One-step microwave-assisted asymmetric cyclisation/hydrogenation of citronellal to menthols using supported nanoparticles on mesoporous materials

Article abstract of DOI:10.1039/c003600e

The selective conversion of citronellal to menthols, with good diastereoselectivities to (-)-menthol for the case of (+)-citronellal as starting material, can effectively be carried out in a one-step reaction under microwave irradiation catalysed by supported nanoparticles on mesoporous materials. 2% Pt/Ga-MCM-41 was found to be the optimum catalyst for the reaction, with a quantitative conversion of starting material and selectivities above 85% to menthols obtained in short reaction times (typically 15 min). These results constitute the first report of a simple microwave-assisted one-step cyclisation/hydrogenation process for the production of menthols.

Full text of DOI:10.1039/c003600e

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
One-step microwave-assisted asymmetric cyclisation/hydrogenation of
citronellal to menthols using supported nanoparticles on mesoporous
materials†
Alina Mariana Balu, Juan Manuel Campelo, Rafael Luque* and Antonio Angel Romero
Received 26th February 2010, Accepted 7th April 2010
First published as an Advance Article on the web 29th April 2010
DOI: 10.1039/c003600e
The selective conversion of citronellal to menthols, with good diastereoselectivities to (-)-menthol for
the case of (+)-citronellal as starting material, can effectively be carried out in a one-step reaction under
microwave irradiation catalysed by supported nanoparticles on mesoporous materials. 2%
Pt/Ga-MCM-41 was found to be the optimum catalyst for the reaction, with a quantitative conversion
of starting material and selectivities above 85% to menthols obtained in short reaction times (typically
15 min). These results constitute the first report of a simple microwave-assisted one-step
cyclisation/hydrogenation process for the production of menthols.
reported to replace the very selective and active conventional
homogeneous methodologies that were able to give isopulegol
Introduction
yields up to 99.3%.6 For the second step (hydrogenation of the
double bond in isopulegol), a conventional metal-reducing step
under hydrogen has been reported to provide high selectivities
to menthols (>93%, with diastereoselectivities of 85%).7 However,
despite several attempts to combine these reactions into a one-step
process,7,8 currently there is no simple, mild, and environmentally
friendly methodology that can provide good yields of menthols in
a one-step process, at short reaction times under mild conditions.
In our recent work in the area of supported nanoparticles on
porous materials, we have demonstrated that a wide range of
functionalities including ketones and aldehydes could be easily
hydrogenated using a simple and efficient microwave-assisted
transfer hydrogenation (TH) system with 2-propanol as a stable
hydrogen donor.9 In parallel to these studies, a significant variety
of supported metal and metal oxide nanoparticles (SMNPs)
including Au, Ag, Fe, Cu, Pt and Pd on a variety of porous
materials have also been prepared and investigated in a number of
heterogeneously catalysed processes.10–12
Menthol is among the most demanded chemicals in industry,
with a wide range of uses in pharmaceuticals, agrochemi-
cals, cosmetics, flavourings and personal-care applications.1 De-
spite the many industrial applications of racemic ( )-menthol,
(-)-menthol is actually the compound that exhibits the char-
acteristic peppermint odour and unique cooling sensation.1 At
an industrial scale, the Takasago process is one of the major
routes currently utilised for menthol production. It involves the
cyclisation and isomerisation of (+)-citronellal to (-)-isopulegol
(Scheme 1, first step) using ZnBr2 as catalyst,1–3 followed by a
hydrogenation of the exo-double bond in the isopulegol to give
(-)-menthol (Scheme 1, second step).
Herein, we report the use of versatile heterogeneous catalytic
systems in the asymmetric chemical transformation of ( )-or
(+)-citronellal to ( )-menthols, with high diastereoselectivities to
(-)-menthol, via one-step cyclisation/hydrogenation promoted
under microwave irradiation and mild reaction conditions.
Experimental
Scheme 1 Production of menthols from ( )-citronellal using heteroge-
neous catalysts.
