Biomimetic oxidation of cyclic and linear alkanes: high alcohol selectivity promoted by a novel manganese porphyrin catalyst

Article abstract of DOI:10.1039/c6nj03072f

A novel third-generation metalloporphyrin, chloro-5,10,15,20-tetrakis-(3′-bromine-4′-methoxyphenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(iii), [Mn^IIIBr₁₂T4(-OMe)PPCl], designated as MnBr₁₂Por, was synthesized and characterized. The catalytic activity of the new manganese porphyrin was investigated in the oxidation of cyclohexane, adamantane and n-hexane by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)₂) in comparison to the catalytic activity of the second-generation catalyst [Mn^IIIT4(-OMe)PPCl], designated as MnPor. All yields were based on oxidants. The MnBr₁₂Por/PhIO system led to higher yields of cyclohexane oxidation products (45%) with high selectivity for cyclohexanol (80%) as compared to the MnPor/PhIO system (23% and 77%, respectively). Addition of imidazole to MnPor/PhIO increased the total product yield from 23 to 43%; addition of imidazole to MnBr₁₂Por/PhIO did not alter the total product yield at all. For the MnPor/PhI(OAc)₂ and MnBr₁₂Por/PhI(OAc)₂ systems, addition of imidazole increased the product yields from 19 to 45% and from 35 to 66%, respectively. Addition of water increased the total product yields during cyclohexane and adamantane oxidation for all the studied systems. In all cases, MnBr₁₂Por performed better than MnPor.

Full text of DOI:10.1039/c6nj03072f

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DOI: 10.1039/C6NJ03072F.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Biomimetic oxidation of cyclic and linear alkanes: high alcohol
selectivity promoted by a novel manganese porphyrin catalyst
Vinicius Santos da Silva,a Warleson Cândido dos Santos Vieira,a Alexandre Moreira Meireles,a
Geani Maria Ucoski,b Shirley Nakagaki,b Ynara Marina Idemori,a Gilson DeFreitas-Silvaa
Material
Suplementary.docx
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A novel third-generation metalloporphyrin, chloro-5,10,15,20-tetrakis-(3’-bromine-4’-methoxyphenyl)-2,3,7,8,12,13,17,18-
octabromoporphyrinatomanganese(III), [MnIIIBr12T4(-OMe)PPCl], designated MnBr12Por, was synthesized and
characterized. The catalytic activity of the new manganese porphyrin was investigated in the oxidation of cyclohexane,
adamantane and n-hexane by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2) in comparison to the catalytic
activity of the second-generation catalyst [MnIIIT4(-OMe)PPCl], designated MnPor. All yields were based on oxidants. The
MnBr12Por/PhIO system led to higher yields of cyclohexane oxidation products (45%) with high selectivity for cyclohexanol
(80%) as compared to the MnPor/PhIO system (23% and 77%, respectively). Addition of imidazole to MnPor/PhIO
increased the total product yield from 23 to 43%; addition of imidazole to MnBr12Por/PhIO did not alter the total product
yield at all. For the MnPor/PhI(OAc)2 and MnBr12Por/PhI(OAc)2 systems, addition of imidazole increased product yields
from 19 to 45% and from 35 to 66%, respectively. Addition of water increased total product yields during cyclohexane and
adamantane oxidation for all the studied systems. In all cases, MnBr12Por performed better than MnPor.
www.rsc.org/
16-18
and H2O2);4,
and (4) metalloporphyrin immobilization on
Introduction
different supports (galactodendritic conjugate porphyrin,
layered double hydroxide (LDH) and silica)19,
20
to obtain
Synthetic metalloporphyrins have been extensively
investigated as catalysts in the oxidation of organic substrates
– they can act as biomimetic models of cytochromes P450
during these reactions.1-4 Cytochromes P450 consist of a
protein matrix and a prosthetic group (heme) in which
ironprotoporphyrin IX is axially coordinated to the sulfur of a
cysteine residue.5-7 In vivo, cytochromes P450 catalyze the
oxidation of xenobiotics with wide specificity and regio- and
stereoselectivity.6, 7
biomimetic systems that are as efficient as cytochromes P450.
About 30 years ago, Traylor and Tsuchyia21 were the first to
lower the electron density over the porphyrin ring by
introducing electron-withdrawing groups at the mesoaryl
and/or β-pyrrole positions of the porphyrin ring, to obtain
second- and third-generation metalloporphyrins, respectively.1
In their work,21 the authors used the extremely stable
5,10,15,20-tetrakis(2,6-dichlorophenyl)-2,3,7,8,12,13,17,18-
octabromoporphyrinatoiron(III) chloride to catalyze the
hydroxylation and epoxidation of organic substrates. In the
following years, many authors described the use of third-
generation iron and manganese porphyrins to catalyze the
oxidation of organic substrates by different oxidants.22-24 For
instance, in 1990, Ellis and Lions23 used 5,10,15,20-
tetrakis(2,3,4,5,6-pentafluorophenyl)-2,3,7,8,12,13,17,18-
octabromoporphyrinatoiron(III) chloride to catalyze the
oxidation of isobutane by molecular oxygen, to obtain over
90% selectivity toward the alcohol product and a turnover
number higher than 13,000.
Groves8 designed the first cytochrome P450 biomimetic
model,
the
first-generation
metalloporphyrin
chloro(5,10,15,20-tetraphenylporphyrinato)iron(III),
or
[FeIIITPPCl], and used it to catalyze the oxidation of
cyclohexane, adamantane, and cis-stylbene by iodosylbenzene
(PhIO). Since then, researchers have adopted various
strategies such as (1) introduction of bulky electron-
withdrawing substituents at the peripheral positions of the
porphyrin macrocycle;9-12 (2) use of additives during the
oxidation reactions, such as pyridine and imidazole;12-15 (3) use
of different oxidants (PhIO, iodobenzene diacetate (PhI(OAc)2)
Additives can boost the catalytic efficiency of
metalloporphyrin-based biomimetic systems. For example,
25
nitrogen (imidazole and pyridine)15, and oxygen (H2O and
OH−) bases10, 14, 26 can positively impact total product yield and
selectivity as well as diminish the degree of catalyst
destruction. Gunter and Tuner25 used imidazole to improve
iron and manganese porphyrin-catalyzed oxidation and.
epoxidation of organic substrates.
