Heteropoly acid catalyzed cyclization of nerolidol and farnesol: Synthesis of α-bisabolol

Article abstract of DOI:10.1016/j.apcata.2015.06.022

Heteropoly acid H₃PW₁₂O₄₀ is an active and environmentally friendly homogeneous catalyst for the synthesis of α-bisabolol, a high-priced and highly demanded ingredient for the fragrance, cosmetic and pharmaceutical industries, starting from more abundant biomass-based sesquiterpenic alcohols. The solvent nature remarkably affects the reaction pathways and product selectivity. In acetone solutions, α-bisabolol can be obtained in 55-60% GC yields from nerolidol and 60-70% GC yields from farnesol at complete substrate conversions, which are probably the best results ever reported for these reactions. α-Bisabolol synthesized by this method contains no farnesol, which is a potentially allergenic compound and should be avoided in the commercially used α-bisabolol. This advantage is especially important because the distillative separation of α-bisabolol and farnesol is a troublesome task. The catalyst shows high turnover numbers and operates under mild nearly ambient conditions.

Full text of DOI:10.1016/j.apcata.2015.06.022

Applied Catalysis A: General 502 (2015) 271–275
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Heteropoly acid catalyzed cyclization of nerolidol and farnesol:
Synthesis of ␣-bisabolol
Augusto L.P. de Meirelesa, Maíra dos Santos Costaa, Kelly A. da Silva Rochab,
Elena V. Gusevskayaa,∗
a Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
b Departamento de Química, Universidade Federal de Ouro Preto, 35400-000 Ouro Preto, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Heteropoly acid H3PW12O40 is an active and environmentally friendly homogeneous catalyst for the
synthesis of ␣-bisabolol, a high-priced and highly demanded ingredient for the fragrance, cosmetic and
pharmaceutical industries, starting from more abundant biomass-based sesquiterpenic alcohols. The
solvent nature remarkably affects the reaction pathways and product selectivity. In acetone solutions,
␣-bisabolol can be obtained in 55–60% GC yields from nerolidol and 60–70% GC yields from farnesol at
complete substrate conversions, which are probably the best results ever reported for these reactions.
␣-Bisabolol synthesized by this method contains no farnesol, which is a potentially allergenic compound
and should be avoided in the commercially used ␣-bisabolol. This advantage is especially important
because the distillative separation of ␣-bisabolol and farnesol is a troublesome task. The catalyst shows
high turnover numbers and operates under mild nearly ambient conditions.
Received 11 March 2015
Received in revised form 26 May 2015
Accepted 17 June 2015
Available online 22 June 2015
Keywords:
Biomass-based feedstock
Homogeneous catalysis
Isomerization
Polyoxometalates
Terpenes
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
reviewed by Kamatou and Viljoen [6]. This non-toxic and non-
allergenic compound is used in over 1000 cosmetic, fragrance
Terpenes represent an important renewable feedstock for flavor
and fragrance and pharmaceutical industries [1,2]. These natu-
ral compounds are available from essential oils of many plants
and fruits. Various useful terpenoid products are commercially
obtained by acid-catalyzed transformations of more abundant ter-
penes. In many of these processes mineral acids are still used as
catalysts, which results in the large amounts of waste. The devel-
opment of more environmentally friendly acid catalysts for the
reactions of terpenes is constantly required for the efficient val-
orization of bio-renewable essential oils.
␣-Bisabolol, a monocyclic sesquiterpenic alcohol with a pleas-
ant floral-sweet aroma, is a major component of the essential
oils of chamomile (Matricaria chamomilla), candeia (Eremanthus
erythropappus) and sage (Salvia runcinata) oils (up to 50, 85 and
90% contents, respectively) [3–6]. ␣-Bisabolol (also known as lev-
omenol) has demonstrated a strong therapeutic potential due to
its anti-inflammatory, skin-soothing, antibiotic, gastro-protective,
analgesic and antioxidant properties [3–8]. The applications and
pharmacological properties of ␣-bisabolol have been recently
and therapeutic products such as dermatological formulations,
decorative cosmetics, shampoos, moisturizes, cleansers, antiper-
spirants, sunscreens and fine fragrances [9]. The natural sources
of ␣-bisabolol do not currently satisfy a high commercial demand
and most ␣-bisabolol on the market is produced synthetically from
other terpenic compounds such as farnesol and nerolidol [10–13].
