Synthetic and mechanistic investigations on manganese corrole-catalyzed oxidation of sulfides with iodobenzene diacetate

Article abstract of DOI:10.1016/j.ica.2018.11.048

Manganese corrole complexes catalyze the efficient oxidation of organic sulfides to sulfoxides with iodobenzene diacetate [PhI(OAc)₂] as a mild oxygen source in the presence of small amounts of water. Various substituted thioanisoles can be eff

Full text of DOI:10.1016/j.ica.2018.11.048

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Research paper
Synthetic and mechanistic investigations on manganese corrole-catalyzed oxi-
dation of sulfides with iodobenzene diacetate
Davis Ranburger, Ben Willis, Benjamin Kash, Haleh Jeddi, Christian Alcantar,
Rui Zhang
PII:
S0020-1693(18)31579-2
https://doi.org/10.1016/j.ica.2018.11.048
ICA 18663
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29 November 2018
Please cite this article as: D. Ranburger, B. Willis, B. Kash, H. Jeddi, C. Alcantar, R. Zhang, Synthetic and
mechanistic investigations on manganese corrole-catalyzed oxidation of sulfides with iodobenzene diacetate,
Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.11.048
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Synthetic and mechanistic investigations on manganese
corrole-catalyzed oxidation of sulfides with iodobenzene
diacetate
Davis Ranburger, Ben Willis, Benjamin Kash, Haleh Jeddi,§ Christian Alcantar and Rui Zhang*
Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101, USA
* Corresponding author: Rui Zhang, Department of Chemistry, Western Kentucky University,
1906 College Heights Blvd #11079, Bowling Green, KY 42101
Phone: 270-745-3803, Fax: 270-745-5361, e-mail: rui.zhang@wku.edu
§ Current address: Department of Chemistry, Duke University, Durham, NC 27708
1

Abstract
Manganese corrole complexes catalyze the efficient oxidation of organic sulfides to sulfoxides
with iodobenzene diacetate [PhI(OAc)2] as a mild oxygen source in the presence of small amounts
of water. Various substituted thioanisoles can be efficiently oxidized to sulfoxides with quantitative
conversions (up to 2500 TONs) and excellent selectivies for sulfoxides. The catalyst effects,
including the corrole ligands and oxidation states of the central manganese, were investigated, and
showed a significant impact on the catalytic sulfoxidations. The previously known high-valent
manganese(V)-oxo corroles were chemically generated and kinetically studied in the oxidation of
thioanisole substrates. The competition results suggested the oxidation state of manganese metal is
crucial to control the nature of active oxidant forms that significantly affected their catalytic
activity.
Keys words
manganese corrole; oxidation; iodobenzene diacetate; sulfide; sulfoxide
2

1.
Introduction
In both nature and the laboratory, organic sulfoxides are valuable synthetic intermediates in
biological systems for production of natural products, ligands for catalysis, and significant biological
molecules [1-3]. Meanwhile, optically active sulfoxides play important roles in medicinal and
pharmaceutical fields for the production of compounds such as antiulcer (proton pump inhibitors),
antibacterial, antifungal, antihypertensive, and cardiotonic agents, as well as psychotonics and
vasodilators.[4] In addition, oxidative desulfurization is a promising method in industry to remove
sulfur-containing air pollutants.[5] To date, various catalysts including metal complexes,
polyoxometalates, and flavins have been reported in the literature for the catalytic oxidation of
sulfides to sulfoxides commonly with H2O2 or organic peroxides.[6-11] However, the use of these
strong oxidants often results in oxidative degradation of the catalyst due to their oxidizing potency.
Moreover, many of these methods cause the over-oxidation to sulfones, and/or involve strong
oxidizing agents and highly toxic oxo-metal oxidants that have proven to be detrimental for the
environment.[12] Thus, there is still a great need for design and development of more selective and
greener sulfoxidation catalysts that can provide an ideal combination of excellent selectivity, high
activity, broad substrate scope, and mild reaction conditions.
In recent years, advances in facile synthesis of the 19-membered macrocyclic triarylcorroles
as trianionic and aromatic porphyrin-like ligands propelled seemingly increased interests in corrole
chemistry, ranging from oxidation chemistry of the corresponding corrole metal complexes
(metallocorroles) to biochemical and practical applications in chemistry and biology. [13-16] In
particular, a large number of metallocorroles and metallocorrolazines (corrolazine = meso-N-
substituted triazocorroles) have been prepared and exploited in a wide variety of catalytic
oxidations including epoxidation, hydroxylation, aziridination, sulfoxidation, and atom transfer
reactions.[17-21] For examples, Gross and coworkers reported the first example of catalytic
3

