Synthesis of novel imidazolium and benzimidazolium salts from halomethyl derivatives of 4-pyrones

Article abstract of DOI:10.2174/157017812803521153

A series of mono- and bis-benzimidazolium and imidazolium salts of 4-pyrone derivatives has been synthesized in good yields, by various quaternization reactions of halomethyl derivatives of 4H-pyran-4-ones. These salts can be used as novel precursors of functionalized N-heterocyclic carbenes and some of them as novel ionic liquids.

Full text of DOI:10.2174/157017812803521153

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Letters in Organic Chemistry, 2012, 9, 677-682
677
Synthesis of Novel Imidazolium and Benzimidazolium Salts from Halo-
methyl Derivatives of 4-Pyrones
Zarrin Ghasemia, Mahdi Gholamhoseini-Nazaria, Maryam Allahvirdinesbata, Mahnaz Saraeib and
Aziz Shahrisa*a
aDepartment of Organic and Biochemistry, Faculty of Chemistry, University of Tabriz, Iran; bDepartment of Chemistry,
Payame Noor University, Tabriz-Iran
Received February 07, 2012: Revised May 02, 2012: Accepted August 04, 2012
Abstract: A series of mono- and bis-benzimidazolium and imidazolium salts of 4-pyrone derivatives has been synthe-
sized in good yields, by various quaternization reactions of halomethyl derivatives of 4H-pyran-4-ones. These salts can be
used as novel precursors of functionalized N-heterocyclic carbenes and some of them as novel ionic liquids.
Keywords: 4-pyrones, benzimidazolium salts, imidazolium salts, KI, kojic acid, substitution reactions.
INTRODUCTION
prepared from benzylic halogenations of the corresponding
4-pyrones according to the literatures [32]. Treatment of
halomethyl derivatives 1a-d with commercial N-
methylimidazole in acetonitrile at 25 ºC for 24 hr, resulted in
the formation of salts 2a-d, which were obtained in good
yields and simple workup. The 4-(bromomethyl)phenyl de-
rivatives 1e,f also react similarly to afford imidazolium salts
2e,f. To evaluate the possibility of reducing the melting
points of these compounds to achieve the new ionic liquids,
anion exchange reactions of these salts with AgBF4 were
carried out according to a known procedure [42], to furnish
the corresponding imidazolium tetrafluoroborates 3a-f
(Schemes 1, 2).
Imidazolium and benzimidazolium salts are an important
class of quaternary ammonium compounds (QACs) which
have extensively achieved considerable attention in different
chemical, biochemical and industrial areas [1-7]. Deprotona-
tion of N,Nꢀ-disubstituted derivatives of these salts, generates
N-heterocyclic carbenes (NHCs) species. These nucleophilic
carbenes as excellent ligands of metal ions are widely used
in coordination chemistry [8, 9]. Resulting metal-NHC com-
plexes have found great applications in medicinal chemistry
[10] and as catalysts in many carbon-carbon bond forming
reactions [11-16]. Furthermore, applications of NHC precur-
sors as organocatalysts in many organic transformations
have rapidly increased [17, 18].
Table 1 shows the yields and uncorrected melting points
of imidazolium salts. According to the table, the salts ob-
tained from protected kojic acid, 2b, 3b, as well as ionic de-
rivatives bearing phenyl group, 3d-f, melt below 100 ºC or
near room temperature. Due to the influence of miscibility of
these salts in their applications, we also examined the misci-
bility of all synthesized salts in water and conventional or-
ganic solvents (Table 2). As shown in the table, good misci-
bility in water and methanol was observed for these salts.
Moreover, the imidazolium salts can serve as ionic liq-
uids as environmentally benign solvents due to their interest-
ing properties such as thermal stability, non-flammability,
very low vapor pressure and reusability [19, 20]. 4-Pyrones
and their heterocyclic derivatives constitute an important
class of biologically active natural and synthetic products,
continuing to attract interest [21-24]. Since a variety of het-
erocycles with tethered imidazolium or benzimidazolium
groups have been synthesized and used as carbene ligands,
[25-29], and through our recent attempts for the synthesis of
heterocyclic and polyfunctional frameworks of 4-pyrones
[30, 31], we were interested in the preparation of imida-
zolium and benzimidazolium salts based on 4-pyrone struc-
tures. Herein we report synthesis of some of these salts start-
ing from various halomethyl derivatives of 4-pyrones.
We
then
examined
the
reactions
of
N-
alkylbenzimidazoles with 2-halomethyl-4-pyrones (Scheme
1). At first, treatment of 1a,c with N-alkylbenzimidazoles in
boiling acetonitrile resulted in trace amounts of salts even
after extended time. But the addition of potassium iodide
(KI, 1.1 eq.) to the mixture, led to the progress of the reac-
tions and the benzimidazolium salts 2g-l were obtained after
10 h at 70oC in good yields (Table 3). Employment of KI as
activator for Nꢀ-alkylation of N-alkylimidazoles with alkyl-
chlorides has also been reported [6]. The utilized N-
alkylbenzimidazoles, were prepared from alkylation of ben-
zimidazole according to the reported procedures [43, 44].
