Heteropoly acid catalysts for the synthesis of fragrance compounds from biorenewables: The alkoxylation of monoterpenes

Article abstract of DOI:10.1002/cctc.201402395

The cesium salt of tungstophosphoric heteropoly acid, Cs_2.5H_0.5PW₁₂O₄₀, is an active and environmentally friendly solid-acid catalyst for the liquid-phase alkoxylation of widespread monoterpenes, such as camphen

Full text of DOI:10.1002/cctc.201402395

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DOI: 10.1002/cctc.201402395
Heteropoly Acid Catalysts for the Synthesis of Fragrance
Compounds from Biorenewables: The Alkoxylation of
Monoterpenes
Augusto L. P. de Meireles,[a] Maꢀra dos Santos Costa,[a] Kelly A. da Silva Rocha,[b]
Elena F. Kozhevnikova,[c] Ivan V. Kozhevnikov,[c] and Elena V. Gusevskaya*[a]
The cesium salt of tungstophosphoric heteropoly acid,
Cs2.5H0.5PW12O40, is an active and environmentally friendly solid-
acid catalyst for the liquid-phase alkoxylation of widespread
monoterpenes, such as camphene, limonene, a-pinene, and b-
pinene. These reactions provide isobornyl or a-terpenyl ethers,
useful as fragrances, in good to excellent yields. The reactions
are equilibrium-controlled and occur with high catalyst turn-
over numbers (TONs) up to 1500–4200 without catalyst leach-
ing. Heteropoly acid H3PW12O40 also efficiently catalyzes the
alkoxylation of these monoterpenes under homogeneous con-
ditions with TONs up to 1500–11300.
Introduction
Monoterpenes are an abundant renewable feedstock for the
pharmaceutical and flavor and fragrance industries.[1–4] Major
terpene sources are essential oils, natural resins, and byprod-
ucts of the pulp, paper, and citrus juice industries. The most
widespread terpene hydrocarbons, considered as platform
molecules in terpene chemistry,[5] are limonene, a-pinene, and
b-pinene. Limonene is extracted from citrus oils, whereas a-
and b-pinene are the main constituents of turpentine oils ob-
tained from coniferous trees. The content of a-pinene can
reach 85%, which depends on the oil source. Camphene is
also available in nature from the essential oils of some plants
but it is synthesized mostly from a-pinene in the liquid phase
over acidic catalysts. Various useful terpenoid compounds are
produced commercially by acid-catalyzed transformations of
more abundant terpene precursors. Mineral acids are still used
as catalysts in many of these processes, which generates
a large amount of waste. The development of more efficient
and environmentally friendly acid catalysts for the conversion
of terpenes can contribute significantly to the valorization of
biorenewable essential oils.
fine and specialty chemicals.[6–8] As a result of their stronger
acidity, HPAs usually show higher catalytic activities than con-
ventional acid catalysts, such as mineral acids, zeolites, and
ion-exchange resins, and cause less corrosion problems than
mineral acids. HPAs are insoluble in nonpolar solvents and can
be used as heterogeneous catalysts in such solvents. To create
heterogeneous catalytic processes in polar media, in which
HPAs are highly soluble, they can be substituted by their in-
soluble acidic salts. In particular, the Cs salt, Cs2.5H0.5PW12O40,
which has strong Brønsted acid sites and a large surface area,
has been used in a number of liquid-phase organic reactions
as an efficient solid-acid catalyst.[9–14] Previously, we have em-
ployed HPA catalysts for a variety of liquid-phase reactions of
terpenic compounds, such as isomerization,[13,15–18] hydration,
and esterification,[19–22] which includes the hydration and
esterification of camphene, limonene, a-pinene, and b-
pinene.[19–21]
Through the acid-catalyzed addition of alcohols, camphene
can be converted to isobornyl ethers, whereas limonene, a-
pinene, and b-pinene are converted to a-terpenyl ethers.
