Classical and green cyclohexane oxidation catalyzed by manganese porphyrins: Ethanol as solvent and axial ligand

Article abstract of DOI:10.1016/j.poly.2020.114627

The cyclohexane oxidation was revisited in an attempt to make greener this catalytic system. We demonstrated how the use of ethanol could change the catalytic performance of first- and second-generation manganese(III) porphyrins by several oxidants. Altho

Full text of DOI:10.1016/j.poly.2020.114627

Journal Pre-proofs
Classical and green cyclohexane oxidation catalyzed by manganese porphyr‐
ins: ethanol as solvent and axial ligand
Alexandre Moreira Meireles, Dayse Carvalho da Silva Martins
PII:
S0277-5387(20)30284-9
DOI:
Reference:
https://doi.org/10.1016/j.poly.2020.114627
POLY 114627
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28 March 2020
25 May 2020
28 May 2020
Please cite this article as: A. Moreira Meireles, D. Carvalho da Silva Martins, Classical and green cyclohexane
oxidation catalyzed by manganese porphyrins: ethanol as solvent and axial ligand, Polyhedron (2020), doi:
https://doi.org/10.1016/j.poly.2020.114627
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Classical and green cyclohexane oxidation catalyzed by manganese porphyrins: ethanol
as solvent and axial ligand
Alexandre Moreira Meireles and Dayse Carvalho da Silva Martins
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais,
3
1270-901, Belo Horizonte, MG, Brazil.
Correspondent author: Dayse Carvalho da Silva Martins
e-mail: daysequimica@ufmg.br
Declarations of interest: none.
Abstract
The cyclohexane oxidation was revisited in an attempt to make greener this catalytic
system. We demonstrated how the use of ethanol could change the catalytic performance of
first- and second-generation manganese(III) porphyrins by several oxidants. Although the
ethanol is a good solvent to replace the more toxic ones, its excess in the reaction system
could favor the coexistence of different species in diverse equilibriums, including a lot of
inactive species, and the obtaining of low products yields. We also performed the reaction in a
solvent-free system and it was possible to notice the use of a more toxic solvents, as
chlorinated ones, it is not necessary. The use of cocatalysts, as imidazole, could ensure
systems with higher total yields when compared to the reactions in the absence of these
species. A better condition for cyclohexane oxidation should be sought to try as hard as
possible to adhere to the Principles of Green Chemistry and we have shown that the solvent-
free systems and the cocatalyst use could be good options in this attempt.
Keywords: Catalysis; KA oil; axial coordination; ethanol; solvent-free.
1

1
.
Introduction
Alkanes are a class of organic compounds of great industrial interest, which can serve
as raw material for obtaining different oxygenated products [1-4]. For example, the
cyclohexane oxidation could lead to the obtention of cyclohexanol and cyclohexane (KA oil),
important precursors for adipic acid [3], caprolactam and nylon production [5]. However, the
C-H functionalization remains a challenge for scientists, due to the low reactivity of this bond
type [6]. The classic industrial process employs soluble cobalt or manganese salts (or even in
the absence of them) in a homogeneous medium, although high pressure and temperature
have been necessary for the obtention of low cyclohexane conversions [7, 8].
In this context, the utilization of metalloporphyrins catalysts emerges as an alternative.
These compounds can act as biomimetic catalysts of the Cytochrome P450, a family of
enzymes able to activate and to transfer one oxygen atom to the C-H of the alkane, leading to
the desirable oxygenated product, in mild reaction conditions [1, 9, 10]. The first work
employing a synthetic metalloporphyrin in the oxidation of alkanes and alkenes by
iodosylbenzene (PhIO) was reported by Groves and co-workers, in 1979 [11]. 40 years later,
Guo and co-workers published the first industrial-scale process for cyclohexane oxidation by
air, in the presence of simple first-generation manganese(III), iron(III) and cobalt(II)
porphyrins [7].
Since this first report of Groves and co-workers, several modifications were made at
the porphyrinic macrocycle to improve the efficiency and robustness of these compounds in
catalytic reactions, in presence of several oxygen sources (such as O , H O , m-CPBA, t-
2
2
2
BuOOH, NaOCl, and KHSO ) [12-15]. Among the modification types, the introduction of
5
electron-withdrawing groups, such as the halogen atoms, in the meso-aryl positions at the
porphyrinic macrocycle contributed to the development of highly active manganese(III) and
iron(III) porphyrins [12, 16-18].
Simultaneously, some principles have been proposed for the development and
implementation of greener processes, in which ways to reduce produced waste, conserve
energy and replace hazardous substances for non-toxic (or less toxic) reactants are desirable
[19]. In this way, the 12 Principles of Green Chemistry were proposed by Anastas and Warner
and they have been addressed in a lot of studies [20-22].
In this work, we reported the cyclohexane oxidation reactions catalyzed by first- and
second-generation manganese(III) porphyrins (Scheme 01), in which fluorine and chlorine
2

atoms have been introduced in the meso-aryl groups at the porphyrinic macrocycle. Aiming to
investigate the catalytic performance of these metalloporphyrins, the reactions were carried in
the presence of dichloromethane (classic), ethanol, or in a solvent-free (green) system, using
®
iodosylbenzene (PhIO), H O , t-BuOOH, and Oxone as oxidants. Spectrophotometric
2
2
titrations were also carried out to investigate the axial coordination effects of ethanol and
imidazole in the manganese(III) porphyrin.
Scheme 01. Cyclohexane oxidation catalyzed by manganese(III) porphyrins studied in this work.
2
2
.
Experimental
.1.
Materials and Equipment
Dichloromethane (Vetec; HPLC grade), ethanol (Synth; HPLC grade), hydrogen
peroxide (H O ; Neon; 35% wt in water), tert-butyl hydroperoxide (t-BuOOH, Sigma-
2
2
®
Aldrich; 70% wt in water) and Oxone (2KHSO KHSO K SO ; Aldrich; 47%), cyclohexane
5
˙
4˙
2
4
(
Aldrich; 99%), triphenylphosphine (Aldrich; 99%) were utilized without further purification.
Iodosylbenzene (PhIO) was obtained by the basic hydrolysis of iodobenzene diacetate
Aldrich; 98%), by the methodology of Sharefkin and Saltzman [23]. H TPP and H TPFPP
(
2
2
were purchased from Aldrich and purified by chromatography on neutral alumina.
H T2,3DCPP [18] and H T2,6CFPP were obtained by the methodology previously described
2
2
3

