Synthesis of stilbene, 1,4-distyrylbenzene and 4,4′-distyrylbiphenyl via Horner-Wadsworth-Emmons reaction in phase-transfer catalysis system

Article abstract of DOI:10.1016/j.dyepig.2013.05.017

Stilbenes, 1,4-distyrylbenzenes and 4,4′-distyrylbiphenyls were synthesized via Horner-Wadsworth-Emmons (HWE) reaction in liquid-liquid (LL) and solid-liquid (SL) phase transfer catalysis (PTC) systems. The effect of the side reaction, reactants and the third phase on the activity of HWE reaction were investigated. For aldehydes bearing electron-donating substitute, the yields were more than 90% and the products were all (E)-isomers in both PTC systems. The SL-PTC system was milder than LL-PTC system for HWE reaction due to the different mechanisms. The side reaction of aldehyde was similar to Cannizzaro reaction, whereas the molar ratio of benzoic acid to benzyl alcohol as the products was not 1:1. The limited third phase was discovered to exist in LL-PTC system. In SL-PTC system, the third phase could increase substantially the reaction rate. Moreover, the aqueous phase in LL-PTC system could be reused four times without sacrifice of the yield and reaction rate.

Full text of DOI:10.1016/j.dyepig.2013.05.017

Dyes and Pigments 99 (2013) 339e347
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of stilbene, 1,4-distyrylbenzene and 4,40-distyrylbiphenyl via
HornereWadswortheEmmons reaction in phase-transfer catalysis
system
,b,
Qiangqiang Zhao a, Jie Sun a, Baojiang Liu a, Jinxin He a
*
a College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai 201620, China
b Key Laboratory of Textile Science & Technology, Ministry of Education, Shanghai 201620, China
a r t i c l e i n f o
a b s t r a c t
Stilbenes, 1,4-distyrylbenzenes and 4,40-distyrylbiphenyls were synthesized via HornereWadsworth
eEmmons (HWE) reaction in liquideliquid (LL) and solideliquid (SL) phase transfer catalysis (PTC)
systems. The effect of the side reaction, reactants and the third phase on the activity of HWE reaction
were investigated. For aldehydes bearing electron-donating substitute, the yields were more than 90%
and the products were all (E)-isomers in both PTC systems. The SL-PTC system was milder than LL-PTC
system for HWE reaction due to the different mechanisms. The side reaction of aldehyde was similar to
Cannizzaro reaction, whereas the molar ratio of benzoic acid to benzyl alcohol as the products was not
1:1. The limited third phase was discovered to exist in LL-PTC system. In SL-PTC system, the third phase
could increase substantially the reaction rate. Moreover, the aqueous phase in LL-PTC system could be
reused four times without sacrifice of the yield and reaction rate.
Article history:
Received 12 March 2013
Received in revised form
13 May 2013
Accepted 16 May 2013
Available online 2 June 2013
Keywords:
HornereWadswortheEmmons reactions
Phase-transfer catalysis
1,4-distyrylbenzene
4,40-distyrylbiphenyl
The third phase
Ó 2013 Elsevier Ltd. All rights reserved.
Stilbene
1. Introduction
Many synthetic methods for stilbenes, 1,4-distyrylbenzenes and
4,40-distyrylbiphenyls have been developed. The condensation re-
Stilbene, 1,4-distyrylbenzene and 4,40-distyrylbiphenyl bear
remarkable optical, photochemical and photophysical properties
[1]. They have widespread applications in many fields ranging
from fine chemistry and materials science to biomedicine.
They can be used as fluorescent whitening agents in textile,
paper manufacturing and household detergents [2], two-photon
absorbing materials [3e6] and blue electroluminescent materials
[7e11] in the optoelectronic device, drugs and antitumor agents in
pharmacology [12,13], molecular probes and labels in bioassay [14]
and etc. With the discovery of new applications, such as molecular
conformational switches [15], integral part of phenylene vinylene-
based oligomers and polymers [16], energy transporter in one-
dimensional channels [17,18] and bridge spanning redox-active
moieties [19,20], they have drawn great interests of more and
more researchers.