These products possess strong characteristic woody or fruit-
berry aromas almost identical to the natural ones and have
many applications in cosmetics, perfumes, and pharmaceutical
formulations. Isobornyl ethers are also used for the production
of synthetic camphor, the compound with more commercial
applications than any other terpene.[1] A number of solid-acid
catalysts, such as zeolites,[23–28] modified clay materials,[29] sulfo-
nated mesoporous silica,[30] and poly(vinyl alcohol) with sulfon-
ic acid groups,[31] have been used for the alkoxylation of cam-
phene, limonene, a-pinene, and b-pinene. Previously, HPAs
have been reported as homogeneous catalysts for the alkoxy-
lation of camphene.[32] In addition, silica-occluded HPAs pre-
pared by a sol–gel method have been reported as heterogene-
ous catalysts for the alkoxylation of camphene[33] and a-
pinene.[34] Silica-occluded HPAs are stable towards leaching in
Heteropoly acids of the Keggin series (HPAs) are well known
as promising acid catalysts for the green synthesis of many
[a] A. L. P. de Meireles, M. dos Santos Costa, Prof. E. V. Gusevskaya
Departamento de Quꢀmica
Universidade Federal de Minas Gerais
31270-901, Belo Horizonte, MG (Brazil)
Fax: (+55)31-34095700
E-mail: elena@ufmg.br
[b] Dr. K. A. da Silva Rocha
Departamento de Quꢀmica
Universidade Federal de Ouro Preto
35400-000, Ouro Preto, MG (Brazil)
[c] Dr. E. F. Kozhevnikova, Prof. I. V. Kozhevnikov
Department of Chemistry
University of Liverpool
Liverpool L69 7ZD (UK)
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alcohol media. However, the reaction procedures are lengthy
and require a high catalyst-to-substrate ratio.[33,34] Therefore,
the development of economically attractive and green cata-
lysts for these reactions remains a challenge.
Within our program aimed to add value to the natural ingre-
dients of essential oils, we report the application of tungsto-
phosphoric heteropoly acid H3PW12O40 (HPW) and its acidic Cs
salt Cs2.5H0.5PW12O40 (CsPW) for the alkoxylation of camphene,
limonene, a-pinene, and b-pinene.
Scheme 1. Alkoxylation of 1a.
Results and Discussion
The results of the alkoxylation of camphene (1a) by butan-1-
ol, ethanol, and methanol in the presence of HPW are present-
ed in Table 1. The reactions occurred with 96–99% selectivity
to the corresponding isobornyl ethers 1b, 1c, and 1d with
only small amounts of isoborneol observed (Scheme 1). The re-
actions were not complicated by oligomerization and isomeri-
zation as often happens with terpenic compounds under
acidic conditions. Blank reactions, without any catalyst added,
showed virtually no conversion of camphene under the condi-
tions used. The alkoxylation occurred with a remarkably high
stereoselectivity without the formation of any borneol ethers,
that is, endo isomers of 1b, 1c, and 1d.
The NOESY spectra revealed that in all these molecules hydro-
gen atoms H-2 give a NOE with hydrogen atoms H-6ax, which
shows their spatial proximity. This clearly indicates the exo con-
figuration for ethers 1b, 1c, and 1d, in which the alkoxy
groups at C-2 and hydrogen atoms H-6ax are at the opposite
sides of the six-membered ring (Scheme 1).
The butoxylation of camphene at 808C occurred with nearly
90% conversion to isobornyl butyl ether 1b in 13 h (Table 1,
run 1). At 1108C, the same conversion to 1b with excellent se-
lectivity was reached in 3 h with no increase in conversion for
the next 4 h (Table 1, run 4). This suggests that the reaction
was equilibrium-controlled. With the same amount of catalyst,
the reaction was equally efficient with increased substrate con-
centrations to give a 90% camphene conversion at almost
100% selectivity to 1b (Table 1, runs 5 and 6). These results
confirm that the incomplete conversion is because of equilibri-
um-control rather than catalyst deactivation. The reaction pre-
sented in run 6 (Table 1) corresponds to a turnover number
(TON) of 1440 per mol of HPW. The time course for this reac-
tion is shown in Figure 1.
The products 1b, 1c, and 1d were isolated by column chro-
matography and identified by GC–MS and NMR spectroscopy.