in the literature [24]. Mn(III)-porphyrins derived from these free-base porphyrins were
obtained by the methodology described by Adler and co-workers [25].
2
.2.
Equipment
Ultraviolet-visible (UV–Vis) spectra (190–1100 nm) were recorded on HP-8453 A
diode-array spectrophotometer. Cyclohexane oxidation products were analyzed by gas
chromatography in a Shimadzu GC, model 17A, equipped with a capillary column
TM
(
SupelcoWax ; 30,0 m of length x 0,32 mm of diameter x 0,25 μm of film thickness) with
polyethylene glycol as stationary phase and a flame ionization detector (FID). A mixture of
synthetic air and hydrogen was used for the flame and hydrogen was used as the carrier gas.
2
.3.
General procedure for cyclohexane oxidation
The cyclohexane oxidation reactions were carried out in microscale, following the
procedure described by Da Silva and co-workers with modifications. Stock solution of
−
3
-1
−2
-1
catalysts (2.0 × 10 mol L ) and oxidants (2.0 × 10 mol L ) were separately prepared in
®
−6
dichloromethane or ethanol as solvent. In the case of PhIO or Oxone as oxidants (2.0 × 10
mol), their masses were directly measured in reaction flasks. A typical oxidation reaction was
carried out in a glass vial (1.8 mL) equipped with a stirring bar. To the vial were added 100
®
µL of cyclohexane, 100 µL of oxidant (H O or t-BuOOH, or the solid PhIO or Oxone ), and
2
2
1
00 µL of catalyst solutions. The reaction vials were kept under magnetic stirring at 25 °C, in
the presence of light. At the end of the reaction, 50 µL of a bromobenzene solution (5.0 × 10−2
-1
mol L ) and 50 µL of a borax solution were added to the reaction flasks. The reaction
mixtures were analyzed by capillary gas chromatography through comparison to authentic
samples of the products and having bromobenzene as internal standard. The yields were
determined based on the initial mass of oxygen donor. Each reaction was carried out at least
three times and the data reported represent the average of these reactions. Control reactions, in
the absence of either oxidant or catalyst, were performed under the same conditions as the
catalytic runs.
Reactions in the absence of solvent were performed employing the methodology
described by Tôrres and co-workers [16]. Reactions in the presence of cocatalyst (imidazole
or ammonium acetate) were performed in the presence of the solvent (by adding 25 µL
4

-2
-1
aliquot of a 4.0 × 10 mol L cocatalyst solution prepared in the appropriate solvent). All the
reactions were performed in triplicate. The degree of catalyst destruction was determined by
UV-Vis absorption spectroscopy, through the value of the molar absorptivity (ε) of the Soret
band of each catalyst at the end of the reaction. The recovery of porphyrinic catalysts after
reaction cessation was not achieved since the reactions were carried out under microscale
conditions.
2
.4.
General procedure for axial coordination to Mn(III)-porphyrin catalyst
Spectrophotometric titrations of [Mn(T2,6CFPP)Cl] were performed following the
procedure described by Milani et al [26], employing ethanol and imidazole as axial ligands,
separately. Mathematical treatment of the data obtained by the titrations was performed using
the software Stability Quotients from Absorbance Data (SQUAD) [27].
3
3
.0. Results and discussion
.1. Cyclohexane oxidation in dichloromethane as solvent
Cyclohexane has been used as a model substrate for oxidation reactions catalyzed by
metalloporphyrins since 1979 [11]. In general, cyclohexanone (C-one) and cyclohexanol (C-
ol) are obtained as products in the presence of metalloporphyrin catalysts [1, 11]. Initially, we
focus the discussion of the classic systems, i.e. manganese(III) porphyrins in the presence of
PhIO and a chlorinated solvent (dichloromethane). Although most of these systems exhibited
good yields for the oxygenated products [16, 28], oxidation reactions by PhIO present low
nd
atom efficiency, according to the 2 Principle of Green Chemistry [21]. In this work, PhIO
was used to evaluate the catalytic activity of first- and second-generation manganese(III)
porphyrins and the results are exhibited in Table 01.
Control reactions in the absence of the oxidant and presence of any catalyst did not
evidence any product formation (i.e. dissolved molecular oxygen did not act as an oxidant).
Reactions in the absence of any catalyst and presence of PhIO, in the same experimental
conditions, led to the formation of C-ol and C-one in total yield less than 3% (Table 01, entry
1
). In this way, the catalysts are responsible for the product's obtention in the other presented
systems.
5

