Novel synthesis of 2,4-bis(2-pyridyl)-5-(pyridyl)imidazoles and formation of N-(3-(pyridyl)imidazo[1,5-a]pyridine)picolinamidines: nitrogen-rich ligands

Article abstract of DOI:10.1016/j.tetlet.2009.09.002

Heating a neat 1:2 mixture of 2-picolylamine and 2-cyanopyridine followed by treatment of the resultant red gummy substance with aqueous KOH resulted in the isolation of 2,4,5-tris(2-pyridyl)imidazole (1a) as the major product and N-(3-(2-pyridyl)imidazo[

Full text of DOI:10.1016/j.tetlet.2009.09.002

Tetrahedron Letters 50 (2009) 6264–6267
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Novel synthesis of 2,4-bis(2-pyridyl)-5-(pyridyl)imidazoles and formation of
N-(3-(pyridyl)imidazo[1,5-a]pyridine)picolinamidines: nitrogen-rich ligands
*
Vijendra Kumar Fulwa, Rojalin Sahu, Himanshu Sekhar Jena, Vadivelu Manivannan
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Heating a neat 1:2 mixture of 2-picolylamine and 2-cyanopyridine followed by treatment of the resultant
red gummy substance with aqueous KOH resulted in the isolation of 2,4,5-tris(2-pyridyl)imidazole (1a)
as the major product and N-(3-(2-pyridyl)imidazo[1,5-a]pyridine)picolinamidine (2a) in small amounts.
Similarly, by using 3-picolylamine, 2,4,-bis(2-pyridyl)-5-(3-pyridyl)imidazole (1b) and N-(3-(3-pyri-
dyl)imidazo[1,5-a]pyridine)picolinamidine (2b) were isolated, and by using 4-picolylamine, 2,4,-bis(2-
pyridyl)-5-(4-pyridyl)imidazole (1c) and N-(3-(4-pyridyl)imidazo[1,5-a]pyridine)picolinamidine (2c)
were isolated. The plausible mechanism of the formation of 1a–c and 2a–c is delineated.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 7 August 2009
Revised 27 August 2009
Accepted 1 September 2009
Available online 4 September 2009
Keywords:
Imidazole
Imidazo[1,5-a]pyridine
Ligand
Amidine
The imidazole ring system is an important function in biology,
chemistry as well as in pharmaceutical, veterinary, and agrochem-
ical products.1–4 They are useful ligands in coordination chemistry
and the synthesis of the compounds containing the imidazole ring
is an important area of scientific investigation.1–3 Multicomponent
reaction (MCR) methods involving the isocyanides as one of the
components for the synthesis of imidazole derivatives have been
reviewed.5 The synthesis of the 1,2,4-trisubstituted imidazole by
palladium-catalyzed cyclization of O-pentafluorobenzoylamidoxi-
mes is reported.6 Addition reaction of imidazolium ylides to elec-
tron-deficient imines is a useful method for the synthesis of 2-
ine)picolinamidines (2a–c) formed in this reaction are described.
The synthesis of compounds containing the imidazo[1,5-a]pyridine
ring is another general area of research.13,14 Formation of the varying
amounts of 2,4,5-trisubstituted imidazole and imidazo[1,5-a]pyri-
dine ring systems from 2,20-pyridil, aromatic aldehyde, ammonium
acetate, and acetic acid is relevant to mention.13
Heatinga neat 1:2mixture of 2-picolylamine and2-cyanopyridine
at 100 °C, followed by the treatment of the resultant red gummy sub-
stance with alkali resulted in the formation of 2,4,5-tris(2-pyri-
dyl)imidazole (1a) and N-(3-(2-pyridyl)imidazo[1,5-a]pyridine)
picolinamidine (2a) (Scheme 1).15 From the alkaline solution, 2a
and 2,4,6-tris(2-pyridyl)1,3,5-triazine (3), which precipitated were
separated by filtration, while 1a remained dissolved in the solution.