actions of arylmethyl compound are the most common methods,
such as Grignard reaction, Knoevenagel reaction, Wittig reaction and
HornereWadswortheEmmons (HWE) reaction. Particularly, HWE
reaction is the main pathway employed in industry. Some other
condensation routes for new stabilized carbanion have also been
reported [21]. Nonetheless, the low atom economy, the high (Z)-
factor and the high cost of solvents are the main disadvantages of the
condensation reaction. Hence, the new synthetic method of stil-
benes, 1,4-distyrylbenzenes and 4,40-distyrylbiphenyls has been a
hot area of research. The dimerization reactions had been used been
used for many years, such as Meerwein arylation [22], Stille coupling
reaction [23] andHeck reaction [24]. As oneofbasic organic synthetic
methods, Heck reactions has been extensively investigated in the
synthesis of stilbene, 1,4-distyrylbenzene and 4,40-distyrylbiphenyl
[25e30]. However, the high cost of catalyst, ligand and dipolar
aprotic solvent limits their application in industry. As an alternative
to these methods, we proposed here an easy and convenient phase
transfer catalysis (PTC) routes via HWE reaction (Scheme 1).
* Corresponding author. College of Chemistry, Chemical Engineering & Biotech-
nology, Donghua University, Shanghai 201620, China. Tel.: þ86 21 67792602;
fax: þ86 21 67792608.
PTC is one of the most widely used synthetic techniques and has
had more than 700 industrial applications [31,32]. The major ad-
vantages of PTC include its simplicity, use of inexpensive reagents
E-mail address: jxhe@dhu.edu.cn (J. He).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.05.017

340
Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
Scheme 1. The synthetic routes of stilbene, 1,4-distyrylbenzene and 4,40-distyrylbiphenyl in PTC system.
under mild conditions, high reaction rate and high selectivity to the
desired product [33]. HWE reaction under PTC conditions was
1H and 13C NMR spectra were recorded at 298 K on a Bruker ARX
400 spectrometer (at 400.13 and 100.62 MHz). CDCl3 was used as
solvent and chemical shifts were internally referenced to Me4Si
(0 ppm). IR spectra were obtained on a Thermo Electron Corpora-
tion Nicolet 380 FT-IR spectrophotometer. Elemental analysis (EA)
was performed on an Elementar Vario (III) to determine the carbon,
hydrogen and nitrogen contents of the product. High performance
liquid chromatography (HPLC) analysis was performed on the
Agilent 1200 system equipped with a Zorbax Eclipse XDB-C18
firstly reported in 1974 [34]. The synthesis of
a,b-unsaturated
compounds via HWE reaction in solideliquid PTC (SL-PTC) system
was then researched [35,36]. At present, HWE reaction in PTC
system have been used in the synthesis of polymer [37] and dipolar
organometallic complexes [38,39]. Nevertheless, those reactions
suffer from one or more problems, such as long reaction time, low
yield and expensive solvent. Moreover, there are few studies on the
mechanism of HWE reaction under PTC conditions and the pa-
rameters influenced the activity of HWE reaction in PTC system
have rarely been investigated.
column (150 mm ꢀ 0.46 mm, 0.5
mm) and a UV variable wave-
length detector (VWD).
In this work, HWE reactions for monophosphonate or
bisphosphonate with aldehyde were preceded in liquideliquid PTC
(LL-PTC) and SL-PTC system (Scheme 1). The yield and geometric
selectivity for HWE reaction in these two PTC systems were
compared. The effects of the side reaction, reactants and the third
phase on the activity of HWE reaction in both PTC systems were
studied. Furthermore, the reusability of the aqueous phase in LL-
PTC system was discussed. These results will provide a potential
application for HWE reaction in industry and show new insights
into the mechanisms of PTC reaction.
2.2. Method
2.2.1. General synthesis of stilbene in LL-PTC system
20 g of 50% (m/m) NaOH aqueous solution was placed in a
100 mL four-necked flask and stirred at 1400 rpm. A solution of
DEBP (10 mmol), TBAB (0.5 mmol) in toluene (30 mL) was added at
35 ꢁC. Simultaneously,10 mmol of aldehyde in 15 mL of toluene was
dropped into the flask over an appropriate period. After completion
of the reaction as indicated by TLC, the resulting mixture was
cooled to room temperature and transferred into a separatory
funnel. The aqueous phase was separated and the organic phase
was washed twice with 10 mL of 3% (v/v) HCl solution and water,
respectively. Subsequently, the organic phase was dried over
MgSO4. The toluene was removed under reduced pressure and the
residue was recrystallized from n-hexane.
