Q. Zhao et al. / Catalysis Communications 36 (2013) 98–103
99
Scheme 1. HWE reaction of DEBP under SL–PTC conditions.
Chemical Reagent Co., Ltd. Dry toluene was obtained by distillation
under nitrogen in the presence of sodium and benzophenone. The
other chemicals were used without further purification. High perfor-
mance liquid chromatography (HPLC) analysis was performed on an
Agilent 1200 equipped with a Zorbax Eclipse XDB-C18 column
(150 mm × 0.46 mm, 0.5 μm) and a UV variable wavelength detector.
interface saturation. Namely, the mass transfer of the active species
and the deprotonation of DEBP reach the maximum value. Therefore,
the interfacial mass transfer resistance between the solid phase and or-
ganic phase can be negligible. The stirring speed was fixed at 1400 rpm
for the following experiments.
3.2. Effect of the quaternary ammonium salts and catalyst amount
2.2. Kinetics of HWE reaction under SL–PTC conditions
Besides TBAB, six other quaternary ammonium salts were also
employed as PTCs, which were listed in Table 1. The order of their catalytic
activity is: TBAB > BTEAB > TOAB > MTOAB > HTAB > DTAB > TEAB.
The results are clearly relevant to the characteristics of the interfacial re-
action mechanism. Those quaternary ammonium salts with low total car-
bon number, such as TEAB, have a low calculated partition coefficient
(ClogP) [26]. They are not sufficiently lipophilic to allow the ion-pair
dissolve in the organic phase. On the other hand, the quaternary ammoni-
um salts containing large alkyl groups, such as TOAB, have a low accessary
parameter (q) [27]. They have an inferior electrostatic accessibility to the
positive charge. Hence, the ion-pair will be not generated in the interfacial
region. For DTMAB and HTMAB, the carbanion in the ion-pair has steric
hindrance due to their very long alkyl chain. Therefore, TBAB was deter-
mined as the optimal PTCs in this system.
Unexpectedly, the reaction rate constant estimated for the reac-
tion in the absence of PTCs is about one-tenth of that with TBAB. It
is attributed to interfacial solid–liquid reaction which is similar to
that in HWE reaction of moderately acidic phosphates. In other
words, the carbanion is generated at the surface of solid base and
the intrinsic reaction occurs at the solid–liquid interface [12]. Howev-
er, the sodium salt of the carbanion cannot be dissolved in toluene
and contacts between molecules at the solid–liquid interface are dif-
ficult. Hence, the reaction rate is relatively low. However, the reaction
rate is enhanced dramatically even with adding a small quantity of
PTCs. It is attributed that the carbanion can be easily transferred
into the bulk organic phase and the mechanism is changed.
The reaction was performed in a 100 mL four-necked flask equipped
with a mechanical stirrer, a thermometer, a reflux condenser and a sam-
pling port. DEBP (0.02 mol), tetrabutyl ammonium bromide (TBAB,
2 × 10−4 mol) and toluene (23 mL) were introduced into flask and
stirred at 500 rpm for 10 min. 0.04 mol of solid NaOH was added at
35 °C. The water or salt was added in the same time with the solid
NaOH. The mixture was stirred at 1400 rpm for 30 min to activate
DEBP. A solution of aldehyde (2 × 10−3 mol) in toluene (12 mL) was
added at zero time. The sample (about 50 μL) was withdrawn from
the reaction mixture at a regular time interval and put into test tube.
Subsequently, 0.2 mL of 10% (v/v) hydrochloric acid was added to
quench the reaction. Finally, the solution was diluted to 10 mL with
50% (v/v) aqueous acetonitrile. The contents of the stilbene and toluene
were estimated by HPLC with an external standard method. The HPLC
was operated with a mobile phase consisting of 50:50 (v/v) water/
acetonitrile at a flow rate of 1.0 mL/min. The detection wavelength
was 260 nm and the column temperature was 30 °C.
2.3. Synthesis of stilbenes in SL–PTC system
A mixture of DEBP (0.01 mol), TBAB (5 × 10−4 mol), solid NaOH
(0.04 mol) and toluene (30 mL) was introduced into a 100 mL flask
and stirred at 1400 rpm. Aldehyde (0.01 mol) in 15 mL of toluene
was added dropwise at 35 °C. The reaction mixture was stirred for an
appropriate time. After completion of the reaction as indicated by TLC,
H2O (10 mL) was added and the layers were separated. The organic
layer was washed with three 10-mL portions of 5% HCl solution. The or-
ganic phase was dried over magnesium sulphate and evaporated to dry-
ness. The residue was chromatographed and the product was identified
by FT-IR and NMR (1H NMR and 13CNMR).
It was observed from Fig. 2 that the reaction rate quickly increased
with the concentration of TBAB varied from 2.1 to 6.5 mol/L. This is
3. Results and discussion
The kinetic experiments of HWE reaction were carried out under
pseudo-first-order conditions, taking DEBP and solid NaOH in excess.
The pseudo-first-order rate constant (kobs) was obtained from the
plot of log(1 − x) versus time, where x was the yield of the stilbene
at a given reaction time t.
3.1. Effect of stirring speed
To determine the influence of mass transfer, the stirring speed was
varied from 200 to 1800 rpm when the amount of TBAB was 4%
(2.06 mmol/L) and 10% (5.2 mmol/L) respectively. It can be seen from
Fig. 1 that a dramatic increase in the reaction rate is produced with a
rise in the stirring speed from 200 to 800 rpm. The possible reason is
that the interfacial area per unit volume of dispersion increases linearly
with the agitation speed increasing [25]. However, the reaction rate in-
creased slightly when the stirring speed was over 1000 rpm due to
Fig. 1. Dependence of kobs on stirring speed (0.02 mol of DEBP, 2 × 10−3 mol of
benzaldehyde, 0.04 mol of solid NaOH, 35 mL of toluene, 35 °C).