RESULTS AND DISCUSSION
Schemes 1 and 2 show the substitution reactions of ha-
lomethyl derivatives 1a-f. These starting compounds were
In order to obtain more interesting compounds from the
standpoint of coordination chemistry, we then decided to
synthesize the 4-pyrone-bridged bis imidazolium and ben-
zimidazolium salts. For this purpose we used 2,6-bis [(4-
bromomethyl)phenyl)]-4-pyrone 4 [45]. Again, treatment of
*Address correspondence to this author at the Department of Organic and
Biochemistry, Faculty of Chemistry, University of Tabriz, Iran; Tel:
+984113393118; Fax: +984113340191; E-mail: ashahrisa@yahoo.com
1875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

678 Letters in Organic Chemistry, 2012, Vol. 9, No. 9
Ghasemi et al.
O
O
O
O
R2
R3
R1
R2
R3
R1
N
N
R
AgBF4 / H2O
rt
2a-d
N
N
CH3CN / 25 oC / 24 h
R=Me
+
X
+
O
N
BF4
N
R
R2
R3
R1
X
R
N
N
O
O
3a-d
R2
R1
R
1a-d
2g-l
R= Me, Et, Bu
R3
O
CH3CN / KI / 70 oC / 10 h
N
+
I
N
R
Scheme 1. Reactions of N-methylimidazole and N-alkylbenzimidazoles with 2-halomethyl-4-pyrones and anion exchanging of imidazolium
products.
O
O
O
R2
R3
R1
R2
R3
R1
R2
R3
R1
N
N
CH3CN
AgBF4 / H2O
rt
+
O
25 oC, 25 h
O
O
CH3
BF4
N
+
N
+
X
X
N
3e,f
N
2e,f
1e,f
H3C
H3C
Scheme 2. Reactions of N-methylimidazole with 2-[(4-bromomethyl)phenyl]-4-pyrones and anion exchanging of imidazolium products.
Table 1. Synthesis of Imidazolium Salts 2a-f and 3a-f.
2a
3b
Entry
R1
R2
R3
X
1
(Mp oC, Yield %)
(Mp oC, Yield %)
1
2
3
4
5
6
H
H
OH
H
H
Cl
Cl
Br
Br
Br
Br
1a
1b
1c
1d
1e
1f
2a (237-239, 88)
2b (93-95, 83)
3a (132-134, 92)
3b (31-32, 90)
3c (123-124, 92)
3d (98-99, 90)
3e (91-93, 89)
3f (60-62, 91)
OCH2Ph
H
H
Ph
H
Ph
2c (218-220, 85)
2d (190-191, 83)
2e (177-179, 88)
2f (138-139, 86)
Ph
H
CH3
CH3
Ph
H
H
Conditions: a 1-methylimidazole (2.2 mmol), 1a-f (2 mmol), acetonitrile (15 ml), r.t., 24 h. b: 2a-f (1 mmol), AgBF4 (2 mmol), H2O (14 ml), r.t., 1 h.
dibromide 4 with N-methylimidazole in acetonitrile at 25 ºC
for 24 h, resulted in the formation of 6a (Scheme 3). For
synthesis of the corresponding bis-benzimidazolium salts,
we used an indirect route. Treatment of 4 with 2 equiv. of
benzimidazole in the presence of sodium hydride in THF at
60°C for 24 h gave the bis(benzimidazolyl) derivative 5 in
90% yield (Scheme 3). Reactions of 5 with alkyl halides
such as methyl iodide, ethyl bromide and n-butyl bromide in
hot acetonitrile, again resulted in the formation of trace
amounts of salts even after prolonged time. Addition of po-
tassium iodide (2.0 eq.) to the reaction mixture, expedited
the reactions and the bis-benzimidazolium iodides 6b-d were

Synthesis of Novel Imidazolium and Benzimidazolium Salts
Letters in Organic Chemistry, 2012, Vol. 9, No. 9
679
Table 2. Miscibility of Imidazolium Salts 2a-f, 3a-f.