In the presence of [Mn(TPP)Cl], a first-generation metalloporphyrin catalyst, a total
yield of 27% was obtained, with 67% of selectivity for C-ol (Table 01, entry 2). These results
are slightly higher than those reported by Silva and co-workers [29], and very similar to those
reported by Friedermann and co-workers employing acetonitrile as solvent [30]. Although this
catalyst is a standard for evaluating the catalytic activity of metalloporphyrins, the absence of
any substituent on the periphery of the porphyrinic macrocycle is responsible for the low
observed yields generally in the presence of this first-generation metalloporphyrin catalyst
[9].
Table 01. Cyclohexane oxidation reactions by PhIO and t-BuOOH in dichloromethane as the
solvent, in the presence of manganese(III) porphyrins.
Yield (%)a
C-ol C-one
Selectivityb
Catalyst
Destruction (%)
Entry
Catalyst
Oxidant
c
(%)
1d
2
-
PhIO
PhIO
< 2
18
25
29
nd
3
< 1
9
-
-
[Mn(TPP)Cl]
[Mn(T2,3DCPP)Cl]
[Mn(T2,6CFPP)Cl]
-
67
74
74
-
45
19
23
-
3
PhIO
9
4
PhIO
10
nd
2
5d
6
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
[Mn(TPP)Cl]
[Mn(T2,3DCPP)Cl]
[Mn(T2,6CFPP)Cl]
60
50
50
27
24
27
7
3
3
8
6
6
Reaction conditions: catalyst = 2.00 × 10-7 mol and oxidant (PhIO or t-BuOOH) = 2.00 × 10-6 mol; molar
catalyst:oxidant ratio of 1:10. Magnetic stirring, 25 °C and 90 minutes; nd = not detected.
a
Yields were determined based on the oxidant amount added.
Selectivity for C-ol was determined from the relation: Selectivity = 100 × [%C-ol/(%C-ol + %C-one)].
Catalyst destruction was determined based on the UV–Vis spectra at the end of the reaction, by monitoring the
b
c
Soret band of each catalyst.
d
The reaction was performed in the absence of manganese(III)-porphyrin catalyst.
In an attempt to improve the yields for the oxygenated products, we evaluated the
catalytic activity of second-generation catalysts [Mn(T2,3DCPP)Cl] and [Mn(T2,6CFPP)Cl],
bearing halogen atoms (chlorine or fluorine) in the ortho- and/or meta-positions on the meso-
aryl groups at the porphyrinic macrocycle. The catalyst [Mn(T2,3DCPP)Cl] was firstly
synthesized by our group [18], while the [Mn(T2,6CFPP)Cl] was firstly reported by
Friedermann and co-workers as a catalyst for the cyclohexane oxidation by PhIO in
acetonitrile [30]. As expected, these catalysts exhibited a higher total yield for the C-ol and C-
one (34 and 39%, respectively) when compared to [Mn(TPP)Cl] catalyst (Table 1, entries 3
6

and 4). In oxidation reactions catalyzed by metalloporphyrins, the high-valence active species
V
manganese(V)oxo-porphyrin, [Mn (O)P] is responsible to transfer an oxygen atom from the
oxidant to organic substrate [31]. Furthermore, the reactivity of this species can be modulated
by the substituents on the periphery of the porphyrinic macrocycle [9]. Halogen atoms
introduced at the meso-aryl groups promote an electron-withdrawing effect, making the active
V
species [Mn (O)P] more reactive to transfer the oxygen atom. This observation can be
associated with the increasing yields for the C-ol and C-one products in the systems in which
second-generation [Mn(T2,3DCPP)Cl] and [Mn(T2,6CFPP)Cl] porphyrins were used as
catalysts.
Friedermann and co-workers verified the influence of the halogen atoms at the meso-
III
II
aryl groups through electrochemical studies [30], comparing the Mn /Mn redox potential
E) for halogenated manganese(III) porphyrins. For [Mn(T2,6CFPP)Cl], these authors
(
reported 32% of C-ol, employing acetonitrile as the solvent, in the same conditions studied by
our group. For the second-generation catalyst, an anodic shift of the E (more positive) was
observed when compared to [Mn(TPP)Cl], in acetonitrile solution [30]. Thus, we could
suggest that the same effect occurs in a dichloromethane solution, justifying the results
observed in this work.
rd
As recommended by the 3 Principle of Green Chemistry (Less Hazardous Chemical
Syntheses), reactants should generate byproducts with little or no toxicity to human health and
the environment [21]. In this way, we replaced the oxidant PhIO (which generates the by-
product PhI that could produces hepatic necrosis, lipid peroxidation and necrosis in the
®
animals [32]) by t-BuOOH, H O , and Oxone , since these oxidants lead to less-toxic
2
2
byproducts. Reactions performed in the presence of t-BuOOH exhibited much lower total
yields (up to 12%, Table 01) when compared to the systems with PhIO. The lower yields
observed for these systems can be attributed to the homolytic cleavage of the O-O bond by the
metalloporphyrin catalysts, as proposed by Nam and co-workers for iron(III)
porphyrins/peroxide systems [33]. By homolytic cleavage, radicals are preferentially
V
generated instead of [Mn (O)P], leading to reactions with low selectivity and low yields for
C-ol and C-one products [33]. In this way, we verified the possibility of the formation of
cyclohexyl-hydroperoxide (C-OOH) in this system (which could be responsible for the low
yields for C-ol and C-one), according to the methodology proposed by Shul’pin [34], by the
addition of triphenylphosphine (PPh ) to the reaction media, once this reactant could reduce
3
R-OOH to R-OH. This methodology provides a more reliable analysis of the content of C-ol
7

present in the reaction [34]; however, the total yield generally remained the same that
observed without the addition of PPh , indicating that the cyclohexyl-hydroperoxide have not
3
been formed at the studied systems.
In general, the oxidation by a radical-chain mechanism is a slow process at the room
temperature (~ 30 °C), mainly for alkanes with strong C-H bonds, such as cyclohexane [35].
Thus, we could suggest that the time of the reaction was insufficient to achieve high yields for
the oxygenated products, in the systems with t-BuOOH/CH Cl /Mn(III) porphyrins.
2
2
The imidazole effect was not evaluated in reactions with t-BuOOH, since the
oxidation of imidazole, by this oxidant (generating N-oxide species), seems to be more
favorable than the substrate oxidation [36].
®
Reactions performed in the presence of H O or Oxone not led to the formation of
2
2
any product, probably due to the low solubility of these oxidants in dichloromethane. Systems
with Oxone® were studied by Meunier and co-workers [37], in a mixture of
dichloromethane:water, and even in the presence of a phase transfer catalyst, these authors
reported very low yields for the oxygenated products.
Concerning the catalyst destruction, [Mn(TPP)Cl] exhibited a higher degradation
value when compared to second-generation catalysts in the reactions with PhIO. In fact, the
absence of substituents on the macrocycle periphery has been held responsible for the highest
destruction values for first-generation metalloporphyrins [9]. The lowest values observed for
[Mn(T2,3DCPP)Cl] and [Mn(T2,6CFPP)Cl] degradation can be attributed to the presence of
chlorine and/or fluorine atoms in the ortho- positions of the meso-aryl groups. In t-BuOOH
systems, all the catalysts presented similar degradation values (~ 25%).
3
.2. Cyclohexane oxidation in ethanol as solvent
Most of the cyclohexane oxidation systems already described in the literature
employed non-environmental friendly solvents, such as dichloromethane, chloroform, and
acetonitrile (or a mixture of these solvents) [12, 38-42], which could provide potential risks
from human health and environmental impact. In this way, the replacement of these
conventional organic solvents should be encouraged for resulting in more eco-friendly
processes. As an alternative, ethanol emerges as a renewable feedstock solvent obtained from
sugarcane, widely produced in Brazil [43]. Besides, the use of renewable feedstocks is
th
recommended by the 7 Principle of Green Chemistry (Use of Renewable Feedstocks) [21].
8

