Selective one-pot carvone oxime hydrogenation over titania supported gold catalyst as a novel approach for dihydrocarvone synthesis

Article abstract of DOI:10.1016/j.molcata.2016.04.013

It was shown for the first time that dihydrocarvone can be selectively produced by gold-catalyzed one-pot transformation of carvone oxime. This reaction was carried out at 100 °C under hydrogen pressure of 9 bar over 1.9 wt.% Au/TiO₂ catalyst using methanol as a solvent. Dihydrocarvone synthesis was shown to occur via carvone formation with the subsequent hydrogenation of its conjugated C=C double bond. Application of Au/TiO₂ catalyst for both deoximation and selective hydrogenation of olefinic C=C functional group is reported for the first time. The combination of these steps provides optimization of the synthetic method for dihydrocarvone production from carvone oxime which is a key intermediate in carvone synthesis from limonene. Despite a lower reaction rate than in the case of carvone, a significant increase in the stereoselectivity towards trans-dihydrocarvone was observed in the case of carvone oxime hydrogenation. The ratio between trans- and cis-dihydrocarvone was close to 4.0 compared to 1.8 achieved in the case of carvone hydrogenation.

Full text of DOI:10.1016/j.molcata.2016.04.013

Journal of Molecular Catalysis A: Chemical 420 (2016) 142–148
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Selective one-pot carvone oxime hydrogenation over titania
supported gold catalyst as a novel approach for dihydrocarvone
synthesis
Yu. S. Demidovaa,b, E.V. Suslovc, O.A. Simakovad, K.P. Volchob,c, N.F. Salakhutdinovb,c
,
I.L. Simakovaa,b,∗, D. Yu. Murzind
a Boreskov Institute of Catalysis, 630090, pr. Lavrentieva, 5, Novosibirsk, Russia
b Novosibirsk State University, 630090, Pirogova 2, Novosibirsk, Russia
c Novosibirsk Institute of Organic Chemistry, 630090, pr. Lavrentieva, 9, Novosibirsk, Russia
d Process Chemistry Centre, Åbo Akademi University, FI-20500, Turku/Åbo, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
It was shown for the first time that dihydrocarvone can be selectively produced by gold-catalyzed one-
pot transformation of carvone oxime. This reaction was carried out at 100 ◦C under hydrogen pressure of
9 bar over 1.9 wt.% Au/TiO2 catalyst using methanol as a solvent. Dihydrocarvone synthesis was shown
to occur via carvone formation with the subsequent hydrogenation of its conjugated C C double bond.
Application of Au/TiO2 catalyst for both deoximation and selective hydrogenation of olefinic C C func-
tional group is reported for the first time. The combination of these steps provides optimization of the
synthetic method for dihydrocarvone production from carvone oxime which is a key intermediate in
carvone synthesis from limonene. Despite a lower reaction rate than in the case of carvone, a signifi-
cant increase in the stereoselectivity towards trans-dihydrocarvone was observed in the case of carvone
oxime hydrogenation. The ratio between trans- and cis-dihydrocarvone was close to 4.0 compared to 1.8
achieved in the case of carvone hydrogenation.
Received 10 November 2015
Received in revised form 14 April 2016
Accepted 15 April 2016
Available online 19 April 2016
Keywords:
Gold catalyst
Chemoselective hydrogenation
Carvone oxime
Dihydrocarvone
Stereoselectivity
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
This novel approach is of significant interest in terms of a pos-
sibility to obtain industrially valuable dihydrocarvone via direct
Dihydrocarvone is a valuable product that has a spearmint-like
odor and is used as a flavoring additive in food industry. It is typi-
cally formed as a mixture of two stereoisomers and may be obtained
starting from biomass derived carvone hydrogenation using molec-
ular hydrogen over Ni, Pt, Pd, Rh, Ru catalysts [1–8]. This reaction
path requires hydrogenation of conjugated C C double bond in
carvone hydrogenation. The highest total selectivity to dihydrocar-
vone (62%) was achieved at a nearly complete carvone conversion
(90%) after 13 h, with the trans-to-cis-dihydrocarvone ratio being
about 1.8.
