C.J.A. Ribeiro et al.
CatalysisTodayxxx(xxxx)xxx–xxx
reaction was switched to the products with a para-menthenic skeleton.
In polar basic solvents (dimethylacetamide) trans-carveol was the major
product, whereas in polar weakly basic solvents (e.g. acetone), the fa-
vored route was the formation of trans-sobrerol and pinol. Although we
previously reported that the PW catalyst allowed to obtain trans-car-
veol, trans-sobrerol and pinol in ca. 70% yield each using an appro-
priate solvent [14], the reactions were performed under homogeneous
conditions due to the high solubility of PW in polar solvents. These
results encouraged us to work further on the isomerization of α-pinene
oxide in order to develop heterogeneous catalytic processes for the
synthesis of products other than campholenic aldehyde with a sufficient
selectivity for practical use.
nitrogen physisorption (Micromeritics ASAP 2010 instrument). The
content of tungsten and phosphorus in the catalyst was confirmed by
inductively coupled plasma atomic emission spectroscopy (ICP) on a
Spectro Ciros CCD instrument.
2.3. Catalytic tests
The catalytic tests were run under air in a 10 mL glass reactor
equipped with a reflux condenser to avoid solvent evaporation. In a
typical run, a mixture of the substrate (0.45–1 mmol), dodecane (GC
internal standard, 0.30 mmol) and the catalyst CsPW (2.5–25 mg,
0.75–7.50 μmol), in a specified solvent (5.0 mL) was magnetically
stirred at 15–40 °C for a specified time. The reaction mixture was per-
iodically analyzed by gas chromatography (GC, Shimadzu QP-2014
instrument, Rtx-Wax capillary column, flame ionization detector). Due
to the small volume of the samples taken for the GC analysis (5 μL) the
changes in catalyst bulk density were insignificant. Conversions and
selectivities were calculated from GC analysis using dodecane as the
internal standard. The difference in mass balance (if any) was attributed
to high-boiling products (probably oligomers which could not be de-
tected by GC). The reaction rate was not dependent on the intensity of
stirring within the range used, suggesting the absence of mass transfer
limitations. In order to control any contribution of homogeneous re-
actions and catalyst leaching, the catalyst was separated from the re-
action mixture by centrifugation (25 °C, 18,000 rpm) and the reaction
was allowed to proceed with another portion of α-pinene oxide. The
lack of substrate conversion indicated the absence of any significant
catalyst leaching. In our previous works, the stability of CsPW in polar
solvents was confirmed by various techniques [33,34].
To design heterogeneous catalytic processes in polar solvents, ex-
cellent alternatives for HPAs could be their acidic salts which are in-
soluble in such media [28–30]. In particular, successful use of
Cs2.5H0.5PW12O40 (CsPW),
a compound which combines strong
Brønsted acidity and a large surface area, as a heterogeneous acid
catalyst has been reported by our group [31–34].
In this work, we report the isomerization of α-pinene oxide in the
presence of CsPW as a heterogeneous catalyst. The reactions were
performed in various solvents, which allowed to obtain trans-carveol,
trans-sobrerol or pinol, each in 60–80% yield. These compounds are
valuable ingredients for the fragrance and pharmaceutical industries.
To our knowledge, no attempt to use CsPW as the catalyst for this re-
action has been made before; moreover, such high yields of trans-car-
veol and pinol have not been achieved with heterogeneous catalysts.
The selectivity obtained for trans-sobrerol in the present work is com-
parable with the best results reported previously in both homogeneous
and heterogeneous systems [14,15].
The products were identified by the comparison with authentic
samples (co-injection tests) and/or GC–MS (Shimadzu, GCMS-QP2010-
PLUS instrument operating at 70 eV). Major reaction products were
isolated by a column chromatography (silica gel 60) using mixtures of
hexane and ethyl acetate as eluents and identified by GC–MS (Shimadzu
QP2010-PLUS spectrometer, 70 eV), 1H, and 13C-NMR (Bruker 400 MHz
spectrometer, CDCl3, TMS). Spectroscopic data for campholenic alde-
hyde (2), trans-carveol (3), trans-sobrerol (4) and pinol (5) have been
reported previously [7,14].
2. Experimental
2.1. Chemicals
All chemicals were purchased from commercial sources and used as
received, unless specially indicated. α-Pinene oxide, dimethylcarbonate
and H3PW12O40 (PW) hydrate were from Aldrich, Aerosil 300 silica
from Degussa and solvents from VETEC or Synth. In some experiments
(indicated in Tables), commercial acetone (0.5 wt% of water) was dried
for 24 h over molecular sieve (4 Å) activated for 3 h at 100 °C.
3. Results and discussion
Acid-catalyzed transformations of α-pinene oxide (1) can yield
several products, such as campholenic aldehyde (2), trans-carveol (3),
trans-carveol (4) and pinol (5) (Scheme 1), as well as p-cymene, iso-
pinocamphenol, and isopinocamphone. Most of the reported work
aimed to obtain campholenic aldehyde, whereas other products have
been usually detected in low yields. In contrast, this work was focused
on the development of heterogeneous catalytic protocols for the
synthesis of para-menthenic compounds 3, 4 and 5 from α-pinene
oxide. Our previous studies [14–16] suggested that the chemoselec-
tivity of the α-pinene oxide rearrangements in acidic solutions could be
controlled by solvent and that the formation of para-menthenic pro-
ducts would be favored in polar solvents such as acetone. In modern
solvent selection guides, acetone is ranked as an environmentally
friendly solvent [37], as it is biodegradable and, although is currently
made from fossil sources, can be produced from biomass. In addition,
2.2. Catalyst preparation and characterization
Cs2.5H0.5PW12O40 (CsPW) was synthesized by the dropwise addition
of aqueous solution of cesium carbonate (0.47 M) to aqueous solution of
PW (0.75 M) at room temperature with stirring, as described previously
[35]. The precipitated CsPW was aged in the aqueous mixture for 48 h
at room temperature and dried in a rotary evaporator at 45 °C/3 kPa
then in an oven at 150 °C/0.1 kPa for 1.5 h. The BET surface area of the
prepared catalyst was 111 m2 g−1, the pore volume 0.07 cm3 g−1 and
the average pore diameter 24 Å. The acid properties of CsPW have been
characterized calorimetrically by ammonia and pyridine adsorption and
discussed previously [35,36]. The catalyst was also characterized by 31
MAS NMR (Bruker Avance DSX 400 NMR, room temperature, spinning
rate of 4 kHz, 85% H3PO4 as the reference), X-ray diffraction (XRD,
Rigaku Geigerflex-3034 diffractometer with CuKα radiation) and
P
Scheme 1. Products of the acid-catalyzed transfor-
mations of α-pinene oxide.
2