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Figure 1. Structural representation of the metalloporphyrins synthetized in this work.
Strategies that increase reaction efficiency and selectivity and attempt to improve the catalytic efficiency of the target
generate less environmentally harmful oxidant byproducts are systems.
desirable. Therefore, deciding which oxidant to employ during
the development of biomimetic cytochrome P450 systems is
Experimental
extremely important. In the context of cytochrome P450
mimics, PhIO is a classic oxidant for in vitro reactions. It usually
replaces the oxidants O2/NAD(P)H or H2O2 employed by
cytochrome P450 systems in vivo.27 Groves et al.8 introduced
the use of PhIO in biomimetic systems in 1979. Since then,
PhIO has been the oxidant of choice in most studies on
metalloporphyrins. Although this classic O-donor leads to
direct formation of the active species, its use has caused
inconveniences, such as: it is poorly soluble in most organic
solvents, it is potentially explosive and it undergoes slow but
progressive disproportionation.28 In this scenario, PhI(OAc)2
has emerged as an alternative to PhIO: basic PhI(OAc)2
hydrolysis generates PhIO, not to mention that PhI(OAc)2
meets the principles of green chemistry because it requires
fewer steps to generate PhI(OAc)2 in relation to PhIO28. In
addition, many research groups have used PhI(OAc)2 without
noting significant alterations in total product yield.11, 15, 29
A. Reagents
5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin (H2P, Figure 1)
was purchased from Aldrich Chemical Co, purified on an
column chromatography (Al2O3:CH2Cl2), and characterized by
1
UV-Vis, FTIR and H NMR (Supplementary Material, Figure S2
and S3). Analytical grade CH3OH, CH2Cl2, and CHCl3 were
obtained from Vetec and distilled prior to use.
Dimethylformamide (DMF) was acquired from Vetec. PhIO was
prepared according to a literature procedure,31 stored at −20
oC in a freezer and periodically controlled for purity by
iodometric titration. All the other reagents and solvents were
of analytical grade and were used without further purification,
unless stated otherwise.
B. Equipment
Ultraviolet–visible (UV–Vis) spectra were registered in the
200–800 nm range on an HP-8453A diode-array
spectrophotometer. Infrared (IR) spectra were registered on a
Perkin Elmer spectrometer model BXFTIR; using KBr pellets.
KBr was crushed with a small amount of the samples. Room-
This study has evaluated the catalytic performance of two
manganese porphyrins (MnP, Figure 1) derived from
5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin H2T4-(OMe)PP
(H2P, Figure 1), namely a second- and a third-generation MnP
designated MnPor and MnBr12Por, respectively, in the
oxidation of alkanes (cyclohexane, adamantane, or n-hexane)
by PhIO or PhI(OAc)2. Based on literature reports,13, 25, 30 we
have also added imidazole or water to the reactions in an
1
temperature (25 oC) H NMR spectra were obtained in CDCl3
on a Bruker DPX-400 Advance spectrometer operating at 400
MHz; tetramethylsilane (TMS) was used as the internal
standard. The ESI-MS analyses were conducted on an LCQFleet
(ThermoScientific, San Jose, CA, USA) mass spectrometer
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equipped with an electrospray ionization (ESI) source The new metalloporphyrin MnBr12Por (Figure 1) was
operating in the positive ion mode; CH3OH was used as synthesized according to the procedure used to prepare
solvent.
Electron
paramagnetic
resonance
(EPR) [MnIIIBr9APTPPCl].13 Briefly, MnPor (101.35 mg; 0.12316 mmol)
measurements of the powder materials were accomplished on and liquid bromine (~0.3 mL, 5.6 mmol, 40-fold molar excess)
an EPR BRUKER EMX microX spectrometer (frequency X, band were dissolved in DMF and kept under magnetic stirring at
9.5 GHz), at room temperature and 77
K (liquid N2); ambient temperature for 12 h. Then, distilled water was added
perpendicular microwave polarization X-band was employed. to the system, which led the MnP to precipitate. MnBr12Por
The ultrasound equipment Unique® MaxiClean 1400, 40 kHz, was purified by column chromatography on SiO2; CHCl3/CH3OH
was also employed in the experiments. Cyclic voltammetry was (10:1) was used as eluent. Subsequently, MnBr12Por was
carried out on an Eco Chemie l-Autolab potentiostat; the GPES precipitated with n-hexane, percolated on a Sephadex column,
software was used. The electrochemical cell contained a and eluted with CHCl3. The yield was 65% (141.69 mg;
homemade glassy carbon working electrode, a platinum wire 0.080054 mmol). Elemental analysis. Found: C, 35.5; H, 1.9; N,
counter electrode, and
a homemade Ag/AgCl reference 2.9%. Calc. for C48H24Br12ClMnN4O4.1C6H14: C, 35.0; H, 2.1, N,
electrode. These analysis was carried out according to the 3.0%. UV–Vis (CHCl3), λmax (nm) (log ε): 371 (4.71); 508 (5.09);
procedure used by Silva et al,13 as recommended by IUPAC for 623 (4.11); 670 (4.30). FTIR (Supplementary Material, Figure
measurements in non-aqueous solvents.32
S5) in KBr (cm-1): (1594)
Cβ–Br; (1255) C-O-C; (1175)
δ
C=C; (1289)
δ
porphyrin skeleton;
All the products from the catalytic oxidation reactions was (1272)
ν
ν
ν
OCH3; (1004) Mn-N.
δ
conducted on a Shimadzu GC-17A chromatograph equipped ESI-TOF [MnIIIBr12T4(-OMe)PP]+ (Supplementary Material,
with a flame ionization detector and a Carbowax capillary Figure S6) m/z 1734.07 (100%).
column (measuring 30.0 m × 0.32 mm, with a film thickness of
0.25 µm). The oven temperature program employed for
determination of the oxidation products from cyclohexane
oxidation started at 80 °C. Then, the temperature was
increased to 150 °C at 5°C min-1, which was maintained for 1
min. For analysis of the products originated from adamantane,
the temperature program started at 100 °C, which was
maintained for 1 min, followed by a temperature elevation to
130 °C at 15 °C min-1, maintained for 5 min, and a further
temperature increase to 180 °C at 15 °C min-1, kept for 1 min.