oxidation with the iron(IV) and manganese(III) complexes of tris(pentafluorophenyl)corrole
(H3TPFC) using iodosobenzene (PhIO) as the sacrificial oxidant.[22] Later, biomimetic oxidations
with [MnIII(TPFC)(OEt2)2],[23] albumin-conjugated manganese,[24] and chromium corroles[25]
were also studied by the same group of authors. Perhalogenation of the corrole macrocycle with
bromine or fluorine results in increased reactivity for the corrole–manganese complexes under
catalytic conditions with PhIO.[26, 27] Catalytic aerobic oxidations by a fluorinated iron(IV) µ-oxo
biscorrole through a photo-disproportionation pathway was also reported.[28]
One problem associated with metallocorrole catalysts is that metallocorroles are prone to
oxidative degradation in the presence of commonly used sacrificial oxygen sources such as meta-
chloroperoxybenzoic acid (mCPBA) and PhIO. In contrast, the less commonly used iodobenzene
diacetate [PhI(OAc)2] does not damage the metal catalysts nor show appreciable reactivity to the
substrate under normal catalytic conditions.[29, 30] Recently, our studies revealed the efficient and
selective oxidations of alkenes and organic sulfides with PhI(OAc)2 catalyzed by electron deficient
iron(III) corroles.[31, 32] The inherently low stability of the corrole-catalyst against degradation
was significantly improved owing to the mild oxidizing power of PhI(OAc)2. In this work, we
further explore the usefulness of PhI(OAc)2 for the efficient and chemoselective oxidation of
sulfides using a series of manganese corrole catalysts (Scheme 1). In most cases, aromatic sulfides
have been successfully oxidized to the corresponding sulfoxides with quantitative conversions and
excellent selectivities for sulfoxides. Furthermore, our mechanistic studies show that a low-
reactivity corrole-manganese(V)–oxo intermediate, which was detected by the oxidation of the
manganese(III) corrole with PhI(OAc)2, is not likely the sole oxidant. Instead, a more reactive and
higher oxidation form oxidant, possibly corrole-manganese(VI)-oxo species, is suggested from the
competition studies as the premier active oxidant for the manganese(IV) corrole-catalyzed
sulfoxidation reactions.
4

Ar
Cl
N
N
N
MnIV
N
Ar
Ar
PhI(OAc)
2, CH3OH, H2O
O
S
S
Ar
R
Ar
R
TPC, a
Ar = C6H5,
tBu-TPC, b
t
Ar = 4- Bu-C6H4, 4-
Ar = 4-NO2-C6H4, 4-
> 99% yield
NO -TPC, c
2
TPFC, d
Ar = C6F5,
Scheme 1. Catalytic sulfoxidations by manganese corroles in the presence of PhI(OAc)2 and water
2.
2.1
Experimental
Materials and instrumentation
All commercial reagents were of the best available purity and used as supplied unless
otherwise specified. Iodobenzene diacetate or (diacetoxyiodo)benzene, i.e. PhI(OAc)2, was
purchased from Aldrich Chemical Co. and used as such. Pyrrole (98%), benzaldehyde and
pentafluorobenzaldehyde from Sigma-Aldrich Chemical Company were distilled prior to use. All
reactive substrates of organic sulfides for catalytic oxidations were passed through a dry column of
active alumina (Grade I) before use. UV-vis spectra were recorded on an Agilent 8453 diode array
spectrophotometer. 1H NMR was performed on a JEOL ECA-500 MHz spectrometer at 298K with
tetramethylsilane (TMS) as internal standard. Gas chromatograph analysis was conducted on an
Agilent GC-7820A/MS-5975 equipped with a flame ionization detector (FID) using a DB-5
capillary column. The above GC-MS system is also coupled with an auto sample injector.
Electrospray ionization-mass spectroscopy (ESI-MS) data was collected on an Agilent 500 LCMS
5

ion trap system. Reactions of MnIII(Cor) with excess of PhI(OAc)2 were conducted in a anaerobic
methanol solution at 23 ± 2 oC, monitored by the UV-vis spectrometer.
Non-metalated corroles used in this study, including 5,10,15-triphenylcorrole (H3TPC, a)
[14] and 5,10,15-tripentafluorophenylcorrole (H3TPFC, d),[13] were prepared following the earlier
reported procedures, and their characterization data (1H NMR and UV-vis) was consistent with
reported values. 5,10,15-Tris(4-tert-butylphenylcorrole [H3(4-tBu-TPC), b], 5,10,15-tris(4-
nitrophenylcorrole [H3(4-NO2-TPC), c] and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin
(H2TPFPP) were purchased from Sigma-Aldrich Co and used as received. The corrole-
manganese(III) dietherate complexes (1) were prepared as previously described.[33] In a typical
procedure, a solution of H3TPC (50 mg, 63 µmol) and a large excess of manganese(II) acetate (200
mg) in dry DMF under argon was heated at reflux for 30 min. Evaporation of solvent followed by
column chromatography on silica gel (diethyl ether) resulted in isolation of the corresponding
[MnIII(TPC)(OEt2)2] (1a) in 60% yield. [MnIII(TPFPP)Cl] was also prepared and purified in a
similar fashion. Following the known procedure,[33, 34] treatment of manganese(III) species 1 with
tris(4-bromophenyl)ammoniumyl hexachloroantimonate gave known corrole–manganese(IV)
chloride salts 2 in varied yields depending on the corrole ligands. Compounds 2 were further
purified through recrystallization from CH2Cl2/hexane followed by column chromatography on
silica gel to remove traces of the amine byproduct. All the known manganese complexes used in
this study were characterized and confirmed by UV–vis spectra and ESI-MS (below), matching
those previously reported.[34] [35]
[MnIII(TPC)(Et2O)2] [1a]: Yield = 70%; UV-vis (CH2Cl2) λmax/nm (relative ε): 403 (0.89), 431
(1.00), 651 (0.18); ESI-MS: m/z: 578.21 [M]+
[MnIII(4-tBu-TPC)(Et2O)2] [1b]: Yield = 82%; UV-vis (CH2Cl2) λmax/nm (relative ε): 400 (0.97),
6

