Fe(NO3)3-catalyzed monoterpene oxidation by hydrogen peroxide: An inexpensive and environmentally benign oxidative process

Article abstract of DOI:10.1007/s10562-013-1189-x

Simple Fe(NO₃)₂·9H₂O was demonstrated to be able to catalyze the oxidation of monoterpenes by hydrogen peroxide in methyl alcohol solution. Compared with the previous iron-catalyzed methods, the present procedure avoids the use of stabilizing ligands, additives, and corrosive peroxide organic oxidants. A novel, simple and highly efficient catalyst system was developed for oxidizing monoterpenes into a valuable derivates using hydrogen peroxide, an environmentally friendly oxidant. Graphical Abstract: Possible steps involved in the Fe(III)-catalyzed oxidation of β-pinene by hydrogen peroxide into myrtenol methyl ether in methyl alcohol[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-013-1189-x

Catal Lett (2014) 144:615–622
DOI 10.1007/s10562-013-1189-x
Fe(NO3)3-Catalyzed Monoterpene Oxidation by Hydrogen
Peroxide: An Inexpensive and Environmentally Benign
Oxidative Process
•
Danieli Marcolan Carari Marcio Jose da Silva
´
´
Received: 3 September 2013 / Accepted: 24 December 2013 / Published online: 25 February 2014
Ó Springer Science+Business Media New York 2014
Abstract Simple Fe(NO3)2Á9H2O was demonstrated to be
able to catalyze the oxidation of monoterpenes by hydro-
gen peroxide in methyl alcohol solution. Compared with
the previous iron-catalyzed methods, the present procedure
avoids the use of stabilizing ligands, additives, and corro-
sive peroxide organic oxidants. A novel, simple and highly
efficient catalyst system was developed for oxidizing
monoterpenes into a valuable derivates using hydrogen
peroxide, an environmentally friendly oxidant.
compatible with a broad scope of catalysts [4]. Moreover, it
presents a higher amount of active oxygen by mass unit and
is lesser corrosive and easier to handle than organic per-
oxides [5].
Among transition metals, iron catalysts have significant
advantages compared to the other noble metals; it is cheap,
abundant, has low toxicity, being therefore an environ-
mentally friendly metal catalyst [6]. Consequently, iron
catalysts have been widely applied in plentiful organic
transformations [7].
Keywords Iron(III) nitrate Á Hydrogen peroxide Á
Monoterpenes Á Catalytic oxidation
Peroxide oxidants have been used in olefin oxidations in
the presence of iron–porphyrin complexes as catalysts [8].
However, porphyrin catalysts have some shortcoming such
as the laborious synthesis, as well as the requirement of co-
catalysts [9]. Alternatively, a nonheme iron complex was
developed and successfully catalyzed the oxidation of
bicyclic and tricyclic terpenoids with hydrogen peroxide
[10]. Actually, the literature has described scarce examples
where iron salts act as ‘‘solo’’ catalyst (i.e., without addi-
tives or radicalar initiators) in oxidation reactions. A rare
instance of iron(III)-catalyzed reaction is the benzylic
substrates oxidation to corresponding carbonyl compounds
with tert-butyl hydroperoxide [11]. However, these FeCl3-
catalyzed reactions take place at a long time (ca. 24 h);
moreover, the major drawback is use pyridine as a solvent,
which is economically and environmentally undesirable
[11].
1 Introduction
Oxidized monoterpenes are valuable raw materials for the
preparation of commercially important products such as
fragrances, perfumes and flavors; in addition, they are
attractive molecules for the pharmaceutical, sanitary, cos-
metic, agrochemical, and food industries [1]. Interest in
these reactions is higher when environmentally benign
oxidants, mainly dioxygen or hydrogen peroxide, are
employed, because they produce only water as the by-
product [2, 3]. Hydrogen peroxide is an inexpensive
commercial product used in numerous industrial processes,
because it is a non-flammable liquid oxidant, and
The development of monoterpene selective oxidation
processes based on clean oxidants and active catalysts is a
goal that has been pursued by our research group [12, 13].
Herein, we wish describe a simple and efficient Fe(III)-
catalyzed monoterpene oxidation process with H2O2 in
methyl alcohol solution, in the absence of bulker nitrogen
stabilizing ligands, additives, or even co-catalysts. We paid
special attention to study factors driving oxidation
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-013-1189-x) contains supplementary
material, which is available to authorized users.