Ga- and Al-MCM-41 supports were prepared using previously
reported protocols,13 in a similar way to the Al- and/or Ga-SBA-
15 employed as supports.14 In this way, Ga- and Al-MCM-41 and
SBA-15 materials with Si/Al 15 ratio were synthesized.
A typical methodology to prepare supported Pd, Pt and Cu
materials was performed under microwave irradiation in a similar
way to a previously reported protocol9 as follows: 0.4 g pre-
synthesized support and 15 mL of a water solution containing the
needed quantity of metal salt precursor (tetraamine platinum(II)
nitrate, palladium(II) acetate or copper(II) chloride) to achieve a
For the first step (citronellal cyclisation), the use of a wide
variety of heterogeneous catalysts including Lewis acid sup-
ported catalysts,2,4 heteropolyacids5 and metal fluorides1 has been
Departamento de Qu´ımica Orga´nica, Universidad de Co´rdoba, Edificio
Marie Curie, Ctra Nnal IV, Km 396, E-14014, Co´rdoba, Spain. E-mail:
q62alsor@uco.es; Fax: +34 957212066; Tel: +34 957212065
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c003600e
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2% loading were added to a round-bottomed flask and microwaved
for 30 min in a CEM-Discover microwave reactor in open-vessel
mode. The final solid was then placed in a rotary evaporator for
around 30 min, subsequently calcined at 550 ◦C for 4 h and further
reduced under hydrogen stream for 2 h (50 mL min-1) at 400 ◦C to
ensure the generation of supported metal nanoparticles.
method was generally power controlled where samples were irra-
diated at different power outputs and various temperatures were
reached. The effect of the power, time of irradiation and quantity
of catalyst in the reaction were investigated under optimised
conditions.
In a typical experiment, 10 mmol citronellal (1.54 g), 5 mL 2-
propanol, 1 mmol K2CO3 and 0.05 g catalyst were microwaved
for 15 min at 100 W. The final mixture was then analysed by
GC and GC-MS. Response factors of reaction products were
determined with respect to the corresponding starting material
from GC analysis using known compounds in calibration mixtures
of specified compositions. Reaction products were also identified
through their MS and 1H NMR spectra (Bruker 300 MHz).
Stereoselectivities were evaluated by GC with the chiral column
that allows good separation of products as well as corroborated
Materials characterisation
Materials were characterised by means of porosimetry, pyridine
titration and elemental analysis. N2 adsorption measurements
were performed in a volumetric adsorption analyser Micromeritics
ASAP 2000. Samples were degassed for 24 h at 100 ◦C under
vacuum (p < 10-2 Pa) and subsequently analysed.
Pyridine (PY) titration was carried out using a previously
reported gas-chromatography methodology.15
1
by H NMR. Materials were also reused following the proposed
Elemental analysis was performed using Inductively Coupled
Plasma (ICP) in a Philips PU 70000 sequential spectrometer
equipped with an Echelle monochromator (0.0075 nm resolution).
Samples were digested inHNO3 and subsequentlyanalysedbyICP.
XPS measurements were performed in a ultra high vacuum
(UHV) multipurpose surface analysis system (SpecsTM model,
Germany) operating at pressures <10–10 mbar using a con-
ventional X-ray source (XR-50, Specs, Mg–K, 1253.6 eV) in a
“stop-and-go” mode to reduce potential damage due to sample
irradiation. Survey and detailed Cu high-resolution spectra (pass
energy 25 and 10 eV, step size 1 and 0.1 eV, respectively) were
recorded at room temperature with a Phoibos 150-MCD energy
analyser. Powdered samples were deposited on a sample holder
using double-sided adhesive tape and subsequently evacuated un-
der vacuum (<10-6 Torr) overnight. Eventually, the sample holder
containing the degassed sample was transferred to the analysis
chamber for XPS studies. Binding energies were referenced to the
C1s line at 284.6 eV from adventitious carbon.
methodology. With this purpose, 0.1 g catalyst (to facilitate the
recovery of the catalyst in successive reuses) was utilised in
the process under identical reaction conditions. Upon reaction
completion, the catalyst was filtered off, washed thoroughly with
acetone and ethanol, and oven-dried overnight at 120 ◦C prior to
its reuse in the reaction.
