Ϫ3537.
G. R. Newkome, E. He, C. N.
removed, and the crude product was recrystallized from acetone.
Yield: 400 mg (0.96 mmol, 96%) of 12 as slightly yellow crystals,
m.p. 146 °C. IR (KBr): ν˜ ϭ 3046 (w), 3007 (w), 1585 (s), 1564 (s),
1465 (s), 1429 (s), 1328 (w), 1148 (m), 1088 (m), 1050 (m), 995 (m),
775 (m), 754 (s), 703 (w), 624 (m), 603 (m) cmϪ1. UV/Vis (CH3CN):
λmax. (lg ε) ϭ 204 (4.91, sh), 207 (4.88, sh), 257 (4.60, sh), 263
(4.65), 269 (4.59, sh), 401 (2.23) nm. 1H NMR (500.1 MHz,
CDCl3): δ ϭ 5.93 (s, 2 H, CH), 7.10 (m, 4 H, 5Ј/5ЈЈ/5ЈЈЈ/5ЈЈЈЈ-H),
7.19 (m, 6 H, 3Ј/3ЈЈ/3ЈЈЈ/3ЈЈЈЈ-H, 3/5-H), 7.50 (td, J ϭ 7.8, 1.9 Hz,
4 H, 4Ј/ 4ЈЈ/ 4ЈЈЈ/4ЈЈЈЈ-H), 7.58 (t, 3J ϭ 7.7 Hz, 1 H, 4-H), 8.53 (ddd,
J ϭ 4.9, 1.8, 0.9 Hz, 4 H, 6Ј/6ЈЈ/6ЈЈЈ/6ЈЈЈЈ-H) ppm. 13C NMR
(125.8 MHz, CDCl3): δ ϭ 63.9 (d, CH), 121.5 (d, C-5Ј/5ЈЈ/5ЈЈЈ/
5ЈЈЈЈ), 122.1 (d, C-3/5), 124.1 (d, C-3Ј/ 3ЈЈ/ 3ЈЈЈ/ 3ЈЈЈЈ), 136.1 (d, C-
4Ј/ 4ЈЈ/ 4ЈЈЈ/ 4ЈЈЈЈ), 137.2 (d, C-4), 149.1 (d, C-6Ј/ 6ЈЈ/ 6ЈЈЈ/ 6ЈЈЈЈ),
160.2 (s, C-2/ 6), 161.4 (s, C-2Ј/ 2ЈЈ/ 2ЈЈЈ/ 2ЈЈЈЈ) ppm. MS (EI, 70 eV):
m/z (%) ϭ 415 (67) [Mϩ], 414 (60), 337 (56), 258 (13), 247 (38), 246
(100), 169 (52), 168 (22). C27H21N5 (415.50): calcd. C 78.05, H 5.09,
N 16.86; found C 78.12, H 5.04, N 16.72.
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crown-6 (530 mg, 2.00 mmol), and K-OtBu (225 mg, 2.00 mmol) in
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1508 (m), 1474 (m), 1443 (s), 1359 (w), 1258 (s), 1228 (s), 1147 (m),
1074 (w), 1002 (s), 821 (w), 798 (w), 755 (m) cmϪ1. UV/Vis
(CH3CN): λmax. (lg ε) ϭ 223 (4.41), 274 (3.87), 323 (2.90) nm. 1H
NMR (500.1 MHz, CDCl3): δ ϭ 3.98 (’’q’’, J ϭ 14.5 Hz, 4 H,
CH2), 6.34 (d, J ϭ 8.1 Hz, 2 H, Py-5ЈЈ/5ЈЈЈ-H), 6.70 (d, J ϭ 7.4 Hz,
Py-3ЈЈ/3ЈЈЈ-H), 7.05Ϫ7.28 (m, 12 H, Ar/Py-H), 7.37 (tm, J ϭ
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M. Yashiro, Bull. Chem. Soc., Jpn. 2002, 75,
1383Ϫ1384. [5n] J. Manzur, A. M. Garcia, R. Letelier, E. Spod-
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[5o]
Chem. Soc., Dalton Trans. 1993, 905Ϫ911.
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1549Ϫ1550.
[6]
[7]
5Ј-H), 8.47 (ddd, J ϭ 4.9, 1.8, 0.9 Hz, 2 H, 6ЈЈЈЈ/6ЈЈЈЈЈ-H) ppm. 13
C
NMR (125.8 MHz, CDCl3): δ ϭ 46.6 (t, CH2), 108.7 (d, C-5ЈЈ/5ЈЈЈ),
117.5 (d, C-3ЈЈ/3ЈЈЈ), 121.4 (d), 121.8 (d), 123.4 (s), 124.0 (d), 124.9
(d), 126.2 (d), 126.5 (d), 127.7 (d), 129.1 (d), 130.9 (s), 134.1
(s), 136.3 (d), 139.2 (d), 149.1 (d), 150.3 (s), 157.4 (s), 159.2 (s),
163.1 (s) ppm. MS (EI, 70 eV): m/z (%) ϭ 622 (15) [Mϩ], 437 (9),
213 (10), 186 (13), 169 (9), 155 (24), 127 (14), 100 (29), 79 (31).
C42H30N4O2 (622.72): calcd. C 81.01, H 4.86, N 8.99; found
C 80.76, H 4.74, N 9.12.
[8]
[9]
[10]
[11]
R. T. Jonas, T. D. P. Stack, J. Am. Chem. Soc. 1997, 119,
8566Ϫ8567.
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2002, 41, 6244Ϫ6248.
The lithium cation is coordinated to the pyridyl nitrogens:
H. Gornitzka, D. Stalke, Angew. Chem. 1994, 106, 695Ϫ698;
Angew. Chem. Int. Ed. Engl. 1994, 33, 693Ϫ695.
Received June 22, 2004
Acknowledgments
This work was supported by the Fonds der Chemischen Industrie.
[12]
[13]
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[14]
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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