Heteropoly acid catalyzed isomerization of caryophyllene
oxide
Augusto L. P. de Meireles1,*, Kelly A. da Silva Rocha2, Elena F. Kozhevnikova3, Ivan V Kozhevnikov3,
Elena V. Gusevskaya1.
1Departamento
de Química, Universidade Federal de Minas Gerais, Belo Horizonte, 31270-901, MG, Brazil
de Ciências Exatas e Biológicas, Universidade Federal de Ouro Preto, Ouro Preto, 35400-000, MG, Brazil
3Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, UK
*Corresponding author:[email protected]
2Departamento
Keywords: Biomass-based feedstock; isomerization; heterogeneous catalysis; heteropoly compounds; caryophyllene oxide
1. Introduction
Natural terpenes and their synthetic derivatives
are very important for perfumery, pharmaceutical
and cosmetic industries. In particular, often valueadded terpenoids are industrially produced through
the acid catalyzed transformations of more abundant
terpenes [1–2].
Heteropoly acids (HPAs) of the Keggin series,
such as H3PW12O40 (HPW), are attractive as acid
catalysts for many important processes in fine
chemistry due to the strong Brønsted acidity and
high chemical stability [3–4]. Supported HPAs can
be used as heterogeneous catalysts in non-polar and
weakly polar media without leaching problems. On
the other hand, for heterogeneous catalysis in polar
solvents, their insoluble acidic salts, as
Cs2.5H0.5PW12O40 (CsPW), represent an excellent
alternative [5–6].
In this work the application of HPAs in the
isomerization of caryophyllene oxide was studied.
Caryophyllene oxide is found in various essential
oils, such as clove or lavender oils [7].
Caryophyllane derivatives are widely applied as
aromatic ingredients (woody, floral, fruity) in
perfumes, food and tobacco products [2,7].
The
development
of
sustainable
and
environmentally benign processes based on the
application of solid acid catalysts is very important
for the chemistry of terpene as it allows extending
the commercial use of easily affordable and biorenewable essential oils.
2. Experimental Part
The 20 wt% H3PW12O40/SiO2 catalyst
(HPW/SiO2) was prepared as previously described
[8]. The acidic heteropoly salt CsPW was prepared
according to the literature procedure [9]
The reactions were run under air in a glass
reactor equipped with a condenser. Typically, a
mixture of caryophyllene oxide, dodecane and the
catalyst (HPW/SiO2 or CsPW) in an indicated
solvent was intensely stirred with a magnetic stirrer
at an indicated temperature. The reactions were
followed by gas chromatography (GC). The mass
balance, conversion and product selectivity were
calculated based on dodecane as an internal standard.
3. Results and discussion
The transformations of caryophyllene oxide over
HPW and CsPW are shown in Figure 1.
Figure 1. HPW and CsPW catalyzed transformations of
caryophyllene oxide.
The results for the transformations of
caryophyllene oxide 1 over HPW/SiO2 in different
solvents are shown in Table 1.
Table 1. Isomerization of caryophyllene oxide (1)
catalyzed by HPW/SiO2.a
[a] Reactions were carried out in the specified solvents, with the
total volume of the reaction mixture of 5.0 mL. Conversion and
selectivity were determined by GC. [b] Caryophyllene oxide –
0.375 mmol. [c] Caryophyllene oxide – 0.750 mmol [d] CsPW
(25 mg; 7.5 mol) was used as the catalyst.
In cyclohexane and iso-octane solutions, the
reactions were performed at 60 °C due to the limited
solubility of caryophyllene oxide in these media at
lower temperature. The conversion of caryophyllene
oxide was relatively fast even in the presence of
small amounts of the catalyst (0.1–0.2 mol% of
HPW); however, the reaction became stagnated after
80–85% conversion (Table 1, runs 1–3). The GC
analysis revealed the formation of a complex
mixture of products (2, 3, 4, 5 and 6).
Five identified products accounted for ca. 80% of
the mass balance. The best individual selectivity of
ca. 45% was obtained for clovenol 2, formed as a
major product in all the runs presented in Table 1. In
both nonpolar solvents, the reaction occurred at
similar rates (Table 1, cf. runs 1 and 3) and with
similar product distributions.
