A biomimetic oxidation catalyzed by manganese(III) porphyrins and iodobenzene diacetate: Synthetic and mechanistic investigations

Article abstract of DOI:10.1016/j.ica.2015.03.003

Abstract With iodobenzene diacetate [PhI(OAc)₂] as the oxygen source, manganese(III) porphyrin complexes exhibit remarkable catalytic activity toward the selective oxidation of alkenes and activated hydrocarbons. Conspicuous is the fact that the readily soluble PhI(OAc)₂ in the presence of a small amount of water is more efficient than the commonly used PhIO and other oxygen sources under same catalytic conditions. High selectivity for epoxides and excellent catalytic efficiency with up to 10,000 TON have been achieved in alkene epoxidations. It was found that the reactivity of manganese(III) porphyrin catalysts was greatly affected by axial ligand and the weakly binding perchlorate gave the highest catalytic activity in the epoxidation of alkenes. A manganese(IV)-oxo porphyrin was detected in the reaction of the manganese(III) porphyrin and PhI(OAc)₂. However, our catalytic competition and Hammett studies have suggested that the more reactive manganese(V)-oxo intermediate was favored as the premier active oxidant, even it is too short-lived to be produced in detectable concentrations.

Full text of DOI:10.1016/j.ica.2015.03.003

Inorganica Chimica Acta 430 (2015) 176–183
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Inorganica Chimica Acta
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A biomimetic oxidation catalyzed by manganese(III) porphyrins
and iodobenzene diacetate: Synthetic and mechanistic investigations
⇑
Ka Wai Kwong, Tse-Hong Chen, Weilong Luo, Haleh Jeddi, Rui Zhang
Department of Chemistry, Western Kentucky University, 1906 College Heights Blvd. #11079, Bowling Green, KY 42101, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
With iodobenzene diacetate [PhI(OAc)2] as the oxygen source, manganese(III) porphyrin complexes exhi-
bit remarkable catalytic activity toward the selective oxidation of alkenes and activated hydrocarbons.
Conspicuous is the fact that the readily soluble PhI(OAc)2 in the presence of a small amount of water
is more efficient than the commonly used PhIO and other oxygen sources under same catalytic condi-
tions. High selectivity for epoxides and excellent catalytic efficiency with up to 10,000 TON have been
achieved in alkene epoxidations. It was found that the reactivity of manganese(III) porphyrin catalysts
was greatly affected by axial ligand and the weakly binding perchlorate gave the highest catalytic activity
in the epoxidation of alkenes. A manganese(IV)-oxo porphyrin was detected in the reaction of the
manganese(III) porphyrin and PhI(OAc)2. However, our catalytic competition and Hammett studies have
suggested that the more reactive manganese(V)-oxo intermediate was favored as the premier active oxi-
dant, even it is too short-lived to be produced in detectable concentrations.
Received 19 January 2015
Received in revised form 27 February 2015
Accepted 5 March 2015
Available online 19 March 2015
Keywords:
Manganese porphyrin
Catalytic oxidation
Iodobenzene diacetate
Manganese-oxo species
Ó 2015 Elsevier B.V. All rights reserved.
t
1. Introduction
H2O2, BuOOH (tert-butylhydroperoxide), KHSO5 and oxaziridines
[9]. Molecular oxygen can also be used in the presence of an elec-
Catalytic oxidation is a pivotal synthetic transformation in
organic laboratories and chemical industries [1]. As the demand
for greener processing increases [2,3], the transition metal cat-
alyzed oxidations of hydrocarbons to oxygenated products with
environmentally friendly oxygen sources such as molecular oxygen
are becoming the most important and rewarding protocols in
chemical industries [4,5]. In Nature, the ubiquitous cytochrome
P-450 enzymes (P450s) [6] can catalyze a wide variety of oxidation
reactions with exceptionally high reactivity and selectivity [7,8]. In
this context, many transition metal catalysts with a core structure
closely resembling that of the iron porphyrin core of P450s, have
been synthesized as models to invent enzyme-like oxidation cata-
lysts as well as to probe the sophisticated mechanism of molecular
oxygen activation [9,10]. Among the most extensively studied sys-
tems are the epoxidation of alkenes and hydroxylation of alkanes
catalyzed by transition metals in macrocyclic ligands, including
those containing chiral auxiliaries for enantioselective oxidations
[11]. Historically, synthetic manganese porphyrin complexes have
shown catalytic promise as P450s enzyme model in oxidation reac-
tions over decades [9,12]. The sacrificial oxidants compatible with
manganese porphyrins were mostly restricted to PhIO, NaOCl,
tron source [13]. The use of H2O2 often results in oxidative degra-
dation of the catalyst due to the potency of this oxidant [9]. In
contrast to epoxidations catalyzed by other metals, the manganese
porphyrin-catalyzed oxidation gave low stereospecificity. For
example, the epoxidation of cis-stilbene catalyzed by MnIII(TPP)Cl
(TPP = tetraphenylporphyrin) and PhIO generated cis- and trans-
stilbene oxide in a ratio of 35:65 [14].
