A novel Fe(III) salt-catalyzed monoterpene aerobic oxidation in methyl alcohol

Article abstract of DOI:10.1016/j.catcom.2013.08.018

The Fe(III)-catalyzed aerobic oxidation of monoterpenes in CH₃OH has been developed, in which simple Fe(III) salts are used as catalysts in the absence of stabilizing ligands. Remarkably, Fe(NO₃)₃ catalyst efficiently promotes the oxidation of monoterpenes under air or dioxygen. The highest TON and TOF reached 476 and 162 h^{- 1}, respectively. In general, reactions with 1.0-9.0 mol% of catalyst reached high conversions (ca. 90-99%) and high oxidation products selectivity (ca. 80%). Notably, α-pinene and β-pinene were selectively oxidized into only allylic product, myrtenol methyl ether. The significant breakthroughs of this simple oxidative process are the use of inexpensive Fe(III) salts as catalysts and environmentally-friendly oxidants (air or dioxygen).

Full text of DOI:10.1016/j.catcom.2013.08.018

Catalysis Communications 42 (2013) 129–133
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A novel Fe(III) salt-catalyzed monoterpene aerobic oxidation in
methyl alcohol
⁎
Fabiano Gomes Ferreira de Paula, Lilian Berllini, Márcio José da Silva
Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais 36590-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 May 2013
Received in revised form 21 August 2013
Accepted 22 August 2013
Available online 29 August 2013
The Fe(III)-catalyzed aerobic oxidation of monoterpenes in CH3OH has been developed, in which simple Fe(III)
salts are used as catalysts in the absence of stabilizing ligands. Remarkably, Fe(NO3)3 catalyst efficiently promotes
the oxidation of monoterpenes under air or dioxygen. The highest TON and TOF reached 476 and 162 h−1
,
respectively. In general, reactions with 1.0–9.0 mol% of catalyst reached high conversions (ca. 90–99%) and
high oxidation products selectivity (ca. 80%). Notably, α-pinene and β-pinene were selectively oxidized into
only allylic product, myrtenol methyl ether. The significant breakthroughs of this simple oxidative process are
the use of inexpensive Fe(III) salts as catalysts and environmentally-friendly oxidants (air or dioxygen).
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Iron catalysts
Dioxygen
Monoterpenes
Aerobic oxidation
1. Introduction
monoterpene oxidation reactions by dioxygen has always involved
nitrogen ligands. Transition metal porphyrin complexes and iron-
Monoterpenes are natural olefins which constitute an abundant raw
material, whose oxygenate derivatives are the ingredients of fragrances,
drug synthesis intermediates, and fine chemical industries [1]. Olefin
oxidation reactions are interesting from the viewpoint of industry, espe-
cially when dioxygen is employed, an environmentally benign oxidant
that produces water as its only by-product [2]. Nevertheless, the use of
dioxygen requires a transition metal catalyst for its activation, which
generally requires special conditions to work, such as the presence of
stabilizing ligands or reversible reoxidants [3,4].
The use of Pd(II) catalysts in olefin aerobic oxidation has been com-
prehensively explored, because the reaction generally involves σ or
π-organopalladium intermediates instead of radical species, which
make selectivity control difficult [5]. An example is the Wacker process,
which is used on an industrial scale to oxidize C2H4 into CH3CHO (PdCl2/
CuCl2/O2) [6]. However, the Wacker catalyst is only ineffective in the
monoterpene oxidation reactions, due to the formation of chlorinated
products and the high Lewis acidity of CuCl2 [7]. Several modifications
in the Wacker system have been proposed, such as replacing the
CuCl2 by other reoxidant [8]. In this regard, we developed a catalytic
system based on a nitrate reoxidant (i.e., Pd(OAc)2/M(NO3)n (M =
Li(I), Cu(II) or Fe(III); n = 1, 2 or 3), which was employed in Pd(II)-
catalyzed monoterpene oxidation by dioxygen [9]. In that work, the
Fe(NO3)3 was used exclusively as a palladium reoxidant; no catalytic
action was reported [10]. Actually, the use of iron as a catalyst in
bipyridine complexes were also employed in monoterpene oxidation
under a continuous flow of dioxygen [11–13]. However, in addition to
the laborious synthesis of porphyrin complexes, most of these reactions
use H2O2 as stoichiometric oxidant [14].