438 (1.00), 500 (0.38), 650 (0.34) ESI-MS: m/z: 746.39 [M]+
[MnIII(4-NO2-TPC)(Et2O)2] [1c]: Yield = 31%; UV-vis (CH2Cl2) λmax/nm (relative ε): 428 (1.00),
480 (0.99), 658 (0.40); ESI-MS: m/z: 713.10 [M]+
[MnIII(TPFC)(Et2O)2] [1d]: Yield = 87 %.UV-vis (CH2Cl2) λmax/nm (relative ε): 400 (0.96), 415
(1.00), 469 (0.69), 598 (0.28); ESI-MS: m/z: 848.19 [M]+
[MnIV(TPC)Cl] [2a]: Yield = 90%; UV-vis (CH2Cl2) λmax/nm (relative ε): 364 (0.72), 430 (1.00),
603 (0.24); ESI-MS: m/z: 578.11 [M – Cl]+, 613.05 [M]+
[MnIV(4-tBu-TPC)Cl] [2b]: Yield = 94%; UV-vis (CH2Cl2) λmax/nm (relative ε): 366 (0.83), 440
(1.00), 605 (0.22); ESI-MS: m/z: 780.42 [M]+, 746.39 [M – Cl]+
[MnIV (4-NO2-TPC)Cl] [2c]: Yield = 36%; UV-vis (CH2Cl2) λmax/nm (relative ε): 363 (0.50), 424
(1.00), 602 (0.12); ESI-MS: m/z: 748.99 [M]+, 713.15 [M – Cl]+
[MnIV(TPFC)Cl] [2d]: Yield = 89%.UV-vis (CH2Cl2) λmax/nm (relative ε): 361 (0.68), 416 (1.00),
593 (0.14); ESI-MS: m/z: 884.05 [M]+, 848.15 [M – Cl]+
2.2
General procedure for catalytic oxidations
Unless otherwise specified, all catalytic reactions were typically carried out in the presence
of a small amount of H2O (15 µL) with 1 µmol of catalyst (ca. 0.2 mol%) in 2 mL of methanol
containing 0.5 mmol of organic substrates. PhI(OAc)2 (0.75 mmol, 1.5 equiv.) was then added to
o
the reaction solution at 23 ± 2 C. Aliquots of the reaction solution at constant time intervals were
analyzed by GC-MS to determine the conversions, products formed, and yields. The pure products
were isolated by a flash column chromatography (silica gel with CH2Cl2 and hexane mixture) to
give isolated yields (Table 2, entries 1 and 2). All reactions were performed at least 3 times, and the
data reported represents the average of these reactions. Monitoring reaction by UV-vis spectroscopy
7

before and after reactions indicated that no significant degradation of the manganese catalysts was
found after catalytic reactions.
2.3
Kinetic studies
Chemical oxidation of corresponding manganese(III) complexes (1) with ca. 10 equivalent
of PhI(OAc)2 was conducted in two corrole systems, which gave the known manganese(V)-oxo
corroles, i.e. [MnV(Cor)O] (3). Following its formation in situ, oxidation reactions of 3 with excess
o
sulfide substrate (> 100 equiv.) were conducted in a solution at 23 ± 2 C. The approximate
concentrations of the chemically-generated 3 were estimated by assuming 100% conversion of
manganese(III) precursors. The rates of the reactions which represent the rates of oxo group transfer
from MnV(Cor)O to substrate were monitored by the decay of the Soret absorption band of the oxo-
species 3. The kinetic traces at λmax of the Soret band displayed good pseudo-first-order behavior for
at least four half-lives, and the data was solved to give pseudo-first-order observed rate constants,
kobs. Plots of these values against the concentration of substrate were linear in all cases. The second-
order rate constants for reactions of the oxo species with the organic sulfides were solved according
to Eq. (1), where k0 is a background rate constant found in the absence of added substrate, kox is the
second-order rate constant for reaction with the substrate, and [Sub] is the concentration of
substrate. All second-order rate constants are averages of 2-3 determinations consisting of 3
independent kinetic measurements. Errors in the rate constants were weighted and are reported at
2.
kobs = k0 + kox[Sub]
(1)
2.4
Competition and Hammett correlation studies
8

A CH3OH solution containing equal amounts of two substrates, e.g. thioanisole (0.5 mmol)
and substituted thioanisoles (0.5 mmol), manganese(III) or manganese(IV) corrole catalyst (1 µmol)
and an internal standard of 1,2,4-trichlorobenzene (0.1 mmol) was prepared (final volume = 2 mL).
PhI(OAc)2 (0.2 mmol) as the limiting reagent, was added and the mixture was stirred at ambient
temperature (23 ± 2 oC) in the presence of a small amount of H2O (5 µL) for 10 to 20 min. Relative
rate ratios for catalytic oxidations were determined by GC based on the amounts of sulfoxide
products as measured against an internal standard. In this work, all the catalytic sulfoxidations
proceeded with good yields (> 99%), mass balance (> 95%), and in all cases no sulfones were
detected. Thus, the ratio of product formation should reasonably reflect the relative sulfide
reactivity toward the corrole-manganese-catalyzed oxidations.
3
Results and Discussion
3.1
Screening studies in the catalytic oxidation of thioanisole
Although the PhI(OAc)2 has been used as a mild oxygen source for some time, apparently it
had hitherto not been employed for manganese corrole-catalyzed sulfoxidation reactions. Thus, the
usefulness of PhI(OAc)2 was first evaluated in the catalytic oxidation of thioanisole (4a) by a series
of manganese corrole catalysts (1 and 2) that differ in oxidation states of manganese metal and in
electronic environment of ligands. In order to work under homogeneous conditions and follow with
GC-MS analysis, catalytic reactions were performed in different solvents at room temperature with
a 0.2 mol% catalyst loading and a 1:1.5 ratio of thioanisole and PhI(OAc)2. The comprehensive
screening results are compiled in Table 1. As expected, methyl phenyl sulfoxide (5a) and methyl
phenyl sulfone (6a) were the only oxidized products detected by GC-MS under catalytic conditions,
whereas other byproducts such as the corresponding disulfide were not detected. Control
9

