Exploring the reaction pathways of Pd(II)-catalyzed cyclohexene oxidation with molecular oxygen: Vinylic and allylic oxidation, disproportionation and oxidative dehydrogenation

Article abstract of DOI:10.1039/c6nj00225k

Palladium(ii) salts are able catalysts to promote different oxidative transformations of cyclohexene in the presence of molecular oxygen: vinylic and allylic oxidation and disproportionation. In this paper, we assessed the main aspects that govern these reactions, by using palladium salts (i.e. PdCl₂, Pd(OAc)₂, Pd(acac)₂ and Pd(CF₃COO)₂) in protic solvents (i.e. CH₃COOH and CH₃OH). When carried out in CH₃COOH solutions at 333 K, the Pd(ii)-catalyzed oxidation reactions preferentially convert cyclohexene to 2-cyclohexenil-1-acetate. Benzene was the secondary product. It was found that the efficiency of the palladium reoxidant followed the trend: Fe(NO₃)₃ > LiNO₃ > CuCl₂ > FeCl₃. Pd(OAc)₂ was the most active catalyst. Replacing Cu(OAc)₂ by Fe(NO₃)₃ notably enhances the conversion of cyclohexene and the selectivity of 2-cyclohexenyl-1-acetate, reducing significantly the reaction time from 22 to 3 h. Conversely, when performed in CH₃OH solutions, the reaction had its selectivity drastically changed; benzene and cyclohexane were the most selectively formed products (i.e. through disproportionation reaction). In this case, the reaction occurred at room temperature and in the absence of a reoxidant. Hence, Pd(CF₃OO)₂ is the most active and selective catalyst. The addition of the Fe(NO₃)₃ reoxidant and increasing the reaction temperature to 328 K resulted in the formation of 1,2-dimethoxycyclohexane and benzene as major products (i.e. allylic oxidation and oxidative dehydrogenation products, respectively). Catalytic cycles for these transformations were proposed based on experimental data and palladium chemistry.

Full text of DOI:10.1039/c6nj00225k

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Journal Name
RSCPublishing
ARTICLE
Exploring the reaction pathways of Pd(II)-catalyzed
cyclohexene oxidation with molecular oxygen: vinylic and
allylic oxidation, disproportionation and oxidative
dehydrogenation
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
M.J. da Silva* and L. Berlinia
DOI: 10.1039/x0xx00000x
Palladium (II) salts are able catalysts to promote different oxidative transformations of the cyclohexene
in the presence of molecular oxygen: vinylic and allylic oxidation and disproportionation. In this paper,
we assessed the main aspects that govern these reactions, by using of palladium salts (i.e. PdCl2,
Pd(OAc)2, Pd(acac)2 and Pd(CF3COO)2 in protic solvents (i.e. CH3COOH and CH3OH). When carried out in
CH3COOH solutions at 333 K, the Pd(II)-catalyzed oxidation reactions preferentially convert cyclohexene
to 2-cyclohexenil-1-acetate. Benzene was the secondary product. It was found that efficiency of
palladium reoxidant followed the trends: Fe(NO3)3> LiNO3> CuCl2> FeCl3. The Pd(OAc)2 was the most
active catalyst. The replacing of Cu(OAc)2 by Fe(NO3)3 notably enhances the conversion of cyclohexene
and selectivity of 2-cyclohexenyl-1-acetate, reducing significantly the reaction time from 22 to 3 h.
Conversely, when performed in CH3OH solutions, the reactions had its selectivity drastically changed;
benzene and cyclohexane were the most selectively formed products (i.e. through disproportionation
reaction). In this case, the reaction occurred at room temperature and in the absence of reoxidant.
Hence, Pd(CF3OO)2 the most active and selective catalyst. The addition of Fe(NO3)3 reoxidant and the
increasing the reaction temperature to 328 K, resulted in the formation of 1,2-dimethoxycyclohexane
and benzene as major products (i.e. allylic oxidation and oxidative dehydrogenation products,
respectively). Catalytic cycles for these transformations were proposed based on experimental data and
palladium chemistry.
www.rsc.org/
Copper(II) chloride is a palladium reoxidant on the Wacker
industrial process (i.e. PdCl2/ CuCl2), where molecular oxygen
Introduction
oxidizes ethylene to acetaldehyde in homogeneous reactions in
aqueous phase [7]. This process requires an extra amount of
chloride ions (i.e. LiCl, HCl), which are corrosive reactants that favor
the formation of chloride products. Moreover, when employed on
oxidation of superior olefins, the CuCl2 reoxidant promotes
undesirable transformations due to its high Lewis acidity,
compromising the oxidation reaction selectivity.
Oxidation of olefins provides commercially important products such
as fragrances, flavor ingredients or attractive molecules for the
pharmaceutical and food industries, and has been goal of numerous
researchers around the world [1]. In this sense, Pd(II) salts are well-
known catalysts widely applied in different oxidative
transformations (i.e. oxidative dehydrogenation, allylic oxidation,
epoxidation) [2,3]. These reactions are synchronized with the
principles of green chemistry and are an attractive synthesis route,
mainly when uses an environmentally benign oxidant and with high
atom economy as molecular oxygen, which produces only water as
by-product [4, 5].
Allylic acetates are highly sought-after intermediates for the
preparation of fragrances, medicines and important synthetic
targets. Bystram et al have assessed the activity of a modified
Wacker system (i.e. Pd(OAc)2/ Cu(OAc)2/ hydroquinone) on
cyclohexene allylic oxidation by O2 in CH3COOH solutions at 323 K
temperature [8]. Those authors verified that cyclohexene was
selectively converted to allyl acetate (ca. 85 %). However, a long
reaction time was vital to achieve a high conversion (ca. 90 %, 22 h).
The metal-catalyzed cyclohexene oxidation by molecular oxygen
can also favor the formation of epoxides or disproportionation
products, depending on the reaction conditions. Several authors
have tried to understand the main aspects that towards the
cyclohexene oxidation selectivity to the allylic, disproportionation
or epoxy products, exploring the using of different metal catalysts
[9-12] For instance, the using of metal porphyrin (i.e. Mn, Cu) as
Molecular oxygen is a highly available, inexpensive and greener
oxidant commonly employed in industrial scale. Nonetheless, the
olefin oxidation by molecular oxygen is a spin forbidden reaction.
Therefore, it always requires a metal catalyst to activate molecular
oxygen and the olefin. However, the palladium oxidation involves
an oxidant reversible, which should be easily recovered by the
oxygen. For these reasons, a lot of attention has been devoted to
the development of efficient aerobic oxidation processes based on
the combination of molecular oxygen, palladium catalysts and
reversible reoxidants [6].
