Anethole isomerization and dimerization induced by acid sites or UV irradiation

Article abstract of DOI:10.3390/molecules15075012

The formation of cis-anethole and various dimers as a result of the exposure of trans-anethole to microporous solid acids (dealuminated HY zeolites), or UV-Vis irradiation was established by means of high resolution gas chromatography coupled to mass spectrometry. 3,4-bis-(4-Methoxyphenyl)-(E)-hex- 2-ene was the most abundant compound among eight different methoxyphenyl- disubstituted hexenes produced by electrophilic addition and elimination reactions induced by HY zeolites. (1a,2a,3b,4b)-1,2-bis(4-Methoxyphenyl)-3,4- dimethylcyclobutane was the principal component in the mixture of 5 methoxyphenyl-disubstituted cyclobutanes found, together with cis-anethole, after UV-Vis irradiation of a trans-anethole solution in toluene.

Full text of DOI:10.3390/molecules15075012

Molecules 2010, 15, 5012-5030; doi:10.3390/molecules15075012
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Anethole Isomerization and Dimerization Induced by Acid Sites
or UV Irradiation
Hans T. Castro, Jairo René Martínez * and Elena Stashenko
Chromatography Laboratory, CIBIMOL, Research Center of Excellence CENIVAM, Building 45,
Universidad Industrial de Santander, Carrera 27 calle 9, Bucaramanga, Colombia
* Author to whom correspondence should be addressed; E-Mail: rene@tucan.uis.edu.co;
Tel.: +57(7)6456737; Fax: +57(7)6358210.
Received: 21 May 2010; in revised form: 21 June 2010 / Accepted: 25 June 2010 /
Published: 22 July 2010
Abstract: The formation of cis-anethole and various dimers as a result of the exposure of
trans-anethole to microporous solid acids (dealuminated HY zeolites), or UV-Vis
irradiation was established by means of high resolution gas chromatography coupled to
mass spectrometry. 3,4-bis-(4-Methoxyphenyl)-(E)-hex-2-ene was the most abundant
compound among eight different methoxyphenyl-disubstituted hexenes produced by
electrophilic addition and elimination reactions induced by HY zeolites. (1a,2a,3b,4b)-1,2-
bis(4-Methoxyphenyl)-3,4-dimethylcyclobutane was the principal component in the
mixture of 5 methoxyphenyl-disubstituted cyclobutanes found, together with cis-anethole,
after UV-Vis irradiation of a trans-anethole solution in toluene.
Keywords: trans-anethole; photoisomerization; HY zeolite; cis-anethole; dimerization
1. Introduction
trans-Anethole (1a) is one of the main constituents of anise, clove, cinnamon and thyme essential
oils [1–3]. trans-Anethole-containing oils are widely used in the food and liquor industries [4],
although their use has been the subject of discussion due to the formation of cis-anethole (1b) when
trans-anethole is exposed to UV radiation or acidic conditions. cis-Anethole is toxic, and possesses an
unpleasant scent and flavor [5]. We have previously reported that isomerization of trans-anethole takes
place during the catalytic transformation of anise oil over zeolite Y [6].