On adjusting the pH of the aqueous solution to 7–8, 1a separated as
a yellow gel-like substance, which afforded yellow crystals from the
ether medium. Since the formation of the triazine ring from cyano-
pyridines is already well established,16 we limit our discussion only
to 1aand2a. Formationofthesetwocompoundsisbelievedtoinvolve
the presence of intermediate adducts IA and IIA (in Schemes 2 and 3),
which may be present in major and minor quantities. In addition 3
could have formed at this stage. In the aqueous solution of KOH, IA
and IIA, respectively, lead to the generation of the imidazole nucleus
and to the imidazo[1,5-a]pyridine ring systems.
The plausible intermediates involved in the formation of 1a are
depicted in Scheme 2. One molecule of 2-cyanopyridine forms an
adduct with 2-picolylamine leading to an amidine, which in turn
forms an adduct with another molecule of 2-cyanopyridine
through the –NH2 nitrogen atom of the amidine function. One of
the methylene hydrogen atoms in IA is removed by the base to
generate IB; then the negative charge attacks the carbon atom of
(a
-substituted-amidoalkyl)imidazoles.7 An efficient, mild one-pot
method for preparing polysubstituted imidazoles from aryl-substi-
tuted tosylmethyl isocyanide (TosMIC) reagents and in situ gener-
ated imines8 as well as a new synthetic approach for the synthesis
of chiral imidazoles using thio-Ugi reaction9 have been reported.
The synthesis of the annulated terpene-imidazole,10 a microwave
assisted organic synthesis (MAOS) method for the synthesis of
2,4,5-triaryl-imidazole11a as well as the synthesis, and p38 MAP ki-
nase inhibitor property11b have been described.
We recently described the Cu(NO3)2ꢀ3H2O-mediated conversion
of the Schiff base N-(2-pyridylmethyl)pyridine-2-methylketimine
to 40-(2-pyridyl)-2,20:60,200-terpyridine and have continuing interest
in the synthesis of nitrogen-rich ligands.12 In this Letter we report a
facile formation of 2,4-bis(2-pyridyl)-5-(pyridyl)imidazoles (1a–c)
using a simple two-component method. In addition the details of
the other minor product, N-(3-(pyridyl)imidazo[1,5-a]pyrid-
* Corresponding author. Tel.: +91 361 258 2306; fax: +91 361 258 2349.
E-mail address: mani@iitg.ernet.in (V. Manivannan).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.002

V. K. Fulwa et al. / Tetrahedron Letters 50 (2009) 6264–6267
6265
Py
N
Py
N
N
N
N
N
C
Py
NH
N
1) Heat
HN
NH
2
+
+
+
N
N
NH2
2) aq. KOH
N
N
N
N
N
2a = 2-pyridyl
2b = 3-pyridyl
2c = 4-pyridyl
1a = 2-pyridyl
1b = 3-pyridyl
1c = 4-pyridyl
~73%
~5%
~20%
Scheme 1. Synthesis of 1a–c and 2a–c. Legend inside the scheme is for the substituent Py.
the amidine function that leads to the formation of the five-mem-
bered central ring in IC. The proton shift occurs in IC that leads to
ID, from which elimination of one molecule of NH3 occurs, result-
ing in the formation of 1a as the final product. The presence of
strong ammonia stench in the reaction mixture is consistent with
its evolution noted in the final step. It is pertinent to note that a
low-yielding method of preparation of 1a by the reduction of 2-
cyanopyridine with sodium borohydride was described earlier.17
1H and 13C NMR spectra of 1a are consistent with the structure.
The ESI mass spectrum shows the presence of a characteristic
N
N
H2N
C
N
N
N
N
Z
N
N
NH2
N
N
NH2
OH
Z
N
N
C
N
IA
N
N
H2N
H2N
H
N
H
N
N
N
N
N
N
N
IB
IC
N
N
H2N
N
N
H+
HN
N
N
HN
N
-NH3
N
ID
1a
Scheme 2.

6266
V. K. Fulwa et al. / Tetrahedron Letters 50 (2009) 6264–6267
H
H
N
N
N
N
N
N
E
N
N
N
OH
HN
N
NH
HN
NH
HN
NH
N
N
IIA
IIC
IIB
H
N
N
N
N
N
N
N
N
N
OH
HN
N
NH
HN
NH
N
N
NH2
N
IID
2a(A)
2a(B)
Scheme 3.
