[BPy]HSO4 acidic ionic liquid as a novel, efficient, and environmentally benign catalyst for synthesis of 1,5-benzodiazepines under mild conditions

Article abstract of DOI:10.1080/00397910600616602

A novel and simple ionic liquid methodology for the synthesis of 1,5-benzodiazepines is described. 1-Butylpyridinium hydrogen sulphate ([BPy]HSO₄), an acidic room-temperature ionic liquid, as a novel and efficient catalyst, was synthesized and

Full text of DOI:10.1080/00397910600616602

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[BPy]HSO4 Acidic Ionic Liquid as a Novel,
Efficient, and Environmentally Benign
Catalyst for Synthesis of 1,5‐Benzodiazepines
under Mild Conditions
Yuying Du a , Fuli Tian a & Wenzhi Zhao a
a Department of Chemistry, College of Chemistry and Chemical Engineering,
Inner Mongolia University, Hohhot, China
Version of record first published: 22 Aug 2006
To cite this article: Yuying Du, Fuli Tian & Wenzhi Zhao (2006): [BPy]HSO4 Acidic Ionic Liquid as a Novel,
Efficient, and Environmentally Benign Catalyst for Synthesis of 1,5‐Benzodiazepines under Mild Conditions,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
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Synthetic Communicationsw, 36: 1661–1669, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600616602
[BPy]HSO4 Acidic Ionic Liquid as a Novel,
Efficient, and Environmentally Benign
Catalyst for Synthesis of 1,5-
Benzodiazepines under Mild Conditions
Yuying Du, Fuli Tian, and Wenzhi Zhao
Department of Chemistry, College of Chemistry and Chemical
Engineering, Inner Mongolia University, Hohhot, China
Abstract: A novel and simple ionic liquid methodology for the synthesis of 1,5-benzo-
diazepines is described. 1-Butylpyridinium hydrogen sulphate ([BPy]HSO4), an acidic
room-temperature ionic liquid, as a novel and efficient catalyst, was synthesized and
used in the preparation of a series of 1,5-benzodiazepine derivatives by the reaction
of o-phenylenediamine with chalcones under mild conditions. This method is easy,
efficient, environmentally friendly, economical, free of toxic catalysts, and has good
yields for the formation of 1,5-benzodiazepines
Keywords: Acidic counterion, 1,5-benzodiazepines, 1-butylpyridinium hydrogen
sulpates, room-temperature ionic liquid
INTRODUCTION
The synthesis of 1,5-benzodiazepines and their derivatives have attracted con-
siderable attention from researchers, including pharmaceutical and organic
synthetic chemists, in recent years because their medicinal properties as
anti-anxiety, hypnotic, antidepressive, tranquilizing, anti-inflammatory, antic-
onvulsant, antifeedant, antibacterial, and analgesic agents.[1,2] In addition,
benzodiazepines derivatives are also used in industry as dyes for acrylic
Received in Japan August 25, 2005
Address correspondence to Fuli Tian, Department of Chemistry, College of
Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021,
China. Tel.: þ 86-471-4993443; E-mail: ceac@imu.edu.cn
1661