In this paper, Fe(III)-catalyzed aerobic monoterpene oxidation reac-
tions were performed using air or dioxygen in CH3OH, in the absence of
bulkier nitrogen ligands. While we were assessing the catalytic activity
of the Pd(OAc)2/Fe(NO3)3 catalyst in oxidation of β-pinene by dioxygen,
we found an unexpected result: the Fe(NO3)3 was an active and selec-
tive catalyst; it promoted monoterpene oxidation, even in the absence
of palladium. The main reaction parameters, such as temperature, na-
ture and catalyst concentration and gas phase were addressed. High
TONs were obtained and α and β-pinenes were almost completely oxi-
dized, with selectivity for myrtenol methyl ether equal to 60%.
2. Experimental procedures
2.1. Materials
All chemicals were purchased from commercial sources. Pd(OAc)2
(99.9% w/w), Cu(NO3)2·6 H2O (99% w/w) and Fe(NO3)3⋅9·H2O
(99. 9% w/w) were acquired from Sigma-Aldrich. Fe2(SO4)3·5 H2O
(97% w/w) and LiNO3·H2O (99% w/w) were purchased from Merck.
FeCl3·6H2O (97%) and FeSO4·7H2O (99% w/w) were acquired from
Sigma-Aldrich. Acetonitrile, methyl, ethyl and propyl alcohols were
acquired from Sigma-Aldrich (99% w/w) and were used as received.
Limonene, (−) β-pinene and (−) α-pinene were substrate selected
(Sigma Aldrich, 99% w/w).
⁎
Corresponding author at: Departamento de Química, Universidade Federal de Viçosa
(UFV), Campus Universitário, Avenida P.H. Rolfs, s/n, Viçosa, MG 36570-000, Brazil.
Tel.: +55 31 38993210, +55 31 3899 3071; fax: +55 31 3899 3065.
E-mail address: silvamj2003@ufv.br (M.J. da Silva).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.08.018

130
F.G. Ferreira de Paula et al. / Catalysis Communications 42 (2013) 129–133
2.2. Catalytic tests
Reactions under air were carried out in a glass reactor (50 mL)
equipped with a magnetic stir bar and a septum. In a typical run, the
catalyst was dissolved in CH3OH (ca. 25 mL), the reactor temperature
was adjusted to 55 °C, and then substrate (12.5 mmol) was added and
the reaction started. All nitrate salts and the iron salts were used in
hydrated form as commercially available. When necessary, the system
was first evacuated and submitted to gas flux (i.e., dioxygen) at room
temperature and then pressurized to 0.10 MPa. Careful optimization
of the stirring rate excluded the possibility of controlling by diffusion.
Scheme 1. Main oxidation products of β-pinene by dioxygen in CH3OH solutions con-
taining Fe(NO3)3 in the presence or absence of palladium.
2.3. Reaction monitoring and product identification
with dioxygen in the presence or absence of Pd(OAc)2, were rigorously
the same.
Reactions were monitored by analyzing aliquots taken at regular
time intervals by gas chromatography (Varian 450 instrument, FID,
Carbowax 20 M capillary column). Conversions were estimated from
the corresponding peak areas in comparison with the corresponding
calibrating curves.
In general, the products were identified on a Shimadzu MS-QP
5050A mass spectrometer instrument operating at 70 eV, coupled
with a Shimadzu 17A GC, by comparing the corresponding retention
times with those of authentic samples and by inspecting the mass spec-
tra. The procedures to purification by silica column as well as the data
of NMR and FT-IR spectroscopy analyses of two main products are in
the supplementary material.
Although of Pd(OAc)2 catalyst has remained stable throughout reac-
tions (i.e., no Pd(0) formation was detected) (Exps. 1–3, Table 1), it was
inactive in the β-pinene oxidation. Actually, the highest conversion and
oxidation selectivity were reached in the only in the reactions where
Fe(NO3)3 was present, regardless of palladium presence. Thus, it can
be concluded that Pd(OAc)2 was an inactive catalyst in the β-pinene
oxidation by dioxygen in CH3OH. Moreover, regardless of the co-
oxidants concentration, only Fe(NO3)3 was an effective catalyst.