Identifying the kinetically competent oxidants could lead to
better control of the catalytic oxidation reactions, especially in
terms of selectivities. In general, high-valent transition metal-oxo
transients are invoked as the active oxidizing species in many
metal-catalyzed oxidations [15]. However, in most catalytic reac-
tions, the concentrations of active metal-oxo oxidants do not build
up to detectable amounts. For example, highly reactive porphyrin-
manganese(V)-oxo derivatives were proposed as the key inter-
mediates in catalytic processes for decades [14,16], but they
eluded detection until 1997 when Groves and co-workers reported
the synthesis of the first manganese(V)-oxo porphyrin complex
[17]. Subsequently, additional examples of manganese(V)-oxo
porphyrins were reported [18,19]. In contrast, the well character-
ized manganese(IV)-oxo derivatives are less reactive than
manganese(V)-oxo species and unlikely to be the dominant oxi-
dants in manganese porphyrin-catalyzed oxidations [20–22]. In
previous laser flash photolysis (LFP) studies reported by
Newcomb and coworkers, both porphyrin-manganese(IV)-oxo
⇑
Corresponding author. Tel.: +1 270 745 3803; fax: +1 270 745 5361.
E-mail address: rui.zhang@wku.edu (R. Zhang).
http://dx.doi.org/10.1016/j.ica.2015.03.003
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
177
species and porphyrin-manganese(V)-oxo species were photochemi-
cally produced as a function of the identity of the counterions bind-
ing to the metal of the manganese(III) [23,24]. For homogeneous
catalysts, where catalytic species have ready access to one another,
high-valent metal-oxo species might be detected, but these species
might not be reactive enough to be the true oxidants. Moreover, as
reported studies of porphyrin-manganese(IV)-oxo species [24], and
corrole-manganese(V)-oxo species [25], indicate, a high-valent
metal-oxo species detected in a reaction might not be the true
oxidant in the system. Specifically, the observable less reactive por-
phyrin-manganese(IV)-oxo species can react via a disproportiona-
tion pathway to give an manganese(V)-oxo intermediate that is
the true oxidant but is not produced in detectable amounts [24].
Our group initiated a general program that aims to fully exploit
the potential of metalloporphyrin complexes toward oxidation
reactions, with an ultimate goal of developing practical metallo-
porphyrin oxidation catalysts for organic synthesis. In contrast to
the sacrificial oxidants in common use for metal-catalyzed reac-
tions, iodobenzene diacetate, i.e. PhI(OAc)2, has been less often
employed for porphyrin-manganese-catalyzed oxidations due to
its mild oxidizing ability. Of note, PhI(OAc)2 is readily soluble in
organic media and safe to use. In particular, it does not show
appreciable reactivity toward organic substrates nor damage the
metal catalysts under the usual catalytic conditions. Collman and
Nam reported, respectively, the use of PhI(OAc)2 as terminal oxi-
dant for the iron(III) porphyrin catalyzed oxidation of hydrocar-
bons [26,27]. Xia and coworkers also described the
manganese(III) porphyrin catalyzed epoxidation of several alkenes
with PhI(OAc)2 in the ionic liquid/CH2Cl2 mixed solvent [12,13].
Adam et al. described a highly selective oxidation of alcohols by
chromium(III) salen with PhI(OAc)2 [28]. In addition, Nishiyama
showed that PhI(OAc)2 is a better oxidant than PhIO in ruthe-
nium–pyridine-2,6-dicarboxylate complex-catalyzed epoxidation
of trans-stilbene [29]. We recently employed PhI(OAc)2 as an effi-
cient oxygen source for the selective catalytic sulfoxidations by
ruthenium porphyrins under visible light irradiation [30]. In our
unpublished work, we have discovered that the electron-deficient
corrole-iron(III) complex catalyzed efficient oxidation of alkenes
with PhI(OAc)2 as a promising oxygen source. The inherent unsta-
ble corrole-catalyst against degradation was much improved
owing to the mild oxidizing power of PhI(OAc)2. In this study, we
present our detailed findings on the usefulness of PhI(OAc)2 for
the highly efficient catalytic oxidation of alkenes and activated
benzylic hydrocarbons by porphyrin-manganese(III) catalysts
(1a–c in Scheme 1). In most cases, quantitative conversions of sub-
strates, excellent selectivities and high turnovers of up to 10,000
TON were obtained. Meanwhile, we show that a low-reactivity
manganese(IV)-oxo porphyrin intermediate which was detected
by the oxidation of the manganese(III) porphyrin with PhI(OAc)2,
is not likely the sole oxidant. Instead, a more reactive porphyrin-
manganese(V)-oxo species is favored as the premier active oxidant.
2. Results and discussions
2.1. Screening studies
The potential of PhI(OAc)2 as an oxygen source was first evalu-
ated in the catalytic epoxidation of cis-cyclooctene (2a) by different
manganese(III) porphyrin catalysts (1a–c in Scheme 1). Under mild
homogeneous conditions, the epoxidations were carried out with a
catalyst: substrate: PhI(OAc)2 ratio of 1:200:300 (Table 1). After
30 min of reaction in CH3CN, cis-cyclooctene oxide (3a) was
obtained as the only identifiable oxidation product (>99% by GC)
Table 1
Manganese(III) porphyrin-catalyzed epoxidation of cis-cyclooctene with PhI(OAc)2.a
MnIII(Por)X (0.01-0.5 mol%)
O
PhI(OAc)2 (1.5 equiv.)