Reaction runs with added chiral modifiers were performed
as follows: the chiral modifier (few milligrams to reach a sub-
strate/modifier 200/1 ratio) was dissolved in 2-propanol (1–
2 mL) under gentle heating (30–50 ◦C), prior to its addition to
a microwave tube. Then, the resulting solution was added to the
typical reaction mixture (10 mmol citronellal, 3–4 mL 2-propanol,
1 mmol K2CO3 and 0.05 g catalyst) and eventually microwaved for
15 min at 100 W.
Results and discussion
Results for N2 physisorption, PY titration and elemental analysis
(metal content) experiments are summarised in Table 1. The metal
content in the materials was close to the theoretical 2 wt% selected
for the reaction, regardless of the metal (Pt, Pd or Cu) employed in
their preparation (Table 1). XPS analysis of the materials showed
Pd and Pt nanoparticles were mostly present as reduced metals
(Pd0 and Pt0) on the supports while Cu nanoparticles were mostly
present as reduced copper species (>60% total copper content,
peaks at ca. 932–933 eV) and CuO (<40% of total copper content,
characteristic shape-up peaks at 940–945 eV, only seen when Cu2+
Catalytic activity
Microwave reaction experiments were carried out in a CEM-
DISCOVER model with PC control and monitored by sampling
aliquots of reaction mixture. Samples were analysed by GC/GC-
MS using an Agilent 6890 N GC model fitted with a Rt-gDEXsaTM
cyclodextrin based column (30 m, 0.25 i.d.) and an FID detector.
Experiments were conducted in closed-vessel mode (10 mL,
pressure controlled) under continuous stirring. The microwave
Table 1 Metal content (wt%, as measured by elemental analysis), textural properties and PY titration data (mmol PY adsorbed per gram of catalyst at
300 ◦C) of different supported metal nanoparticles on acidic mesoporous supports
Entry
Catalyst
Metal content (wt%)
Surface area/m2 g-1
Pore size/nm
Pore volume/mL g-1
PY titration at 300 ◦
C
1
2
3
4
5
6
7
8
Pd/Al-MCM-41
Pd/Ga-MCM-41
Pd/Al-SBA-15
Pd/Ga-SBA-15
Pt/Al-MCM-41
Pt/Ga-MCM-41
Pt/Al-SBA-15
Pt/Ga-SBA-15
Cu/Al-MCM-41
Cu/Ga-MCM-41
Cu/Al-SBA-15
Cu/Ga-SBA-15
1.88
1.96
1.96
1.92
2.02
1.99
1.87
1.89
1.85
1.78
1.98
1.93
928
796
903
799
905
807
910
812
752
816
890
824
1.9
2.0
5.9
4.5
1.9
2.1
6.0
4.6
2.0
1.9
6.0
4.6
0.69
0.76
0.81
0.83
0.71
0.76
0.82
0.89
0.72
0.69
0.85
0.87
193
38
185
65
199
45
181
70
210
43
230
68
9
10
11
12
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Table 2 Catalytic performance [total conversion and selectivity to prod-
ucts (Sproduct, mol%) of supported nanoparticles on porous materials
in the one-step microwave-assisted tandem cyclisation/hydrogenation of
( )-citronellala
Table 3 Effect of various parameters in the catalytic performance of
2% Pt–Ga-MCM-41 in the one-step microwave-assisted tandem cyclisa-
tion/hydrogenation of ( )-citronellala
Conversion Selectivity to
Reaction conditions
(mol%)