D. M. Carari Á M. J. da Silva (&)
Chemistry Department, Federal University of Vic¸osa, Avenida
PeterHenry Rolfs, s/n, Vic¸osa, MG 36570-000, Brazil
e-mail: silvamj2003@ufv.br; silvamj2003@yahoo.com.br
123

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D. M. Carari, M. J. da Silva
selectivity and to optimizing the main reaction parameters.
Remarkably, after a short time reaction (1 h) the Fe(NO3)3-
catalyzed oxidation reactions of a and b-pinene by H2O2
reached a very high conversion (i.e., 98 and 89 %,
respectively), and combined selectivity for the two main
products of ca. 73 %, a result superior to the described
recently by us in the literature [14]. The maximum indi-
vidual selectivity was of ca. 49 and 24 % for myrtenol
methyl ether and a-terpineol, respectively. This result is
lower than one reported in monoterpene oxidation reac-
tions with H2O2 where epoxides are main products [15].
However, recent literature have described that oxidation of
both a and b-pinene by H2O2 commonly result in the
formation of a number allylic products, compromising the
reaction selectivity [16–18]. To the best of our knowledge,
this is the first report of monoterpene oxidation reactions
with hydrogen peroxide catalyzed by Fe(III) cations in the
absence of bulker nitrogen ligands, additives, or nitrogen
solvent.
Reactions were monitored by analyzing aliquots taken at
regular time intervals by gas chromatography (Varian 450
instrument, FID, Carbowax 20 M capillary column).
Dodecane was the internal standard. Reaction conversions
were estimated from the corresponding chromatographic
peak areas comparing with the corresponding calibrating
curve. The quantification of reaction products was made
via co-injection in GC and response factors of product
samples (pure or stander). The identification of products
was done by GC–MS analyses operating at impact elec-
tronic mode (70 eV).
In order to determine concentrations of all oxidation
products and decompose the intermediate hydroperoxides
possibly present in the reaction solutions the aliquots were
analyzed twice by mass spectroscopy (i.e., before and after
their treatment with PPh3), as described in the literature
[19].
3 Results and Discussion
2 Experimental Procedures
3.1 General Aspects
2.1 Materials and Physical Methods
Literature has described the use of iron(III) nitrate as
reoxidant of the palladium catalysts in monoterpene oxi-
dation reactions by dioxygen, where it also transfers oxy-
gen atoms from the oxidant to olefin via intermediates
organopalladium [20]. On the other hand, iron(III) com-
plexes with bulk nitrogen ligands have been employed as
catalysts in olefin oxidation reactions by both dioxygen and
hydrogen peroxide [21, 22].
All of chemicals and solvents are commercially available and
used without additional purification. The salts LiNO3 (99 %),
Fe(NO3)3Á9H2O (98 %), and Cu(NO3)2Á3H2O (98 %) were
purchased from Sigma-Aldrich. Camphene (95 %), b-pinene
(99 %), a-pinene (99 %), and limonene (99 %) were also
acquired from Sigma-Aldrich. 1H NMR and 13C NMR spectra
were recorded on the Mercury-300 Varian Spectrometer
300 MHz for 1H instrument, with chemical shifts (ppm)
reported relative to the internal standard tetramethylsilane.
Major reaction products were purified by column chromatog-
raphy using silica gel (60G). Gas chromatography analyses
were carry out on a Varian 450 instrument with a FID detector
Herein, we assessed the Fe(NO3)3-catalyzed monoter-
pene oxidation reactions using hydrogen peroxide as stoi-
chiometric oxidant, in bulky nitrogen ligand-free methyl
alcohol solutions and in the absence of co-catalysts. The
use of methanol as solvent in olefin oxidations reactions by
H2O2 was previously described in the literature, and no
formation of their oxidized derivatives was not reported
[23, 24].
and
a
CP-WAX capillary chromatographic column
(25 m 9 0.32 mm 9 0.30 lm). GC–MS spectra were recor-
ded on a Shimadzu 5050 gas chromatography–mass spec-
trometry instrument (i.e., 70 eV). The active oxygen content of
the hydrogen peroxide oxidant (34 wt% aqueous solution;
Vetec) was determined by permanganimetry prior to use.