The major product of β-pinene oxidation was isolated and identi-
fied by 1H and 13C NMR spectroscopy as myrtenol methyl ether (1a),
a product previously obtained by us via Pd(II)-catalyzed oxidations
(Scheme 1) [15].
Remarkably, among the nitrate salts employed, only Fe(NO3)3 pro-
moted the oxidation of β-pinene by dioxygen. It should be noted that
the possibility of stoichiometric oxidation of β-pinene by NO−3 anions
is discarded because both LiNO3 and Cu(NO3)2 failed in the same reac-
tion, even when used in stoichiometric proportions (i.e., considering
the total reduction of NO−3 ions, entries 5 and 7, Table 1). Thus, it can
be concluded that Fe(NO3)3 salt truly acted as a catalyst in β-pinene
oxidation by dioxygen.
Beyond promoting β-pinene oxidation by dioxygen, Fe(NO3)3 cata-
lyst also provoked its isomerization into two other monoterpenes and
the conversion them into respective ethers (i.e., identified by GC-MS
analyses) (Scheme 2).
3. Results and discussion
3.1. General aspects
Fe(NO3)3 has been used as the reoxidant in the oxidation reactions
of monoterpenes by dioxygen using palladium catalysts [10]. Generally,
CH3COOH was used as solvent because the Pd(0) oxidation into Pd(II)
occurs more quickly in protic solvent. However, oxidation selectivity is
generally compromised by the solvent's acidity, which provokes con-
current reactions, such as carbon skeletal rearrangement followed by
nucleophilic addition, lowering the yield of oxidation products [9,10].
For these reasons, the first objective herein was investigating the effect
of CH3COOH replacement by CH3OH, in Pd(OAc)2/Fe(NO3)3-catalyzed
monoterpene oxidation by dioxygen.
3.3. Effect of Fe(NO3)3 catalyst concentration on the oxidation of β-pinene
3.2. β-Pinene oxidation by dioxygen in Pd(OAc)2/M(NO3)n/CH3OH
(M = Li (I), Cu(II) or Fe(III); n = 1, 2 or 3) system: Fe(III) cations
as the real catalysts
The discovery of Fe(NO3)3-catalyzed β-pinene oxidation by dioxygen
was extremely welcome because the Pd(OAc)2 is much more expensive
than Fe(NO3)3. The Fe(NO3)3 concentration effects on the conversion
and selectivity of β-pinene oxidation were assessed in a broad range of
concentrations (i.e., 0.010–1.200 mmol). The main results are summa-
rized in the Table 2.
As a general tendency, it can be observed that either conversion,
as well as the selectivity obtained from the oxidation of β-pinene
Table 1
β-pinene oxidation by dioxygen in Pd(OAc)2/M(NO3)n (M = Li(I), Cu(II) or Fe(III); n = 1, 2, or 3) system.a
Exp.
Nitrate
Concentration (mmol)
Conversion (%)
Products selectivity (%)
Isomersb
Ethersc
Myrtenol methyl ether
62
Othersd
1e
2e
3e
4
5
6
LiNO3
3.6
1.8
1.2
3.6
6.25
1.8
3.125
1.2
b3
b3
97
b3
b3
b3
3
100
100
6
100
100
100
100
5
Cu(NO3)2
Fe(NO3)3
LiNO3
12
20
20
Cu(NO3)2
Fe(NO3)3
7
8
97
13
62
a
Reaction conditions: β-pinene (12.5 mmol); methyl alcohol (25 mL); 55 °C; dioxygen (0.10 MPa); 8 hour reaction.
α and γ-terpinenes were formed in almost equimolar amounts.
α and γ-terpinyl methyl ethers were formed in almost equimolar amounts.
Complex mixture of non-identified minority products.
b
c
d
e
Reaction containing Pd(OAc)2 (0.010 mmol); no Pd(0) species were formed.

F.G. Ferreira de Paula et al. / Catalysis Communications 42 (2013) 129–133
131
Scheme 2. Isomerization and oxidation products obtained in the Fe(NO3)3-catalyzed β-pinene oxidation reactions by dioxygen.
