Selective Oxygenation of Cyclohexene by Dioxygen via an Iron(V)-Oxo Complex-Autocatalyzed Reaction

Article abstract of DOI:10.1021/acs.inorgchem.7b00220

An iron complex with a tetraamido macrocyclic ligand, [(TAML)Fe^III]^?, was found to be an efficient and selective catalyst for allylic oxidation of cyclohexene by dioxygen (O₂); cyclohex-2-enone was obtained as the major product along with cyclohexene oxide as the minor product. An iron(V)-oxo complex, [(TAML)Fe^V(O)]^?, which was formed by activating O₂ in the presence of cyclohexene, initiated the autoxidation of cyclohexene with O₂ to produce cyclohexenyl hydroperoxide, which reacted with [(TAML)Fe^III]^? to produce [(TAML)Fe^V(O)]^? by autocatalysis. Then, [(TAML)Fe^V(O)]^? reacted rapidly with [(TAML)Fe^III]^? to produce a μ-oxo dimer, [(TAML)Fe^IV(O)Fe^IV(TAML)]^2-, which was ultimately converted to [(TAML)Fe^V(O)]^? when [(TAML)Fe^III]^? was not present in the reaction solution. An induction period was observed in the autocatalytic production of [(TAML)Fe^V(O)]^?. The induction period was shortened with increasing catalytic amounts of [(TAML)Fe^V(O)]^? and cyclohexenyl hydroperoxide, whereas the induction period was prolonged by adding catalytic amounts of a spin trapping reagent such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The allylic oxidation of cycloalkenes was also found to depend on the allylic C-H bond dissociation energies, suggesting that the hydrogen atom abstraction from the allylic C-H bonds of cycloalkenes is the rate-determining radical chain initiation step. In this study, we have shown that an iron(III) complex with a tetraamido macrocyclic ligand is an efficient catalyst for the allylic oxidation of cyclohexene via an autocatalytic radical chain mechanism and that [(TAML)Fe^V(O)]^? acts as a reactive intermediate for the selective oxygenation of cyclohexene with O₂ to produce cyclohex-2-enone predominantly.

Full text of DOI:10.1021/acs.inorgchem.7b00220

Article
pubs.acs.org/IC
Selective Oxygenation of Cyclohexene by Dioxygen via an Iron(V)-
Oxo Complex-Autocatalyzed Reaction
Muniyandi Sankaralingam,† Yong-Min Lee,† Wonwoo Nam,*,† and Shunichi Fukuzumi*,†,‡
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
‡Faculty of Science and Engineering, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi
468-8502, Japan
S
* Supporting Information
ABSTRACT: An iron complex with a tetraamido macrocyclic
ligand, [(TAML)FeIII]−, was found to be an efficient and
selective catalyst for allylic oxidation of cyclohexene by
dioxygen (O2); cyclohex-2-enone was obtained as the major
product along with cyclohexene oxide as the minor product.
An iron(V)-oxo complex, [(TAML)FeV(O)]−, which was
formed by activating O2 in the presence of cyclohexene,
initiated the autoxidation of cyclohexene with O2 to produce
cyclohexenyl hydroperoxide, which reacted with [(TAML)-
FeIII]− to produce [(TAML)FeV(O)]− by autocatalysis. Then,
[(TAML)FeV(O)]− reacted rapidly with [(TAML)FeIII]− to
produce a μ-oxo dimer, [(TAML)FeIV(O)FeIV(TAML)]2−,
which was ultimately converted to [(TAML)FeV(O)]− when
[(TAML)FeIII]− was not present in the reaction solution. An induction period was observed in the autocatalytic production of
[(TAML)FeV(O)]−. The induction period was shortened with increasing catalytic amounts of [(TAML)FeV(O)]− and
cyclohexenyl hydroperoxide, whereas the induction period was prolonged by adding catalytic amounts of a spin trapping reagent
such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The allylic oxidation of cycloalkenes was also found to depend on the allylic
C−H bond dissociation energies, suggesting that the hydrogen atom abstraction from the allylic C−H bonds of cycloalkenes is
the rate-determining radical chain initiation step. In this study, we have shown that an iron(III) complex with a tetraamido
macrocyclic ligand is an efficient catalyst for the allylic oxidation of cyclohexene via an autocatalytic radical chain mechanism and
that [(TAML)FeV(O)]− acts as a reactive intermediate for the selective oxygenation of cyclohexene with O2 to produce cyclohex-
2-enone predominantly.
