Detection of reactive intermediates in manganese(III) porphyrin catalyzed oxidation reaction using 2,4,6-tri-tert-butylphenol as probe substrate

Article abstract of DOI:10.1016/j.ica.2019.119004

The 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphine-Mn(III) chloride (F₂₀TPPMn(III)Cl) catalyzed oxidation of 2,4,6-tri-tert-butylphenol (TTBP) by pentafluoroiodosylbenzene (C₆F₅IO) in dichloromethane (CH_2<

Full text of DOI:10.1016/j.ica.2019.119004

InorganicaChimicaActa495(2019)119004
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Research paper
Detection of reactive intermediates in manganese(III) porphyrin catalyzed
oxidation reaction using 2,4,6-tri-tert-butylphenol as probe substrate
Bhawna Rania, Amit Singhb, Rahul Shrivastavaa, Arunava Agarwalaa,⁎
a Department of Chemistry, Manipal University Jaipur, VPO-Dehmi-Kalan, Off Jaipur-Ajmer Expressway, Jaipur, Rajasthan 303007, India
b Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Mn(III)-Pophyrin
2,4,6-Tri-tert-butylphenol
Oxidation reaction
Pentafluoroiodosylbenzene
The 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphine-Mn(III) chloride (F20TPPMn(III)Cl) catalyzed
oxidation of 2,4,6-tri-tert-butylphenol (TTBP) by pentafluoroiodosylbenzene (C6F5IO) in dichloromethane
(CH2Cl2) and acetonitrile (CH3CN) medium was studied. The reactive intermediates of these reactions were
trapped by 2,4,6-tri-tert-butylphenol (TTBP) with formation of 2,4,6-tri-tert-butylphenoxy radical (TTBP% ra-
dical). It was noted that the formation of TTBP% radical in CH2Cl2 was distinctly biphasic in nature, demon-
strating the presence of two intermediates. Whereas the same reaction became monophasic in more polar
acetonitrile medium. The rate constants for the formation of TTBP% radical in CH2Cl2 were dependent on the
substrate (TTBP) concentrations. However, the rate constants of the formation of TTBP% radical
(15
1 × 10−3 s−1) in CH3CN solvent is independent of TTBP concentrations. The rate constants for CH3CN
solvents were calculated directly from −ln(A∞ − At) vs. time (t) plots. For CH2Cl2 solvent the biphasic plot was
analyzed in two steps.
1. Introduction
studies, intermediate structures were proposed mainly from product
profile analysis. Herein we report a direct kinetic evidence of the ex-
The selective oxygenation reaction of organic compounds is an
important step in various biological and industrial processes. These
reactions include CeH hydroxylation, epoxidation, hetero-atom oxi-
dation etc [1–4]. Nature employs a number of heme enzymes to catalyze
various mono-oxygenation reactions. Among the heme enzymes Cyto-
chrome P450 is the most important, so the understanding of cyto-
chrome P450 mediated oxidation reactions in biology has been a very
interesting topic and a subject of extensive investigation in last several
decades [5–7]. Several metalloporphyrins, which mimic cytochrome
P450, have been used extensively to investigate the major reaction
pathway of the cytochrome P450 [8,9]. Among the metalloporphyrins,
manganese (III) porphyrins have received considerable attention be-
cause of their versatile and efficient role as catalyst for alkane hydro-
xylation, olefin epoxidation and CeH bond activation. Mechanistic
studies on Mn(III) porphyrin catalyzed reactions indicated the presence
of oxo-manganese(V) and manganeporphyrin-oxidant adduct as major
intermediates, which transfer an oxygen atom to the substrate [10–14].
In these studies, percids, hydroperoxides, hydrogen peroxide and io-
dosylarenes are mainly used as terminal oxidants [15–17]. Recent re-
sults have indicated definitive involvement of multiple reactive inter-
mediates in both enzymatic and model systems [18–21]. In these
istence of at least two reactive intermediates, its probable structure and
reactivity. In this study the reaction of an electronegatively substituted
porphyrin complex of manganese (III) (5,10,15,20-tetrakis-(2,3,4,5,6-
pentafluorophenyl)-porphine-Mn(III) chloride (F20TPPMn(III)Cl), with
C6F5IO using 2,4,6- tri-tert-butylphenol as probe substrate has been
investigated. It has been noted that solvent composition is important
factor in directing the nature of intermediate.
2. Experimental section
2.1. Solvents and reagents
Commercially available CH2Cl2 and CH3CN were dried over CaCl2
and were freshly distilled from P4O10. TTBP was purchased from
Aldrich and was purified by recrystallization in water:ethanol (5:95)
and then dried in vacuum over CaCl2. C6F5IO [22,23] and F20TPPMn
(III)Cl [24–26] were synthesized according literature method. UV–vi-
sible spectral measurements were performed with a Perkin–Elmer
Lambda (2S) spectrometer.
⁎ Corresponding author.
E-mail addresses: arunava.agarwala@jaipur.manipal.edu, arunava.agarwala@gmail.com (A. Agarwala).
https://doi.org/10.1016/j.ica.2019.119004
Received 13 April 2019; Received in revised form 1 July 2019; Accepted 5 July 2019
Availableonline06July2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

