Fast and green microwave-Assisted conversion of essential oil allylbenzenes into the corresponding aldehydes via alkene isomerization and subsequent potassium permanganate promoted oxidative alkene group cleavage

Article abstract of DOI:10.3390/molecules14093411

Essential oil allylbenzenes from have been converted quickly and efficiently into the corresponding benzaldehydes in good yields by a two-step "green" reaction pathway based on a solventless alkene group isomerization by KF/Al₂O₃ to form the corresponding l-arylpropene and a subsequent solventless oxidation of the latter to the corresponding benzaldehyde by KMnO₄/CuSO_4-5H₂O. The assistance by microwave irradiation results in very short reaction times (<15 minutes). The green conversion of eugenol (4-allyl-2-methoxyphenol) into vanillin (4-hydroxy-3-methoxybenzaldehyde) has been carried out in a similar way, requiring however two additional microwave-assisted synthetic steps for acetylation of the hydroxy group prior to the oxidation reaction, and for the final deacetylation of vanillin acetate (4-acetoxy-3-methoxybenzaldehyde) by KF/Al₂O₃ under solvent-free conditions, respectively.

Full text of DOI:10.3390/molecules14093411

Molecules 2009, 14, 3411-3424; doi:10.3390/molecules14093411
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Fast and Green Microwave-Assisted Conversion of Essential Oil
Allylbenzenes into the Corresponding Aldehydes via Alkene
Isomerization and Subsequent Potassium Permanganate
Promoted Oxidative Alkene Group Cleavage
Thi Xuan Thi Luu 1, Trinh To Lam 2, Thach Ngoc Le 2 and Fritz Duus 1,*
1
Department of Science, Systems, and Models, Roskilde University, P.O. Box 260, DK-4000,
Roskilde, Denmark; E-mail: luu@ruc.dk (T.X.T.L.)
2
Department of Organic Chemistry, University of Science, National University of Ho Chi Minh City,
227 Nguyen Van Cu, Ho Chi Minh City, Vietnam; E-Mail: trinh.lam@bluescopesteel.com (T.T.L.);
lenthach@hcm.vnn.vn (T.N.L.)
* Author to whom correspondence should be addressed; E-Mail: fd@ruc.dk.
Received: 12 August 2009; in revised form: 2 September 2009 / Accepted: 2 September 2009 /
Published: 3 September 2009
Abstract: Essential oil allylbenzenes from have been converted quickly and efficiently
into the corresponding benzaldehydes in good yields by a two-step “green” reaction
pathway based on a solventless alkene group isomerization by KF/Al2O3 to form the
corresponding 1-arylpropene and a subsequent solventless oxidation of the latter to the
corresponding benzaldehyde by KMnO4/CuSO4‚5H2O. The assistance by microwave
irradiation results in very short reaction times (<15 minutes). The green conversion of
eugenol (4-allyl-2-methoxyphenol) into vanillin (4-hydroxy-3-methoxybenzaldehyde) has
been carried out in a similar way, requiring however two additional microwave-assisted
synthetic steps for acetylation of the hydroxy group prior to the oxidation reaction, and for
the final deacetylation of vanillin acetate (4-acetoxy-3-methoxybenzaldehyde) by
KF/Al2O3 under solvent-free conditions, respectively.
Keywords: microwave irradiation; solvent-free reaction; alkene isomerization; alkene
oxidation