Scheme 1. Oxidation Products of Cyclohexene
INTRODUCTION
■
Allylic oxidation reactions are fundamental and important for
C−H bond functionalizations, accompanied by a significant
increase in the synthetic and/or commercial value of the target
molecules, because the resulting oxidation products are
attractive synthetic intermediates for pharmaceuticals.1−3
Classically, allylic oxidations are performed with stoichiometric
amounts of chromium reagents.3 Due to the high toxicity of the
chromium reagents, however, the stoichiometric use of toxic
oxidants should be avoided and replaced by environmentally
friendly methods whenever possible.4−14 Among various
oxidants, dioxygen (O2) is an ideal oxidant because it is
ubiquitous and environmentally totally benign. Thus, a number
of metal-catalyzed methods have been developed for allylic
oxidations by O2.15−18 However, the catalytic oxidation of
alkenes by O2 is normally not selective.18 For example, the
catalytic oxidation of cyclohexene by O2 afforded mixtures of
cyclohex-2-enol, cyclohex-2-enone, cyclohexene oxide, and
other products, including cyclohexenyl hydroperoxide and
1,2-cyclohexanediol (Scheme 1).19 Thus, there has been no
report on the selective, catalytic oxidation of cyclohexene by O2
under mild conditions. In addition, the mechanism of the
catalytic oxidation of cyclohexene by O2 has yet to be clearly
clarified.
We report herein the catalytic and selective allylic oxidation
of cyclohexene by O2 to produce cyclohex-2-enone as the major
product together with cyclohexene oxide as the minor product
in the presence of an iron complex bearing a tetraamido
macrocyclic ligand, Na[(TAML)FeIII], in O2-saturated acetoni-
trile (MeCN) at 298 K (Scheme 2a for the structure of
[(TAML)FeIII]− and Scheme 2b for the products formed in the
autoxidation of cyclohexene by [(TAML)FeIII]−). The catalytic
mechanism is proposed on the basis of a detailed kinetics study
Received: January 30, 2017
© XXXX American Chemical Society
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Scheme 2. (a) Molecular Structure of Iron(III) Tetraamido
Macrocyclic Complex ([(TAML)FeIII]−) and (b) Allylic
Oxidation Products Formed in the Oxidation of
Cyclohexene with [(TAML)FeIII]− in O2-Saturated
Acetonitrile (MeCN) at 298 K
Table 1. Catalytic Oxidation of Cyclohexene by O2 in the
a
Presence of [(TAML)FeIII]− in O2-Saturated Acetonitrile
a
Reaction conditions: the catalytic activity of [(TAML)FeIII]− toward
and the detection of intermediates in the catalytic oxidation
reactions. This is the first time the catalytic and selective
oxidation of cyclohexene by O2 has been achieved under
ambient temperature and pressure with the elucidation of the
autocatalytic reaction mechanisms.
cyclohexene was examined by bubbling O2 into the reaction solution
of deuterated acetonitrile (CD3CN) for 4 h at 298 K. Products were
1
analyzed and quantified by taking H NMR spectra of the reaction
solution.
RESULTS AND DISCUSSION
■
analyses (see Figure S1 in the Supporting Information for the
analysis of products using H NMR).
1
Catalytic Oxidation of Cyclohexene by Dioxygen in
the Presence of [(TAML)FeIII]−. The [(TAML)FeIII]−
complex with Na+ countercation20,21 exhibited no reactivity
toward oxygen in acetonitrile, although the [(TAML)-
As shown in Table 1, an increase in the substrate
concentration to 200 mM resulted in the formation of
cyclohex-2-enone as the major product together with cyclo-
hexene oxide and cyclohex-2-enol as the oxygenated products.
Then, the decrease of the catalyst (20 μM) in an O2-saturated
MeCN solution containing cyclohexene (200 mM) resulted in
the formation of cyclohex-2-enone as the major product
together with cyclohex-2-enol, cyclohexene oxide, and cyclo-
hexenyl hydroperoxide as the oxygenated products. The
formation of cyclohex-2-enone, cyclohex-2-enol, cyclohexenyl
FeIII(H2O)]− complex with PPh4 countercation was reported
+
to be oxidized by oxygen to produce the μ-oxo dimer in
dichloromethane.22 In this study, the oxidation reaction was
carried out with Na[(TAML)FeIII] (2.0 mM) and cyclohexene
(50 mM) in an O2-saturated MeCN solution at 298 K, yielding
cyclohex-2-enone (90%) and cyclohexene oxide (10%) without
the formation of cyclohex-2-enol, cyclohexenyl hydroperoxide,
or cyclohexanediol (Figure 1 and Table 1). The solvent MeCN
molecule may be coordinated to [(TAML)FeIII]−. The
1
hydroperoxide, and cyclohexene oxide was confirmed by H
NMR measurements (Figure S1 in the Supporting Informa-
tion). The turnover number (TON) reached >104 by a
decrease in the catalyst concentration, although the selectivity
of the allylic oxidation products was somewhat diminished
(Table 1). We have also examined the effect of concentrations
of the catalyst and substrates (Table 1): the concentration of
cyclohexenyl hydroperoxide decreased with increasing concen-
tration of [(TAML)FeIII]−, because cyclohexenyl hydroper-
oxide reacts with [(TAML)FeIII]− to produce an iron(V)-oxo
complex, [(TAML)FeV(O)]− (vide infra). In the absence of the
[(TAML)FeIII]− catalyst, no formation of the oxygenated
products was observed. In addition, iron salts (e.g., Fe-
(CF3SO3)2) and iron complexes, such as [Fe(TMC)]2+
(TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetrade-
cane),23 [Fe(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-
bis(pyridyl)methylamine),24 and [Fe(Bn-TPEN)]2+ (Bn-TPEN
= N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diamino-
ethane),25 did not yield the oxygenated products under the
catalytic conditions described above. These results demonstrate
that [(TAML)FeIII]− is a highly efficient catalyst for the
oxidation of cyclohexene by O2. We have also performed the
catalytic reaction in the presence of isotopically labeled
H218O.26 In the reaction mixture, nearly 50% incorporation of
1
products were identified and quantified by H NMR as well
as gas chromatography (GC) and gas chromatography linked to
a mass spectrometer (GC-MS); the yields of products were
1
within experimental errors ( 10%) in the H NMR and GC
Figure 1. Reaction time profiles for the formation of cyclohex-2-enone
(green) and cyclohexene oxide (red) and for the consumption of
cyclohexene (black) in the oxidation of cyclohexene (50 mM) in the
presence of [(TAML)FeIII]− (2.0 mM) in O2-saturated MeCN at 298
K.
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18O-labeled cyclohexene oxide was observed (Figure S2 in the
Supporting Information).
Supporting Information), whose mass and isotope distribution
pattern corresponds to [(TAML)FeV(O)]− (calculated m/z
442.1). When the reaction was carried out with isotopically
labeled H218O, a mass peak appeared corresponding to
[(TAML)FeV(18O)]− (m/z 444.1) (Figure S4), indicating
that the intermediate contains only one oxygen atom. Thus,
the stability of [(TAML)FeV(O)]− at 298 K increased under
our reaction conditions, such as using O2 as an oxidant. It
should be also noted that the [(TAML)FeV(O)]− complex
generated by other oxidants, such as cumene hydroperoxide
and cyclohexenyl hydroperoxide, at 298 K (see Figures S5 and
S6 in the Supporting Information) is much more stable than
that produced by m-CPBA,21,27 probably indicating that the
presence of H+ derived from m-CPBA may decrease the
stability of [(TAML)FeV(O)]−.
Autocatalytic Formation of [(TAML)FeV(O)]− in the
Reaction of [(TAML)FeIII]− with Cyclohexene and
Dioxygen. Addition of [(TAML)FeIII]− (0.10 mM) to an
O2-saturated MeCN solution containing cyclohexene (2.5 mM)
resulted in UV−vis spectral changes of the iron complex
(Figure 2a), where the absorption band at 400 nm due to
Addition of a catalytic amount of [(TAML)FeV(O)]− to an
MeCN solution of [(TAML)FeIII]− containing cyclohexene
resulted in a significant decrease in the induction period, which
became shorter with an increase in the catalytic amount of
[(TAML)FeV(O)]− (Figure 3a). Because [(TAML)FeV(O)]−
is the product of the oxidation of [(TAML)FeIII]− with O2 in
the presence of cyclohexene, the decrease in the induction
period in Figure 3a indicates an autocatalytic behavior; an
autocatalytic reaction is defined as one in which the product
acts as the catalyst for its own formation.30 Because the rate of
reaction is proportional to the concentration of product, the
time course of the reaction exhibits an induction period with a
sigmoidal curvature, as shown in Figure 3a.30 Autocatalysis has
been extensively studied because it is central to the propagation
of living systems.31−38
Addition of a catalytic amount of the μ-oxo dimer
([(TAML)FeIV(O)FeIV(TAML)]2−) to an MeCN solution of
[(TAML)FeIII]− containing cyclohexene decreased the in-
duction period in a similar way (Figure 3b). The induction
period was also reduced by addition of a catalytic amount of
cyclohexenyl hydroperoxide (Figure 3c), which was independ-
ently prepared by photoinduced autoxidation of cyclohexene by
O2 with xyloquinone,39 and the concentration was determined
on the basis of iodometric titration (Figures S1e and S7 in the
Supporting Information).40 Addition of a catalytic amount of
cumene hydroperoxide also resulted in the reduction of the
induction period with an increase in the concentration of
cumene hydroperoxide (Figure S8 in the Supporting
Information), because cumene hydroperoxide reacted with
[(TAML)FeIII]− to produce [(TAML)FeV(O)]− (Figure S5 in
the Supporting Information). The induction period was also
reduced by increasing the concentration of cyclohexene (Figure
4a). In contrast, the induction period increased with increasing
concentration of [(TAML)FeIII]− (Figure 4b). Since the
[(TAML)FeV(O)]− complex rapidly reacts with [(TAML)-
FeIII]− to yield [(TAML)FeIV(O)FeIV(TAML)]2−, which is in
equilibrium with [(TAML)FeV(O)]− and the equilibrium
shifted to the right-hand side for the formation of the μ-oxo
dimer,41,42 the concentration of [(TAML)FeV(O)]−, which
acted as the autocatalyst, decreased with increasing concen-
tration of [(TAML)FeIII]− when the induction period increased
(Figure 4b). These autocatalytic behaviors suggest that a radical
chain autoxidation is responsible for the formation of
[(TAML)FeIV(O)FeIV(TAML)]2− and [(TAML)FeV(O)]− in
the oxidation of [(TAML)FeIII]− by O2 in the presence of
cyclohexene, as shown in Scheme 3 for the autocatalytic radical
chain reaction induced by the [(TAML)FeV(O)]− complex
(see below).