Despite that (+)- and (−)-carvones can be isolated by fractional
distillation of caraway and spearmint oils, respectively, currently
carvones are mainly produced semi-synthetically starting from
(+)- and (−)-limonenes through intermediate formation of carvone
oxime (Scheme 1) [10,11]. Acid hydrolysis of the oxime group in the
presence of a hydroxylamine acceptor, such as acetone, yields the
target compound-carvone. From the viewpoint of improving the
chemical synthesis efficiency, a one-pot multistep carvone oxime
transformation to dihydrocarvone can be a promising approach.
According to the literature data there are a few examples of
direct carvone oxime conversion into dihydrocarvone including
attempts of consecutive reduction via a corresponding ketone using
LiAlH4 in hexamethylphosphoramide (HMPA) [12] or by appli-
cation of cell suspension cultures of Nicotiana tabacum, which
promoted a slow formation of the target dihydrocarvone in a rela-
tively low yield as a part of a complex mixture [13]. Despite a higher
the presence of other unsaturated functional groups, such as C
O
and an isolated C C bonds. Application of the above mentioned
catalysts typically favors hydrogenation of other unsaturated func-
tional groups, resulting in formation of a complex mixture, with
the target dihydrocarvone yield being less than 30%. In our pre-
vious work stereo- and chemoselective carvone hydrogenation to
dihydrocarvone catalyzed by Au/TiO2 catalyst with predominant
formation of the trans-isomer was reported for the first time [9].
∗
Corresponding author at: Boreskov Institute of Catalysis, 630090, pr. Lavrentieva,
5, Novosibirsk, Russia.
E-mail addresses: simakova@catalysis.ru, simakova-home@mail.ru
(I.L. Simakova).
http://dx.doi.org/10.1016/j.molcata.2016.04.013
1381-1169/© 2016 Elsevier B.V. All rights reserved.

Yu.S. Demidova et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 142–148
143
Scheme 1. Industrial method of carvone synthesis [10,11].
Scheme 2. Possible reaction pathways for carvone formation from carvone oxime over Au/TiO2.
dihydrocarvone yield achieved by using the LiAlH4/HMPA system,
the method has significant disadvantages in terms of practical and
ecological considerations.
heterogeneous catalysts for both deoximation and selective hydro-
genation of olefinic C C functional group.
Thus the goal of this work was to study regularities of carvone
oxime transformation over Au/TiO2 catalyst with the main aim
to explore feasibility of the one-pot synthesis of dihydrocarvone
starting from carvone oxime.
As it is well known, oximes possess a relatively high hydrolytic
stability compared to imines [14]. According to the literature
oximes can be also catalytically hydrogenated to produce imines
or other nitrogen-containing derivatives followed by their further
hydrolysis to form ketones [15–17]. Recently gold nanoparticles
supported on TiO2 or Fe2O3 were shown by Corma and co-workers
to be highly active in chemoselective hydrogenation of a nitro-
group in the presence of other reducible functional groups [18–21].
Furthermore, highly catalytically active yolk–shell Au-CeO2@ZrO2
nanoreactors were applied recently for the 4-nitrophenol selec-
tive reduction to 4-aminophenol [22]. More conventional platinum
group metal catalysts along with the nitro-group hydrogenation
can also activate olefinic or carbonyl groups due to stronger inter-
actions with other functional groups [19,21,23–25]. Noteworthy is
also, that application of more inexpensive metal as an active com-
ponent, such as copper-containing catalysts, results in a preferable
hydrogenation of a carbonyl group into hydroxyl group [26]. In
this connection application of gold catalysts for oxime consecutive
transformations to dihydrocarvone may be of high relevance.