Table 1. Alkoxylation of 1a catalyzed by HPW.[a]
Run
HPW
[mmol]
1a
[mmol]
T
[8C]
t
[h]
Conv.
[%]
TOF[b]
TON[b]
[hÀ1
]
Butan-1-ol
1
8.3
4.0
80
7
13
7
7
3
7
3
7
3
76
91
68
76
90
90
86
90
87
90
65
450
540
610
The reaction with ethanol and methanol gave the corre-
sponding isobornyl ethers 1c and 1d, which are also useful as
fragrances. Under optimized conditions, ethers 1c and 1d
were obtained in 80–85% yields, and these reactions were also
equilibrium-controlled (Table 1, runs 7 and 10; Figure 1).
2
3
4
5.0
5.0
5.0
4.0
4.0
4.0
90
100
110
160
280
440
720
1080
1440
5
5.0
5.0
6.0
8.0
110
110
660
880
6
7
Ethanol
7
10.0
10.0
8.0
110
110
3
10
24
3
60
80
80
40
65
240
480
640
8
32.0
10
2080
Methanol
9
10.0
16.0
8.0
90
3
7
3
10
24
40
77
59
86
86
240
720
1230
10
5.0
100
1380
[a] Reactions were performed in the specified alcohol as the solvent with
the total volume of the reaction mixture of 5.0 mL in butan-1-ol (glass re-
actor) and 20 mL in methanol and ethanol (stainless-steel autoclave). The
conversion and selectivity were determined by GC. Selectivity for ethers
1b–1d was 96–99% in all runs. [b] TON in moles of substrate converted
per mole of HPW. TOF is the initial turnover frequency (mol of the sub-
strate converted per mol of HPW per hour).
Figure 1. Alkoxylation of camphene catalyzed by HPW in various solvents:
~
&
*
butan-1-ol ( ), ethanol ( ), and methanol ( ). The kinetic curves correspond
to runs 6, 7, and 10 in Table 1, respectively. Reaction conditions are given in
Table 1.
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Table 2. Alkoxylation of 1a catalyzed by CsPW.[a]
Run
CsPW
[mmol]
1a
[mmol]
T
[8C]
Conv.
[%]
TOF[b]
[hÀ1
]
TON[b]
Butan-1-ol
1
2
3
4[c]
5
6
7
7.5
7.5
7.5
7.5
9.0
6.0
4.5
8.0
8.0
8.0
8.0
8.0
8.0
8.0
80
90
26
59
77
85
86
82
84
50
140
210
370
480
530
750
280
630
820
910
760
100
110
110
110
110
1090
1490
Ethanol
8
9.0
9.0
32.0
32.0
125
125
50
710
550
61[d]
2170
1560
*
~
&
Figure 2. Alkoxylation of camphene catalyzed by 9.0 ( ), 7.5 ( ), 6.0 ( ), or
!
Methanol
9
4.5 ( ) mmol CsPW in butan-1-ol (8 mmol of camphene, 5 mL of solvent,
41
44[d]
110 8C).
[a] Reactions were performed for 7 h in the specified alcohol as the sol-
vents with the total volume of the reaction mixture of 5.0 mL in butan-1-
ol (glass reactor) and 20 mL in methanol and ethanol (stainless-steel auto-
clave). The conversion and selectivity were determined by GC. Selectivity
for ethers 1b–1d was 96–99% in all runs. [b] TON in moles of substrate
converted per mole of CsPW. TOF is the initial turnover frequency (mol of
the substrate converted per mol of CsPW per hour). [c] After run 4, the
catalyst was removed, the solution was recharged with fresh substrate
(8 mmol), and the reaction was allowed to proceed further, and no fur-
ther conversion was observed thereupon. [d] 24 h reaction time.
The CsPW catalyst was also effective for the methoxylation
and ethoxylation of camphene to give the corresponding
ethers. Under optimized conditions, isobornyl ethers 1c and
1d were obtained with virtually 100% selectivity at nearly
equilibrium conversions and high TONs (Table 2, runs 8 and 9).