Figure 2. UV−visible absorption spectral changes observed in the
oxygenation of [(TAML)FeIII]− (0.10 mM) with O2 in the presence of
cyclohexene (2.5 mM) in O2-saturated MeCN at 298 K. The two-step
changes are separated by the time regions of (a) 0−720 s and (b)
720−800 s.
[(TAML)FeIII]− decreased with isosbestic points at 372 and
415 nm, accompanied by the formation of a μ-oxo dimer,
[(TAML)FeIV(O)FeIV(TAML)]2−,27−29 with an induction
period (Figure 2a, inset). The latter species was then converted
rapidly to an iron(V)-oxo complex, [(TAML)FeV(O)]−, with
isosbestic points at 572 and 692 nm in the time region of 720−
780 s (Figure 2b). The yield of the iron(V)-oxo complex was
determined to be >95% on the basis of the extinction
coefficient at 630 nm.21,27−29 The formation of the iron(V)-
oxo complex has also been confirmed by electron paramagnetic
resonance (EPR) spectroscopy and cold spray ionization time-
of-flight mass (CSI-MS) spectrometry. The major anisotropic
EPR signals with gxx = 1.98, gyy = 1.97, and gzz = 1.76 in the EPR
spectrum recorded in acetone at 77 K belong to S = 1/2
[(TAML)FeV(O)]− species (Figure S3 in the Supporting
Information). A simulated EPR spectrum using the anisotropic
g-tensors of gxx = 1.979, gyy = 1.973, and gzz = 1.763 was also
represented (Figure S3a, pink dotted line). The spin amount of
the iron(V)-oxo complex (78(4)%) was determined by a
comparison of the doubly integrated value of the EPR signal
with that of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•)
species as a reference (Figure S3). The signals with g values of
2.06 and 2.03 originated from the minor species.27 The CSI-MS
spectrum of [(TAML)FeV(O)]− exhibits a prominent ion peak
at a mass to charge ratio of m/z 442.1 (Figure S4 in the
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Figure 4. (a) Time profiles monitored for the change of absorbance at
630 nm due to [(TAML)FeV(O)]− in the autocatalytic oxygenation of
[(TAML)FeIII]− (0.10 mM) with O2 in the presence of cyclohexene
(2.5−10.0 mM). (b) Time profiles monitored for the change of
absorbance at 630 nm due to [(TAML)FeV(O)]− in the reaction of
cyclohexene (5.0 mM) with [(TAML)FeIII]− (0.10−0.25 mM) in
aerated MeCN at 298 K.
Scheme 3. Autocatalytic Radical Chain Mechanism for
Catalytic Oxidation of Cyclohexene by O2 in the Presence of
[(TAML)FeIII]−
Figure 3. Time profiles monitored for the change of absorbance at 630
nm due to [(TAML)FeV(O)]− in the autocatalytic oxygenation of
[(TAML)FeIII]− (0.10 mM) with O2 in the presence of cyclohexene
(2.5 mM) with catalytic amounts of (a) [(TAML)FeV(O)]− (0−10%),
(b) [(TAML)FeIV(O)FeIV(TAML)]2− (0−15%), and (c) cyclohexenyl
hydroperoxide (0−1.5%) in aerated MeCN at 298 K.