For analysis of the products originated from n-hexane, the
temperature program started at 45 °C, which was maintained
for 4 min, followed by a temperature elevation to 100 °C at 7
°C min-1, kept for 5 min.
C.3 Alkane oxidation reactions
All the catalytic reactions were realized according to da Silva et
al.26 All the catalytic reactions were performed in 2 mL
Wheaton® vials sealed with teflon-faced silicon septa.
Reactions were conducted under magnetic stirring, at 25 °C,
for 90 min. Oxidation was carried out in air; either PhI(OAc)2 or
PhIO were used as oxygen donor. Reaction mixtures comprised
2.0 × 10−4 mmol of the catalyst (MnP); 2.0 × 10−3 mmol of the
oxidant (PhIO or PhI(OAc)2); 100 µL of cyclohexane (0.93
mmol), or 100 µL of n-hexane (0.76 mmol), or 100 µL of
adamantane solution in CH2Cl2 (0.2 mol L-1; 0.02 mmol); and
200 µL of CH2Cl2. The catalyst/oxidant/substrate molar ratio
was 1:10:4650 for cyclohexane, and 1:10:3800 for n-hexane.
As for adamantane, the catalyst/oxidant/substrate molar ratio
was 1:10:100, because adamantane was poorly soluble in
CH2Cl2. When deemed necessary, the reaction was quenched
by addition of sulfite and borax solution44. The reaction
mixtures were analyzed by capillary gas chromatography;
bromobenzene was the internal standard. The retention times
of the products were confirmed by comparison with the
retention times of authentic product samples. The yields were
based on either initial PhIO or PhI(OAc)2. Each reaction was
accomplished at least three times, and the reported data
represent the average of the results of these reactions. Errors
in yields and selectivity were calculated on reactions
reproducibility basis. The degree of MnP destruction
(bleaching) was determined by UV–vis spectroscopy at the end
of the catalytic run. Control reactions were conducted in three
ways: (1) absence of the catalyst, (2) absence of the oxidant
and (3) substitution of the MnP for a manganese salt
(Mn(OAc)2•4H2O). The effect of imidazole was studied by
adding a 10-µL aliquot of an imidazole (Im) 1.0 × 10−2 mol L-1
solution in CH2Cl2 to the reaction medium. To test the effect of
water on cyclohexane and adamantane oxidation, 0.5 µL of
water was added to the reaction mixture. The molar ratio
MnP:Im was 1:0.5, whereas for MnP:H2O it was 1:139.
C. Synthesis of metalloporphyrin catalysts
C.1. Chloro-5,10,15,20-tetrakis(4’-methoxyphenyl)porphyrina-
tomanganese(III) - [MnIIIT4(-OMe)PPCl] or MnPor
MnPor (Fig. 1) was synthesized according to the procedure
used to prepare [MnIIIAPTPPCl],13 with some modifications.
The free-base porphyrin, H2P, (99.66 mg; 0.136 mmol) was
dissolved in
a
minimum volume of CHCl3. Then,
Mn(OAc)2•4H2O (333.0 mg; 1.359 mmol) dissolved in CH3OH
was added, and the system was kept under magnetic stirring
and reflux for 12 h. The porphyrin complex was washed with
distilled water, extracted with CHCl3, and purified by
chromatography on SiO2 column; CHCl3/CH3OH (10:1) was
used as eluent. Yield 92% (102.1 mg; 0.124 mmol). UV–Vis
(CHCl3), λmax (nm) (log ε): 382 (4.69); 480 (4.98); 587 (3.91);
627 (4.09). FTIR (Supplementary Material, Figure S5) in KBr
(cm-1): (1604)
O-C, (1173)
δ
ν
C=C; (1294)
δ
OCH3; (1004)
porphyrin skeleton; (1249)
Mn-N. ESI-TOF [MnIIIT4(-
ν C-
δ
OMe)PP]+ m/z 787.21 (100%).
C.2. Chloro-5,10,15,20-tetrakis(3’-bromo-4’-methoxyphenyl)-
2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III) -
[MnIIIBr12T4(-OMe)PPCl] or MnBr12Por
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orbitals by conformational changes (planar to saddle shape),
which diminished the conjugation of the orbitals.
Results and discussion
π
Consequently, the energy between the frontier orbitals
decreased.39, 41, 42
A. Obtainment of metalloporphyrin catalysts
The second-generation catalyst MnPor was obtained by the
chloroform/methanol method33 and characterized by thin
layer chromatography. When dichloromethane was the eluent,
MnPor remained close to the application point (Rf ~ 0.04),
whereas the free-base porphyrin ran along the plate (Rf ~ 0.6).
Another evidence of metal insertion into H2P was the loss of
fluorescence (red) typical of the free-base porphyrin under
irradiation with ultraviolet light.34 The novel third-generation
catalyst MnBr12Por was prepared according to the method
described by Richards et al.,35 with modifications. The use of
dimethylformamide and excess bromine favored complete
polybromination of the porphyrin macrocycle.35 The activating
character of the methoxy (-OCH3) substituents favored
addition of a bromine atom at the ortho- position relative to -
OCH3. Sankar et al.36 observed the same outcome when they
added 15 or 16 bromine atoms (at the β-pyrrole positions and
the mesoaryl groups) to [ZnIIT3,5DMPP] . In a recent paper by
The infrared vibrational spectra (Supplementary Material,
Figure S5) helped to identify the -OCH3 groups in the structure
of the porphyrin macrocycle and to confirm metal insertion
into the porphyrin ring: the band due to N-H (pyrrole) bond
deformation (966 cm-1) disappeared, and a new band (1004
cm-1) due to axial deformation of the Mn-Npyrrole bond
appeared.43
The analysis of mass spectra of MnPor and MnBr12Por
(Supplementary Material, Figure S6) confirms the proposed
structure because presents the peaks at m/z 787.21 (100%)
and 1734.07 (100%), respectively, associated with loss of the
1
chloride ion. The H NMR (Supplementary Material, Figure S4)
spectrum of the brominated free-base porphyrin H2Br12Por
confirmed that eight and four bromine atoms were introduced
at the
the
β-pyrrole positions and at one ortho- position relative to
OCH3 groups in the mesoaryl substituents of the
−
porphyrin macrocycle, respectively. This is observed because
the introduction of bromine atoms at the two ortho- positions,
relative to the –OCH3, should be hampered by steric
hindrance. In addition, since the aryl group is less reactive than
the β-pyrrole positions, a longer reaction time may be required
for the substitution take place at the two ortho- positions.
