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[12]. The multi-site phase-transfer catalyst contains more than one
active center, and has the potential to be used in solid–liquid sys-
tem. The most significant merit for MPTC’s is that it has an ability
to transfer more number of anionic species (M+Yꢀ) from aqueous
or solid phase to organic phase. In contrast, the single-site quater-
nary onium phase-transfer catalyst can transfer only one molecule
of anionic species, i.e., M+Yꢀ from aqueous or solid phase per cycle.
Especially, nowadays much emphasis has been given to economy
of scale and efficiency of onium salts particularly for the industrial
scale preparation of organic compounds. In our laboratory too, sev-
eral multi-site phase transfer catalysts are reported for various
reactions [13–15]. Ultrasonic irradiation in biphase reaction can
increase interfacial area coupled with local hot-spot generation,
and has been demonstrated to promote high reaction rate in
organic synthesis [16–19]. Under ultrasonic irradiation, the liquid
jet could be favorable to employ for removing the surface-
deposited side-product in SLPTC and increasing the formation of
the catalytic intermediate. Ultrasound irradiation combined with
solid–liquid PTC has revealed significant improvement in the reac-
tion rate. In solid–liquid PTC system, the overall reaction rate can
also be effectively raised with or without multi-site phase-transfer
catalyst [20–23]. But the application of ultrasound in SLPTC espe-
cially catalyzed by multi-site phase-transfer catalyst was rarely
reported.
Our interest was entered on first time evaluating the influence
of ultrasound in association with multi-site phase-transfer catalyst
(MPTC) on the synthesis of 1-(benzyloxy)-4-nitrobenzene from
potassium salt of benzyl alcohol with 4-chloronitrobenzene
(CNB) under heterogeneous solid–liquid condition. Since, the
kinetic study of arylation of benzyl alcohol using 4-chloronitroben-
zene under controlled MPTC reaction conditions will be interest-
ing, we followed the kinetic study using a newly synthesized
multi-site phase-transfer catalyst (MPTC) viz., 1,3,5-triethyl-
1,3,5-trihexyl-1,3,5-triazinane-1,3,5-triium trichloride, as a cata-
lyst under ultrasonic condition (40 kHz; 300 W). Further, to the
best of our knowledge, there is no literature report’s regarding
4-nitrophenylation of benzyl alcohol under combined MPTC–
ultrasonic irradiation condition. An alternative procedure (method
II) also tried to get same product (1-(benzyloxy)-4-nitrobenzene)
under a new MPTC assisted by ultrasound (40 kHz, 300 W) using
4-nitrophenol, benzyl chloride and potassium carbonate.
2.3. Ultrasonic process equipment
Ultrasonic energy is transmitted to the process vessel through
the liquid medium, usually water in the tank. For safety purpose,
the sonochemical reactor consisted of two layers stainless steel
body. The sonochemical reactor configuration used in the present
work is basically an ultrasonic bath. The internal dimension of
the ultrasonic cleaner tank is 48 ꢁ 28 ꢁ 20 cm with liquid holding
capacity of 5 L. Two types of frequencies of ultrasound were used
in these experiments, which are 28 and 40 kHz with each output
as 300 W. Both ultrasounds separately produces through a flat
transducer mounted at the bottom of the sonicator. The reactor
was a 250 mL three-necked Pyrex round-bottom flask. This reac-
tion vessel was supported at the centre of the ultrasonic cleaning
bath 2 cm above from the position of the transducer to get the
maximum ultrasound energy. All the experimental parameters
were done at 40 kHz with output power of 300 W.
2.4. Synthesis of 1,3,5-triethyl-1,3,5-triazinane
A mixture of 83 g of ethylamine, and 40 g of paraformaldehyde
were placed in a 250 mL three necked round bottomed Pyrex flask.