Runs performed at lower substrate concentrations resulted in
lower conversions over the same time. Importantly, CsPW oper-
ated heterogeneously in these reactions without leaching. Al-
though CsPW is insoluble in alcohols, the possibility of leach-
ing and any contribution of homogeneous reactions were veri-
fied in special experiments. After reaction, the catalyst was sep-
arated by centrifugation, and the filtrate was recharged with
fresh camphene and allowed to react (Table 2, run 4). No fur-
ther reaction was observed after catalyst removal, which im-
plies the lack of any significant leaching.
The reactions presented in Table 1 are catalyzed homogene-
ously by the dissolved HPW. The use of solid catalysts based
on HPW in alcohol solvents was not possible because of HPW
leaching. To overcome this problem, we tested the insoluble
CsPW salt as the catalyst for the alkoxylation of camphene.
This allowed the truly heterogeneous synthesis of isobornyl
ethers in alcohol solvents. Representative results are given in
Table 2. The reaction in butan-1-ol at 808C in the presence of
small amounts of CsPW (ꢀ0.1 mol%) reached a rather moder-
ate conversion of 26% in 7 h; however, isobornyl ether 1b was
the only product observed (Table 2, run 1). At higher tempera-
tures (Table 2, runs 2–4), the reactions were also highly selec-
tive to ether 1b, and they were much faster. At 1108C, the re-
action was complete in 7 h to give an 85% conversion.
The alkoxylation of limonene (2a), a-pinene (3a), and b-
pinene (4a) in solutions of butan-1-ol that contained dissolved
HPW resulted in the formation of the same product: a-terpenyl
butyl ether (2b; Scheme 2, Table 3). A possible mechanism of
To verify the catalyst stability, we performed runs with differ-
ent catalyst amounts (Table 2, runs 4–7, Figure 2). With the
same amount of substrate, the reaction was equally efficient
with approximately 0.05 mol% of CsPW to give 84% cam-
phene conversion in 7 h (Table 1, run 7). These results indicate
the equilibrium-control of the reaction and also show the
high-performance stability of the CsPW catalyst, which gave
a TON value up to approximately 1500 per mol of CsPW. If we
consider that some acid sites may be located in the bulk of
the solid phase and hence not accessible to the substrate, the
real efficiency of the surface active sites could be even higher.
The reaction rate depended only slightly on the catalyst
amount (Figure 2), so that the initial turnover frequencies (TOF,
mol of the substrate converted per mol of CsPW per hour)
were higher in the reaction with smaller catalyst amounts
(Table 2, run 7 vs. run 5).
Scheme 2. Alkoxylation of 2a, 3a, and 4a.
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CsPW can also be used in these reactions as heter-
ogeneous catalyst to give a-terpenyl butyl ether 2b
with yields comparable to those obtained in the ho-
mogeneous reaction (Table 4).
Table 3. Alkoxylation of 2a, 3a, and 3a catalyzed by HPW.[a]
Run HPW Substrate T
t
Conv.
Selectivity [%]
TOF[b] TON[b]
[hÀ1
]
[mmol] [mmol]
[8C] [h] [%]
Isomerization
Addition
2b Others Total
1
2
3
4
5
6
7
8
6.6
6.6
6.6
10.0
1.7
1.6
1.7
0.7
1.7
1.7
0.7
0.7
2a (8.0)
2a (8.0)
2a (8.0) 100 2 71
2a (8.0)
3a (4.0)
3a (4.0) 100 2 96
3a (4.0)
3a (4.0)
4a (4.0)
4a (4.0)
4a (4.0)
4a (8.0)
80 7 60
90 7 75
15
17
52
22
37
60
40
32
15
36
37
38
76
9
85
83
48
78
63
40
60
68
85
65
63
62
300
730
n.d.
200
500
n.d.
250
470
175
500
n.d.