The oxidation of organic substrates catalyzed by
metalloporphyrins depends on the oxidation state and redox
potential of the metal ion, which will determine the formation,
stability, and reactivity of the high-valence active species and
will account for oxygen transfer to the substrate.44 Therefore,
we submitted MnBr12Por in the solid state as well as in DMF
solution to Electron Paramagnetic Resonance (EPR) at 77 K
(Figure 3, Frequency X, band 9.5 GHz) by perpendicular
polarization with microwave X-band. Metalloporphyrins
bearing electron-withdrawing bromine atoms at the β-pyrrole
positions should favor lower metal ion oxidation states (e.g.,
Mn(II)).45-47 According to the EPR spectra illustrated in Figure 3,
Mn(II) EPR signals did not emerge in the spectrum of solid-sate
MnBr12Por, suggesting that MnBr12Por only contained Mn(III),
an EPR silent species, in the measurement conditions.48, 49 This
behavior has been observed for other β-halogenated MnP.29
Indeed, the Mn(III) ion is paramagnetic and has four unpaired
d electrons (S=2). Mn(III) typically exhibits pronounced Jahn-
Teller distortion, which results in substantial spin-orbit
coupling.50 The final outcome of this coupling and rapid Mn(III)
spin relaxation processes is the absence of signals in
conventional Mn(III) EPR spectra conducted in the X-band
(microwave field (H1) perpendicular to the static magnetic
field (H0)) even at 77 K,50 even though spin transitions may
our group, [MnIIIT3,5(
−OMe)PPCl] bromination also introduced
bromine atoms at the ortho- position relative to the −OCH3
groups.29
The UV-Vis spectra of MnPor and MnBr12Por (Figure 2) agreed
with the spectra of other MnP described in the literature.9, 10, 26
Comparison of the spectra of H2P and MnPor revealed
alterations in the spectral profile: MnPor displayed fewer Q
bands, and there was a bathochromic shift of the Soret band.
Strong interaction between the orbitals of the porphyrin
macrocycle and the orbitals of the metal could account for
these alterations.37 Comparison of the spectra of MnPor and
Figure 2. UV-Vis absorption spectra of H2Por (3.15
10-5 mol L-1) and MnBr12Por (4.79 10-6 mol L-1) in CH2Cl2.
× ×
10-6 mol L-1), MnPor (1.05
×
MnBr12Por showed further bathochromic shift of the Soret arise during measurements in the parallel mode (H1 ll H0).51
band, which agreed with the characterization of other β-
octabrominated MnP as compared to their non-brominated
counterparts.10,
38-40
The bromine atoms at the β-pyrrole
positions promoted the bathochromic shift of the Soret band:
bromine stabilized the HOMO and LUMO orbitals of the
porphyrin ring by inductive effect and destabilized the HOMO
4 | J. Name., 2012, 00, 1-3
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B. Catalytic studies
Selective oxidation of inert organic molecules, like alkanes,
under mild conditions has been one of the most investigated
topics over the last decades for both practical and economic
reasons.2 Oxidation of cyclic alkanes, such as adamantane and
cyclohexane, is important because their oxidation products are
precursors in various syntheses (1-adamantanol (1-adol), 2-
adamantanol (2-adol), and 2-adamantanone (2-adone))53 and
constitute raw materials for the production of nylon-6 and
nylon-66 (cyclohexanol and cyclohexanone).54 Nevertheless,
the oxidation of cyclohexane is one of the least efficient
industrial processes.55 As for the oxidation of n-hexane,
interest in this process stems from the difficulty in
transforming highly stable linear alkanes into high-value
products.
Figure 3. EPR spectra of MnBr12Por in the solid state (black line) and DMF
solution (red line).
B.1. Oxidation of cyclohexane
The voltammograms in Figure 4 helped to evaluate the
MnII for the
reversibility of the process MnIII + e‒
⇌
The oxidation of cyclohexane by different oxidants catalyzed
by metalloporphyrins generally produces cyclohexanol (Cy-ol)
and cyclohexanone (Cy-one) as majority products and adipic
acid may arise in some systems.56 The catalytic systems
studied here generated Cy-ol and Cy-one only.
brominated MnBr12Por and its non-brominated counterpart
MnPor. The process was quasi-reversible for both MnP – it did
not meet some parameters that characterize reversibility, such
as the difference between the anodic and cathodic potentials,
which was not constant, and the ratio between the anodic and
cathodic peaks and the scanning rate, which were not close to
one.52
Monitoring of the oxidation of cyclohexane along time (Figure
5) suggest that the MnBr12Por/PhIO system produced Cy-ol
faster than the MnPor/PhIO system. The bromine atoms at the
Comparison of the half-wave potentials of the MnIII/MnII
process (Table 1 and Figure 4) revealed a 367 mV anodic shift
for MnBr12Por as compared to MnPor. Introduction of bromine
β
-pyrrole positions of MnBr12Por withdrew electron density
from the porphyrin ring and the metal ion, not to mention that
they modified the macrocycle conformation. Consequently,
the energy level of the LUMO (eg) and HOMO (a2u) orbitals
decreased and increased, respectively, favoring the oxidation
reaction. Indeed, increased energy of the HOMO (a2u) orbitals
favors formation of the high-valence active species MnV(O)P. 49
atoms at the
β-pyrrole positions of the porphyrin ring
destabilized HOMO and LUMO via an inductive effect and
HOMO via conformational changes. As a consequence, the
energy of the LUMO orbitals decreased, favoring reduction.