Miscibility in Solvents
Salt
H2O
MeOH
CH3CN
CHCl3
EtOAc
THF
2a
2b
2c
2d
2e
2f
m
m
m
m
m
m
m
m
m
m
m
m
m
m
pm
pm
pm
m
im
im
im
pm
im
im
pm
im
pm
m
im
im
im
pm
im
im
pm
im
pm
m
im
im
im
im
im
im
im
im
im
im
im
im
m
m
m
pm
pm
m
m
3a
3b
3c
3d
3e
3f
m
m
m
m
m
pm
pm
m
m
m
pm
im
pm
im
m
m: miscible, pm: partly miscible, im: immiscible
O
O
O
N
N
H
O
NaH / THF / 60 oC / 24 h
90%
BrH2C
CH2Br
N
N
N
4
N
5
25 oC, 24 h
CH3CN
N
N
RX / KI
CH3CN / 70 oC
R
R=Me
O
O
O
O
N
N
N
N
N
2 Br-
2
I-
N
N
R
N
R
R
R
6a
6b-d
Scheme 3. Synthesis of 4-pyrone-bridged bis imidazolium and benzimidazolium salts.
CONCLUSION
achieved at 70oC in 74-84% yields (Scheme 3). Table 4
shows the yields and melting points of the 4-pyrone-bridged
salts.
In conclusion, we have designed and synthesized a series
of novel imidazolium and benzimidazolium salts containing

680 Letters in Organic Chemistry, 2012, Vol. 9, No. 9
Ghasemi et al.
[4]
Kim, S.Y.; Kim. S.; Park, M.J. Enhanced proton transport in nanos-
tructured polymer electrolyte/ionic liquid membranes under water-
free conditions. Nat. Commun., 2010, 1-7.
Sun, P.; Armstrong, D.W. Ionic liquids in analytical chemistry.
Analytica Chimica Acta, 2010, 661, 1-16.
Oertel, A.M.; Ritleng, V.; Chetcuti, M.J. Nꢀ-activation of N-
arylimidazoles: facile syntheses of N-alkyl-Nꢀ-arylimidazolium io-
dides from less expensive chloro substrates. Synthesis, 2009, 1647-
1650.
Shibata, N.; Okamoto, M.; Yamamoto, Y.; Sakaguchi, S. Reversal
of stereoselectivity in the Cu-catalyzed conjugate addition reaction
of dialkylzinc to cyclic enone in the presence of a chiral azolium
compound. J. Org. Chem., 2010, 75, 5707-5715.
Table 3. Synthesis of Benzimidazolium Salts 2g-l.
Entry
1
R
2 (Mp oC, Yield %)
[5]
[6]
1
2
3
4
5
6
1a
1c
1a
1c
1a
1c
Me
Me
Et
2g (200-202, 88)
2h ( 199-201, 67)
2i (150-152, 68)
2j (150-152, 73)
2k ( 158-160, 90)
2l ( 218-220, 87)
[7]
Et
Bu
Bu
[8]
Hahn, F.E.; Jahnke, M.C. Heterocyclic carbenes: synthesis and
coordination chemistry. Angew. Chem. Int. Ed., 2008, 47, 3122-
3172.
[9]
Diez-Gonzalez, S.; Marion, N.; Nolan, S.P. N-Heterocyclic car-
benes in late transition metal catalysis. Chem. Rev., 2009, 109,
3612-3676.
Conditions: 1a,c (2 mmol), N-alkylbenzimidazoles (2.2 mmol), acetonitrile (15 ml), KI
(2.2 mmol), 70°C, 10 h.
[10]
[11]
Teyssot, M.L.; Jarrousse, A.S.; Manin, M.; Chevry, A.; Roche, S.
Metal-NHC complexes: a survey of anti-cancer properties. Dalton
Trans., 2009, 6894-6902.
Gulcemal, S.; Kahraman, S.; Daran, J.C.; Cetinkaya, E. The syn-
thesis of oligoether- substituted benzimidazolium bromides and
their use as ligand precursors for the Pd- catalyzed Heck coupling
in water. J. Organometal. Chem., 2009, 694, 3580-3589.
Demir, S.; Ozdemir, I.; Cetinkaya, B. Synthesis and catalytic activ-
ity of novel xylyl-linked benzimidazolium salts. Appl. Or-
ganometal. Chem., 2009, 23, 520-523.
Zou, G.; Huang, W.; Xiao, Y.; Tang, J. Heck reaction catalysed by
palladium supported with an electron-rich benzimidazolylidene
generated in situ: remarkable ligand electronic effects and control-
lable mono- and di-arylation. New J. Chem., 2006, 30, 803-809.
Altava, B.; Burguete, M.I.; Karbass, N.; Luis, S.V. Palladium N-
methylimidazolium supported complexes as efficient catalysts for
the Heck reaction. Tetrahedron Lett., 2006, 47, 2311.
Yiꢁit, B.; Yiꢁit, M.; Özdemir, I.; Çetinkaya, E. The Synthesis of
Some Benzimidazolium Salts and Use as Carbene Precursors in the
Heck and Suzuki Reactions. Heterocycles, 2010, 81, 943-953.