Molecules 2010, 15
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Zeolites are porous crystalline aluminosilicates formed by [SiO4]-4 and [AlO4]-5 tetrahedral units.
The relative amounts of these two building blocks determine the negative charge of the framework.
This charge is compensated by the presence of counterions such as sodium, calcium or proton.
Protonic dealuminated zeolites have found important applications as petroleum cracking catalysts.
They contain Brønsted-type acid sites associated to framework Al [7]. Their use as catalysts is mainly
related to their high acidity [8]. In this work we present the results of GC-MS analysis of the products
(which included anethole dimers) resulting from the exposure of trans-anethole to HY zeolites, at 30,
60 and 90 °C. Details of the hydrogen transfer between zeolite and trans-anethole were based on GC-
MS analysis of the reaction products obtained when deuterated acid zeolites, HDY, were used.
trans-Anethole exposure to UV radiation and its chemical transformation have been investigated by
different authors. Lewis and Kojima [9,10] studied trans-anethole photoisomerization at two excitation
wavelengths, λexc (281 and 313 nm), under different reactant concentrations, and in the absence or
presence of various electron acceptors. Among their results, the authors highlighted the formation of
cis-anethole and five dimers, products of [2+2] cycloaddition and electron transfer. One dimer (6e in
our work) was reported as the only one formed from trans-anethole; whereas a different dimer (6c in
our work) was formed from trans-anethole only in the presence of electronic acceptors
(cyanoanthracene, CA, 9,10-dicyanoanthracene, DCA, and 1,4-dicyanobenzene, DCB). We present
results on the study of UV-Vis-induced isomerization and dimerization of trans-anethole in toluene,
which indicate that dimer formation could take place without the addition of electronic acceptors.
2. Results and Discussion
2.1. trans-Anethole treatment with HY zeolites
A total of nine compounds (> 0.1%) were tentatively identified by means of GC-MS of the mixture
obtained after exposure of trans-anethole to HY zeolites at 30, 60 and 90 ºC, for 5 h. The distinction
between geometrical isomers was based on the comparison of their molecular ion abundances in their
mass spectra, and some characteristic ion-fragments. (E)-geometry was associated with the more stable
molecular ion of the two isomers. Since there were no standard substances available, dimer structure
assignment relied strongly on the comparison of typical fragment losses found in their mass spectra
and on the structures which appear in Scheme 1. Table 1 presents the relative amounts (%) of trans-
anethole and its transformation products. The structural formulae of dimers 2a–5b appear in Scheme 1.
Dimer 5b (20–63%) was the most abundant compound obtained under the conditions presented in
Table 1. cis-Anethole (1b, 1–2%), the dimers [M+•, m/z 296] 2a (0.4–2.7%), 2b (0.7–4%), dimers 3a
and 3b (1–23%), 4a (0.4–5%), 4b (0.71–11%) and 5a (<0.1%) were also found as transformation
products.

Molecules 2010, 15
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Scheme 1. Proposed formation routes for dimers 2a - 5b from trans-anethole (1a).
O
O
O
O+
O
O
+
H
O
2a
O
O
O
H
+
O+
H
O
2b
H+ (FAU)
O
O
O
H
O
O
O
O
O+
O
O+
+
H
H
+
H
+
3a
O
O
H
O
O
O+
+
+
H
1,2 H-shift
O
3b
O
O
O
O
O
O+
O
H
H
O
H
+
4a
O
H
O
O+
+
H
O
4b
O
O
O
O
O
+
O+
H
H
+
5a
O
O
O
O+
+
H
5b

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5015
Table 1. Relative amounts of trans-anethole and its transformation products (>0.1%), after
its exposure to HY zeolites (5 h).
Relative amount, %a
HY2
Zeolite
HY1
60
HY3
60
Compd. T, °C
30
90
30
60
90
30
90
cis-Anethole
0.3 0.02 1.9 0.24 0.8 0.01 0.2 0.01 1.2 0.80 0.9 0.11 0.2 0.03 1.2 0.08 0.4 0.06
trans-Anethole
99 0.02 45.0 0.80 9.0 0.86 99 0.01
30 23
2.7 0.92
1.7 0.74
12 1.1 99.0 1.7
76 1.4
8.0 1.9
2a
2b
---
---
---
---
---
---
---
2
0.17
3
0.16
---
---
---
---
---
---
---
2
4
9
5
0.08
0.24
0.09
0.06
1.3
---
---
---
---
---
---
---
0.4 0.18 1.4 0.17
0.7 0.05 2.6 0.27
1.0 0.08 0.9 0.03
2.0 0.06 9.0 0.03
3a. 3b
4a
5
2.02
1.0 0.19
23 2.2
1.0 0.08
2.0 0.05 11 0.11
tr tr
45 0.30 63 0.92
3
0.04
1.0 0.38
3.0 1.14
tr
0.45 0.05 2.2 0.18
0.71 0.06 5.9 1.2
4b
7
5a
tr
60 0.4
tr
tr
5b
55 18
20 1.2
57 5.6
a Mean s (n = 3); tr = trace (<0.1%)
The relative amount of dimers increased with temperature (Table 1), whereas cis-anethole was
formed in higher amount at 60 °C than at either 30 or 90 °C. This suggests that competition between
isomerization and dimerization exists and it is consistent with the existence of a common intermediate,
i.e. protonated anethole, for both processes. The increase of the dimers as the temperature was
incremented could be explained by a higher reaction rate for dimerization than for isomerization.
There are very few publications in the scientific literature on the dimers presented in Table 1.
Structures 3a–5b appeared in studies on biological activity and synthetic estrogens carried out in the
first half of the last century [11,12]. Dimer 5b was mentioned by Whitmore in the context of a study on
olefin polymerization with acid catalysts [13]. Scheme 1 contains the proposed formation scheme for
dimers 2a–5b. The two intermediary carbocations resulting from protonation of the trans-anethole
double bond bind each one with a neutral molecule at two different locations. The most stable
carbocation, stabilized by resonance, leads to dimers 4a–5b. The second carbocation produces the less
abundant dimers 2a–3b. The orientation of the attack of the incoming trans-anethole molecule on the
carbocation determines whether the a or b isomer is formed. Although only the trans-isomer was
employed in the scheme proposed in Scheme 1, in principle, some of the dimers 2a–5b could also
result from the addition of a molecule of cis-anethole to either cationic intermediary. However, the
very low cis-anethole amounts make this a rare event.
2.2. Transformation of trans-anethole exposed to deuterated zeolite, HDY
Based on the in situ 13C-NMR determination of alkoxides, various studies [14–20] have proposed a
carbenium-mediated mechanism for trans-anethole isomerization or oligomerization. Since both
reactions take place in the same medium, it is possible that some dimers result from the reaction of a
just-formed cis-anethole molecule with the cation obtained from the highly abundant trans-anethole.
As evidenced in their mass spectra (Figure 1), peaks corresponding to cis- or trans-anethole