Thus, brominated MnP are systematically easier to reduce
than their corresponding non-brominated analogue.13, 39, 41, 46
Figure 5. Monitoring of cyclohexanol production along time during the oxidation
of cyclohexane by PhIO catalyzed by MnP in CH2Cl2 under aerobic condition,
MnP/Oxidant/cyclohexane/CH2Cl2 molar ratio
Figure 4. Cyclic voltammograms of MnPor and MnBr12Por (CH2Cl2, [MnIIIP] = 5.0 ×
10-4 mol L-1, [TBABF4] = 0.1 mol L-1, scan rate 0.1 V s-1).
=
1:10:4650:15550, 25 oC,
magnetic stirring, 90 min. of reaction. The yields are based on PhIO.
Table
2 lists the results concerning the oxidation of
Table 1. Potentials of the anodic peak (Epa) and cathodic peak (Epc) and of the half-wave
potential (E1/2) vs Fc+/Fc for Mn(III)/Mn(II) in MnP (in CH2Cl2, 100 mV s-1).
cyclohexane by PhIO catalyzed by the MnP prepared in this
work, all yields were based on oxidant (PhIO or PhI(OAc)2).
MnPor (entry 1) was slightly more efficient than [MnIIITPPCl], a
first-generation metalloporphyrin, that is the classic reference
in studies on the catalytic activity of MnP8 and gives Cy-ol yield
of 14% and selectivity of 56% in the same reaction
Complex
MnPor
Epa (mV)
Epc (mV)
E1/2 (mV)
ΔE1/2 (mV)
−
−
750
368
−
−
930
578
−
−
840
473
-
MnBr12Por
367
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conditions.13 The
−OCH3 group at the para-mesoaryl positions Cy-ol and lower bleaching than MnP/PhIO systems (Table 2,
of MnPor must have anodic shifted the MnIII/MnII redox entries 1 and 2). Distinct active species may have emerged
potential and increased the reactivity of the high-valence during the oxidation reactions conducted in the presence of
active species. A similar behavior emerged after comparison of different oxygen donors for the same substrate, as proposed in
the catalytic activity of MnPor with the activity of a kinetic study reported by Collman et al.58
[MnIIIT3,5DMPPCl] (21% total yield), which bears a
−OCH3 In the presence of PhIO or PhI(OAc)2, the third-generation
group at the meta-mesoaryl positions of the porphyrin ring.29 catalyst MnBr12Por was much more efficient than its second-
Compared to the [MnIIITCMPPCl], the MnP containg groups generation counterpart, MnPor (Table 2). However, in
carbomethoxy (−
COOCH3) in the para-mesoaryl positions,9 presence of PhI(OAc)2 the catalyst MnBr12Por was more
MnPor gave practically the same total product yield. However, destroyed. As MnBr12Por is in a saddle-shaped conformation, it
MnPor was less efficient than metalloporphyrins bearing is more distorted, which due to steric hindrance makes it more
26, 57
groups like
−
Substituents containing nitrogen atoms may coordinate with species react with the catalyst). However, this distortion of the
NH2 in the para-mesoaryl positions.13,
resistant to oxidative destruction (the high-valence active
the metal ion of another MnP complex and weaken the Mn
bond.25
=
O
macrocycle causes a smaller overlap of the π orbitals,
consequently, this compound is more susceptible to be
In an attempt to develop more robust catalytic systems that oxidized. The PhI(OAc)2 may form different oxidant species in
abide by the principles of green chemistry, we evaluated the the reaction medium59, thus the third generation
efficiency of MnPor and MnBr12Por in the oxidation of metalloporphyrin in the presence of this oxidant shows a
cyclohexane by PhI(OAc)2 (Table 2, entries 7 and 8). This higher destruction (bleaching). This behavior is not observed in
oxidant is commercially available and it does not undergoes the presence of PhIO, which is polymeric and insoluble in the
disproportionation, differently of PhIO. Some literature papers solvent used, then this oxidant only favors the formation of
have described that PhI(OAc)2 can efficiently substitute the high valence active species.
PhIO.11, 13, 29 Here, MnP/PhI(OAc)2 systems (Table 2, entries 7
and 8) gave slightly lower product yields, higher selectivity for
Table 2. Product yields obtained during the oxidation of cyclohexane by PhIO or PhI(OAc)2 catalyzed by MnP in CH2Cl2 under aerobic condition, with or without water or imidazole
as additive.
Product Yield (/%)
Cy-ol1,4 Cy-one1
Entry
Catalyst
System
Total Yield (/%)
Select.2 (/%)
TON5
Bleac.3 (/%)
2.3
4.6
5.9
4.5
3.4
5.3
1.9
3.5
4.5
6.6
4.5
4.2
1
2
MnPor
MnBr12Por
MnPor
PhIO
PhIO
18
37
28
34
22
39
14
27
32
50
30
32
5
23
46
40
45
34
53
19
35
45
66
45
42
77
80
70
75
65
74
74
77
71
76
67
77
61
44
44
38
43
40
68
74
67
52
67
55
9
3
PhIO/Im
12
11
12
14
5
4
MnBr12Por
MnPor
PhIO/Im
5
PhIO/H2O
PhIO/H2O
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2/Im
PhI(OAc)2/Im
PhI(OAc)2/H2O
6
MnBr12Por
MnPor
7
8
MnBr12Por
MnPor
8
9
13
16
15
10
10
11
12
MnBr12Por
MnPor
MnBr12Por PhI(OAc)2/H2O
Reaction conditions: [MnP] = 5 × 10-4 mol L-1, [PhIO or PhI(OAc)2] = 5 × 10-3 mol L-1, MnP/Oxidant/cyclohexane/CH2Cl2 molar ratio =
1:10:4650:15550, 25 oC, magnetic stirring, 90 min. of reaction. MnP/Im molar ratio in Reactions 3, 4, 9, and 10 = 1:0.5. MnP/H2O molar
ratio in reactions 5, 6, 11, and 12 = 1:139 (0.5 µL). 1Yields based on the oxidant. Cy-ol: cyclohexanol and Cy-one: cyclohexanone. 2Selectivity
for the alcohol was determined from the relation: Selectivity = 100 × [%Cy-ol / (%Cy-ol + %Cy-one)]. 3The degree of catalyst destruction was
4
calculated from the UV-Vis absorption spectra recorded at the end of the reaction. The maximum error is ± 3% with confidence level of
95%. 5TON = total moles products per mol catalyst.