Molecules 2009, 14
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Introduction
Allylbenzenes such as e.g., methylchavicol, safrole, and eugenol are the main ingredients of several
essential oils found in plants in tropical areas [1]. The usefulness of these compounds as starting
materials in the synthesis of important flavourings has been amply demonstrated. Thus, in 1924
Aoyama and Otake reported the preparation of vanillin from eugenol via KOH promoted alkene group
isomerization and subsequent oxidation of isoeugenol by ozonolysis [2]. In 1949 Mayer prepared
vanillin by alkene isomerization of eugenol followed by oxidation of isoeugenol by nitrobenzene [3].
In 1992 Bao treated sassafras oil, containing 90% of safrole, with aqueous KOH and oxidized the
isomerized product by sodium dichromate in 32% aqueous H2SO4 [4]. In fact, the synthetic
transformation of naturally occurring allylbenzenes into flavouring aromatic aldehydes by alkene
group isomerization and subsequent oxidative cleavage of the new C=C double bond has been studied
quite extensively, both with respect to the nature of the base catalyst used for alkene isomerization
[5-15], and the nature of the oxidation agent used for formation of the aldehyde [15-20].
In recent years the principles of “green chemistry” [21-26] have gained increasing attention for the
benefit of environmental mildness and savings of materials and energy resources. New concepts such
as "green reagents", "solvent-free reactions", and "microwave or ultrasound supported reactions" are
now generally accepted and widely used. In this paper we present a new completely green microwave-
assisted synthetic procedure for the conversion of allylbenzenes (whether or not procured from
essential oils) into the corresponding benzaldehydes in two steps: solvent-free isomerization of the
former into the corresponding 1-arylpropenes by KF/Al2O3, and green solvent-free oxidation of the
latter into the benzaldehydes by KMnO4/CuSO4‚5H2O (Scheme 1, Scheme 2). For the conversion of
eugenol into vanillin (Scheme 2), two additional subordinate steps were required for protection of the
phenolic OH group: microwave-assisted acetylation of isoeugenol by acetic anhydride, and
microwave-assisted solvent-free ester hydrolysis [27] by KF/Al2O3. For comparison, all the above
mentioned reactions were also performed under conventional heating conditions.
Results and Discussion
Isomerization of allylbenzenes
In 1990 Radhakrishna and coworkers introduced KF/Al2O3 as an efficient and versatile reagent for
isomerization of olefins under heterogeneous reaction conditions [7]. Three years later, on the basis of
a comparative study of the efficiencies of KF/Al2O3, KOH, and t-BuOK as isomerization reagents
towards olefins, Le and coworkers called attention to the usefulness of the first mentioned reagent, in
particular under solvent-free reaction conditions [8]. The assistance of microwave irradiation was
introduced in 1993 by Loupy and Le in connection with their investigation of the base-catalysed
isomerization of eugenol [9]. The microwave-assisted isomerization of safrole was reported by
Salmoria and his coworkers in 1997 [10]. The applicability of microwave-activated isomerization of
organic compounds adsorbed onto inorganic solids was demonstrated by Villemin and coworkers in
1989 [28].

Molecules 2009, 14
Scheme 1. Conversion of allylbenzenes into the corresponding benzaldehydes under
3413
solvent-free conditions.
CHO
KF/Al2O3
KMnO4/CuSO4.5H2O
+
MW or Δ
MW or Δ
Y
Y
Y
Y
X
X
X
X
3
(E)-2
(Z)-2
1
X
Y
1-3
a
H
H
b
c
OCH3
H
OCH3
O
OCH3
O
d
CH2
Scheme 2. Conversion of eugenol into vanillin under solvent-free conditions.
KF/Al2O3
Ac2O
+
+
MW or Δ
MW or Δ
OCH3
OCH3
OCH3
OCH3
OCH3
OAc
OAc
OH
1e
OH
OH
(Z)-2e'
(E)-2e'
(E)-2e
(Z)-2e
CHO
CHO
KF/Al2O3
MW or Δ
KMnO4/CuSO4.5H2O
MW or Δ
OCH3
OCH3
OH
OAc
3e
4e
Altogether four allylbenzenes, 1a-1d, were subjected to isomerization by KF/Al2O3 under solvent-
free reaction conditions. Two different methods were used. In Method A (Table 1) the mixture of the
allylbenzene and KF/Al2O3 was irradiated by microwaves for 6-15 minutes. For each of the
allylbenzenes, the optimum molar ratio of the allylbenzene and KF/Al2O3 was determined. In general,
high yields were obtained within short reaction times. In method B (Table 1) the same reaction was
performed without microwave irradiation, the reaction vessel being instead placed in an oil bath kept at
the same temperature as measured for the reaction mixture at the end of the microwave-assisted

Molecules 2009, 14
3414
reaction. The results of Method B (Table 1) clearly demonstrate that high reaction temperatures and
long reaction times are required for the complete conversion of allylbenzenes into the isomeric 1-aryl-
propenes. On the other hand, the high capacity of KF/Al2O3 for microwave energy absorption may lead
to incipient decomposition of the product before the isomerization is fully completed. The attempted
isomerization of eugenol (1e) into isoeugenol under solvent-free reaction conditions gave only a low
yield of the product (14%), probably due to the extensive deprotonation of the phenolic OH group.
However, according to Radhakrishna and his coworkers [7], eugenol can be converted into isoeugenol
in the yield of 78% by KF/Al2O3 upon reflux in ethylene glycol for 90 minutes. Evidently, the solvent
ethylene glycol has the effect of being an inhibitor towards phenolic OH group deprotonation.
Therefore, the microwave-assisted KF/Al2O3-promoted isomerization of eugenol into isoeugenol was
performed under heterogeneous conditions, using ethylene glycol as the solvent. In order to determine
the optimum reaction conditions, a series of experiments were carried out, in which the amount of the
solvent was varied in proportion to the amount of KF/Al2O3 (Figure 1).
Figure 1. Influence of solvent volume relative to the quantity of KF/Al2O3 on the
efficiency of the isomerization of eugenol into isoeugenol under heterogeneous reaction
conditions (eugenol: 2 mmol, KF/Al2O3: 40 mmol, 5.8 g).
100.00
80.00
60.00
40.00
20.00
0.00
0.5
0.6
0.7
0.8
1.0
Ratio between ethylene glycol and catalyst (mL/g)
All experiments were carefully monitored by GC/MS. Thus, it appeared that the isomerization
reactions in all cases gave a mixture of the geometrically isomeric cis- and trans-1-arylpropenes, (Z)-2
and (E)-2, respectively. The trans-1-arylpropenes were the predominant species in all cases (Table 1).
The identity of all products was confirmed by 1H- and 13C-NMR spectroscopy (Experimental Section).