As it was mentioned above an outstanding activity of Au/TiO2
catalyst in carvone hydrogenation to valuable dihydrocarvone was
previously demonstrated in our work. As a next step this study is
focused on one-pot process consisting of sequential transforma-
tions of carvone oxime which is a key intermediate in carvone
synthesis from limonene to dihydrocarvone. The combination of
these steps can provide optimization of the synthetic method for
dihydrocarvone production and moreover will exclude acid hydrol-
ysis in an aqueous medium making dihydrocarvone synthesis more
efficient in line with the guidelines of green chemistry. To the best
of our knowledge there were no reports in the open literature on
2. Experimental
2.1. Starting materials
(−)-Carvone (Aldrich, ≥97%), (5R)-dihydrocarvone (SAFC, ≥97%,
mixture of trans- and cis-isomers 4:1) and methanol (J.T. Baker,
≥99.8%) were purchased from commercial suppliers and used as
received. (−)-Carvone oxime and (2R,5R,E)-dihydrocarvone oxime
were synthesized according to the methods presented below. 1H
NMR and 13C NMR spectra of the compounds were recorded by
Bruker AV-400 spectrometer (400.13 MHz (1H), 100.61 MHz (13C))
in the CDCl3 solutions.
2.2. Synthesis of (−)-carvone oxime and
(2R,5R,E)-dihydrocarvone oxime
First to obtain (−)-carvone oxime a 100-ml round-bottom flask
was charged with hydroxylamine hydrochloride (13.4 mmol,
0.93 g), sodium acetate (13.4 mmol, 1.10 g), (−)-carvone
(6.67 mmol, 1.00 g, [˛]2D0 = −58 (c = neat)), methanol (20 ml) and
water (17 ml). The solution was stirred for 4 days at r.t. and then
water (100 ml) was added. The reaction mixture was extracted
with ethyl acetate (EtOAc) (3 × 50 ml). The combined organic
phase was washed with saturated sodium bicarbonate (2 × 25 ml),
brine (2 × 25 ml) and then dried (Na2SO4). As a result, (−)-carvone
oxime (0.83 g (75%)) was obtained. The synthesized (−)-carvone
oxime was characterized by 13C and 1H NMR (supplementary data,

144
Yu.S. Demidova et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 142–148
Fig. 1. TEM micrographs (left) and histogram of particles size distribution (right) of Au/TiO2 catalyst.
40
35
30
25
20
15
10
5
80
70
60
50
40
30
20
10
0
trans-Dihydrocarvone
cis-Dihydrocarvone
Carvone
0
0
200
400
600
800
1000
1200
1400
1600
Time, min
Fig. 2. Carvone oxime hydrogenation over Au/TiO2. Conversion of carvone oxime (᭿) and selectivity towards carvone ( ), trans- ( ) and cis-dihydrocarvone ( ). Reaction
conditions: T = 373 K, p (H2) = 9 bar, carvone oxime (5 mmol), methanol (15 ml), Au/TiO2 (140 mg).
Figs. S1 and S2 , respectively). The 13C NMR spectral characteristics
of (−)-carvone oxime corresponded to those described in the
literature [27].
dihydrocarvone oximes corresponded to those described in the
literature [28].