Doꢁan, Ö.; Gürbüz, N.; Özdemir, ꢂ.; Çetinkaya, B.; ahin, O. Syn-
thesis, characterization and catalytic activity of novel N-
heterocyclic carbene-palladium complexes. Dalton trans., 2009,
35, 7087-7093.
Table 4. Synthesis of Salts 6a-d.
Entry
R
6 (Mp oC, Yield %)
1
2
3
4
Me
Me
Et
6a (209-211, 74)
6b (198-200, 74)
6c (238-240, 80)
6d (278-280, 84)
[12]
[13]
Bu
Conditions: for 6a: dibromide 4 (1 mmol), 1-methylimidazole (1.1 mmol), acetonitrile
(15 ml), r.t., 24 h., for 6b-d: compound 5 (2 mmol), alkylhalides (12 mmol), KI (4
mmol), acetonitrile (10 ml), 70°C, 48 h.
[14]
[15]
[16]
a 4-pyrone moiety, by reaction of different halomethyl de-
rivatives of 4-pyrones with N-methyl imidazole and N-
methyl benzimidazoles, followed by anion exchanging of
imidazolium salts with AgBF4. These salts can be used as
novel precursors of N-heterocyclic carbenes and some of
them as novel ionic liquids. Solubility of all imidazolium
salts has been studied in water and conventional organic sol-
vents and remarkable results have been obtained. We have
also synthesized the 4-pyrone bridged bis-imidazolium and
benzimidazolium salts, from dibromide 4 in one or two steps
respectively. Spectroscopic data of the selected salts are re-
ported [47-49].
[17]
Shimakawa, Y.; Morikawa, T.; Sakaguchi, S. Facile route to
benzils from aldehydes via NHC-catalyzed benzoin condensation
under metal-free conditions. Tetrahedron Lett., 2010, 51, 1786-
1789.
Enders, D.; Niemeier, O.; Henseler, A. Organocatalysis by N-
Heterocyclic carbenes. Chem. Rev., 2007, 107, 5606-5655.
Plechkova, N.V.; Rogers, R.D.; Seddon, K.R. Ionic liquids: from
knowledge to application (Book Review). J. Am. Chem. Soc., 2010,
132, 12518.
[18]
[19]
[20]
[21]
Giernoth, R. Task-specific ionic liquids. Angew. Chem. Int. Ed.,
2010, 49, 2834-2839.
Wilk, W.; Waldmann, H.; Kaiser, M. ꢃ-Pyrone natural products-A
privileged compound class provided by nature. Bioorg. Med.
Chem., 2009, 17, 2304-2309.
CONFLICT OF INTEREST
The author(s) confirm that this article content has no con-
flicts of interest.
[22]
[23]
Yan, Y.L.; Cohen, S.M. Efficient Synthesis of 5-amido-3-hydroxy-
4-pyrones as inhibitors of matrix metalloproteinases. Org. Lett.,
2007, 9, 2517-2520.
ACKNOWLEDGEMENTS
South, M.S., Ma, C.C.; Koeller, K.J., Rahman, H.K.; Neumann,
W.L. Polycyclic aryl and heteroaryl substituted 4-pyrones useful
for selective inhibitors of the coagulation cascade. U.S. Patent
6,916,847 B2, July 12, 2005, Chem. Abstr, 2001, 135, 303776e.
Sengoku, T.; Takemura, T.; Fukasawa, E.; Kigoshi, H. Aldol-type
reaction of a 4-pyrone: a straightforward approach to 4-pyrone con-
taining natural products. Tetrahedron Lett., 2009, 50, 325-328.
Jahnke, M.C.; Hussain, M.; Hupka, F.; Cavell, K.J. Synthesis of
pyrazine-bridged diimidazolium salts and their application in palla-
dium catalyzed Heck-type coupling reactions. Tetrahedron, 2009,
65, 909-913.
Zhang, X.; Gu, S.; Xia, Q.; Chen, W. New structural motifs of
silver and gold complexes of pyridine-functionalized benzimida-
zolylidene ligands. J. Organometal. Chem., 2009, 694, 2359-2367.
Lee, H.M.; Chiu, P.L.; Hu, C.H.; Lai, C.L. Synthesis and structural
characterization of metal complexes based on pyra-
We thank research affair of University of Tabriz for fi-
nancial support.
[24]
[25]
REFERENCES
[1]
[2]
[3]
Lee, J.P.; Yoo, B.; Suresh, T.; Kim, K.J. Novel silane-substituted
benzimidazolium iodide as gel electrolute for dye-sensitized solar
cells. Electrochimica Acta, 2009, 54, 4365-4370.
Xia, M.; Shi, C.; Sun, R.; Cai, M.; Liu, Z. Solvent-free synthesis of
alkylbenzimidazolium iodides and their applications in dye-
sensitized solar cells. Solar Energy, 2010, 84, 1828-1832.