Molecules 2009, 14
3415
Table 1. Optimized results of allylbenzene isomerization by KF/Al2O3.
Method Aa Method Ba
Selectivityb (%) Selectivityb (%)
1: (KF/Al2O3)
(mol/mol)
Yieldb (Conv.,b
time,c Pd)
Yieldb (Conv.,b
1
time,c∗ tempf)
cis
trans
85
86
83
77
cis
10
13
13
12
trans
1a 2 : 4
1b 2 : 10
1c 2 : 4
1d 2 : 8
96(100, 11, 630) 15
94 (100, 13, 400) 14
99 (100, 2.5, 140)
96 (100, 3.5, 150)
99 (100, 2.5, 140)
98 (100, 2.5, 150)
90
87
87
88
67
96 (99, 15, 400)
96 (96, 6, 630)
97 (97, 9, 80) a∗
16
19
6
1e 2 : 40
91
77 (78, 1.3, 203)a∗∗ 11
a
Method A: The reaction was assisted by microwave irradiation under solvent-free conditions;
a∗
Method B: The reaction was assisted by heating in an oil bath under solvent-free conditions;
Method A1: The reaction was assisted by microwave irradiation under heterogeneous conditions;
a∗∗ Method B1: The reaction was assisted by heating in an oil bath under heterogeneous conditions;
b
c
Yield, Conv. (conversion yield), and Selectivity were determined by GC/MS; time = reaction
time in minutes; c∗ time = reaction time in hours; d P = power of microwave oven (W); f temp = oil
bath temperature (°C).
Oxidation of 1-arylpropenes
In 1979 Menger and Lee announced that under heterogeneous conditions a mixture of KMnO4 and
CuSO4‚5H2O was unreactive towards the carbon-carbon double bond in the oxidation reaction of
unsaturated secondary alcohols into the corresponding ketones using the above mentioned reagent
[29]. Nevertheless, inspired by later reports on successful KMnO4 promoted oxidation reactions of
1-aryl-1-alkenes [17,19], we investigated the possibility of using the mixture of KMnO4 and
CuSO4·5H2O for oxidation of 1-phenylpropene (our pertinently chosen model substance) into
benzaldehyde under solvent-free reaction conditions. In order to find the most efficient oxidation
agent, a series of experiments was performed, where the molar ratio between KMnO4 and
CuSO4‚5H2O was varied. The most efficient of the oxidants investigated (all of them being prepared
by the vaporization method [30]), appeared to be KMnO4 absorbed on a fourfold molar amount of
CuSO4‚5H2O. This oxidant, in the following referred to as PP/4CSP (potassium permanganate
absorbed on a fourfold molar amount of copper sulfate pentahydrate) was chosen as the standard
oxidant in our subsequent experiments. In order to achieve the very best yield of benzaldehyde, we
paid attention also to the molar ratio between 1-phenylpropene and PP/4CSP. The most appropriate
molar ratio (3:12) was also chosen for the oxidation of the four other 1-arylbenzenes. All
1-arylpropenes used were mixtures of the geometrically isomeric forms (Table 1).
The solvent-free oxidation of the 1-arylpropenes into the corresponding benzaldehydes was
performed under the assistance of microwave irradiation (Method C), as well as under conventional
heating conditions (Method D). The results are listed in Table 2. For both methods, it is clear that the
1-arylpropenes containing electron donating substituents are oxidized most efficiently. Not
surprisingly, the microwave-assisted oxidations proceed markedly faster than the corresponding
reactions driven by conventional heating. However, the yields obtained by Method C also appear to be
generally higher than those of Method D. Minor amounts of overoxidation products were easily