Nerolidol and farnesol are acyclic sesquiterpenic allylic alcohols
readily available on the industrial scale. Both compounds have a
delicate sweet floral odor and are widely used as fragrance and
food-flavor ingredients [1]. Nerolidol and farnesol are found in
essential oils of various plants and flowers, such as neroli, lemon
grass and jasmine oils, and also produced industrially from acetone
and acetylene [1]. The data on the synthesis of ␣-bisabolol avail-
able in the literature are very scarce [11–23]. All the works (mostly
patents) report the use of nerolidol or farnesol as starting materials
[11–16] except for the articles describing total syntheses [17–23].
In most of the procedures, the synthesis of ␣-bisabolol from far-
nesol or nerolidol is performed in the presence of large amounts of
strong acids, usually mineral acids, and gives the product in 30–50%
yields.
Heteropoly acids of the Keggin series (HPAs) are well known as
attractive acid catalysts for the green synthesis of many fine and
specialty chemicals [24–27]. A stronger Brønsted acidity usually
∗
Corresponding author. Fax: +55 31 34095700.
E-mail address: elena@ufmg.br (E.V. Gusevskaya).
http://dx.doi.org/10.1016/j.apcata.2015.06.022
0926-860X/© 2015 Elsevier B.V. All rights reserved.

272
A.L.P. de Meireles et al. / Applied Catalysis A: General 502 (2015) 271–275
renders HPAs higher catalytic activities than conventional acid cat-
alysts, such as mineral acids, zeolites and ion-exchange resins. HPAs
as catalysts cause less corrosion and environmental problems than
mineral acids and do not promote undesirable side reactions. Even
from homogeneous systems HPAs can be recovered and recycled
without neutralization because they are highly soluble in polar sol-
vents, on the one hand, and insoluble in non-polar solvents (e.g.,
hydrocarbons), on the other hand. For this reason, HPAs can be
recovered from polar organic solutions by precipitating with a non-
polar solvent.
Previously, we have applied HPA catalysts in a variety of liquid-
phase reactions of terpenic compounds, such as isomerization
[28–32], hydration, etherification and esterification [33–35], and
cycloaddition [36]. Within our program on the valorization of nat-
ural ingredients of essential oils by catalytic transformations, we
report now the application of tungstophosphoric heteropoly acid
H3PW12O40 (HPW), the strongest HPA in the Keggin series, as a
homogeneous catalyst for the isomerization of nerolidol (1) and
farnesol (2) aiming to obtain ␣-bisabolol (3). To our knowledge, no
attempt to use HPA catalysts for these reactions has been made so
far. Moreover, none of the previous works, as far as we know, have
achieved such a high yield of ␣-bisabolol.
was predominantly formed under these conditions rather than the
desired ␣-bisabolol. The selectivity for ␣-bisabolol did not exceed
40%. At 50 ◦C, the results were even less encouraging in terms of
selectivity (Table 1, run 2). On the other hand, to run the reaction
at 30 ◦C allowed to direct more efficiently the transformation of
nerolidol to ␣-bisabolol. The formation of farnesol in significant
amounts was detected at low conversions. Then, the concentra-
tion of farnesol decreased, whereas the selectivity of ␣-bisabolol
increased and reached 60% at a 90% conversion (Table 1, run 3).