experiments showed that no oxidized products were formed in the absence of either the catalyst or
the PhI(OAc)2, or in the presence of PhI(OAc)2 and water without catalyst. Similar to previously
reported works, a small of amount of water with PhI(OAc)2 gave an accelerating effect in all
manganese corrole-catalyzed sulfoxidations. In the absence of water, the reaction was relatively
slow and 51% conversion of sulfide was observed after 1 h (entry 1 in Table 1). In contrast, the
same reaction proceeded more rapidly with a small amount of H2O (5 µL), and thus, 93%
conversion was obtained within 1h at ambient temperature (entry 2). Apparently, addition of 15 to
20 µL of water resulted in a maximum acceleration in the rate of catalytic reaction without reducing
the selectivity for sulfoxide (entry 3). According to early studies,[31, 36] the trace amount of water
can induce the steady and slow formation of more oxidizing PhIO from the PhI(OAc)2,[30] while
allowing for the release of stable HOAc (detected by GC) instead of anhydride Ac2O under
anhydrous conditions, thus, resulting in a faster reaction. Additionally, water acts as the dissociation
solvent and may help with removal of the axial ligand and binding of the oxygen source, in this
case PhI(OAc)2, more efficiently to the metal center of the corrole catalyst. [32]
Table 1. Catalytic oxidation of thioanisole by manganese corroles with iodobenzene diacetatea
O
S
O O
S
Mn(Cor) (0.2 mol%)
S
Ph
Me
Ph
Me
Ph
Me
PhI(OAc)2 (1.5 eqiv.)
Solvent, 23 oC
4a
5a
6a
Entry Catalyst
Solvent
H2O
(µL)
Time (h) Convn.
(%)b
Selectivity
(5a:6a)b
1
2
3
4
[MnIV(TPC)Cl]
CH3OH
0
1
51
95:05
95:05
96:04
90:10
5
1
93
18
5
0.5
1
100
10
CHCl3
10

5
CH3CN
CH3OH
CH3OH
5
1
43
85:15
93:07
96:04
95:05
93:07
95:05
90:10
68:32
6
[MnIII(TPC)]
18
18
18
18
18
18
18
1
21
7
[MnIV(4-tBu-TPC)Cl]
0.5
2
100
100
36
8c
9
[MnIII(4-tBu-TPC)]
[MnIV(4-NO2-TPC)Cl]
[MnIV(TPFC)Cl]
CH3OH
CH3OH
CH3OH
CH3OH
1
10
11
12
1
80
1
80
[MnIII(TPFPP)Cl]
1
54
a All reactions were performed in solvent (2 mL) at ca. 23 °C with a 1:1.5 molar ratio of thioanisole
versus PhI(OAc)2 and 0.2 mol% catalyst at an initial substrate concentration of 0.25 M.
b
Conversions (convn.) and product selectivity were determined by quantitative GC-MS analysis
with a DB-5 capillary column on the crude reaction mixture.
c
0.04 mol% catalyst loading.
Among the solvents studied, methanol was the most effective with 100% conversion and
excellent selectivity for methyl phenyl sulfoxide (5a) (> 95% by GC) achieved in a short reaction
time, which manifests the high catalytic efficiency of this oxidation. The use of other solvents such
as CHCl3 or CH3CN resulted in reduced activity as well as selectivity (Table 1, entries 4 and 5),
presumably due to the low solubility of PhI(OAc)2. In this study, we found a significant catalyst
effect on the sulfoxidation of 4a in CH3OH. Fig. 1 shows the reaction profiles for the catalytic
sulfoxidation reactions of 4a by different catalysts that differ in the corrole ligands and oxidation
states of the manganese metal. For the four corrole systems studied here, the [MnIV(TPC)Cl] (2a)
and [MnIV(4-tBu-TPC)Cl] (2b) without electron-deficient substituents on the corrole ligands were
apparently most effective with quantitative yields and excellent selectivities achieved within 30 min
(entries 3 and 7). Remarkably, the catalyst 2b promoted the catalytic oxidation under low loadings
11

(down to 0.04 mol% in entry 8) without loss of activity, reaching a high efficiency with up to 2500
TON. The electron-deficient catalysts [MnIV(4-NO2-TPC)Cl] (2c) and [MnIV(TPFC)Cl] (2d)
catalyzed the oxidations in a slower fashion compared to less electron-demand 2a and 2b, and a
prolonged reaction time was required to obtain relatively good conversion (entries 10 and 11). This
activity trend correlates well with the redox potentials recorded for different corrole ligands in
which the electron-deficient groups such as F and NO2 on corrole ligands resulted in much higher
oxidation potentials to access high-valent oxidation states. This observation implied that the rate-
determining step in the manganese-corrole catalytic system is the oxygen atom transfer from
PhI(OAc)2 to generate the active oxidant (metal-oxo) and not from the proposed metal-oxo oxidant
to the sulfide substrate.
100
MnIV(TPC)Cl
MnIV(4-NO2TPC)Cl
80
MnIV(4-NO2TPC)Cl
MnIV(TPFC)Cl
60
40
20
0
MnIII(TPC)
0.0
0.2
0.4
0.6
0.8
1.0
Time (h)
Fig. 1. Time courses of oxidation of thioanisole (0.5 mmol) with PhI(OAc)2 (0.75 mmol) in CH3OH
(2 mL) at room temperature) in the presence of H2O (15 µL) catalyzed by different manganese
corroles ((1 µmol ). Aliquots were taken at selected time intervals for product analyses with GC-
MS.
Next, oxidation states of manganese catalyst have a profound impact on the catalytic
12