2a
3a
Entry Catalyst
Solvent
t (min)
Convn
(%)b
Yields(%)c
1d
2
3
MnIII(TPFPP)Cl CH3CN
30
30
10
22
100
100
100
100
100 (93)e
(50 °C)
200
(50 °C)
30
30
30
4f
100
100
5
6
7
CH3OH
CH2Cl2
CH2Cl2/CH3CN
37
67
90
99
100
100
(v/v = 1)
CH3CN
CH3CN
CH3CN
8
9
10
MnIII(TPP)Cl
MnIII(TMP)Cl
30
30
10
7
8
23
100
100
100
(50 °C)
a
Unless otherwise specified, all reactions were carried out in solvent (0.5 mL)
with H2O (5 L) at 23 °C with cis-cyclooctene (0.20 mmol), 1.5 equiv. of PhI(OAc)2
l
and 0.5 mol% of manganese(III) porphyin catalysts.
b
Determined by GC–MS analysis of the crude reaction mixture with a capillary
column (J&W Scientific Cyclodex B).
c
Based on the amount of substrate consumed; material balances > 95%.
Without H2O.
Isolated yield after column chromatography (silica gel).
0.01 mol% catalyst loading.
d
e
f
Scheme 1. Biomimetic oxidations catalyzed by manganese(III) porphyrins (1) with PhI(OAc)2.

178
K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
with ca. 22% conversion (Table 1, entry 1). The most striking fea-
ture of these catalytic epoxidations by MnIII(TPFPP)Cl (1a;
TPFPP = tetrakis(pentafluorophenyl)porphyrin) is the remarkable
enhancement of the reaction rate by addition of H2O. Thus, the
same epoxidation proceeded much more rapidly with a small
2.2. Comparison of various oxygen sources
The promising results with the PhI(OAc)2 in Table 1 prompted
us to evaluate other common oxygen sources in the
manganese(III) porphyrin-catalyzed epoxidation of cyclohexene
for the intended purpose of comparison. Cyclohexene not only is
common substrate for epoxidation reactions but also offers the
opportunity to test the chemoselectivity of the oxidation in terms
of epoxide versus allylic alcohol formation, that is, oxygen transfer
to the double bond or oxygen insertion into the allylic C–H bond. A
screening of diverse oxygen source under identical experimental
conditions disclosed that the mild oxygen source PhI(OAc)2 was
especially effective for selective oxidation of cyclohexene to the
corresponding epoxide, as representative results are shown in
Table 2. Although iodosobenzene (PhIO) is a more common oxygen
source used for metalloporphyrin-catalyzed oxidations, it was
found that the use of the soluble PhI(OAc)2 under the same condi-
tions led to the epoxide in higher substrate conversion and selec-
tivity for epoxide (Table 2, entries 1 and 3). The use of more
oxidizing NaOCl gave lower conversion and selectivities (entry 4).
amount of H2O (5 lL), and quantitative conversions were obtained
within 30 min at 23 °C (entry 2) or 10 min at 50 °C (entry 3). Fig. 1
depicts the time courses for the epoxidation in the presence and
absence of water with 1a and 1c, respectively. The effect of water
amounts on the epoxidation of cis-cyclooctene 2a with 1a in ace-
tonitrile was further investigated (see Fig. S1 in Supporting
Information). Clearly, addition of 5–10
maximum acceleration in the rate of catalytic reaction, whereas
addition of over 100 L water slowed the reaction. Similar water
lL of water resulted in a
l
accelerating effect observed in the previously reported iron(III)
porphyrin/corrole-catalyzed oxidations was rationalized in terms
of the formation of more oxidizing PhIO [27]. Another striking cat-
alytic enhancement caused by the addition of water was observed
in the alkene expoxidation with IOꢀ4 catalyzed by 2-methylim-
idazole and MnIII(TPFPP)(OAc), where deprotonation of imidazoles
to imidazolate may play a significant role in the presence of water
[11]. These findings indicate that the access of the oxygen source to
the metal center might be crucial. Water is a dissociating solvent
and helps removal of the axial ligand and efficiently binding oxy-
gen source.
To test the synthetic utility of the method, the epoxidation of
cis-cyclooctene was scaled up to 4.0 mmol and 100% conversion
and 95% isolated yield exclusively for epoxide were still obtained.
Remarkably, the catalyst loading of 1a can be as low as of
0.01 mol% (entry 4) without significant loss of activity, illustrating
an unequivocally the high efficiency (10,000 TON!). The use of
CH3OH or CH2Cl2 as solvent instead of CH3CN resulted in reduced
t
With BuOOH and H2O2, significant amounts of allylic oxidation
products, i.e. 2-cyclohexenol (4b) and 2-cyclohexenone (5b) were
obtained with minor epoxide (entries 5 and 6). Using heterocyclic
additives such as pyrazole can improve the chemoselectivity for
epoxide (entry 7). The most likely explanation is that these oxygen
sources might generate different reactive oxidizing intermediates
and/or reaction mechanisms with manganese porphyrin catalyst
than that with PhI(OAc)2. Besides the high selectivity and catalytic
efficiency, an additional advantage of the PhI(OAc)2 compared to
the other oxidants, is the fact that it will not cause the degradation
of the porphyrin catalysts. Again, a very high catalytic activity with
a TON of 9400 was achieved at the very low catalyst loading of only
0.01 mol% (entry 2).
activity (entries
5
and 6). When the non-halogenated
MnIII(TPP)Cl (1b) and MnIII(TMP)Cl (1c; TMP = tetramesityl-
porphyrin) were used as catalyst, poor activities were obtained
within 30 min at ambient temperature (entries 8 and 9). Control
experiments demonstrated that no epoxide was formed in the
absence of either the catalyst or PhI(OAc)2 even at elevated
temperature (50 °C).