Molecules 2009, 14
3416
removed by filtration of the crude product mixture through a layer of NaHCO3/SiO2 (Experimental
Section). Thus the 1-arylpropenes 2a-2d could be smoothly converted into the aldehydes 3a-3d.
However, upon treatment with PP/4CSP, isoeugenol (2e) partly decomposed with the development
of some black smoke. Obviously, the phenolic OH group of isoeugenol should be protected. Thus a
mixture of isoeugenol and a minor excess of acetic anhydride was reacted to form isoeugenol acetate
(2e’, Scheme 2) under the assistance of microwave irradiation, as well as under conventional heating
conditions. As expected, the crucial factor for obtaining maximum yield by both methods is the
reaction time, which was found to be considerably shorter for the microwave-assisted reaction. The
100% conversion of isoeugenol (2e) into isoeugenol acetate (2e’) under the assistance of microwave
irradiation (80 W) could be performed within 20 minutes, whereas the same reaction supported by
conventional heating at 150 °C required 90 minutes.
Table 2. Oxidation of 1-arylpropenes into corresponding benzaldehydes.
Method Ca
Method Da
KMnO4 CuSO4‚5H2O
2 (mmol)
Yieldb (Conv.,b time,c Pd) Yieldb (Conv.,b time,c tempf)
(mmol)
(mmol)
24
2a (1.5)
2b (1.5)
2c (1.5)
2d (1.5)
6
6
6
6
6
52 (60, 8, 240)
70 (86, 7.3, 350)
79 (96, 7.5, 350)
73 (90, 17, 450)
58 (77, 16, 450)
50 (57, 60, 90)
80 (95, 60, 96)
75 (94, 13, 100)
77 (93, 60, 95)
56 (75, 90, 94)
24
24
24
24
2e’(1.5)
a
Method C: The reaction was assisted by microwave irradiation; Method D: The reaction
b
was assisted by heating in an oil bath at the appropriate temperature; Yield and Conv.
c
(conversion yield) were determined by GC/MS; time = reaction time in minutes;
d P = power of microwave oven (W); f temp = oil bath temperature (°C)
The oxidation of isoeugenol acetate (2e’) into vanillin acetate (3e; Scheme 2) was performed in the
same way as for the other 1-arylpropenes 2a-2d (Table 2), i.e., under the assistance of microwave
irradiation (Method C), as well as with provision of energy from conventional heating (Method D).
The yields of 3e were in both cases higher than the corresponding yields of benzaldehyde (3a), but still
lower than the corresponding yields of the aldehydes 2b-2d. Evidently, the presence of the acetoxy
group has a hindering influence on the oxidation sensitivity of 2e’.
The last step, the hydrolysis of vanillin acetate (3e) into vanillin (4e), was carried out under solvent-
free conditions using KF/Al2O3 as base catalyst [27]. In order to determine the reaction conditions
leading to the highest yield, the influence of the molar ratio between vanillin acetate and KF/Al2O3 was
investigated initially under intuitively chosen microwave irradiation conditions (Figure 2).
Subsequently, the optimum conditions with respect to the duration of the microwave irradiation were
determined (Table 3, Method E). For comparison, a series of hydrolysis experiments was carried out
under conditions of conventional heating (Table 3, Method F). Thus, while the 100% conversion of
vanillin acetate into vanillin with the assistance of microwave irradiation could be performed within 14
minutes, the same reaction supported by conventional heating required about one hour (Table 3).

Molecules 2009, 14
Figure 2. Influence of the molar ratio between vanillin acetate and KF/Al2O3 on the
3417
efficiency of the microwave-assisted hydrolysis of vanillin acetate into vanillin under
solvent-free reaction conditions (irradiation time: 14.5 minutes; power: 300 W).
100
80
60
40
20
0
2 : 7
2 : 10
2 : 12
2 : 14
2 : 15
(3e):KF/Al2O3 (mol:mol)
Table 3. Optimization of the hydrolysis of vanillin acetate by KF/Al2O3.
Methoda
Method E
Method E
Method E
Method F
Method F
Method F
Time (min)
Temp. (°C)c
110
115
122
115
115
115
Conv.b (%)
100
100
99.5
82
Yieldb (%)
14.5
14.0
13.0
14
30
60
92
94
96
76
91
91
97
100
a
Method E: The hydrolysis was assisted by microwave irradiation at 300 W; Method F: The
hydrolysis was supported by conventional heating to 115 °C; b Conv. and yield were determined by
c
GC/MS; Temp. = temperature measured when the microwave irradiation was stopped (Method
E); Temp. = oil bath temperature (Method F)
1
The identity and purity of all products reported were confirmed by H-NMR and 13C-NMR
spectroscopy, as well as by GC/MS. Since all benzaldehyde products are well-known and already
spectroscopically well-characterized [31-35], a further presentation and discussion of their spectral
characteristics has been found superfluous. On the other hand, the 1-arylpropenes formed (as mixtures
of geometrically isomeric cis-and trans-forms) by the isomerization of the allylbenzenes appear not to
1
have been well-characterized spectroscopically [36,37], for which reason full details of both the H-
NMR spectra (600 MHz) and the 13C-NMR spectra are presented in the Experimental.

