Palladium-catalyzed aerobic oxidation of naturally occurring allylbenzenes as a route to valuable fragrance and pharmaceutical compounds

Article abstract of DOI:10.1002/adsc.201000050

A palladium-catalyzed, aerobic oxidation of naturally occurring allylbenzenes, i.e., eugenol, methyleugenol, safrole, and estragole, in dimethylacetamide/water solutions under mild conditions has been developed, in which palladium(II) chloride is used in the absence of co-catalysts or special stabilizing ligands as the sole and recyclable catalyst. Methyl ketones that are important for the flavour and pharmaceutical industries have been obtained in good to excellent yields with low catalyst loadings (1-2 mol%) and high average turnover frequencies. This simple catalytic method represents an ecologically benign and economically attractive route to industrially valuable compounds starting from renewable substrates easily available from essential oils.

Full text of DOI:10.1002/adsc.201000050

FULL PAPERS
DOI: 10.1002/adsc.201000050
Palladium-Catalyzed Aerobic Oxidation of Naturally Occurring
Allylbenzenes as a Route to Valuable Fragrance and
Pharmaceutical Compounds
Luciana A. Parreira,a Luciano Menini,b Joyce C. da Cruz Santos,a
and Elena V. Gusevskayaa,*
a
Departamento de Quꢀmica, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
Fax : (+55)-31-3409-5700; phone (+55)-31-3409-5741; e-mail: elena@ufmg.br
Coordenadoria do Curso Tꢁcnico em Quꢀmica, Campus Vitꢂria, Instituto Federal do Espꢀrito Santo, 29040-780 Vitꢂria,
b
ES, Brazil
Received: January 20, 2010; Published online: May 28, 2010
Abstract: A palladium-catalyzed, aerobic oxidation good to excellent yields with low catalyst loadings
of naturally occurring allylbenzenes, i.e., eugenol, (1–2 mol%) and high average turnover frequencies.
methyleugenol, safrole, and estragole, in dimethyl- This simple catalytic method represents an ecologi-
ACHTUNGTRENNUNGacetamide/water solutions under mild conditions has cally benign and economically attractive route to in-
been developed, in which palladium(II) chloride is dustrially valuable compounds starting from renewa-
used in the absence of co-catalysts or special stabiliz- ble substrates easily available from essential oils.
ing ligands as the sole and recyclable catalyst.
Methyl ketones that are important for the flavour Keywords: allylbenzenes; methyl ketones; oxidation;
and pharmaceutical industries have been obtained in oxygen; palladium
Introduction
often show biological and phytosanitary activities and
are useful in the pharmaceutical industry.[6]
The functionalization of naturally occurring olefins
Palladium-catalyzed selective oxidations represent
can provide oxygenated compounds that are valuable a versatile method to introduce an oxygen-containing
in the fine chemicals industry. We have recently re- functionality into organic molecules. These reactions
ported that various fragrance alcohols, ketones, alde- are especially attractive when molecular oxygen is in-
hydes, and esters can be obtained in good yields by volved as a final oxidant, which is usually achieved by
the metal complex-catalyzed oxidation[1,2] and hydro- using CuCl2 as co-catalyst to re-oxidize reduced palla-
formylation[3–5] of natural terpenic olefins. Substituted dium species (Wacker catalyst).[7] However, the
allylbenzenes that are easily available from biomass, Wacker process requires large amounts of chloride
such as eugenol (1a), methyleugenol (2a), safrole ions and acid to maintain a catalytic cycle; therefore,
(3a), and estragole (4a) (Scheme 1), are also an im- the system is highly corrosive and often causes the
portant renewable feedstock for flavour and fragrance formation of chlorinated side products. Although sys-
industries; in addition, their oxygenated derivatives tems with various alternative halide-free co-catalysts,
such as CuACTHNURGTNEUNG(OAc)2, heteropoly acids, nitrates, and ben-
zoquinone, have been intensively studied in attempts
to reduce the environmental impact of these process-
es,[8–14] the reoxidation of palladium(0) during the cat-
alytic cycle under more friendly conditions remains a
critical challenge.
Important recent advances in this field consist in
the use of oxidatively robust ligands to stabilize re-
duced palladium and promote its regeneration direct-
ly by molecular oxygen avoiding the use of corrosive
and waste-generating additives.[14–20] Nevertheless,
only a few examples of ligand-modulated palladium-
catalyzed oxidations of terminal alkenes into methyl
Scheme 1. Oxidation of allylbenzenes into methyl ketones.