(2R,5R,E)-Dihydrocarvone oxime was synthesized starting
from a mixture of trans- and cis-dihydrocarvone. 500-ml flask
was charged with hydroxylamine hydrochloride (67.4 mmol,
4.65 g), sodium acetate (67.7 mmol, 5.53 g), (5R)-dihydrocarvone
(33.2 mmol, 5.05 g, a mixture of trans- and cis-dihydrocarvone
(4:1)), methanol (100 ml) and water (85 ml). The solution was
stirred for 4 days at r.t. The precipitate formed was filtered and dis-
solved in diethyl ether (Et2O). The diethyl ether phase was dried
over Na2SO4, the drying agent was filtered off and washed with
Et2O, and then the solvent was removed. As a result, (2R,5R,E)-
dihydrocarvone oxime (3.07 g (55%)) was obtained. Moreover, the
mixture of (2R,5R,E)-, (2S,5R,E)- and (2S,5R,Z)-isomers of dihy-
drocarvone oxime (2.36 g, 43%; 4.4:2.1:1) was obtained from the
mother liquor by extraction with EtOAc (4 × 70 ml). 1H NMR spec-
trum of (2R,5R,E)-dihydrocarvone oxime is presented in Fig. S3
(supplementary data). The 1H NMR spectral characteristics of
2.3. Catalytic experiments
(−)-Carvone, (−)-carvone oxime and (2R,5R,E)-dihydrocarvone
oxime were used as substrates, while methanol was applied as a
solvent. The reactions were carried out in a stainless steel reactor
equipped with an electromagnetic stirrer (1100 rpm) and the sam-
pling system. In a typical experiment, a mixture of the substrate
(5 mmol), solvent (15 ml) and 1.9 wt.% Au/TiO2 catalyst (140 mg)
was intensively stirred at 100 ◦C under H2 or N2 atmosphere (9 bar).
At regular time intervals, the samples were withdrawn and ana-
lyzed by gas chromatography (Tsvet-500) using a Carbowax-20 M
column (length 50 m, inner diameter 0.2 mm and film thickness
0.5 ␮m) at 160 ◦C and a flame ionization detector operating at
250 ◦C. Additionally the structure of reaction products was con-
firmed by analysis with a gas chromatography-mass spectrometer
(Agilent Technologies 7000 GC/MS Triple Quad, HP-5MS column)
as well as by NMR spectroscopy. The chemical shifts of the cis- and

Yu.S. Demidova et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 142–148
145
9
trans-dihydrocarvones were determined in accordance with [29].
1H NMR spectra were recorded by Bruker AV-400 spectrometer
(400.13 MHz (1H)) in the CDCl3 solutions of the reaction mixture.
Dihydrocarvone oxime
Carvone oxime
8
7
6
5
4
3
2
1
0
Carvone oxime+H O
2
2.4. Catalyst preparation
Carvone
The 1.9 wt.% Au/TiO2 catalyst was prepared from HAuCl4 aque-
ous solution (5 × 10−4 M) by deposition-precipitation method with
urea at 81 ◦C during 24 h over TiO2 (Degussa AG, Aerolyst 7708,
anatase >70%, SBET = 45 m2/g) [30]. The obtained slurry was washed
with NH4OH aqueous solution (4 M) and deionized water. There-
after, the catalyst was dried at 60 ◦C for 12 h and calcined at 300 ◦C
for 4 h. The obtained catalyst was characterized by a variety of state
of the art physical methods described in detail elsewhere [30].
Electron microphotographs of the samples were taken by LEO
912 OMEGA energy-filtered transmission electron microscope by
using 120 kV acceleration voltage. A histogram of the particle size
distribution was obtained by counting at least 100 gold particles on
micrographs. The obtained gold catalyst was chemically analyzed
with an inductively coupled plasma atomic emission spectroscopy
(ICP-AES) using PerkinElmer, Optima 5300DV spectrometer. The
surface analysis of the catalysts was performed by X-ray photo-
electron spectroscopy (XPS) in a PHI Quantum 2000 Scanning ESCA
Microprobe spectrometer with Al anode to clarify the chemical
state of the gold species formed on the catalyst surface.
0
200
400
600
800
Time, min
Fig. 3. Effect of substrate structure and water addition (0.15 ml) on trans-/cis-
dihydrocarvone ratio. Reaction conditions: T = 373 K, p (H2) = 9 bar, carvone or
carvone oxime or dihydrocarvone oxime (5 mmol), methanol (15 ml), Au/TiO2
(140 mg).
The recycling experiments were performed in order to study the
catalyst stability. The catalyst after the first reaction run was fil-
tered, washed with ethanol and dried at 373 K before reusing. The
second reaction run was performed at the same reaction conditions
keeping the ratio between substrates, the solvent and the catalyst
constant. During the second reaction run the catalyst exhibited car-
vone oxime conversion 20% and total selectivity to dihydrocarvone
75% with trans-/cis-dihydrocarvone = 4 after 23 h indicating its sta-
ble performance and absence of significant deactivation during the
reaction.