To accelerate the reaction, the catalyst amount was increased
(Table 1, runs 4 and 5). In both runs, ␣-bisabolol was obtained in
good selectivities at 90% conversions (60–66%). However, attempts
to complete the conversion resulted in a drastic drop in ␣-bisabolol
selectivity (36% at 97% conversion; run 5, Table 1). Although, far-
nesol was detected in the reaction solutions in significant amounts
at early reaction stages (up to 30% of the converted nerolidol), final
reaction mixtures contained no even traces of farnesol. Thus, a vari-
ety of reversible transformations occur with nerolidol in acetone
solutions under acidic conditions. These transformations are con-
nected in a complex manner so that a task to obtain ␣-bisabolol in
high selectivity becomes really challenging.
Trying to inhibit the formation of dehydration products, i.e.,
bisabolenes, we performed the reaction in the presence of extra
water (Table 1, runs 6 and 7). However, the addition of only 1 eq. of
water significantly decelerated the reaction. Moreover, the 7-fold
molar excess of extra water completely suppressed the reactivity
of nerolidol. A further attempt to improve the selectivity for ␣-
bisabolol was decreasing the reaction temperature. However, the
reaction at 15 ◦C was not only much slower (expectedly) but also
much less selective toward the desired ␣-bisabolol (Table 1, run
8). The HPW recovered after the reaction by precipitation with
hexane and centrifugation was reused without loss of activity and
selectivity (Table 1, run 11).
In summary, using the HPW catalyst, nerolidol can be converted
to ␣-bisabolol in 55–60% GC yields under optimized conditions.
This value is among the best results reported for this reaction so
far [11,15]. Owing to low catalyst loading, which is an important
advantage of the method along with mild reaction conditions, high
turnover numbers (TONs) can be achieved in the substrate conver-
sion (up to 450 per mol of HPW). It is very important that final
reaction mixtures contain virtually no farnesol (by GC analysis).
The problem is that farnesol, differently from ␣-bisabolol, has an
allergenic potential; therefore, its presence in cosmetic products
is highly undesirable from a dermatological standpoint [37]. The
purification of ␣-bisabolol by distillation is difficult because ␣-
bisabolol and one of the isomers of farnesol (cis, cis) have almost
identical boiling points. Removing farnesol from natural and syn-
thetic ␣-bisabolol for practical applications is usually performed
by the selective esterification of farnesol in the mixtures and sub-
sequent distillative separation of farnesol esters from ␣-bisabolol
[37–39]. It should be mentioned that ␣-bisabolol synthesized in our
work is most probably racemic.
2. Experimental
All chemicals were purchased from commercial sources and
used as received, unless otherwise stated. Nerolidol (3,7,11-
trimethyl-1,6,10-dodecatrien-3-ol, mixture of isomers) and far-
nesol (3,7,11-trimethyl-2,6,10-dodecatrien-1-ol, mixture of iso-
mers) as well as H3PW12O40 hydrate were acquired from Aldrich.
The reactions were carried out in a 10 mL glass reactor equipped
with a magnetic stirrer and a condenser. In a typical run, the solu-
tion (5.0 mL) of the substrate (0.38–1.50 mmol, 0.075–0.3 M), HPW
(5–20 mg, 1.65–6.67 ␮mol) and dodecane (0.5 mmol, 0.1 M, GC
internal standard) in the indicated solvent was intensively stirred
under air at a specified temperature (15–50 ◦C). The reaction rate
was not dependent on the intensity of stirring within the range
used. The reactions were followed by gas chromatography (GC)
using a Shimadzu 17 instrument fitted with a Carbowax 20 M cap-
illary column and a flame ionization detector. After an appropriate
reaction time, aliquots were taken and analyzed by GC. Conver-
sion and selectivity were determined by GC based on substrate
converted. The GC mass balance was based on the charged sub-
strate using dodecane as an internal standard. Some experiments
were run for several times to confirm reproducibility. Turnover fre-
quencies (TOF) were measured by GC at low conversions (up to
20–40%) by taking aliquots at short reaction times. The products
were identified by mass spectroscopy using a Shimadzu QP2010-
PLUS instrument operating at 70 eV by comparison with authentic
compounds.