It has been known that the initiation of autoxidation of
cyclohexene by O2 occurs without any catalyst.43 The radical
chain reaction is then started by hydrogen atom abstraction of
the allylic C−H bonds of cyclohexene by [(TAML)FeV(O)]−
to produce [(TAML)FeIV(OH)]− and cyclohexenyl radical
(Scheme 3, reaction a), which readily reacts with O2 to afford
cyclohexenylperoxyl radical (Scheme 3, reaction b). The
produced [(TAML)FeIV(OH)]− may be dimerized to produce
the μ-oxo dimer. The cyclohexenylperoxyl radical is the chain
carrier, which abstracts a hydrogen atom from cyclohexene to
produce cyclohexenyl hydroperoxide and the regeneration of
cyclohexenyl radical (Scheme 3, reaction c), constituting the
radical chain reactions.44 Cyclohexenyl hydroperoxide oxygen-
ates [(TAML)FeIII]− to produce cyclohex-2-enol and
[(TAML)FeV(O)]− (Scheme 3, reaction d). It was confirmed
that addition of cyclohexenyl hydroperoxide to an MeCN
solution of [(TAML)FeIII]− resulted in the generation of
[(TAML)FeV(O)]− (Figure S6 in the Supporting Information).
In the presence of [(TAML)FeIII]−, [(TAML)FeV(O)]− is
known to react rapidly with [(TAML)FeIII]− to produce the μ-
oxo dimer [(TAML)FeIV(O)FeIV(TAML)]2− (Scheme 3,
reaction e).21,27−29,41,42 When [(TAML)FeIII]− was completely
consumed, [(TAML)FeIV(O)FeIV(TAML)]2− was rapidly
converted to [(TAML)FeV(O)]− (Scheme 3, reaction f), as
shown in Figure 2b.
The overall chain reaction is given in eq 1, where
cyclohexene reacts with O2 and [(TAML)FeIII]− to yield
[(TAML)FeV(O)]−. Then, cyclohexene is directly oxygenated
by [(TAML)FeV(O)]− to produce cyclohex-2-enol (Scheme 4,
reaction a), which is further oxidized by [(TAML)FeV(O)]− to
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Scheme 4. Stepwise Oxidation of Cyclohexene with
[(TAML)FeIII]− (Major Pathway)
Figure 5. (a) UV−visible spectral changes observed in the reaction of
[(TAML)FeV(O)]− (0.20 mM) with cyclohex-2-enol (5.0 mM) in
MeCN at 233 K. [(TAML)FeV(O)]− was generated by the O2-
activation reaction of [(TAML)FeIII]− (0.20 mM) with cyclohexene
(5.0 mM) in O2-saturated MeCN. (b) UV−visible spectral changes
observed in the reaction of [(TAML)FeV(O)]− (0.20 mM) with
cyclohexene (5.0 mM) in O2-saturated MeCN at 233 K. Insets show
the time courses monitored at 840 nm.
yield cyclohex-2-enone (Scheme 4, reaction b), accompanied by
the regeneration of [(TAML)FeIII]−, which is in equilibrium
with [(TAML)FeIV(O)FeIV(TAML)]2− in the presence of
[(TAML)FeV(O)]− (Scheme 4). The rapid oxidation of
cyclohex-2-enol by [(TAML)FeV(O)]− was independently
confirmed (Figure 5a; see also Figure S9 in the Supporting
Information), in which the rate of the reaction of [(TAML)-
FeV(O)]− with cyclohex-2-enol was about 10 times faster than
that of [(TAML)FeV(O)]− with cyclohexene at 233 K (Figure
5b). As a minor reaction pathway, cyclohexene was epoxidized
by [(TAML)FeV(O)]− to give cyclohexene epoxide (Scheme
5). Once the autocatalytic radical chain reaction starts, the
initiation step by [(TAML)FeV(O)]− produced in an
autocatalytic pathway (Scheme 3) becomes dominant in
comparison with the O2 activation pathway. Previously we
have reported the formation of [FeIV(O)(TMC)]2+ by reacting
[FeII(TMC)]2+ with dioxygen in the presence of cyclo-
alkenes.23c In the case of dioxygen activation by [FeII(TMC)]2+,
however, no autocatalytic pathway was involved, in contrast to
the present study of [(TAML)FeIII]−.
Scheme 5. Epoxidation of Cyclohexene with [(TAML)FeIII]−
(Minor Pathway)
The UV−vis spectral changes were also monitored under the
same conditions as in Figure 1 to clarify the rate-determining
step in Scheme 3. The results are shown in Figure S10 in the
Supporting Information, where spectral changes were observed
similar to those seen in Figure 2. When the cyclohexene
concentration was 50 mM, which was much larger than that in
Figure 2 (2.5 mM), no induction period was observed (Figure
S10a). The nearly complete formation of [(TAML)FeV(O)]−
via the formation of [(TAML)FeIV(O)FeIV(TAML)]2− from
[(TAML)FeV(O)]− and [(TAML)FeIII]− indicates that the
oxygenation of cyclohexene with [(TAML)FeV(O)]− is the
rate-determining step (Scheme 3, reaction a). After the
completion of the oxidation of cyclohexene, [(TAML)FeIII]−
was regenerated (Figure S10b). The regenerated [(TAML)-
FeIII]− was confirmed by EPR and CSI-MS (Figure S11 in the
Supporting Information).