M++H peak at m/z = 300. The single crystal X-ray structure was
established18 and a perspective view of 1a is shown in Figure 1.
Formation of the small quantities of 2a is interesting, which has
separated as a solid from the basic reaction mixture. Pure fibrous
solids of 2a were obtained after chromatographic separation on ba-
sic alumina using 3:7 ethyl acetate–hexane mixtures. Probable
intermediates involved are represented in Scheme 3.
The intermediate IIA (E) is a geometrical isomer of IA (Z) with
respect to –N@C— group attached to the methylene group and
hence the 2-pyridyl ring instead of the amidine group is in the
vicinity of the methylene group. Abstraction of a proton from the
methylene group generates a negative charge which is delocalized
into the 2-pyridyl ring as shown in IIB. This leads to the concentra-
tion of electron density on the nitrogen atom of 2-pyridyl ring, IIC.
The resultant negative charge on the nitrogen atom attacks the sp2
carbon and generates the fused five-membered ring in IID. The
proton abstraction by a base in IID leads to the formation of 2a that
contains the imidazo[1,5-a]pyridine nucleus, which could exist in
two tautomeric forms 2a(A) and 2a(B). The 1H NMR spectrum
shows two broad signals for the NH protons and therefore is con-
sistent with the 2a(A) form.19
A double triplet at d = 6.81,
6.85 ppm in the 1H NMR, and the presence of M++H peak at m/
z = 315 in the ESI mass spectra of 2a are characteristic.
Under the same experimental conditions and using the same
quantities of the respective reagents, the reaction proceeded very
well with 3- and 4-picolylamines.20 The products isolated using
3-picolylamine are 2,4,-bis(2-pyridyl)-5-(3-pyridyl)imidazole (1b)
and N-(3-(3-pyridyl)imidazo[1,5-a]pyridine)picolinamidine (2b).
With 4-picolylamine, 2,4,-bis(2-pyridyl)-5-(4-pyridyl)imidazole
(1c) and N-(3-(4-pyridyl)imidazo[1,5-a]pyridine)picolinamidine
(2c) were isolated in similar yields. The 1H, 13C NMR, and ESI mass
spectra of 1b, 1c, 2b, and 2c are in accordance with the structures.
In conclusion, we have described an efficient and simple
two-component method for the synthesis of 2,4-bis(2-pyridyl)-5-
(pyridyl)imidazoles (1a–c) and for the formation of N-(3-(pyridyl)
imidazo[1,5-a]pyridine)picolinamidines (2a–c) in small amounts
as the minor product. This method could provide insights into the
synthesis of derivatives of the imidazole as well as the substituted
Figure 1. ORTEP (30% probability) diagram of 1a. All the hydrogen atoms except H1
were omitted for clarity. Selected distances, Å: N1–C1 1.3462(16), N1–C3
1.3702(16), N2–C1 1.3256(15), N2–C2 1.3832 (15), C1–C2 1.3434(15).

V. K. Fulwa et al. / Tetrahedron Letters 50 (2009) 6264–6267
6267
was eluted with ethyl acetate–hexane (3:7) mixture and the yellow fibrous
solid of 2a was obtained after removal of the solvents. Yield: 150 mg (5%). Then
tris(2-pyridyl)triazine (3) was eluted using methanol. Yield: 390 mg (20%). The
filtrate had a strong ammonia smell, the pH of which was adjusted to 7–8 using
5 M HCl. 1a separated as yellow gel-like substance and was separated from the
aqueous solution by decantation. The gel on crystallization from ether afforded
single-crystals suitable for X-ray diffraction studies. Yield: 1.42 g. The decanted
aqueous solution was extracted with ether affording another crop of 1a. Yield:
600 mg, which was recrystallized from ether. Combined yield of 1a: 73% 2,4,5-
imidazo[1,5-a]pyridine nucleus. Our investigations in this regard
and the coordination chemistry of these ligands are in progress.
Acknowledgments
We are grateful to the Council of Scientific and Industrial Re-
search (CSIR), New Delhi, for financial grant and the Department
of Science and Technology (DST), New Delhi, for establishing single
crystal X-ray diffractometer facility under FIST scheme. The
authors are thankful to Mr. Babulal Das for the X-ray data.