Aiming to suppress the use of these non-environmental friendly solvents, we verified
the effect of ethanol in cyclohexane oxidation catalyzed by metalloporphyrins, in the presence
of the same oxidants studied in dichloromethane. Ethanol is a polar protic solvent, in which
metalloporphyrins, substrate, and oxidants present high solubility, ensuring a better
homogeneous media for oxidation reaction when compared to dichloromethane.
Control experiments performed in the absence of manganese(III) porphyrin catalysts
and the presence of oxidants, separately, did not lead to the formation of the products,
indicating that the replacement of dichloromethane by ethanol does not affect the non-
catalyzed reaction course (Table 02, entries 1, 9, and 15).
®
Table 02. Yields of cyclohexane oxidation reactions by PhIO, t-BuOOH, Oxone and H O
,
2
2
in ethanol as the solvent, in the presence of manganese(III) porphyrins
Entry
Yield (%)a
C-ol C-one
Selectivityb
Catalyst
destruction (%)
Catalyst
Oxidant
c
(%)
1d
2
-
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
H O
nd
5
nd
3
-
-
[Mn(TPP)Cl]
63
70
87
79
97
27
23
38
24
48
20
19
-
3
[Mn(T2,3DCPP)Cl]
[Mn(T2,6CFPP)Cl]
[Mn(TPFPP)Cl]
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
-
7
3
4
20
15
36
35
32
nd
< 1
< 1
< 1
< 1
< 1
nd
2
3
5e
6f
7g
8h
9
4
1
0
100
3
92
-
nd
nd
nd
nd
nd
nd
nd
3
2
2
2
2
2
2
2
1
0
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
-
H O
2
-
47
60
52
64
82
-
1
1f
H O
2
-
1
1
2g
3h
H O
2
-
H O
2
-
1
4i
5d
H O
2
-
1
t-BuOOH
t-BuOOH
Oxone®
-
1
6
7
[Mn(T2,6CFPP)Cl]
[Mn(T2,6CFPP)Cl]
40
-
30
20
1
< 1
1
Reaction conditions: catalyst = 2.00 × 10-7 mol and oxidant (PhIO or t-BuOOH) = 2.00 × 10-6 mol; molar
catalyst:oxidant ratio of 1:10. Magnetic stirring, 25 °C and 90 minutes; nd = not detected.
a
Yields were determined based on the oxidant amount added.
Selectivity for C-ol was determined from the relation: Selectivity = 100 × [%C-ol/(%C-ol + %C-one)].
Catalyst destruction was determined based on the UV–Vis spectra at the end of the reaction, by monitoring the
b
c
Soret band of each catalyst.
9

d
e
f
The reaction was performed in the absence of manganese(III)-porphyrin catalyst.
Chlorido-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinatomanganese(III) was used as catalyst.
The reaction was performed at a molar catalyst:oxidant ratio of 1:25.
The reaction was performed at a molar catalyst:oxidant ratio of 1:50.
The reaction was performed at a molar catalyst:oxidant ratio of 1:100.
The reaction was performed at a molar catalyst:oxidant ratio of 1:1000.
g
h
i
We expected the highest yields for reactions with PhIO in ethanol since this oxidant is
more soluble in this solvent when compared to dichloromethane (Table 01), which is justified
by the fast solubilization of the PhIO solid in the polar reactional media. Initially, we could
suggest that ethanol will be acting as a substrate, competing with cyclohexane by the
V
[
Mn (O)P] species. In this way, we performed reactions in the absence of cyclohexane to
verify the formation of some oxygenated products from ethanol. We did not observe the
presence of any peak in the chromatograms, in the analysis conditions employed in this work.
Although the alcohol dehydrogenation could be catalyzed by the active species [16, 44],
primary alcohols with small substituents (such as ethyl or methyl groups) seem to be stable
against this transformation type [44], in the presence of manganese(III)-porphyrins.
Among the catalysts employed in this work, [Mn(T2,6CFPP)Cl] presented the highest
total yield (23%) and selectivity for C-ol production (87%), probably due to the higher
reactivity of the high-valence active species derived from this catalyst. As reported by
Friedermann and co-workers, the catalyst bearing fluorine atoms in all the five positions of
the meso-aryl groups, [Mn(TPFPP)Cl], presented the highest yield for C-ol production (38%,
in acetonitrile) [30]. Thus, we verified the performance of this catalyst in reactions with
ethanol as solvent (Table 02, entry 5). Surprisingly, the total yield obtained by this catalyst
was lower when compared to the [Mn(T2,6CFPP)Cl] (19 and 23%, respectively). In fact, the
III
II
[
Mn(T2,6CFPP)Cl] presented a more positive Mn /Mn potential redox value than the
Mn(TPFPP)Cl] catalyst (0.09 and 0.05 mV, respectively) [30], justifying the highest
[
reactivity of this species. Based on these results, we have chosen to continue the studies using
the [Mn(T2,6CFPP)Cl] as the catalyst, since the highest yields and selectivity for C-ol were
observed in its reaction systems (ethanol and dichloromethane).
Leanord and Lindsay-Smith verified that the oxidation of an alkene in methanolic
solution reached a maximum yield with a higher amount of PhIO [45] (about molar
catalyst:PhIO ratio of 1:200). Thus, we verified the effect of the amount of PhIO in reactions
with ethanol (Table 02). The reactions with a molar catalyst:PhIO ratio of 1:25, 1:50 and
1
:100 exhibited similar yields for the oxygenated products (Table 02, entries 6, 7, and 8,
1
0