Molecules 2010, 15
5016
fragmentation products which do not contain methoxy groups, incorporate deuterium. The presence of
deuterium in both cis- and trans- anethole after the treatment of pure trans-anethole with deuterated
faujasite supports the proposed formation scheme in which isomer interconversion occurs (Scheme 1).
Figure 1. Mass spectra (EI, 70eV) of anethole isomers and their deuterated analogs formed
after the exposure of trans-anethole to deuterated acid faujasite, HDY. A. trans-Anethole
and its isotopomer. B. cis-Anethole and its isotopomer.
M+·
148
M+·
148
I,%
100
O
100
I,%
O
B
A
75
50
25
0
75
50
25
0
[M-OCH3]+
[M-CH3]+
117
77
105
[M-OCH3]+
[M-CH3]+
133
91
117
51
77
63
105
39
133
91
51
30 40 50 60 70 80 90 100 110 120 130 140 150
63
39
m/z
30 40 50 60 70 80 90 100 110 120 130 140 150
m/z
148
148
I,%
100
100
75
I,%
O
O
75
50
25
0
+·
Md1
+·
Md1
50
C3H4
D
[M-OCH3]+
117
C3H4D
[M-OCH3]+
+
117 [M-CH3]+
133
77
[M-CH3]
133
77
25
105
123
105
91
63
51
57
63
39
39
51
91
0
30 40 50 60 70 80 90 100 110 120 130 140 150
m/z
30 40 50 60 70 80 90 100 110 120 130 140 150
m/z
The cis-anethole mass spectra of Figure 1 were obtained from the GC-MS analysis of the products
of trans-anethole exposure to either acidic faujasite, HY, or deuterated acidic faujasite, HDY, allowing
us to affirm that cis-anethole formation was due to the zeolite treatment. The ratio of characteristic
fragment ion abundances in the mass spectrum of deuterated cis-anethole (m/z 117/118 = 22/26; m/z
133/134 = 19/18; m/z 148/149 = 100/77), compared to those of cis-anethole formed on the non-
deuterated faujasite HY (m/z 117/118 = 33/5; m/z 133/134 = 25/4; m/z 148/149 = 100/14), showed an
increased abundance of deuterated cis-anethole when HDY was used. Thus, an electrophilic deuterium
addition to the sp2 carbon of the trans-anethole double bond, takes place during the isomerization or
dimerization processes.
The ratios of the ion abundances at m/z 148 and 149 of deuterated trans-anethole are similar to
those observed in the deuterated cis-anethole mass spectrum. This agrees with the existence of an
intermediary carbocation that gives rise to either isomer at a similar rate. An isomerization scheme is
proposed in Scheme 2. The relatively high abundance of m/z 149 in the mass spectra of isomerization
products (both cis- and trans-antehole), indicative of the presence of deuterium, suggests that the

Molecules 2010, 15
5017
neutral product may be formed by the loss of a proton different from that which participated in the
electrophilic addition.
Scheme 2. Proposed scheme for trans-anethole isomerization over deuterated faujasite, HDY.
Typical mass spectra of the dimers formed after trans-anethole exposure to deuterated faujasite
HDY, appear in Figure 2. Ions at m/z 297 and 298 in the mass spectra of all dimers indicate single and
double deuteration, respectively, which results from the participation of one or two deuterated
molecules of either cis- or trans-anethole. The signal of the non-deuterated molecular ions (m/z 296)
was in all cases of lower intensity than that of the monodeuterated dimers (m/z 297). Scheme 3 shows a
mechanism proposed for the dimerization process, applied to the case of dimer 5b, which was the most
abundant product. In the observed mass spectra (electron impact ionization, 70 eV), peaks of the
various deuterated ions derived from these structures were of high intensities (Figure 2).