Actually, a novel oxidation process based on the use of Fe(NO3)3 in
catalytic amounts was developed (Table 2). The Fe(NO3)3 catalyst was
noticeably active; the highest TON and TOF were achieved (ca. 476
and 162 h−1, respectively; entry 1, Table 2). The results suggest that
this catalyst is more effective than other transition metal salt catalysts
described in literature [16]. The TOF displayed in the Table 2 was calcu-
lated to take into account the conversion reached within the first hour
reaction; this is because, after this period, the decrease of substrate con-
centration affects the reaction rate.
Kinetic curves shown in the Fig. 1 reveal that a lowering on catalyst
concentration resulted in a reduction of reaction rate, which reduced
the final conversion in the reaction period studied herein. However,
high TON values were achieved if compared to the other metal-
catalyzed aerobic oxidation reactions [15].
oxidation by dioxygen. In addition, another auspicious result is that
the catalytic activity of Fe(III) cations was not affected when air was
the oxidant. Thus, this oxidative process is even more attractive from
an economic and environmental viewpoint.
3.5. Effect of iron catalyst nature on the oxidation of β-pinene
In Fig. 2, the kinetic curves obtained in the presence of different iron
catalysts are shown. It is important to note that all of reactions were
performed with the same iron concentration (0.50 mmol).
The results obtained reveals that the oxidation state of iron catalyst
is a key aspect in the iron-catalyzed β-pinene oxidation reactions by
dioxygen. Remarkably, Fe(III) salts were much more efficient catalysts
than Fe(II) one. It was observed that FeCl3 and mainly Fe2(SO4)3 were
the more active catalysts, while the FeSO4 was the least effective.
Only a poor conversion (ca. 8%) after a 6-hour reaction was achieved
in presence of Fe(II) catalyst.
3.4. Effect of gas phase and the source of Fe(III) ions on the oxidation of
β-pinene
3.6. Effect of temperature on the Fe(III)-catalyzed oxidation of β-pinene
The effect of gas phase was assessed using catalytic amounts of
Fe(NO3)3 or Fe(SO4)3 under both inert or oxidant atmosphere (Table 3).
The changes of the gas phase does not impact either conversion or
selectivity of reactions catalyzed by Fe(III) cations. The same effect
was observed in changing the source of Fe(III) ions: Fe(NO3)3 as well
as Fe2(SO4)3 were equally active and selective on β-pinene oxidation
under air or dioxygen (Table 3).
The reactions were performed using a low concentration catalyst
(ca. 0.020 mmol), with the aim of making the effect of temperature
more visible (Table 4).
Notably, under the lowest temperature (ca. 25 °C), the lowest con-
version was reached (ca. 8%); in addition, reaction selectivity shifted
in favor of the isomer formation (ca. 51% selectivity), with a consequent
reduction in the selectivity obtained for ether (1a) (ca. 14%). Conversely,
for temperatures equal to or higher than 35 °C, a sharp increase in the
reaction conversions has been observed; oxidation selectivity was
again favored and similarly to what was described in previous sections,
ether (1a) was again the major product with selectivity in the range
of 60%.
The catalytic activity of Fe2(SO4)3 reinforces the observation made
previously that Fe(III) cations are effective catalysts in the β-pinene
Table 2
Effect of Fe(NO3)3 concentration on the conversion and selectivity of β-pinene oxidation
by dioxygen.a
Exp. Fe(NO3)3 Conv. (%) Selectivity (%)
(mmol)
Isomersb Myrtenol
Ethersc Othd TONe TOFf
(h−1
100
Fe(NO 3)3 mmol
methyl ether
)
0.010
0.020
0.085
0.120
0.250
0.500
0.850
1.200
1
2
3
3
4
5
6
7
8
0.010
0.020
0.030
0.085
0.120
0.250
0.500
0.850
1.200
48
66
70
77
89
91
93
99
97
12
11
9
9
8
59
22
22
22
17
15
15
19
21
20
7
9
10
11
8
7
8
5
5
476
330
236
91
78
39
19
12
8
162
105
60
68
52
30
16
11
8
80
60
40
20
0
58
59
63
69
70
61
61
62
8
12
13
13
a
Reaction conditions: β-pinene (12.5 mmol); methyl alcohol (25 mL); 55 °C; dioxygen
(0.10 MPa); 8 hour reaction.
b
α and γ-terpinenes were formed in almost equimolar amounts.