2. Experimental
2.1. Chemical and instruments
Diethyl benzylphosphonate (DEBP), diethyl 2-cyanobenzyl-
phosphonate (DCBP), tetraethyl (1,4-phenylenebis(methylene))
bis(phosphonate) (TEPP) and tetraethyl ([1,10-biphenyl]-4,40-diyl-
bis(methylene))bis(phosphonate) (TEBP) were prepared according
to the literature [40e42]. Other chemicals were purchased from
Sinopharm Chemical Reagent Co. Ltd. and used directly as received
without further purification. Dry toluene was obtained by distilla-
tion under nitrogen in the presence of sodium and benzophenone.
2.2.2. General synthesis of 1,4-distyrylbenzene and 4,40-
distyrylbiphenyl in LL-PTC system
The procedure was similar to that for the synthesis of stilbenes
in LL-PTC system. The resulting mixture was transferred to a sep-
aratory funnel. After the phases had been separated, the organic
phase was filtered. The precipitate was rinsed twice using 3% HCL
solution and ethanol, respectively.

Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
341
2.2.3. General synthesis of stilbenes in SL-PTC system
the mixture was cooled to room temperature and transferred to a
separatory funnel. The aqueous phase was separated and the
treatment of the organic phase was similar to that described in
Section 2.2.1. The weight of the aqueous phase was adjusted to 20 g
by adding 50% (m/m) NaOH aqueous solution. Subsequently, the
resulting solution was reused by adding the fresh organic phase.
A mixture of the DEBP (10 mmol), TBAB (0.5 mmol), solid NaOH
(40 mmol) and dry toluene (30 mL) was stirred at 1400 rpm.
Aldehyde (10 mmol) in 15 mL of toluene was added dropwise at
35 ꢁC. The resulting mixture was stirred for the appropriate time.
After completion of the reaction as indicated by TLC, 10 mL of H2O
was added to dissolve the excess of solid NaOH. The resulting
mixture was cooled to room temperature and transferred to a
separatory funnel. The aqueous layer was separated and the
treatment of the organic phase was similar to that in the synthesis
of stilbenes in LL-PTC system.
3. Result and discussion
3.1. Synthesis of stilbene, 1,4-distyrylbenzene and 4,40-
distyrylbiphenyl in PTC system
2.2.4. General synthesis of 1,4-distyrylbenzene and 4,40-
distyrylbiphenyl in SL-PTC system
A
series of stilbenes, 1,4-distyrylbenzenes and 4,40-dis-
The procedure was similar to that for the synthesis of stilbenes
in SL-PTC system. After completion of the reaction as indicated by
TLC, 10 mL of H2O was added. The aqueous layer was separated. The
organic layer was filtered and the precipitate was washed twice
with 3% HCL solution and ethanol, respectively.
tyrylbiphenyls were prepared according to the synthetic route 1
and 2 shown in Scheme 1. The yield and geometric selectivity for
each reaction were listed in Table 1. The structure of the product
was fully characterized by IR, 1H NMR and 13C NMR spectrometry
and element analysis. The datum were given in the Supplementary
Information.
2.2.5. The side reaction of aldehyde in PTC system
It can be observed from Table 1 that the activity of HWE reaction
is varied with PTC system. The yield of stilbenes with electron
donating group did not change significantly with PTC system,
whereas the yield of stilbenes containing electron-withdrawing
group in SL-PTC system was higher than that in LL-PTC system.
All the yields of 1,4-distyrylbenzenes and 4,40-distyrylbiphenyls in
SL-PTC system were lower than those in LL-PTC system. These may
be attributed to the different mechanisms of HWE reaction in these
two PTC systems. In LL-PTC system, HWE reaction undergoes the
extraction mechanism. Hydroxide ion (OH‾) which is extracted by
PTCs into the organic phase can deprotonate phosphonate quickly.
Subsequently, the reaction of the carbanion and aldehyde proceed
in the organic phase. However, the interfacial mechanism is
preferred in SL-PTC system. The phosphonate molecules aggregate
in the interfacial region and the available OH‾ on the surface of
solid NaOH can deprotonate the phosphonate. Subsequently, the
PTCs cation in the interfacial region couples with the carbanion.