Molecules 2010, 15
Figure 2. Mass spectra (EI, 70 eV) of anethole dimers formed as a result of trans-anethole
5018
exposure to deuterated faujasite HDY.
O
O
[M-C12H14DO]+
121
I, %
D
100
75
50
25
0
I, %
100
D
D
D
75
50
25
0
O
[M-C10H11DO]+
O
+
·
Md1
2a
[M-CH3-C8H9DO]+
159
[M-C12H11DO]+
148
121
3a
297
+
·
+·
+
·
Md2
Md1
297
Md2
298
238
282
176
189
252
77
209
223
104
77
51
146
134
91
91
103
267
176189
202 226 251
39
64
51 65
39
40 60 80 100 120 140 160 180 200 220 240 260 280 300
m/z
40 60 80 100 120 140 160 180 200 220 240 260 280 300
m/z
[M-C2H4D]+
267
O
[M-C2H4D]+
O
I, %
267
100
75
50
25
0
I, %
100
[M-C2H4D-C7H8O]+
O
159
D
O
268
75
50
25
0
D
D
4a
[M-C2H4D-C7H8O]+
159
D
[M-C12H14DO]+
+·
Md1
5b
[M-C12H14DO]+
+·
268
Md1
297
+
·
297
Md2
29
121
+
121
·
189
Md2
298
238
252
144
252
238
144
77
91
174
221
209
103
178
190
77 91
209
221
8
104
63
283
39 51 63
40 60 80 100 120 140 160 180 200 220 240 260 280 300
40 60 80 100 120 140 160 180 200 220 240 260 280 300
m/z
m/z
2.3. trans-Anethole phototransformation
Figure 3 shows the chromatographic profiles of the mixtures that resulted from the irradiation of a
trans-anethole toluene solution with UV-Vis at –5, 5 and 15 °C for 2 h. In addition to cis-anethole, five
other compounds were identified at relative amounts above 0.1% in all cases. Anethole (cis- and
trans-) and their dimers were tentatively identified according to reported data [9,10] and the
comparison of their mass spectra (EI, 70 eV) fragmentation patterns, and calculated dipole moments
and ionization potentials (Table 2). The interaction of an S1 excited t-A molecule with another one in
its ground state, So in a [2 + 2] cycloaddition leads to dimers 6c (anti head-to-head) and 6e (syn head-
to-head). The structure of dimer 6e, the most abundant product (peak 7) in the chromatograms (Figure
3), was assigned by Nozaki et al. [21] based on independent synthesis and NMR spectra. This dimer
was the only sensitizer-free trans-anethole dimerization (313 nm) product reported by Lewis and
Kojima [9,10]. The mass spectrum of dimer 6c had been reported by Meyer and Metzger [22]. Dimer
6c (peak 5), the second most-abundant product in the chromatograms (Figure 3), was obtained in
Lewis and Kojima’s experiments when a sensitizer (1,4-dicyanobenzene, cyanoanthracene) was
present. Marquez et al. included dimer 6c as one of the products of trans-anethole electron transfer
reaction with radical cation tri-(4-bromophenyl) ammonium [23].

Molecules 2010, 15
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Scheme 3. Proposed mechanism for the formation of singly and doubly deuterated dimers
5b obtained after the exposure of trans-anethole to deuterated faujasite, HDY.

Molecules 2010, 15
Figure 3. Chromatographic profiles of the different products obtained during trans-
5020
anethole photoreaction (UV-Vis) in toluene at –5, 5, and 15 °C for 2 h. 1. cis-Anethole; 2.
trans-anethole; 3. Dimer 6a; 4. Dimer 6b; 5. Dimer 6c; 6. Dimer 6d; 7. Dimer 6e.
1b
1a
6a
6b
6c
6d
6e
Structures
Peak N°
1
2
3
4
5
6
7
Table 2. Anethole dimers 6a–6e formed during trans-anethole photoreaction.
DB-5 column
tR, min
(Polar column,
DB-WAX)
---
Dipole
moment,
Debye
2.11
Molecular structure,
Dimer
tR, min
RI
EI (eV)
7.90
R = p-Methoxy-phenyl
(Fig. 3)
29.6
2139
6a
R
R