The reaction was carried out at 40 °C for 4 h and was gently
refluxed in the nitrogen atmosphere. After reaction completed
the solvent was completely removed under vacuum and we get
1,3,5-triethyl-1,3,5-triazane (Scheme 1) the colorless liquid was
stored in a refrigerator. Yield: 92%; 1H NMR (300 MHZ, CDCl3); d
1.054–1.102 (t, 9H–CH2–CH3), 2.449–2.522 (q, 6H–CH2–CH3),
3.432 (s, 6H, N–CH2). 13C NMR (75 MHZ, CDCl3): d 12.62 (CH2–
CH3), 46.50 (CH2–CH3), 73.72 (N–CH2). Elemental analysis: Calcu-
lated: C, 62.98%; H, 12.16%; N, 24.23%; and Found: C, 63.11%; H,
12.36%; N, 24.53%.
2.5. Synthesis of a new MPTC
A mixture of 8.7 g (50.7 mmol) of 1,3,5-triethyl-1,3,5-triazane,
29.4 g (24.9 mL, of hexyl chloride, and 80 mL of ethanol was placed
in a 250 mL three necked round bottomed Pyrex flask. The reaction
was carried out at 50 °C for 30 h and was gently refluxed in the
nitrogen atmosphere. The solvent was then completely removed
under vacuum and the onium salt, i.e., 1,3,5-triethyl-1,3,5-tri-
hexyl-1,3,5-triazinane-1,3,5-triium trichloride, (MPTC; Scheme 1)
was washed with n-hexane (4 ꢁ 25 mL). The Brownish oily liquid
was stored in CaCl2 desiccator. Yield: 96%; 1H NMR (300 MHZ,
CDCl3); d 0.8(t, 9H, –CH3 in hexyl group), 1.088–1.136 (t, 9H, –
CH3 in ethyl group), 1.168–1.367 (m, 24H, –CH2 in hexyl group),
2.585–2.622 (t, 6H, N+–CH2 in hexyl group), 3.302–3.398 (q, 6H,
N+–CH2 in ethyl group), 5.203 (s, 6H, N+–CH2–N+) 13C NMR
(75 MHZ, CDCl3): d 5.5(–CH3 in hexyl group), 13.40 (–CH3 in Ethyl
group), 21.81 (N+–CH2–CH2), 23.25 (C5- in hexyl group), 26.55
(C3- in hexyl group), 30.52(C4- in hexyl group), 51.58(N+–CH2 in
ethyl group), 69.55 (N+–CH2 in hexyl group); Elemental analysis:
Calculated: C, 60.86%; H, 11.42%; Cl, 19.10%; N, 7.92%; and Found:
C, 60.83%; H, 11.34%; Cl, 19.95%; N, 7.88%.
2. Experimental
2.1. Chemicals
All the reagents, including, benzyl alcohol, 4-chloronitrobenzene
(CNB), benzyl chloride, 4-nitrophenol, ethylamine, paraformalde-
hyde, n-hexyl chloride, biphenyl, tetrabutylammonium chloride
(TBAC), tetrabutylammonium bromide (TBAB), benzyltriethylam-
monium chloride (BTEAC), tetraethylammonium chloride (TEAC),
tetraethylammonium bromide (TEAB), potassium hydroxide,
potassium carbonate, n-hexane, toluene, chlorobenzene, anisole,
diethyl ether and other reagents for synthesis were guaranteed
grade (GR) chemicals and were used without further purification.
3. Synthesis of 1-(benzyloxy)-4-nitrobenzene under mechanical
stirring
2.2. Instrumentation
1H NMR and 13C spectra were recorded on a Bruker 300 and
75 MHz respective using TMS as an internal standard. Gas chroma-
tography was carried out using a GC-Varian 3700 model. Ultrasonic
water bath, Equitron, Media Instrument Manufacturing Company,
Chennai, India-600 004. The ultrasonic generator was a thermo-
static bath equipped with dual frequencies (28/40 kHz) and elec-
tric power 300 W with 0.0126 W/mL of power density.
3.1. Method-I
To the well powdered KOH (10 g) 1 mL of water and (1.0 g,
9.25 mmol) of benzyl alcohol was added under overhead stir-
ring for few minutes to generate the benzyl alcohol anion. Then
4-chloronitrobenzene (1 g, 6.35 mmol) and the newly synthesized
MPTC (0.3 g) in chlorobenzene (30 mL) were added slowly. The