Finally, the carbanion is transferred into the organic phase and
reacts with the aldehyde. The basicity of OH‾ in the interfacial re-
gion is weaker than that extracted into the organic phase [43].
Consequently, the rate of the deprotonation reaction in LL-PTC
system is higher than that in SL-PTC system, and the reactants
are prone to undergo side reactions in LL-PTC system, especially for
aldehyde containing electron-withdrawing group. Hence, SL-PTC
system is milder than LL-PTC system for HWE reaction. In HWE
reaction for TEBP or TEPP, the products would easily deposit on the
surface of solid NaOH due to their poor dissolution in toluene. Less
free OH‾ is available and in turn the carbanion could not be
generated efficiently. Consequently, the extensive side reaction
occurs and the yield is decreased in SL-PTC system.
A solution of TBAB (0.5 mmol), aldehyde (10 mmol) in toluene
(30 mL) was placed in a 100 mL flask and stirred at 1400 rpm. 20 g
of 50% (m/m) NaOH aqueous solution or 40 mmol of solid NaOH
was added at 35 ꢁC. After the completion of the reaction as indi-
cated by TLC, the aqueous phase was separated and washed twice
with 5 mL of toluene. The pH value of the aqueous phase was
adjusted to 2 with HCl solution. The resulting precipitate was
filtered and rinsed twice with 3% HCl solution. The organic layer
was washed with three 5 mL-portions of 3% HCl solution. The
organic phase was dried over MgSO4 and evaporated to dryness.
The crude products were purified by recrystallization from ethanol.
2.2.6. The effect of reactants on the rate of HWE reaction in PTC
system
A solution of phosphonate, aldehyde and TBAB (0.5 mmol) in
toluene (30 mL) was placed in a 100 mL four-necked flask equipped
with a mechanical stir, a thermometer and a sample port, and
stirred at 1400 rpm. 20 g of 50% (m/m) NaOH aqueous solution or
40 mmol of solid NaOH was added at 35 ꢁC. The sample (about
50 mL) 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, aldehyde 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
and a flow rate of 1.0 mL/min. The detection wavelength was set at
260 nm and the column temperature was 30 ꢁC.
2.2.7. The effect of the third phase(
PTC system
u
phase) on HWE reaction in SL-
The products were predominantly the (E)-isomers in both PTC
systems, especially for aldehyde with electron-donating substitute
or nitro group. In HWE reaction, the four-center transition state (TS)
has two conformation - cis TS and trans TS. The cis TS is later
(more oxaphosphetane-like) and hence less flexible. The cis TS is
constrained to be closer to planar which leads to unfavorable
1,2 interactions. Although the trans TS is semi-puckered, it is
A mixture of solid NaOH (40 mmol), dry toluene (15 mL) and
water was stirred at 1400 rpm. A solution of benzaldehyde
(10 mmol), DEBP (10 mmol) and TBAB (0.5 mmol) in toluene
(15 mL) was added at 35 ꢁC. The sample (about 50
mL) was with-
drawn from the reaction mixture at a regular time interval. The
treatment and analysis of sample were similar to that described in
Section 2.2.6.
less hindered and favored (Scheme 2) [44]. Therefore,
E
selectivity in HWE reactions was found. For HWE reaction of
2-halobenzaldehyde or 2,4-dihalobenzaldehyde with DEBP, the
proportion of (Z)-stilbene increased due to the cooperative ortho
halo effect [45]. However, there was no (Z)-isomer product in HWE
reaction for DCBP, TEPP or DPP. The steric hindrance of the cyano
2.2.8. The reuse of the aqueous phase in LL-PTC system
20 g of 50% (m/m) NaOH aqueous solution was placed in a
100 mL flask and stirred at 1400 rpm. A solution of DEBP (10 mol),
benzaldehyde (10 mol), TBAB (0.5 mmol) in toluene (30 mL) was
added at 35 ꢁC. After completion of the reaction as indicated by TLC,
group and the conjugation of the
erative ortho halo effect.
p bond can overcome the coop-

342
Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
Table 1
The yield and E/Z ratio of stilbene, 1,4-distyrylbenzene and 4,40-distyrylbiphenyl in HWE reaction under PTC conditions.