α and γ-terpinyl methyl ethers were formed in almost equimolar amounts.
Complex mixture of non-identified minority products.
Calculated in relation to Fe(III) cations: TON = [(% myrtenol methyl ether + %
c
d
0
60 120 180 240 300 360 420 480
time (min)
e
epoxides) / 100 × % conv/100] × 12.5/n mmol Fe(III) catalyst.
f
Calculated in relation to the conversion obtained after 1 hour reaction (see
Fig. 2): TOF = [(% myrtenol methyl ether + % epoxides)/100 × % conv (1 h)/100] ×
12.5/n mmol Fe(III) catalyst.
Fig. 1. Effects of Fe(NO3)3 catalyst concentration on the β-pinene oxidation reactions by
dioxygen.

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F.G. Ferreira de Paula et al. / Catalysis Communications 42 (2013) 129–133
70
Table 3
Effect of gas phase on the conversion and selectivity of the Fe(III)-catalyzed β-pinene
oxidation reactions.a
60
50
40
30
20
10
0
Exp. Gas phase Catalyst
Conv. (%) Products selectivity (%)
298 K
308 k
318 K
328 K
Isomersb Myrtenol
methyl ether
Ethersc Othersd
1
2
3
4
O2
Air
O2
Air
Fe(NO3)3 93
Fe(NO3)3 93
Fe2(SO4)3 93
Fe2(SO4)3 92
12e
11
10
10
61
58
62
60
19
19
18
19
8
12
10
11
a
Reaction conditions: β-pinene (7.5 mmol); methyl alcohol (25 mL); Fe(III) catalyst
(0.5 mmol); 8 hour reaction.
b
α and γ-terpinenes were formed in almost equimolar amounts.
α and γ-terpinyl methyl ethers were formed in almost equimolar amounts.
Complex mixture of non-identified minority products.
Calculated in relation to Fe(III) cations.
c
d
e
0
100
200
300
400
500
The kinetic curves (Fig. 3) show that the initial rate was affected by
the variations in the reaction temperature. Notably, within the first
hour, all of the reactions occurred at the highest rates, regardless
of the temperature employed. Nevertheless, after this period, a remark-
able decrease was observed. Although the increasing of conversions
continued, it occurred at a slower rate than the initial reaction period.
time (min)
Fig. 3. Effect of temperature on Fe(NO3)3-catalyzed β-pinene oxidation reaction rate
by dioxygen.a. aReaction conditions: β-pinene (12.5 mmol); Fe(NO3)3 (0.020 mmol);
methyl alcohol (25 mL); dioxygen (0.10 MPa); 8 hour reaction.
The formation of methyl ether (1a) is linked to the use of methyl
alcohol. For this reason, when using another solvent, the formation of
this product was not observed. However, similar to that observed
using methyl alcohol as solvent, the main product obtained in the reac-
tion with ethyl alcohol was also an ether, namely myrtenol ethyl ether,
which was obtained with modest selectivity (46%). The catalyst was
completely insoluble in propyl alcohol and consequently the reaction
was not performed. In CH3CN, the catalyst was partially soluble; the so-
lution became slightly yellow, however, there still remained a part of
solid catalyst. At the end of the reaction, although no ether was obtained,
α-terpineol (15%) was the major product (see footnote Table 5).
The catalyst was completely insoluble in propyl alcohol and conse-
quently the reaction was not performed. In CH3CN, the catalyst was par-
tially soluble (i.e. the colorless pale yellow was observed and a solid
remained in the reaction). At reaction end, although no ether has been
obtained, myrtenic acid (15%) was the major product (see footnote
Table 5).
3.7. Effect of solvent
Aiming to improve the oxidation selectivity, reactions using different
solvents were performed and the results are summarized in Table 5.
100
80
60
Fe 2(SO4)3
40
20
0
FeCl 3
FeSO
4
3.8. Insights into a possible mechanism of Fe(III)-catalyzed oxidation
β-pinene by dioxygen in CH3OH
Iron catalysts containing nitrogen ligands are frequently used for
dioxygen activation in the oxidation reactions [13]. Different mecha-
nisms have been proposed in which is postulated that these reactions
0
60
120
180
240
300
360
time (min)
Table 5
Fig. 2. Effect of iron catalyst nature on the β-pinene oxidation reactions by dioxygen.