To study whether the reaction can occur non-catalytically or can
be promoted by the support per se, TiO2 was examined for carvone
oxime hydrogenation at the same reaction conditions. As a result
the support per se did not promote any carvone oxime transforma-
tion demonstrating that carvone formation from oxime occurs only
in the presence of Au/TiO2. In addition TiO2 as such was also shown
to be inactive in both carvone hydrogenation and dihydrocarvone
isomerization [9]. Thus, both steps of carvone formation from car-
vone oxime and its subsequent hydrogenation a require presence
of the gold catalyst.
Since carvone accumulation was observed during the reac-
tion, carvone oxime hydrogenation was proposed to occur first
via carvone formation with the subsequent hydrogenation of its
conjugated C C double bond (route A, Scheme 2). At the same
time a parallel route B in Scheme 2 proceeding via dihydrocarvone
oxime formation should be taken into account as a possible route
resulting in dihydrocarvone. To improve understanding of gold-
catalyzed carvone oxime transformation and to clarify the reaction
scheme, hydrogenation of dihydrocarvone oxime was also stud-
ied over Au/TiO2 catalyst at the same reaction conditions as for
carvone oxime (Scheme 2). As a result, the reaction rate of dihydro-
carvone oxime conversion to dihydrocarvone was lower compared
to carvone oxime transformation (Fig. 4), while the ratio between
trans- and cis-dihydrocarvone slightly increased (Fig. 3). To sum
up absence of dihydrocarvone oxime in the reaction mixture along
with a lower rate of dihydrocarvone formation from dihydrocar-
vone oxime with the saturated endocyclic C C bond was observed.
These results excluded C C bond hydrogenation in carvone oxime
prior to ketone formation and confirmed dominance of the route A
(Scheme 2).
3. Results
Carvone oxime hydrogenation was studied in the presence
of Au/TiO2 catalyst synthesized by the deposition-precipitation
method. The Au content in the synthesized catalyst determined by
ICP-AES was 1.9 0.2 wt.%. The TEM micrograph and histogram of
the particle size distribution are presented in Fig. 1. According to
TEM the mean diameter of gold nanoparticles was 1.9 nm. Accord-
ing to XPS analysis the catalyst was mainly characterized by the
presence of metallic gold species with the corresponding binding
energy values for Au 4f5/2 and 4f7/2 components at 87.8 and 84.1 eV,
respectively. A shift of binding energy by 0.9 eV can be attributed to
the presence of Au␦− species, probably located in the gold-support
interface discussed earlier [30].
The catalytic experiments were performed at 100 ◦C under
hydrogen pressure 9 bar in the kinetic regime, which was
established according to the previously described procedure
[9]. Moreover, in our previous work [9] the effect of sol-
vent on the catalytic performance in carvone hydrogenation
to dihydrocarvone was studied in detail. Both catalytic activity
and trans- to cis-dihydrocarvone ratio were shown to strongly
depend on the solvent and increase in the following order: 2-
propanol < ethanol < methanol. Such behavior was quantitatively
described based on the transition state theory and the Kirkwood
treatment [9]. Therefore, in the current work, methanol was used
as an optimal solvent providing the highest yield of dihydrocar-
vone starting from carvone as well as higher stereoselectivity to
the trans-isomer among other alcohols.
As a result, carvone oxime transformation under hydrogen
atmosphere catalyzed by Au/TiO2 was found to result in for-
mation of carvone and trans-, cis-dihydrocarvones (Scheme 2).
Concomitant with a lower initial reaction rate of carvone oxime
transformation (3.3 × 10−3 mol/h) in comparison to that of carvone
hydrogenation (2.4 × 10−2 mol/h) [9], the increase in the stere-
oselectivity towards trans-dihydrocarvone was observed (Fig. 2).