Molecules 2010, 15
5021
Table 2. Cont.
R
30.2
30.9
30.5
31.1
2106
---
3.00
2.54
2.53
2.37
6b
8.30
8.10
R
R
2266
2289
2255
142.8
6c
6d
6e
R
R
---
8.29
8.27
R
151.4
R
R
Dimers 6a, 6b and 6d have been found among cis-anethole photodimerization (at 313 nm) products,
but not among trans-anethole irradiation products, formed in the presence or absence of sensitizer
[10,24]. Their formation during trans-anethole UV-Vis irradiation may be explained by the reaction of
cis-anethole formed during the experiment. Figure 4 shows a continuous increase of cis-anethole
concentration during the experiment (2 h) at the three temperatures employed. The relative amounts of
6a, 6b and 6d dimers showed a small increase. Thus, we conclude that the rate of trans-anethole
photoisomerization is higher than that of cis-anethole photodimerization. Dimer 6d formation was
linked in Lewis and Kojima’s work with oxygen sensitization of trans-anethole. In the present work,
trans-anethole UV-Vis irradiation experiments were conducted in a sealed reactor that had been
maintained under dry nitrogen flow to remove molecular oxygen. No epoxides were found among the
UV-Vis irradiation products. The scheme of formation proposed for the various photodimerization
products is shown in Scheme 4.
Caldwell et al. [24] observed cis-anethole configuration retention in [2+2] reactions with singlet
9-cyanophenanthrene. The dimer structures proposed here maintain the cis-anethole stereochemistry.
1
1
Dimerizations result from t-A*-1t-A, and to a lesser extent, from t-A*-1c-A interactions. The latter
happens with lower frequency due to the low cis-anethole concentration. Table 3 presents conversions
and selectivities determined after UV-Vis trans-anethole toluene solution irradiation for 2 h at -5, 5
and 15 °C. cis-Anethole and dimer 6e were the most abundant products. While trans-anethole
conversion increased with temperature, the selectivity towards cis-anethole decreased, whereas those

Molecules 2010, 15
5022
of 6c and 6e dimers increased. This shows the competition that exists between isomerization and
dimerization.
Figure 4. Variation of cis-anethole concentration during trans-anethole UV-Vis iiradiation.
0,25
15 ºC
5 ºC
- 5 ºC
0,20
0,15
0,10
0,05
0,00
1000
1500
2000
2500
3000
3500
4000
4500
5000
Time (s)
Table 3. Conversion and selectivities of trans-anethole (in toluene) photoreaction products
(UV-Vis, 120 min, -5, 5 and 15 °C).
Selectivity, %
trans- Anethole
T ºC
conversion, %
cis-anethole
6a
<1
<1
6b
2
6c
5
6d
1
6e
50
54
-5
5
30
35
52
42
38
34
2
5
2
15
<1
<1
8
1
57
Scheme 4. Scheme proposed for trans-anethole photodimerization products.

Molecules 2010, 15
5023
2.4. Quantum yield of trans-anethole photoisomerization
Quantum yields of the formation of cis-anethole (
Φ
1a→1b ) and 6c (
Φ
1a→2c ) and 6e (
Φ
1a→2e ) dimers
at -5, 5 and 15 °C, were calculated as the ratio of reaction rate over flux of absorbed photons (Table 4).
In each case, the reaction rate was calculated as the slope of the plot of concentration Vs time. Figures
4 and 5 show the linear nature of the concentration profiles of cis-anethole and dimers 6c and 6e.
Potassium ferrioxalate solutions were employed in the actinometric determination of flux as 3 × 10-4
2 × 10-5 mol photon L-1 s-1.
Figure 5. Concentration profiles of dimers 6c and 6e during trans-anethole UV-Vis
irradiation for 2 h.
Table 4. Quantum yields determined for trans-anethole isomerization and dimerization.
cis-Anethole
a Φ1a→1b
6c Dimer
b Φ1a→2c × 10 6
6e Dimer
c Φ1a→2e
T (ºC)
-5
+5
0.09
0.11
0.12
6
5
5
0.08
0.09
0.10
+15
a CV(%) = 1.1%; b CV(%) = 1.6%; c CV(%) = 1.2%; n = 3