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Y. Du, F. Tian, and W. Zhao
fibers[3] in photography. Moreover, 1,5-benzodiazepines are valuable
synthons used for the preparation of other fused ring compounds such as
triazolo, oxazino, and furano-benzodiazepines.[4] Thus, the synthesis of
these heterocyclic nuclei is still of much interest.
1,5-Benzodiazepines have commonly been synthesized by the reaction
of o-phenylenediamine with a,b-unsaturated carbonyl compounds, b-halo
ketones, or ketones. There are many methods for the preparation of 1,5-benzo-
diazepines in the literature, including BF3–etherate,[5] NaBH4,[6] polypho-
sphoric acid or SiO2,[7] Amberlyst-15[8], Yb(Otf)3,[9] MgO/POCl3,[10]
Al2O3/P2O5 or acetic acid under microwave irradiation,[11] TiCl4/Sm/THF
system,[12] [bbim]Br ionic liquid,[13] sulfated zirconia,[14] silica gel,[15] and
CeCl3/NaI/silica gel.[16] Many of these processes suffer from limitations
such as requiring harsh conditions, expensive reagents, high catalyst
loading, corrosive reagent, or toxic ions; low to moderate yields; and occur-
rence of several side reactions. It is also necessary to find a milder,
selective, nonhazardous, and inexpensive reagent.
Room-temperature ionic liquids (RTILs) have increasingly attracted
attention as the green, high-tech reaction media of the future.[17,18] ILs are
used as green solvents and have unique properties such as wide liquid
range, good solvating ability, tunable polarity, and high thermal stability.
As a result of their green credentials and potential to enhance rates and selec-
tivity, ILs based on imidazolium salts are finding increasing applications in
organic synthesis,[19] but there are no examples of synthesis and application
of ILs based on pyridinium and iso-quinolinium salts in the area of functiona-
lized acidic ILs, especially with hydrogen sulphate counteranion. However,
ILs based on imidazolium salts with HSO24 counteranion as catalyst have
been investigated many acid-catalyzed reactions.[20–22] To extend the scope
and decrease the cost of ILs, we synthesized and explored the utility of two
novel acidic ILs 1-butylpyridinium hydrogen sulphate ([BPy]HSO4) and
1-butyl-iso-quinolinium hydrogen sulphate ([iso-BQu]HSO4), as efficient
catalysts to promote the formation of 1,5-benzodiazepines. Herein, we
report a facile, efficient, and environmentally benign method for the prep-
aration of 1,5-benzodiazepine derivatives by condensation of o-phenylenedia-
mine with ketones using a catalytic amount of [BPy]HSO4 and [iso-
BQu]HSO4 acidic ILs under mild conditions (Schemes 1 and 2).
Scheme 1. Synthesis of 1,5-benzodiazepines from o-phenylenediamine and
chalcones.

[BPy] HSO4 Acidic Ionic Liquid
1663
Scheme 2. Acidic ionic liquids used in the present work.
EXPERIMENTAL
Melting points are uncorrected. IR spectra were recorded on a Shimadzu 8600
1
FT spectrometer in KBr with absorptions in cm21. H NMR spectra were
recorded on a Bruker AV400 spectrometer and TMS was used as internal
standard. Mass spectra were taken on a Bruker BiFlexIII mass spectrometer.
Elemental analyses were carried out on an EA 1110 instrument. Products
were characterized by comparison of their spectroscopic data (1H NMR, IR,
and mass) and meltine points with those reported in the literature.
Preparation of Ionic Liquids as Catalyst for 1,5-Benzodiazepines
[BPy]HSO4 and [iso-BQu]HSO4 derived from chloride salts were obtained by
a dropwise addition of 1 equiv of concentrated sulphuric acid (98%) to a cooled
solution of the corresponding 1-butylpyridinium or iso-quinolinium chloride
(1 equiv) in anhydrous methylene chloride. The mixture was refluxed for
48 h, and the HCl by-product formed in the reaction was distilled out of the
condenser and dissolved in 10% NaOH solution. When the formed HCl had
been completely removed, the solution was cooled to room temperature, and
methylene chloride was evaporated with a rotary evaporator. The ionic
liquid was dried under high vacuum at 808C for 2 h. After elimination of
water, it is recommended that it be handled under an inert atmosphere.
[BPy]HSO4: viscous oil, 1H NMR (400 MHz, d6-DMSO) dH (ppm): 0.86
(t, 3H), 1.25 (m, 2H), 1.87 (m, 2H), 4.63 (m, 2H), 7.61 (1H, br s, OH), 8.61
(m, 1H), 8.13 (m, 2H), 9.16 (m, 2H).
1
[iso-BQu]HSO4: viscous oil, H NMR (400 MHz, d6-DMSO) dH (ppm):
0.87 (t, 3H), 1.26 (m, 2H), 1.95 (m, 2H), 4.74 (m, 2H), 8.00–8.85 (m, 7H),
10.20 (1H, s).
Other protic acidic ionic liquids (Table 1) were synthesized and purified
according to the literature.[20]