B. Rani, et al.
InorganicaChimicaActa495(2019)119004
2.2. F20TPPMn(III)Cl catalyzed oxidation of 2,4,6-tri-tert-butylphenol by
C6F5IO in CH2Cl2
Table 1
F20TPPMn(III)Cl catalyzed oxidation of TTBP by C6F5IO in CH2Cl2 at
25 1 °C.
Entry Catalyst
C6F5IO
(mM)
TTBP
(mM)
Yielda (%) kobs (slow)
(s−1
kobs (fast) (s−1
)
In
a typical kinetic experiment solid 2,4,6-tri-tert-butylphenol
(2 mg) was taken in a low volume quartz cell fitted with silicone
septum. The cell was deaerated by purging argon gas through it with
fine needle for 20 min. CH2Cl2 was taken in a glass syringe and was
deaerated by bubbling argon gas through it. This deaerated CH2Cl2
(1.5 mL) was used to dissolve the TTBP in the cell. The final con-
centration of TTBP was 5 mM. A standard solution of F20TPPMn(III)Cl
was prepared in degassed CH2Cl2. An aliquot volume (6–8 µL) of this
solution was added to the cell so that the final concentration of the
catalyst was 100 µM. A stock solution of C6F5IO was prepared by dis-
solving 9–10 mg of the oxidant in 200 µL of degassed CF3CH2OH in a
small vial. An aliquot volume (~10 µL) of the oxidant solution was
introduced to the reaction mixture in the quartz cell to initiate the
oxidation reaction. The cell was shaken and was immediately placed in
the cell holder of the spectrophotometer. The progress of the reaction
was monitored by evolution of 630 nm peak due to formation of TTBP
radical [27,28]. The absorbance at 630 nm was recorded at an interval
of 10 s for 30 min. The A∞ was obtained when there was no change in
absorbance for at least 30 min. The yield of TTBP% radical in this re-
action was 53%. Believing that the lower yield of TTBP% radical could
be due to the insufficient amount of substrate in the reaction medium,
the above experiment was repeated with systematically increasing the
concentration of TTBP. Thus the concentration was increased to 51 mM,
the catalyst and oxidant concentration were kept constant. In all these
experiments the yield of TTBP% radical was varied from 53% to 98%. In
all these experiments the formation of TTBP% radical was a clean bi-
phasic reaction in all these experiments.
(μM)
)
1
2
3
4
100
100
100
100
1
1
1
1
5
53
76
97
99
2.5 × 10−3
3.6 × 10−3
5.9 × 10−3
8.7 × 10−3
5.1 × 10−3
7.1 × 10−3
12.4 × 10−3
21.5 × 10−3
11
28
51
a
Yields were based on total oxidants. Averages of duplicate sets of experi-
ments are given.
solvent, 0.1 mM catalyst was treated with 1.0 mM of C6F5IO in con-
taining variable amounts of 2,4,6-tri-tert-butylphenol (TTBP). The basic
objective was to find out the reaction conditions under which at least
50% of the terminal oxidant was accountable in terms of the only
product 2,4,6-tri-tert-butylphenoxy radical (TTBP% radical). Thus in the
first reaction where TTBP concentration was ~5 mM the total yield of
the TTBP% radical was ~53
2% (Table 1, entry 1). The yield was