þ
Tris(2-pyridyl)imidazole (1a): mp 144 °C; ESI-MS: m/z calcd for C18H13N5
299.117 found (M++H) 300.114. 400 MHz 1H NMR (d (J, Hz), CDCl3): 11.4 (1NH,
s), 8.69 (1H, d, 4.8), 8.61 (2H, d, 4.4), 8.54 (1H, d, 8.0), 8.29 (1H, d, 8.0), 8.09 (1H,
d, 7.6), 7.80 (2H, dd, 7.5), 7.65 (1H, t, 7.6), 7.27 (2H, dd, 4.8), 7.17 (1H, t, 6.2).
100 MHz 13C NMR (d, CDCl3): 148.9, 148.7, 148.4, 147.8, 145.3, 136.6, 136.4,
136.1, 123.2, 122.4, 122.0, 120.0. FTIR (KBr, cmꢁ1): 3439, 3053, 1586, 1566,
1530, 1478, 1452, 1435, 1422, 1387, 1291, 1274, 1250, 1212, 1151, 1119, 1075,
1042, 993, 980, 968, 897, 800, 789, 776, 737, 716, 696, 658, 626, 605, 552, 507,
486, 413, 400. N-(3-(2-Pyridyl)imidazo[1,5-a]pyridine)picolinamidine (2a): mp
Supplementary data
CIF for 1a and 1H NMR, 13C NMR, and ESI Mass spectra of 1a–c
and 2a–c. Crystallographic data (excluding structure factors) for
the structure 1a in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
no. CCDC 742608. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 (0)1223 336033 or e-mail: deposit@ccdc.cam.a-
c.uk). Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2009.09.002.
186 °C. ESI-MS: m/z calcd for C18H14N6 314.128 found (M++H) 315.135.
þ
Rf = 0.66. 400 MHz 1H NMR (d (J, Hz), CDCl3): 9.94 (1H, d, 7.2), 9.32 (1NH, s),
8.63 (3H, m), 8.28 (1H, d, 8.0), 7.99 (1H, d, 8.8), 7.80 (2H, qu, 8.0), 7.51 (1NH, s),
7.35 (1H, t, 5.6), 7.15 (1H, t, 5.6), 6.85 (1H, t, 7.6), 6.81 (1H, t, 6.4). 100 MHz 13C
NMR (d, CDCl3): 152.9, 151.3, 151.2, 148.4, 148.2, 140.7, 136.6, 136.5, 130.0,
127.5, 125.4, 124.5, 121.7, 121.6, 121.0, 118.9, 118.7, 115.2. FTIR (KBr, cmꢁ1):
3366, 3250, 3058, 1689, 1621, 1587, 1563, 1542, 1495, 1469, 1454, 1429, 1397,
1252, 1190, 1137, 1090, 1048, 995, 879, 816, 785, 747, 730, 700, 685, 622, 594,
460, 421, 404. Anal. Calcd for C18H14N6: C, 68.78; H, 4.49; N, 26.74. Found: C,
68.71; H, 4.44; N, 26.68.
16. Case, F. H.; Koft, E. J. Am. Chem. Soc. 1959, 81, 905–906.
17. Yamada, S.; Kuramoto, M.; Kikugawa, Y. Tetrahedron Lett. 1969, 10, 3101–3104.
18. X-ray diffraction data: 1a (296(2) K, Mo
a = 16.3582(6), b = 11.5830(4), c = 7.6079(3) Å, b = 92.996(2)°, V = 1439.55(9) Å3,
Z = 4, Dc = 1.381 g cm–3 = 0.87 mmꢁ1, F(0 0 0) = 624, Rint = 0.0336, R1 (I > 2
= 0.0425, wR2 (I > 2 ) = 0.1140 (260 param., 2832 obsd refl., 3958 unique).
19. We could obtain a very weakly diffracting crystals of Ni(II) complex of 2a and
from the data, only the structure of 2a could be ascertained.
Ka): C18H13N5, 299.33, P21/c,
References and notes
,
l
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Heterocycl. Chem. 1970, 12, 103–183.
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65, 1516–1524; (c) Sisko, J. J. Org. Chem. 1998, 63, 4529–4531.