730
910
860
67 16
36 12
57 21
46 17
23 17
42 18
50 18
70 15
44 21
47 16
42 20
Conclusions
80 7 80
80 9 85
640
Heteropoly acid H3PW12O40 and its acidic Cs salt,
Cs2.5H0.5PW12O40, are efficient catalysts for the liquid-
phase alkoxylation of camphene, limonene, a-pinene,
and b-pinene. These reactions provide effective
routes for the synthesis of isobornyl and a-terpenyl
ethers, important fragrances that possess a strong
characteristic aroma, from renewable biomass-based
substrates. The reaction yield is equilibrium-con-
trolled, the selectivity of which is virtually 100% for
isobornyl ethers and up to 70–75% for the a-terpen-
yl ether. As it is insoluble in the reaction mixtures,
the Cs salt acts as a truly heterogeneous and environ-
mentally friendly catalyst, which can be recycled
easily.
2000
2260
1670
1490
1110
2020
5660
70 7 71
70 7 26
40 7 47
60 5 86
80 2 99
80 2 99
9
10
11
12
n.d. 11320
[a] Reactions were performed in butan-1-ol, and the total volume of the reaction mix-
ture was 5.0 mL in a glass reactor. The conversion and selectivity were determined by
GC; 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).
this reaction is shown in Scheme 3. The protonation of a- and
b-pinene gives carbenium ion A, which isomerizes to a-terpen-
yl carbenium ion (B). This ion can also form directly
from limonene by protonation of its exocyclic double
Table 4. Alkoxylation of 2a, 3a, and 3a catalyzed by CsPW.[a]
bond (Scheme 3). The nucleophilic attack of alcohol
on B gives product 2b. The alkoxylation of limonene,
a-pinene, and b-pinene is more complicated than the
alkoxylation of camphene because of the isomeriza-
tion of these substrates. As a result, numerous minor
alkoxylation products were formed in these reactions
along with a-terpenyl ether. The nature of the minor
products was suggested based on their GC retention
times and mass spectra. Under optimized conditions,
ether 2b was obtained with 70–75% selectivity from
limonene and 40–50% selectivity from a- and b-
pinene. Blank reactions, without any catalyst added,
showed virtually no conversion of limonene, a-
pinene, and b-pinene under the conditions used.
Run CsPW Substrate T
t
Conv.
Selectivity [%]
TOF[b] TON[b]
[hÀ1
[mmol] [mmol] [8C] [h] [%]
Isomerization
Addition
2b Others Total
]
1
2
3
4
5
6
7
7.5
7.5
1.5
1.5
1.5
1.5
1.5
2a (4.0)
2a (8.0)
3a (4.0) 100
3a (8.0)
3a (8.0)
4a (4.0)
4a (4.0)
80
80
7
7
7
75
75
78
13
17
60
30
40
58
42
73 14
67 16
30 10
52 18
44 16
30 12
37 21
87
83
40
70
60
42
58
133 400
210 800
590 2080
220 2270
213 4270
1050 2480
530 1600
80 24 85
70 24 80
80
60
5
7
93
60
[a] Reactions were performed in butan-1-ol, and the total volume of the reaction mix-
ture was 5.0 mL in a glass reactor. The conversion and selectivity were determined by
GC. [b] TON in moles of substrate converted per mole of CsPW. TOF is the initial turn-
over frequency (mol of the substrate converted per mol of CsPW per hour).
Experimental Section
All chemicals were purchased from commercial sources and used
as received, unless otherwise stated. Camphene (1a), R-(+)-limo-
nene (2a), (À)-a-pinene (3a), (À)-b-pinene (4a), and H3PW12O40 hy-
drate were from Aldrich.
31P magic-angle spinning (MAS) NMR spectra were recorded at RT
and a 4 kHz spinning rate by using a Bruker Avance DSX 400 NMR
spectrometer using 85% H3PO4 as a reference. Powder XRD pat-
terns of the catalysts were obtained by using a Rigaku Geigerflex-
3034 diffractometer with CuKa radiation. The textural characteris-
tics were determined from N2 physisorption measured by using
a Micromeritics ASAP 2010 instrument at 77 K. The W and P con-
tent was determined by inductively coupled plasma atomic emis-
sion spectroscopy by using a Spectro Ciros CCD spectrometer.