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catalysts have been extensively studied [13]. A drawback of these
We followed the reaction progress through the GC analysis of
catalysts is that generally they involve a laborious synthesis. aliquots collected at regular time intervals. Gas chromatography
Copper-MOFs (i.e. metal organic frameworks) catalysts were also analyses were performed on the Shimadzu 2010 GC instrument
efficient to oxidize cyclohexene to allylic products; nevertheless, a with a FID detector and a CP-WAX capillary chromatographic
long reaction time was required (ca. 15 h) [14].
column (25 m×0.32 mm x 0.30 µm).
In a typical run, catalyst palladium and the reoxidant were
dissolved in a reactor containing dodecane (ca. 0.5 mL) solved in
acetic acid solution (ca. 10 mL). After the pressurization with 0.10
MPa of oxygen and heating at the reaction temperature, the
addition of olefin started the reaction
Our research group has developed selective oxidation processes
of olefins based on clean oxidants and simple catalysts [15].
Previously, we have studied the use of nitrate salts as palladium
reoxidant for reactions of monoterpene oxidation by molecular
oxygen [16,17]. However, among the natural olefins employed as
substrate (i.e. camphene, β-pinene and limonene), only camphene
Products quantification and conversion calculations
was successfully oxidized, suffering
a highly stereoselective
oxidative coupling resulting in a conjugate diene [18]. By your turn,
diene was consecutively oxidized to ketone [19]. These two tandem
transformations occurred in one pot reaction.
Analysis of the GC peak area of substrate through the
calibrating curve provided the reaction conversion. Reaction
conversions were calculated by the Equation 1, where A0
correspond to the GC peak area of the substrate at the beginning
of the reaction, and Acons correspond to the GC peak area of
consumed substrate. The correction factor (i.e. cf) was calculated
using dodecane as an internal standard.
Herein, we have investigated the oxidation by molecular oxygen
of short chain liquid olefins (i.e. 1 and 2-hexene, cyclohexene) in the
presence of Pd(OAc)2/ Fe(NO3)3 catalytic combination. Cyclohexene
is a valuable model platform to understand and improve the
oxidation processes of natural cyclic olefins (i.e. monoterpenes),
which are attractive raw material to synthesize valuable chemicals.
As cyclohexene was not water soluble (i.e. the solvent used in
Wacker reaction), we used CH3OH and CH3COOH solvents, which
allowed assessment the palladium-catalyzed reactions pathway. We
paid special attention to study factors that drive the oxidation
selectivity and to optimizing the main reaction parameters (i.e.,
temperature, concentration of reoxidant and catalyst, reaction
time). The solvent played a pivotal role, being responsible for
driving the reaction selectivity toward allylic oxidation (i.e.
CH3COOH) or disproportionation (i.e. CH3OH). We compared the
activity of different reoxidants in reactions with cyclohexene in
CH3COOH solutions, as well as the effect of nature of anionic ligand
on palladium. Based on literature and experimental data, we
proposed a mechanism to allylic acetoxylation carried out in Pd(II)/
Fe(NO3)3/ O2/ CH3COOH system. Finally, we have investigated the
mechanism of disproportionation suffered by the cyclohexene in
Pd(II)-catalyzed reactions in CH3OH solutions at room temperature
in the absence or presence of reoxidant.
Conversion (%) = A0 - Acons x cf x 100
(Equation 1)
A0
Selectivity of main products was calculated following the
equation 2, where rf correspond to the response factor that was
obtained from calibration curves built with pure products.
Selectivity (%) = Aprod. x rf /Acons (Equation 2)
To determining selectivity (%) of oligomers, which are not
detectable by GC analysis, the equation 3 was used:
Oligomers (%) = Acons - ΣAproduts/ Acons x 100 (Equation 3)
As in the Equation 1, Acons correspond to the GC peak area of
consumed substrate. The Aproduts correspond to the total area of
Σ
GC peaks of the detected products, which were corrected via
calibration curves. However, a small amount of detected products
remained unidentified. Because they were always few significant,
(ca. 3 to 15 % in almost of runs), no correction on GC peak areas
was done. These unidentified products were grouped with the
oligomers and named as “others” in all the Tables.
Experimental
Chemicals
The main products yield % was calculated through Equation 4:
All chemicals and solvents are commercially available and used
without additional purification. The salts palladium PdCl2 (99 %),
Pd(OAc)2 (99 %), Pd(CF3COO)2 (99 %), Pd(acac)2 (99 %) and
cyclohexene (99 %), were purchased from Sigma-Aldrich.
yield (%) = selectivity x conversion /100 (Equation 4)
Products identification
We isolated major products by column chromatography using silica
gel (60G). These products were analyzed on a Shimadzu MS-QP
2010 ultra mass spectrometer instrument operating at 70 eV,
coupled with a Shimadzu 2010 GC.
Catalytic runs
Oxidation runs were carried out under an oxygen atmosphere in a
glass reactor (50 mL) equipped with a magnetic stirrer and sampling
septum, and unit to measure oxygen uptake. After pressurizing the
reactor at 0.10 MPa with oxygen and adjust the temperature to 333
K using an oil bath, we carefully optimize the stirring speed to
prevent the possibility of the diffusion control the reaction rate.
1
The H and 13C NMR spectra were recorded on the Mercury-300
Varian Spectrometer at 300 and 75 MHz respectively, in CDCl3
solution using TMS as internal standard. FT-IR spectroscopy
analyses carried out in Varian 660 FT-IR Spectrometer. The
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6NJ00225K
spectroscopic data of the cyclohex-2-enyl-acetate (Figure 1) are preferentially to the oligomers [22]. Herein, oligomers were formed
summarized as follow:
at 10:1 proportion in relation to the other side-products. Oligomers
and side-products were named as “others” in all the Tables (i.e.
Tables 1 to 7).
2
8
O
7
1
3
4
O
6
Table 1. Effect of Pd(II) catalyst on the cyclohexene oxidation by
5
molecular oxygen
Figure 1. Cyclohex-2-enyl-acetate structure
NMR 1H (300 MHz,CD3OD): 5.94 (dd, CH); 5.67 (dd, CH); 5.19 (m,
CH); 2.01 (s, CH3); 1.83 (m, CH2); 1.72 (m, CH2); 1.66 (m, CH2).
NMR 13C (75 MHz, CD3OD): 171.16 (C7); 132.10 (C2); 125.28 (C3);
68.11 (C1); 27.93 (C6); 24.40 (C4); 19.80 (C8); 18.43 (C5).
Run
Catalyst
Conv. a
(%)
Yielda
(%)
Products selectivitya
(%)
FT-IR) ν
(cm-1): 2937; 1726; 1370; 1265.