2.3. Substrate scope
In view of the efficient epoxidation of cyclohexene and cis-
cyclooctene with PhI(OAc)2 catalyzed by manganese(III) por-
phyrins, the catalytic activity of MnIII(TPFPP)Cl (1a) toward oxi-
dations of a variety of organic substrates were investigated in
Table 2
Catalytic oxidation of cyclohexene by the MnIII(TPFPP)Cl (1a) with various oxygen
sources.a
OH
O
MnIII(TPFPP)Cl (0.5 mol%)
O +
+
OS (1.5 equiv.), CH3CN, 23oC
2b
3b
4b
5b
Entry Oxygen source
(OS)
t (h)
Convn
Product ratiob
(3b:4b:5b)
(%)b
1
PhI(OAc)2
2
3
95
94
93:7:0
86:2:12
2c
(50 °C)
3
4
5
PhIO
2
2
2
6
2
86
61
53
5
90:2:8
NaOCl (8% aq.)
tBuOOH (70% aq.)
H2O2 (30% aq.)
85:4:11
2:85:13
11:63:26
86:7:7
6
7d
14
a
Unless otherwise specified, all reactions were carried out in CH3CN (0.5 mL)
with H2O (5 L) at 23 °C with cyclohexene (0.20 mmol), 1.5 equiv. of PhI(OAc)2 and
l
0.5 mol% of manganese(III) porphyrin (1a).
Fig. 1. Time courses of epoxidation of cis-cyclooctene (0.2 mmol) with PhI(OAc)2
b
(0.30 mmol) in CH3CN (0.5 mL) at room temperature catalyzed by MnIII(TPFPP)Cl
Determined by GC–MS analysis of the crude reaction mixture with a capillary
without water (cube); MnIII(TMP)Cl with 5 L of water (diamond); MnIII(TPFPP)Cl
l
column (J&W Scientific Cyclodex B); material balances > 95%.
c
0.01 mol% of catalyst loading and 92% isolated yield.
With 5 mg of pyrazole.
with 5
lL of water (circle). Aliquots were taken at selected time intervals for GC
d
analyses.

K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
179
the presence of a small amount of H2O at elevated temperature
(50 °C). Table 3 lists the oxidized products and corresponding
substrate conversions and product yields including isolated
yields using 1a as catalyst with PhI(OAc)2. As evident in
Table 3, in most cases, quantitative conversions, excellent selec-
tivity and rapid turnovers (up to 20 TOF minꢀ1) were observed
within a short reaction time. In the oxidation of alkenes, epox-
ides were afforded as the major products with high efficiency.
For example, epoxidation of norbornene was completed within
10 min, giving primarily exo epoxide with negligible amounts
of endo products (Table 3, entry 1). Significant for preparative
purposes, most reactions gave comparable yields in isolated
products (entries 1–3, 8, 10, 11, 13 and 14). In the epoxidation
of styrene and substituted styrene, 100% conversions were
observed albeit with small amounts of aldehyde products
(entries 2–7). Epoxidation of cis-stilbene gave epoxide product
with a ratio of cis/trans = 91:9 (entry 8). In contrast, trans-alkenes
afforded corresponding trans-epoxides exclusively with complete
stereoretention (entries 10 and 11). This catalytic activity and
product selectivity is major improvement over previously
a
reported oxygen sources including PhIO and NaOCl [9,12].
Remarkably, the MnIII(TPFPP)Cl effectively catalyzes the epox-
idation of the allylic alcohol mainly to the epoxy alcohol with
a ratio of cis/trans = 1:4 in a quantitative conversion (entry 12).
Although the rate of oxidation reactions was markedly influ-
enced by the presence of water, the product yields and dis-
tributions in most cases were not affected by the presence and
absence of H2O. Similarly, the oxidation of secondary benzylic
alcohols gave the corresponding ketones with excellent catalytic
activities (entries 13 and 14), similar to the alkene epoxidation.
Activated alkanes including triphenylmethane, ethylbenzene
and diphenylmethane were oxidized to the corresponding alco-
hols and/or ketones from over-oxidation with lowest activity
(entries 15–17). It is noteworthy that monitoring catalytic reac-
tions by UV–Vis spectroscopy indicated no appreciable catalyst
bleaching in the end of reactions.
Table 3
Catalytic oxidation with PhI(OAc)2 in the presence of H2O by MnIII(TPFPP)Cl.a.