Effect of solvent on the conversion and selectivity of the Fe(NO3)3-catalyzed β-pinene
oxidation reactions.a
Exp. Solvent Conv. (%) Products selectivity (%)
Table 4
Isomersb Oxidation Myrtenol alkyl Ethersd Otherse
Products ethers
Effect of temperature on the conversion and selectivity of β-pinene oxidation by
dioxygen.a
Exp. Temperature Conv. Products selectivity (%)
TONd
1
CH3OH 93
C2H5OH 70
12
28
–
–
61
46
–
19
21
–
8
5
(°C)
(%)
2
–
Epoxidesb Myrtenol
methyl ether
Ethers Othersc
3f
4g
C3H7OH
–
–
29c
–
CH3CN 25
46
–
–
25
1
2
3
4
25
35
45
55
8
24
34
66
23
23
23
22
14
51
9
9
12
10
11
9
19
122
174
288
a
Reaction conditions: β-pinene (7.5 mmol); solvent (25 mL); Fe(III) catalyst
58
59
58
(0.5 mmol); 8 hour reaction.
b
α and γ-terpinenes were formed in almost equimolar amounts.
The oxidation products were formed with the selectivity as follow: pinocarveol (9%);
11
c
a
Reaction conditions: β-pinene (12.5 mmol); Fe(NO3)3 (0.020 mmol); methyl alcohol
α-terpineol (15%); myrtenic acid (5%).
d
(25 mL); dioxygen (0.10 MPa); 8 hour reaction.
α and γ-terpinyl methyl ethers were formed in almost equimolar amounts.
Complex mixture of non-identified minority products.
The reaction in C3H7OH was not performed because the catalyst was totally insoluble.
The Fe(NO3)3 catalyst was partially soluble in CH3CN.
b
e
α and γ-terpinenes were formed in almost equimolar amounts.
α and γ-terpinyl methyl ethers were formed in almost equimolar amounts.
Complex mixture of non-identified minority products.
c
f
d
g

F.G. Ferreira de Paula et al. / Catalysis Communications 42 (2013) 129–133
133
Fe3+
CH3O
+
Fe4+OOH
O2 +
CH3OH
+
OCH3
4+
Fe3+
+
H2O
1/2O
+
+
2
Fe OOH
+
+
CH3O
Myrtenol
methylether
-pinene
Scheme 3. Proposed mechanism for Fe(III)-catalyzed oxidation of β-pinene to myrtenol methyl ether by dioxygen.
involve the formation of iron-hydroperoxo adduct between the catalyst
and olefin [13,17]. In addition, it should be highlighted that the large
number of products obtained supports the hypothesis that autoxidation
also takes place in those reactions.
(see Supplementary material) into myrtenol methyl ether or epoxide
derivatives in CH3OH. Notably, the catalytic oxidation of these monoter-
penes was performed in the absence of palladium. High TONs, TOFs,
and selectivity were obtained in oxidation reactions, and assessed at
different conditions. This protocol is a straightforward synthesis method
of monoterpene ethers in a nitrogen ligand-free system, which uses mo-
lecular oxygen as a stoichiometric oxidant, and cheap and commercially
available iron salts as catalysts.
In general, mechanisms involving intermediate species containing
iron cations with high oxidation numbers (i.e., Fe(V) = O) have been
reported [13,16–18]. However, different from the most of the results
described in the literature, the Fe(III)-catalyzed oxidation reactions
described herein occurred in the absence of nitrogen ligands; thus,
we think that Fe(V) = O species are not sufficiently stable to be
formed. Actually, a detailed mechanism of iron-catalyzed oxidation
reactions described herein is not clear at the present stage. However,
mechanisms based on stabilization of Fe(IV) oxo intermediate spe-
cies in Fe(acac)2-catalyzed oxidation by dioxygen in alcohol medium
reactions were recently described [19]. Thus, based on our experi-
mental findings and on the literature, a mechanism could be proposed
(Scheme 3).
