Tuning selectivity in terpene chemistry: Selective hydrogenation versus cascade reactions over copper catalysts

Article abstract of DOI:10.1002/adsc.200404351

The selectivity of Cu/Al₂O₃ under very mild catalytic hydrogenation conditions can be tuned only by switching the solvent. Geraniol can be converted in a one-pot one-step process into a mixture of citronellol and menthol in hydrocarbon solvents or reduced to citronellol with 98% selectivity in 2-propanol without any additive. Both reactions can be applied to essential oils or synthetic mixtures containing geraniol, citronellal and nerol.

Full text of DOI:10.1002/adsc.200404351

FULL PAPERS
Tuning Selectivity in Terpene Chemistry: Selective
Hydrogenation versus Cascade Reactions over Copper Catalysts
a
b,
a
a
b
F. Zaccheria, N. Ravasio, * A. Fusi, M. Rodondi, R. Psaro
a
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universit a` di Milano, Via Venezian 21,
0133 Milano, Italy
CNR-ISTM, via Golgi 19, 20133 Milano, Italy
2
b
Fax: (þ39)-(0)2-5031-4405, e-mail: n.ravasio@istm.cnr.it
Received: November 19, 2004; Accepted: April 4, 2005
Abstract: The selectivity of Cu/Al O under very mild sential oils or synthetic mixtures containing geraniol,
2
3
catalytic hydrogenation conditions can be tuned only citronellal and nerol.
by switching the solvent. Geraniol can be converted in
a one-pot one-step process into a mixture of citronel-
lol and menthol in hydrocarbon solvents or reduced Keywords: copper catalysts; essential oils; geraniol;
to citronellol with 98% selectivity in 2-propanol with- menthol; selective hydrogenation; trifunctional catal-
out any additive. Both reactions can be applied to es- ysis
[
8]
Introduction
acids, Ni/diatomaceous earth and alkali hydroxides
[
9]
or carbonates or Raney Ni and alkali or alkaline earth
[
10]
Terpenes are very cheap precursors to fragrances, fla- metal hydroxides. In these studies yields never ex-
vours, drugs and agrochemicals but their high reactivity ceeded 85%. Thus, this reaction is complicated not
often makesit difficult toachieve aselectivetransforma- only by undesired saturation of the isolated C¼C bond,
[1]
tion. However, the target is worth pursuing as almost but also by hydrogenolysis and other reactions taking
all their derivatives can be useful. We have long been in- place on the catalyst surface.
volved in catalytic transformations of terpenes and ter-
Here we report that Cu/Al O catalysts allow both the
2 3
[2]
penoids studying acid-catalyzed rearrangements of specific reduction of geraniol to citronellol and its one-
terpenic epoxides over different amorphous silica-alu- step transformation into valuable mixtures of citronellol
minas, hydrogenation reactions over supported copper and menthol. The selectivity towards one reaction or the
catalysts, selective epoxidation of terpenic alcohols other can be switched simply by changing the solvent.
over titanium silicates. In particular, we have successful- Results obtained with citronellal under the same exper-
ly set up several bifunctional processes by exploiting imental conditions are also reported for the sake of com-
[
3]
acidic and hydrogenation sites or acidic and epoxida- parison.
[4]
tion sites present on the catalyst surface.
Following our continued interest in catalytic transfor-
mations of renewable starting materials we began to
study the behaviour of the terpenic allylic alcohol gera- Results and Discussion
niol over copper systems under catalytic hydrogenation
conditions.
Multifunctional Processes
The selective catalytic homogeneous hydrogenation
of geraniol (1) to citronellol (2) has been widely investi-
gated, focussing in particular on chiral catalysts. Rh- and During our studies on catalytic hydrogenation over Cu/
Ru-based catalysts have been predominately utilized, SiO we found that hydrocarbon solvents are best suited
2
Noyoriꢀs BINAP ligand being the most frequently to enhance the reaction rate; however, they also favour
[5]
used. As far as heterogeneous catalysts are concerned, acid-catalyzed reactions. When the hydrogenation of
the earliest literature for the preparation of citronellol geraniol (1) was carried out over Cu/SiO these side-re-
2
from geraniol reported the use of Pt black or Pt oxide actions could not be controlled and very complex mix-
[6]
and Ni, while more recent papers report on the use tures were obtained including dehydration, cyclisation
of more complex catalytic systems such as platinum- and rearrangement products. On the contrary by using
[
7]
containing zeolite, Pt/Al O modified with carboxylic a Cu/Al O catalyst the reaction could be addressed to-