Entry
1
Substrate
Products
t (min)
Convn. (%)b
100
Yields (%)c (isolated yield)
100e (88)
TOFd (minꢀ1
20
)
10
O
2
3
10
10
100
100
91 (85)
92f (90)
20
20
O
O2N
O2N
O
O
O
4
5
6
10
10
10
100
100
100
87f
92f
95f
18
19
20
F
F
Cl
Cl
Me
O
O
Me
7
8
10
10
100
100
100
20
20
MeO
MeO
91(cis):9(trans) (82)
Ph
Ph
Ph
Ph
O
9
10
20
100
100
90
18
20
O
10
99 (trans) (91)
Ph
Ph
O
O
Ph
Ph
Ph
Ph
11
12
10
10
100
100
99 (94)
78g:22
20
20
Me
O
Me
OH
OH
O
O
13
14
10
10
97
100 (92)
100 (95)
19
20
O
OH
Ph
Ph
Ph
Ph
100
OH
O
Ph
Ph
Ph
Ph
OH
Ph
15
24 (h)
43
100
0.06
Ph
Ph
Ph
16
17
24 (h)
24 (h)
82
86
24:76
67:33
0.1
0.1
O
Ph
OH
Ph
Ph
Ph
Ph
O
OH
Ph
Ph
Ph
Ph
a
b
c
d
e
f
All reactions were carried out at 50 °C with 0.5 mol% catalyst of MnIII(TPFPP)Cl in 0.5 mL of CH3CN containing 0.2 mmol of substrate, 0.3 mmol of PhI(OAc)2 and 5
Determined by GC–MS analysis of the crude reaction mixture with an internal standard (1,2,4-trichlorobenzene); material balances > 95%.
Based on 100% conversion of substrate; isolated yields listed in the parenthesis.
TOF = turnovers frequency (minꢀ1).
Isomeric ratio (exo:endo) > 95:5.
A trace amount of benzaldehyde was detected (<5%).
lL H2O.
g
A diastereomeric ratio of cis/trans = 1:4.

180
K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
2.4. Axial ligand effect
valent manganese(V)-oxo porphyrins (6) in aqueous solutions or
organic solvents in the presence of base [17,19]. To probe the iden-
tity of the active oxidizing species, we conducted the chemical
oxidation reaction of manganese(III) catalyst 1a by PhI(OAc)2 in
CH3CN or CD3CN in the absence of substrate (Fig. 2A). As shown
in Fig. 2A, with 5–10 equivalent of PhI(OAc)2, the precursor (1a)
was converted to a species 7a which then slowly decayed back to
MnIII precursor with clearly resolved isosbestic points. The absorp-
tion spectrum of 7a with a strong Soret band at 418 nm and weak
absorption band around 538 nm was essentially identical to that of
the known manganese(IV) mono-oxo porphyrin, which was
independently prepared from a reported method (Fig. 2S in
Supporting Information) [24]. In principle, MnIV-oxo derivatives
might have been formed by comproportionation reactions of the
MnV-oxo porphyrin with the residual MnIII products. Previous
studies with manganese-oxo species found that porphyrin-
manganese(V)-oxo species comproportionate rapidly with
manganese(III) species [24], and corrole-manganese(V)-oxo spe-
cies reacted with corrole-manganese(III) species to give
manganese(IV) species [25]. As thermodynamically favored, the
comproportionation reactions were important under our condition
with the mild oxidizing PhI(OAc)2. Since the chemical conversions
of 1a by mild oxidizing PhI(OAc)2 to MnV-oxo was relatively slow
process, therefore, the MnIV-oxo derivative was formed from the
fast reactions of MnV-oxo with residual MnIII to give MnIV-oxo
species.
It is literature known that axial ligand has a marked influence
on the reactivity of the high-valent metal-oxo porphyrin inter-
mediate [31,32]. It has been shown that axial ligands of iron(III)
porphyrin complexes play an important role in the catalytic oxida-
tion of hydrocarbons by various terminal oxidants, in which the
yields of oxidized products were markedly dependent on the axial
ligands of the iron(III) porphyrin catalysts [33]. In this regard, the
effect of axial ligand on the catalytic reactivity of MnIII(TPFPP)X
(X = Clꢀ, ClOꢀ4 , ClO3ꢀ, NOꢀ3 , and NOꢀ2 ) had been investigated in the
epoxidation of cis-cyclooctene and cis-stilbene (Table 4).
Reactions of the porphyrin-manganese(III) chloride with
corresponding silver salts, i.e. AgX (X = ClOꢀ4 , ClO3ꢀ, NO3ꢀ, and
NOꢀ2 ), gave solutions of the corresponding MnIII(TPFPP)X salts;
the formation of these species was indicated by the UV–Vis spec-
tra, matching those literature reported values [24]. To make a
quantitative comparison, epoxidations were catalyzed under iden-
tical conditions. The results in Table 4 reveal that the axial ligand of
the manganese catalysts has a significant effect on the rate of prod-
ucts formation. The MnIII(TPFPP)(ClO4) was the best catalyst, which
gave a complete conversion of cis-cyclooctene within 10 min.
Slower conversions were observed when MnIII(TPFPP)X (X = ClO3ꢀ,
NOꢀ3 , and NOꢀ2 ) were used instead. The lowest conversion was
obtained with Clꢀ as the axial ligand. It should be pointed out that,
among all axial ligands that we studied, ClOꢀ4 has the weakest,
albeit the Clꢀ has the strongest coordinating ability to the metal
of manganese. Thus, the effect of axial ligand is likely due to a rapid
reaction of MnIII(TPFPP)(ClO4) with PhI(OAc)2 to generate the
active oxidizing species. The dependence of the oxidation rates
product ratios on the axial ligands of manganese(III) porphyrin
catalysts may also suggest the involvement of different reactive
species in olefin epoxidation reactions [33].
The MnIV-oxo species (7a) in the presence of substrates decayed
to give MnIII product with no evidence for formation of MnII species
in any of our studies, similar to that of self-decay shown in Fig. 2A.