Molecules 2010, 15
5024
The monotonous increase of trans-anethole photoisomerization and dimerization products (Figures
4 and 5) is consistent with a higher rate for the reaction of excited singlet trans-anethole (1t-A*) with
trans-anethole than that observed for its reaction with cis-anethole. Since cis-anethole is being formed,
encounters of 1t-A* with cis-anethole are much less likely than those with trans-anethole. Additionally,
cis-anethole has been reported as less reactive than trans-anethole in the photocycloaddition with 9-
cyanophenanthrene and 6-methylphenantridine [24]. The much higher abundance of 6c and 6e dimers
in comparison with 6a and 6b dimers is another consequence of this reaction rate difference.
The quantum yields obtained (Table 4) are close to those reported for trans-anethole
photoisomerization in hexane and acetonitrile (Φ1a→1b = 0.12 and 0.13, respectively) [26]. The increase
of quantum yields with temperature indicates the existence of an energy barrier in photoisomerization
and photodimerization. The Φ1a→1b values determined after multichromatic UV-Vis irradiation of
trans-anethole in toluene (0.35 M) are similar to the values reported for trans-anethole
photoisomerization after monochromatic (254, 281, or 313 nm) irradiation [9,10]. Thus, there is no
appreciable dependence of quantum yield on excitation wavelength. The quantum yield for trans-
retinal photoisomerization ( t→c ∼ 0.67) has been found to be independent of excitation wavelength
Φ
[25]. On the other hand, conformational photoisomerization [26] of trans-stilbene and stilbene
derivatives [27], some fluorinated compounds [28], and azobenzene and its dimers [29–34] has been
reported as irradiation wavelength dependent.
3. Experimental
3.1. General
trans-Anethole (99%), ammonium nitrate (98%) D2O (99.96% deuteration), NaY zeolite, were
obtained from Aldrich Chemical Co. Inc (Milwaukee, WI, USA). Toluene (99%, reagent grade) and
DCl (20% in D2O, 99.5% deuteration) were purchased from Merck (Darmstadt, Germany). cis-
Anethole chromatographic reference data were obtained from hydrodistilled anise seed essential oil.
HCl (ACS grade, 37%) was obtained from J. T. Baker (Phillipsburg, NJ, USA).
Chromatographic quantification was performed with an HP 5890A Series II (Hewlett-Packard, Palo
Alto, CA, USA) gas chromatograph equipped with a split/splitless injection system (split ratio 6:1), a
flame ionization detector (FID) and a DB-5 (J&W Scientific, Folsom, CA, USA) capillary column
(30 m × 0.32 mm × 0.25 µm) coated with 5%-phenyl-poly(methylsiloxane). Synthetic air (zero grade),
H2 (99.995%), N2 (99.995%), and He (99.995%), were obtained from Aga-Fano S.A. (Bucaramanga,
Colombia). The oven temperature was programmed from 80 ºC (1 min) @10 ºC/min to 230 ºC
(5 min). The injection port and detector temperatures were maintained at 250 and 280 ºC, respectively.
Chromatographic data were processed with HP ChemStation A.06.03 (Hewlett-Packard, Palo Alto,
CA, USA) software.
Two GC–MS systems were employed: an Agilent Technologies 6890 Plus gas chromatograph (Palo
Alto, CA) equipped with an Agilent Technologies 5973N mass selective detector (EI, 70 eV,
m/z 40–350) and an Agilent Technologies 6890 gas chromatograph coupled to an Agilent
Technologies 5975 mass selective detector (EI, 70 eV, m/z 40–350). Both systems were equipped with
a split/splitless injector (split ratio 1:30), a 7863 automatic injector and an MS-ChemStation G1701-