Compound number
LL-PTC
Yield
SL-PTC
Yield
Compound number
LL-PTC
Yield
SL-PTC
Yielda
E/Zb
E/Zb
E/Zb
E/Zb
1a
1b
1c
1d
1e
1f
1g
1h
1ac
2a
2b
2c
2d
2e
2f
0.95
0.96
0.95
0.89
0.71
0.45
0.37
0.56
0.20f
0.96
0.94
0.95
0.91
0.84
0.61
0.58
0.70
0.30d
0.65
>99/1
>99/1
>99/1
>99/1
83/17
>99/1
>99/1
78/22
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
0.96
0.97
0.93
0.90
0.85
0.65
0.55
0.70
0.23
0.96
0.93
0.94
0.92
0.86
0.65
0.61
0.78
0.35
0.58
>99/1
>99/1
>99/1
>99/1
83/17
>99/1
>99/1
78/22
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
3a
3b
3c
3d
3e
3f
3g
3h
3ac
4a
4b
4c
4d
4e
4f
0.93
0.94
0.95
0.92
0.57
0.33
0.28
0.43
0.14
0.93
0.94
0.94
0.92
0.66
0.34
0.30
0.46
0.15
0.78
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
e
0.88
0.84
0.85
0.80
0.55
0.24
0.20
0.36
0.20
0.86
0.84
0.83
0.81
0.58
0.30
0.25
0.36
0.20
0.64
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
e
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
2g
2h
2ac
3ae
4g
4h
4ac
3ie
a
LL-PTC condition: Aldehyde (10 mmol), Phosphonate (10 mmol) or Disphosphonate (5 mmol), TBAB (0.5 mmol), 50% (m/m) NaOH (20 g) and Toluene (30 mL); SL-PTC
condition: Aldehyde (10 mmol), Phosphonate (10 mmol) or Disphosphonate (5 mmol), TBAB (0.5 mmol), Solid NaOH (40 mmol) and Toluene (30 mL).
b
The Z/E ratio of 1a-2h was detected by HPLC; The Z/E ratio of 3a-4h was detected by 1H NMR.
The experiments were carried out without phase-transfer catalyst.
The yield was detected by HPLC.
3a and 3i were synthesized with the route 2.
c
d
e
Although HWE reaction could occur in both PTC systems
without PTCs, the yield of the product was relatively low. This may
be attributed to the heterogeneous reaction [46]. Few carbanions
are generated in the interface of two liquid phases or the surface of
solid NaOH, and they could not be transferred into the organic
phase or the interfacial region. Hence, the reaction of carbanion
with aldehyde is heterogeneous. The probability of effective colli-
sion between the carbanion and aldehyde is much less than that in
homogeneous phase. Therefore, the yields of 1a, 2a, 3a and 4a were
much lower than that in PTC system with PTCs.
two factors: 1) the accompanying side reaction occurs more easily
for the aldehyde containing electron-withdrawing substitute; 2)
the insoluble product dispersing in the organic phase would in-
crease the thickness of interfacial layer, thus decreasing the transfer
rate of the active intermediate and enhancing the side reactions.
The benzoic acid was detected in the aqueous phase when
3-nitrobenzaldehyde or 4-nitrobenzaldehyde was used in the both
PTC systems.
3.2. The side reaction of aldehyde
Interestingly, the effect of aldehyde or phosphonate on the yield
and geometric selectivity in LL-PTC system was similar to that in SL-
PTC system. The yield of the product containing electron-donating
group was more than 90% and much higher than that of the product
with electron-withdrawing group. Meanwhile, the yield of the
stilbene was higher than that of 1,4-distyrylbenzene or 4,40-dis-
tyrylbiphenyl. These phenomena could be ascribed to the following
The experiments on the side reaction of aldehyde in PTC system
were carried out in absence of phosphonate. It was demonstrated in
Table 2 that the side reaction of aldehyde was similar to Cannizzaro
reaction, whereas the molar ratio of carboxylic acid to benzyl
alcohol was not 1:1. The benzyl alcohol was only detected by HPLC
and could not be isolated in the reaction for benzaldehyde or
Scheme 2. The transition state structure model and rationale of selectivity in HWE reaction for DEBP under PTC conditions.

Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
343
Table 2
For aldehydes containing electron-withdrawing substitute, the
Cannizzaro intermediate can be generated in both the organic
phase and the interfacial region. In the organic phase, the “naked”
OH‾ can react quickly with aldehyde. The electron-withdrawing
substitute in the benzene ring can increase the electrophilicity of
the carbon in the aldehyde group. Because the activity of the
nucleophilic addition reaction for the carbonyl carbon increases,
the OH‾ in the interfacial region can react with aldehyde group.