The
reaction
was
performed
in
1,2-dichloroethane solutions where HPW/SiO2 can
be used as a heterogeneous catalyst without any
leaching problems [10]. The caryophyllene oxide
conversion in 1,2-dichloroethane was much faster
than in nonpolar media (Table 1, runs 4 and 5 vs.
runs 1–3). At 30 °C, the reaction was finished in 0.5
h showing turnover numbers (TONs) of ca. 1000
(Table 1, run 4). The combined selectivity for five
identified products was ca. 80% with nearly the
same product distribution as in cyclohexane and isooctane.
The alternative CsPW catalyst was also tested in
the isomerization of caryophyllene oxide (Table 1,
run 6). The advantage of using CsPW was an
increase in the relative amounts of clovane products
(2 and 3) as compared to caryophyllane derivatives
(4, 5 and 6) from ca. 2/1 to 3.7/1.
In a further work, we have decided to run the
reaction in acetone solutions using the CsPW
catalyst, which is virtually insoluble in acetone. The
reaction with 2 mol% of CsPW occurred rapidly at
30 °C (Table 2, run 1). The overall conversion of 1
was 81% for 30 min.
Table 2. Isomerization of caryophyllene oxide (1)
catalyzed by CsPW in acetone solutionsa
Run
CsPW
(mol)
T
(°C)
1
15.0
30
2
3
4
5
6b
7b
3.0
1.5
1.5
1.5
3.0
1.5
30
30
40
50
40
40
Time
(h)
0.5
1
1
3
1
0.5
1
6
Conv.
(%)
81
83
78
76
78
77
76
60
Selectivity for (%)
2
3
4
5
27 43 1
5
19 42 1
6
23 50 4
5
22 55 5
6
17 60 8
5
21 44 6
6
21 55 4
8
23 47 5 11
6
6
6
5
7
5
6
7
8
[a]Total volume of the reaction mixture - 5.0 mL, caryophyllene
oxide – 0.75 mmol. Conversion and selectivity were determined
by GC. [b] Caryophyllene oxide – 2.25 mmol.
The decrease in catalyst loading proportionally
decelerated the reaction and improved the total
selectivity for five identified products from 82 to
95% (Table 2, run 3 vs. runs 1 and 2). We observed
that the clovanes/caryophyllanes ratio in acetone
was ca. 4.5/1. At 40 °C, the reaction was still highly
selective; however, at 50 °C the selectivity began to
decrease (Table 2, runs 4 and 5). Clovanediol 3 was
formed in acetone solutions with the selectivity of
up to 60%, which is the best result reported so far
for this compound. The combined selectivity for
clovane products 2 and 3 was ca. 75%. The CsPW
catalyst was perfectly stable under the reaction
conditions showing TONs of up to 1000 (Table 2,
runs 3–7).
Clovanediol 3 is formally the product of the
water addition to the caryophyllene oxide molecule
accompanied by its skeletal rearrangement. Thus,
the amounts of hydration water that present in the
catalyst and commercial caryophyllene oxide and
solvents were sufficient for the formation of 3.
4. Conclusions
Both 20% H3PW12O40/SiO2 and Cs2.5H0.5PW12O40
are active and environmentally benign solid acid
catalysts for the isomerization of caryophyllene
oxide in a liquid phase to give rare polycyclic
oxygenated sesquiterpenes potentially useful for
fragrance
and
pharmaceutical
applications.
Compounds with a tricyclic clovane structure,
clovenol 2 and clovanediol 3, were obtained with up
to 80% combined selectivity and 60% individual
selectivity each (under different reaction conditions),
which is the best result reported for these
compounds as far as we know. The reactions occur
under mild conditions at low catalyst loadings
without leaching problems. Solid catalysts can be
simply centrifuged from the reaction mixture and
relatively low boiling points of the solvents allow
their easy removal by distillation.
Acknowledgments
CNPq, PROPP/UFOP, CAPES, FAPEMIG, and INCT-Catálise
(Brazil).
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