1a
1b
0
1a
0
1b
0
oth.d
100
100
18
GC-MS (m/z/ relative intensity): 43/77; 70/29; 79/100; 98/69;
140/6
1b
2c
3
-
39
42
51
30
47
57
0
0
-
0
0
0
Results and discussion
Pd(OAc)2
PdCl2
39
2
3
76
6
6
General aspects
4
0
0
94
Palladium (II) salts are extensively explored catalysts in a number of
organic transformations. The olefin oxidation by molecular oxygen
is a spin-forbidden reaction, and therefore the palladium has the
pivotal role for that the reaction occurs [20,21]. Nonetheless, the
reduced palladium species needs a reversible reoxidant that should
be easily recoverable by the oxygen.
5
Pd(CF3CO2)2
Pd(acac)2
29
12
3
62
21
6
32
6
0
0
79
aConversions, selectivity and yields were determined by gas
chromatography using dodecane as internal standard.
bIn absence of the catalyst and reoxidant.
In this work, our strategy was firstly assessing the catalytic
activity of palladium salts on cyclohexene oxidation reactions by
molecular oxygen in CH3COOH solutions in the presence of different
metal reoxidant. Secondly, we have investigated the Pd(II)-
catalyzed disproportionation of cyclohexene to cyclohexane and
benzene in CH3OH solutions at room temperature.
cFe(NO3)3 (1.2 mol %).
doth. = mixture of oligomers and minority products at 10: 1
proportion (see section experimental).
Pd(acac)2
60
Pd(OAc)2
Effect of palladium catalyst on cyclohexene oxidation by
molecular oxygen
40
20
0
Pd(CF3COO)2
PdCl2
The activity of PdX2 salts (X = Cl-, CH3COO-, CF3COO- and acac) on
the oxidation reactions of cyclohexene by O2 in CH3COOH solutions,
and the main results are summarized in Table 1.
Although weak, the acidity strength of CH3COOH was enough to
promote the cyclohexene oligomerization (runs 1 and 2, Table 1).
However, the presence of the palladium catalysts minimized this
concurrent reaction, favoring the oxidation reaction.
0
30
60
time (min)
90
120
Figure 2. Kinetic curves of Pd(II)-catalyzed cyclohexene oxidation by
molecular oxygena
It is important to note that we do not have adjusted the
reaction conditions to achieve maximum conversion. Our intention
was only to find the most active and selective palladium catalyst.
We verified that while the Pd(acac)2-catalyzed reactions achieved
the highest conversion (Figure 2), the Pd(OAc)2-catalyzed reactions
were the most selective toward the oxidation products (Table 1).
These studies lead us to propose that the role of the anionic
ligand on the palladium is to control which step of the mechanism
will be determining for the formation of oxidation products
aReaction conditions: Pd(II) catalyst (2 mol %); cyclohexene (5
mmol), Fe(NO3)3 (1.2 mol %), temperature (333 K), HOAc (15 mL).
The amount of oligomers was calculated comparing the
consumed area of GC peak of substrate with the sum of
corrected area of GC peak of products detected. In addition,
the use of an internal standard (i.e. dodecane) allowed
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ensuring that no substrate or product was lost throughout the
reaction.
Table 2. Effect of reoxidant on Pd(OAc)2-catalyzed cyclohexene
It was hope that catalysts containing electron withdrawing oxidation by molecular oxygena
ligands on palladium would be more efficient, favoring its
coordination to the olefin. We found that when the palladium is
coordinated to the chloride ion, a less electron withdrawing ligand
-
than acetylacetonate (acac), acetate (i.e. OAc) or trifluoroacetate
(i.e. CF3COO-) ligands, it was the least active catalyst. In terms of
conversion, the reactions in the presence of Pd(CF3COO)2 or
Run Reoxidant Conversiona
Yielda
(%)
Products selectivitya
(%)
Pd(OAc)2 had similar behavior, attaining closer values. On the other
hand, Pd(acac)2-catalyzed reactions achieved highest conversion
(%)
and it was the most active catalyst.
In general, two oxidative transformations occurred throughout
the reactions; allylic oxidation was the major and gave cyclohex-2-
enyl-acetate. Conversely, aromatization of cyclohexene provided
1a 1b 1a
1b
0
0
0
0
0
2
0
-
oth.e
100
100
28
1
-
30
82
57
72
77
57
79
>90
19
37
0
0
0
0
0
0
0
0
0
0
0
1
0
0
benzene as minority product. We will separately discuss these two
reactions (Table 1).
2b
3c
4b
5c
6b
7c
8d
9b
10c
LiNO3
LiNO3
41
0
72
0
Pd(II)-catalyzed cyclohexene allylic oxidation by molecular oxygen
Fe(NO3)3
Fe(NO3)3
Cu(NO3)2
Cu(NO3)2
Cu(OAc)2
FeCl3
100
28
After the screening of palladium catalyst, we have assessed the
behavior reaction in the presence or absence of iron (III) nitrate
reoxidant. In general, the conversions of these reactions were
almost the same (Figure 3 a). Nonetheless, the selectivity was
noticeably different (Figure 3 b).
56
0
72
0
98
oligomers
2-cyclohexenyl-1-acetate
30
77
0
38
85
0
62
benzene
unidentified
50
80
60
40
20
0
Fe(NO3)3
Pd(OAc)2/Fe(NO3)3
blank-reaction
-
40
30
20
10
0
0
3
100
97
FeCl3
0
0
0
20 40 60 80 100 120
time (min)
blank-reaction Fe(NO3)3 Pd(OAc)2/ Fe(NO3)3
aConversions, selectivity and yields were determined by gas
chromatography using dodecane as internal standard.
(b) selectivity
(a) conversion
bReaction conditions: no catalyst; NO3- reoxidant (10.8 mol %).
cPd(OAc)2 catalyst (2 mol %).
dPd(OAc)2 catalyst (5 mol %); Cu(OAc)2 (5 mol %); hydroquinone (10
mol %); conversion after 22 h reaction, data from ref. 8.
eoth. = mixture of oligomers with minority products at 10: 1
proportion (see section experimental).
Figure 3. Conversion (a) and selectivity (b) of cyclohexene oxidation
by molecular oxygena
aReaction conditions: catalyst (2 mol %), cyclohexene (5 mmol),
Fe(NO3)3 (1.2 mol %), temperature (333 K), HOAc (15 mL), 2 h.
The nitrate reoxidant without palladium was not enough to
oxidize the cyclohexene (Table 2). Indeed, regardless action
potential as stoichiometric oxidant, none of the nitrate reoxidant
was efficient to oxidize cyclohexene, even in the presence of
molecular oxygen (ca. 0.10 MPa pressure).