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r
20. 2,4,-Bis(2-pyridyl)-5-(3-pyridyl)imidazole (1b): Yield: 2.05 g, 75% mp 193 °C;
ESI-MS: m/z calcd for C18H13N5 299.117 found (M++H) 300.124. 400 MHz 1H
þ
NMR (d (J, Hz), CDCl3): 11.3 (1NH, s), 8.95 (1H, s), 8.63 (3H, m), 8.25 (1H, d, 8.0),
8.07 (1H, d, 8.0), 7.80 (1H, t, 7.8), 7.55 (1H, t, 7.8), 7.41 (2H, m), 7.30 (1H, t, 6.2),
7.16 (1H, t, 6.2). 100 MHz 13C NMR (d, CDCl3): 149.9, 149.2, 148.9, 148.1, 146.7,
137.2, 136.7, 136.5, 123.7, 122.4, 120.6. FTIR (KBr, cmꢁ1): 3458, 1641, 1625,
1590, 1562, 1530, 1502, 1472, 1438, 1389, 1321, 1302, 1254, 1181, 1140, 1097,
1029, 955, 797, 727, 702, 624. Anal. Calcd for C18H13N5: C, 72.23; H, 4.38; N,
23.40. Found: C, 72.15; H, 4.36; N, 23.36. N-(3-(3-Pyridyl)imidazo[1,5-
a]pyridine)picolinamidine (2b): Yield: 150 mg, 5% mp 195 °C; ESI-MS: m/z
calcd for C18H14N6 314.128 found (M++H) 315.135. Rf = 0.64. 400 MHz 1H
þ
NMR (d (J, Hz), CDCl3): 9.21 (1NH, s), 9.14 (1H, s), 8.63 (2H, m), 8.22 (1H, d, 8.0),
8.17 (1H, d, 12.0), 7.95 (1H, d, 11.2), 7.82 (1H, t, 8.8), 7.46 (2H, dd, 7.0), 7.35
(1H, t, 6.6), 6.74 (1H, t, 8.0), 6.67 (1H, t, 7.6). 100 MHz 13C NMR (d, CDCl3):
152.8, 151.3, 149.0, 148.6, 148.2, 141.4, 136.6, 134.8, 129.8, 127.1, 126.2, 124.6,
123.9, 121.7, 120.1, 120.0, 117.4, 115.5. FTIR (KBr, cmꢁ1): 3414, 3052, 2853,
1651, 1589, 1562, 1489, 1469, 1447, 1427, 1393, 1289, 1263, 1185, 1137, 1090,
1084, 1046, 1025, 995, 967, 847, 790, 743, 719, 700, 621, 500. Anal. Calcd for
C18H14N6: C, 68.78; H, 4.49; N, 26.74. Found: C, 68.68; H, 4.42; N, 26.70. 2,4,-
10. Kulhánek, J.; Bureš, F.; Šimon, P.; Schweizer, W. B. Tetrahedron: Asymmetry
2008, 19, 2462–2469.
11. (a) Sparks, R. B.; Combs, A. P. Org. Lett. 2004, 6, 2473–2475; (b) Boehm, J. C.;
Smietana, J. M.; Sorenson, M. E.; Garigipati, R. S.; Gallagher, T. F.; Sheldrake, P.
L.; Bradbeer, J.; Badger, A. M.; Laydon, J. T.; Lee, J. C.; Hillegass, L. M.; Griswold,
D. E.; Breton, J. J.; Chabot-Fletcher, M. C.; Adams, J. L. J. Med. Chem. 1996, 39,
3929–3937.
12. Padhi, S. K.; Manivannan, V. Inorg. Chem. 2006, 45, 7994–7996.
13. Wang, J.; Mason, R.; VanDerveer, D.; Feng, K.; Bu, X. R. J. Org. Chem. 2003, 68,
5415–5418.
14. (a) Shibahara, F.; Sugiura, R.; Yamaguchi, E.; Kitagawa, A.; Murai, T. J. Org. Chem.