improved systematically up to almost 99% when TTBP concentration
was increased up to ~51 mM (Table 1, entry 2–4). This almost quan-
titative yield of the product, TTBP% radical remained constant even if
the TTBP concentration was increased up to almost 100 mM. Interest-
ingly the formation of TTBP% radical was a clean biphasic reaction in all
these experiments. A representative plot of −ln(A∞ − At) versus time
(t) is depicted in Fig. 2.
The slow phase rate constants were directly calculated from −ln
(A∞ − At) vs. time (t) plots whereas fast phase rate constants were
determined from −lnΔ(A∞ − At) vs. time plots [30,31].
presentative plot of −lnΔ(A∞ − At) vs. time is given in Fig. 3. The
results of four such sets of experiments are presented in Table 1.
Similar reactions were then conducted in CH3CN solvent. It was
observed that all the plots in the concentration range of TTBP of
5–51 mM were distinctly single exponential up to at least 85% of the
total reaction. The rate constants for pseudo first order reactions were
calculated from the slope of –ln(A∞ − At) vs. time plots.
presentative plot is depicted in Fig. 4. The details of four such reactions
are given in Table 2.
A re-
2.3. F20TPPMn(III)Cl catalyzed oxidation of 2,4,6-tri-tert-butylphenol by
C6F5IO in CH3CN
The reaction was performed in the same manner as has been de-
scribed in above section. With 5 mM TTBP, the yield of TTBP. radical
was 77% and the yield was increased with increasing TTBP con-
centration. Unlike CH2Cl2 medium, in CH3CN medium the reactions
were clearly monophasic. A representative spectral growth plot is de-
picted in Fig. 1.
A re-
The kobs values of the slow and fast phases in CH2Cl2 solvent were
plotted against the corresponding TTBP concentration (Fig. 5). Both the
plots were distinctly linear and the bimolecular rate constants were
0.1329 M−1 s−1 and 0.3539 M−1 s−1 for the slow and fast reactions
respectively. The closer analysis of the Fig. 5, where the kobs values are
not passing through the origin indicating a saturation behavior in the
3. Results and discussion
In order to investigate the nature of reactive intermediate formed in
the reaction of Mn(III) porphyrins with iodosylarenes the reaction of
F20TPPMn(III)Cl (meso-tetrakis (pentafluorophenyl)porphinato manga-
nese(III) chloride) with C6F5IO has been investigated in details in two
solvents: CH2Cl2 and CH3CN with considerable difference in their po-
larity [29]. In a typical reaction performed in a quartz cell in CH2Cl2
Fig. 1. Absorbance (at 630 nm) vs time plot of 2,4,6-tri-tetr-butylphenoxy ra-
dical formation in acetonitrile. Concentration of TTBP = 28 mM,
Fig. 2. The plot of −ln(A∞ − At) vs. time in CH2Cl2 solvent for the formation of
TTBP% radical. Concentration of TTBP = 28 mM, C6F5IO = 1.0 mM, F20TPPMn
C6F5IO = 1.0 mM, F20TPPMn(III)Cl = 0.1 mM; temp = 25
1 °C.
(III)Cl = 0.1 mM; temp = 25
1 °C.
2