In the absence of Pd(OAc)2, the Fe(NO3)3 reoxidant triggered
undesirable reactions such as oligomerization. Conversely, the
combination Pd(OAc)2/ Fe(NO3)3 was more effective, resulting in
selective formation of the unique oxidation product (i.e. cyclohex-2-
enyl-acetate, Figure 1)
Runs in the absence of palladium catalyst but in the presence of
the reoxidant provided side-products, among which predominated
oligomers (ca. 10:1 proportion in relation to minority products,
Table 2). No oxidation product was obtained using “sole” nitrate; it
is evidence that the formation of oxidation products is as one
palladium mediated-step [23].
The reoxidant had a crucial function on palladium-catalyzed
reactions. When ketone or alcohols are main oxidation products,
nitrate reoxidant may act transferring the oxygen atom of nitrate
group to the olefin [20,21]. Conversely, there are very reactions
where the solvent itself provides the oxygen atom to the olefin,
likewise in the traditional Wacker process, where the water is who
donate oxygen to the ethylene be converted to acetaldehyde [22].
In several cases, the anion on the metal reoxidant is more than
an innocent bystander. The addition of halide or nitrate ions to the
olefin double bond may compromise the oxidation products
selectivity. We investigate this possible effect and main resulted are
in Table 2 and Figure 4.
Grubbs et al have developed a system PdCl2(CH3CN)2/ CuCl2/
AgNO2/ t-BuOOH/ CH3NO2 for aldehyde-selective Wacker-type
oxidation of unbiased alkenes by using of nitrite as co-catalyst at
room temperature and molecular oxygen [23]. Under those
reaction conditions, they verified that possibly a radical mechanism
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DOI: 10.1039/C6NJ00225K
OAc-
H2O
may explain the anti-Markovnikov selectivity for aldehydes of these
reactions. However, herein the major product obtained was the
allylic acetate.
O
O
2+
Pd
O
O
2 HOAc
OAc
-
NO3
1/2 O2
Fe3+
HOAc
OAc-
(i)
others
-
(ii)
90
NO2
(v)
2-cyclohexenyl-1-acetate
oligomers
90
75
60
45
30
15
0
Cu(NO3)2
Fe(NO3)3
75
60
45
30
15
0
LiNO3
FeCl3
OAc
4+
+
4
H
AcO
O
O
O
Pd
Pd
AcO
O
OAc
0
30
60
90
120 150 180
Fe(NO3)3 Cu(NO3)2
FeCl3
LiNO3
time (min)
(iii)
(iv)
(a) conversion
(b) selectivity
+
4
OAc
AcO
AcO
OAc-
Pd
OAc-
Scheme 1. Mechanism proposed for Pd(OAc)2-catayzed oxidative
acetoxylation of cyclohexene in system Fe(NO3)3 / O2 / HOAc [25,26]
Figure 4. Effect of reoxidant on conversion (a) and selectivity (b) of
Pd(OAc)2-catalyzed cyclohexene oxidation by molecular oxygena
aReaction conditions: Pd(OAc)2 (2 mol %), cyclohexene (5 mmol),
NO3- (3.6 mol %), temperature (333 K), HOAc (15 mL), 3 h.
The formation of Pd(IV) intermediate species in reactions of
olefin oxidation is widely reported [25,26]. The oxidation by the
molecular oxygen of reduced nitrite anions quickly recovers the
nitrate anions, mainly when Fe(III) cations are in solution (step (i),
Scheme 1). Therefore, the highest acetate selectivity was achieved
in catalytic reactions in the presence of pair Pd(OAc)2/ Fe(NO3)3,
and can be attributed to the participation of Fe(III) cations on this
step.
Notably, the palladium has a pivotal role as catalyst; however,
the contribution of the nitrate reoxidant was also unquestionable.
The source of nitrate played a key aspect for the conversion rates
and selectivity [22]. A comparison against literature data shows that
replacing of Cu(OAc)2 by M(NO3)n (M = Li+, Cu2+, Fe3+; n = metal nox)
remarkably improved the conversion rate of reaction [8]. We have
found that copper(II) and iron(III) cations are enough able to
accelerate the oxidation of nitrite anions to nitrate by the molecular
oxygen more than lithium(I) cation. These results agree with the
literature [24].
Although the conversion values have been almost identical, the
reactions with Fe(NO3)3 were much more selective for oxidation
than those in the presence of Cu(NO3)2. It is suggestive than iron
cations could be involved in the formation of 1a product.
Nonetheless, when iron(III) cations are in solution in the absence of
nitrate anions (i.e. as FeCl3), the Pd(OAc)2-catalyzed reactions did
not result in the formation of the oxidation product (i.e. 2-
cyclohexenyl acetate).
After olefin coordinate to the Pd(IV) specie (i.e. type-π
intermediate, step iii), this intermediate suffer β-hydrogen shift
reaction to result in the π-allylic intermediate (step iv), Scheme 1).
This proposal is supported by the results of Backvall and Greenberg,
who prompted demonstrated that Pd(II)-catalyzed acetoxylation of
cyclohexene proceed through π-allylic route, preferably to the
oxypalladation route [27]. Finally, after the reductive elimination
step (step v), the 1a product is formed releasing Pd(OAc)2 catalyst.
Pd(OAc)2-catalyzed oxidation of cyclohexene by molecular oxygen
in CH3COOH solutions: Effect of catalyst concentration
Thus, we can conclude than iron cations have a role that is to
promote the nitrate-nitrite redox coupling, which favors the
formation of 1a oxidation product. Based on data experimental we
In general, an increase of the catalyst concentration resulted in
a greater conversion, an indicative that reactions did not achieve
equilibrium (Figure 5).
propose
a mechanism to rationalize the formation of 2-
100
cyclohexenyl acetate in the system Pd(OAc)2/ Fe(NO3)3/ O2/ HOAc
[25,26] (Scheme 1).
3.0 mol %
80
60
40
20
0
2.0 mol %
The transformation of cyclohexene to 2-cyclohexenyl acetate
utilizes as Pd(OAc)2 catalyst in conjunction with Fe(NO3)3 as a redox
co-catalyst. The two acidic hydrogens of the acetate group are
eliminated together with the oxygen atom provided by the nitrate
group. Probably, likewise to the traditional Wacker reaction, where
the solvent provided the oxygen atom to the olefin, the acetyl
group incorporated into product derives from HOAc solvent [22].
We assume that during the first stage of the reaction,
palladium(II) acetate in the presence of nitrate ions undergoes a
step of oxidative addition of the acetate ligand, forming an active
species Pd(IV)-complex (i.e. [Pd(OAc)4], step ii, Scheme 1).