activity. [MnIV(Cor)Cl] proved to be much more active compared to MnIII catalysts that gave much
lower activities in the oxidation of thioanisole under identical conditions (entries 6 and 9). These
results suggested that manganese(IV) catalysts have a faster access to form the active oxidants by
PhI(OAc)2, and /or generate the more reactive oxidizing metal-oxo intermediates in comparison to
the manganese(III) complexes. For comparison, one of the best-known porphyrin catalysts
[MnIII(TPFPP)Cl] [TPFPP = tetra(pentafluorophenyl)porphinato] performed the sulfoxidation with
a much lower activity and selectivity under similar reaction conditions (entry 12).
3.2
Substrate scope
The promising results in Table 1 prompted us to apply the manganese corroles with the
PhI(OAc)2 to the catalytic sulfoxidations of various sulfides under optimized conditions. Table 2
lists the oxidized products and corresponding yields using 2a as the catalyst. Of note, catalyst 2b
gave comparable results under similar conditions. In analogy with what we observed for 4a, all
catalytic oxidation of substituted thioanisoles proceeded with a quantitative conversion into the
corresponding sulfoxides as the major identifiable products. In some cases, traces of sulfones (< 5
% by GC) were detected. Significant for preparative purposes, 100% conversions and excellent
yields (> 90%) in isolated products were obtained under scale-up conditions (entries 1 and 2). The
excellent catalytic efficiency displayed by manganese(IV) corroles is ascribed in part to good
solubility of the PhI(OAc)2 in solution and the slow and steady-state formation of PhIO in the
presence of small amount of water, which may dramatically improve stability of the corrole catalyst
against degradation, and at same time, permit the fast access to generate the active oxidant.
Table 2. Catalytic oxidation of substituted thioanisoles by manganese(IV) corrole (2a) and
a
PhI(OAc)2
13

Entry
1
Substrate
Time
(h)
Convn.
(%)b
Product
Yields
(%)b
95 (92)c
S
O
1
100
S
4a
5a
O
S
2
3
4
5
6
1
100
98 (95)c
S
F
4b
4c
F
5b
S
O
2
100
100
100
100
97
95
95
91
S
Cl
Me
Cl
5c
S
O
0.5
0.4
12
S
4d
Me
MeO
5d
O
S
S
MeO
4e
5e
S
O
S
O2N
4f
O2N
5f
S
O
S
7
4
100
100
96
66
4g
5g
8d
0.5
S
O
S
O
4h
O
5h
9e
24
100
96
S
O
Ph
4i
S
Ph
5i
14

O
S
S
10
1
6
100
15
99
HO
4j
HO
5j
S
O
S
11
98
4k
5k
a
o
Unless otherwise specified, all reactions were performed in CH3OH (2 mL) at ca. 23 C with 1.5
equiv. of PhI(OAc)2 (0.75 mmol), substrate (0.5 mmol), 0.2 mol% catalyst in the presence of H2O
(18 µL); sulfoxides and trace amounts of sulfone were detected by GC analysis of the crude
reaction mixture.
b Based on the conversion of substrates and determined by quantitative GC-MS analysis with an
internal standard on the crude reaction mixture after the reaction; material balance > 95%.
c Isolated yield in the parenthesis from a scaled-up reactions with 5 mmol substrate.
d Ring-opening products (30%) were detected by GC-MS.
e Using 2b as catalyst in CHCl3 due to the poor solubility of the substrate in CH3OH.
As evident in Table 2, the reactivity of thioanisoles was affected by the electronic effects of
the substituents on the aromatic rings of 4a-f. For examples, 4-methoxythioanisole (4e) resulted in the
fastest reaction among all substrates studied (entry 5). On the contrary, the oxidation of 4-
nitrothioanisole (4f) proceeded slowest and required much longer reaction time (12 h) to reach the
complete conversion (entry 6). This can be ascribed to the reduced nucleophilicity of sulfide due to the
electron-withdrawing substituents on the substrate. In addition, the relative rates (krel) for the catalytic
oxidation of several substituted thioanisoles (Y-thioanisole, Y = 4-MeO, 4-Me, 4-F, 4-Cl and 4-
NO2) were evaluated by monitoring the formation of sulfoxides by GC (see the Experimental
Section). Fig. 2 depicts a linear correlation (R = 0.92) of log krel [krel = k(Y-
thioanisole)/k(thioanisole)] versus Hammett σ+ substituent constant in the competitive oxidation by
[MnIV(TPC)Cl] catalyst. The slope (ρ+) of the plot is – 1.75, suggesting a single mechanism with
involvement of an electrophilic oxidant, most likely a corrole-manganese-oxo intermediate.
15

Furthermore, negative ρ+ value also indicates the transition states for rate-limiting steps involve
positive charge development. Similar negative values were also found for the sulfide oxidations
with the corrole-manganese(V)-oxo species with halogen-substituted ligand (ρ+ = –1.06 to –1.34),
[37] iron(IV) corrole catalysts (ρ+ = –0.85 to – 0.98), and inorganic and organic hydroperoxides (ρ+
= –0.65 to –1.90).
1.0
TPC
+
 = -1.75
p-Me
0.5
0.0
p-MeO
R = 0.92
p-H
p-F
-0.5
-1.0
-1.5
-2.0
p-Cl
p-NO
2
-1.0
-0.5
0.0
0.5
1.0
+