respectively), and a higher yield when compared to the reaction with a molar catalyst:oxidant
ratio of 1:10 (Table 02, entry 4). However, we verified the most selective system to the C-ol
formation (100%) when the molar catalyst:PhIO ratio of 1:50 was employed.
Similar to the reported by Leonard and Lindsay-Smith, the increase in the amount of
PhIO does not lead to more efficient systems for obtaining oxidized products. In this way, we
could suggest the occurrence of by-side reactions, such as the solvolysis of PhIO, generating
the iodobenzene ethoxide and diethoxyde - PhI(OH)(OCH CH ) and PhI(OCH CH ) ,
2
3
2
3 2
respectively - as observed for this oxidant in the presence of methanol [45, 46]. Furthermore,
as reported for iron(III)-porphyrins, the excess of iodosylbenzene could facilitate the oxygen
transfer for another PhIO molecule by the high-valence active species, leading to the
iodoxybenzene (PhIO ) formation, an inactive species derived from this oxidant [45].
2
Reactions in the presence of H O led to yields of less than 1%. Initially, we evaluated
2
2
the number of H O additions (one, two or three aliquots, added in 30 min intervals). By the
2
2
obtained yields (data not shown), the number of additions of H O does not influence the
2
2
reaction course. Similar behavior was observed by Karasevich [47], employing iron(III)
porphyrins as catalysts for cyclohexane oxidation. Then, we also verified the effect of the
molar catalyst:H O ratio employed in the reactions, similar to that performed for PhIO
2
2
systems (Table 02, entries 10-14). However, we did not observe any increase in reaction yield
and C-ol selectivity for the studied systems.
The low yields observed for the systems with ethanol/H O could be associated to the
2
2
high catalyst destruction values (Table 02, entries 10-14). For these systems, we could suggest
that the homolytic cleavage of the O-O bond would be facilitated, generating ·OH radicals
responsible for the porphyrinic macrocycle oxidation [33].
The other employed oxidants did not lead to higher total yields in ethanol systems
(Table 02, entry 16 and 17). When t-BuOOH as utilized, C-one was observed as the major
product, although the yields were lower than 6%. In these systems, we could suggest that the
ethanol stabilize the oxidant by forming hydrogen bonds, avoiding the formation of high-
®
valence active species [48]. Reactions performed in the presence of Oxone did not lead to
the formation of any product, as observed for the systems with dichloromethane (Table 02,
®
entry 17). Indeed, Oxone also presented a low solubility in ethanol, avoiding the formation
of high-valence active species in the reactional media.
In relation to the catalyst destruction, the [Mn(T2,6CFPP)Cl] presented the highest
value in the systems with ethanol/PhIO (Entry 4, Table 2), while [Mn(TPP)Cl] and
1
1

[Mn(T2,3DCPP)Cl] catalysts exhibited similar destruction values (Table 02, entries 2 and 3).
In the t-BuOOH/ethanol systems, we could suggest that the radicals generated by the
homolytic cleavage of the oxidant interact preferentially with the unsubstituted meso-aryl
groups of the catalyst, leading to the macrocycle destruction. The presence of chlorine and
fluorine atoms in the structure of the [Mn(T2,6CFPP)Cl] and [Mn(T2,3DCPP)Cl] hindering,
in some extension, the macrocycle destruction by the radicals.
3
.3. Cyclohexane oxidation in ethanol as a solvent in the presence of a cocatalyst
Although the use of auxiliary substances should be avoided wherever as possible (4th
Principle of Green Chemistry: Safer Solvents and Auxiliaries), the reactions catalyzed by
metalloporphyrins have presented a higher efficiency when cocatalysts are added to the
reactions [21]. Some papers already described imidazole as an efficient cocatalyst for systems
with metalloporphyrins [29, 39, 49, 50]. Intending to evaluate the effect of a cocatalyst, we
carried out the cyclohexane oxidation in the same conditions (i.e. dichloromethane or ethanol
as the solvent, and PhIO or H O as the oxidant) in the presence of [Mn(T2,6CFPP)Cl], since
2
2
this catalyst has conducted to the highest total yields.
Initially, we verified the imidazole effect in the systems with dichloromethane, at a 1:5
molar catalyst:imidazole ratio. Silva and co-workers verified the influence of imidazole
coordination for several manganese(III) porphyrins types, for which a total yield of 74%
(associated to 68% of C-ol selectivity) was obtained for the second-generation catalyst
[Mn(T4CMPP)Cl] (bearing carbomethoxyl substituents at para-positions on the meso-aryl
groups), in the same 1:5 molar catalyst:imidazole ratio [29].
In the present work, when imidazole was added at this molar ratio, a total yield of 95%
was achieved, with 86% of C-ol selectivity (Table 03, entry 1). As described by Gunter and
V
Turner [51], when imidazole is coordinated in trans-position to oxygen at [Mn (O)P] species,
an electronic density can be donated to the manganese ion, weakening the Mn=O bond,
making this species more unstable and reactive. The same effect was observed when
ammonium acetate was used instead of imidazole (Table 03, entry 2), leading to 87% of the
total yield.
In contrast to those observations for the reactions in dichloromethane, the cocatalyst
did not present a positive influence in the total yield for the oxygenated products in systems
with ethanol (Table 03, entry 3). Furthermore, in reactions with NH OAc (Table 03, entry 4)
4
1
2