1.0 mol %
0.5 mol %
0
20
40
60
80
100 120 140 160 180
time (min)
Figure 5. Kinetic curves of Pd(OAc)2-catalyzed cyclohexene
oxidation in system Fe(NO3)3/ O2/ HOAca
aReaction conditions: Pd(OAc)2 catalyst, cyclohexene (5 mmol),
Fe(NO3)3 reoxidant (1.2 mol %), 333 K, HOAc (15 mL), 3 h.
The effect of catalyst concentration was assessed in the range
of 0.5 to 3.0 mol % and the results are summarized in Table 3. The
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reaction temperature was lowered to 323 K aiming become more within first hour of run (Figure 5). After this time, the conversion
evident this effect.
remained almost constant. The high oxidation selectivity (ca. 93 %),
the high conversion (ca. 87 %), the using of a biodegradable solvent
and an environmentally benign oxidant (i.e. O2) comprises positive
aspects of Pd(OAc)2/ Fe(NO3)3/ O2/ HOAc system.
Table 3. Effect of Pd(OAc)2 concentration on oxidation of the
cyclohexene by molecular oxygen
Effect of iron(III) nitrate reoxidant concentration on the Pd(OAc)2-
catalyzed cyclohexene oxidation reactions in acetic acid solutions
Thermal heating can decompose the nitrate ions to gaseous
nitrogen oxides mainly in acidic medium reactions. To evaluate this
effect, we have performed reactions with different loads of nitrate
reoxidant (Table 4). The reactions were carried out to 323 K to do
more visible this effect.
Run Pd(OAc)2 Conversiona
Yielda
Products
Selectivitya
(%)c
(mol %)
(%)
Table 4. Effect of Fe(NO3)3 reoxidant concentration on Pd(OAc)2-
catalyzed cyclohexene oxidation by molecular oxygen
1a
1b
0
1a 1b
othb.
1
0
42
58
59
74
87
0
0
0
0
0
0
0
100
49
43
7
2
0.5
1.0
2.0
3.0
29
34
69
53
0
51
57
93
61
Run
Fe(NO3)3
(mol %)
Conversiona
(%)
Yielda
Products
selectivitya
(%)
3
4
5c
0
39
1a
19
40
57
60
72
75
1b
0
1a
46
85
92
90
93
95
1b
1
nib
53
10
5
aConversions, selectivity and yields were determined by gas
chromatography using dodecane as internal standard.
1
2
3
4
5
6
0.6
1.2
2.4
3.0
3.6
4.2
42
47
62
67
77
79
both. = mixture of minority products with oligomers (see section
experimental).
2
5
cFormation of palladium(0) mirror at reaction end.
The selectivity for 1a oxidation product improved when the
palladium concentration was increased until 2 mol % (ca. 93 %).
Nonetheless, when the reactions were carried out with the highest
concentration (ca. 3 mol % of Pd(OAc)2) the 1a selectivity was
decreased from ca. 93 to 61 %.
1
3
0
1
9
0
1
6
This unexpected effect could be explained when we analyzing
the Scheme 1. We supposing that active species are Pd(IV)
intermediates, whose formation depends on the presence of nitrate
ions. Therefore, when we have a high concentration of palladium, it
is possible that some species of Pd(II) (i.e. Pd(OAc)2) are sufficiently
able to directly oxidize the olefin. In this case, the palladium would
then be reduced to Pd(0). So this justifies the formation of Pd(0)
mirror at reaction end (run 5, see footnote Table 3) and,
consequently, the lower 1a selectivity.
0
1
4
aConversions, selectivity and yields were determined by gas
chromatography using dodecane as internal standard.
bni = mixture of minority products with oligomers (see section
experimental).
An increase of reoxidant concentration improved the catalytic
performance of palladium acetate in oxidation reactions. The
highest nitrate ions concentration gave the highest conversion and
selectivity to 1a product. Moreover, the increasing of the reoxidant
concentration increased the initial rate of reactions (Figure 6).
The beneficial effect of increasing concentration of nitrate ions
supports the hypothesis of that they are essential to form the 1a
product, as suggested in the mechanism proposed in Scheme 1. In
an early work, we verified that metal nitrate participates of the rate
determining step, besides palladium catalyst and reactants (i.e.
oxygen and olefin) [17,28].
The profile of kinetic curves showed in Figure 5 suggests that
the conversion rates reach the maximum after the first hour of
reaction, and then substantially remain constant after this initial
period. We concluded that the deactivation of the iron(III) nitrate
reoxidant may are compromising the efficiency of the reaction. We
did not observe the formation of gaseous nitrogen oxides
throughout the reaction. Nevertheless, it is possible that the
reoxidation step of palladium rapidly consumes the Fe(NO3)3 at the
beginning reaction. We will address this point in the next section.
The analysis of kinetic curves is showing that regardless of
catalyst concentration, the reactions achieved high conversion rates
Recently, Sthal and Zultanski demonstrated that different NOx
sources serve as efficient redox mediators for palladium-catalyzed
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6NJ00225K
aerobic acetoxylation of benzene under room pressure of oxygen oligomerization of substrate, which is favored at these conditions.
[28]. In that work, the authors assessed several aspects related to We suppose that the partial decomposition of nitrate ions provoked
the decomposition of nitrite and nitrate anions under different by an increase of temperature may have compromised the
reaction conditions, as well as the participation of Pd(II)/Pd(IV) reoxidation of the palladium catalyst, decreasing thus the
species in recovery steps.
concentration of catalytically active species.
90
80
70
60
50
40
30
20
10
0
Table 5. Effect of temperature on Pd(OAc)2-catalyzed cyclohexene
oxidation by molecular oxygena
4.2 mol %
3.6 mol %
3.0 mol %
2.4 mol %
1.2 mol %
0.6 mol %
Run
Temperature
(K)
Conv.a
(%)
Yielda
Products
selectivitya
(%)
0
30
60
90
120
150
180
time (min)
Figure 6. Effects of Fe(NO3)3 reoxidant concentration on conversion
of Pd(OAc)2-catalyzed oxidations reactions of cyclohexene by
molecular oxygena
1a
1b 1a 1b
oth.b
aReaction conditions: Pd(OAc)2 catalyst (2 mol %), cyclohexene (5
mmol), Fe(NO3)3 reoxidant, 323 K, HOAc (15 mL), 3 h.