Molecules 2009, 14
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Experimental
General
Microwave irradiations were performed by means of a CEM MDS 200 batch microwave oven.
GC/MS analyses were performed on a Hewlett Packard 5890 GC 5971A MS apparatus equipped with
a J&W DB-5MS capillary column (30 m, 0.25 mm i.d., 0.25 μm film thickness) and a Hewlett Packard
7673A autosampler. NMR spectra were recorded on a Varian Mercury 300 NMR spectrometer and/or
a Varian Inova 600 NMR spectrometer on ca. 0.2 M solutions (solvent: CDCl3) at 25 °C. All chemicals
used were from obtained from Aldrich or Fluka.
Preparation of base catalyst KF/Al2O3 (40% w/w)
KF (20 g) was dissolved completely in de-ionized water (150 mL, pH of solution: 6.5). Al2O3 (30 g)
was stirred regularly in de-ionized water (150 mL) for around 5 minutes (pH of solution: 7.6). Then the
KF solution was poured into the solution containing Al2O3 under continuous stirring for
30-45 minutes, until the pH of the mixture of the solutions was 11.5-11.7. Subsequently, water was
removed from the solution mixture by rotatory evaporation, until the weight of the remaining solid
mass was 53-55 g. The wet solid mass was dried under vacuum (200 mbar) in an oven at 140-150 °C
for 6 hours. Finally, the obtained solid mass (50.5-51.5 g) was ground in a mortar into a fine
homogeneous powder (Note: the use of laboratory protective gloves and an inhalation mask is
recommended!).
Preparation of oxidation agent PP/4CSP
Copper sulfate pentahydrate (0.1 mol, 25 g) was dissolved completely in de-ionized water. Then
KMnO4 (0.025 mol, 3.95 g) was added, followed by a sufficient volume of de-ionized water to obtain a
homogeneous solution. The solution was stirred for 10 minutes at 80 °C. Subsequently water was
removed from the solution by rotatory evaporation, until the weight of the remaining solid mass was
equal to the sum of the weights of the original ingredients. The obtained solid mass was ground in a
mortar into a fine homogeneous powder (Note: the use of laboratory protective gloves and an
inhalation mask is recommended!).
Isomerization of allylbenzenes into the corresponding 1-arylpropenes by KF/Al2O3 under solvent-free
reaction conditions with microwave irradiation (Method A)
A suitable quantity of finely ground KF/Al2O3 was added to a test tube (h = 16 cm, d = 1.4 cm;
alternatively: h = 20 cm, d = 3 cm) containing the allylbenzene (2.00 mmol). The test tube was placed
in an alumina bath, equipped to support test tubes, in the microwave oven. For each of the
allylbenzenes, an irradiation programme was applied to determine the most efficient reaction
conditions, see Table 1. For every experiment performed, the temperature of the reaction mixture was
measured immediately after the termination of the reaction. After cooling, the reaction mixture was
extracted with ether (4 × 15 mL). The combined extracts were filtered, washed with water until neutral,

Molecules 2009, 14
3419
and then dried by anhydrous Na2SO4. After removal of the solvent by rotatory evaporation, the
remaining crude product was analysed by GC/MS and NMR spectroscopy.
Isomerization of allylbenzenes into the corresponding 1-arylpropenes by KF/Al2O3 under solvent-free
reaction conditions with conventional heating (Method B)
A suitable quantity of finely ground KF/Al2O3 was added to a test tube (h = 16 cm, d = 1.4 cm;
alternatively: h = 20 cm, d = 3 cm) containing the allylbenzene (2.00 mmol). The test tube was placed
in an oil bath heated to the desired temperature, and kept there for the pertinent period of time, see
Table 1. Subsequently, the reaction mixture was worked up as described in Method A.
Isomerization of eugenol into isoeugenol by KF/Al2O3 under heterogeneous reaction conditions with
microwave irradiation (Method A1)
The necessary volume of ethylene glycol [calculated on the basis of the quantity (g) of KF/Al2O3]
was added into a test tube (h = 20 cm, d = 3 cm) containing the finely ground KF/Al2O3, and then
eugenol (0.328 g, 2.00 mmol) was added under shaking. The test tube was placed into a beaker
equipped to support test tubes and placed in the microwave oven. An irradiation programme was
applied to determine the most efficient reaction conditions, see Table 1. For every experiment
performed, the temperature of the reaction mixture was measured immediately after the end of the
reaction. After cooling, the reaction mixture was dissolved in water (50 mL), and the aqueous solution
was neutralized (pH = 7) with 10% aqueous HCl. Then toluene (50 mL) was added, and the new
solution mixture was stirred for 15-30 minutes, and then filtered through a glass filter funnel. The two-
layer filtrate was separated by means of a separating funnel. The toluene layer was washed with water,
and then dried by anhydrous Na2SO4. After removal of the solvent by rotational evaporation, the
remaining crude product was analysed by GC/MS and NMR spectroscopy. The pure isoeugenol (as a
mixture of cis and trans isomeric forms) was isolated by flash column chromatography (4-7 g silica
gel, Davisil, grade 710, 4-20 μm, 60 A, 99%) using as eluent a mixture of hexane and dichloromethane
(9:1 v/v).
Isomerization of eugenol into isoeugenol by KF/Al2O3 under heterogeneous reaction conditions with
conventional heating (Method B1)
The pertinent volume of ethylene glycol was poured into a test tube (h = 20 cm, d = 3 cm)
containing the finely ground KF/Al2O3, and then eugenol (0.328 g, 2.00 mmol) was added under
shaking. The test tube was placed in an oil bath heated to the temperature measured by the reaction
stop of the parallel reaction run under microwave irradiation, and kept there for the pertinent period of
time. After cooling, the reaction mixture was worked up as described in Method A1.