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Luciana A. Parreira et al.
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ketones using molecular oxygen as the sole oxidant of palladium and found that the solution still con-
have been published.[15,16,20] Recently, Kaneda and co- sumed dioxygen. As a result of that study, we have
workers have discovered that the use of dimethylacet- discovered the novel process of nuclear oxychlorina-
ACHTUNGTRENNUNGamide as a solvent allows one to perform an aerobic tion of phenolic compounds and, then, aromatic
oxidation of a number of terminal alkenes without amines catalyzed by CuCl2.[29,30] On the other hand,
the need for additional co-catalysts or special li- the problem with the selective oxidation of olefinic
gands.[21] In this system, the solvent seems to stabilize bonds in the molecules of eugenol and safrole re-
a palladium catalyst preventing its precipitation into mained unsolved.
inactive metal.
Inspired by these disclosures and in the context of ventional palladium-based systems, such as Pd
our program aimed at adding value to natural ingredi- LiNO3/O2, Pd(OAc)2/BQ, Pd(OAc)2/BQ/CuACHTNUGTRENNUNG
Our various attempts to oxidize eugenol in the con-
(OAc)2/
(OAc)2/
AHCTUNGTRENNUNG
A
U
ents of renewable essential oils, we decided to study O2, gave no promising results due to a great differ-
the oxidation of naturally occurring substituted allyl- ence in the reaction mass balance; in other words, eu-
benzenes 1a–4a. Although palladium-catalyzed oxida- genol underwent unselective transformations with the
tions of these alkenes are expected to give phenyl-2- predominant formation of the products which were
propanones, which are widely used as intermediates not detectable by GC. Encouraged by recent advance-
in pharmaceutical synthesis and particularly impor- ments in palladium-catalyzed aerobic oxidations,[17]
tant for the production of antihypertensive a-methyl- we have decided to work with these problematic sub-
dopa,[23,24] the data published on this subject are really strates using molecular oxygen as the sole oxidant.
scarce.[25–27] A probable reason for this is the reported
The oxidations of eugenol (1a), methyleugenol
anomalous behaviour of eugenol and estragole in the (2a), safrole (3a), and estragole (4a) with dioxygen
conventional PdCl2/CuCl2 Wacker system, as the reac- were studied in dimethylacetamide (DMA) or dime-
tion does not lead to expected carbonyl compounds thylformamide (DMF) solutions containing 1–2 mol%
but rather gives dimeric and/or oxidative cleavage of PdCl2 and 15–20 vol% of water. In most of the
products.[26] A corresponding methyl ketone was ob- runs, elevated pressures of oxygen were used in order
tained from safrole in ca. 50% yield only by using p- to ensure efficient capture of the reduced palladium
benzoquinone (BQ) as a stoichuiometric oxidant in and prevent metal precipitation. Some experiments
methanol solutions with catalytic amounts of PdCl2.[27] were performed at atmospheric pressure. The reac-
In the present paper, we report a simple and effi- tions with each substrate resulted in corresponding
cient palladium-catalyzed direct-dioxygen-coupled ox- methyl ketones 1b–4b (Scheme 1) as major products,
idation of eugenol, methyleugenol, safrole, and estra- which were obtained in 70–90% yields in most of the
gole which gives valuable methyl ketones in good to runs. A GC mass balance was based on the substrate
excellent yields. The reactions do not need additional charged using bornyl acetate as an internal standard.
co-oxidants or special ligands and proceed under mild The difference in the mass balance in the runs pre-
aerobic conditions. The use of PdCl2 as the sole cata- sented in Table 1 and Table 2 was usually less than
lyst and inexpensive high boiling amidic solvents as 5%. It was attributed to unidentified high boiling
well as molecular oxygen as the final oxidant are sig- products. Thus, a high stability of these delicate sub-
nificant practical advantages of the process. All ob- strates toward oligomerization under the conditions
tained products have a pleasant scent with flower or used for their oxidation is especially noteworthy
fruit tinge and could be useful as components of syn- taking into account the problems with the mass bal-
thetic perfumes in addition to their applications in ance found in the conventional palladium systems.
pharmaceutical industry.