B. Rani, et al.
InorganicaChimicaActa495(2019)119004
Fig. 6. −ln(A∞ − At) vs. time plot for the formation of TTBP% radical in CH2Cl2
solvent containing 40 μL CF3CH2OH. Concentrations of TTBP = 28 mM,
Fig. 3. −lnΔ(A∞ − At) vs. time plot for the formation of TTBP% radical in
CH2Cl2 solvent. Concentration of TTBP = 28 mM, C6F5IO = 1.0 mM, F20TPPMn
C6F5IO = 1.0 mM, F20TPPMn(III)Cl = 0.1 mM; temp = 25
1 °C.
(III)Cl = 0.1 mM; temp = 25
1 °C.
shape of kobs = k(complex or oxidant)(TTBP)/[1 + K(TTBP)]. The re-
action with 0.1 mM catalyst 1.0 mM C6F5IO and 28 mM TTBP in CH2Cl2
was next repeated in the same solvent but containing only 40 μL of
CF3CH2OH. The previous biphasic plot observed in neat CH2Cl2 was
converted to a clean single exponential plot. This result is given in
Fig. 6. The rate constant observed for this reaction was
21
1 × 10−3 s−1 and the Yield of TTBP% radical was 91
The solubility of C6F5IO in CF3CH2OH has been proposed to be due
1%.
to solvated species: C6F5IO(CF3CH2OH), as shown in Scheme 1, struc-
ture 1 [32]. We believe that in CH2Cl2 solvent this solvated oxidant 1
was binding fast with the catalyst to form 2. We assume that in CH2Cl2
solvent 2 will be highly unstable and it will be in equilibrium with a
desolvated species with the probable structure 3. This proposition has
been supported by the fact that when the reaction was conducted in
CH2Cl2 containing an excess of CF3CH2OH (40 µL) the reaction became
monophasic (Fig. 6). The slope of this plot gives pseudo first order rate
Fig. 4. The plot of −ln(A∞ − At) vs. time in CH3CN solvent for the formation of
TTBP% radical. Concentration of TTBP = 28 mM, C6F5IO = 1.0 mM, F20TPPMn
constant of 21
1 × 10−3 s−1
. The enhancement of the rate
(III)Cl = 0.1 mM; temp = 25
1 °C.
(21
1 × 10−3 s−1) from the corresponding rate in neat CH2Cl2 was
most probably due to the polarity change of CH2Cl2 solvent in presence
of 40 μL of CF3CH2OH. The terminal oxidant has indeed been activated
with new structures 2 and 3. However one was solvated and other was
not. Depending upon the equilibrium they can certainly show difference
in reactivity and we believe that this was what in fact observed in the
form of biphasic production of TTBP% radical in CH2Cl2 solvent. Ob-
viously in the beginning, 2 will be the dominating species, so we
Table 2
F
20TPPMn(III)Cl catalyzed oxidation of TTBP by C6F5IO in CH3CN at
25 1 °C.
Entry Catalyst (μM)
C6F5IO (mM)
TTBP (mM)
Yielda (%)
kobs (s−1
)
1
2
3
4
100
100
100
100
1.0
1.0
1.0
1.0
5
77
85
97
99
15.4 × 10−3
16.1 × 10−3
15.1 × 10−3
14.9 × 10−3
11
28
51
a
Yields were based on total oxidants. Averages of duplicate sets of experi-
ments are given.
Fig. 5. Plots of kobs vs. TTBP conc. (For the F20TPPMn(III)Cl catalyzed oxidation
of TTBP with C6F5IO in CH2Cl2 solvent. Conc. of TTBP = 5, 11, 28 and 51 mM).
Scheme 1. Probable reactive intermediates in the reaction of F20TPPMn(III)Cl
with C6F5IO in CH2Cl2.
3

B. Rani, et al.
InorganicaChimicaActa495(2019)119004
demonstrated the involvement of multiple reactive intermediates and
the formation of these intermediates are very much solvent dependent.
Acknowledgement
R.S. and A. A. acknowledge Manipal University Jaipur for providing
infrastructure facility. B.R. acknowledges Manipal University Jaipur for
teaching assistantship. A.A. and A.S acknowledge Prof. D.
Bandyopadhyay for his guidance and encouragement.
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4. Conclusions
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4