Shimakawa, Y.; Morikawa, T.; Sakaguchi, S. Facile route to ben-
zyls from aldehydes via NHC-catalyzed benzoin condensation un-
der metal-free conditions. Tetrahedron Lett., 2010, 51, 1786-1789.
[26]
[27]

Synthesis of Novel Imidazolium and Benzimidazolium Salts
Letters in Organic Chemistry, 2012, Vol. 9, No. 9
681
zole/imidazolium chlorides. J. Organometal. Chem., 2005, 690,
[45]
2,6-bis[(4-bromomethyl)phenyl)]-4-pyrone was prepared by
4
403-414.
benzylic bromination of 2,6-bis(4-methylphenyl)-4-pyrone in the
presence of NBS (2 eq.) and catalytic amounts of benzoyl peroxide
(BPO) in boiling CCl4 for 48 h [46]. 2,6-Bis(4-methylphenyl)-4-
pyrone was itself prepared through dehydrative cyclization of 1,5-
bis(4-methylphenyl)-1,3,5-pentane-trione in the presence of conc.
H2SO4 [38-41].
Marei, M.G.; Mishrikey, M.M.; El-Kholy, I.E.S. Synthesis of a
new series of 4-pyrones. Indian J. Chem., 1987, 26B, 163-5. Chem.
Abstr., 108, 5817e.
[28]
[29]
Nielsen, D.J.; Cavell, K.J., Viciu, M.S., Nolan, S.P. Furan- and
thiophene-functionalised bis-carbene ligands: Synthesis, silver(I)
complexes, and catalysis. J. Organometal. Chem., 2005, 690, 6133-
6142.
Scheele, U.J.; Dechert, S.; Meyer, F. A versatile access to pyridazi-
nes with tethered imidazolium groups-new precursors for mono-
and binucleating NHC/pyridazine hybrid ligands. Tetrahedron
Lett., 2007, 48, 8366-8670.
Shahrisa, A.; Ghasemi, Z.; Saraei, M. Synthesis of 2,6-Bis(1H-
indole-6-yl)-4H-pyran-4-ones via Leimgruber–Batcho Indole Syn-
thesis. J. Heterocycl. Chem., 2009, 46, 273-277.
Shahrisa, A.; Ghasemi, Z. Synthesis of fused pyrimidone deriva-
tives of 4-pyrones from the acetates of Baylis-Hillman adducts.
Chem. Hetrocycl. Comp., 2010, 46, 30-36.
2-Chloromethyl-5-hydroxy-4-pyrone 1a (chlorokojic acid) and 5-
bezyloxy-2-chloromethyl-4-pyrone 1b were prepared from chlori-
nation of commercially available kojic acid and kojic acid-benzyl
ether respectively according to the literature [33]. Kojic acid-
benzyl ether itself was achieved by benzylation of phenolic OH of
kojic acid [34]. 2-Bromomethyl-6-phenyl-4-pyrone 1c was synthe-
sized by heating of 2-hydroxymethyl-6-phenyl-4-pyrone in
HBr/H2SO4 at 70 ꢀC for 18 h which itself was prepared from reduc-
tion of the corresponding ethoxycarbonyl derivative [35]. Bromides
1d-f were obtained from benzylic bromination of the corresponding
4-pyrones in the presence of NBS (1 eq.) and catalytic amounts of
benzoyl peroxide (BPO) in boiling CCl4 for 48 h [36]. 2,6-
Dimethyl-3,5-diphenyl-4-pyrone, required for preparation of 1d,
was itself prepared according to the reported procedure [37]. The
starting 4-pyrones, required for preparation of 1e,f, were achieved
through dehydrative cyclization of related 1,3,5-triketones in the
presence of conc. H2SO4, as a significant procedure for the synthe-
sis of a variety of 4-pyrone structures [38-41].
Tam, T. F.; Li, W. Processes for the manufacturing of 3-hydroxy-
N,1,6-trialkyl-4-oxo-1,4-dihydropyridine-2-carboxamide.
U.S.Patent 6,426,418 B1, July 30, 2002, Chem. Abstr, 137, 12089q.
Kazuo, M.; Masanao, S.; Shoichiro, O.; Yutaka, W. Ascorbic acid-
Kojic acid conjugate and production. Jpn. Patent JP08081462,
March 26, 1996. Chem. Abstr., 125, 58988u.
Rudas, M.; Fejes, I.; Nyerges, M.; Toke, L. Substituent effects on
the 4ꢁ + 2ꢁ cycloadditions of 4H-pyran-4-one derivatives. J. Chem.
Soc. Perkin Trans. 1, 1999, 9, 1167-1172.