Molecules 2009, 14
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Oxidation of 1-arylpropenes into the corresponding aldehydes by PP/4CSP under solvent-free reaction
conditions with microwave irradiation (Method C)
Finely ground PP/4CSP (13.896 g, 12.00 mmol) was added to a test tube (h = 20 cm, d = 3.0 cm)
containing the 1-arylpropene (3.00 mmol). The test tube was placed into a beaker, equipped to support
test tubes in the microwave oven. For each of the 1-arylpropenes, an irradiation programme was
applied to determine the most efficient reaction conditions, see Table 2. For every experiment
performed, the temperature of the reaction mixture was measured immediately after the irradiation
ceased. After cooling, the reaction mixture was stirred with diethyl ether (50 mL) for 15 minutes, and
then filtered through a layer of Celite (2 cm). The ethereal extract was concentrated by rotational
evaporation until a total volume of around 10 mL, and then filtered through a layer of NaHCO3/SiO2 (3
cm). The layer of NaHCO3/SiO2 was rinsed with diethyl ether (10-15 mL), after which the combined
ethereal layers were dried by anhydrous Na2SO4. After removal of the solvent by rotatory evaporation,
the remaining crude product was analysed by GC/MS and NMR spectroscopy. The aldehyde was
isolated by flash column chromatography (4-7 g silica gel, Davisil, grade 710, 4-20 μm, 60 A, 99%)
using as eluent a mixture of hexane and ethyl acetate (9:1 v/v).
Oxidation of 1-arylpropenes into the corresponding aldehydes by PP/4CSP under solvent-free reaction
conditions with conventional heating (Method D)
A test tube (h = 20 cm, d = 3.0 cm) containing finely ground PP/4CSP (13.896 g, 12.00 mmol) and
the 1-arylpropene (3.00 mmol) was placed in an oil bath heated to the temperature measured by
reaction stop of the parallel reaction run under microwave irradiation, and kept there for the pertinent
period of time. After cooling, the reaction mixture was worked up as described in Method C.
Hydrolysis of vanillin acetate into vanillin by KF/Al2O3 under solvent-free reaction conditions with
microwave irradiation (Method E)
A suitable quantity of finely ground KF/Al2O3 was added to a test tube (h = 16 cm, d = 1.4 cm)
containing vanillin acetate (2.00 mmol). The test tube was placed into a beaker, equipped to support
test tubes in the microwave oven. An irradiation programme was applied to determine the most
efficient reaction conditions. For every experiment performed, the temperature of the reaction mixture
was measured immediately after the irradiation was stopped. After cooling, the reaction mixture was
dissolved in water (50 mL) and the aqueous solution was neutralized with 10% aqueous HCl. Then
dichloromethane (50 mL) was added, the two-layer solution mixture was stirred for 15-30 minutes, and
filtered through a glass filter funnel. The dichloromethane layer was separated and dried by anhydrous
Na2SO4. After removal of the solvent by rotational evaporation, the remaining crude product was
analysed by GC/MS and NMR spectroscopy. Vanillin was isolated by flash column chromatography
(4-7 g silica gel, Davisil, grade 710, 4-20 μm, 60 A, 99%) using as eluent a mixture of
dichloromethane and ethyl acetate (9:1 v/v).

