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R. Kore, R. Srivastava / Catalysis Communications 12 (2011) 1420–1424
reused in a further run. In the case of benzoic acid, ethyl acetate was
added in the reaction mixture to separate the product and BAILs.
2.2.3. Synthesis of 7-hydroxy, 4-methylcoumarins
In a typical experiment, resorcinol (5 mmol), ethylacetoacetate
(5 mmol) and catalyst (0.5 mmol) were taken in 50 ml round bottom
flask. The reaction was conducted at 333 K for 1 h. Ethyl acetate was
added in the reaction mixture to separate the product and BAILs.
3. Results and discussion
BAILs were synthesized by multi-step synthetic protocol
(Scheme 1). N-methylimidazole/imidazole was benzylated with
benzyl chloride and the resulted quaternary salt was transformed
into BAILs by using concentrated sulfuric acid as anion exchange and
sulfonating reagents, respectively. In this study, BAILs were prepared
with equivalent amounts of the H2SO4. When samples were prepared
with excess amount of H2SO4, yields of the products were more than
the theoretical yields. This may be due to the presence of excess H2SO4
in the product. When equivalent amount of H2SO4 was taken, then the
yield was very good but slightly less than the theoretical yield. For
comparative study, solid acid catalysts such as H-ZSM-5, H-BEA and
SBA-15-pr-SO3H were prepared and characterized by using XRD, N2-
adsorption and XRF (Table 1).
Acidity of BAILs was measured using Analytikjena Specord 250
PLUS UV-visible spectrophotometer with a basic indicator by
following the concept reported in literature [21,22]. With the increase
of acidity of the BAILs, the absorbance of the unprotonated form of the
basic indicator decreased, whereas the protonated form of the
indicator could not be observed because of its small molar
absorptivity and its location, so the [I]/[IH+] (I represents indicator)
ratio can be determined from the differences of measured absorbance
after the addition of BAILs and Hammett function, H0, can be
calculated using Eq. (1). This value can be regarded as the relative
acidity of the BAILs.
Fig. 1. Absorbance spectra of 4-nitroaniline for various BAILs in water.
BAIL-5: B.P.N360 °C. IR (KBr, υ, cm−1)=3395, 3149, 3084, 2943,
2867, 2525, 1702, 1563, 1498, 1455, 1146, 1022, 875, 708. 1H NMR
(D2O) δ=8.59 (s, 2H), 7.23–7.27 (m, 12H), 5.19 (s, 4H), 3.94 (t, 4H),
1.61 (quint, 4H), 1.05 (quint, 4H). 13 C NMR (D2O) δ=134.64, 133.37,
131.52, 128.86, 128.09, 122.15, 121.96, 52.36, 48.96, 28.34, 24.1.
Elemental analysis for C26H34N4O14S4: theoretical (%): C 41.38, H 4.51,
N 7.43; experimental (%): C 41.58, H 4.24, N 7.55.
H-ZSM-5 [18], H-BETA [19] and SBA-15-pr-SO3H [20] were
synthesized following the reported procedures. 1H, 13 C & DEPT
NMR and FT-IR spectra of unknown BAILs are given in the
supplementary information.
2.2. Catalytic reaction procedure
h
ꢀ
ꢁi
H0 = pKðIÞaq + log ðIÞ= IHþ
ð1Þ
2.2.1. Synthesis of 3-phenacylphthalide
In a typical synthesis, phthalaldehydic acid (5 mmol), acetophe-
none (5 mmol) and catalyst (0.125 mmol) were mixed in a 50 mL
round bottom flask. Reaction was conducted at 353 K for 30 min. After
the reaction, the mixture was poured over crushed ice. The solid was
filtered and recrystallized using hexane: ethyl acetate (80:20). Water
layer was washed with ethyl acetate. The aqueous layer was dried
under vacuum at 343 K to obtain the BAILs for further use.
Under the same concentration of 4-nitroanline (5 mg/L, pK
(I)aq =pKa =0.99) and BAILs (25 mmol/L) in water, H0 values of all
BAILs were determined. The maximal absorbance of the unprotonated
form of the indicator was observed at 380 nm in water. When the BAIL
was added, the absorbance of the unprotonated form of the basic
indicator decreased. As shown in Fig. 1, the absorbance of the
unprotonated form of the indicator on addition of BAILs decreased as
follows: BAIL-1NBAIL-3NBAIL-2NBAIL-4NBAIL-5. Calculations sug-
gest that the Hammett acidity (H0) of these ionic liquids follows the
order: BAIL-5NBAIL-4NBAIL-2NBAIL-3NBAIL-1 (Table 2).
The catalytic activity of BAILs was investigated in the synthesis of
3-phenacylphthalide (Scheme 2), aromatic esters (Scheme 3) and 7-
hydroxy-4-methylcoumarin (Scheme 4). All BAILs investigated in this
study were found to be active. The activity of Brönsted acidic ILs in
these catalytic investigations follows the order BAIL-1bBAIL-3bBAIL-
2bBAIL-4≈BAIL-5 (Tables 3, 4 and 5), which is consistent with the
acidity measurement using UV–visible spectroscopy. Under the
optimized condition for these reactions, solid acid catalysts H-ZSM-
5, H-BETA and SBA-15-pr-SO3H were found to be either inactive or
weakly active (Tables 3, 4 and 5). As shown in Tables 3–5, the activity
of BAIL-4/BAIL-5 is comparatively higher than that of BAIL-2.
However, it may be noted that each BAIL have different numbers of
acid sites (SO3H and HSO4−), hence all of them have different TOF per
mol. If the number of acid sites (SO3H and HSO4−) present in BAIL-4/
BAIL-5 is taken into account for calculating TOF, the activity of BAIL-2
is found to be higher than BAIL-4/BAIL-5. Acidity-activity relationship
with increase in the number of sulphonic acid group clearly illustrates
2.2.2. Esterification reaction
In a typical reaction, benzyl alcohol (10 mmol), acids (10 mmol)
and catalyst (0.5 mmol) were mixed in a 50 mL round bottom flask.
Depending upon substrates, reactions were conducted at desired
temperature (323 K or 353 K) for stipulated time period (30, 45 or
90 min). The reaction mixture became biphasic (in case of hexanoic
acid and acetic acid). Upper layer containing the ester can be isolated
just by simple decantation. The lower layer consisting of the BAIL was
Table 2
Calculation and comparison of H0 values of different BAILs in water at 298 K.
E. No.
ILs
Amax
[I] (%)
[IH+] (%)
H0
1
2
3
4
5
6
No BAIL
BAIL-1
BAIL-2
BAIL-3
BAIL-4
BAIL-5
1.696
1.588
1.260
1.543
1.041
1.001
100
0
–
93.6
74.3
90.9
61.4
59.0
06.4
25.7
09.1
38.6
41.0
2.155
1.451
1.989
1.192
1.148
Indicator: 4-nitroanline.