3. Results and discussion
Farnesol was also used as the starting material for the synthe-
sis of ␣-bisabolol in our work (Table 2). ␣-Bisabolol was obtained
in 60–70% GC yields, the highest values reported for the syn-
thesis ␣-bisabolol from either nerolidol or farnesol as far as we
know. It is noteworthy that in the very beginning of the reaction,
most of farnesol rapidly isomerized to nerolidol, which was further
slowly consumed to give ␣-bisabolol along with small amounts
of bisabolenes. For example, in run 1 in Table 2, a 83% conver-
sion of farnesol occurred for 1 h, from which 67% was converted to
nerolidol, 25% to ␣-bisabolol and only 8% to bisabolenes. Then, the
relative amounts of ␣-bisabolol gradually increased at the expense
of nerolidol so that after 9 h a 72% selectivity for ␣-bisabolol was
obtained. It is important that the combined GC yield of ␣-bisabolol
and nerolidol was as high as 91%, with only small amounts of bis-
The results on the isomerization of nerolidol in acetone solu-
tions containing HPW are presented in Table 1. The reaction gave
␣-bisabolol with up to 66% selectivity along with small amounts of
farnesol, which usually disappeared by the end of the nerolidol con-
version (Scheme 1). The rest of the substrate was converted mainly
in bisabolenes, which formally are the products of the dehydration
of ␣-bisabolol. The reactions were not significantly complicated by
oligomerization, as often happens with terpenic compounds under
acidic conditions.
The transformation of nerolidol in the presence of ca. 0.2 mol% of
HPW occurred smoothly at 40 ◦C resulting in a nearly complete con-
version for 4 h (Table 1, run 1). However, a mixture of bisabolenes

A.L.P. de Meireles et al. / Applied Catalysis A: General 502 (2015) 271–275
273
Table 1
Isomerization of nerolidol (1) to ␣-bisabolol (3) and farnesol (2) catalyzed by HPW.a
Run
HPW
T
1
Time
(h)
Conversion
(%)
Selectivity (%)
TOFb
TONb
436
GC yield
(␮mol)
(◦C)
40
(mmol)
0.75
3
2
(h−1
n.d.a
)
(%)
36
1
1.65
0.5
4
3
3
13
1
9
3
5
7
61
96
100
37
91
29
91
90
97
0
40
50
75
100
94
40
38
35
55
60
40
66
60
36
–
–
–
–
22
4
31
–
–
–
–
2
3
1.65
1.65
50
30
0.75
0.75
n.d.a
68
454
414
35
55
4
5
3.30
6.60
30
30
0.75
0.75
68
205
60
35
n.d.a
6c
7d
8
3.30
3.30
3.30
3.30
3.30
1.65
30
30
15
30
30
30
0.75
0.75
0.75
1.50
0.38
0.75
0
0
90
114
340
115
426
6
5
9
5
24
17
65
55
67
5
3
4
–
23
45
99
70
n.d.a
10
9
46
55
63
9
10
11e
13
–
a
Reactions were carried out in acetone solutions, with the total volume of the reaction mixture of 5.0 mL; conversion and selectivity were determined by GC based on
substrate converted; n.d. – not determined.
b
TON in moles of substrate converted per mole of HPW. TOF is the initial turnover frequency (mol of the substrate converted per mol of HPW per hour).
100 ␮l (5.55 mmol) of water was added.
13.5 ␮l (0.75 mmol) of water was added.
HPW recovered after run 3 by the precipitation with hexane was used as the catalyst.
c
d
e
abolenes being detected in the reaction solutions at this stage.
Keeping the reaction mixture in the contact with the catalyst for
one hour more did not result in a further increase in the ␣-bisabolol
yield.