Due to the relatively high area of the interfacial region and the
excess amount of OH‾, the alkoxide in the interfacial region is
deprotonated quickly. The generated Cannizzaro intermediate is
stable in the interfacial region. Subsequently, the reaction of the
Cannizzaro intermediate with another aldehyde occurs to give the
products e carboxylic acid and benzyl alcohol. However, the reac-
tion rate in the interfacial region increases with the electron
withdrawing ability of the substitute in benzaldehyde. Especially,
the Cannizzaro reaction in the interfacial region is main reaction for
3-nitrobenzaldehyde and 4-nitrobenzaldehyde in both PTC system.
Therefore, the molar ratio of product is close to 1:1.
Because the area of the interfacial region in LL-PTC system is
relatively high, the reaction rate of the Cannizzaro intermediate in
the interfacial region under LL-PTC conditions are higher than
those under SL-PTC conditions. Consequently, the rate of side re-
action for aldehyde and the content of benzyl alcohol in products
are relatively high in LL-PTC system. Moreover, due to the high
lattice energy of the crystalline structure for solid NaOH, the rate of
generating available OH‾ in SL-PTC system is lower than that in LL-
PTC system. Both HWE reaction and the side reaction have a high
rate in LL-PTC system.
The reaction time and consists of product in the side reaction of aldehyde.
Entry Aldehyde
LL-PTC
SL-PTC
Time(min) Acid/Alcohol Time Acid/
(mol/mol)
55/45
(min) Alcohol
(mol/mol)
1
10
35
65/35
2
3
4
5
6
3
50
50/50
82/18
78/22
>99/1
>99/1
25
90
55/45
92/8
30
65
88/12
>99/1
>99/1
120
240
160
320
* LL-PTC condition: Aldehyde (10 mmol), TBAB (0.5 mmol), 50% (m/m) NaOH (20 g)
and Toluene (30 mL); SL-PTC condition: Aldehyde (10 mmol), TBAB (0.5 mmol),
NaOH (40 mmol) and Toluene (30 mL).
3.3. The effect of aldehyde on the activity of HWE reaction in PTC
system
4-methoxylbenzaldehyde. However, the molar ratio was close to
1:1 in the reaction for 3-nitrobezaldehyde or 4-nitrobezaldehyde.
In both PTC systems, the molar ratio of benzyl alcohol to carboxylic
acid and the reaction rate increased with the electron withdrawing
ability of the substitute in benzaldehyde. Moreover, the molar ratio
in LL-PTC system was higher than that in SL-PTC system for the
aldehyde containing the electron-withdrawing substitute.
The effect of aldehyde on the activity of HWE reaction was
studied taking DEBP and PTCs in excess. Interestingly, which reac-
tion, HWE reaction or side reaction, dominates in PTC system is
decided by the substitute in benzaldehyde. In the reaction of
3-nitrobenzaldehyde or 4-nitrobenzaldehyde under LL-PTC condi-
tions, the stilbene could be detected, whereas more than 60% of
aldehyde underwent the side reaction (Fig. 1). Conversely, no
Cannizzaro reaction is a base-induced disproportionation of an
aldehyde. The nucleophilic addition of hydroxide anion to the
carbonyl carbon of the aldehyde is the first reaction step. The
resulting alkoxide is deprotonated to form a dianion, known as the
Cannizzaro intermediate, in a strong basic environment. Then the
intermediate reacts with another aldehyde to give the products -
carboxylic acid and benzyl alcohol [47]. However, in the PTC sys-
tem, the region where the Cannizzaro intermediate is generated is
varied with the reactivity of aldehydes, resulting in that the
following reactions and the consists of final products are different.
For aldehyde bearing electron-donating substitute, the Canni-
zzaro intermediate is generated in the organic phase. The electron-
donating substitute can decrease the electrophilicity of the carbon
in the aldehyde group. The OH‾ which can react with the aldehyde
group must have a relatively high nucleophilicity. Due to the
hydrogen-bond interaction with water molecule, the OH‾ in the
interfacial region of PTC system is not enough nucleophilic to react
with the aldehyde group and the Cannizzaro intermediate cannot
be generated. Conversely, the OH‾ which is extracted into the
organic phase by PTCs is “naked” and has a stronger nucleophilicity
and basicity than those in the interfacial region [47e49]. Therefore,
the alkoxide and Cannizzaro intermediate are generated in the
organic phase and couple with the cation of PTCs. However, due to
the limited amount of the naked OH‾ in the organic phase, the
intermediate is unstable and quickly decompose to the carboxylic
acid [50].
