Acceleration of disproportionation of aromatic alcohols through self-emulsification of reactants in water

Article abstract of DOI:10.1002/cssc.201200562

Exploration of new and effective routes to conduct organic reactions in water using the special properties of water/organics is of great importance. In this work, we performed the disproportionation of various aromatic alcohols in water and in different organic solvents. It was demonstrated that the disproportionation reactions of the alcohols were accelerated more effectively in water than organic-solvent-based or solvent-free reactions. A series of control experiments were conducted to study the mechanism of the accelerated reaction rate in water. It was shown that the reactants could emulsify the reactant/water systems at the reaction conditions owing to their amphiphilic nature. The regularly orientated reactant molecules at the water/reactant droplet interface improved the contact probability of the reactive groups and the Pd nanocatalysts, which is one of the main reasons for the enhanced reaction rate in water. Controlling the self-emulsification of amphiphilic reactant/water systems has great application potential for optimizing the rate and/or selectivity of many organic reactions.

Full text of DOI:10.1002/cssc.201200562

DOI: 10.1002/cssc.201200562
Acceleration of Disproportionation of Aromatic Alcohols
through Self-Emulsification of Reactants in Water
Binbin Zhang, Jinliang Song, Huizhen Liu, Buxing Han,* Tao Jiang, Honglei Fan,
[a]
Zhaofu Zhang, and Tianbin Wu
Exploration of new and effective routes to conduct organic re-
actions in water using the special properties of water/organics
is of great importance. In this work, we performed the dispro-
portionation of various aromatic alcohols in water and in differ-
ent organic solvents. It was demonstrated that the dispropor-
tionation reactions of the alcohols were accelerated more ef-
fectively in water than organic-solvent-based or solvent-free re-
actions. A series of control experiments were conducted to
study the mechanism of the accelerated reaction rate in water.
It was shown that the reactants could emulsify the reactant/
water systems at the reaction conditions owing to their amphi-
philic nature. The regularly orientated reactant molecules at
the water/reactant droplet interface improved the contact
probability of the reactive groups and the Pd nanocatalysts,
which is one of the main reasons for the enhanced reaction
rate in water. Controlling the self-emulsification of amphiphilic
reactant/water systems has great application potential for opti-
mizing the rate and/or selectivity of many organic reactions.
Introduction
In recent years, reactions using water as the medium have at-
tracted considerable attention because it is abundantly avail-
water nucleophilic substitution of alcohols obtained satisfacto-
[17]
ry results without the use of acidic catalysts. McErlean and
co-workers developed a domino in-water–on-water process
that shuttles molecules between the aqueous and organic
phases based on the solubility difference of the reactants and
[
1]
able, inexpensive, non-toxic, and non-inflammable. A variety
of organic reactions have been shown to have improved reac-
tivity and selectivity in aqueous solutions. For example, Diels–
Alder cycloadditions and Claisen rearrangements of nonpolar
compounds in dilute aqueous solutions were accelerated sig-
nificantly compared to the organic-solvent-based or solvent-
[18]
products. It was also reported that diarylprolinolsilyl ether
salts could be used as the catalysts for asymmetric Michael ad-
dition of aldehydes to nitroolefins on water with excellent dia-
[
2]
[19]
free reactions. The reactions of azodicarboxylates with vari-
ous aldehydes to produce hydrazine imide products were per-
stereo- and enantioselectivities.
Various interactions and effects may exist in the aqueous re-
action systems, such as electrostatic effects, polarization, hy-
drogen bonding, and hydrophobic effects. All of these interac-
[3]
formed very efficiently in water without the use of a catalyst.
The asymmetric desymmetrization of meso-epoxides with
amines showed a higher enantiomeric excess (ee) in water
[1,10a]
tions and effects can influence the reactions in water;
[
4]
than in organic solvents. High yield and selectivity for the
synthesis of ladder polyether moieties were achieved in
which factors are dominant depends on the natures of the re-
actants and reaction conditions. The mechanisms with which
water enhances the efficiency of different reactions have been
studied. For example, rate acceleration in water for the reac-
tion with a negative activation volume, such as Diels–Alder re-
actions, was ascribed to the internal pressure from water wrap-
[
5]
water. Knoevenagel condensation of aromatic aldehydes in
water could be accelerated dramatically by a cationic coordina-
[
6]
tion cage under neutral conditions. 2-Naphthols and substi-
tuted phenols could be converted to the corresponding biaryl
compounds efficiently in water using molecular oxygen as the
[2b,20]
ping the hydrophobic reactant aggregates.