In the case of carvone oxime, the ratio between trans- and
cis-dihydrocarvone was ca. two fold compared to carvone hydro-
genation (Fig. 3). Thus the use of carvone oxime as a starting
material for dihydrocarvone preparation led to an increase of the
synthesis efficiency in terms of stereoselectivity to the trans-isomer
as well as a possibility to have an one-pot multistep process.
Note that previously the C C bond conjugation with C O in car-
vone was shown to be important for the initial C C activation and

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24
22
20
18
16
14
12
10
8
Carvone oxime
Dihydrocarvone oxime
Carvone oxime+H O
2
6
4
2
0
0
200
400
600
800
Time, min
Fig. 4. Effect of substrate structure and water addition (0.15 ml) on conversion. Reaction conditions: T = 373 K, p (H2) = 9 bar, carvone or carvone oxime or dihydrocarvone
oxime (5 mmol), methanol (15 ml), Au/TiO2 (140 mg).
substrate adsorption on the catalyst surface resulting in selective
dihydrocarvone formation [9]. The results obtained for dihydrocar-
vone oxime additionally confirmed an influence of double bonds
conjugation on the initial molecule activation on the catalyst sur-
face probably due to a stronger adsorption through both groups
with, however, a more favorable initial activation of the C N group
followed by C C bond hydrogenation. Nevertheless, to clarify the
reaction mechanism in terms of molecular interactions on the sur-
face resulting in chemo- as well as stereo selective hydrogenation of
carvone oxime over Au/TiO2 catalyst a theoretical adsorption study
is still required.
Taking into account the obtained data the next logical step was
to explore in detail a possible mechanism of carvone formation
from carvone oxime, while carvone hydrogenation to dihydrocar-
vone was mainly investigated earlier [9]. In this connection a series
of experiments was performed. In fact, theoretically there are sev-
eral transformations including hydrogenation and hydrolysis that
can occur in different sequences resulting in ketone formation from
oxime. Acid-catalyzed hydrolysis usually is applied for obtaining
ketones from corresponding oximes. At the same time some other
approaches are well known such as biotransformations or oxida-
tive deoximation [31] as well as reductive deoximation [15,32,33].
To the best of our knowledge, Au catalysts have never been applied
before for reductive deoximation. According to the literature data
in the case of gold catalyzed hydrogenation of C N group to pro-
duce imines or other nitrogen-derivatives followed by its further
hydrolysis to form ketone seems to be more feasible [15–17].
Nevertheless, to determine an impact of the direct carvone
oxime hydrolysis as an initial reaction step, conversion of carvone
oxime was studied under nitrogen instead of hydrogen keeping
the other conditions constant. As a result, no carvone oxime trans-
formation was observed under nitrogen atmosphere. Even water
addition (0.15 ml, 1 vol.%) under nitrogen atmosphere did not lead
to carvone formation as well as in general did not promote any
carvone oxime hydrolysis.
decrease in the reaction rate (Figs. 4 and 5) and selectivity to dihy-
drocarvone. The ratio between trans- and cis-dihydrocarvone was
not significantly affected by water addition (Fig. 3).
Sensitivity of the catalytic performance to water in the case of
gold catalysts was actively studied, especially for oxidation reac-
tions [34–38]. Catalytic activity of Au/TiO2 catalyst in CO [34–37]
and alcohols [38] oxidation was shown to be promoted significantly
by water due to oxygen activation and/or beneficial role of sur-
face OH groups, originating from water. At the same time water
addition to the gas feed during propylene oxidation over Au/TiO2
caused decrease in catalytic activity because of competitive adsorp-
tion of water and propylene oxide [38]. In the case of carvone
oxime transformation water addition most likely leads to partial
active catalyst sites blocking through competitive water adsorption
preventing carvone oxime adsorption and suppressing its hydro-
genation, favouring, however, further hydrolysis to form carvone
from intermediate nitrogen containing derivatives.