1664
Y. Du, F. Tian, and W. Zhao
Table 1. Comparison of the effect of catalysts in the synthesis of 1,5-benzodiazepine
(a) from o-phenylenediamine and chalcone
Catalyst loading
(mol%)a
Ratio of
substrateb
Time
(h)
Yield
(%)
Entry
Catalyst
1
2
3
4
5
6
7
8
[Bpy][HSO4]
[iso-Bqu]HSO4
[HMIm]BF4
5
5
1.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3
3
3
3
3
3
3
3
69
84
43
59
57
44
87
88
5
[HMIm]PhSO3
[HMIm]CF3CO22
[HMIm]ClO4
[Bpy][HSO4]
[Bpy][HSO4]
5
5
5
5
10
aCalculated based on chalcone (M/M).
bRatio of o-phenylenediamine to chalcone.
General Procedure for the Synthesis of 1,5-Benzodiazepines
In a typical experiment, ionic liquid (0.05 mmol) was dissolved into the
mixture of chalcone (1.0 mmol) and o-phenylenediamine (1.5 mmol) in
ethyl acetate (3 mL) in a 25-mL round-bottom flask equipped with a distilla-
tion condenser. The reaction mixture was stirred vigorously at 808C for the
desired time under atmosphere. The completion of reaction was followed by
TLC using 20% EtOAc in petroleum ether as eluent. After completion,
methanol or ethanol was added to the reaction mixture, and the content was
filtered to afford the 1,5-benzodiazepines. The products, thus isolated, were
pure (single spot on TLC). They were subjected to further purification by
chromatography through a column of silica gel using n-hexane/ethyl
acetate (1:4) as eluent.
After reaction, it was possible to reuse the IL catalyst in another run after
washing twice with EtOAc or Et2O and eventually drying under high vacuum
at 70 8C for 0.5 h (for elimination of the unreacted starting products because of
their immiscibility with the ionic liquid).
SPECTROSCOPIC DATA FOR SELECTED PRODUCTS
2,3-Dihydro-2,4-diphenyl-1H-1,5-benzodiazpine (a): yellow solid, mp 129–
1
130 8C (lit.[23] 129–129.58C). H NMR (400 MHz, d6-DMSO), dH (ppm)
3.06 (dd, 1H, J ¼ 9.2 Hz, J ¼ 7.2 Hz), 3.38 (dd, 1H, J ¼ 8.4 Hz,
J ¼ 6.8 Hz), 3.66 (s, 1H), 5.88 (s, 1H), 6.83 (m, 1H), 7.00–7.42 (m, 11H),
7.77 (d, 2H, J ¼ 1.6 Hz, J ¼ 3.6 Hz). n/cm21 1609 (C55N), 3366 (N-H);
MS: m/z ¼ 298 (Mþ), 283, 221, 194, 119, 103, 91, 77, 65, 51; C21H18N2
(298.387): calcd. C, 84.53; H, 6.08; N, 9.39; found C, 84.76; H, 6.22; N, 9.01.

[BPy] HSO4 Acidic Ionic Liquid
1665
2,3-Dihydro-2-phenyl-4-(40-methylphenyl)-1H-1,5-benzodiazpine (b): yellow
1
solid, mp 126–1278C (lit.[23] 1278C). H NMR (400 MHz, d6-DMSO), dH
(ppm) 2.30 (s, 3H), 2.53 (m, 2H), 3.04 (m, 1H), 5.16 (s, 1H), 6.83-7.84
(m, 13H). n/cm21 1607 (C55N), 3360 (N-H); MS: m/z ¼ 312 (Mþ), 297,
246, 221, 208, 119, 103, 91, 77, 65, 51; C22H20N2 (312.413): calcd. C,
84.58; H, 6.45; N, 8.97; found C, 84.69; H, 6.28; N, 9.02.
2,3-Dihydro-2-phenyl-4-(40-nitrophenyl)-1H-1,5-benzodiazpine (e): yellow
1
solid, mp 104–1058C (lit.[23] 104–1058C). H NMR (400 MHz, d6-DMSO),
dH (ppm) 2.59 (m, 2H), 3.08 (m, 1H), 5.41 (s, 1H), 7.46–8.28 (m, 13H). n/
cm21 1600 (C55N), 3346 (N-H); MS: m/z ¼ 343 (Mþ), 328, 266, 240, 221,
194, 119, 103, 91, 77, 65, 51; C21H17N3O2 (343.385): calcd. C, 73.45; H,
4.99; N, 12.24; found C, 73.29; H, 5.16; N, 12.01.
2,3-Dihydro-2-(30,40-dichloropheny)l-4-phenyl-1H-1,5-benzodiazpine
(d):
1
yellow solid, mp 150–1518C. H NMR (400 MHz, d6-DMSO), dH (ppm)
2.49 (m, 2H), 3.02–3.18 (m, 1H), 5.26 (s, 1H), 6.97–7.77 (m, 13H). n/
cm21 1612 (C55N), 3341 (N-H); MS: m/z ¼ 366 (Mþ), 351, 289, 263, 221,
119, 103, 91, 77, 65; C21H16N2Cl2 (367.277): calcd. C, 68.68; H, 4.39; N,
7.63; found C, 68.79; H, 4.44; N, 7.80.
2,3-Dihydro-2-(40-methoxyphenyl)-4-phenyl-1H-1,5-benzodiazepine (h): yellow
1
solid, mp 145–1468C (lit.[23] 146–1478C). H NMR (400 MHz, d6-DMSO),
dH (ppm) 3.05 (d, 2H, J ¼ 3.8 Hz), 3.63 (s, 3H), 3.77 (br s, 1H), 5.03 (dd, 1H,
J ¼ 4.7, 4.6 Hz), 6.71–7.94 (m, 13H). n/cm21 1610 (C55N), 3362 (N-H);
MS: m/z ¼ 328 (Mþ), 297, 221, 194, 119, 103, 91, 77, 65, 51; C22H20N2O
(328.413): calcd. C, 80.46; H, 6.14; N, 8.53; found C, 80.37; H, 6.22; N, 8.58.
RESULTS AND DISCUSSION
We initially tested the reaction of o-phenylenediamine with chalcone at 80 8C
in the presence of [BPy]HSO4 ionic liquid. The conversion was only 69%
when using equimolar amounts of chalcone and o-phenylenediamine
(Table 1, entry 1). Fortunately, the desired reaction was efficiently accom-
plished when the reaction was carried out using 1.5 equiv. of o-phenylenedia-
mine (Table 1, entry 2). When the catalytic amount of ILs was increased, the
conversion reached 88% (Table 1, entry 8) with only a marginal increase in the
reaction rate. The subsequent condition optimization experiments revealed
that the 5 mol% of catalyst amount was necessary to complete the reaction
(Table 1, entry 7).
The result of synthesis of 1,5-benzodiazepines from o-phenylenediamine
with chalcone suggested that the catalytic performance of the [BPy]HSO4 and
[iso-BQu]HSO4 could be much better than that of the other protic acidic ILs
under the same reaction conditions. However, it should be noted that the