menthols (mol%)
Effect of the added quantity of catalyst
100 W, 120–130 ◦C, 0.05 g cat., 15 min >90
87
75
69
100 W, 120–130 ◦C, 0.1 g cat., 15 min
100 W, 120–130 ◦C, 0.2 g cat., 15 min
Effect of time of microwave irradiation
>99
>99
100 W, 120–130 ◦C, 0.05 g cat., 10 min 78
100 W, 120–130 ◦C, 0.05 g cat., 15 min >90
100 W, 120–130 ◦C, 0.05 g cat., 30 min >99
100 W, 120–130 ◦C, 0.05 g cat., 45 min >99
Effect of microwave power
89
87
80
77
Conversion Sisopulegols Smenthols Sothers
Entry Catalyst
(mol%)
(mol%)
(mol%) (mol%)c
50 W, 100 ◦C, 0.05 g cat., 15 min
100 W, 125 ◦C, 0.05 g cat., 15 min
150 W, 129 ◦C, 0.05 g cat., 15 min
200 W, 130 ◦C, 0.05 g cat., 15 min
82
88
87
80
70
1
2
3
4
5
6
7
8
No catalystb
Al-MCM-41
Ga-MCM-41
Al-SBA-15
Ga-SBA-15
Pd/Al-MCM-41
<5
78
59
85
65
—
70
>90
65
85
20
15
15
20
15
8
10
10
30
50
25
35
—
Traces
—
Traces
—
50
75
40
60
60
87
60
80
30
40
30
50
—
30
<10
35
15
30
10
45
20
25
5
35
10
40
10
45
15
>90
>95
>99
a Reaction conditions: 10 mmol citronellal, 5 mL 2-propanol, 1 mmol
K2CO3, 120–130 ◦C. Main reaction byproducts were citronellal etherifica-
tion, cracking and dehydration compounds.
>90
Pd/Ga-MCM-41 80
Pd/Al-SBA-15
Pd/Ga-SBA-15
Pt/Al-MCM-41
Pt/Ga-MCM-41
Pt/Al-SBA-15
Pt/Ga-SBA-15
Cu/Al-MCM-41 90
Cu/Ga-MCM-41 75
Cu/Al-SBA-15
Cu/Ga-SBA-15
>95
75
95
>90
>99
85
9
10
11
12
13
14
15
16
17
citronellal dehydration, cracking and/or etherification with 2-
propanol employed as solvent/hydrogen donor in the reaction
(Table 2, entries 3 and 5). These byproducts have been observed in
catalysts containing higher and stronger acidities (e.g. Al-MCM-
41 and Al-SBA-15).
>95
85
2% Pt–Ga-MCM-41 was selected as an optimum material in
terms of conversion and selectivity to menthols for the reaction,
with its balanced combination of Lewis acidity of the support13a,14
as well as superior hydrogenation activity of Pt compared to Cu
and Pd nanoparticles. These findings were in good agreement
with previous reports that claimed the cyclisation step is readily
catalysed by strong Lewis and/or weak Brønsted acid sites2,17 and
it is actually the rate determining step in this process.18
The effect of different microwave parameters in the activity of
the optimum 2% Pt–Ga-MCM-41 was subsequently investigated.
The results in Table 3 gave an interesting insight into the effect
of microwaves in the activity and selectivity of the systems.
Microwaves have been reported to improve rates of reaction
in many organic syntheses as well as sometimes selectivities to
targeted products.19
An increase in the quantity of catalyst increased the conversions
in the systems at the expense of menthol selectivity (Table 3, first
three entries). Similar trends were found at increasing microwave
power and times of reaction, although relatively high selectivities
to menthols could still be obtained changing the latter (Table 3,
middle entries).