The asymmetric nature of the Soret absorbance at 470 nm indi-
cates that the products are a mixture of manganese(III) porphyrin
species containing different axial ligands.
The isosbestic points at 384, 442, 492 and 595 nm demonstrate
that the conversion of MnIV-oxo (7a) to MnIII species (1a) does not
involve the accumulation of any intermediates.
2.5. Mechanistic investigations
The observed rate constants for decay of 7a with the substrates
such as ethylbenzene were fit reasonably well by pseudo-first-
order solutions. The rate constants increased as a function of sub-
strate concentration, and plots of kobs versus substrate concentra-
tion were linear (Fig. 2B). As solved by Eq. (1), where kobs is the
observed rate constant, k0 is rate constant of background reaction
and kox is second-order rate constant for oxidation of substrate,
reactions of 7a gave a kox = (7.12 0.03) ꢁ 10ꢀ1 Mꢀ1 sꢀ1 for ethyl-
benzene, and kox = (7.16 0.01) ꢁ 10ꢀ2 Mꢀ1 sꢀ1 for ethylbenzene-
Prior to the present studies, conspicuous is the fact that use of
PhI(OAc)2 for manganese porphyrin-catalyzed oxidation has not
been well studied before [12,13]. We now show that
manganese(III) porphyrins catalyze the highly efficient oxidation
of alkenes and activated hydrocarbons by PhI(OAc)2 in the pres-
ence of a small amount of H2O. The preparative utility and syn-
thetic value of the new catalytic system presented above are
indisputable, but mechanistic understanding of the complex oxy-
gen-transfer processes is important for the design of still more
effective and selective oxidants with general applicability.
d10
,
thus revealing
a
kinetic isotope effect (KIE) of kH/kD
=
9.9 0.2 at 298 K.
It is known that the reactions of manganese(III) porphyrin com-
plexes with more oxidizing oxidants such as m-chloroperoxyben-
zoic acid (m-CPBA), iodosylarenes and H2O2, produced high-
kobs ¼ k0 þ kox½Substrateꢂ
ð1Þ
2.6. Competitive kinetics
The directly observed MnIV-oxo species (7a) in above kinetic
studies is not necessarily the active oxidant under catalytic turn-
over conditions. One method to evaluate whether the same species
is active in the two sets of conditions is to compare the ratios of
products formed under catalytic turnover conditions to the ratios
of rate constants measured in the direct kinetic studies [34]. If
the same oxidant is present in both cases, the ratios of absolute
rate constants from direct measurements and relative rate con-
stants from the competition studies should be similar, although a
coincident similarity for two different oxidants cannot be
excluded. When the ratios are not similar, however, the active oxi-
dants under the two sets of conditions must be different.
Table 4
Effect of axial ligand on the manganese(III) porphyrin-catalyzed cis-alkene
epoxidations.a.
Entry
MnIII(TPFPP)X Xꢀ
cis-Cyclooctene
cis-Stilbene
Convn
(%)b
Yields
(%)b
Convn
(%)b
Epoxides (%)b
cis:trans
1
2
3
4
5
ClOꢀ4
ClOꢀ3
NOꢀ3
NOꢀ2
Clꢀ
100
82
65
76
18
100
100
100
100
100
74
45
24
57
17
91:9
86:14
92:8
93:7
90:10
a
All reactions were carried out in CH3CN (0.5 mL) over 10 min at 23 °C with
substrate (0.20 mmol), 1.5 equiv. of PhI(OAc)2 and 0.5 mol% of manganese(III)
To evaluate the identity of the active oxidant during the cat-
alytic conditions, the competition studies with MnIII(TPFPP)X with
different axial ligands (Clꢀ and ClO4ꢀ) and PhI(OAc)2 were
porphyin (1a).
b
Determined by GC–MS analysis of the crude reaction mixture with an internal
standard (1,2,4-trichlorobenzene); material balances > 95%.

K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
181
Fig. 2. (A) Time-resolved spectrum following the self-decay of 7a generated by reacting 1a (1.0 ꢁ 10ꢀ5 M) with PhI(OAc)2 (ca. 5 equiv.) over 300 s in CH3CN at 23 2 °C; (B)
observed rate constants for reactions of 7a with ethylbenzenes in CH3CN at 23 2 °C, monitored at 418 nm.
Table 5
Relative ratios from absolute rate constants of kinetic studies and from competition
catalytic oxidations.a.
Substrate
Oxidant
Method
krel
PhEt/PhEt-d10
MnV(TPFPP)O
Kinetic ratio 2.3b
Kinetic ratio 9.9c
Competition 3.3
Competition 2.9
MnIV(TPFPP)O
MnIII(TPFPP)Cl/PhI(OAc)2
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
cis-stilbene/Ph2CH2
MnV(TPFPP)O
Kinetic ratio 4.7b
Competition 5.2
Competition 4.9
MnIII(TPFPP)Cl/PhI(OAc)2
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
4-fluorostyrene/styrene
4-chlorostyrene/styrene
MnIII(TPFPP)Cl/PhI(OAc)2
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
Competition 1.1
Competition 1.0
MnIII(TPFPP)Cl/PhI(OAc)2
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
Competition 1.1
Competition 1.0
Fig. 3. Hammett correlation studies (logkrel vs r
+) for the MnIII(TPFPP)Cl-catalyzed
epoxidation of substituted styrenes by PhI(OAc)2 in CH3CN at 23 2 °C.