Molecules 2009, 14
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Hydrolysis of vanillin acetate into vanillin by KF/Al2O3 under solvent-free reaction conditions with
conventional heating (Method F)
A suitable quantity of finely ground KF/Al2O3 was added into a test tube (h = 16 cm, d = 1.4 cm)
containing vanillin acetate (2.00 mmol). The test tube was placed in an oil bath heated to the
temperature measured by reaction stop of the parallel reaction run under microwave irradiation, and
kept there for the pertinent period of time. After cooling, the reaction mixture was worked up as
described in Method E.
1H- and 13C-NMR spectroscopic data
1
(E)-1-phenylpropene (E)-2a: H-NMR (600 MHz) δ (ppm) = 7.31 (dd, J = 8.4 Hz, J = 1.3 Hz, 2H),
7.26 (dd, J = 8.4 Hz, J = 7.2 Hz, 2H), 7.16 (tt, J = 7.2 Hz, J = 1.3 Hz, 1H), 6.38 (dq, J = 15.6 Hz,
13
J = 1.2 Hz, 1H), 6.22 (dq, J = 15.6 Hz, J = 6.6 Hz, 1H), 1.86 (dd, J = 6.6 Hz, J = 1.2 Hz, 3H); C-
NMR (75 MHz) δ (ppm) = 137.95, 131.05, 128.47, 126.73, 125.82, 125.66, 18.47.
1
(Z)-1-phenylpropene (Z)-2a: H-NMR (600 MHz) δ (ppm) = 7.30 (dd, J = 8.4 Hz, J = 1.5 Hz, 2H),
7.26 (dd, J = 8.4 Hz, J = 7.2 Hz, 2H), 7.20 (tt, J = 7.2 Hz, J = 1.5 Hz, 1H), 6.43 (dq, J = 11.4 Hz,
13
J = 1.8 Hz, 1H), 5.78 (dq, J = 11.4 Hz, J = 7.2 Hz, 1H), 1.89 (dd, J = 7.2 Hz, J = 1.8 Hz, 3H); C-
NMR (75 MHz) δ (ppm) = 137.62, 129.87, 128.83, 128.11, 126.76, 126.41, 14.61.
(E)-1-(4-Methoxyphenyl)propene (E)-2b: 1H-NMR (600 MHz) δ (ppm) = 7.24-7.26 (m, 2H), 6.81-6.83
(m, 2H), 6.33 (dq, J = 16.0 Hz, J = 1.8 Hz, 1H), 6.08 (dq, J = 16.0 Hz, J = 6.6 Hz, 1H), 3.78 (s, 3H),
1.85 (dd, J = 6.6 Hz, J = 1.8 Hz, 3H); 13C-NMR (75 MHz) δ (ppm) = 158.59, 130.82, 130.35, 126.88,
123.47, 113.90, 55.25, 18.41.
(Z)-1-(4-Methoxyphenyl)propene (Z)-2b: 1H-NMR (600 MHz) δ (ppm) = 7.22-7.24 (m, 2H), 6.86-6.88
(m, 2H), 6.36 (dq, J = 11.4 Hz, J = 1.8 Hz, 1H), 5.69 (dq, J = 11.4 Hz, J = 7.2 Hz, 1H), 3.79 (s, 3H),
1.88 (dd, J = 7.2 Hz, J = 1.8 Hz, 3H); 13C-NMR (75 MHz) δ (ppm) = 158.12, 130.34, 129.89, 129.28,
125.09, 113.55, 55.23, 14.59.
(E)-1-(3,4-Dimethoxyphenyl)propene (E)-2c: 1H-NMR (600 MHz) δ (ppm) = 6.89 (d, J = 1.8 Hz, 1H),
6.85 (dd, J = 8.4 Hz, J = 1.8 Hz, 1H), 6.79 (d, J = 8.4 Hz, 1H), 6.33 (dq, J = 15.6 Hz, J = 1.6 Hz, 1H),
6.10 (dq, J = 15.6 Hz, J = 6.6 Hz, 1H), 3.88 (s, 3H), 3.86 (s, 3H), 1.86 (dd, J = 6.6 Hz, J = 1.6 Hz, 3H);
13C-NMR (75 MHz) δ (ppm) = 148.99, 148.17, 131.16, 130.62, 123.77, 118.66, 111.20, 108.49, 55.91,
55.77, 18.37.
(Z)-1-(3,4-Dimethoxyphenyl)propene (Z)-2c: 1H-NMR (600 MHz) δ (ppm) = 6.87 (d, J = 1.8 Hz, 1H),
6.83-6.85 (m, 2H), 6.37 (dq, J = 11.4 Hz, J = 1.8 Hz, 1H), 5.71 (dq, J = 11.4 Hz, J = 7.2 Hz, 1H),
3.878 (s, 3H), 3.877 (s, 3H), 1.91 (dd, J = 7.2 Hz, J = 1.8 Hz, 3H); 13C-NMR (75 MHz)
δ (ppm) = 148.52, 147.67, 131.16, 129.53, 125.40, 121.36, 112.17, 110.93, 55.88, 55.80, 14.66.