The acidic heteropoly salt CsPW was prepared according to the lit-
erature procedure[35] by the dropwise addition of the required
Scheme 3. Schematic representation of acid-catalyzed alkoxylation of 2a,
3a, and 4a.
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amount of an aqueous solution of cesium carbonate (0.47m) to an
aqueous solution of HPW (0.75m) at RT with stirring. The precipi-
tate obtained was aged in an aqueous mixture for 48 h at RT and
dried by using a rotary evaporator at 458C/3 kPa and an oven at
1508C/0.1 kPa for 1.5 h. CsPW thus prepared had a surface area of
111 m2 gÀ1, a pore volume of 0.07 cm3 gÀ1, and a pore diameter of
24 ꢂ. The acid strength of CsPW was characterized calorimetrically
using ammonia and pyridine adsorption as discussed previously.[36]
1H NMR (400 MHz, CDCl3, 258C, Me4Si): d=0.78, (s, 3H; C8H3), 0.88
(s, 3H; C10H3), 0.97 (s, 3H; C9H3), 0.92–1.02 (m, 2H; C5HH and
C6HaxH), 1.13 (t, 3J=7.0 Hz, 3H; C12H3), 1.40–1.50 (m, 1H; C6HHeq),
1.55–1.60 (m, 1H; C3HH), 1.65–1.70 (m, 2H; C4H and C5HH), 1.70–
1.75 (m, 1H; C3HH), 3.17 (dd, J=3.6 Hz, J=7.6 Hz, 1H; C2H), 3.31
(dq, 2J=9.6 Hz, 3J=7.0 Hz, 1H; C11HH), 3.46 ppm (dq, 2J=9.6 Hz,
3J=7.0 Hz, 1H; C11HH); 13C NMR (100 MHz, CDCl3, 258C, Me4Si): d=
11.51 (C10), 15.29 (C12), 19.96 (C9), 20.03 (C8), 27.04 (C5), 34.28 (C6),
38.54 (C3), 45.75 (C4), 46.09 (C7), 48.75 (C1), 64.10 (C11), 86.48 ppm
(C2).
3
3
The reactions were performed in a 10 mL glass reactor equipped
with a magnetic stirrer and a condenser or in a homemade 100 mL
stainless-steel reactor with a magnetic stirrer. The latter reactor
was used in the runs performed at a higher temperature than the
solvent boiling point. In a typical run, a mixture (5.0 mL in the
glass reactor or 20.0 mL in the stainless-steel rector) of the sub-
strate (4.0–32.0 mmol, 0.4–1.6m), dodecane or undecane (2.0–
8.0 mmol, 0.4m, GC internal standards), and the catalyst [HPW (2–
30 mg, 0.7–10.0 mmol) or CsPW (5–30 mg, 1.5–9.0 mmol)] in alcohol
solvent was stirred intensely under air at a specified temperature
(40–1258C). The reactions were followed by GC by using a Shimad-
zu 17 instrument fitted with a Carbowax 20m capillary column and
a flame ionization detector. After the appropriate reaction time,
the stirring was stopped, the catalyst settled quickly, and aliquots
were taken and analyzed by CG. The mass balance and the prod-
uct selectivity and yield were determined using dodecane or unde-
cane as internal standards. Any difference in the mass balance was
attributed to the formation of oligomers, which were unobservable
by GC. Turnover frequencies (TOF) were measured by GC at low
conversions (up to 20–40%) by taking aliquots at short reaction
times (usually within the first 30–60 min).