1
2
3
4
5
303
313
323
333
343
14
34
47
51
75
13
33
40
39
33
0
0
0
2
0
95
96
85
76
44
0
0
0
4
2
5
Effect of temperature on Pd(OAc)2-catalyzed cyclohexene oxidation
by O2
4
15
20
54
The kinetic curves showed in Figure 7 reveals that an increasing
of temperature enhanced the initial rate of the reactions. In all
runs, the reactions achieved high conversion within the initial
period (ca. 60 min). However, only a little increasing was observed.
It suggests that the reoxidation of the palladium catalyst by the
nitrate anions became less efficient with the reaction progress.
aConversions, selectivity and yields were determined by gas
chromatography using dodecane as internal standard.
both. = mixture of minority products with oligomers (see section
experimental).
80
343 K
70
60
333 K
0,0
-0,1
-0,2
-0,3
-0,4
-0,5
-0,6
-0,7
-0,8
-0,9
R2= 0.99
R2= 0.97
303 K
313 K
-9,2
-9,3
-9,4
-9,5
-9,6
-9,7
-9,8
50
323 K
40
313 K
323 K
333 K
R2= 0.98
R2= 0.96
30
k
20
303 K
343 K
35
R2= 0.99
10
0
5
10
15
20
25
30
40
45
2.90 2.95 3.00 3.05 3.10 3.15 3.20
1/T (10-3
0
30
60
90
120
150
180
time (min)
)
reaction time (min))
(a) First-order plot
(b) Arrenhius plot
Figure 8. Plots of first order (a) and ln [cyclohexene]/
[cyclohexene]initial versus reaction time of cyclohexene oxidation in
Pd(OAc)2/ Fe(NO3)3/ O2 system at different temperatures (Arrhenius
plot) (b)a
Figure 7. Effects of temperature on conversion Pd(OAc)2-catalyzed
oxidations reactions of ciclohexene by molecular oxygena
aReaction conditions: Pd(OAc)2 catalyst (2.0 mol %), cyclohexene (5
mmol), Fe(NO3)3 reoxidant (0.3 mol %), 3 h.
aReaction conditions: Pd(OAc)2 catalyst (2.0 mol %), cyclohexene (5
mmol), Fe(NO3)3 reoxidant (0.3 mol %), 3 h.
On the other hand, while a raise of temperature always
increased the initial rate of reaction, the selectivity of allylic
oxidation products was differently affected. Table 5 shows that
although reactions had achieved lower conversions, the selectivity
for 1a acetate was higher when the reactions were carried out at
low temperatures.
We should emphasize that kinetic measurements were
obtained while the Pd(OAc)2-catalyzed reactions were far from their
equilibrium position, a crucial condition for the attainment of
consistent data.
Although Figure 7 shows the kinetic curves obtained after
monitoring of the reaction throughout 3 h, only the points collected
within the initial period (i.e. 45 minutes of the reaction) at each
reaction were used to build the plots displayed in Figure 8a.
Indeed, heating the reactions to highest temperature (ca. 343 K)
gave the lowest selectivity of 1a acetate. It can be attributed to the
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The Figure 8a confirms a first-order dependence of the reaction cyclohexene in the absence of iron(III) nitrate at room temperature.
rate in relation to cyclohexene concentration. The angular In this section, the Pd(acac)2 catalyst were not studied because it
coefficients of the curves provided the rate constant (k) using the was almost insoluble in the reaction.
pseudo-first-order rate law.
Table 6. Pd(II)-catalyzed cyclohexene disproportionation of
cyclohexene to benzene and cyclohexane in CH3OH solutions under
aerobic conditions in the absence of Fe(NO3)3 at 298 Ka
We have used the k values to build Arrhenius plot presented in
Figure 8b. From angular coefficient (-Ea/ R) obtained by plotting ln k
x 1/ T (i.e. linear Arrhenius equation and the k values determined at
different temperatures), activation energy value (Ea) of Pd(OAc)2-
catalyzed cyclohexene oxidation by molecular oxygen can be
determined.
Although collinear with the other points, the first point
measured at 303 K was excluded by us because of low rate reaction.
However, the high linearity coefficient (R2 = 0.99, omitted by
simplification) confirmed that the Arrhenius plot is acceptable. The
slope of curve showed in Figure 8b gave a value of activation energy
of 16.3 kJ mol-1. There are scarce results in the literature related to
activation energy of palladium-catalyzed olefin oxidation reactions
in acetic acid as solvent, hampering a comparison with other
catalysts or substrates.
Run
Cat.
Conv.a
(%)
Time
until
Pd(0)
TONb
Products
selectivity a
(%)
(min)
an
54
41
60
en
39
21
40
oth.
1
2
3
PdCl2
55
43
68
10
56
1
27
13
35
6
38
0
Palladium-catalyzed disproportionation of cyclohexene to
benzene and cyclohexane in presence of molecular oxygen in
acetic acid solutions
Pd(OAc)2
Pd(CF3CO2)2
The disproportionation of cyclohexene to benzene and
cyclohexane is a reaction studied since long time ago. Trost et al
reported the results of Pd(CF3COO)2-catalyzed disproportionation of
cyclohexene to benzene by using the maleic acid as a hydride
acceptor reactant [29]. Alternatively, others authors have tried to
create reaction conditions wherein cyclohexene may be preferably
dehydrogenate (i.e. providing only benzene), instead to undergo
disproportionation to cyclohexane and benzene [29].
aConversions and selectivity were determined by gas
chromatography using dodecane as internal standard.
bTON calculate as follows: TON = (mmol cyclohexane formed) + 2
(mmol of benzene formed)/ mmol of Pd catalyst.
cane = cyclohexane; ene = benzene; oth. = mixture of minority
products with oligomers (see section experimental).
Previously, Labinger et al evaluated the activity of Pd(CF3COO)2
catalyst by using of proton acceptor sacrifice solvent (i.e. acetone)
[34]. They verified that at room temperature, cyclohexene was
selectively converted to benzene. However, only 25 % conversion
was achieved after 24 h reaction under oxygen pressure (ca.
0.1MPa). They found that under those reaction conditions, the
presence of molecular oxygen inhibited the reaction of
disproportionation [34].
The interesting on this known reaction persists and it has been
assessed in homogeneous or heterogeneous phase (i.e. reactions in
gas or liquid phase) [31-33]. Recently, Iosub and Stahl developed a
Pd(II) catalyzed-system to aerobically dehydrogenate a number of
substituted cyclohexenes, generating the corresponding arene-
derivatives. They have used
a sodium salt (i.e. sodium
anthraquinone-2-sulfonate) as co-catalyst, which improved the
formation of desired products [32].
Herein, we realized that although our solvent is not a hydride
acceptor such as other solvents (i.e. maleic acid, acetone), in
methanol the disproportionation of cyclohexene proceeded
selectively, achieving a molar proportion between cyclohexane and
benzene equal to 1.4, 1.9 and 1.5, in the presence of PdCl2,
Pd(OAc)2 and Pd(CF3COO)2, respectively.