2
3
2
3
Adv. Synth. Catal. 2005, 347, 1267–1272
DOI: 10.1002/adsc.200404351
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FULL PAPERS
F. Zaccheria et al.
of citronellal (4) into menthol (3) has already been stud-
[
3a]
[11]
ied over Cu/SiO , Ru-based catalysts and Ir-beta-
2
[
12]
zeolite and recently also the synthesis of menthols
from citral over Ni-Al-MCM-41 has been reported.
[13]
To the best of our knowledge this is the first report of
a process starting from geraniol (1).
The results are summed up in Table 1. When the reac-
tion is carried out over the 8% catalyst citronellol (2)
and isopulegol (5) are soon formed and in 2 hours also
the hydrogenation of isopulegol (5) to menthol (3) is
Scheme 1.
ward the formation of two main products, citronellol (2) complete, thus giving a 56% yield in citronellol (2) and
and menthol (3) (Scheme 1). about 30% in menthol (3). The geometric isomer of ger-
Citronellol (2) is formed through selective hydrogena- aniol (1), nerol (entry 2), reacts in a very similar way. It
tion of the C¼C bond activated by the presence of the should be underlined that none of the products deriving
OH group, whereas menthol (3) is likely the product from the hydrogenation of the C6/C7 double bond could
of a three-functional process involving isomerisation of be detected in the reaction mixtures, the only by-prod-
the allylic alcohol 1 to citronellal (4), acid-catalyzed ucts, almost exclusively 3,7-dimethyl-1,6-octadiene,
ene reaction to isopulegol (5) and final hydrogenation coming from dehydration of citronellol (2).
to menthol (3) (Scheme 2). The direct transformation
Scheme 2.
Table 1. Polyfunctional transformation of geraniol, nerol and citronellal over different catalysts under hydrogenation condi-
tions.
[a]
[b]
Entry Substrate Experimental Conditions t [h] 2 [% mol] 5 [% mol] 3 [% mol] Stereoselectivity [%] Others [%]
1
2
3
4
5
6
7
geraniol
nerol
geraniol
geraniol
geraniol
8% Cu/Al O , H
2
4
7
5
24
8
56
58
46
21
–
3
4
1
59
–
7
30
23
41
–
–
66
–
81
81
80
73
–
11
15
12
2
15
–
2
3
2
2
2
2
8% Cu/Al O , H
2
3
5% Cu/Al O , H
2
3
8% Cu/Al O , N
2
3
Al O , H
2
3
2
citronellal 8% Cu/Al O , H
citronellal Al O , H
2
27
–
75
71
2
3
2
1
95
–
2
3
[
[
a]
Amount of (ꢀ )-menthol.
b]
Amount of dehydration products (mainly 3,7-dimethyl-1,6-octadiene).
1268
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Tuning Selectivity in Terpene Chemistry
FULL PAPERS
In order to increase the yield in menthol (3), we car- gave the same products but the mixture was richer in
ried out the reaction over a catalyst with a lower Cu menthol (3) thus showing that citronellol (2) is produced
loading (5%) to reduce its hydrogenation activity. In from both geraniol (1) and its isomerization product cit-
this case the reaction is much slower, the intermediate ronellal (4).
formation of citronellal (4) can be observed (Figure 1)
The selective one-pot transformation of citronellal (4)
and an almost equimolar mixture of citronellol (2) and into menthol (3) over Cu/Al O is not possible under
2
3
menthol (3), very easy to separate the one from the oth- these conditions as the hydrogenation and cyclisation
er, can be obtained at the end of the transformation. rates are comparable. On the contrary the acid-cata-
A similar switching effect between the hydrogenation lyzed ene reaction is much faster than the hydrogenation
[
3a]
activity and the acidic features of the catalyst was report- of citronellal (4) over Cu/SiO2, thus allowing very fast
ed for the reduction of citronellal (4) with 2-propanol and complete conversion of 4 into isopulegol (5) before
over alkali metal-exchanged X-zeolite. This reaction the hydrogenation reaction begins and giving menthol
gave mainly citronellol (2) or mainly isopulegol (5) de- (3) with a 90% chemical yield [based on starting citro-
[
14]
pending on the compensating cation used the bulkier nellal (4)]. It is worth noting that also in the present
ones favouring the hydrogen transfer reaction. The ef- case a small but definite improvement in stereoselectiv-
fect of Cu content on product distribution is reported ity towards the all-equatorial isomer was observed when
in Figure 2.
the reaction was carried out under H (entry 1 vs. 4).