Minor products of these reactions were vanillin
(1c), methylvanillin (2c), piperonal (3c), and p-anisal-
dehyde (4c) (Scheme 2) probably formed due to the
isomerization of allyl benzenes 1a–4a into the corre-
sponding propenylbenzenes followed by the oxidative
Results and Discussion
For several years, our research group has been inter- cleavage of their olefinic bonds. The cleavage reac-
ested in the palladium-catalyzed aerobic oxidations of tions seem to occur through a radical auto-oxidation
naturally occurring olefins.[12,14,28] Studying the reactiv- mechanism. At least, in the related palladium-cata-
ity of eugenol and safrole we observed that their lyzed oxidation of styrene, the formation of benzalde-
acetic acid solutions with catalytic amounts of PdCl2, hyde was found to be suppressed by adding radical in-
CuCl2, and LiCl readily consumed dioxygen; however, hibitors to the system.[31] Aldehydes 1c–4c are also
only small amounts of the corresponding methyl ke- useful compounds important for flavour and/or phar-
tones were detected. Most of the reacted substrates maceutical industries.
were converted into high boiling products which were
We chose eugenol 1a as a standard substrate to find
not detectable by GC. Trying to clarify these observa- optimal conditions for the reaction. Eugenol was
tions, we ran the reaction with eugenol in the absence found to react readily with PdCl2 in DMA solutions
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Palladium-Catalyzed Aerobic Oxidation of Naturally Occurring Allylbenzenes
under these conditions for 4 h showing an almost dou-
bled initial rate and average turnover frequency
(TOF) of 17.8 hÀ1 (Table 1, run 2). This value is con-
siderably higher that those usually reported for con-
ventional Wacker oxidations using co-catalysts.[21] The
enhancing effect of water was also observed in the
Pd/(À)-sparteine/DMA system for the oxidation of
other terminal alkenes[20] and in our previous work
with styrene.[32] However, a further increase in the
water concentration cannot be recommended due to
the appearance of problems with the miscibility of the
substrate. The relative amounts of ketone 1b are in-
creased with the water content increase at the ex-
pense of vanillin, with the combined yield of these
Scheme 2. Minor products of the oxidation of allylbenzenes
1a–4a.
in the presence of water. A virtually complete conver- two fragrance compounds remaining almost quantita-
sion has been attained after 6 h at 608C and 10 atm tive.
resulting in an 84% yield of methyl ketone 1b and
The kinetic data show that the reaction is roughly
16% yield of aldehyde 1c (vanillin) (Table 1, run 1). first order in palladium, as a 2-fold increase in the
The reaction is catalytic in palladium and shows a di- concentration of the catalyst leads to a 2-fold increase
oxygen-coupled turnover number (TON) of 80, with in the reaction rate (Table 1, run 2 vs. run 3 and run 4
no palladium mirror being observed on the walls of vs. run 5). On the other hand, the initial reaction rate
the autoclave after the reaction. Thus, DMA as a co- depends also on the oxygen pressure showing a posi-
ordinating solvent successfully prevents zerovalent tive order (Table 1, runs 6 and 7). At 5 atm, no palla-
palladium species from clustering into inactive bulk dium mirror has been observed and the reaction has
metal and their re-oxidation by molecular oxygen been completed with only 2.5 mol% of palladium.
occurs faster than aggregation.
These observations suggest that the step which deter-
At higher water concentration (20 vs. 15 vol%), the mines the rate of the whole process can be the re-oxi-
reaction is significantly faster and can be completed dation of the reduced palladium species by molecular
Table 1. Palladium-catalyzed oxidation of eugenol 1a by molecular oxygen.[a]
Run
G
Temperature
[oC]
Pressure
[atm]
Time
[h]
Conversion
[%]
Rate[b]
TON[c]
Selectivity [%]
Ketone 1b Aldehyde 1c
N
[10À2 M·hÀ1]
1[d]
2
0.40
0.40
0.40
0.40
0.40
0.20
0.20
0.20
0.20
0.40
0.20
0.20
0.20
0.20
0.20
60
60
60
50
50
60
60
60
60
80
80
100
25
60
60
10
10
10
10
10
10
5
6
4
3
9
4
4
5
7
7
2
4
2
26
6
4
100
99
8.0
80
80
40
80
40
40
40
6
13
40
40
40
40
30
80
84
90
90
91
92
90
94
90
93
80
86
83
95
84
89
16
9
9
8
7
7
6
3
5
18
10
9
3
6
15.5
32.0
10.0
20.3
6.4
4.4
0.5
1.5
64.0
10.7
20.0
2.0
5.0
6.0
3[e]
4
100
100
100
100
99
15
32
99
98
5[e]
6
7
8[f]
9
1
1
10[e]
11
12
13
14[g]
15[h]
10
10
10
10
10
10
100
100
76
100
8
[a]
Conditions: [eugenol]=0.20M; [PdCl2]=0.005M; gas phase O2, 10 atm; solvent DMA/H2O (20 vol%); conversion and
selectivity were determined by GC and based on the consumed eugenol.