5,5′-dimethyl-1-pyrroline N-oxide (DMPO), which is a typical
spin-trapping reagent.45−47 The addition of only 0.001%
DMPO to an aerated MeCN solution of [(TAML)FeIII]− and
cyclohexene resulted in an increase in the induction period, and
the addition of 2% DMPO inhibited the reaction completely, as
shown in Figure 6. Such a strong inhibition of the reaction by
DMPO suggests a long radical chain length. Since the rate
constants of the spin-trapping reactions of peroxyl radicals with
DMPO (103−104 M−1 s−1)48 are much larger than those of
allylic hydrogen atom abstraction from alkenes by peroxyl
radicals (e.g., 2.7 M−1 s−1 for the reaction of tert-butylperoxyl
radical with tetralin),49,50 the rate of consumption of DMPO in
the induction period ([DMPO]/(t4 − t2)) corresponds to the
initiation rate. The propagation rate is obtained as the fast rate
after the induction period (i.e., [[(TAML)FeIII]−]/(t3 − t1))
(see Figure 7). Then, the chain length of the propagation step
in Scheme 3 is evaluated as the ratio of the propagation rate to
the initiation rate, {[[(TAML)FeIII]−]/(t3 − t1)}/{[DMPO]/
Inhibition of the Radical Chain Reaction. The chain
carrier radical (cylcohexenylperoxyl radical) was trapped by
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Figure 6. Time profiles monitored for the change of absorbance at 630
nm due to [(TAML)FeV(O)]− in the oxidation of [(TAML)FeIII]−
(0.10 mM) with O2 in the presence of cyclohexene (2.5 mM) with
[(TAML)FeIII]− (0.10 mM) and a catalytic amount of DMPO (0−
2.0%) with O2 in aerated MeCN at 298 K.
Figure 8. (a) UV−vis spectral changes observed in the reversible
interconversion between [(TAML)FeV(O)]− and [(TAML)FeIV(O)-
FeIV(TAML)]2− by changing the temperature between 253 and 233 K.
Initially, [(TAML)FeV(O)]− was generated by an O2 activation
reaction of [(TAML)FeIII]− (0.20 mM) with cyclohexene (5.0 mM) in
O2-saturated MeCN at 298 K. (b) Time profiles monitored at 840 nm
due to [(TAML)FeIV(O)FeIV(TAML)]2− by changing the reaction
temperature between 253 and 233 K.
Figure 7. Chain length evaluated as the ratio of the propagation rate to
the initiation rate. The initiation rate is equal to [DMPO]/(t4 − t2),
whereas the propagation rate is equal to [[(TAML)FeIII]−]/(t3 − t1):
t1 = 580 s; t2 = 630 s; t3 = 830 s; t4 = 2240 s. The chain length was
calculated from {[[(TAML)FeIII]−]/(t3 − t1)}/{[DMPO]/(t4 − t2)}
to be 650.
(Scheme 3). When the temperature was increased to 253 K,
however, the absorbance at 840 nm due to [(TAML)FeIV(O)-
FeIV(TAML)]2− decreased, accompanied by an increase in
absorbance at 630 nm due to [(TAML)FeV(O)]−. This result
indicates that, at a higher temperature, the autocatalytic
formation of [(TAML)FeV(O)]− (Scheme 3) is faster than
the oxidation of cyclohexene by [(TAML)FeV(O)]− (Schemes
4 and 5). Such interconversion between [(TAML)FeV(O)]−
and [(TAML)FeIV(O)FeIV(TAML)]2− can be repeated by
changing the temperature between 233 and 253 K (Figure 8).
Catalytic Oxygenation of Other Cycloalkenes by O2.
Other cycloalkenes, such as cycloheptene, were also oxygenated
by O2 in the presence of a catalytic amount of [(TAML)FeIII]−
to produce the corresponding allylic oxidation products with
epoxides (Table 2). In the case of cyclooctene and deuterated
(t4 − t2)}, which is ca. 650 (Figure 7). Thus, the chain length is
long enough to obtain large TONs in Table 1. It should be
noted that the autocatalytic radical chain reaction is for the
formation of [(TAML)FeV(O)]−, which acts as an efficient
catalyst for the selective oxidation of cyclohexene to cyclohex-2-
enone as the major oxidized product, in contrast with the
autoxidation pathway to produce cyclohexenyl hydroperoxide.