the yields were lower than those observed in the absence of cocatalyst (Table 02, entry 4),
probably due to solvation effects promoted by the solvent into the acetate anion.
Table 03. Yields of cyclohexane oxidation reactions by PhIO or H O catalyzed by
2
2
[Mn(T2,6CFPP)Cl], in the presence of a cocatalyst
a
Selectivityb
Entry
Yield (%)
Cocatalyst
Imidazole
Oxidant
Solvent
C-ol
C-one
13
15
3
(%)
86
83
90
85
98
96
94
-
1
2
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
H O
Dichloromethane
Dichloromethane
Ethanol
82
72
26
11
52
54
46
< 1
2
NH OAc
4
3
Imidazole
4
NH OAc
Ethanol
2
4
5c
6d
7e
8
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
Imidazole
Ethanol
1
Ethanol
2
Ethanol
3
Ethanol
nd
nd
nd
nd
2
2
2
2
2
2
2
9
H O
2
Ethanol
-
1
1
1
0
1
2
H O
2
Ethanol
2
-
H O
2
Ethanol
3
-
H O
2
Ethanol
16
89
Reaction conditions: catalyst = 2.00 × 10-7 mol and oxidant (PhIO or H ) = 2.00 × 10-6 mol; molar
O
2 2
catalyst:oxidant ratio of 1:10. Magnetic stirring, 25 °C and 90 minutes; nd = not detected.
a
Yields were determined based on the oxidant amount added.
Selectivity for C-ol was determined from the relation: Selectivity = 100 × [%C-ol/(%C-ol + %C-one)].
The reaction was performed at a molar catalyst:oxidant ratio of 1:25.
The reaction was performed at a molar catalyst:oxidant ratio of 1:50.
b
c
d
e
The reaction was performed at a molar catalyst:oxidant ratio of 1:100.
Then, we verified the imidazole effect in systems with the highest amount of PhIO
(Table 03, entries 6, 7, and 8). These experiments were performed at the same molar
catalyst:PhIO ratio described for the reactions in the absence of cocatalyst. In these systems,
we observed an increase in the total yields, associated with more selective systems for C-ol
formation. However, these results were lower than those observed for reactions in the
presence of dichloromethane as solvent. The observed increase in selectivity could be justified
by the interaction between the C-ol and the solvent (by hydrogen bond interaction, for
V
example), avoiding the C-ol oxidation by the [Mn (O)P], leading to the C-one production.
1
3

As observed for PhIO systems, imidazole has shown to be more efficient by promoting
an increase in total yields and selectivity for C-ol when compared to NH OAc. Thus, this
4
cocatalyst was chosen for the studies with H O in ethanol (Table 03, entries 8 – 12).
2
2
In our work, the presence of cocatalysts led to an increase of the total yields for the
oxygenated products when compared to the reactions in the absence of these additives,
although the total yields were much lower than those observed in the reactions with PhIO. We
verified significative yields only at the highest 1:1000 molar catalyst:H O ratio. This
2
2
behavior was reported by Battioni and co-workers for imidazole (52% of total yield) [52] and
by Thellend and co-workers [53] for NH OAc and imidazole (52% and 38% of total yield,
4
respectively) as additives, employing higher amounts of H O than that utilized in our work.
2
2
Faria and co-workers [54] also investigated the effect of imidazole in cyclohexane oxidation
by H O and manganese(III) porphyrin, obtaining 25% of C-ol after 24 h of reaction and
2
2
absence of any product when the reaction was performed without imidazole as an additive. As
observed for the reactions in the PhIO-dichloromethane system, the cocatalysts should
V
facilitate the heterolytic cleavage of peroxide, making the high-valence species [Mn (O)P]
the responsible for the oxidation of cyclohexane, avoiding the hydroxyl radical formation by
the homolytic cleavage. Furthermore, imidazole can act as a basic catalyst, capturing a proton
from H O and facilitating its conversion into H O at another reaction step [52].
2
2
2
In systems with imidazole, the catalyst destruction could not be determined due to the
presence of dark green solids at the end of the reaction, indicative of the partial
metalloporphyrin precipitation, visible to the naked eye. Similar behavior was reported by
Silva and co-workers [29], for which the catalyst has precipitated at the 1:5 molar
catalyst:imidazole ratio (the same molar ratio employed in this work). In reactions with
NH OAc, the UV-Vis spectra recorded at the end of the reaction have presented the Soret
4
band of the porphyrin at a different wavelength, preventing a reliable determination of the
catalyst destruction values for this system.
3
.3 Ethanol as an axial ligand during cyclohexane oxidation by manganese porphyrins
It is important to highlight that all reactions in the presence of ethanol exhibited lower
total yields for the oxygenated products when compared to those systems with
dichloromethane as solvent (Table 01). Based on these results, we investigated the effect of
ethanol as an axial ligand for the manganese(III) porphyrin catalyst. Ethanol is a Lewis base
1
4

(with a Donor Number of 32.0; pKa = 15.9) and can interact with a Lewis acid, such as the
manganese(III) ion coordinated to the porphyrinic macrocycle, since the catalyst presents its
axial positions free for coordination.
To verify the coordination effects of ethanol to free axial positions of manganese(III)
porphyrin catalyst (Equations 01 and 02), we carried out spectrophotometric titrations of
[Mn(T2,6CFPP)Cl] in dichloromethane (a non-coordinating solvent), using ethanol as an
axial ligand (L). The data were analyzed by SQUAD software and fitted by the non-linear
least square method. The stability constants are presented in Table 04 and the parameters
[
αdata and Σ(Obsvd.-Calc)2] are within the acceptable limits. The same study was performed to
imidazole-MnP interaction. The titration curves and the output data obtained from the
SQUAD analysis can be seen in the Supplementary Material.
[
MnPCl] + L  [MnPL ]Cl
β = K (Equation 01)
1 1
1
[
MnPL ]Cl + L  [MnPL ]Cl
β = K ⅹ K (Equation 02)
2 1 2
1
2
Table 04. Parameters obtained by the SQUAD program for the spectrophotometric titration of
Mn(T2,6CFPP)Cl] with ethanol or imidazole as ligand.a
[
Ligand
Ethanol
logβ1b Logβ2c
K1
K2
K /K
αdata
Σ(Obsvd.-Calc)2
1.94 x 10-2
7.00 x 10-3
2
1
0.49
3.39
0.63
7.28
2.03
1.57
0.7
2.3
3.65 x 10-3
2.00 x 10-3
Imidazole
2.45 x 103 6.94 x 103
a
The titrations were carried out in dichloromethane at room temperature (~ 25 °C), in triplicate.
b
c
β
β
1
= K
= K
1
.
2
1
2
x K .
The ethanol coordination is associated with low values for K and K in the formation
1
2
of penta- and hexacoordinated species, [Mn(T2,6CFPP)L]Cl (MnPL1) and
Mn(T2,6CFPP)L ]Cl (MnPL ) respectively. Mu and Schultz reported the methanol axial
[
2
2
coordination to [Mn(TPP)Cl] [55], for which the values of K and K (determined by the
1
2
Benesi-Hildebrand method) were similar to the obtained in our work.
The low values observed for K and K could be justified by the weak interaction of
1
2
manganese(III) ion and ethanol molecules. This observation could be justified by the fact of
ethanol can interact with manganese(III) ion only by σ-bonds, due to the absence of p orbitals
in the oxygen atom, and the formation of a strong π-bond among these species is not allowed
[56].
1
5