1666
Y. Du, F. Tian, and W. Zhao
a
Table 2. Synthesis of 1,5-benzodiazepines over [BPy]HSO4
Time
(h)
Yield
(%)
Entry
1
Chalcone
Product (num.)
3.0
87
2
3
4
2.5
1.5
3.0
89
95
80
5
6
7
1.5
3.0
2.5
93
91
90
8
9
2.5
2.0
3.0
88
92
84
10b
aReaction conditions: o-phenylenediamine, 1.5 mmol; chalcone, 1.0 mmol; ionic
liquid, 5 mmol%; temperature: 808C.
bReused four times.

[BPy] HSO4 Acidic Ionic Liquid
1667
reaction rate in ILs is dependent upon the IL chosen, and in this case,
[HMIm]BF4, [HMIm]PhSO3, [HMIm]CF3COO, and [HMIm]ClO4 give a
lower rate enhancement (Table 1, entries 3–6) than the [BPy]HSO4 and
[iso-BQu]HSO4 ILs as catalysts (Table 1, entries 1, 2). Probably this is due
to the higher Brønsted acidity of the hydrogen sulphate counteranion. [iso-
BQu]HSO4 also has good activities for this reaction, but [BPy]HSO4 seems
to be slightly better compared with [iso-BQu]HSO4 (Table 1, entries 2, 7).
To investigate the scope and limitation of [BPy]HSO4 as a catalyst for the
synthesis of 1,5-benzodiazepines, other chalcones as a substrate were also
tested, and the results are summarized in Table 2. Various chalcones and
o-phenylenediamine were efficiently converted to the corresponding 1,5-
benzodiazepines in the presence of 5 mol% [BPy]HSO4 acidic IL. Yields
of these reactions are almost quantitative in most cases. The presence of
electron-donating and electron-withdrawing groups on the aromatic ring of
chalcones reacted with o-phenylenediamine without any significant difference
to give the corresponding 1,5-benzodiazepines in good yields (Table 2, entries
1–9). It is noteworthy that [BPy]HSO4 acidic IL could be recycled and used in
the next run. In the reaction of chalcones, [BPy]HSO4 could be easily reused
for four cycles with little decrease in activity.
CONCLUSION
In summary, [BPy]HSO4 acidic IL was found to be a novel, environmentally
benign, and highly efficient acid catalyst for the synthesis of 1,5-benzo-
diazepines under mild conditions. The advantages of the present protocol are
air stability of the catalyst, ready availability, environmental friendliness, low
cost, mild conditions, and easy workup, which make the procedure an attractive
alternative to the existing methods for the synthesis of 1,5-benzodiazepines.
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