Fig. 1. The effect of various aldehydes bearing electron-withdrawing substitute on the
rate of HWE reaction in LL-PTC system: 10 mmol of DEBP, 2.5 mmol of benzaldehyde,
0.5 mmol of TBAB, 20 g of 50% (m/m) NaOH, 30 mL of toluene, 1400 rpm, 35 ꢁC.

344
Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
byproducts were detected for the other aldehydes, even for benz-
aldehyde in SL-PTC system (Fig. 2). These results demonstrate that
the generating rate of Cannizzaro intermediate is higher than
that of the carbanion and the side reaction dominates for
3-nitrobenzaldehyde or 4-nitrobenzaldehyde in LL-PTC system. For
the other aldehydes, the rate of HWE reaction is much higher than
that of the side reaction and the side reaction occurs hardly.
Therefore, the inhibiting role of the competing side reaction turns
out to be less significant when aldehyde with electrophilic substi-
tute is used in both PTC systems. In the following experiment,
benzaldehyde and phosphonate could be added simultaneously
into the flask.
The reaction rate in LL-PTC system increased in the sequence:
4-(dimethylamino)benzaldehyde
< 4-methoxybenzaldehyde <
4-methylbenzaldehyde < benzaldehyde< 2-choloraldehyde < 2,4-
dichlorobenzaldehyde, which was the opposite to the order of the
electron donating ability of the substitutes in benzene ring. In view
of nucleophilic nature of HWE reaction, aldehydes containing
electron-withdrawing substituents have a higher reactivity than
that containing electron-donating substitute owing to the increase
of the positive charge of the carbon in the aldehyde group.
Fig. 3. The effect of phosphonate on the process of HWE reaction in PTC system:
10 mmol of phosphonate or 5 mmol of disphosphonate, 10 mmol of benzaldehyde,
0.5 mmol of TBAB, 20 g of 50% (m/m) NaOH or 40 mmol of solid NaOH, 30 mL of
toluene, 1400 rpm, 35 ꢁC.
3.4. The effect of phosphonate on the activity of HWE reaction in
PTC system
Therefore, HWE reaction for DCBP, TEPP or TEBP has a higher ac-
tivity than that for DEBP in both PTC systems.
To explore the effect of phosphonate on the activity of HWE
reaction in PTC system, the process of HWE reaction was investi-
gated (Fig. 3). Although the yield of 1a had little change with PTC
system, the reaction rate in SL-PTC system was much lower than
that in LL-PTC system. It is also attributed to different mechanisms
in these two systems. The carbanion generated easily in LL-PTC
system due to high reactivity of the available OH‾ (“naked OH‾”).
The reaction rate in LL-PTC system decreased in the following or-
der: DCBP > TEPP > TEBP > DEBP which was the same as the order
of pKa value for phosphonates. The lower the pKa value of phos-
phonates is, the more easily the deprotonation reaction occurs.
3.5. The third phase in LL-PTC system
The conversion of the traditional LL-PTC into liquideliquide
liquid PTC (TL-PTC) offers several advantages, such as catalyst re-
covery, waste reduction, better selectivity and improving cost-
effectiveness. Above all, the third phase (the middle catalyst-rich
phase) can enhance the reaction rate by several orders of magni-
tude [51]. In the synthesis of 1a and 2a, there was one phase
exciting between the organic phase and the aqueous phase (Fig. 4a
and b), and the volume of the third phase was less than 1 mL.
However, in the synthesis 3a and 4a, the third phase was not found.
The 3a or 4a is dispersed in the organic phase owning to their poor
dissolubility. Thus, the viscosity of the organic phase is increased
and the third phase could not be separated. The aggregation of the
PTCs in the third phase may be one of the reasons for the high
activity of HWE reaction in LL-PTC system. The constituents of the
third phase and their effect on the volume of third phase will be
discussed in a separated work.
Meanwhile, the electron-withdrawing substitute and
p-conjuga-
tion can increase the stability of the oxaphosphetane intermediate.