Focusing on the formation of ether (1a) as the main oxidation
product, it is concluded that the presence of methoxy group makes the
participation of a solvent obligatory. We suggest that, in the presence
of dioxygen and Fe(III) cations, the CH3OH is converted into highly ac-
tive methoxyl species, which in the presence of Fe(IV)–OOH intermedi-
ates reacts with β-pinene, oxidizing it into ether (1a) and regenerating
Fe(III) cations (Scheme 3). Similar to that described in the literature,
we think that the presence of CH3OH stabilizes the Fe(IV) intermediate
species [19–21].
Acknowledgments
The authors are grateful to CAPES, CNPq, FAPEMIG and FUNARBE
(Brazil).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.08.018.
References
[1] P. Gallezot, Catal. Today 121 (2007) 76–91.
[2] U. Neuenschwander, I. Hermans, J. Catal. 287 (2012) 1–4.
[3] C. Chapuis, D. Jacoby, Appl. Catal. A 221 (2001) 93–117.
[4] Z. Shi, C. Zhang, C. Tanga, N. Jiao, Chem. Soc. Rev. 41 (2012) 3381–3430.
[5] J.A. Gonçalves, M.J. da Silva, D. Pilo-Veloso, O.W. Howarth, E.V. Gusevskaya,
J. Organomet. Chem. 690 (2005) 2996–3003.
[6] C.N. Cornell, M.S. Sigman, Inorg. Chem. 46 (2007) 1903–1907.
[7] J.A. Gonçalves, E.V. Gusevskaya, Appl. Catal. A 258 (2004) 93–98.
[8] S.S. Stahl, Angew. Chem. Int. Ed. 43 (2004) 3400–3420.
[9] M.J. da Silva, E.V. Gusevskaya, J. Mol. Catal. A 176 (2001) 23–27.
[10] M.J. da Silva, D.M. Carari, R.R. Teixeira, J. Organomet. Chem. 694 (2009) 3254–3261.
[11] C.-C. Guo, W.-J. Yang, Y.-L. Mao, J. Mol. Catal. A Chem. 226 (2005) 279–284.
[12] A. Sobkowiak, D. Narog, D.T. Sawyer, J. Mol. Catal. A 159 (2000) 247–256.
[13] D. Narog, A. Szczepanik, A. Sobkowiak, Catal. Lett. 120 (2008) 320–325.
[14] Ned A. Stephenson, Alexis T. Bell, J. Mol. Catal. A Chem. 275 (2007) 54–62.
[15] M.J. da Silva, L.M.M. Vieira, A.A. Oliveira, M.C. Ribeiro, Monatsh. Chem. 144 (2013)
321–332.
It is important to highlight that the allylic radical formed initially
from β-pinene oxidation (which was omitted in the Scheme 3 by
simplification), may be isomerized into a more stable allylic radical
with the carbon skeletal similar to the α-pinene (i.e. where the
unpaired electron occupies the exocyclic allylic position in relation to
internal double bond) [2]. This proposal is supported by the fact that
the same product (1a) was the major oxidation product obtained in
the α-pinene oxidation reactions (see supplementary material).
[16] M.J. da Silva, P.A. Robbles-Dunthefner, L. Menini, E.V. Gusevskaya, J. Mol. Catal. A 201
(2003) 71–77.
[17] M. Costas, K. Chen, L. Que, Coord. Chem. Rev. 517 (2000) 200–202.
[18] I.I. Moiseev, J. Mol. Catal. A 127 (1997) 1–23.
4. Conclusions
[19] B. Li, A.-H. Liu, L.-N. He, Z.-Z. Yang, J. Gao, K.-H. Chen, Green Chem. 14 (2012)
130–135.
[20] M. Selke, M.F. Sisemore, R.J.S. Valentine, J. Mol. Catal. A 117 (1997) 71–82.
[21] J. Yoon, S.A. Wilson, Y.K. Jang, M.S. Seo, K. Nehru, B. Hedman, K.O. Hodgson, E. Bill,
E.I. Solomon, W. Nam, Angew. Chem. Int. Ed. 48 (2009) 1257–1260.
A nitrogen ligand-free protocol was developed for Fe(III)-catalyzed
oxidation of monoterpenes by dioxygen in methyl alcohol. Simple iron
salts (i.e., Fe(NO3)3 or Fe2(SO4)3 hydrate) were highly efficient catalysts
in promoting selective oxidation by dioxygen of β-pinene and α-pinene