Under the reaction conditions, the methyl alcohol used
as a solvent could be oxidized to volatile (i.e. formalde-
hyde, carbon oxides) or non-volatile products (i.e. formic
acid), and compromise the b-pinene oxidation reaction
because of extra consumption of H2O2. We checked this
possibility carrying out reactions at the same conditions
described in Table 1, but without the monoterpene pre-
sence, using different H2O2 load (i.e. 10, 20, 30 and
40 mmol of H2O2) and we find out that the methyl alcohol
concentration remained almost constant, regardless of
peroxide concentration used. Gas chromatography analyses
not detected formic acid or methyl alcohol derivatives in
the reaction solutions. Literature describes that the com-
bination FeBr3/H2O2 was inefficient on primary alcohols
2.2 Catalytic Oxidation of Monoterpenes
Catalytic runs were carried out under air in a glass reactor
(50 mL) equipped with a magnetic stirrer and sampling
septum. In a typical run, an adequate amount of Fe(III)
catalyst and monoterpene was dissolved in methyl alcohol
(15 mL), the reactor temperature was adjusted to 55 °C and
then the reaction was initiated by the adding of hydrogen
peroxide 34 wt%.
123

Fe(NO3)3-Catalyzed Monoterpene Oxidation
617
OCH3
Table 1 Effect of metal catalyst oxidation state on b-pinene oxida-
tion by hydrogen peroxide in CH3OH
Run Catalyst
Conversiona (%) Products selectivityb (%)
Fe(NO3)3
55 °C; 4h
Isomers
1a
1b
Ni
+
1
–
\3
0
0
0
2
LiNO3
0
0
0
00
30
18
17
9
0
3
Cu(NO3)2 56
11
13
15
16
45
50
48
31
14
17
20
44
OH
1
1a
1b
4
5c
Fe(NO3)3
Fe(NO3)3
FeSO4
98
97
11
Fig. 2 Main products of Fe(NO3)3-catalyzed reactions of b-pinene
with H2O2 in CH3OH. Reaction conditions: b-pinene (5 mmol);
Fe(NO3)3 (0.21 mmol); methyl alcohol (15 mL); 55 °C; hydrogen
peroxide (20.0 mmol)
6
Reaction conditions: b-pinene (5 mmol); catalysts (0.21 mmol);
methyl alcohol (15 mL); 558C; hydrogen peroxide (20.0 mmol); 4 h
reaction
a
Determined by GC
H2O2 load was possible perceive a slightly higher con-
b
Determined by GC; isomers (i.e., a and c-terpinene) were formed in
sumption of oxidant due to the presence of b-pinene.
almost equimolar amounts; (1a) myrtenol methyl ether and a-ter-
pineol (1b); complex mixture of non-identified minority products (Ni)
3.2 b-Pinene Oxidation by Hydrogen Peroxide
in the Presence of Different Salts
c
Reaction performed under N2 atmosphere (0.10 MPA)
The activity of M(NO3)n (M = Li(I), Cu(II), or Fe(III);
n = 1, 2 or 3) nitrate salts was evaluated in the b-pinene
oxidation reactions by H2O2 in CH3OH solutions and the
main results are summarized in Table 1.
[25]. Actually, methyl alcohol anol has lowest oxidation
rate among all of alcohols [26].
In addition, monitoring these same reactions via titration
against KMnO4 0.02 mol L-1 solution, we verified that
hydrogen peroxide concentration quickly decreased within
first reaction moments; instead of start 1.3 and 2.7 mol L-1
(i.e. initial concentration values), in the first aliquot titrated
the concentration of remaining H2O2 was equal to 0.94 and
1.4 mol L-1, respectively (Fig. 1a, b). It is noteworthy that
KMnO4 was normally used because it do not oxidize
monoterpene in absence of acid catalysts.
Even though an oxidant excess have been used (i.e., b-
pinene: H2O2 molar ratio equal to 1: 6), only a poor con-
version was obtained in the reaction-blanck (Run 1,
Table 1). On the other hand, the ineffectiveness of LiNO3
discarded a possible stoichiometric oxidation of b-pinene
by the nitrate anion (Run 2, Table 1). Conversely, in the
presence of transition metal cations, such as Cu(II) and
more remarkably Fe(III), the oxidation reactions became
truly catalytic. Notably, the Fe(NO3)3 catalyst was the most
efficient and promoted the selective oxidation of b-pinene
Figure 1 suggests that H2O2 undergo thermal decom-
position, because the kinetic curves obtained with or
without b-pinene are close or coincident; only using lower
1,0
with β-pinene
without β-pinene
with β-pinene
without β-pinene
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0
15
30
45
60
75
90
105
120
0
15
30
45
60
75
90
105
120
Time (min)
Time (min)
(b) 40 mmol H2O2
(a) 20 mmol H2O2
Fig. 1 Consumption of H2O2 determined by titration with KMnO4 in Fe(NO3)3-catalyzed reactions in presence or absence of b-pinene.