Molecules 2009, 14
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1
(E)-5-(Prop-1-enyl)benzo[d]-1,3-dioxole [(E)-Isosafrol, (E)-2d]: H-NMR (600 MHz) δ (ppm) = 6.87
(d, J = 1.2 Hz, 1H), 6.73 (dd, J = 8.1 Hz, J = 1.2 Hz, 1H), 6.72 (d, J = 8.1 Hz, 1H), 6.30 (dq,
J = 15.9 Hz, J = 1.8 Hz, 1H), 6.05 (dq, J = 15.9 Hz, J = 6.6 Hz, 1H), 5.91 (s, 2H), 1.84 (dd, J = 6.6 Hz,
13
J = 1.8 Hz, 3H); C-NMR (75 MHz) δ (ppm) = 147.92, 146.51, 132.51, 130.57, 123.92, 120.06,
108.20, 105.34, 100.90, 18.33.
1
(Z)-5-(Prop-1-enyl)benzo[d]-1,3-dioxole [(Z)-Isosafrol, (Z)-2d]: H-NMR (600 MHz) δ (ppm) = 6.82
(d, J = 1.2 Hz, 1H), 6.78 (d, J = 7.8 Hz, 1H), 6.75 (dd, J = 7.8 Hz, J = 1.2 Hz, 1H), 6.35 (dq,
J = 11.4 Hz, J = 1.8 Hz, 1H), 5.94 (s, 2H), 5.69 (dq, J = 11.4 Hz, J = 7.2 Hz, 1H), 1.87 (dd, J = 7.2 Hz,
13
J = 1.8 Hz, 3H); C-NMR (75 MHz) δ (ppm) = 147.40, 146.01, 131.79, 129.45, 125.59, 122.53,
109.05, 108.08, 100.90, 14.61.
(E)-1-(4-Hydroxy-3-methoxyphenyl)propene, [(E)-Isoeugenol, (E)-2e]: 1H-NMR (600 MHz) δ (ppm) =
6.80-6.86 (m, 3H), 6.31 (dq, J = 15.6 Hz, J = 1.8 Hz, 1H), 6.07 (dq, J = 15.6 Hz, J = 6.6 Hz, 1H), 5.59
13
(s, 1H), 3.87 (s, 3H), 1.84 (dd, J = 6.6 Hz, J = 1.8 Hz, 3H); C-NMR (75 MHz) δ (ppm) = 146.57,
144.76, 130.75, 130.66, 123.42, 119.31, 114.37, 107.90, 55.83, 18.34.
1
(Z)-1-(4-Hydroxy-3-methoxyphenyl)propene [(Z)-Isoeugenol, (Z)-2e]: H-NMR (600 MHz) δ (ppm) =
6.89 (d, J = 8.4 Hz, 1H), 6.83-6.86 (m, 1H), 6.80 (d, J = 1.8 Hz, 1H), 6.35 (dq, J = 15.6 Hz, J = 1.8 Hz,
1H), 5.69 (dq, J = 12.0 Hz, J = 7.2 Hz, 1H), 5.63 (s, 1H), 3.86 (s, 3H), 1.89 (dd, J = 7.2 Hz, J = 1.8 Hz,
13
3H); C-NMR (75 MHz) δ (ppm) = 146.14, 144.29, 130.08, 129.64, 125.14, 122.09, 114.10, 111.43,
55.87, 14.64.
1
(E)-1-(4-Acetoxy-3-methoxyphenyl)propene, [(E)-Isoeugenol acetate, (E)-2e’]: H-NMR (600 MHz) δ
(ppm) = 6.94 (d, J = 8.2 Hz, 1H), 6.92 (d, J = 1.8 Hz, 1H), 6.88 (dd, J = 8.2 Hz, J = 1.8 Hz, 1H), 6.36
(dq, J = 15.6 Hz, J = 1.2 Hz, 1H), 6.18 (dq, J = 15.6 Hz, J = 6.6 Hz, 1H), 3.82 (s, 3H), 2.29 (s, 3H),
1.87 (dd, J = 6.6 Hz, J = 1.2 Hz, 3H); 13C-NMR (75 MHz) δ (ppm) = 169.13, 151.00, 138.60, 137.07,
130.46, 126.06, 122.66, 118.36, 109.63, 55.76, 20.64, 18.40.
1
(Z)-1-(4-Acetoxy-3-methoxyphenyl)propene, [(Z)-Isoeugenol acetate, (Z)-2e’]: H-NMR (600 MHz) δ
(ppm) = 6.99 (d, J = 8.4 Hz, 1H), 6.87-6.94 (m, 2H), 6.39 (dq, J = 12.0 Hz, J = 1.8 Hz, 1H), 5.78 (dq,
13
J = 12.0 Hz, J = 7.2 Hz, 1H), 3.82 (s, 3H), 2.31 (s, 3H), 1.90 (dd, J = 7.2 Hz, J = 1.8 Hz, 3H); C-
NMR (75 MHz) δ (ppm) = 166.40, 150.65, 138.19, 136.57, 129.33, 127.06, 122.35, 121.25, 112.97,
55.80, 22.12, 14.62.
Acknowledgements
We thank DANIDA, the Danish International Aid Organization, for financial support via the
ENRECA (Enhancement of Research Capacity) Programme.

Molecules 2009, 14
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