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13
The higher efficiency of MnBr12Por systems was attributed to In agreement with the results of Silva et al. and Balcells et
the presence of the bromine atoms at the β-pyrrole positions al.,14 addition of water (0.5
ꢀL) to the MnPor/PhIO and
of the macrocycle. These substituents withdrew electron MnBr12Por/PhIO systems (entries 5 and 6) enhanced total
density from the metal center and modified the macrocycle product yield from 23 to 43% and from 44 to 53%,
conformation, increasing the energy level of the HOMO (a2u) respectively) and decreases the degree of catalysts
orbitals and turn the high-valence active species more destruction. Water must have coordinated to an axial position
reactive.22, 49, 59
in the metalloporphyrin, weakening the Mn=O bond and
Comparison of the catalytic activity of MnBr12Por to the activating the high-valence species (MnV(O)P), as described by
activity of other third-generation MnP revealed that Balcells et al.14 Addition of water to the MnPor/PhI(OAc)2 and
MnBr12Por
provided
higher
similar
product
product
yields
yields
than MnBr12Por/PhI(OAc)2 systems increased the total product yield
to from 19 to 45% and 35 to 42% and diminished bleaching
[MnIIIBr8T4CMPPCl],9
[MnIIIBr9APTPPCl],13 and lower product yields than cis- (entries 11 and 12, respectively), evidencing that water may
[MnIIIBr12DAPDPPCl]26 and [MnIIIBr12T3,5DMPPCl].29
have partialy hydrolyzed PhI(OAc)2, to generate PhIO in situ.60,
62
In contrast, addition of 5 ꢀL of H2O to the MnPor/PhIO and
Overall, third-generation brominated MnP are more efficient
than second-generation MnP, so the synthesis of third- MnBr12Por/PhIO systems reduced Cy-ol yields by 10% and
generation MnP via direct bromination of second-generation 20%, respectively, as compared to the systems without added
MnP should constitute an attractive strategy to develop novel water. These results corroborated the hypothesis that water
metalloporphyrin catalysts.
acted as axial ligand, but its excess can promote
In attempt to develop more efficient catalytic systems to hexacoordination in the metalloporphyrin, leaving no positions
oxidize cyclohexane, we investigated how additives such as available for reaction with the oxidant for consequent
imidazole and water affected the catalytic systems evaluated substrate oxidation.
here. The additive should coordinate to the metal center in the The values of turnover number, TON (total moles products per
metalloporphyrin catalyst, weakening the Mn=O bond and mol catalyst), are small to all systems. This is because the
boosting the reactivity of the high-valence active species oxidant was used in small amount relative to the substrate. As
MnV(O)P toward the substrate.14 Moreover, the added ligand an attempt to increase TON values, using our best catalyst
should prevent metal ion displacement from the porphyrin (MnBr12Por), two strategies were used: (1) we increased the
macrocycle plane, reducing catalyst destruction.15, 25
Addition of imidazole to the MnPor/PhIO (entry
molar ratio of oxidant: MnP:oxidant:cyclohexane 1:50:4650
in (entry 47 and 48, Table S2, ESI). It was observed that yields and
3
comparison to entry 1) system increased the total product selectivity for alcohol were diminished (entry 2 and 8, Table 2).
yield (Cy-ol + Cy-one) from 23 to 40% and and diminished the The smaller yields can be explained by the formation of
degree of catalyst destruction, indicating that imidazole made MnIVP(oxo), which is little reactive63 or by increased oxidative
the high-valence species more reactive.
destruction. However, there was an increase in TON. This
Addition of imidazole to the MnBr12Por/PhIO system did not shows that increasing the amount of oxidant can generate
alter significantly the total product yield, selectivity and better catalytic systems, but this increment leads to catalyst
bleaching (entry 4 in comparison to entry 2). The saddle-shape destruction. (2) We diminished the molar ratio of cyclohexane
conformation of MnBr12Por probably made it difficult for the and
increased
molar
ratio
of
oxidant,
approach of the additive (Im) or the substrate the metal ion.15 MnP:oxidant:cyclohexane molar ratio = 1:50:50. In this system
Addition of imidazole increased the total product yield for it was observed the inactivation of catalyst since we did not
both the MnPor/PhI(OAc)2 from 19 to 45% (entry 9 in observe the oxidation products (entry 45 and 46, Table S2,
comparison to entry 7), and MnBr12Por/PhI(OAc)2 from 35 to ESI). Furthermore, catalyst was completely destroyed during
66% (entry 10 in comparison to entry 8) systems. The fact that both strategies. The same behavior was noted in the attempt
the MnBr12Por/PhIO and MnBr12Por/PhI(OAc)2 systems to identify active species (ESI – Figure S8 and S9) in absence of
behaved differently upon addition of imidazole corroborated substrate. This result can be justified by the prevalence of
the proposal of Collman et al, who suggested that different catalyst oxidative destruction.
oxidants generate distinct high-valence active species.58
Besides that, addition of imidazole diminished the degree of B.2. Adamantane Oxidation
Groves et al. (1979)8 were the first to employ
catalyst destruction, showing that small quantities of this
additive generated more efficient catalytic systems.15
metalloporphyrins as catalysts for the oxidation of
adamantane. They used [FeIIITPPCl],
Apart from imidazole, other additives such as water can be
a first-generation
introduced into catalytic systems to enhance catalyst
efficiency. As proposed in a theoretical study by Balcells et al.
(2008)14, water as axial ligand could act like imidazole by
coordinating to the central metal ion and weakening the M=O
bond. Furthermore, in systems that employ PhI(OAc)2 as
oxidant, water could hydrolyze PhI(OAc)2, to generate PhIO in
situ. 60-62
metalloporphyrin, as catalyst and observed the formation of 1-
adamantanol without production of ketone.