Summarizing it can be proposed that, carvone oxime hydro-
genation appeared to be the first step initializing the overall
one-pot consecutive transformations catalyzed by Au/TiO2. Note
that no intermediate products were observed during carvone oxime
transformations to carvone that can be related to trace amounts
of these compounds in the reaction mixture or to a more compli-
cated reaction mechanism over Au/TiO2 catalyst via a simultaneous
oxime protonation followed by an intramolecular rearrangement.
Analysis of the reaction mixture by 13C NMR did not allow any
additional reaction products determination.
Finally, it is known that both molecular hydrogen and methanol
used as a solvent can act as hydrogen donors during hydrogena-
tion. A possibility of conjugated C C double bond reduction in
carvone by hydrogen transfer from alcohol in the presence of
alumina as a catalyst was demonstrated recently [39]. To eluci-
date the role of hydrogen, taking part in hydrogenation of each
substrate, transformations of the corresponding substrates were
studied under nitrogen pressure instead of hydrogen. First, as it
was mentioned above, no carvone oxime hydrogenation occurred
without hydrogen presence. As for carvone, maintaining it under
nitrogen pressure at 100 ◦C for 6 h did not result in any transforma-
At the same time water addition to the reaction mixture
(0.15 ml to 15 ml of methanol, 1 vol.%) under hydrogen atmo-
sphere increased selectivity to carvone (Fig. 5), concomitant with a

Yu.S. Demidova et al. / Journal of Molecular Catalysis A: Chemical 420 (2016) 142–148
147
40
35
30
25
20
15
10
5
80
70
60
50
40
30
20
10
0
trans-Dihydrocarvone
Carvone
cis-Dihydrocarvone
0
0
200
400
600
800
Time, min
Fig. 5. Effect of water addition on oxime hydrogenation over Au/TiO2. Conversion of carvone oxime (᭿) and selectivity towards carvone ( ), trans- ( ) and cis-dihydrocarvone
). Reaction conditions: T = 373 K, p (H2) = 9 bar, carvone oxime (5 mmol), methanol (15 ml), water (0.15 ml), Au/TiO2 (140 mg).
(
tion also, whereas subsequent nitrogen replacement by hydrogen
led to 26% of carvone conversion after 4 h. A higher carvone conver-
sion compared to the previously obtained value (17% after 4 h [9])
was achieved probably due to preliminary substrate activation on
the catalyst surface. Nevertheless these results demonstrated that
both reduction steps required the presence of molecular hydrogen.
The results obtained in the present work showed for the first
time that dihydrocarvone can be selectively produced by gold-
catalyzed one-pot transformation of carvone oxime. This method
has a significant advantage in terms of stereoselectivity, since more
than two fold trans- to cis-dihydrocarvone ratio was achieved com-
pared to direct carvone hydrogenation.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.04.
013.
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Catal. Today 66 (2001) 289–295.
Gold nanoparticles supported on TiO2 were shown to catalyze
selective carvone oxime transformation to dihydrocarvone. The
process was proposed to occur via catalytic carvone formation
followed by its hydrogenation to the target product. A possible
mechanism of carvone oxime conversion to dihydrocarvone over
Au/TiO2 catalyst was discussed. Application of Au/TiO2 catalyst for
both deoximation and selective hydrogenation of olefinic C C func-
tional group is reported for the first time. Utilization of carvone
oxime as a starting material allowed increasing of stereoselectiv-
ity to trans-dihydrocarvone up to trans-/cis-dihydrocarvone = 4.0.
At the same time, the reaction rate was lower compared to car-
vone hydrogenation and after 23 h carvone oxime conversion was
22% contrary to total conversion after 18 h in the case of carvone.
Nevertheless, application of the gold catalyst for dihydrocarvone
synthesis from carvone oxime in particular and in general to control
consecutive chemoselective hydrogenation of different functional
groups seems to be promising due to high selectivity. Moreover,
catalytic one-pot mode dihydrocarvone synthesis from carvone
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