2
In order to verify if citronellol (2) is produced through
The chance of obtaining selectively isopulegol (5)
hydrogenation of geraniol (1) or citronellal (4) we car- from geraniol (1) by working in the absence of H was
2
ried out the reaction starting from 4 (entry 6). This also tested, but this reaction is not quantitative as it gives
also citronellol (2) (entry 4).
This can be due to dehydrogenation of geraniol (1) to
citral making some amount of hydrogen available. Thus,
this same catalyst is able to dehydrogenate secondary
and allylic alcohols under the experimental conditions
[
15]
used. When a hydrogen acceptor is present the reac-
tion of secondary alcohols can reach complete conver-
sion and total selectivity, whereas in the particular case
of geraniol (1) competition with other reactions does
not allow the selective oxidation.
From Figure 3 it is apparent that citral produced
through the dehydrogenation reaction is not reduced
under these conditions, therefore citronellal (4) does
not form in this way. On the contrary, all the hydrogen
formed is used to produce citronellol (2). The presence
of citronellal (4) and of its cyclization product isopulegol
(
5) thus shows the existence of a direct isomerization of
Figure 1. Hydrogenation of geraniol over 5% Cu/Al O in n- the allylic alcohol to the aldehyde. The isomerization of
2
3
heptane.
Figure 3. Reaction of geraniol over 8% Cu/Al O under an
inert atmosphere.
2
3
Figure 2. Effect of Cu loading on the product distribution.
Adv. Synth. Catal. 2005, 347, 1267–1272
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F. Zaccheria et al.
allylic alcohols into carbonyl compounds is catalyzed by the yield in citronellol (2) increases. In particular by us-
[
16]
a variety of transition metal complexes. However, the ing 2-propanol as solvent a quantitative yield can be ach-
case of geraniol (1) represents a challenging one. Al- ieved starting from both geraniol (1) and nerol. In this
though the isomerisation of geraniol to g-geraniol under way also citronellal (4) can be selectively reduced, as
catalytic hydrogenation conditions in the presence of a all acid-catalyzed reactions are inhibited. Such high se-
[
5b]
Ru chiral complex has been reported achiral Ru phos- lectivities are unprecedented over heterogeneous cata-
phine complexes failed to isomerise geraniol (1) to citro- lysts. As already mentioned, yields in citronellol (2) dur-
[17]
nellal (4). Good yields in this particular isomerisation ing the hydrogenation of geraniol (1) in the heterogene-
could be obtained only with tetrapropylammonium per- ous phase never exceed 85% even when alkali is added
[
18]
[6–10]
ruthenate and 2-undecanol.
In our case, experiments carried out under H over the
to suppress hydrogenolysis.
As far as the hydrogenation of citronellal (4) is con-
2
bare support, activated as the metal catalyst, show that cerned, recently a 91% yield in citronellol (2) has been
in fact Al O is an effective promoter of the ene-reaction reported over an Ru/Fe/C catalyst when working in
2
3
[
21]
converting quantitatively citronellal (4) into isopulegol methanol/trimethylamine solution.
Cu/Al O in 2-
2 3
[
19]
(
5) as already reported, whereas no reaction could propanol represents a very simple and clean alternative
be observed starting from geraniol (1) apart from to these systems as it works under very mild conditions
some dehydration. and does not require the presence of any additive. Al-
This unambiguously shows that the isomerisation of though under the experimental conditions here report-
geraniol (1) to citronellal (4) is to be ascribed to the pres- ed only 1 g of product per g of catalyst can be obtained
ence of reduced copper on the catalyst. Therefore at in a single run, a productivity of 8.7 mmolsproduct/gcatalyst
/
least two steps in the multifunctional process leading h can be computed for the hydrogenation of citronellal
from geraniol to menthol depend on the presence of (4). Although reduction of the catalyst loading results
the metal.
in a slower reaction, the catalyst can be effectively recy-
cled without significant losses in activity or in selectivity,
[15]
as already reported for the dehydrogenation reaction
[
22]
Selective Hydrogenations
and for hydrogenation ones on Cu/SiO2.