Reactions condition: Fe(NO3)3Á9H2O (0.02 mmol); b-pinene (5.0 mmol); methanol (15.0 mL); temperature (55°C)
123

618
D. M. Carari, M. J. da Silva
OPPh3
PPh3
Fig. 3 GC–MS analyses of aliquots collected after 30 min reaction from Fe(NO3)3-catalyzed oxidation of b-pinene by hydrogen peroxide in
CH3OH. Reaction conditions: b-pinene (5 mmol); Fe(NO3)3 (0.21 mmol); methyl alcohol (15 mL); 55 °C; hydrogen peroxide (20.0 mmol)
(1) to myrtenol methyl ether (1a) and a-terpineol (1b),
achieving highest conversion (ca. 98 %) with selectivity of
50 and 18 % for these products, respectively (Fig. 2).
Although Cu(NO3)2 was also effective, it was lesser
selective and active than Fe(NO3)3 catalyst.
main results in Fig. 3. Only the chromatograms of the
aliquots collected after 30 min were shown by
simplification.
We find out that although triphenylphosphine oxide
have been formed no significant variation was observed in
the area of the GC–MS peaks (Fig. 2). We monitored the
reactions during the two first hours (i.e. aliquots were
analyzed at 30 min intervals), and verified that only the
amounts of triphenylphosphine oxide and triphenylphos-
phine were appreciably modified. Initially, all PPh3 was
converted to oxide, however, with an increase of reaction
time, the OPPh3 quantity formed was lowering. As showed
in Fig. 1a, this behavior can be attributed to the decrease of
H2O2 concentration that remained in solution.
It is important to note that a-terpineol normally corre-
sponds the skeletal rearrangement product of b-pinene
followed by the water addition [27]. Nevertheless, herein
its formation only occurred in presence of the hydrogen
peroxide oxidant and of the catalyst Fe(NO3)3, then it will
be named ‘‘oxidized product’’.
Recently, we have reported the oxidation of b-pinene by
hydrogen peroxide into the same product 1a, in PdCl2-
catalyzed reactions [14]. However, the palladium is an
expensive catalyst, and in those reactions, the ether 1a was
obtained with other three allylic products. Whereas, the
Fe(NO3)3-catalyzed reactions has low cost and the good
combined selectivity to two main products, making this
oxidative process much more attractive. Moreover, a-ter-
pineol 1b is a valuable commercially product because it is
used as fragrance and flavor ingredient [28].
3.3 Fe(NO3)3-Catalyzed Oxidation of b-Pinene
by Hydrogen Peroxide in CH3OH: Effect
of Catalyst Concentration
The effect of catalyst concentration was investigated in the
range of 0.02–0.21 mmol and the results are summarized in
Table 2.
No epoxide product was detected herein. These data are
in agreement with the literature, which reports a highly
selective formation of allylic products from b-pinene and
no epoxide formation in oxidation reactions with H2O2 [12,
14].
Different from PdCl2-catalyzed b-pinene oxidation, in
all runs catalyzed by Fe(III), the high initial reaction rate
allows maximum conversion within of first hour reaction,
remaining almost constant from this point forward [12, 14].
It can be seen that a decreasing on catalyst concentration
did not result in any significant change to the conversion or
reaction selectivity (Table 2; Fig. 4). Regardless of catalyst
concentration, the combined selectivity of products 1a and
1b remained in the range of 68–73 %, with myrtenol
methyl ether (1a) as the major product.
A crucial point in alkane or olefin oxidation reactions
with H2O2 is the probable hydroperoxides formation,
which can be decomposed into ketone in GC injector,
masking the selectivity results [29]. Hydroperoxide
amounts can be determined via a double injection, with and
without reduction of the reaction mixture by trimethyl-
phosphine. From the obtained increase in alcohol content,
the corresponding hydroperoxide content can be deter-
mined [30]. We addressed this issue and summarized the
The reaction selectivity was mostly independent on
catalyst concentration; myrtenol methyl ether and a-ter-
pineol were always the main products. Similar results were
123

Fe(NO3)3-Catalyzed Monoterpene Oxidation
619
Table 2 Effect of Fe(NO3)3 concentration on the oxidation of b-pinene by H2O2 in CH3OH
Exp.
