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P. Sarma et al.
Table 3 Comparison of the activity of ILs methodology with traditional acids
Entry
Catalyst/medium (amount/mol/%)
Time/min (Temp./°C)
Solvent
Yield 3/%
Recyclability [Ref]
1
2
4
5
6
7
8
Cu(OTf)
TFA (10)
Y(OTf) (20)
2
(20)
300 (25)
15 (100)
300 (25)
90 (70)
No
No
80
92
No [15]
No [16]
3
CH
No
No
No
No
3
CN
83
Yes [17]
Sulfamic acid (5)
Oxalic acid (10)
94
Yes [18]
120 (80)
45 (100)
70 (100)
89
No [19]
[Msim][OOCCCl
3
] (40)
100
100
Yes [present]
Yes [present]
[Msim]Cl (1.3 mmol)
After optimizing the amounts of ILs and trichloroacetic
acid, we extended these conditions to a variety of 2-amino-
aryl ketones and b-keto esters (Table 4, entries 3–6, 14). In
addition, various 1,3-diketones reacted with 2-aminoaryl
ketones to give the corresponding quinolines (Table 4,
entries 1–2, 7–10, 15, 16). Subsequently, we directed our
study to use simple cyclic ketones and it was observed that all
these reactions completed within the specified time with
excellent yields (Table 4, entries 11–13, 17–18). These
results indicate that the acidities of both ILs are sufficient to
give excellent yields with various types of a-methylene
group containing carbonyl compounds under optimized
conditions irrespective of the nature of 2-aminoaryl ketones.
The catalytic activity of trichloroacetic acid was also found
to be good with all systems under solvent-free condition
except 2-aminoacetophenone (Table 4, entries 14–18).
X-ray structure analysis was performed on the quinoline
derivative 3d (Table 4, entry 4) which indicated the presence
of basic quinoline moiety in the molecule (Fig. 3).
function of [Hmim][HSO ] to give excellent yield of pro-
4
ducts for the two recycling reactions of [Msim][OOCCCl3].
Interestingly, the [Msim]Cl ionic liquid retains its activity
up to three cycles. The trichloroacetic acid catalyst was
recycled for three times after regeneration from each cycle
of the reaction under solvent-free method.
Conclusions
The present study emphasized on the syntheses of two acidic
ionic liquids [Hmim][OOCCCl ] and [Msim][OOCCCl ]
3
3
and their characterization by various analytical techniques.
The order of activity of these ILs for the Friedl a¨ nder syn-
theses was identical with the acidity order of Hammett plot.
The use of ILs methods for this annulation have many
advantages, such as single product formation, high yields,
easy work up, short reaction time, and recycling of ILs as
medium or catalyst. Furthermore, the use of trichloroacetic
acid as catalyst needed more time to complete the quinoline
synthesis as compared to ionic liquid-mediated reactions.
The reusability of the two acidic ILs and trichloroacetic
acid was represented by the bar diagram in Fig. 4. The two
acidic ILs were easily recovered from the 1st cycle of
reactions and reused for two cycles with the reaction
between 2-amino-benzophenone and 5,5-dimethyl-1,3-
cyclohexanedione under optimized condition. In this study,
we observed slight loss in the activity of [Msim][OO-
Experimental
The NMR spectroscopic data of all products were recorded
on a JEOL 400 MHz spectrometer using TMS as internal
standard. Coupling constants are (J) expressed in Hertz
(Hz). IR spectra of all compounds were recorded on a
Nicolet Impact-410 spectrometer. The measurement of the
acidic scale of these Brønsted acidic ILs was conducted on
an UV 2550 spectrophotometer according to the reported
literature [13, 14]. Single crystal data of compound 3d
were collected on a Bruker SMART APEX II CCD dif-
fractometer; CCDC-958618 contains the supplementary
crystallographic data. These data can be obtained free of
ues of the ionic liquids were determined using digital pH
meter 802 [20]. Thermogravimetric analyses of the three
CCCl ] ionic liquid in next two runs of recycling which
3
consumed 30–45 min more time than 1st run to show
equivalent results. These results were also supported by the
observation of change in the FT-IR absorption bands of the
fresh (A) and reused (B, C) spectra of [Msim][OOCCCl3]
ionic liquid in Fig. 5. The observed loss of activity can be
explained by the proposed mechanism of [Hmin][HSO4]
formation (Scheme 3) in presence of atmospheric moisture
to some extent. The comparison of fingerprint region of
reused spectra were almost identical with the spectra of
[
[
Hmim][HSO ] ionic liquid (D). Using 0.4 mmol of
4
Hmim][HSO ] for the same reaction produced only 50 %
4
of product during 2.5 h reaction time at 100 °C in solvent-
free medium. This result did not support the catalytic
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23