4-methylstyrene/styrene MnIII(TPFPP)Cl/PhI(OAc)2
Competition 1.4
Competition 1.3
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
4-methoxystyrene/
styrene
MnIII(TPFPP)Cl/PhI(OAc)2
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
Competition 1.6
Competition 1.5
observed by chemical oxidations in mixing experiments [24].
Table 5 also includes the ratios of absolute rate constant of less
reactive manganese(IV)-oxo porphyrin (7a) that was observed
from the reaction of 1a and PhI(OAc)2 (this work), and competition
reactions where PhI(OAc)2 was employed as sacrificial oxygen
source. In competition studies, a limiting amount of PhI(OAc)2
was used to keep the conversion less than 25% to avoid the effect
of substrate concentration on the product ratios. As evident in
Table 5, the results of the competition reactions between ethylben-
zene and ethlybenzene-d10 and between cis-stilbene and diphenyl-
methane are in close agreement with the ratios of the absolute rate
constants found in direct kinetic studies of the MnV(TPFPP)O from
LFP studies. Notably, the competitive catalytic oxidation of PhEt-d0
and PhEt-d10 revealed a kinetic isotope effect (KIE) of kH/kD = 3.3
(X = Cl) and 2.9 (X = ClOꢀ4 ) at 298 K, much smaller than the KIE of
kH/kD = 9.9 for the same reaction with the observed MnIV-oxo spe-
cies (7a). From standard reactivity–selectivity considerations, a
more reactive porphyrin-manganese(V)-oxo species would be
expected to display smaller ratios of substrate selectivity than
the less reactive manganese(IV)-oxo porphyrin. The similarity in
relative rate constants suggests that MnV(TPFPP)O species (6a)
were the primary active oxidants in the catalytic reactions, as
3-nitrotyrene/styrene
MnIII(TPFPP)Cl/PhI(OAc)2
MnIII(TPFPP)(ClO4)/
PhI(OAc)2
Competition 0.7
Competition 0.6
a
All competition reactions were conducted in CH3CN (0.5 mL) in the presence of
H2O (5 L) containing equal amounts of two substrates, e.g., PhEt (0.2 mmol) and
PhEt-d10 (0.2 mmol), PhI(OAc)2 (0.1 mmol) and catalyst MnIII(TPFPP)X (X = Cl and
ClO4) (1.0 mol). Ratios of relative rate constants from competition reactions were
l
l
determined based on the conversions of substrates. All competition ratios are
averages of 2–3 determinations with standard deviations smaller than 5% of the
reported values.
b
From LFP kinetic studies (see Ref. [24]).
From kinetic studies of this work.
c
conducted. Table 5 contains the relative ratios of absolute rate con-
stant of manganese(V)-oxo porphyrin complex, i.e. MnV(TPFPP)O,
from our previous laser flash photolysis (LFP) studies[24], whereas
irradiation of MnIII(TPFPP)(OClO3) perchlorate resulted in heteroly-
tic cleavage of O–Cl bonds to give porphyrin-manganese(V)-oxo
cations. The inherent good temporal resolution of LFP methods
permits the production, spectral characterization, and kinetic stud-
ies of highly reactive MnV(TPFPP)O intermediates that cannot be

182
K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
Scheme 2. Proposed catalytic cycle.
commonly assumed, and demonstrates that the kinetic results can
be used in a predictive manner for estimating the relative reactivi-
ties of substrates under catalytic conditions. Differences in inher-
ent reactivities of the reductants 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 experi-
ment strongly suggests that manganese(V)-oxo species were the
active oxidants in the catalytic oxidation reactions, even though
they could not be detected during the reactions.
identities of the multiple oxidants are speculative points [26,38–
40]. The relative populations of manganese(V)-oxo (6) and
manganese(IV)-oxo species (7) were determined by the rates of
the subsequent comproportionation reactions. Of note, the
manganese(V)-oxo porphyrin does react rapidly with the chloride
salt of its manganese(III) precursor, but the comproportionation
reaction of the manganese(V)-oxo species with the perchlorate salt
is slower [24]. This apparently reflects the observed axial ligand
effect (Table 3) that a weak-binding ligand perchlorate is more
active catalyst, whereas a tight-binding ligand chloride is less
effective.
2.7. Hammett correlation
A further reflection of the more reactive manganese(V)-oxo
intermediate involved in the catalytic oxidations instead of
manganese(IV)-oxo complex is seen in the linear Hammett plot
for competitive oxidations of the series of substituted styrenes
(Y-styrene, Y = 4-MeO, 4-Me, 4-F, 4-Cl, and 3-NO2). Fig. 3 depicts
a linear correlation (R = 0.996) of logkrel[krel = k(Y-styrene)/k(styr-
3. Conclusions
In summary, we have demonstrated that manganese(III) por-
phyrins catalyze the highly efficient oxidation of alkenes and acti-
vated hydrocarbons with PhI(OAc)2 in the presence of small
amount of water. A variety of alkenes and activated hydrocarbons
were oxidized to afford epoxides or alcohols and ketones with
excellent yields of up to 100% and high selectivities. It was found
that the reactivity of MnIII(TPFPP)X was greatly affected by axial
ligand and the weakly binding perchlorate gave the highest cat-
alytic activity in the epoxidation of cis-alkenes. The catalytic
competition and Hammett correlation studies have suggested that
the observed MnIV-oxo species (7a) in direct kinetic studies is not
likely the sole oxidant under catalytic turnover conditions. A
high-valent manganese(V)-oxo intermediate was indicated as the
premier active oxidant, even it does not accumulate to detectable
concentrations due to its high reactivity. A catalytic mechanism
involving two possible oxidants with different reactivities and
selectivities, i.e. manganese(V)-oxo and manganese(IV)-oxo com-
plexes, has been suggested.