The heterogeneous nature of the catalyst was then investigated
under the reaction conditions. The results included in Fig. 1
demonstrated that 2% Pt–Ga-MCM-41 is a highly stable and
reusable catalyst under the investigated microwave conditions,
preserving over 90% of its initial activity after 3 reuses.
a Reaction conditions: 10 mmol citronellal, 5 mL 2-propanol, 1 mmol
K2CO3, 0.05 g catalyst, 100 W, 120–130 ◦C, 15 min reaction. b Blank
run after 1 h microwave irradiation. c Other detected products include
citronellol and byproducts from citronellal etherification and cracking.
species are present), in good agreement with our previous reports
both for Pd and/or Pt systems9,11a,16 and Cu.12a,12c
Pyridine titration data showed different acidities between ma-
terials. These could be well correlated with the nature of the
supports. Al-MCM-41 and Al-SBA-15 exhibited a remarkably
higher total acidity compared to those of the analogous Ga-
systems, while acidities of MCM-41 and SBA-15 materials were
almost comparable (Table 1). Ga materials have been previously
reported to have mostly Lewis acid sites13a,14 while Al materials
have a combination of both Lewis and Brønsted acid sites.2,13b,15
Materials were then tested for activity in the one-step cycli-
sation/hydrogenation of ( )-citronellal to ( )-menthols. Table 2
summarises the preliminary screening of catalysts and conditions
in the proposed reaction. Acidity of the supports is believed to be
sufficient to promote the cyclisation step, while the microwave
conditions and the presence of supported nanoparticles could
catalyse the hydrogenation of the double bond generated upon
formation of isopulegol via hydrogen transfer (Scheme 1).
Blank runs (without catalyst) gave minimum conversion of
starting material after 60 min microwave irradiation. Al and Ga
materials provided relatively good conversions of starting material,
with relatively high selectivities to isopulegols (Table 2, entries 2
to 5, superior to 80% for Ga materials) and minor selectivities
to other products including citronellol (via isomerisation of
citronellal on weak acid sites) as well as related byproducts from
Interestingly, the selectivity to menthols was not significantly af-
fected by the reuse of the catalyst. Nanoparticle sizes also remained
unaltered upon reuse, with no aggregation of nanoparticles under
the investigated reaction conditions. Pt nanoparticles were also
found to be highly stable with no significant quantities of metal
leached in solution (<2 ppm) after the reaction, as confirmed by
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Fig. 1 Reuses of 2% Pt–Ga-MCM-41 catalyst in the one-step conversion
of citronellal to menthols under microwave irradiation. Reaction condi-
tions: 10 mmol citronellal, 5 mL 2-propanol, 1 mmol K2CO3, 0.1 g catalyst,
100 W, 120–130 ◦C, 15 min reaction.
Fig. 2 Diastereoisomer distribution in the microwave-assisted tandem
cyclisation/hydrogenation of ( )-citronellal to ( )-menthols using opti-
mised catalytic systems. Reaction conditions: 10 mmol citronellal, 5 mL
2-propanol, 1 mmol K2CO3, 0.05 g catalyst, MW, 100 W, 120–130 ◦C
(maximum temperature reached), 15 min reaction.
ICP and the fact that no hydrogenation activity was observed in
the final mixture filtrate (after 1 h of microwave irradiation) upon
catalyst reuse.
Further studies were performed in order to investigate the
diastereoselectivities in the optimised systems as well as the
possibility to obtain (-)-menthol using (+)-citronellal and chiral
catalysts/modifiers as alternatives. Stereoisomers that can be
obtained in the reaction are included in Scheme 2 (for the
particular case of enantiomerically pure citronellal), namely
(-)-menthol, (+)-neomenthol, (-)-isomenthol and (+)-neoiso-
menthol.
very similar to those obtained from ( )-citronellal, implying the
purity and diastereoselectivity of the starting material was not as
important as the non chiral catalysts employed in the microwave-
assisted process. In any case, these findings were found to be in
good agreement to previous reports by Ravasio et al. in which
(-)-menthol could be achieved in high yields with high selectivities
using Cu/SiO2 catalysts.18
In any case, we were inspired by previous reports showing the
addition of chiral modifiers including alkaloids (quinine, cincho-
nine, cinchonidine) could significantly improve selectivities and
enantiomeric excess in asymmetric reactions including enantiose-
lective hydrogenations.20,21 We then devised a similar microwave
protocol in which a chiral modifier [including quinine (QN),
cinchonine (CN) and cinchonidine (CD)] was added to investigate
its potential effect in the distribution of the diastereoisomers using
enantiomerically pure (+)-citronellal as starting material. Results
are summarised in Fig. 3. The addition of cinchonidine (CD)
had a remarkable effect in the diastereoselectivity of the poor
(-)-menthol systems, with a maximum of a 75% selectivity to the
target menthol achieved with the 2% Pt/Ga-MCM-41 catalyst.