It should be underlined that under these conditions
Solvents are very effective tools to tune the selectivity of the hydrogenation of citronellal (4) is very much faster
copper catalysts in hydrogenation reactions. Thus, we al- than that of geraniol (1). Moreover, no citronellal for-
ready reported that unsaturated ketonesproneto under- mation could be observed during the hydrogenation of
go secondary reactions under catalytic hydrogenation geraniol (1) in the oxygenated solvent. This suggests
conditions can be selectively reduced to unsaturated al- that the isomerisation of geraniol (1) to citronellal (4)
cohols by using a solvent that poisons the catalyst acidic requires the presence of both Cu, as shown in the previ-
[20]
sites. Data reported in Table 2 show how the yield in ous subheading, and acidic sites. Also the isomerisation
citronellol (2) can be increased only by tuning the sol- of allylic alcohols catalyzed by nickel in the homogene-
[
23]
vent basicity.
ous phase requires the presence of protic acids and in
As already mentioned, the use of hydrocarbon sol- the case of geraniol (1) gives mixture of citronellal (4)
vents allows the acidic properties of the catalyst to and isopulegol (5). This feature represents the peculiar-
show. Therefore, even though the reaction is fast, yields ity of our catalytic system. Thus, all the one-pot transfor-
in citronellol (2) are quite low in these solvents (entries 1 mations of citronellal (4) into menthols (3) reported up
[
3a,11,12]
and 2).
to now
rely on the presence of two functions on the
By using oxygenated solvents both menthol (3) and catalyst surface, one active in the cyclisation step and the
dehydration products formation are suppressed and other in the hydrogenation one. The proper choice of the
metal and of the support should then always result in a
[
a]
Table 2.
and citronellal over 8% Cu/Al O .
Solvent effect in the hydrogenation of geraniol bifunctional system effective for this reaction.
We herein describe a unique, single-pot transforma-
tion of geraniol (1) into menthol (3) by a trifunctional
catalyst. The diverse multifunctional activities are char-
acteristic features of a synergic action between the metal
centres and the supportꢀs active site.
2
3
Entry
Substrate
Solvent
t [h]
2 [% mol]
1
2
3
4
5
5
6
geraniol
geraniol
geraniol
geraniol
nerol
n-heptane
toluene
dioxane
2-propanol
2-propanol
2-propanol
2-propanol
2
5
6
56
61
79
>98
>98
100
96
The two reactions reported here can be also applied to
complex mixtures of terpenes such as essential oils.
There has recently been an increasing search for new
catalytic processes for fine chemical production based
12
12
0.75
1
citronellal[
b]
citronellal
[
24]
on renewable resources but up to now papers dealing
[25]
[
[
a]
b]
Complete conversion.
Catalyst activated at 1808C.
with the transformation of essential oils are very rare.
Moreover, recently a selective hydrogenation process
1270
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Tuning Selectivity in Terpene Chemistry
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Table 3. Hydrogenation of essential oils at 908C and 1 atm H under different conditions.
2
Experimental Conditions
t [h]
1 [% mol]
4 [% mol]
2 [% mol]
3 [% mol]
Citronella oil
Palmarosa oil
0
1
18
16
52
2
12
64
80
37
8
5
% Cu/Al O , 2-PrOH
2 3
1
6
% Cu/Al O , n-heptane
11
0
20
13
32
31
2
3
81
8
5
% Cu/Al O , 2-PrOH
% Cu/Al O , n-heptane
75
38
2
3
2
3
has been proposed to increase the value of distillation synthesis. In particular, the use of Cu/Al O under trans-
2
3
[
26]
residues from terpene syntheses. Reactions promoted fer dehydrogenation conditions shows that this simple
by Cu/Al O allow one to convert complex mixtures of system constitutes a rare example of selective oxidation
products that are very difficult to separate into two val- of secondary alcohols in the presence of primary ones.