Comparison between MnPor (entry 13) and [FeIIITPPCl]
catalytic results showed that MnPor provided slightly better
product yields than [FeIIITPP]Cl.8 On the other hand, MnPor
was less efficient than the second-generation catalyst
MnIIIPFTDCPP]Cl obtained by Doro et al. (2000) (entry 13
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versus 35% 1-adol 10% 2-adol, and 5% 2-adone, respectively). Both MnPor and MnBr12Por exhibited over 95% regioselectivity
45
Such behavior might have been associated with the for 1-adol because the tertiary carbon was more reactive than
electron-withdrawing substituents (fluoro and/or chloro) in the secondary carbon.66 The ketone (2-adone) emerged in
the mesoaryl positions of the macrocycle in [MnIIIPFTDCPP]Cl small quantities (~0.5%).
in comparison to MnPor, which made the high-valence active Addition of imidazole to the MnPor/PhIO and MnBr12Por/PhIO
species more reactive. This result agreed with observations systems (entries 21 and 22, respectively) at a MnP/Im molar
made by Rayati et al. when they compared the catalytic ratio of 1:0.5 did not improve product (1-adol and 2-adol)
activities of two partially brominated MnP, namely yields or diminish the degree of catalyst destruction. Doro et
[MnIIIBr4TPPCl] and [MnIIIBr4T4(-OMe)PP]Cl: these authors al. (2000) 45 obtained markedly superior yields of hydroxylated
found that electron-deficient MnP were better catalysts than products (61% 1-adol and 14% 2-adol) when they used the
electron-rich MnP.64
third-generation metalloporphyrin
[MnIICl8PFTDCPP] to
In comparison to MnPor/PhIO (Table 3, entry 13) system, the catalyze the oxidation of adamantane by PhIO with addition of
MnPor/PhI(OAc)2 system (Table 3, entry 19) afforded higher imidazole at a MnP/Im ratio of 1:30. The MnP/Im ratio used
product yields probably because that the different oxidants here (1:0.5) was much lower than Doro et al. report and may
generated distinct high-valence active species.58 On the other not have sufficient to generate the pentacoordinated catalytic
hand, the MnBr12Por/PhIO and MnBr12Por/PhI(OAc)2 systems species that was necessary to improve the catalytic efficiency.
gave practically the same efficiency (entries 14 and 20 The substituents present at the mesoaryl positions of the
respectively). Compared to MnPor (entries 13 and 19), macrocycle may also have influenced the amount of additive
MnBr12Por performed better (entries 14 and 20), giving higher
required to form the pentacoordinated species.15 Moreover,
1-adol and 2-adol yields and lower degree of catalyst [MnIICl8PFTDCPP] contain electron-withdrawing substituents
destruction regardless of the oxidant. The superior efficiency (Cl and F) in the mesoaryl positions of the macrocycle, which
of MnBr12Por can be associated with the presence of bromine greatly increased the reactivity of the high-valence active
atoms at the β-pyrrole positions of the macrocycle – these species as compared to the active species originating from the
substituents withdrew electron density from the metal center MnP employed here. When PhI(OAc)2 was the oxidant,
and distorted the metalloporphyrin ring,65 increasing the imidazole raised product yields only slightly, and the degree of
reactivity of the high-valence active species (MnV(O)P) and catalyst destruction decreased very little (entries 21 and 22).
diminishing the degree of catalyst destruction.9, 10, 13, 26
Table 3. Product yield obtained during the oxidation of adamantane by PhIO or PhI(OAc)2 catalyzed by MnP in CH2Cl2 under aerobic condition, with or without water or imidazole
as additives.
Alcohol yield (/%)
Entry
Catalyst
System
TON
Regiosel.2 (/%)
Bleac.3 (/%)
1-adol1,4
17
39
19
39
22
41
22
38
25
44
28
43
0
2-adol1
1.9
4.3
2.1
4.2
2.4
4.5
2.4
4.2
2.6
4.6
3.0
4.7
-
13
14
15
16
17
18
19
20
21
22
23
24
25
26
MnPor
MnBr12Por
MnPor
PhIO
PhIO
2
4
2
3
2
4
2
4
1
2
2
3
0
0
97
97
97
97
97
97
97
97
98
99
98
98
-
52
40
57
39
61
42
52
48
57
33
61
39
-
PhIO/Im
MnBr12Por
MnPor
PhIO/Im
PhIO/H2O
PhIO/H2O
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2/Im
PhI(OAc)2/Im
PhI(OAc)2/H2O
PhI(OAc)2/H2O
PhIO
MnBr12Por
MnPor
MnBr12Por
MnPor
MnBr12Por
MnPor
MnBr12Por
-
-
-
PhI(OAc)2
0
-
-
Reaction conditions: [MnP] = 5 × 10-4 mol L-1, [PhIO or PhI(OAc)2] = 5 × 10-3 mol L-1, MnP/Oxidant/adamantane/CH2Cl2 molar ratio =
o
1:10:100:23325, 25 C, magnetic stirring, 90 min. of reaction. MnP/Im molar ratio in Reactions 15, 16, 21, and 22 = 1:0.5. MnP/H2O molar
ratio in Reactions 17, 18, 23, and 24 = 1:139 (0.5 µL).1Yields based on the oxidant. 1-adol: 1-adamantanol, 2-adol: 2-adamantanol.
2Regioselectivity was determined from the relation 1-adol [= 100 × (%) 1-adol / (% 1-adol + (% 2-adol/3)]. 3The degree of catalyst
destruction was calculated from the UV-Vis absorption spectra recorded at the end of the reaction. 4The maximum error is ± 3% with
confidence level of 95%. 5TON = total moles products per mol catalyst.