Increasing the catalyst concentration allowed to accelerate the
transformation of nerolidol, which was rapidly formed from far-
nesol at early reaction stages, to ␣-bisabolol without a significant
loss in selectivity for the target ␣-bisabolol (Table 2, runs 2 and 3).
Interestingly, the combined selectivity of ␣-bisabolol and nerolidol
was only 49% after the complete farnesol conversion in run 3 for
1 h, with bisabolenes accounting for the rest of the mass balance.
At longer reaction times, the selectivity of ␣-bisabolol increased to
70% at the expense of both nerolidol and bisabolenes showing that
the formation of bisabolenes in our system is also reversible.
To improve the catalyst efficiency in terms of TONs, the amount
of the starting farnesol was doubled in run 4. In this run, ␣-bisabolol
H+
-H2O
H+
-H2O
OH
OH
Nerolidol
B
Farnesol
A
mixture of bisabolenes
mainly:
O
O
-H+
H+
D
C
-Bisabolene
H2O
-H+
H2O
- acetone
-H+
OH
-Bisabolene
-Bisabolol
Scheme 1. Schematic representation of acid-catalyzed transformations nerolidol and farnesol.

274
A.L.P. de Meireles et al. / Applied Catalysis A: General 502 (2015) 271–275
Table 2
Isomerization of farnesol (2) to ␣-bisabolol (3) and nerolidol (1) catalyzed by HPW.a
Run
HPW
2
Time
(h)
Conversion
(%)
Selectivity (%)
3
TONb
GC yield
(%)
(␮mol)
(mmol)
1
1
2
3
4
3.30
4.95
6.60
3.30
0.75
0.75
0.75
1.50
1910
1613
1 2 4
1 12
83 100 100
90 100 100
98 100 100
90 100
257272
306058
244770
22 60
671911
60 24 4
252113
227
151
114
454
72
58
70
60
47
6
a
Reactions were carried out in acetone solutions, with the total volume of the reaction mixture of 5.0 mL; 30 ◦C; conversion and selectivity were determined by GC based
on substrate converted.
b
TON in moles of substrate converted per mole of HPW.
Table 3
Isomerization of nerolidol (1) to ␣-bisabolol (3) and farnesol (2) catalyzed by HPW in different solvents.a
Run
Solvent
HPW
Conversion
(%)
Selectivity (%)
3
TOFb
TONb
(␮mol)
2
(h−1
)
1
2
3
4
5
6
Acetone
3.30
3.30
3.30
1.65
4.95
4.95
4.95
3.30
3.30
76
70
58
97
0
0
0
75
70
65
12
14
12
–
–
–
50
52
5
1
3
–
–
–
–
68
39
23
250
–
–
–
110
99
172
160
132
440
–
–
–
340
320
1,2-Dichloroethane
Dichloromethane
Ethyl acetate
Acetonitrile
Dimethylacetamide
Dimethylformamide
Methyl ethyl ketone
Acetone
7
8c
9c
22
15
a
Total volume of the reaction mixture: 5.0 mL; nerolidol: 0.75 mmol; 30 ◦C; reaction time: 6 h; conversion and selectivity were determined by GC based on substrate
converted.
b
TON in moles of substrate converted per mole of HPW. TOF is the initial turnover frequency (mol of the substrate converted per mol of HPW per hour).
Nerolidol: 1.50 mmol.
c
was obtained in 60% GC yield at the complete substrate conversion,
which corresponded to the TON of 454 per mol of HPW (Table 2,
run 4). No farnesol was detected in the final solutions at the end of
the reactions.
the reaction solutions to give C30 carbenium ions initiating an irre-
versible oligomerization process. Fortunately, under the conditions
used in our work the reactions were not significantly complicated
by oligomerization. Under optimized conditions, ␣-bisabolol was
obtained in 55–60% GC yields from nerolidol and 60–70% GC yields
from farnesol in acetone solutions. Within this mechanistic scheme,
the presence of water in the reaction mixtures should disfavor
the formation of carbenium ions A and B from nerolidol and far-
nesol, what was really observed in the experiments described above
(Table 1, runs 6 and 7).
