Anethole Isomerization and Dimerization Induced by Acid Sites or UV Irradiation

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.


Introduction
trans-Anethole (1a) is one of the main constituents of anise, clove, cinnamon and thyme essential oils [1][2][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].

OPEN ACCESS
Zeolites are porous crystalline aluminosilicates formed by [SiO 4 ] -4 and [AlO 4 ] -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, λ exc (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.

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 transanethole 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. 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.

Transformation of trans-anethole exposed to deuterated zeolite, HDY
Based on the in situ 13 C-NMR determination of alkoxides, various studies [14][15][16][17][18][19][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 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).
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 neutral product may be formed by the loss of a proton different from that which participated in the electrophilic addition.
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).   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 S 1 excited t-A molecule with another one in its ground state, S o in a [2 + 2] cycloaddition leads to dimers 6c (anti head-to-head) and 6e (syn headto-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].    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.

trans-Anethole phototransformation
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. Dimerizations result from 1 t-A*-1 t-A, and to a lesser extent, from 1 t-A*-1 c-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 of 6c and 6e dimers increased. This shows the competition that exists between isomerization and dimerization.  Scheme 4. Scheme proposed for trans-anethole photodimerization products.

Quantum yield of trans-anethole photoisomerization
Quantum yields of the formation of cis-anethole ( ) 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 .  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 ( 1 t-A*) with trans-anethole than that observed for its reaction with cis-anethole. Since cis-anethole is being formed, encounters of 1 t-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 9cyanophenanthrene 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.

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.

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, W M+ , 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:

trans-Anethole treatment with deuterated HY zeolite
Zeolite F1 (10 mg) was suspended in DCl/D 2 O (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 (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).

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].

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.