2009, 74, 3566–3568; (b) Salassa, L.; Garino, C.; Albertino, A.; Volpi, G.; Nervi,
C.; Gobetto, R.; Hardcastle, K. I. Organometallics 2008, 27, 1427–1435; (c)
Shibahara, F.; Kitagawa, A.; Yamaguchi, E.; Murai, T. Org. Lett. 2006, 8, 5621–
5624; (d) Wang, J.; Dyers, L.; Mason, R., Jr.; Amoyaw, P., Jr.; Bu, X. R. J. Org.
Chem. 2005, 70, 2353–2356; (e) Katritzky, A. R.; Qiu, G. J. Org. Chem. 2001, 66,
2862–2864; (f) Sasaki, K.; Tsurumori, A.; Hirota, T. J. Chem. Soc., Perkin Trans. 1
1998, 3851–3856; (g) Krapcho, A. P.; Powell, J. R. Tetrahedron Lett. 1986, 27,
3713–3714.
15. Synthesis: 2-Picolylamine (1 g, 9.26 mmol) and 2-cyanopyridine (1.93 g,
18.6 mmol) were heated at 100 °C in an oil-bath for 12 h. The resultant red
gummy oil was suspended in 30 mL of water, KOH (3.11 g, 55.6 mmol) was
added and stirred for 12 h. The yellow solid obtained was filtered, washed with
water and dried in vacuum over fused CaCl2. The dry solid was subjected to
chromatographic separation using basic alumina column. Firstly compound 2a
Bis(2-pyridyl)-5-(4-pyridyl)imidazole (1c): Yield: 2.02 g, 73% mp 196 °C; ESI-MS:
+
m/z calcd for C18H13N5 299.117 found (M +H) 300.123. 400 MHz 1H NMR (d (J,
þ
Hz), CDCl3): 11.41 (1NH, s), 8.66 (2H, m), 8.59 (2H, d, 4.0), 8.25 (1H, d, 8.0), 7.81
(1H, t, 7.2), 7.68 (2H, d, 5.2), 7.60 (1H, t, 7.4), 7.53 (1H, d, 7.6), 7.30 (1H, t, 7.6),
7.20 (1H, t, 6.0). 100 MHz 13C NMR (d, CDCl3): 149.8, 149.7, 149.0, 147.9, 146.8,
137.1, 136.6, 124.0, 123.0, 122.5, 121.5, 120.5. FTIR (KBr, cmꢁ1): 3498, 3218,
1644, 1608, 1574, 1567, 1496, 1471, 1447, 1418, 1398, 1332, 1320, 1293, 1272,
1246, 1218, 1143, 1105, 1090, 1049, 1001, 994, 917, 822, 807, 790, 764, 754,
738, 720, 705, 683, 666, 632, 612, 547, 520, 498, 459. Anal. Calcd for C18H13N5:
C, 72.23; H, 4.38; N, 23.40. Found: C, 72.17; H, 4.33; N, 23.34. N-(3-(4-
Pyridyl)imidazo[1,5-a]pyridine)picolinamidine (2c): Yield: 135 mg, 4.5% mp
183 °C; ESI-MS: m/z calcd for C18H14N6 314.13 found (M++H) 315.14.
þ
Rf = 0.60. 400 MHz 1H NMR (d (J, Hz), CDCl3): 9.19 (1NH, s), 8.70 (2H, d, 6.0),
8.57 (2H, d, 8.8), 8.34 (1H, d, 7.2), 7.95 (1H, d, 10.0), 7.77 (3H, m), 7.48 (1NH, s),
7.34 (1H, t, 6.8), 6.79 (1H, t, 8.0), 6.74 (1H, t, 6.8). 100 MHz 13C NMR (d, CDCl3):
152.7, 151.6, 150.6, 148.2, 141.9, 137.7, 136.6, 129.7, 127.2, 124.7, 121.7, 120.8,
120.7, 120.1, 118.0, 116.0. FTIR (KBr, cmꢁ1): 3438, 3305, 1643, 1624, 1598,
1564, 1527, 1497, 1472, 1454, 1325, 1289, 1247, 1218, 1182, 1112, 998, 961,
815, 800, 724, 703, 658, 507, 484. Anal. Calcd for C18H14N6: C, 68.78; H, 4.49; N,
26.74. Found: C, 68.70; H, 4.40; N, 26.65.