Isoborneol methyl ether (1d): MS (70 eV, EI): m/z (%): 168 (2) [M]+,
153 (3) [MÀCH3]+, 136 (11) [MÀC4H7OH]+, 121 (19), 110 (18), 108
(13), 95 (100), 93 (16), 67 (11), 55 (11); 1H NMR (400 MHz, CDCl3,
258C, Me4Si): d=0.80, (s, 3H; C8H3), 0.89 (s, 3H; C10H3), 0.96 (s, 3H;
C9H3), 0.97–1.03 (m, 2H; C5HH and C6HaxH), 1.42–1.52 (m, 1H;
C6HHeq), 1.52–1.58 (m, 1H; C3HH), 1.62–1.70 (m, 2H; C4H and C5HH),
3
3
1.70–1.75 (m, 1H; C3HH), 3.11 (dd, J=6.0 Hz, J=9.0 Hz, 1H; C2H),
3.23 ppm (s, 3H; C11H3); 13C NMR (100 MHz, CDCl3, 258C, Me4Si): d=
11.91 (C10), 20.35 (C8), 20.40 (C9), 27.50 (C5), 34.80 (C6), 38.02 (C3),
45.08 (C4), 46.57 (C7), 49.30 (C1), 56.66 (C11), 89.10 ppm (C2).
a-Terpenyl butyl ether (2b): MS (70 eV, EI): m/z (%): 195 (0.3)
[MÀCH3]+, 136 (19) [MÀC4H7OH] +, 121 (26), 115 (24), 93 (27), 81
1
(9), 79 (9), 68 (11), 67 (13), 59 (100); H NMR (400 MHz, CDCl3, 258C,
3
Me4Si): d=0.85 (t, J=4.4 Hz, 3H; C14H3), 1.02 (s, 3H; C9H3), 1.03 (s,
3H; C10H3), 1.15–1.20 (m, 1H; C5HH), 1.25–1.35 (m, 2H; C13H2), 1.38–
1.45 (m, 2H; C12H2), 1.57 (s, 3H; C7H3), 1.55–1.60 (m, 1H; C4H), 1.70–
1.80 (m, 2H; C3HH and C5HH), 1.85–1.95 (m, 3H; C3HH and C6H2),
3.24 (t, 3J=3.6 Hz, 2H; C11H2), 5.31 ppm (brs, 1H; C2H); 13C NMR
(100 MHz, CDCl3, 258C, Me4Si): d=14.00 (C14), 19.80 (C13), 22.52
(C10), 22.66 (C9), 22.87 (C7), 23.95 (C5), 26.84 (C3), 31.13 (C6), 32.77
(C12), 41.99 (C4), 60.26 (C11), 76.08 (C8), 121.00 (C2), 133.88 ppm (C1).
In reactions with CsPW, to control catalyst leaching and the possi-
bility of a homogeneous reaction, the CsPW catalyst was removed
by centrifugation of the reaction mixture at the reaction tempera-
ture, the supernatant was added to a fresh portion of substrate, if
necessary, and allowed to react on. The absence of a further reac-
tion in such experiments indicated the absence of active compo-
nent leaching.
Acknowledgements
Financial support and scholarships from CNPq, CAPES, FAPEMIG,
and INCT-Catꢁlise (Brazil) are acknowledged.
Ethers 1b, 1c, 1d, and 2b were separated by column chromatog-
raphy (silica gel 60) using mixtures of hexane and CH2Cl2 as eluents
1
1
and identified by GC–MS and H and 13C NMR spectroscopy. The H
and 13C NMR signals were assigned by using 2D techniques. NMR
spectra were recorded in CDCl3 by using a Bruker 400 MHz spec-
trometer with Me4Si as an internal standard. Mass spectra were ob-
tained by using a Shimadzu QP2010-PLUS instrument that operat-
ed at 70 eV.
Keywords: biomass · cesium · fragrances · polyoxometalates ·
renewable resources
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Salts for smells: The cesium salt (CsPW)
of tungstophosphoric heteropoly acid
(HPW) is an active and environmentally
friendly solid catalyst for the liquid-
phase alkoxylation of widespread
monoterpenes, such as camphene, limo-
nene, a-pinene, and b-pinene, to pro-
vide monoterpenic ethers with strong
characteristic woody or fruit-berry
aromas.
A. L. P. de Meireles, M. dos Santos Costa,
K. A. da Silva Rocha, E. F. Kozhevnikova,
I. V. Kozhevnikov, E. V. Gusevskaya*
&& – &&
Heteropoly Acid Catalysts for the
Synthesis of Fragrance Compounds
from Biorenewables: The Alkoxylation
of Monoterpenes
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 7
&
7
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These are not the final page numbers!