B.M. Godajdar, B. Ansari / Journal of Molecular Liquids 202 (2015) 34–39
37
Scheme 2. Synthesis of benzyl azide from benzyl halide in the presence of magnetic phase transfer catalyst.
are peaks at about 1165 cm−1, which were assigned to the characteristic
absorption of N–CH in functionalized PEG-MDIL. The absorption bands at
cleanly. On the other hand, in the absence PEG-MDIL, the reaction was
sluggish and, even after prolonged reaction time, a considerable amount
of starting material remained. Moreover, the reaction mixture was con-
taminated with alcohol. This observation was confirmed with the pres-
ence of alcohol on the TLC plate.
To evaluate the catalytic activity of PTC, the reaction of benzyl ha-
lides with NaCN was examined as a model reaction. The results are sum-
marized in Table 2.
The effect of the reaction temperature on the reaction time of benzyl
bromide was investigated at reaction temperatures ranging from 25 to
90 °C. The results show that the suitable reaction temperature is 90 °C.
The reaction was carried out in diethyl ether, n-hexane, acetonitrile,
dichloromethane, ethyl acetate and water. From the results, it follows
that the best solvent for this reaction is water. Thus, water is an excel-
lent solvent in terms of cost, availability, and environmental impact
and shorter reaction times. Benzyl halide bearing activated and
deactivated groups was quickly and efficiently converted to the virtually
pure corresponding products in high isolated yields. No evidence for the
formation of alcohol by products of the reactions was observed and the
products were obtained in pure form without further purification. As
2
−
1
3152 and 3013 cm (imidazolium CH stretching modes) presented in
the inset of Fig. 3 demonstrate modification of the PEG. The absorption
−
1
2
at 2911 cm is usually assigned to CH stretching of the polyether link-
−
1
age chains. The absorption observed at 1571 cm is also characteristic of
the imidazolium ring and is assigned to imidazolium ring stretching.
Moreover, this PEG-MDIL was characterized via Raman spectroscopy
and proved to be identical with an authentic material reported in liter-
−
1
−
ature [18]. The peak at 333 cm
is due to FeCl
(Fig. 4).
To evaluate the catalytic activity of PEG-MDIL, the reaction of benzyl
halides with NaN was examined as a model reaction. The results are
4
, indicating that the
−
anion of this PEG-MDIL is FeCl
4
3
summarized in Table 1.
We first examined the catalytic ability of PEG-MDIL for conversion of
3
benzyl halides to benzyl azide with NaN in water at room temperature
and under reflux conditions (Scheme 2). This catalyst acted very efficient-
ly and it converts different benzyl halides to their corresponding benzyl
azide in high isolated yields. The obtained results of the reaction are
given in Table 1.
1H NMR spectra of the crude products clearly showed the formation
of benzyl azide and no evidence for the hydrolysis of benzyl halides to
the alcohols was observed, which proved that the reactions proceeded
N
expected the typical steric effect on the rate of S 2 reactions was
observed. The primary alkyl halides could be efficiently converted to
the corresponding alkyl azide, whereas secondary alkyl halides such as
bromo cyclohexane did not convert after 5 h (Table 1, entry 8). All the
products were characterized and identified by comparison of their spec-
Table 2
1
13
tral data (IR, H NMR and C NMR) with those of authentic samples.
The success of the above reactions prompted us to investigate the re-
cyclability of catalyst. We carried out our study by using the reaction
3
benzyl bromide with NaN and under optimal conditions as a model
study. The aqueous phase was then subjected to distillation at 80 °C
under reduced pressure (10 mm Hg) for 4 h to recover the PEG-MDIL al-
most completely. The hydrophobic IL was purified by repeated washing
with deionized water and evaporation. The catalyst could be reused for
the fourth time without significant decrease in catalytic activity
(
Table 3). The IR, UV, and TGA analyses of reused catalyst indicated
that no detectable changes of the catalyst occurred during the reaction
and the recycling stages (Figs. 5, 6).
PEG-MDIL showed remarkable reactivity as a Lewis acid reagent and
considerably accelerated the reactions. It seems that polyethylene glycol
units in PEG-MDIL encapsulate alkali metal cations, much like crown
ethers, and these complexes cause the anion to be activated. The 1-
methylimidazol-3-ium units introduced ionic liquid property to the
−
catalyst. In addition, FeCl
4
groups of the IL, probably, facilitate the sub-
stitution reaction (Scheme 4).
3. Experimental
3.1. Material and methods
Melting points were measured on an Electro thermal 9100 appara-
tus and are uncorrected. H NMR & C NMR spectra were recorded on
a Bruker Advanced DPX 400 MHz instrument spectrometer using TMS
1
13
Table 3
3
The reusability of the catalyst in the reaction benzyl bromide with NaN .
a
Products were identified by comparison of their physical and spectral data with those of
Cycle
Fresh
86
First
84
Second
81
Third
76
Fourth
72
authentic samples.
Yield (%)
b
Isolated yields.