Interconversion between [(TAML)FeV(O)]− and
[(TAML)FeIV(O)FeIV(TAML)]2− Depending on Temper-
ature. The balance for the autocatalytic formation of
[(TAML)FeV(O)]− in Scheme 3 and the oxidation of
cyclohexene with [(TAML)FeV(O)]− (eq 1) is changed
depending on the temperature. When the reaction temperature
was lowered to 233 K after the formation of [(TAML)-
FeV(O)]− at 298 K, the absorbance at 630 nm due to
[(TAML)FeV(O)]− decreased, accompanied by the appearance
of an absorption band at 840 nm due to the formation of the μ-
Table 2. Products Formed in the Catalytic Oxidation of
Cycloalkenes by O2 in the Presence of [(TAML)FeIII]− in
a
O2-Saturated Acetonitrile at 298 K
product (%)
substrate
alcohol
ketone
epoxide
cyclohexene
cycloheptene
cyclooctene
0
10(1)
0
89(2)
22(2)
0
11(1)
22(1)
0
oxo dimer [(TAML)FeIV(O)FeIV(TAML)]2− in the reaction of
−
[(TAML)FeV (O)]− and [(TAML)FeI II
]
(Figure
8);21,27−29,41,42 cyclohexene is oxidized by [(TAML)FeV(O)]−
to produce cyclohex-2-enol and [(TAML)FeIII]−, which reacts
with [(TAML)FeV(O)]− rapidly to yield the μ-oxo dimer
species [(TAML)FeIV(O)FeIV(TAML)]2− (Schemes 4 and 5).
The reaction of [(TAML)FeV(O)]− and [(TAML)FeIII]− is
faster than the autocatalytic formation of [(TAML)FeV(O)]−
a
Reaction conditions: the oxygenation of cycloalkenes was performed
by bubbling O2 into a CD3CN solution containing [(TAML)FeIII]−
(2.0 mM) and cycloalkenes (50 mM) at 298 K for 4 h. Products were
analyzed and quantified by taking H NMR spectra and comparing
1
them with those of authentic samples.
F
DOI: 10.1021/acs.inorgchem.7b00220
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
collected on a JMS-T100CS (JEOL) mass spectrometer equipped with
a CSI source. Typical measurement conditions are as follows: needle
voltage, 2.2 kV; orifice 1 current, 50−500 nA; orifice 1 voltage, 0−20
V; ring lens voltage, 10 V; ion source temperature, 5 °C; spray
temperature, −40 °C. The product analyses were also performed with
an Agilent 6890N gas chromatograph (GC) and a FOCUS DSQ
(dual-stage quadrupole) mass spectrometer (Thermo Finnigan,
Austin, TX, USA) interfaced with a Finnigan FOCUS gas chromato-
graph (GC-MS).
Formation of Fe(V)-Oxo Intermediate. Reactions were run in a
1 cm UV cuvette by monitoring UV−vis spectral changes of reaction
solutions. Addition of a catalytic amount of [(TAML)FeIII]− (0.10
mM) to an O2-saturated MeCN solution containing cyclohexene (2.5
mM) resulted in the formation of [(TAML)FeV(O)]− at 298 K in a
good yield, which was determined using the extinction coefficient at
630 nm due to [(TAML)FeV(O)]−. An authentic [(TAML)FeV(O)]−
complex was synthesized by reacting [(TAML)FeIII]− with m-
chloroperbenzoic acid (m-CPBA) according to the published
procedure.27
cyclohexene (cyclohexene-d10), however, the oxidation by O2
with [(TAML)FeIII]− did not occur under the identical reaction
conditions (Table 2). No [(TAML)FeV(O)]− was produced in
the reaction of [(TAML)FeIII]− (0.10 mM) with O2 in the
presence of cyclooctene (50 mM) in O2-saturated MeCN at
298 K for 4 h due to the much slower autoxidation to start the
autocatalytic reaction (Figure S12 in the Supporting
Information). The reactivity of cycloalkenes was correlated
with the allylic C−H bond dissociation energies (BDEs) of
cycloalkenes, since the yields of oxygenated products decrease
with an increase in the C−H BDEs, such as cyclohexene
(100%, BDE = 81.0 kcal mol−1), cycloheptene (54%, BDE = 83
kcal mol−1), and cyclooctene (0%, BDE = 85.0 kcal mol−1)
(Table 2).51 Thus, the hydrogen atom abstraction from
cycloalkenes by [(TAML)FeV(O)]− is the initiation step in
the autoxidation radical chain reactions, as proposed in Scheme
3.
Catalytic Oxygenation of Cycloalkenes by O2 in the
Presence of [(TAML)FeIII]−. The catalytic activity of [(TAML)FeIII]−
toward cycloalkenes was examined by bubbling dioxygen into the
reaction solution of acetonitrile (MeCN or CD3CN). Oxygenated
products were identified by 1H NMR and GC and then compared with
those of authentic samples to determine product yields.