3.6. The third phase in SL-PTC system
As water is added, SL-PTC system can be changed to solideliquid
(
u)-liquid (org) PTC system. It is known that the u phase also en-
hances the reaction rate in SL-PTC, and there is an optimum
quantity beyond which the rate falls dramatically [52]. Thus the
experiments were done by carefully adding trace quantities of
water in 30 mL of the organic phase. It has been shown in Fig. 5 that
the small amounts of water increases substantially the reaction
rates compared to those under anhydrous conditions, whereas the
reaction rate decreases with the addition of excess water. The polar
H2O molecule can overcome the lattice energy of the crystalline
structure for the solid NaOH. Hence, with the increasing in the
amount of available NaOH, much more carbanions are generated in
the interfacial region. When an excessive amount of water has been
added, there is sufficient water for saturation of the hydration shell
of OH‾. Therefore, the basicity of OH‾ decreases considerably and
the carbanion cannot be generated efficiently. Meanwhile, a little
water can distribute into the organic phase and the degree of
Fig. 2. The effect of aldehydes bearing electron-donating substitute on the rate of HWE
reaction in PTC system: 10 mmol of DEBP, 2.5 mmol of benzaldehyde, 0.5 mmol of
TBAB, 20 g of 50% (m/m) NaOH or 40 mmol of solid NaOH, 30 mL of toluene, 1400 rpm,
35 ꢁC.

Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
345
Fig. 4. The image of the third phase in LL-PTC system (a) the reaction of DCBP with benzaldehyde; (b) the reaction of DCEP with benzaldehyde; (c) the reaction of TEBP with
benzaldehyde: 10 mmol of benzaldehyde, 10 mmol of phosphonate or 5 mmol of disphosphonate, 0.5 mmol of TBAB, 20 g of 50% (m/m) NaOH and 30 mL of toluene.
hydration of the ion-pair increases, resulting in the decrease of its
reactivity. Thus, the dehydration treatment for the organic solvent
is not required in SL-PTC system, which can decrease the cost of
organic solvent.
3.7. Reusability of the liquid phase in LL-PTC system
In LL-PTC system, the aqueous phase was reused by adding the
fresh organic phase and PTCs. It can been observed that the rate of
HWE reaction in LL-PTC system decreases slightly and the yield of
the product is unchanged with the reuse times of the aqueous
phase (Fig.6). The yield and reaction rate when the aqueous phase
was reused five times were still higher than that where the aqueous
phase contained 40% NaOH. The result shows that the decreasing
concentration of NaOH and the increasing concentration of salt in
the reused aqueous phase decrease only slightly the generating rate
of available OH‾ and the transferring rate of active intermediate.
The aqueous phase can be reused four times without sacrifice of the
yield and the reaction rate. Unexpectedly, due to the high viscosity
of the aqueous phase, the separation of two phases needed a long
time and the weight of the aqueous phase was less than 20 g.
Accordingly, the following research will be targeted at the explo-
ration of new PTC system in which the aqueous phase can be
separated and reused easily.
Fig. 5. The effect of water on the process of HWE reaction in SL-PTC system: 10 mmol
of DEBP, 10 mmol of benzaldehyde, 0.5 mmol of TBAB, 40 mmol NaOH, 30 mL of
toluene, 1400 rpm, 35 ꢁC.

346
Q. Zhao et al. / Dyes and Pigments 99 (2013) 339e347
generated by the addition of a trace amount of water in SL-PTC
system could increase the reaction rate. In addition, the aqueous
phase can be recycled four times without sacrifice of the yield in
HWE reaction under LL-PTC condition, thereby leading to high cost-
effectiveness and waste reduction.
It is anticipated that this simple, controllable and economical
method will provide a potential application of HWE reaction in
chemical industry.
Acknowledgements
One of the authors is thankful to Dr. Guimin Zhang, Shanghai
Heliya Fine Chemicals Co., Ltd., for his constant encouragement.
Financial assistance from Donghua University is acknowledged
gratefully.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.05.017.
Fig. 6. The dependence of the yield, the reaction time and the weight of aqueous phase
on the reuse time: 10 mmol of DEBP, 10 mmol of benzaldehyde, 0.5 mmol of TBAB,
30 mL of toluene, 1400 rpm, 35 ꢁC.
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