Fig. 2. Hammett correlation studies (log(krel) vs σ+) for [MnIV(TPC)Cl] (2a) catalyzed sulfoxidation
of thioanisole and substituted thioanisoles by PhI(OAc)2 in CH3OH at 23 ± 2 °C.
Sulfoxidation of diphenyl sulfide 4g was also found to be relatively slow (entry 7), possibly due
to the steric effect of two bulky phenyl groups making it harder to access the oxidizing intermediate for
the oxo transfer reaction. In this study, the seemingly large and rigid substrate dibenzothiophene was
shown to be inert toward this catalytic system (data not shown) that might be ascribed to sterics. Its
efficacy for sulfoxidation was further demonstrated in the oxidation of allyic sulfide 4i (entry 9). In
many oxidants such as peroxy acids, NaIO4, and traditional catalytic systems, the oxidation of
16

sulfides is often interfered by the presence of electron-rich double bonds. With catalyst 2b and
PhI(OAc)2 in the presence of H2O, we found that no epoxidation took place and only sulfoxide 4i
was obtained in a quantitative yield even with slower reaction rate (entry 9). Similarly, the presence
of the hydroxyl group did not disturb the selective oxidation of sulfide and no alcohol oxidation was
observed (entry 10). Surprisingly, the oxidation of alkyl sulfide (2j) proceeded much slower and only
15% conversion was obtained after 6 h (entry 11), presumably owing in part to its reduced
nucleophilicity and poor solubility in CH3OH.
3.3
Kinetic studies
The synthetic value of the new catalytic system presented above is indisputable, but
mechanistic understanding of the complex OAT processes is important for the design of still more
effective and selective oxidants. High-valent metal-oxo corroles have been proposed as the key
intermediates in many metallocorrole-catalyzed reactions. [32, 38, 39] Of note, the possible
generation of the reactive manganese(V)-oxo corroles (3) was reported in previous studies.[23, 27,
37] To this end, we conducted the chemical oxidation of MnIII(Cor) by PhI(OAc)2 in CH3OH in the
absence of sulfides to probe the nature of the active oxidizing intermediate. In metal-oxo chemistry,
one typically can expect that the formed metal-oxo intermediates may behave differently due to the
different electronic environments of the ligands.
As shown in Fig. 3A, addition of 10 equivalents of PhI(OAc)2 resulted in a fast conversion
of [MnIII(TPC)(OEt2)2] to a known manganese(V)-oxo 3a, i.e. [MnV(TPC)O], which displays
distinct split Soret bands, matching the spectra reported previously from ozone, m-CPBA or
photochemical oxidation reactions.[35, 37, 40] In a fashion similar to that described for the
generation of 3a, the highly electron-deficient [MnV(TPFC)O] (3d) was also formed with a slightly
17

slower rate than that of 3a due to the expected higher oxidation potential of TPFC system. (Fig.
3B). It is worth noting that using the same procedure by reacting [MnIV(Cor)Cl] with PhI(OAc)2 did
not give the corresponding [MnV(Cor)O] intermediates among all corrole systems studied here.
B
A
2.0
O=MnVTPFC
MnIIITPFC
V
O=Mn TPC
III
Mn TPC
1.5
1.0
0.5
0.0
300
700
400
500
600
700
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. UV-vis spectra for chemically generated oxo-species (A) [MnV(TPC)O] (3a, solid red) from
reaction of [MnIII(TPC)(OEt2)2] (dash black); (B) [MnV(TPFC)O] (3d, solid red) from reaction of
[MnIII(TPFC)(OEt2)2] (dash black) with excess PhI(OAc)2 (10 equiv.) in CH3OH solutions.
In the presence of organic substrates, the time-resolved spectra show the clean conversion of
3a to regenerate the MnIV species that was characterized by its known λmax at 430 nm (Fig. 4A).
Again, this observation is in accord with previous photochemical studies of 3a. [35, 40] In kinetic
studies, we monitored the absorbance in the Soret band region at 350 nm, which decreased over the
course of reaction without overlap with other peaks (Fig. 3B). The traces were fit to single-
exponential decays, as expected for reactions under pseudo-first-order conditions. Plots of kobs
versus concentration of thioanisole as the substrate typically gave straight lines with near-zero
18

intercepts (Inset in Fig. 4B). In contrast, the reaction of more electron-deficient 3d with thiosanisole in
CH3OH regenerated a MnIII product only, and there was no evidence for the accumulation of any MnIV
species during the reaction course (Fig. 5). As indicated in previous reports,[37, 40] this reaction
sequence is consistent with a direct two-electron reduction from MnV to MnIII. Regardless of the
complexity of the decay reactions in different corrole systems, the kinetics of the reactions of 3a and 3d
with thioanisoles followed a simple second-order rate law. We measured the kinetics in four sets of
studies for several substrates with three independent kinetic runs, and the second-order rate
constants are listed in Table 3. Noticeably, the kinetic data of 3a in this work from the chemically
generated method were comparable with the literature reported kox for the same oxidant produced
from photochemical generation.[40]
MnVI
A
B
MnV
0.30
0.15
0.00
0.45
0.30
0.15
0.00
0.00
0.02
0.04
Thioanisole (M)
300
400
500
600
100
200
300
400
500
Wavelength (nm)
Time (s)
Fig. 4. (A) Time-resolved spectra for the reaction of 3a with thioanisole (20 mM) over 5 min (cycle
time =50 s per spectrum) in CH3OH at 23 oC. (B) Kinetic traces monitored at max = 350 nm in the
presence of 0.01 M (black), 0.02 M (blue), 0.03 M (red) and 0.04 M (green) thioanisole. Inset
19