Fe(NO3)3 (mmol)
Conversion (%)
TON
Selectivity (%)
Isomersa
1a
1b
Ni
RSboxidized
1
2
3
4
5
0.21
0.11
0.07
0.04
0.02
98
95
90
92
91
16
30
45
78
166
13
13
13
12
10
50
48
47
49
49
18
22
22
19
24
19
17
18
19
17
68
70
69
68
73
Reaction conditions: b-pinene (5.0 mmol); H2O2 (20.0 mmol); methyl alcohol (15 mL); 55 8C; 8 h reaction
a
Determined by GC; TON (turnover number) = n mol substrate converted/1 mol Fe(III) catalyst
b
Determined by GC; isomers (i.e., a and c-terpinene) were formed in almost equimolar amounts; (1a) are myrtenol methyl ether and (1b) a-
terpineol; complex mixture of non-identified minority products (Ni); RSoxidized is the selectivity combined for the main products
3.4 Effect of Hydrogen Peroxide Concentration
100
on the Fe(NO3)3-Catalyzed b-Pinene Oxidation
Reactions in CH3OH
80
Normally, in homolyitic oxidation reactions catalyzed by
transition metal type Mn?/Mn?1, the presence of a large
Fe(NO3)3 catalyst
60
0.020 mmol
0.040 mmol
0.070 mmol
amount of hydroperoxo radicals may reduce the reaction
selectivity favoring concurrent reactions, such as oligo-
0.112 mmol
0.210 mmol
merization, isomerization, as well as promoting the for-
40
20
0
mation of manifold oxidized products. These features are
great drawbacks to be overcome when monoterpenes,
which reactivity is generally hardly controllable, are the
substrates to oxidize. This effect was investigated by
varying the oxidant concentration in the range of
5–30 mmol (i.e., H2O2 34 wt% aqueous solution), and the
major results are displayed in Table 3.
0
60
120
180
240
300
360
420
480
time (min)
It was observed that an increase of H2O2 amount until
20 mmol (ca. substrate/oxidant molar ratio equal to 1:4)
did not compromise oxidation selectivity (Runs 1–3,
Table 3). However, when amounts higher than 30 mmol of
oxidant were employed other products were formed,
resulting in a decrease of oxidation selectivity (Run 4,
Table 3) (Fig. 5).
Fig. 4 Effect of catalyst concentration on the conversion of the
Fe(NO3)3-catalyzed b-pinene oxidation reactions with H2O2 in
CH3OH solutions. Reaction conditions: b-pinene (5.0 mmol); H2O2
(20.0 mmol); CH3OH (15 mL); 55 8C
described in both homolytic or heterolytic b-pinene oxi-
dation previously assessed by us [12, 31] Probably, the
selectivity is governed by stability of intermediate alkyl
hydroperoxides, which are promptly decomposed by
Fe(III) cations and therefore was not affected by their
concentration.
Notably, reaction rates were highly dependent of the
hydrogen peroxide concentration employed. The maximum
initial reaction rate was achieved with 20 mmol of oxidant
that resulted in the highest conversion and selectivity of b-
pinene oxidation. On the other hand, the greater amount of
oxidant (ca. 30 mmol) was apparently less efficient
because it lowered not only the reaction rate (Fig. 3), but
also the oxidation selectivity (Run 4, Table 3).
The kinetic curves analysis shows that, regardless of
catalyst concentration, high initial reaction rates were
achieved and maximum conversion was reached within the
first three hours of reaction (Fig. 4). This fact, in addition
to the TON obtained, comprises positive aspects of these
reactions. Figure 4 supports the hypothesis that iron (III)
catalyst promote the intermediate alkyl hydroperoxides
decomposition into oxidation products at beginning of the
reaction; consequently the hydroperoxides were not
detected via reduction tests with triphenylphosphine.