Hronec and Cvengrosova evaluated the conversion of
cyclohexene to benzene and cyclohexane in reactions carried out
over Pd or Pt-solid supported [33]. Those authors investigated this
reaction under high oxygen pressure (ca.
2 to 6 oxygen
atmosphere) and verified that although an increase of conversion
has occurred, only a little amount of products was detected.
As suggested in Scheme 2, we could expect that
disproportionation of cyclohexene to cyclohexane and benzene
could occur giving these products at 2:1 molar ratio. Nonetheless,
the cyclohexene may undergo straight dehydrogenation without
that the hydrogen generated reacts with cyclohexene to produce
Herein, we have investigated the reaction in homogeneous
liquid phase. On this section, benzene (1b) was the main target,
different of the first part of this work, where (1a) product was the
major goal. We have found that replacing of the solvent (i.e. acetic
acid by methanol), avoided the formation of allylic acetate and
favors the disproportionation reaction of cyclohexene to benzene.
As described in literature, we initially carried the reactions
under air and room temperature, slowing its deactivation as black
palladium and extending the action time of catalyst [34]. Table 6
shows the main results of Pd(II)-catalyzed disproportionation of
-
cyclohexane (i.e., it may react with OAc ligand released by the
palladium(II) catalyst). Consequently,
a
lower amount of
cyclohexane will be formed, and a molar ratio lower than 2.0 was
obtained in reactions assessed herein.
8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6NJ00225K
system, reaching 27 % of conversion, 70 % selectivity toward
Wacker oxidation (i.e. cyclohexanone), after 2 h of reaction. In
these reaction conditions, the cyclohexanone initially formed was
consecutively dehydrogenated to 2-cyclohexen-1-one and,
afterwards, through oxypalladation mechanism, the cyclohexanone
was converted to 3-ethoxy-2-cyclohexen-1 one (i.e. an allylic keto-
ether).
Pd(II)
+
H2
+
H2
Pd(0)
Pd(0)
Table 7. Palladium(II)-catalyzed cyclohexene oxidation by molecular
oxygen in methanol solutions in the presence or absence of
Fe(NO3)3 reoxidant
+
H2
Pd(II)
+
H2
Scheme 2. Proposed plausible mechanism for palladium-catalyzed
disproportionation of cyclohexene to cyclohexane and benzene
under aerobic conditions at room temperature methanol solutions
[adapted from refs 29, 34].
Run
Pd(II)
Time Conv.a
Products selectivitya,b
(%)
catalyst
(min)
(%)
It is noteworthy that only the Pd(OAc)2-catalyzed was less
selective toward disproportionation, resulting in oligomers
formation and minority products. Nonetheless, a key aspect is that
even in the absence of palladium reoxidant or molecular oxygen
atmosphere, the reactions take place through the catalytic
pathway. Its means that products were generated at higher
quantity than stoichiometric amount, if we considered the molar
proportion between olefin: palladium. The TON values measured
were 27, 13 and 35 for the PdCl2, Pd(OAc)2 and Pd(CF3COO)2
catalysts, respectively (Table 6).
3-
mc
1,2-
mc
an en
oth.
1c
2
PdCl2
32
120
2
14
50
˂2
32
6
6
1
14
3
4
1
0
7
8
2
15
45
˂1
11
7
61
50
0
PdCl2
3c
4
Pd(OAc)2
Pd(OAc)2
Pd(C2F3O2)
Pd(C2F3O2)
˂1
5
0
120
1
19
58
4
5c
6
Hronec et al suggested that that the disproportionation of
cyclohexene under the Wacker conditions may occur through Pd(0)
species intermediately formed, after the stoichiometric oxidation of
cyclohexene by Pd(II) species. Because there is no reoxidant present
in the reaction, these intermediates were not reoxidized. Thus, they
can partly aggregate and then, similarly to that occur on metallic
surfaces, Pd(0) species can dehydrogenate cyclohexene to 1,2-
cyclohexadiene and afterwards, benzene. The hydrogen species
formed migrates on the surface of Pd(0) and hydrogenates the
cyclohexene to cyclohexane [35].
24 57
34
120
37
5
2
57
aConversions and selectivity were determined by gas
chromatography using dodecane as internal standard.
b3-mc
=
3-methoxycyclohexene;
1,2-dmc
=
1,2-
dimethoxycyclohexane; ane = cyclohexane; ene = benzene; mixture
of minority products with oligomers (see section experimental).
cReaction without Fe(NO3)3 reoxidant; reaction time until the
beginning of the Pd(0) species formation.
Therefore, as reported in the literature, we can suppose that
there was a coupling oxidative between the Pd(II) and Pd(0) species
formed throughout the reaction, which are requires to
dehydrogenate the cyclohexene to benzene (i.e. Pd(II) species) and
afterwards to hydrogenate the cyclohexene to cyclohexane (i.e.
Pd(0) species). Scheme 2 depicts all these transformations.
Those authors did not report the formation of oligomers;
however, it can be presumable that oligomers were the other
products not assigned by them (ca. 30 % of selectivity was not
attributed to any product). In that work, the maximum conversion
of cyclohexene was 27 %, and the focus was the dehydrogenation
products [36].
We also assessed the role of reoxidant Fe(NO3)3 on Pd(II)-
catalyzed cyclohexene disproportionation with molecular oxygen,
performing the reactions at the same previous conditions (i.e.
methanol solutions) (Table 7). Once more, the Pd(acac)2 catalyst
was not evaluated because it was insoluble in the reaction medium.
The activity of PdCl2 catalyst on cyclohexene oxidation by molecular
oxygen in an alcoholic solution containing different reoxidant metal
was studied by Takehira et al [36]. Nonetheless, instead use
In this present work the reactions were carried out with
Fe(NO3)3 in HOAc solutions at 323 K (ca. 3 mol %, run 4, Table 4), or
303 K temperature (ca. 1.2 mol %, run 1, Table 5). The reactions
were highly selective, achieving ca. 95 % to cyclohex-2-enyl-acetate.
When we replaced the acetic acid by methanol, the reaction
pathway undergoes
a
drastic change. In addition to
methanol as solvent, those authors carried out the reactions in disproportionation products, was also formed allylic oxidation
ethanol. They verified that PdCl2-CuCl2 was the best catalytic
product (3-methoxycyloexene) and product of vinylic oxidation (1,2-
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dimethoxycyclohexane). Moreover,
a significant amount of
H2O
-
NO2
oligomers (ca. 29 to 46%) was also obtained (Table 7).