Shahrisa. A.; Saraei. M. Synthesis of pyrone carbaldehydes, pyrone
sulfonium ylides and related epoxides. J. Heterocycl. Chem., 2009,
46, 268-272.
Emmick, T.L.; Latsinger, R.L. 2,6-Dimethyl-3,5-diphenyl-4H-
pyran-4-one. Organic synthesis, 1968, collective vol. v. Wiley,
450-452.
Sakakura, A.; Watanabe, H.; Ishihara, K. Rate-accelerating effect
by the neighboring-group participation of protecting groups in the
dehydrative cyclization of 1,3,5-triketones. Org. Lett., 2008, 10,
2569-2572.
Tyvorskii, V.I.; Bobrov, D.N.; Kulinkovich, O.G. New synthetic
approaches to 2-perfluoroalkyl-4H-pyran-4-ones. Tetrahedron,
1998, 54, 2819-2826.
Arimoto, H.; Nishiyama, S.; Yamamum, S. Mild conditions for
cyclization of ꢂ-triketides to the corresponding ꢃ-pyrones carrying
adjacent chiral centers toward biomimetic synthesis of fully substi-
tuted ꢃ-pyrone-containing natural products. Tetrahedron Lett.,
1990, 31, 5619-5620.
Shahrisa, A.; Hemmati, S. Chemistry of pyrones: Part I-Synthesis
of triketones and triketoesters and related 4H-pyran-4-ones. Indian
J. Chem., 2000, 39B, 190-197.
Wilkes, J.S.; Zaworotko, M.J. Air and water stable 1-ethyl-3-
methylimidazolium based ionic liquids. J. Chem. Soc., Chem.
Commun., 1992, 965-967.
Liu, Q.X.; Zhao, X.J.; Wu, X.M.; Guo, J.H. New mercury(II) and
silver(I) complexes containing NHC metallacrown ethers with the
ꢁ–ꢁ stacking interaction J. Organometal. Chem., 2007, 692, 5671-
5679.
[46]
[47]
[30]
[31]
[32]
General procedure for preparation of compounds 2a-f and 3a-f. To
a stirred solution of 1-methylimidazole (0.18 gr, 2.2 mmol) in dry
acetonitrile (5 ml) was added dropwise a solution of bromomethyl
derivative 1a-f (2 mmol) in dry acetonitrile (10 ml). The reaction
mixture was stirred at room temperature for 24 h and then concen-
trated under reduced pressure. Ethyl acetate (20 ml) was added to
the mixture and after stirring for 10 min, the precipitated solid was
filtered. The solid product washed twice with EtOAc (10 ml) to
remove remaining starting materials and then was kept at 45 ˚C un-
der vacuum for 12 h. Imidazolium bromides 2a-f were produced in
83-88% yields. For preparation of salts 3a-f, a solution of AgBF4
(2 mmol) in distilled water (7 ml) was added to a solution of imida-
zolium salts 2a-f (1 mmol) in H2O (7 ml)( as well as 2 ml acetoni-
trile in the case of 2d,e). The mixture was stirred at rt for 1 h and
then white precipitate was filtered. The filtrate was allowed to
stand overnight at room temperature. The solution was passed
through filter paper to removed small amounts of solid impurities.
Water was removed under reduced pressure by rotary evaporator
and acetonitrile (10 ml) was added to the residue. Insoluble black
precipitate was filtered and the filtrate was dried over MgSO4. Ace-
tonitrile was removed under reduced pressure. Imidazolium tetra-
fluoroborates 3a-f were obtained in 89-92% yields. Data for 1-[(5-
Hydroxy-4-oxo-4H-pyran-2-yl)methyl]-3-Methyl-imidazolium
tetrafluoroborate 3a: Gray solid, yield 92%, mp 132-134˚C; FT-
IR(KBr), ꢄ: 3223 (OH), 3135, 3085, 2987, 2943, 1648, 1616, 1445,
1270 cm-1; 1H NMR(400 MHz, D2O), ꢀ: 3.88 (s, 3H, N-CH3), 5.48
(s, 2H, CH2-N), 6.52 (s, 1H, H-3 pyrone), 7.78 (s, 1H, imida-
zolium-H), 7.84 (s, 1H, imidazolium-H), 9.34 (s, broad, 1H, OH),
8.09 (s, 1H, H-6 pyrone), 9.35 (s, 1H, H-2 imidazolium) ppm; 13C
NMR(100 MHz, D2O), ꢀ: 36.0, 48.8, 113.5, 122.7, 124.1, 137.5,
140.1, 146.1, 159.5, 173.6 ppm; Elemental analysis: calculated for
C10H11BF4N2O3, C 40.84, H 3.74, N 9.53; found, C 40.63, H 3.55,
N 9.78. Data for 1-[(5-Benzyloxy-4-oxo-4H-pyran-2-yl)methyl]-3-
Methyl-imidazolium tetrafluoroborate 3b, Bright brown solid, yield
90%, mp 31-32˚C; FT-IR(KBr), ꢄ: 3098, 2930, 1648, 1623, 1434,
1220 cm-1; 1H NMR (400 MHz, D2O), ꢀ: 3.87 (s, 3H, N-CH3), 4.92
(s, 2H, OCH2Ph), 5.47 (s, 2H, CH2-N ), 6.52 (s, 1H, H-3 pyrone),
7.33-7.39 (m, 5H, Ph-H), 7.75 (s, 1H, imidazolium-H), 7.82 (s, 1H,
imidazolium-H), 8.22 (s, 1H, H-6 pyrone), 9.31 (s, 1H, H-2 imida-
zolium), ppm; 13C NMR(100 MHz, D2O), ꢀ: 36.0, 48.6, 70.6,
114.6, 122.7, 124.1, 128.1, 128.3, 128.5, 135.9, 137.6, 141.5,
146.9, 159.8, 172.8 ppm; Elemental analysis: calculated for
C17H17BF4N2O3, C 53.15, H 4.43, N 7.29; found, C 53.42, H 4.31,
N 7.42.