uable products with very different physical properties or The catalyst works under very mild conditions and not
to selectively reduce them to a single product. only on pure substrates but also on natural mixtures
2
3
[15]
[
27]
Table 3 sums up the results obtained by using the read- such as vegetable and essential oils. Moreover the cat-
ily available Citronella oil containing citronellal (4) and alyst preparation is not onerous or expensive and the
geraniol (1) and Palmarosa oil containing mainly gera- material obtained is not pyrophoric or pyrogenic even
niol (1) as starting materials. Both oils gave very similar in the reduced form. Therefore, in all these reactions
mixtures of citronellol (2) and menthol (3) when hydro- these catalysts allow the setting-up of a simple, safe
genated in n-heptane and good yields in citronellol (2) and clean protocol.
when hydrogenated in 2-propanol, showing once more
that over this catalyst geraniol (1) and citronellal (4) be-
have very similarly. In particular, the hydrogenation of Conclusion
Citronella oil (Figure 4) represents more evidence that
in the presence of 2-propanol the reactions of citronellal Cu/Al O is an effective catalyst for the transformation
2
3
and geraniol are completely independent from each oth- of geraniol, nerol and citronellal or mixtures of them
er.
into mixtures of two valuable products, citronellol and
The results reported in this paper show once more the menthol. Moreover, merely by changing the solvent, it
high versatility of these materials. Thus, copper catalysts allows the specific hydrogenation of geraniol, nerol
prepared with this technique exhibited remarkable per- and citronellal to citronellol. This would allow one to ob-
formances in different kind of transformations, varying tain these two products from one or the other starting
from hydrogenation to acid-catalysed reactions under material, or from a mixture of them, both from synthetic
anaerobic oxidation, always showing excellent selectiv- or from renewable sources, depending on their availabil-
ity and good productivity – the basic features for the ap- ity.
plication of heterogeneous catalysts to fine chemicals
Experimental Section
General Remarks
2
Al O (BET¼300 m /g, PV¼1,0 mL/g) was kindly supplied by
2
3
GRACE Davison (Worms, Germany). Geraniol (98%), nerol
(
97%) and Citronella oil (Java 85/35%) were purchased from
Aldrich; citronellal (>80–90%) and Palmarosa oil were
from Fluka.
Catalysts used, with metal loadings of 5% and 8% were pre-
[28]
pared as already reported with a non-conventional chemi-
2
þ
sorption-hydrolysis technique starting from a [Cu(NH ) ]
3
4
aqueous solution.
Before reaction, catalysts (100 mg) were treated for 20 min
in air at 2708C, for 20 min under reduced pressure and there-
after reduced at the same temperature in H . The substrates
2
(
100 mg) were dissolved in n-heptane (8 mL) and the solution
Figure 4. Hydrogenation of Citronella oil over 8% Cu/Al O .
transferred under N into the reaction vessel containing the
2
3
2
Adv. Synth. Catal. 2005, 347, 1267–1272
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FULL PAPERS
F. Zaccheria et al.
pretreated catalyst. Catalytic tests were carried out at 908C
with magnetic stirring. After the reaction, the catalyst was sep-
arated by filtration, washed with acetone, dried and reactivated
by hydrogenation before recycling.
[11] C. Milone, C. Gangemi, R. Ingoglia, G. Neri, S. Galvag-
no, Appl. Catal. A 2000, 184, 89.
[12] F. Iosif, S. Coman, V. Parvulescu, P. Grange, S. Del Sarte,
D. De Vos, P. Jacobs, Chem. Commun. 2004, 1292.
Reaction mixtures were analysed by GC (non-bonded bis-
cyanopropyl-polysiloxane capillary column, 100 m) and GC-
MS (HP-5MS cross-linked 5% phenyl methyl silicone, 30 m)
[
[
[
[
[
[
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1
analysis and by H NMR (Bruker, 300 MHz) spectroscopy.
3
Stereoselectivity given in Table 1, expressed as % (ꢀ)-men-
thol/% all (ꢀ) stereoisomers, was evaluated through both
1
GC and H NMR analysis.
5
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