Hydrogen
[
7]
oxidant. The direct alkyne–imine addition promoted by cop-
bonding could make an important contribution to the acceler-
[21]
per(I)-based catalysts showed a high ee and a good yield in
ation of Diels–Alder reactions in aqueous solution. It was
suggested that a rate increase of the Diels–Alder reactions of
diene carboxylate salts with dienophiles in aqueous suspen-
sions originated from micellar catalysis, resulting in mutual
[
8]
water. Indium(0) could catalyze the allylation of ketones effi-
[
9]
ciently in water under mild conditions.
Interestingly, many reactions of hydrophobic reactants that
are insoluble in water have been conducted more effectively in
aqueous suspensions than in organic solvents, such as Diels–
[
a] B. Zhang, Dr. J. Song, Dr. H. Liu, Prof. B. Han, Prof. T. Jiang, Dr. H. Fan,
Dr. Z. Zhang, Dr. T. Wu
[
10]
[10a,11]
Alder reactions, Claisen rearrangements,
Passerini reac-
[
12]
[13]
tion, iodine transfer cyclization of a-iodoacetates, direct al-
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Colloid and Interface and Thermodynamics
Institute of Chemistry, Chinese Academy of Sciences
Beijing, 100190 (PR China)
Fax: (+86)10-62559373
E-mail: hanbx@iccas.ac.cn
[
14]
kynylation of isatins, oxidation of aldehydes by using oxygen
[
15]
[16]
as the oxidant, and direct arylation of thiazoles. The term
on-water has been used to describe the substantial rate accel-
[
10a]
erations in aqueous suspensions.
It was reported that on-
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2469

B. Han et al.
[
10b,c]
binding of the reactants in an aggregate.
Engberts and
reactant/water systems at suitable conditions. Therefore, in the
reactant/water emulsions emulsified by the reactant(s) itself,
the reactant droplets are dispersed in water, the reactant mole-
cules oriented in a regular fashion at the interface, and the
special orientation of the reactant molecules is favorable to en-
hance the contact probability of the reactants, which may in-
fluence the reactions significantly. In this work, we performed
the first work to provide evidence for the validity of the idea
by conducting disproportionation of different alcohols
(Scheme 1) already extensively studied in different solvents,
Blokzijl provided evidence that some Diels–Alder reactions
were accelerated by an enforced hydrophobic effect, which re-
sulted in the destabilization of the reactants as well as a hydro-
gen-bonding stabilization of the polarizable activated com-
[
22]
plex.
The on-water effect has also been explained by an
acid-catalysis mechanism facilitated by the strong adsorption
of the hydroxide ion at the oil/water interface, leading to acid
[
23]
catalysis and an enhanced rate. Transition-state theory and
energy-minimization studies of the on-water cycloaddition of
quadricyclane and dimethyl azo-
dicarboxylate showed that the
reaction rate was accelerated
À
because the free OH groups of
interfacial
formed
water
molecules
hydrogen
Scheme 1. Disproportionation of alcohols.
stronger
bonds with the transition state
[
24]
than with the reactants.
[
28]
[29]
[30]
Mixed quantum and molecular mechanical methodology stud-
ies indicated that on-water rate enhancement for Claisen rear-
rangements was derived from the ability of the interfacial
such as THF, methanol, toluene, and at solvent-free con-
[31]
dition. Pd nanoparticles are efficient catalysts for the dispro-
[29]
portionation reactions.
[
25]
water to stabilize the polar transition state. A quantum me-
chanical/molecular mechanical modeling study indicated that
some Diels–Alder reactions are accelerated less on the surface
Results and Discussion
[
26]
of water than in water. The origins of water improving the
efficiency of different reactions are very complex and still need
to be studied further.
Self-emulsification of the reactant/water systems
We first studied the phase behaviors of the reaction systems of
A surfactant molecule consists of a polar head and apolar
1
2
the disproportionation of 1-phenylethanol (R =Ph, R =CH in
[
27]
3
tail.
Oil/water systems can form oil-in-water emulsions at
Scheme 1) in different solvents without the catalyst. The pho-
tographs are presented in Figure 2 as insets. It can be seen
that the reactant was miscible with the organic solvents and
transparent solutions were formed.. However, the reactant and
water were not miscible, and the reactant/water mixture
became milky after stirring. Our experiment indicated that the
emulsion could be stable for at least 24 h. The stable emulsion
was probably formed because the reactant has the nature of
a surfactant with a polar group and apolar tail. The formation
of the emulsion was further confirmed by laser scanning con-
focal microscopy (LSCM), which is a commonly used technique
suitable conditions with the aid of surfactants, and the surfac-
tant molecules orientate regularly at the water/oil droplet in-
terface, with polar groups facing the water phase and the
[
27]
apolar tails facing the oil phase, as shown schematically in
Figure 1. The regularly orientated polar groups of the surfac-
tant molecules at the oil/water interface have a high local den-
sity.