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[48]
General procedure for preparation of compounds 2g-l. To a stirred
mixture of 2-halomethyl-4-pyrones 1a or 1c (2 mmol) and potas-
sium iodide (2.2 mmol), in dry acetonitrile (10 mL), was added
dropwise a solution of N-alkylbenzimidazoles (alkyl=Me, Et, n-Bu)
(2.2 mmol) in dry acetonitrile (5 ml). The mixture was heated at
70°C for 10 h and then concentrated by rotary evaporator. Ethyl
acetate (20 mL) was added to the residue and after stirring for 10
min, the precipitated product was filtered, washed with ethyl ace-
tate (2ꢅ10 mL) and dried in vacuo to give the products 2g-l. Data
[41]
[42]
[43]
for
1-[(5-Hydroxy-4-oxo-4H-pyran-2-yl)methyl]-3-Methyl-
benzimidazolium iodide 2g, White solid, yield 88%, mp 200-202
°C; FT-IR (KBr), ꢄ: 3423 (OH), 3142, 3086, 2987, 1644 (pyrone
1
C=O), 1617, 1585, 1364 cm-1; H NMR (400 MHz, DMSO-d6), ꢆ:
4.11 (3H, s, N-CH3), 5.83 (2H, s, CH2-N), 6.68 (1H, s, H-3 py-
rone), 7.69-7.73 (2H, m, H-5,6 benzimidazolium), 8.04-8.13 [3H,
m, (2H, H-4,7 benzimidazolium), (1H, H-6 pyrone)], 9.43 (1H, s,
OH), 9.91 (1H, s, H-2 benzimidazolium) ppm; 13C NMR (100
MHz, DMSO-d6), ꢆ: 33.7, 46.9, 113.5, 113.6, 114.0, 126.9, 127.2,
130.9, 131.9, 140.3, 143.6, 146.2, 159.2, 173.8 ppm; Elemental
analysis: calculated for C14H13IN2O3, C 43.76, H 3.39, N 7.29;
found, C 44.08, H 3.27, N 7.33. Data for 1-Methyl-3-[(4-oxo-6-
phenyl-4H-pyran-2-yl)methyl]-benzimidazolium iodide 2h, White
[44]
Starikova, O.V.; Dolgushin, G.V.; Larina, L.I.; Komarova, T.N.
Synthesis of new stable carbenes from the corresponding 1,3-
dialkylimidazolium and benzimidazolium salts ARKIVOC, 2003,
(xiii), 119-124.

682 Letters in Organic Chemistry, 2012, Vol. 9, No. 9
Ghasemi et al.
solid, yield 67%, mp 199-201 °C; FT-IR (KBr), ꢀ: 3390 (OH),
3038, 2898, 1654 (pyrone C=O), 1622, 1567, 1451, 1194 cm-1; 1H
NMR (400 MHz, DMSO-d6), ꢁ: 4.12 (3H, s, N-CH3), 5.93 (2H, s,
CH2-N), 6.62 (1H, d, J=1.6 Hz, pyrone-H), 6.91 (1H, d, J=1.9 Hz,
pyrone-H), 7.46-7.56 (3H, m, phenyl-H), 7.71-7.77 (2H, m, H-5,6
benzimidazolium), 7.80 (2H, d, J=7.3 Hz, phenyl-H), 8.05 (1H, d,
J=6.4 Hz, H-7 benzimidazolium), 8.23 (1H, d, J=7.8 Hz, H-4 ben-
zimidazolium), 9.96 (1H, s, H-2 benzimidazolium) ppm; 13C
NMR(100 MHz, DMSO-d6), ꢁ: 33.8, 47.0, 110.9, 113.7, 114.0,
115.1, 126.2, 126.9, 127.1, 129.2, 130.4, 131.2, 131.9, 132.0,
143.7, 160.3, 163.3, 178.8 ppm; Elemental analysis: calculated for
C20H17IN2O2, C 54.07, H 3.83, N 6.31; found, C 54.34, H 3.75, N
6.37.