shows plot of the observed pseudo-first-order rate constants versus the concentration of thioanisole.
The standard deviations were at 2 .
2.0
MnV
1.6
MnIII
1.2
0.8
0.4
300
400
500
600
700
Wavelength (nm)
Fig. 5. Time-resolved spectra for the reaction of 3d (2.0 × 10-5 M) with thioanisole (20 mM) over 6
min (cycle time = 60 s per spectrum) in CH3OH at 23 oC.
Table 3. Second-order rate constants for reactions of corrole-manganese(V)-oxo species a
Entry
Substrate
Rate constant (M-1 s-1)
3ab
3ac
6.0 ± 0.8
24.8 ± 1.5
-
3db
1
2
3
4
PhSCH3
7.9 ± 0.3
25.8 ± 2.6
12.0 ± 0.9
91.0 ± 5.1
5.3 ± 0.1
7.0 ± 0.4
3.5 ± 0.2
62.0 ± 5.7
p-F-PhSCH3
p-Cl-PhSCH3
p-MeO-PhSCH3
85.5 ± 2.4b
20

a In CH3OH at 23 ± 2 oC. The values are average of 2-3 runs with 2σ standard deviation; b Formed
by PhI(OAc)2 (ca. 10 equiv.) oxidation in this work. c Photo-generated in CH3CN from ref. 41.
In addition to decaying back to MnIV species, the second-order rate constants determined
from pseudo-first-order kinetic studies for reactions of 3a with thioanisoles are apparently greater in
comparison with those of more electron-deficient species 3d with same reductants (Table 3).
Typically, the reactivity of the electrophilic high-valent metal-oxo species is increased with the
more electron-deficient ligands. When the rates of 3a and 3d are compared, however, the reactivity
order is inverted with the system of less electron demand, i.e. the TPC complex 3a reacting faster
with any given substrate than the more electron-deficient TPFC 3d. According to the previously
proposed mechanistic model,[40] these kinetic results can be rationalized by disproportionation of
3a to give MnIV corrole and a more reactive corrole-manganese(VI)-oxo cationic species as the
predominant oxidant, which transfers an oxygen atom to organic sulfides and generates another
equivalent of the MnIV species. In the more electron-deficient TPFC system, MnV-oxo corrole is the
electrophilic oxidant which directly oxidized sulfides to form the MnIII product in the end of
reaction as we observed in Fig. 5.
3.4
Competition studies
One objective of our studies is to identify the active oxidants during catalytic turnover
conditions with these manganese corrole catalysts, which could lead to better control of the
catalytic oxidation reactions, especially in terms of selectivities. In a typical catalytic reaction,
however, the concentrations of active oxidants do not build up to detectable concentrations.
Moreover, as our kinetic studies of porphyrin-manganese(IV)-oxo species,[41] corrole-
21

manganese(V)-oxo species,[35] and porphyrin-iron(IV)-oxo species[42] indicate, a high-valent
metal-oxo species detected in a reaction might not be the true oxidant in the system.
To evaluate whether the directly observed manganese(V)-oxo species 3 in the above kinetic
studies is also the active oxidant under catalytic turnover conditions, the competitive sulfoxidation
reactions catalyzed by manganese(III) corroles (1) and chloro-manganese(IV) corroles (2) with
PhI(OAc)2 as the sacrificial oxygen source were conducted as described in the experimental section.
In competition studies, a limiting amount of sacrificial oxidant was used to keep the conversion less
than 20%. The amounts of oxidation products formed were determined by GC analysis. Each
sulfide substrate was oxidized to the corresponding sulfoxide in nearly quantitative yield based on
the oxidant consumed without forming the over-oxidation product sulfone. Thus, the rate of
sulfoxide formation should reasonably reflect the thioanisole reactivity toward the manganese
corrole-catalyzed oxidations.
If the ratio of products in the competitive oxidation reaction is the same as the ratio of
absolute rate constants, then one can infer that the same oxo species is active under catalytic and
single-turnover conditions. For example, it was found that the ratio of rate constants for reaction of
TPFC-manganese(V)-oxo species 3d with organic sulfides is similar to the ratio of products found
in competitive catalytic oxidations (Table 4). Differences in inherent reactivities of the thioanisoles
obviously influence the ratio of products, but the close match between the ratios of absolute rate
constants and the ratios of products from the competition experiment strongly suggests that species
manganese(V)-oxo 3d was likely the active oxidant in the catalytic oxidation reactions by
MnIII(TPFC) with PhI(OAc)2, consistent with the previous report.[37]
Table 4. Relative rate constants from kinetic studies and competitive catalytic studies.a
b
Catalyst
Substrates
Method
krel
22