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Addition of water during the oxidation of adamantane by PhIO It is worth noting that despite the strong nature of the
or PhI(OAc)2 catalyzed by MnPor or MnBr12Por (entries 17, 23, oxidants used (PhIO or PhI(OAc)2), the formation of oxidation
18, and 24, respectively) increased product yields. The rise in products was not detected for any of the studied substrates
product yields was more pronounced for systems involving when the reactions were performed in the absence of the MnP
PhI(OAc)2, which corroborated the hypothesis that water acts (Table S1, entry 33 and 34 for n-hexane; Table S2, entry 37 and
in two different ways: as axial ligand in the MnP, as proposed 368 for cyclohexane; Table 3, entry 25 and 26 for
by Barcells et al.14 and Silva et al.13, 26, and in the hydrolysis of adamantane). If we increase the amount of oxidant 5 times
PhI(OAc)2, as described by In et al.60 and Kwong et al.61
(oxidant:substrate = 50:4650), only small quantities of
products are formed for cyclohexane oxidation (Table S2, entry
43 and 44). In addition, the substitution of the MnP by a
manganese salt (Mn(OAc)2•4H2O) also does not lead to
oxidation of the substrates (Table S1, entry 35 and 36 for n-
hexane; Table S2, entry 41 and 42 for cyclohexane; Table S3,
entry 49 and 50 for adamantane). Besides, the systems
without oxidants (PhIO or PhI(OAc)2), which contain only the
MnP in the presence of molecular oxygen from the air, also did
not promote the substrate oxidation (Table S1, entry 31 and 32
for n-hexane; Table S2, entry 39 and 40 for cyclohexane; Table
S3, entry 51 and 52 for adamantane). Therefore, the catalytic
activity reported in all systems presented is due to the MnP
synthesized (MnPor and MnBr12Por) in presence of the
oxidants, in low molar ratios (1:10).
B.3. -Hexane Oxidation
n
The high dissociation energy of the C-H bond in linear alkanes
makes functionalization of these species one of the most
interesting and challenging chemical transformations. Many
research groups have attempted to develop catalysts that can
increase the reactivity of these chemical compounds.
Therefore, we have evaluated the catalytic performance of
MnPor and MnBr12Por in the oxidation of n-hexane, a linear
alkane that gives 2-hexanol (2-ol), 3-hexanol (3-ol), 2-
hexanone (2-one), and 3-hexanone (3-one), Table S1 (ESI), as
oxidation products. The product 1-hexanol (1-ol) did not
emerge in any of the reactions probably because the primary
carbon had higher C-H bond dissociation energy as compared
to secondary and tertiary carbons.66
MnBr12Por/PhIO system (Table S1, ESI) was more efficient and
more selective for the alcohol products (2-ol and 3-ol) than
Conclusions
The usual synthetic route to obtain the brominated third-
generation MnP involves five steps.11, 23 In this work, we obtain
the brominated third-generation catalyst MnBr12Por in two
steps, by direct bromination of the second-generation catalyst
MnPor. Hence, this methodology meets the requirements of
green chemistry. The yield of products, which were based on
oxidants, resulting from cyclohexane and adamantane
oxidation catalyzed by MnPor and MnBr12Por are good,
however to n-hexane the yield of products are lower than
cyclohexane and adamantane, which attests to the high
stability of this linear alkane. In the oxidation of adamantane
catalyzed by both MnP, verify regioselective for 1-Adol over
95%. The third-generation catalyst MnBr12Por presents lower
degree of destruction than the second-generation MnPor
during the oxidation of cyclohexane, adamantane, and n-
hexane by PhIO. When PhI(OAc)2 is the oxidant, the degree of
destruction of MnBr12Por is higher as compared to MnPor, but
MnBr12Por destruction is not complete, in contrast to
literature reports9. Addition of imidazole increases the product
yields obtained with MnPor regardless of the oxidant and with
the MnBr12Por/PhI(OAc)2 system. Addition of water improves
product yields for most of the studied systems, regardless of
the oxidant, which shows that water acts as axial ligand and
also hydrolyzes PhI(OAc)2, to generate PhIO in situ. Overall,
MnPor/PhIO (entries 28 and 27, respectively). As discussed
previously,49,
67-70
this resulted from the introduction of
bromine atoms at the
β-pyrrole positions of the MnBr12Por
macrocycle. Additionally, MnBr12Por had lower degree of
destruction than MnPor. Less catalyst destruction is associated
with a decrease in oxidative processes, which preferentially
occur with planar metalloporphyrins bearing few substituents
at the mesoryl positions of the macrocycle71.
Comparison between MnPor and MnBr12Por in terms of
product distribution showed alcohol products amounted to
53−56% of the total product yield, regardless of the oxidant.
These results contrasted with the findings of Silva et al.26, who
obtained ~71% and ~74% alcohol products for the second- and
the third-generation catalysts cis-[MnIIIDAPDPP]Cl and cis-
[MnIIIBr12DAPDPP]Cl, respectively. The results achieved by Silva
et al. could be attributed to intermolecular coordination of the
amino group of a metalloporphyrin molecule to the metal
center of another metalloporphyrin molecule. The use of
PhI(AOc)2 as oxidant lowered product yields (entries 29 and 30)
This fact probably can be related to that n-hexane was a more
stable substrate in comparison
to adamantane and
cyclohexane. The degree of catalyst destruction increased in
the presence of n-hexane and accounted for the decreased
reaction products.
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31 H. Saltzman and J. G. Sharefkin, Org. Synth., 1963, 43.
MnBr12Por performs better than MnPor, leading to higher
product yields, higher selectivity toward the main product, and
lower degree of catalyst destruction. Nevertheless, additives
favor the activity of the second-generation catalyst to a larger
extent as compared to the third-generation catalyst; the
exception is the MnBr12Por/PhI(OAc)2 system in the oxidation
of cyclohexane, which affords the highest product yields (66%)
among the evaluated systems.
32 G. Gritzner and J. Kuta, Pure Appl. Chem., 1984, 56, 461-466.
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Acknowledgements
Financial support from CNPq, and FAPEMIG is gratefully
acknowledged. We are grateful to Prof. Dr. Paulo Jorge
Sanches Barbeira for use of the potentiostat Eco Chemie l-
Autolab during cyclic voltammetry analysis. We are grateful to
Prof. Guilherme Sippel Machado, UFPR – Campus Litoral for
the elemental analysis.
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2000, 164, 97-108.
46 K. Prakash, R. Kumar and M. Sankar, RSC Adv., 2015, 5, 66824-66832.
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New Journal of Chemistry
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DOI: 10.1039/C6NJ03072F
A novel β-brominated Mn-porphyrin act as a good catalyst for alkanes oxidation in biomimetic
systems. The oxidation of cyclohexane and adamantane takes place with excellent selectivity.