Initial rate of the substrate conversion.
TON=moles of the substrate converted/moles of Pd.
Solvent DMA/H2O (15 vol%).
[b]
[c]
[d]
[e]
[f]
[PdCl2]=0.01M.
[(À)-Sparteine]=0.01M.
[g]
[h]
DMF was used as the solvent.
After run 6, the products were separated by extraction with n-heptane, the reactor was recharged with fresh eugenol and
the reaction was allowed to proceed further. TON is given for two reaction cycles.
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Luciana A. Parreira et al.
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oxygen. On the other hand, the oxygen-dependence 50–808C (Table 1, runs 3, 5, and 10; Figure 1). Anoth-
can also reflect the competition between the re-oxida- er set of the runs (Table 1, runs 6, 11–13; Figure 1)
tion of the Pd(0) intermediates and their aggregation collaborates with this value for activation energy and
to catalytically inactive palladium black, as it has allows us to extend the temperature range to 25–
been observed at the aerobic oxidation of alcohols in 1008C. At 1008C, the reaction is very fast but less se-
related palladium systems.[33–35] Additional kinetic lective, as the combined yield of two main products
studies are necessary to clarify the mechanism of decreases to 92% (Table 1, run 12). It is important
these reactions. No substantial changes in the product that the catalytic system efficiently operates at room
selectivities have been observed with varying pressure temperature. Although the reaction expectedly be-
and catalyst concentration.
comes slower, it can be completed with only
The reaction is also catalytic in palladium at an am- 2.5 mol% palladium and shows a 95% yield of methyl
bient pressure of oxygen in the presence of (À)-spar- ketone, which is the highest value obtained in the
teine (1 equivalent to Pd) used to stabilize reduced present work (Table 1, run 13).
palladium species[20] (Table 1, run 8). No formation of
The palladium-catalyzed oxidation of eugenol also
palladium mirror occurs, but the reaction is slow and shows a dioxygen-coupled TON in another amidic
becomes stagnated at near 15% conversion (TON= solvent, DMF (Table 1, run 14 vs. run 6). It occurs at
6). In the absence of the auxiliary ligand, the reaction a slightly lower rate and not with the same efficiency
also occurs at 1 atm of oxygen, with ca. 30% of the as in DMA becoming stagnated at 75% conversion of
substrate being gradually converted during 7 h the substrate; however, no formation of palladium
(TON=13) (Table 1, run 9). However, then the con- mirror has been observed. This result is remarkable in
version again became stagnated. Thus, an elevated the light of a previously reported observation that 1-
pressure of oxygen, at least 5 atm, must be used to decene gives only a trace yield of the corresponding
keep the catalytic system active in the oxidation of methyl ketone in DMF under similar conditions.[21]
eugenol.
Thus, DMF also promotes the regeneration of the pal-
The reaction can be significantly accelerated by the ladium species at the oxidation of eugenol without
increase in temperature, being completed within 2 h the need of co-catalysts or special ligands; however,
at 808C without loss in the combined yield of methyl the reaction stagnation could indicate a partial cata-
ketone and vanillin which remains excellent (Table 1, lyst decomposition that is not visible by simple visual
cf. runs 3 and 10). On the other hand, at higher tem- inspection. Selectivity for methyl ketone in DMF is
perature the contribution of the carbon-carbon bond slightly lower than in DMA (84 vs. 90%), but the re-
cleagave giving vanillin becomes higher. A treatment action variables have not been optimized yet.
of the kinetic data revealed the temperature depend-
The DMA solution of PdCl2, in principle, can be
ence for this reaction. The data on the reaction rates re-used without any treatment after the separation of
expressed by means of the Arrhenius equation yield the oxygenated products by extraction with n-heptane
an activation energy of ca. 38 kJ·molÀ1 in the range of upon completion of the reaction. The residual solu-
tion after run 6 was recharged with fresh eugenol and
stirred under identical conditions. The reaction rate
with the recharged substrate varied only slightly and
the selectivity for methyl ketone 1b was 89%
(Table 1, run 15).