Pd(CF3COO)2
Fe3+
In methanol and in the presence of the Fe(NO3)3 reoxidant, the
reactions between cyclohexene and molecular oxygen had a
behavior that depended of the anionic ligand nature on palladium.
Therefore, each catalyst generated different results. The PdCl2
catalyst was the most stable in the absence of Fe(NO3)3; the Pd(0)
formation was slowest (i.e. 32 minutes, entry 1, Table 7). This high
stability was also observed at room temperature (entry 1, Tables 6).
Moreover, whereas to observed in the absence of Fe(NO3)3,
wherein disproportionation products were preferentially formed
(i.e. entry 2, Table 6), the PdCl2-catalyzed reaction in the presence
of Fe(NO3)3 gave a vinylic oxidation product (i.e. entry 2, Table 7).
Analysis of GC-MS confirmed the formation of 1,2-
dimethoxycyclohexane. A small amount of 3-metoxycyclohexene
(i.e. allylic product) was also obtained.
O2
+ 2 CF3COOH
-
NO3
Pd(0) + 2 CF3COOH
Pd(0) + 2 CF3COOH
-
NO3
H2O
Fe3+
Pd(CF3COO)2
-
NO2
O2
Scheme 4. Proposed plausible mechanism for formation of benzene
through the Pd(CF3COO)2-catalyzed cyclohexene oxidative
dehydrogenation in system Fe(NO3)3/ O2/ CH3OH.
Moiseev and Vargaftik reported that oxidations of ethylene by
PdX2 involves a key stage (i.e. isomerization of the palladium π-
palladium-alkene complex to σ-organopalladium compound, XPd-
CH2CH2OR; R = alkyl (in alcohol), generating CH3CH(OR)2 as the final
product [37]. They supposed that palladium σ-organopalladium
could undergo fast redox decomposition, including the heterolysis
of the σ-Pd-C bond to give the acetal (Scheme 3).
We suppose that although Pd(0) species could be generated
-
during the reaction, they preferentially react with NO3 anions,
being then converted to Pd(II) species. Thus, a quick recovery of
-
reduced palladium species by the NO3 anions minimize the
formation of cyclohexane, which involves Pd(0). The species of
hydrogen formed can thus be oxidized to water (Scheme 4) [35].
Fe(NO3)3/ O2/ 2 HCl/ CH3OH
Conclusions
-Pd(0)
-HCl
PdCl2/CH3OH
PdCl
OCH3
OCH3
PdClOCH3
In this paper, we have assessed the main aspects that govern the
reaction pathways of Pd(II)-catalyzed cyclohexene oxidation with
molecular oxygen. Depending on the solvent, reaction temperature,
and of the presence or absence of reoxidant, the palladium-
catalyzed reactions of cyclohexene and molecular oxygen provided
disproportionation products, oxidative dehydrogenation, vinylic and
allylic oxidation derivatives. The activity of palladium salts catalysts
(i.e. PdCl2, Pd(OAc)2, Pd(acac)2 and Pd(CF3COO)2 was evaluated on
the reactions carried out in protic solvents (i.e. CH3COOH and
CH3OH), at room temperature or 333 K. In CH3COOH solutions at
333 K, the cyclohexene was preferentially converted to cyclohex-2-
enyl-acetate. Benzene was the secondary product. Under these
conditions, it was verified that efficiency of reoxidant followed the
trends: Fe(NO3)3.> LiNO3 > CuCl2 > FeCl3. In these reactions,
Pd(OAc)2 was the most effective catalyst. The replacing of Cu(OAc)2
by Fe(NO3)3 notably improved the conversion and the reaction
selectivity toward cyclohex-2-enyl-acetate, reducing drastically the
reaction time from 22 to 3 h. Conversely, carrying out the reaction
at room temperature and in the absence of reoxidant in CH3OH
solutions, the reaction selectivity was driven toward the benzene
and cyclohexane. In this case, Pd(CF3OO)2 the most active and
selective catalyst. Performing the reactions in CH3OH solutions
containing Fe(NO3)3 at 328 K temperature, the selectivity of the
reaction was shifted toward 1,2-dimethoxycyclohexane and
benzene. We proposed catalytic cycles for all these
transformations, aiming to explain the different selectivity.
OCH3
-HCl
CH3OH
Scheme 3. Proposed plausible mechanism for formation of 1,2-
dimethoxycyclohexane [36,37].
Nonetheless, herein a 1,2-dieter disubstituted alkyl was the
major product. Therefore, we propose
a similar mechanism
involving the same intermediate but with a final step where the
nucleophilic attack of methanol occurs on carbon coordinated to
the palladium (Scheme 3). A similar reaction was reported by Jiang
et al [38]. Using acetic acid as solvent, those authors described that
the combination of Pd(OAc)2 and different additives (i.e. KBr, KI)
provided disubstituted styrene derivative (i.e. 1,2-diacetate styrene)
[38]. However, a long reaction time (24 h) was essential to
complete the reaction.
In methanol solutions, the Pd(OAc)2 was less stable than PdCl2,
even in the presence of reoxidant (entries 3, Tables 6 and 7). The
selectivity was too completely distinct; in addition to the
disproportionation products, the Pd(OAc)2-catalyzed reactions gave
allylic and vinylic oxidation derivatives.
Finally, likewise observed in the lack of reoxidant, the
Pd(CF3OO)2 catalyst was who provided the highest selectivity for
benzene in the presence of Fe(NO3)3. However, only a small amount
of cyclohexane was obtained, different from observed in reactions
without reoxidant (Table 7). Therefore, this reaction can be namely
as a simple oxidative dehydrogenation. The Scheme 4 describes this
distinct behavior.
Acknowledgements
The authors are grateful for the financial support from CAPES,
CNPq, Rede Minaira de Quimica and FAPEMIG (Brazil).
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18 M. J. da Silva, E. V. Gusevskaya, J. Mol. Catal. A, 2001, 176,
Notes and references
23.
aGrupo de Catalise Homogenea e Heterogenea, Departamento
de Química, CCE, Universidade Federal de Viçosa, Viçosa, MG,
Brazil, 36570-900.
19. M. J. da Silva, E. V. Gusevskaya, J. Braz. Chem. Soc., 2003, 14,
83.
*Corresponding
author:
silvamj2003@ufv.br
or
20. J. Guoa, P. Teo, Dalton Transl. 2014, 43, 6952.
silvamj2003@yahoo.com.br; Fax: +55 31 3899 3065, Tel: +55 31
3899 3210.
21. Z. K. Wickens, K. Skakuj, B. Morandi, R. H. Grubbs, J. Am.
Chem. Soc. 2014, 136, 890–893.
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