[MnIII(TPFC)] 1d
[MnIV(TPC)Cl] 2a
[MnIV(TPFC)Cl] 2d
p-F-PhSMe/PhSMe
p-Cl-PhSMe/PhSMe
p-F-PhSMe/PhSMe
p-MeO-PhSMe/PhSMe
p-F-PhSMe/PhSMe
p-MeO-PhSMe/PhSMe
Kinetic results c
PhI(OAc)2
1.32
1.35
0.66
0.70
3.30
0.62
11.52
4.55
1.32
0.95
11.70
4.76
Kinetic results c
PhI(OAc)2
Kinetic results c
PhI(OAc)2
Kinetic results c
PhI(OAc)2
Kinetic results c
PhI(OAc)2
Kinetic results c
PhI(OAc)2
a Equal amounts of two substrates, e.g. thioanisole (0.25 mmol) and substituted
thioanisole (0.25 mmol), catalyst (1 μmol), H2O (5 μL), and PhI(OAc)2 (0.1 mmol) in
CH3OH (2 mL) was stirred for ca. 20 to 30 min at 23 ± 2 °C.
b Relative ratios of absolute rate constants from kinetic results and competitive
oxidations at ambient temperature.
c Kinetic results were calculated based on data collected in the Table 3 of this work.
The situation with manganese(IV) catalysts is quite different. Table 4 also lists some results
from competition studies with [MnIV(TPC)Cl] (2a) and [MnIV(TPFC)Cl] (2d) catalysts and the
ratios of absolute rate constants for reactions of the same substrates with the observed
manganese(V)-oxo transient 3a and 3d. As evidenced in Table 4, the ratios of absolute rate
constants found in direct kinetic studies differed substantially from the oxidation ratios for
competition oxidation reactions of the two substrates in both systems studied here. Substantially
23

different behavior is particularly apparent for oxidations of para-fluorothioanisole and thioanisole,
in which case the absolute rate constant for oxidation of para-fluorothioanisole by 3a is greater than
that for oxidation of thioanisole by the same oxo species, but para-fluorothioanisole is oxidized
faster than thioanisole in competition studies with manganese(IV) corrole as the catalyst. One
obvious explanation for this behavior is that a manganese(V)-oxo species is unlikely to serve as the
active oxidant under catalytic conditions with PhI(OAc)2. Furthermore, the ratios of rate constants
for oxidations by 3a or 3d are much larger than the observed product ratios from competition
studies, suggesting that the manganese(V)-oxo corrole is less reactive (higher selectivity) than the
species formed in the catalytic reactions. Thus, the competition studies might explain that catalytic
oxidation reactions catalyzed by [MnIV(Cor)Cl] were much more efficient than those by the
[MnIII(Cor)(OEt2)2] catalyst (Table 1 and Fig. 1). Reaction of the corrole-manganese(IV) catalysts
with the sacrificial oxidant PhI(OAc)2 possibly give a more reactive manganese(VI)-oxo species
that is the major oxidant of the sulfide substrate, even though the definitive detection and
assignment of the elusive oxidant needs to be spectroscopically characterized in the future studies.
4
Conclusion
In conclusion, we have demonstrated that the corrole-manganese complexes effectively
catalyzed the oxidation of aryl sulfides into sulfoxides with PhI(OAc)2 in the presence of a small
amount of water. On the basis of catalyst effects, the corrole ligands and the oxidation states of
manganese metal played important roles in the catalytic sulfoxidations, in which manganese(IV)
catalysts containing non-electron deficient ligands were found to be more effective than electron-
deficient manganese(III) corroles. The readily soluble and mild oxidative nature of PhI(OAc)2
overcame the limitations for corrole- manganese catalysts, which exhibited high activity due to the
improved stability against catalyst degradation. The previously known manganese(V)-oxo corroles
were produced by chemical oxidations in mixing experiments and their kinetic data with
24

thioanisoles were accumulated in this work. The competition studies have suggested the different
oxidant forms might be involved under catalytic turnover conditions. The corrole-manganese(V)-
oxo intermediates observed in the kinetic studies appear to be involved as the active oxidants when
manganese(III) catalysts were employed, since the product ratios in catalytic reactions closely
matched the ratios of rate constants from the kinetic studies. However, a more reactive oxidant,
possibly manganese(VI)-oxo derivatives might be the oxidants in the manganese(IV) corrole-
catalyzed sulfoxidation reactions. Further studies to define synthetic applications and detect the
oxidizing transient species are underway in our laboratory.
ACKNOWLEDGMENTS
This research is supported by the National Science Foundation (CHE 1464886 and 1764315)
D. Ranburger and B. Kash are thankful to the WKU Office of Research for awarding internal FUSE
grants. We also thank Ms. Pauline Norris of the Advanced Material Institute at WKU for ESI-MS
measurements.
25

Table of Abbreviations
Corrole
Cor
TPC
5,10,15-triphenylcorrole trianion
5,10,15-tris(4-tert-butylphenyl)corrole trianion
5,10,15-tris(4-nitrophenyl)corrole trianion
5,10,15-tripentafluorophenylcorrole trianion
5,10,15, 20-tetrakis(pentafluorophenyl)porphyrin dianion
iodobenzene diacetate
4-tBu-TPC
4-NO2-TPC
TPFC
TPFPP
PhI(OAc)2
PhIO
iodosylbenzene
m-CPBA
TONs
m-chloroperoxybenzoic acid
turnover numbers
OAT
oxygen atom transfer
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Graphical Abstract:
Manganese(IV) corroles catalyze efficient and selective sulfoxidations with iodobenzene diacetate
as the mild oxygen source. The mechanistic studies suggested the different active oxidant forms
were involved and significantly affected the catalytic activity.
27

H2O
PhI(OAc)2
PhI + HOAc
Ar
Ar
O
Mnn+2
N
N
N
N
N
N
Mnn
Ar
Ar
Ar
Ar
N
N
n = 3 and 4
O
S
S
Ar
R
Ar
R
100% conversion
> 90% yield
Research Highlights:
 Manganese corroles catalyzed efficient sulfoxidations with iodobenzene diacetate
 The reaction conditions, substrate scope and catalyst effects were investigated
 The synthetic utility of this catalytic method was demonstrated
 Kinetic and competition studies were conducted to probe the active oxidants
28