Further investigations are currently ongoing to unravel the
involvement of the alkaloid in the microwave-assisted process to
further extend the protocol to other chiral modifiers including
quinidine, quinoline, lepidine, etc. and/or catalysts (e.g. supported
nanoparticles on chitosan and other chiral supports).
Scheme 2 Stereoisomers obtained in the one-step tandem cyclisa-
tion/hydrogenation of (+)-citronellal.
The diastereoselectivity of the reaction was originally probed
using racemic citronellal. Results included in Fig. 2 show two very
interesting and different trends for the chosen catalytic systems
(those providing the best activities/selectivities to menthols in
Table 2).
For the Pd and Pt catalysts (showing the optimum selectivities
to menthols), a maximum of 55% ( )-menthol was observed (2%
Pt/Ga-MCM-41, out of the 87% selectivity to menthols), with
varying quantities (5–20%) of other diastereisomers. Surprisingly,
the poorly selective 2% Cu/Ga-SBA-15 material (only 50% men-
thol production) was found to have an outstanding 43% selectivity
to ( )-menthol. Only minor quantities of ( )-neomenthol and
( )-isomenthol were observed in the reaction (Fig. 2).
The remarkable increase in (-)-menthol stereoselectivity found
for CD might be related to the higher interaction of CD within the
system due to its higher adsorption strength, in good agreement
with previous reports.22,23 More work is needed to verify and
support these findings.
Conclusions
We were also prompted to investigate whether higher di-
astereoselectivities to (-)-menthol could be obtained from enan-
tiomerically pure (+)-citronellal. Studies showed the selectivities
obtained for the investigated catalysts were slightly higher [ca. 60%
(-)-menthol for the 2% Pt/Ga-MCM-41 material] but in any case
A simple, efficient and environmentally friendly one-step pro-
cess has been developed for the microwave-assisted conversion
of ( )-citronellal to ( )-menthols. ( )-Citronellal cyclisation to
( )-isopulegols is favoured on Lewis acid sites, while the presence
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Fig. 3 Effect of the addition of chiral modifiers in the distribution
of stereoisomers of the 2% Pt–Ga-MCM-41 catalysed production of
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of supported nanoparticles under hydrogen transfer conditions (2-
propanol + base) further promoted the subsequent hydrogenation
of the double bond in isopulegols to ( )menthols. Reaction rates
on both steps are improved under microwave irradiation to give
high yields of menthols in short times of reaction (typically
15 min). 2% Pt–Ga-MCM-41 exhibited the optimum conversion
and selectivity to menthols, combining high isopulegol yields (via
Lewis promoted cyclisation) with the superior performance of Pt
in the hydrogenation of the double bond to ( )-menthols.
When enantiomerically pure (+)-citronellal was employed as
starting material instead of the racemic compound, a 55%
maximum selectivity to (-)-menthol was observed. The addition
of small quantities of a chiral modifier such as cinchonidine
increased the selectivity to (-)-menthol up to a 75%. Supported
nanoparticles on such mesoporous materials were also proved
to be highly stable and reusable under investigated conditions,
offering a potentially interesting alternative to the conventional
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Acknowledgements
Authors gratefully acknowledge support from Ministerio de
Ciencia e Innovacio´n (Project CTQ2007-65754/PPQ). Rafael
Luque is also grateful to Ministerio de Ciencia e Innovacio´n for
the concession of a Ramon y Cajal contract (RYC-2009-04199).
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The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2845–2849 | 2849
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