CONCLUSION
■
The selective catalytic oxidation of cyclohexene by O2 has been
achieved by using an iron complex bearing a tetraamido
macrocyclic ligand, [(TAML)FeIII]−, which is oxidized by O2 in
the presence of cyclohexene to yield [(TAML)FeV(O)]− and
the corresponding allylic oxidation products via an autocatalytic
radical chain mechanism.52 The nonselectivity in the oxidized
products in Scheme 1 results mainly from free radical
decomposition reactions of the autoxidation product (i.e.,
cyclohexenyl hydroperoxide). Cyclohexenyl hydroperoxide
rapidly oxygenates [(TAML)FeIII]− to produce [(TAML)-
FeV(O)]−, which can selectively oxygenate cyclohexene to yield
mainly cyclohex-2-enone, which would otherwise be difficult to
obtain selectively. Thus, the present study provides a unique
autocatalytic reaction mechanism for the selective catalytic
oxygenation of substrates by O2 without the involvement of
free radical decomposition of alkyl hydroperoxides.
Synthesis and Characterization of Cyclohexenyl Hydro-
peroxide. Cyclohexenyl hydroperoxide was generated using cyclo-
hexene (450 μL, 9.8 M) and xyloquinone (2.0 mM) after 15 h of
irradiation using a xenon lamp (300 W) at 298 K according to the
literature method for the photocatalytic production of hydro-
peroxides.39 It was confirmed by 1H NMR spectroscopy; the
hydroperoxide peaks match with the previously reported literature.54
Iodometric Titration of Cyclohexenyl Hydroperoxide. The
amount of cyclohexenyl hydroperoxide was determined by titration
with iodide ion;40 10 μL of the reaction solution containing
cyclohexenyl hydroperoxide generated by cyclohexene (450 μL, 9.8
M) and xyloquinone (2.0 mM) with irradiation was taken and diluted
up to 2.0 mL with MeCN. The reaction solution was then treated with
an excess of sodium iodide under an Ar atmosphere. The amount of
−
−
I3 formed was quantified by absorbance at 361 nm due to I3 (λmax
361 nm, ε = 2.5 × 104 M−1 cm−1).40
EXPERIMENTAL SECTION
■
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00220.
Materials. Commercially available chemicals and solvents were
used without further purification unless otherwise indicated. The
complex Na(H2O)x[(TAML)FeIII] was purchased from GreenOx
Catalyst, Inc. (Pittsburgh, PA, USA), and recrystallized from an
isopropyl alcohol/H2O mixture before use.20 Acetonitrile, cyclo-
hexene, cyclohex-2-enone, cyclohex-2-enol, cyclohexene oxide, cyclo-
heptene, cyclooctene, and 5,5′-dimethyl-1-pyrroline N-oxide were
purchased from Aldrich Chemical Co. 2,5-Dimethyl-4-benzoquinone
was purchased from Alfa-Aesar Chemical Co. Olefins were refluxed
and distilled under Ar and filtered through an active alumina column
prior to use.53 H218O (95% 18O enriched) was purchased from ICON
Services Inc. (Summit, NJ, USA).
Instrumentation. UV−vis spectra were recorded on a Hewlett-
Packard 8453 diode array spectrophotometer equipped with a
UNISOKU Scientific Instruments USP-203A Cryostat for low-
temperature experiments. X-band electron paramagnetic resonance
(EPR) spectra were recorded at 5 K using an X-band Bruker EMX-
plus spectrometer equipped with a dual mode cavity (ER 4116DM).
Low temperature was achieved and controlled with an Oxford
Instruments ESR900 liquid He quartz cryostat with an Oxford
Instruments ITC503 temperature and gas flow controller. The
experimental parameters for EPR measurement were as follows:
microwave frequency, 9.647 GHz; microwave power, 1.0 mW;
modulation amplitude, 10 G; gain, 1 × 104; modulation frequency,
100 kHz; time constant, 40.96 ms; conversion time, 81.00 ms. Nuclear
magnetic resonance (NMR) spectra were measured with a Bruker
model digital AVANCE III 400 FT-NMR spectrometer. The cold
spray ionization time-of-flight mass (CSI-MS) spectral data were
■
S
Figures S1−S12 as described in the text (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for W.N.: wwnam@ewha.ac.kr.
*E-mail for S.F.: fukuzumi@chem.eng.osaka-u.ac.jp.
ORCID
Yong-Min Lee: 0000-0002-5553-1453
Wonwoo Nam: 0000-0001-8592-4867
Shunichi Fukuzumi: 0000-0002-3559-4107
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NRF of Korea through CRI
(NRF-2012R1A3A2048842 to W.N.) and GRL (NRF-2010-
00353 to W.N.), a Grant-in-Aid (no. 16H02268 to S.F.) from
the Ministry of Education, Culture, Sports, Science and
G
DOI: 10.1021/acs.inorgchem.7b00220
Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
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DOI: 10.1021/acs.inorgchem.7b00220
Inorg. Chem. XXXX, XXX, XXX−XXX