ene)] versus Hammett r
+ substituent constant (Table 5). The slope
(q
+) of the plot is ꢀ0.42, which indicates transition states for rate-
limiting steps which involve very little charge separation. Again,
this observed q+ value is about 2-fold smaller in magnitude than
the previously reported value (ꢀ0.99) found for a related por-
phyrin-manganese(IV)-oxo complex [35]. Apparently, the competi-
tive product analysis and Hammett correlation studies strongly
suggest that the observed manganese(IV)-oxo species is unlikely
to be the sole oxidant. Instead, the highly reactive porphyrin-
manganese(V)-oxo species as the premier reactive intermediate
is plausible, even though it could not be detected during the cat-
alytic reactions.
On the basis of the present experimental facts, a catalytic
cycle is proposed with a high-valent manganese(V)-oxo species
as the active oxidant that might describe catalytic oxidation
reactions under turnover conditions (Scheme 2). Reaction of the
porphyrin-manganese(III) salt with the sacrificial PhI(OAc)2
might give a porphyrin-manganese(V)-oxo species (6) that is the
major oxidant of substrate. In a fast competing process, the por-
phyrin-manganese(V)-oxo species might react with residual
manganese(III) salt to form a manganese(IV)-oxo complex (7) that
also serves as a sluggish oxidant [35]. When reactive substrates
were not present, porphyrin-manganese(IV)-oxo derivatives might
be the only observable products because the porphyrin-
manganese(V)-oxo species are too short-lived and do not accumu-
late to detectable concentrations.
If this proposed reaction mechanism is reasonable, then it is
possible that catalytic oxidations could involve two active oxidants
that have different reactivities and selectivities in, for example,
competition reactions [36,37]. Thus, the competition results in
Table 5 are consistent with a multiple oxidant model [38,39].
Involvement of the less reactive manganese(IV)-oxo species may
explain the loss of stereoselectivity as we observed in the epox-
idation of cis-stilbene (Table 5). Much evidence has accumulated
in recent years that multiple oxidants can be involved in oxidations
by ligand–metal catalysts under turnover conditions, although the
4. Experimental
4.1. Materials and methods
Acetonitrile was obtained from Fisher Scientific and distilled
over P2O5 prior to use. All reactive substrates for catalytic reactions
were from Sigma–Aldrich Chemical Co. and purified by passing
through a dry column of active alumina (Grade I) before use.
Iodobenzene diacetate or (diacetoxyiodo)benzene, i.e. PhI(OAc)2,
H2O2 (30%) and tert-butyl hydroperoxide (tBuOOH, 70%) were pur-
chase from Aldrich Chemical Co. and used as such. Iodosylbenzene
(PhIO) was purchased from the TCI America Co. and was used as
obtained.
Free porphyrin ligand, 5,10,15,20-tetrakis(pentafluorophenyl)-
porphyrin (H2TPFPP) was purchased from Sigma–Aldrich and used
as received. 5,10,15,20-tetramesitylporphyrin (H2TMP) was pre-
pared according to the known methods [41]. MnIII(TPFPP) Cl and
MnIII(TMP)Cl were prepared according to the literature procedure
[42] and purified by chromatography on silica-gel. MnIII(TPP)Cl
was obtained from Aldrich Chemical Co. and used without further

K.W. Kwong et al. / Inorganica Chimica Acta 430 (2015) 176–183
183
purification. All perchlorate salts, chlorate salts, nitrate and nitrite
salts of manganese(III) porphyrin complexes were prepared by
stirring equimolar amounts of MnIII(TPFPP)Cl with AgClO4,
AgClO3, AgNO3, and AgNO2, respectively. The resulting mixtures
were filtered and used for catalytic studies immediately after
preparation.
20-tetraphenylporphyrin dianion; m-CPBA = meta-chloroperoxy-
benzoic acid; TON = turnover number; KIE = kinetic isotope effect;
LFP = laser flash photolysis.
Acknowledgement
UV–Vis spectra were recorded on an Agilent 8453 diode array
spectrophotometer. 1H NMR was performed on a JEOL ECA-
500 MHz spectrometer at 298 K with tetramethylsilane (TMS) as
internal standard. Gas Chromatograph analyses were conducted
on an Agilent GC7820A/MS5975 equipped with a flame ionization
detector (FID) using a J&W Scientific capillary column. The above
GC/MS system is also coupled with an auto sample injector.
Reactions of MnIII(TPFPP) with excess of PhI(OAc)2 were conducted
in a anaerobic acetonitrile solution at 23 2 °C.
This work was supported by a National Science Foundation
Grant (CHE-1213971) and an internal Grant from WKU Office of
Research (FUSE 14-FA109).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2015.03.003.
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Abbreviations used in this work
TPFPP = 5,10,15,20-tetrapentafluorophenylporphyrin dianion;
TMP = 5,10,15,20-tetramesitylporphyrin dianion; TPP = 5,10,15,