3.5 Fe(III)-Catalyzed b-Pinene Oxidation Reactions
by H2O2 in CH3OH: Insights into the Role
of Fe(III) Cations
Homolytic oxidations involve intermediate free radicals
being frequently catalyzed by first-row transition metals
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620
D. M. Carari, M. J. da Silva
Table 3 Effect of hydrogen peroxide concentration on the Fe(NO3)3-catalyzed oxidation of b-pinene in CH3OH
Run
Hydrogen peroxide (mmol)
Conversiona (%)
Selectivityb (%)
Isomers
1a
1b
Others
RSboxidized
1
2
3
4
5.0
10.0
20.0
30.0
54
70
91
85
14
13
10
8
53
52
49
38
17
20
24
21
16
15
17
33
70
72
73
59
Reaction conditions: b-pinene (5.0 mmol); Fe(NO3)3 (0.020 mmol); methyl alcohol (15 mL); 55 8C; 8 h reaction
a
Determined by GC
b
Determined by GC; isomers a and c-terpinene formed in almost equimolar amounts; (1a) myrtenol methyl ether; (1b) a-terpineol; (others)
complex mixture of other oxidation products (myrtenal, myrtenol, and myrtenoic acid) and non-identified minority products; RSoxidate is the
selectivity combined for the main products
100
90
80
Fig. 6 Metal-catalyzed decomposition of hydroperoxides (Haber–
Weiss mechanism)
70
60
50
40
30
20
10
0
H2O2 concentration
5.0 mmol
10.0 mmol
20.0 mmol
30.0 mmol
0
60
120
180
240
300
360
420
480
time (min)
Fig. 5 Kinetic curves of Fe(NO3)3-catalyzed b-pinene oxidation
reactions with different amount of H2O2 oxidant in CH3OH. Reaction
conditions: b-pinene (5.0 mmol); Fe(NO3)3 (0.020 mmol); CH3OH
(15 mL); 55 8C
Fig. 7 Possible steps involved in the Fe(III)-catalyzed oxidation of b-
pinene by hydrogen peroxide into myrtenol methyl ether
characterized by one-electron redox steps (e.g., Co(II)/
Co(III), Mn(II)/Mn(III), and Cu(I)/Cu(II) [32]. In these
reactions, the substrate is generally not coordinated to the
metal and it is oxidized outside the coordination sphere via
a radical chain mechanism.
stabilize these intermediates [34]. Thus, we suggest that a
similar mechanism (type Waber-Weiss) based on the initial
generation of cations H? could be involved herein. From
this point, it is reasonable to assume that the three species
present in the solution could react with Fe(III) cations to
provide H? cations (Fig. 7).
The main role of the metal catalyst is generally
decompose intermediate hydroperoxides, which are spon-
taneously formed or through the action of an initiator agent
generating radicals to sustain the reaction. This behavior is
also known as the Haber–Weiss mechanism (Fig. 6) [33].
A significant difference herein is that iron catalyst has the
highest oxidation number, which excludes the first reaction
proposed in Fig. 6.
Because myrtenol methyl ether was always the main
oxidation product, it can be concluded that participation of
the solvent is obligatory. We suggested that, in the pre-
sence of hydrogen peroxide and Fe(III) cations, the methyl
alcohol is converted into highly active methoxy radical,
which reacts with b-pinene radical, oxidizing it into myr-
tenol methyl ether (Fig. 7). We suppose that regenerating
Although the literature describes the formation of
Fe(IV) species in iron-catalyzed reactions with peroxo
oxidants, in general they require nitrogen ligands to
123

Fe(NO3)3-Catalyzed Monoterpene Oxidation
621
Table 4 Effect of temperature on the Fe(NO3)3-catalyzed b-pinene oxidation reaction with H2O2 in CH3OH
Exp.
Temperature (°C)
Conversiona (%)
Selectivityb (%)
Isomersa
1a
1b
Ni
RSoxidized
2
3
4
35.0
45.0
55.0
15
66
91
18
14
10
40
45
49
19
19
24
23
22
17
59
64
73
Reaction conditions: b-pinene (5 mmol); Fe(NO3)3 (0.020 mmol); methyl alcohol (25 mL); hydrogen peroxide (20.0 mmol); 8 h reaction
a
Determined by GC
b
Determined by GC. Isomers a and c-terpinene were formed in almost equimolar amounts; (1a) myrtenol methyl ether and (1b) a-terpineol;
(Ni) complex mixture of non-identified minority products; RSoxidate: selectivity combined into main products
Table 5 Fe(NO3)3-catalyzed monoterpene oxidation reactions with H2O2 in CH3OH
Run
1
Substrate
Chemical structure
Conversion (%)
98
Selectivitya (%)
PI
PO
68
Others
22
Olig.