On the other hand, imidazole is a strong π-donor ligand with a small steric volume,
allowing a more effective superposition of p orbital of the nitrogen atom with the d and d
xz
yz
orbitals of manganese(III) ion [56], which justifies the higher values for K and K (Table 04).
1
2
Furthermore, the introduction of the second imidazole ligand is more favorable when
compared to ethanol coordination. Silva and co-workers verified the imidazole coordination
equilibrium for first-, second- and third-generation manganese(III) porphyrins and reported
lower values for K and K than that observed for [Mn(T2,6CFPP)Cl] [29]. We could suggest
1
2
that the presence of chlorine and fluorine atoms in the ortho-positions of the meso-aryl groups
makes the manganese(III) ion more acid, ensuring a strong interaction between
[Mn(T2,6CFPP)Cl] and imidazole ligand.
In the presence of imidazole as cocatalyst, we observed different maximum total
yields when PhIO was employed with dichloromethane and ethanol (95 and 56%, respectively
in the best conditions). Based on this, we could suggest that the existence of several species in
the reactional medium, since the imidazole and ethanol can be coordinated to the
manganese(III) porphyrin catalyst, generating catalytically inactive species (Scheme 02).
Scheme 02. Possible generated species in the reaction medium in the presence of coordinative species.
To support this supposition, we obtained the distribution curves for the present species
in the [Mn(T2,6CFPP)Cl]/ethanol system by the spectrophotometric titrations data
1
6

(Supplementary Material). In this system, we verified the predominance of MnP(Ethanol)2
species (about 80%) for the ethanol concentration in the reactions. The MnPL (possibly an
2
inactive catalytic species) predominance could justify the low yields obtained for the
oxygenated products. In the presence of imidazole, we could suggest a competition between
the two ligands for the active site of the catalyst, making the formation of high-valence active
species more difficult. Although the values of K and K for imidazole coordination were
1
2
higher than those obtained for ethanol, the excess of this solvent in the reaction medium could
favor the coexistence of different species in diverse equilibriums, including a lot of inactive
species. For the systems with dichloromethane (a non-coordinating solvent), less coordinated
species will be formed at the solution and there will have less inactive species.
3
.4 Cyclohexane oxidation in solvent-free systems
Reactions in the absence of auxiliary substances, such as additives or solvents, are
th
recommended by the 5 Principle of Green Chemistry (Safer Solvents and Auxiliaries) [21].
These systems have as an advantage, the reduction of residues, energy cost (for the removal of
solvents and other substances), and the absence of toxic solvents.
Based on this, some research groups reported the cyclohexane oxidation by
metalloporphyrins and oxygen sources in the absence of solvents [16, 17, 57-59]. In our work,
we carried out the cyclohexane oxidation reactions by PhIO, t-BuOOH, and H O in the
2
2
absence of solvent, employing the [Mn(T2,6CFPP)Cl] as catalyst (Table 05). Control
reactions in the absence of the catalyst and the presence of each oxidant, separately, did not
lead to the formation of any oxygenated product (Table 05, entries 1 and 3). Reactions in the
presence of H O led to the formation of C-ol and C-one in very low yields (<1%,
2
2
respectively). We could suggest that the low solubility of the oxidant in cyclohexane could
prevent the interaction of oxidant and catalyst and, consequently, the formation of the
V
[
Mn (O)P] species, justifying the low yields observed for this reaction.
1
7

Table 05. Solvent-free cyclohexane oxidation reactions by PhIO or t-BuOOH, with
[
Mn(T2,6CFPP)Cl] as the catalyst.
Yield (%)a
Selectivityb
Catalyst destructionc
(%)
Entry
Oxidant
C-ol
C-one
(%)
1d
2
PhIO
PhIO
nd
37
nd
4
nd
2
-
-
95
-
47
-
3d
4
t-BuOOH
t-BuOOH
PhIO
nd
6
60
79
24
72
5e
15
4
Reaction conditions: catalyst = 2.00 × 10-7 mol and oxidant (PhIO or t-BuOOH) = 2.00 × 10-6 mol; molar
catalyst:oxidant ratio of 1:10. Magnetic stirring, 25 °C and 90 minutes; nd = not detected.
a
Yields were determined based on the oxidant amount added.
Selectivity for C-ol was determined from the relation: Selectivity = 100 × [%C-ol/(%C-ol + %C-one)].
Catalyst destruction was determined based on the UV–Vis spectra at the end of the reaction, by monitoring the
b
c
Soret band of each catalyst.
d
The reaction was performed in the absence of manganese(III) porphyrin catalyst.
e
[Mn(TPP)Cl] was employed as a catalyst, in the same conditions utilized in this work. Data are available in
reference [22].
When the reactions were carried out in the presence of PhIO, a total yield of 39% was
achieved, similar to the systems with dichloromethane (Table 01, entry 4); however, the
system in the absence of solvent presented 95% of selectivity for C-ol production, while 74%
was obtained in the same conditions with dichloromethane. Guimarães and co-workers
verified that the cyclohexane oxidation by PhIO and [Mn(TPP)Cl] as a catalyst could lead to a
total yield of 19% and 79% of selectivity for C-ol [22], in the same conditions employed in
this work. The highest values obtained in the reactions with [Mn(T2,6CFPP)Cl] can be
V
attributed to the more reactive [Mn (O)P] promoted by the halogen atoms in the ortho-
positions of the meso-aryl groups. Furthermore, at the end of the reaction, we observed a light
green color in the reaction flask, which indicates that the catalyst and/or oxidant present some
solubility in cyclohexane, favoring the formation of high-valence active species in this
system. The increasing of the selectivity for C-ol (Table 05, entry 2) could be justified by the
absence of solvent, which could favor the interaction between the C-ol and the high-valence
active species for another oxidation to generate the C-one.
Lastly, we verified the t-BuOOH efficiency for the cyclohexane oxidation in solvent-
free systems catalyzed by [Mn(T2,6CFPP)Cl]. Bagchi and Bandyopadhyay reported the
activation of the molecular oxygen (O ) in the presence of t-BuOOH and phosphate ions,
2
leading to alkanes oxidation [60], even in low yields.
1
8