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DA data system that included the spectral libraries WILEY 138K, NIST 2002, and QUADLIB 2004. A
fused-silica 5% phenylpoly(dimethylsiloxane) capillary column (DB-5MS, J&W Scientific) of 60 m,
0.25 mm i.d., 0.25 µm, df, and a fused-silica cross-linked, bonded poly(ethylene glycol) capillary
column (DB-WAX, J & W Scientific, Folsom, CA, USA) of 60 m, 0.25 mm, i.d. 0.25 µm, df, were
employed. The oven temperature program employed with the DB-5MS column was from 100 °C (3
min) to 200 °C (5 min) at 10 °C/min, then to 280 °C (40 min) at 15 ºC/min. For the DB-WAX column,
the oven temperature was programmed from 100 °C (3 min) to 200 °C (5 min) at
10 °C/min, then to 220 °C (40 min) at 4 °C/min. The ionization chamber and the transfer line
temperatures were kept at 230 °C and 285 °C, respectively. The following are the mass spectral data of
the various dimers encountered as trans-anethole photoreaction products.
(Z)-1,4-bis(4-Methoxyphenyl)-2,3-dimethylbut-1-ene (2ª). WM+· (4.8%). m/z (I%): M+· 296(12), [M-
C10H12O]+ 267(7), [M-C2H5-C7H8O]+ 159(28), [M-C12H15O]+ 121(100).
(E)-1,4-bis(4-methoxyphenyl)-2,3-dimethylbut-1-ene (2b). WM+· (5.8%). m/z (I%): M+· 296(13), [M-
C2H5]+ 267(5), [M-C2H5-C7H8O]+ 159(24), [M-C10H12O]+ 148(1), [M-C12H15O]+ 121(100).
(Z)-3,5-bis(4-methoxyphenyl)-4-methylpent-2-ene (3a). WM+· (1.7%). m/z (I%): M+·296(4), [M-C2H5-
C7H8O]+ 159(2), [M-C10H12O]+ 148(100), [M-C12H15O]+ 121(38).
(E)-3,5-bis(4-methoxyphenyl)-4-methylpent-2-ene (3b). WM+· (1.3%). m/z (I%): M+· 296(3), [M-C2H5-
C7H8O]+ 159(3), [M-C10H12O]+ 148(100), [M-C12H15O]+ 121(34).
(E)-1,3-bis(4-methoxyphenyl)-2-methylpent-1-ene (4a). WM+· (7.6%). m/z (I%): M+· 296(28), [M-
C2H5]+ 267(100), [M-C2H5-C7H8O]+ 159(28), [M-C10H12O]+ 148(2), [M-C12H15O]+ 121(17).
(Z)-1,3-bis(4-methoxyphenyl)-2-methylpent-1-ene (4b). WM+· (5.0%). m/z (I%): M+· 296(24), [M-
C2H5]+ 267(100), [M-C2H5-C7H8O]+ 159(31), [M-C10H12O]+ 148(1), [M-C12H15O]+ 121(19).
(Z)-3,4-bis(4-methoxyphenyl)hex-2-ene (5a). WM+· (6.5%). m/z (I%): M+· 296(34), [M-C2H5]+
267(100), [M-C2H5-C7H8O]+ 159(82), [M-C12H15O]+ 121(42).
(E)-3,4-bis(4-methoxyphenyl)hex-2-ene (5b). WM+· (5.2%). m/z (I%): M+· 296(28), [M-C2H5]+
267(100), [M-C2H5-C7H8O]+ 159(98), [M-C10H12O]+ 148(3), [M-C12H15O]+ 121(30).
(1a,2a,3a,4b)-1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (6a). WM+· (6.6%). m/z (I%): M+·
296(4), [M-CH3]+ 267(1), [M-C4H8-H]+ 249(1), [M-CH3-C7H8O]+ 175(4), [M-C10H12O]+· 148(100),
[M-C12H15O]+ 121(12).
(1a,2b,3a,4a)-1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (6b). WM+· (4.7%). m/z (I%): M+·
296(81), [M-CH3]+ 267(9), [M-C4H8-H]+ 249(15), [M-CH3-C7H8O]+ 175(36), [M-C10H12O]+· 148(67),
[M-C12H15O]+ 121(100).
(1a,2b,3a,4b)-1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (6c). WM+· (0.6%). m/z (I%): M+·
296(<1), [M-C10H12O]+· 148(100), [M-C12H15O]+ 121(4).