–
b-Pinene
10
2
3
a-Pineneb
89
45
10
18
65
22
25
25
–
Camphenec
35
Reaction conditions: monoterpene (5.0 mmol); Fe(NO3)3 (0.210 mmol); methyl alcohol (25 mL); hydrogen peroxide (30.0 mmol); 2 h reaction;
55°C
a
Products selectivity: PI isomers, PO oxidation products, Others complex mixture of non-identified minority products, oligomers oligomeri-
zation products non-detectable by GC
b
a-Pinene: isomers a and c-terpinene were formed in almost equimolar amounts; (1a) myrtenol methyl ether and (1b) a-terpineol were the main
products, respectively, with selectivity almost equal to b-pinene oxidation selectivity
c
Camphene: main products were identified by GC–MS analysis as limonene, bornyl ether and fenchil ether
Fe(III) cations occur via reaction of Fe(II) and H? cations
and perhydroxyl radical, resulting in water and another
active specie (i.e., hydroxyl radical). We know that other
radicalar steps and other intermediates should be involved
[35, 36]. However, the steps described here are adequate to
explain formation of main product, as well as the partici-
pation of Fe(III) cations.
than those obtained at 55 °C. In general, radicalar reactions
are highly sensitive to temperature increases, which
also promote the decomposition of intermediate hydro-
peroxides.
3.7 Fe(NO3)3-Catalyzed Oxidation Reactions
with Hydrogen Peroxide in CH3OH: Effect
of Substrate Nature
3.6 Effect of Temperature on the Fe(NO3)3-Catalyzed
b-Pinene Oxidation Reactions by H2O2 in CH3OH
The main factors that affect the monoterpenes reactivity are
the level of substitution of the olefinic double bonds, the
type of carbon skeletal, and the accessibility of yours
vinylic and allylic hydrogens. All of substrates with allylic
hydrogens easily replaceable promptly reacted in
Fe(NO3)3/H2O2/CH3OH system (Table 4). Contrarily, the
camphene that have only one allylic hydrogen that occu-
pies the bridgehead position, and thus is consequently not
easily abstractable was the less reactive monoterpene. In
The changes on reaction temperature drastically affected
the efficiency of Fe(III)-catalyzed reactions conversion. At
room temperature, the system becomes biphasic; however,
after solvent extraction, it was verified that only a poor
conversion of b-pinene was obtained (ca. 5 %). Even when
the reaction temperature was increased to 35 or 45 °C, both
maximum conversion and oxidation selectivity were lower
123

622
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Table 5, we summarized the data of conversion and
selectivity obtained in these reactions.
It was observed that a-pinene has a similar reactivity to
the b-pinene, being mostly oxidized into myrtenol methyl
ether and pinocarveol, with high conversion (ca. 89 %) and
oxidation selectivity (ca. 65 %). However, in the oxidation
reactions of camphene the undesirable formation of olig-
omers occurred. These products were not detectable by GC
analysis; however, their formation was confirmed by
monitoring the mass balance, and by the precipitation of a
white solid, formed after reaction aliquots cooling to
temperatures lower than room temperature.
´
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A Fe(III)-catalyzed natural olefin oxidation process based
on an inexpensive and commercially available catalyst and
an environmentally benign oxidant (i.e., hydrogen perox-
ide) was developed. In these reactions, a and b-pinene
were preferentially oxidized into an allylic ether (i.e.,
myrtenol methyl ether). The high TONs achieved revealed
that the Fe(NO3)3 was more effective catalyst than bioin-
spired (i.e., porphyrins) or noble catalysts (i.e., palladium)
described in the literature. b and a-pinene were mostly
oxidized within a 1–2 h reaction, with conversion close to
90 %. Compared with the previous iron-catalyzed method,
the present procedure avoids the use of stabilizing ligands,
additives, and corrosive peroxide organic oxidants.
Although the oxidation reactions homolytically proceeded,
only two products were obtained with combined selectivity
of 73 % (i.e. myrtenol methyl ether and a-terpineol),
always with conversions of 91–98 %, regardless of
Fe(NO3)3 catalyst concentration employed.
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Acknowledgments The authors are grateful for the financial sup-
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