We have not found any correlation between the effect of solvent
on the selectivity of ␣-bisabolol formation in these reactions and
either polarity or basicity of the solvent. The unique performance
of ketonic solvents could be related to the ability of ketone to trap
bisabolyl carbenium ion C to give carbenium ion D (Scheme 1).
Such an interaction, suggested previously by Uneyama et al. [15]
could prevent a competitive proton lost by carbenium ion C to give
bisabolenes, the main by-products of the process. The consequent
hydrolysis of the adduct D would give the desired ␣-bisabolol. This
hypothesis for the explanation of the solvent effect in the case of
acetone is pictured in Scheme 1.
The transformations of nerolidol and farnesol presented in
Tables 1 and 2 were performed in acetone solutions. The nature
of the solvent was found to exert a remarkable effect on these reac-
tions. In the runs collected in Table 3, a variety of solvents was
used in the reaction with nerolidol. None of them except methyl
ethyl ketone gave results comparable in terms of selectivity with
those obtained in acetone. In dichloroethane and dichloromethane
solutions, nerolidol reacted smoothly at 30 ◦C; however, to give pre-
dominantly the mixture of bisabolenes (Table 3, runs 2 and 3). In
ethyl acetate, the consumption of nerolidol was so fast that we had
to decrease the amount of the catalyst (Table 3, run 4). Nevertheless,
the initial turnover frequency (TOF) was ca. 4 times higher than that
for the reaction in acetone (cf. runs 1 and 4, Table 3). Unfortunately,
very poor selectivity for ␣-bisabolol was also obtained in this sol-
vent (ca. 10%). In the solutions of acetonitrile, dimethylacetamide
and dimethylformamide, nerolidol was stable at 30 ◦C for at least
6 h (Table 3, runs 5–7). Then, the same runs were conducted with a
stepwise increase in temperature (by 10 ◦C each hour) up to 80 ◦C
(not shown in Table); however, the conversions did not exceed
10%. ␣-Bisabolol was detected in only trace amounts in all these
solvents. On the other hand, the reaction in another ketonic sol-
vent, methyl ethyl ketone, gave the results comparable with those
obtained in acetone solutions in terms of both the reaction rate and
selectivity for ␣-bisabolol (Table 3, cf. runs 8 and 9).
4. Conclusions
Heteropoly acid H3PW12O40 is an efficient and environmen-
tally benign catalyst for the liquid-phase cyclization of nerolidol
and farnesol into the highly valuable ␣-bisabolol. A good control
of chemoselectivity can be achieved through the choice of the
solvent, whose nature strongly affects the reaction pathways. In
acetone solutions, ␣-bisabolol was obtained in 55–60% GC yields
from nerolidol and 60–70% GC yields from farnesol at complete
substrate conversions. Main by-products were ␣-bisabolene and
␤-bisabolene, also useful compounds. The results obtained are
probably the best ever reported for these reactions. The catalyst
shows high turnover numbers (up to 450), acts under mild nearly
ambient conditions and, in principle, can be recycled. This simple
and low cost method provides an effective route for the synthesis
A possible mechanism for the acid-catalyzed transformations of
nerolidol and farnesol is shown in Scheme 1. The protonation and
dehydration of nerolidol and farnesol give carbenium ions A and
B, respectively, which can rearrange to each other or to bisabolyl
carbenium ion C. The latter can undergo several competing trans-
formations: the nucleophilic attack of water to give ␣-bisabolol and
deprotonation to give the mixture of bisabolenes. In our reactions,
mainly ␣-bisabolene and ␤-bisabolene were detected. In principle,
all carbenium ions can react with another olefin molecule present in

A.L.P. de Meireles et al. / Applied Catalysis A: General 502 (2015) 271–275
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Acknowledgments
Financial support and scholarships from CNPq, CAPES, FAPEMIG,
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