When compared to the systems with ethanol (5% of total yield) or dichloromethane
12% of total yield), we observed that the solvent has very little influence when this oxidant is
(
employed, since for solvent-free reactions only 10% of total yield was achieved. Surprisingly,
one minute after the t-BuOOH addition to substrate and catalyst, it was possible to observe a
dark brown color in the reaction flask, possibly it indicates the formation of high-valence
active species, similar to the dichloromethane/PhIO systems. Furthermore, the C-OOH is
usually the major product in the cyclohexane oxidation with peroxides and the molecular
oxygen [35]. To verify the cyclohexyl-hydroperoxide (C-OOH) formation, the
chromatographic analysis was repeated after the PPh addition. According to the observed
3
results, we could discard the C-OOH formation in this system.
The catalyst destruction was evaluated at the end of the reactions. In PhIO systems,
[Mn(T2,6CFPP)Cl] was destructed in 47%, a higher value when compared to the reactions in
the presence of dichloromethane and ethanol (23 and 27%, respectively). We could suggest
that the absence of any solvent facilitates the interaction between the high-valence active
species and the porphyrinic macrocycle, leading to the catalyst destruction results observed
for this system. The reactions in the presence of t-BuOOH, presented 24% (Table 05, entry 4)
and 22% (data not presented) of catalyst destruction, in the presence or absence of
atmospheric air, respectively. These values were similar to the reactions with dichloromethane
(27%) and ethanol (30%), indicating that the solvent has little influence in the catalyst
stability in reactions with t-BuOOH.
4
.0 Conclusion
In summary, we demonstrated the catalytic performance of first- and second-
generation manganese(III) porphyrins in cyclohexane oxidation by several oxidants. Also, the
axial coordination studies allowed the determination of stability constants for [MnPL ] and
1
[
MnPL ] species, with ethanol or imidazole as ligands.
2
Some considerations could be highlighted about this work: (1) the greener systems
ethanol or solvent-free, in the presence of peroxides) lead to the lowest yields for the
(
oxygenated products, attributed to the slow activation of the radicals generated in the reaction
medium at the room temperature; (2) although the ethanol is a good solvent to replace the
more toxic dichloromethane, the formation of hydrogen bonds between this solvent and the
oxidant could prevent the formation of high-valence active species; (3) when PhIO was
1
9

employed in the solvent-free catalytic reaction, 39% of total yield and a more selective system
95% of C-ol) was obtained, similarly to the reactions with dichloromethane, indicating that
(
the use of a more toxic solvent it is not necessary; (4) even it not been recommended by the
Principles of Green Chemistry, the use of cocatalysts ensure systems with higher total yields
when compared to the reactions in the absence of these species; (5) also, reactions with
peroxides lead to low yields due to the slow reactivity of the radicals generated, at the room
temperature. In the published literature, the cyclohexane oxidation in greener systems has
been performed in the presence of high pressures (air or O ) or temperatures, leading to
2
systems in which more oxygenated products are obtained associated with a loss of selectivity
for C-ol/C-one.
In this way, to ensure a completely green system for cyclohexane oxidation, catalyzed
by metalloporphyrins, is hard work since it depends on several factors. It is important to study
the better response for each catalyst/solvent/co-catalyst combination which could be most
suitable for application at different oxidation systems.
Acknowledgments
This work was financially supported by the Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior – Brasil (CAPES) - Finance Code 001, by Fundação de Amparo à Pesquisa do
Estado de Minas Gerais (FAPEMIG), Financiadora de Estudos e Projetos (FINEP), and by
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
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[
Classical and green cyclohexane oxidation catalyzed by manganese porphyrins: ethanol
as solvent and axial ligand
Alexandre Moreira Meireles and Dayse Carvalho da Silva Martins
2
3

Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas
Gerais, 31270-901, Belo Horizonte, MG, Brazil.
Correspondent author: Dayse Carvalho da Silva Martins
e-mail: daysequimica@ufmg.br
Highlights
The cyclohexane oxidation was performed in a greener catalytic system.
Ethanol could alter the catalytic performance of Mn(III)-porphyrins.
Solvent-free system has shown to be a good option for cyclohexane oxidation.
Cocatalyst use could be good option to adhere to the Principles of Green Chemistry.
Classical and green cyclohexane oxidation catalyzed by manganese porphyrins: ethanol
as solvent and axial ligand
Alexandre Moreira Meireles and Dayse Carvalho da Silva Martins
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas
Gerais, 31270-901, Belo Horizonte, MG, Brazil.
Correspondent author: Dayse Carvalho da Silva Martins
e-mail: daysequimica@ufmg.br
The Synopsis
The cyclohexane oxidation was catalyzed by manganese porphyrins in a greener catalytic
system. The solvent-free system has shown to be a good option for cyclohexane oxidation and
a cocatalyst use could be a good option to also adhere to the Principles of Green Chemistry.
2
4

Classical and green cyclohexane oxidation catalyzed by manganese porphyrins: ethanol
as solvent and axial ligand
Alexandre Moreira Meireles and Dayse Carvalho da Silva Martins
Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas
Gerais, 31270-901, Belo Horizonte, MG, Brazil.
Correspondent author: Dayse Carvalho da Silva Martins
e-mail: daysequimica@ufmg.br
CRediT author statement
Alexandre Moreira Meireles: Conceptualization, Methodology, Software, Formal analysis,
Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization
Dayse Carvalho da Silva Martins: Conceptualization, Methodology, Software, Formal
analysis, Writing - Review & Editing, Resources, Supervision, Project administration,
Funding acquisition.
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5