[32]
to study the droplet size of emulsions. According to Figure 2
the size of the droplets in the emulsion was in the range of
0.2–2 mm.
Disproportionation of 1-phenylethanol in different solvents
We studied the disproportionation of 1-phenylethanol in differ-
ent solvents at the same condition for the phase behavior
studied above, and a commercial Pd/C catalyst was used,
[
33]
Figure 1. Diagram of oil-in-water emulsions.
which was characterized in detail previously. The results are
also presented in Figure 2. It shows that the conversion of the
reactant in water was substantially higher than in the organic
solvents or solvent-free under the same reaction conditions.
We also conducted the reaction at longer times in different sol-
vents, and the conversion in water was also higher than those
in organic solvents (entry 1, Table 1). In this work, no byprod-
uct was detected in the reaction.
Exploration into effective routes to conduct organic reac-
tions in water using the special properties of water/organic
systems, as well as further studying the mechanism is of great
importance. Some organic reactants are amphiphilic, and these
types of reactants may also act as surfactants to emulsify the
2
470
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Disproportionation of Aromatic Alcohols
and the contact probability of
two hydroxyl groups with the
Pd catalyst. Figure 2 indicates
that the conversions in different
organic solvents were similar,
suggesting that the solvation
effect was not dominant in the
reaction. In this work, the dis-
proportionation of 1-phenyle-
thanol in different solvents was
also conducted at a dilute reac-
tant concentration, at which the
reactant was also soluble in
water. The results indicated that
the conversions in the organic
solvents and in water were
nearly the same (Table 1,
Figure 2. a) The conversion of the disproportionation of 1-phenylethanol in different solvents; reaction conditions:
1-phenylethanol 2 g (16 mmol), solvent 3 mL, Pd/C with 5 wt% Pd (23.5 mmol), 1008C, 2 h; the insets are photo-
graphs of the corresponding reactant/water mixtures without the catalyst. b) LSCM image of 1-phenylethanol/
water emulsion at room temperature.
Table 1. Disproportionation of alcohols in water.
entry 4), which further supported the conclusion that the sol-
vation effect was not the main factor for the reaction. There-
fore, it can be postulated that the contact probability of two
hydroxyl groups and the Pd catalyst was the dominant factor
in the reaction. In the organic solvents and at the solvent-free
condition, the reactant molecules were oriented randomly,
whereas they were oriented in a regular fashion at the water/
reactant droplet interface in water, as shown schematically in
Figure 3. In the aqueous system the contact probability of the
two hydroxyl groups and the catalyst at the water/reactant
droplet interface was much larger than in organic solvents and
solvent-free conditions. This can be attributed to the ordered
orientation, which leads to the larger reaction rate in water.
[
c]
Entry
Substrates
t
Conversion [%]
[h]
heptane
propanol
water
S
S
E
1
2
12
51.3
48.9
96.2
S
S
E
4
12
2
68.5
67.3
94.9
S
S
E
3
4
60.7
58.0
98.8
S
S
S
20.6
20.3
20.2
[
5
5
a] Reaction conditions: substrate 16 mmol, solvent 3 mL, Pd/C with
wt% Pd (23.5 mmol), 1008C. [b] 1-phenylethanol 0.8 mmol, solvent
mL, Pd/C with 5 wt% Pd (4.7 mmol), 1008C. [c] All conversions were de-
The reaction in 1-propanol-water mixed solvents
To further support the mechanism proposed above, in which
water accelerates the reaction, we also performed the reaction
in 1-propanol/water mixed solvents of different compositions.
In combination with the phase behavior study of the reaction
system, the results are illustrated in Figure 4. The reaction
system experienced a transformation from a transparent single
phase to a milky solution upon increasing the water volume
termined by GC; S=single phase; E=emulsion is formed.
The catalytic reaction studied in this work involves two reac-
tant molecules. The two hydroxyl groups and the Pd catalyst
must contact for the reaction to proceed. The reaction rate
probably depends mainly on two factors: the solvation effect
fraction in the mixed solvent (V ). This was caused by the in-
w
Figure 3. Schematic diagrams of the reactant orientation in organic solvents (a), solvent-free (b), and water (c).
ChemSusChem 2012, 5, 2469 – 2473
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B. Han et al.
The disproportionation of other aromatic alcohols
We also studied the disproportionation of other aro-
matic alcohols in different solvents, and the results
are presented in Table 1. 1-Indanol (entry 2) and 1-
(
4-methoxylphenyl)ethanol (entry 3) were soluble in
the organic solvents and could emulsify the aqueous
reaction systems (Figure 5). The conversions of the
reactions in water were much higher than those in
organic solvents at the same conditions, further sup-
porting the proposed mechanism, in which the
larger contact probability, originating from self-emul-
sification, accelerated reactions substantially. Con-
versely, controlling self-emulsification is an effective
tool to optimize the reaction conditions.