Other naturally occurring allylbenzenes, i.e., meth-
yleugenol, safrole, and estragole, can also be oxidized
smoothly by molecular oxygen in DMA/H2O solu-
tions containing PdCl2 (Table 2, runs 2–4). The indus-
trially important fragrance ketones and aldehydes
were obtained in near 90% combined yields, with the
ketones accounting for 70–85% of the mass balance
(Scheme 1 and Scheme 2). Although the values for
the initial rates for all four substrates are close, the re-
actions with methyleugenol, safrole, and estragole
need more time to be completed than that with euge-
nol. The relative amounts of the corresponding alde-
hydes, especially of the one derived from estragole,
are larger than in the case of eugenol (Table 2, run 1),
so that the selectivities for the ketones are lower, but
the reaction variables have not been optimized yet.
Figure 1. Palladium-catalyzed oxidation of eugenol: temper-
ature effect. Conditions: gas phase O2, 10 atm; solvent
DMA/H2O (20 vol%).
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Palladium-Catalyzed Aerobic Oxidation of Naturally Occurring Allylbenzenes
Table 2. Palladium-catalyzed oxidation of eugenol (1a), methyleugenol (2a), safrole (3a), and estragole (4a) in dimethyl-
ACHTUNGTRENNUNG
acetamide (DMA) by molecular oxygen.[a]
Run Substrate
Time [h] Conversion [%] Rate[b] [10À2 M·hÀ1]
Selectivity [%]
Ketone 1b–4b Aldehyde 1c–4c Total[c]
1
2
3
4
eugenol (1a)
methyleugenol (2a)
safrole (3a)
4
7
7
7
100
97
95
6.4
6.2
7.0
6.0
90
80
86
72
7
11
8
97
91
94
91
estragole (4a)
97
19
[a]
Conditions: [substrate]=0.20M; [PdCl2]=0.005M; 608C; gas phase O2, 10 atm; solvent DMA/H2O (20 vol%); conver-
sion and selectivity were determined by GC and based on the consumed substrate.
Initial rate of the substrate conversion.
[b]
[c]
Total selectivity for corresponding ketone and aldehyde.
the components are given in Table 1 and Table 2. The glass
Conclusions
reactor was connected to a gas burette containing molecular
oxygen to measure the gas uptake. The autoclave was pres-
surized with oxygen to the total pressure indicated in
Table 1 and Table 2. The reactors were placed in an oil bath;
then, the solutions were stirred at the specified temperature.
At appropriate time intervals, aliquots were taken via spe-
cial sampling systems without depressurization of the reac-
tors and analyzed by GC using a Shimadzu 17A instrument
fitted with a Carbowax 20 m capillary column and a flame
ionization detector. After carrying out the reaction and
cooling to room temperature, the excess of oxygen was
slowly vented from the autoclave. The products were isolat-
ed by a column chromatography (silica gel 60) using mix-
In summary, we have reported a novel and efficient
method for the oxidation of naturally occurring allyl-
benzenes into the corresponding methyl ketones,
under mild aerobic conditions. The use of renewable
biomass-based feedstock substrates, low-cost high
boiling solvents, and molecular oxygen as the final ox-
idant are especially relevant to the green chemistry
concept. It is also important that PdCl2 operates as
the sole, recyclable catalyst and the reaction does not
require co-oxidants, often corrosive, which must be
employed in the conventional palladium-catalyzed ox-
idations of alkenes. This simple and clean catalytic
method represents an attractive synthetic pathway to
the compounds of industrial importantance for the
flavour and pharmaceutical industries. Further studies
are targeted towards the development of solid palladi-
um catalysts resistant to leaching in polar solvents in
order to facilitate catalyst separation.
1
tures of hexane and CH2Cl2 as eluents and identified by H,
and 13C NMR and/or GC-MS.
Acknowledgements
We gratefully acknowledge the financial support from the
CNPq, FAPEMIG and INCT-Catꢀlise (Brazil) and scholar-
ships from CNPq and FAPEMIG.
Experimental Section
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