Molecules 2010, 15
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(1a,2b,3b,4a)-1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (6d). WM+· (1.2%). m/z (I%): M+·
296(88), [M-CH3]+ 267(8), [M-C4H8-H]+ 249(9), [M-CH3-C7H8O]+ 175(38), [M-C10H12O]+· 148(34),
[M-C12H15O]+ 121(100).
(1a,2a,3b,4b)-1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (6e). WM+· (0.5%). m/z (I%): M+·
296(<1), [M-C10H12O]+· 148(100), [M-C12H15O]+ 121(4).
3.2. Faujasite preparation and characterization
NaY zeolite (10 g) was suspended at 60 ºC for 2 h in an aqueous ammonium nitrate solution (1M,
150 mL) under continuous stirring (1,000 rpm). It was dried afterwards (100 °C, 12 h) and placed in a
quartz tube (1 cm I.D.) in a cylindrical oven and heated to 550 °C under a steam flow at 1.38 bar.
Heating periods of 2, 4 and 6 h led to the formation of zeolite samples HY1, HY2 and HY3,
respectively. They were subsequently treated with HCl (0.5 M, 6 h, continuous stirring) to remove
extra-framework aluminum.
3.3. Zeolite characterization
Dealuminated HY zeolites were characterized by means of elemental analysis (energy dispersive
X-ray fluorescence, Shimadzu EDX 800 HS; Columbia, MD, USA), infrared spectroscopy (Bruker,
Tensor 27; Billerica, MA, USA) and X-ray diffraction (Rigaku D-MAX-III/B; Tokyo, Japan).
3.4. trans-Anethole treatment with dealuminated HY zeolites
trans-Anethole toluene solutions (2 mL, 0,1 M) were placed into three screw-capped test tubes (10
mL), each one containing a different zeolite (zeolites HY1, HY2 and HY3, 25 mg). Zeolites had been
previously activated (100 °C, 12 h). Reactions were performed by heating the tubes in a water bath
(30, 60 and 90 °C, 5 h). After chromatographic analysis of the resultant mixtures, conversion and
selectivities were calculated as follows:
Molecular ion stabilities, WM+, were calculated from the dimers’ mass spectra and used to
distinguish configuration isomers: according to equation 4, the molecular ion abundance is divided by
the sum of abundances of all fragment ions > m/z 50:
3.5. trans-Anethole treatment with deuterated HY zeolite
Zeolite F1 (10 mg) was suspended in DCl/D2O (1:9, 1 mL) and heated to 60 °C for 6 h. It was then
vacuum filtered and dried (100 °C, 12 h). An infrared spectrum of the resulting material was acquired

Molecules 2010, 15
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(Bruker Tensor FT-IR spectrometer). This zeolite sample was then suspended in a test tube (screw cap,
10 mL) with a trans-anethole toluene solution (0,1 M, 2 mL) and heated to 60 °C for 5 h under
continuous stirring. The tube was filled under dry nitrogen flow. The mixture was decanted and
analyzed by GC-MS in selected ion monitoring mode (SIM). The selected ions corresponded to the
molecular ion and its main fragments, together with their deuterated analogs (m/z 148, 149, 117, 118,
133, 134).
3.6. Phototransformation kinetics
A trans-anethole toluene solution (0.35 M, 15 mL) was irradiated in an Ace Glass (Vineland, NJ,
USA) 7880 photoreactor assembly with a 5.5 Watt Pen-Ray gaseous Hg discharge UV-Vis lamp, at
5 °C for 120 min under constant stirring (100 rpm). trans-Anethole toluene solution aliquots (10 µL)
were withdrawn every 20 min and analyzed by GC-FID, using the external standard (trans-anethole)
method for quantification.
3.7. Quantum yield determination
The Pen Ray UV lamp emission spectra were recorded with a high resolution HRG000c6-UV-NIR
spectrometer (Ocean Optics, Dunedin, FL, USA) at the Atomic and Molecular Spectroscopy
Laboratory of the Industrial University of Santander (Colombia). Discrete emission lines were found at
255, 313, 366, 405, 437, 548, and 579 nm. Quantum yields were determined as the quotient between
the time derivative of the concentration and the flux, according to the following equation, in which X
stands for cis-anethole, dimer 2c, or dimer 2e:
(5)
Potassium ferrioxalate was used for the actinometric determination of Ia, the number of quanta
absorbed by trans-anethole per unit concentration, per unit time, according to the IUPAC
recommended procedure [35].
4. Conclusions
Isomerization and dimerization are concurrent processes in the trans-anethole transformation
observed during its exposure to acid sites in a zeolite or UV-Vis irradiation. Proton transfer from the
zeolite to trans-anethole generates cationic intermediaries which lead the formation of eight different
anethole dimers (methoxyphenyl-disubstituted hexenes). Their relative amounts agreed with those
expected from the relative stability of the intermediary cationic species. The use of deuterated zeolites
showed that the proton eliminated during the isomerization process is not necessarily the same proton
transferred to the trans-anethole molecule when the cationic intermediary is formed. cis-Anethole and
five isomeric methoxyphenyl-disubstituted cyclobutanes were observed as products of the
transformation induced by the UV-Vis irradiation of trans-anethole.

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Acknowledgements
Financial support from Colciencias (Grant RC-432-2004) is gratefully acknowledged. The
collaboration and guidance of Jose Antonio Henao and Olger Mendoza, M. Sc., in the X-ray
diffraction and fluorescence experiments are acknowledged, as well as the oven loan from the
Condensed Materials Laboratory of Jorge Martínez.
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Sample Availability: Samples of the compounds are available from the authors.
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