Figure 4. Conversion and solubility of 1-phenylethanol in 1-propanol/water mixed sol-
vents with different water volume fractions (V ). Reaction conditions: 1-phenylethanol
g (16 mmol), solvent 3 mL, Pd/C with 5 wt% Pd (23.5 mmol), 1008C, 2 h. The insets are
w
2
photographs of the reactant/1-propanol–water mixtures.
Conclusions
The disproportionation reactions of aromatic alco-
hols in water are faster than those in organic sol-
crease of the hydrophilicity of the mixed solvent. We also stud-
ied the solubility of the reactant in the mixed solvents, and the
results are also presented in Figure 4. In the one phase region
vents, mainly because the reactants are amphiphilic and can
emulsify the reactant/water systems. The regularly orientated
reactant molecules at the water/reactant droplet interface give
rise to a larger contact probability of the reactive groups with
the catalyst, which is one of the main reasons for the accelerat-
ed reactions. We believe that the self-emulsification ability of
amphiphilic reactant/water systems provides a useful tool to
control the reaction rate and/or selectivity of many reactions in
aqueous media, especially if the contact probability is a domi-
nant factor in the reaction.
(
Vw <0.33), the conversion was nearly independent of the com-
position of the mixed solvent, indicating again that the solva-
tion effect did not affect the reaction considerably. The conver-
sion increased significantly with water content, since the reac-
tion system started to self-emulsify as Vw approached 0.66. In
the V range of 0.33 to 0.66, the solubility of the reactants de-
w
creased substantially with increasing V , that is, the reactant
w
molecules existing at the water/reactant interface increased
with water content significantly, leading to dramatic increase
of the conversion with V . The solubility of the reactant in the Experimental Section
w
mixed solvent was very low in the Vw range of 0.66 to 1.00,
Materials: The commercial Pd/C catalyst was provided by Baoji
Rock Pharmachem Co., Ltd., China (5 wt% Pd, Product No. D L ),
and thus most of the reactants existed as dispersed droplets in
water, and the reactant molecules at the interface did not
5
3
which was the same as that used previously for the hydrogenation
[33]
change considerably with the variation in V . Therefore, the
of phenol to cyclohexanone and was characterized in detail. N,N-
dimethylformamide (A. R. grade), toluene (A. R. grade), n-heptane
w
conversion was, again, almost independent of the water con-
tent. These results further support the proposed mechanism,
in which the regular orientation of the reactant molecules at
the interface enhanced the reaction significantly.
(A. R. grade), 1-propanol (A. R. grade), were purchased from Beijing
Chemical Reagent Company. 1-(4-Methoxylphenyl)ethanol (A. R.
grade), 1-indanol (A. R. grade) and 1-phenylethanol (A. R. grade)
were obtained from Alfa Aesar China (Tianjin).
Methods: All the reactions were performed under nitrogen (1 bar)
in a stainless steel autoclave (15 mL), which was similar to that
[33]
used previously. Only the procedures for the disproportionation
of 1-phenylethanol are described because those of the other sub-
strates were similar. In the experiment, suitable amounts of 1-phe-
nylethanol (reactant) and Pd/C (catalyst) were loaded into the au-
toclave. The autoclave was sealed and placed in a silicon oil bath
(1008C), and the reaction mixture was stirred using a magnetic stir-
rer (400 rpm). After the desired reaction time, the autoclave was
cooled in a cold water bath. If an organic solvent was used for the
reaction, the reaction mixture was analyzed directly. If water was
the reaction solvent, the reaction mixture was extracted with
CH Cl (20 mL). The organic layer was analyzed.
Figure 5. The photographs of the reaction mixtures of a) 1-indanol/n-hep-
tane, b) 1-indanol/1-propanol, c) 1-indanol/water, d) 1-(4-meth-oxylphenyl)e-
thanol/n-heptane, e) 1-(4-methoxylphenyl)ethanol/1-propanol, f) 1-(4-me-
thoxylphenyl)ethanol/water, and the LSCM images of g) 1-indanol/water and
h) 1-(4-methoxylphenyl)ethanol/water.
2
2
The quantitative analysis of the reaction mixture was performed
using GC (Agilent 6820) equipped with a flame ionization detector
(FID) and a PEG-20m capillary column (0.25 mm in diameter, 30 m
2
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Disproportionation of Aromatic Alcohols
in length).Benzene was used as internal standard. Identification of
the products and reactants was performed by using GC–MS (SHI-
MADZU-QP2010) as well as by comparing the retention times to
respective standards in GC traces. The conversion was calculated
from the GC data.
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Published online on October 22, 2012
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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