5.65 (4H, s, CH2-N), 6.96 (2H, s, pyrone-H), 7.24-7.26 (4H, m, H-
5,6 benzimidazole), 7.45 (4H, d, J=8.0, aryl-H), 7.55-7.57 (2H, m,
H-7 benzimidazole), 7.69-7.71 (2H, m, H-4 benzimidazole), 8.00
(4H, d, J=8.0, aryl-H), 8.65 (2H, s, H-2 benzimidazole) ppm; 13C
NMR (100 MHz, DMSO-d6), ꢁ: 47.3, 110.8, 110.9, 119.5, 121.9,
122.7, 126.5, 128.0, 130.3, 133.6, 140.4, 143.4, 144.3, 161.9, 178.9
ppm. Elemental analysis: calculated for C33H24N4O2, C 77.94, H
4.76, N 11.02; found, C 77.68, H 4.73, N 11.21. General procedure
for preparation of compounds 6b-d, A stirred mixture of compound
5 (1.0 g, 1.97 mmol), fresh distilled alkyl halide (12 mmol), potas-
sium iodide (0.66 g, 4 mmol), in dry acetonitrile (10 ml), was
heated at 70°C for 48 h. The mixture was then concentrated by ro-
tary evaporator and ethyl acetate (20 ml) was added to the residue.
After stirring for 10 min, the precipitated product was filtered,
washed with ethyl acetate (3ꢃ20 ml) and dried in vacuo to give the
products 6b-d. Data for 1,1ꢂ-Dimethyl-3,3ꢂ-{[4-(2,6-diyl-4-oxo-
4H-pyran)diphenyl]dimethyl}-dibenzimidazolium diiodide 6b,
Yellow solid, 74% yield; mp 198-200 °C. FT-IR (KBr), ꢀ: 3046,
[49]
Preparation
pyran)diphenyl]dimethyl}-dibenzimidazolium diiodide 6a was
similar to 2a-f. Preparation of 2,6-Bis [4-(1-
of
1,1ꢂ-Dimethyl-3,3ꢂ-{[4-(2,6-diyl-4-oxo-4H-
benzimidazolylmethyl)phenyl]-4H-pyran-4-one 5: To a stirred sus-
pension of sodium hydride 80% (0.25 g, 8.5 mmol) in THF (15 ml)
was added a solution of benzimidazole (1.0 g, 8.5 mmol) in THF
(10 mL) under Ar. After stirring at room temperature for 0.5 h, a
1
2996, 1644 (pyrone C=O), 1604, 1566, 1456, 1259, 947 cm-1; H
NMR (400 MHz, DMSO-d6), ꢁ: 4.12 (6H, s, N-CH3), 5.89 (4H, s,
CH2-N), 7.05 (2H, s, pyrone-H), 7.63-7.72 [8H, m, (4H, aryl-H),
(4H, H-5,6 benzimidazolim)], 7.93 (2H, d, J=8.0 Hz, H-7 benzimi-
dazolim), 8.05-8.09 [6H, m, (4H, aryl-H), (2H, H-4 benzimida-
zolim)], 9.88 (2H, s, H-2 benzimidazolium) ppm; 13C NMR (400
MHz, DMSO-d6), ꢁ: 33.5, 49.3, 111.4, 113.6, 113.9, 126.7, 126.8,
128.9, 130.7, 131.2, 132.1, 137.4, 143.2, 162.0, 179.0 (pyrone
C=O) ppm; F Elemental analysis: calculated for C35H30I2N4O2, C
53.05, H 3.79, N 7.07; found, C 53.32, H 3.63, N 6.76.
solution of 2,6-bis[4-(bromomethyl)phenyl]-4H-pyran-4-one
4
(1.82 g, 4.2 mmol) in THF (20 ml) was added dropwise to the mix-
ture. The mixture was stirred at 60 °C under Ar for 24 h, and then
concentrated by a rotary evaporator. Water (150 ml) was added to
the residue and after stirring, the mixture was left at rt for 24 h. The
precipitate was filtered, washed with acetone (10 ml) and dried in
vacuo to give 1.9 g, 90% yield of 5 as white solid, mp 250-252 °C;
FT-IR (KBr), ꢀ: 3115, 2972, 2861, 1643 (pyrone C=O), 1600,
1
1422, 1383, 1259, 944 cm-1; H NMR (400 MHz, DMSO-d6), ꢁ: