Mechanism of Oxygen Atom Transfer from FeV(O) to Olefins at Room Temperature

Article abstract of DOI:10.1021/ic503053q

In biological oxidations, the intermediate Fe^V(O)(OH) has been proposed to be the active species for catalyzing the epoxidation of alkenes by nonheme iron complexes. However, no study has been reported yet that elucidates the mechanism of direct O-atom transfer during the reaction of Fe^V(O) with alkenes to form the corresponding epoxide. For the first time, we study the mechanism of O-atom transfer to alkenes using the Fe^V(O) complex of biuret-modified Fe-TAML at room temperature. The second-order rate constant (k₂) for the reaction of different alkenes with Fe^V(O) was determined under single-turnover conditions. An 8000-fold rate difference was found between electron-rich (4-methoxystyrene; k₂ = 216 M^-1 s^-1) and electron-deficient (methyl trans-cinnamate; k₂ = 0.03 M^-1 s^-1) substrates. This rate difference indicates the electrophilic character of Fe^V(O). The use of cis-stilbene as a mechanistic probe leads to the formation of both cis- and trans-stilbene epoxides (73:27). This suggests the formation of a radical intermediate, which would allow C-C bond rotation to yield both stereoisomers of stilbene-epoxide. Additionally, a Hammett ρ value of -0.56 was obtained for the para-substituted styrene derivatives. Detailed DFT calculations show that the reaction proceeds via a two-step process through a doublet spin surface. Finally, using biuret-modified Fe-TAML as the catalyst and NaOCl as the oxidant under catalytic conditions epoxide was formed with modest yields and turnover numbers. (Graph Presented).

Full text of DOI:10.1021/ic503053q

Article
pubs.acs.org/IC
Mechanism of Oxygen Atom Transfer from FeV(O) to Olefins at Room
Temperature
Kundan K. Singh,† Mrityunjay k. Tiwari,‡ Basab B. Dhar,† Kumar Vanka,*,‡ and Sayam Sen Gupta*,†
‡
†Chemical Engineering Division and Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory,
CSIR-National Chemical Laboratory, Pune 411008, India
S
* Supporting Information
ABSTRACT: In biological oxidations, the intermediate
FeV(O)(OH) has been proposed to be the active species for
catalyzing the epoxidation of alkenes by nonheme iron
complexes. However, no study has been reported yet that
elucidates the mechanism of direct O-atom transfer during the
reaction of FeV(O) with alkenes to form the corresponding
epoxide. For the first time, we study the mechanism of O-atom
transfer to alkenes using the FeV(O) complex of biuret-modified
Fe−TAML at room temperature. The second-order rate
constant (k2) for the reaction of different alkenes with FeV(O)
was determined under single-turnover conditions. An 8000-fold
rate difference was found between electron-rich (4-methoxystyr-
ene; k2 = 216 M−1 s−1) and electron-deficient (methyl trans-
cinnamate; k2 = 0.03 M−1 s−1) substrates. This rate difference indicates the electrophilic character of FeV(O). The use of cis-
stilbene as a mechanistic probe leads to the formation of both cis- and trans-stilbene epoxides (73:27). This suggests the
formation of a radical intermediate, which would allow C−C bond rotation to yield both stereoisomers of stilbene−epoxide.
Additionally, a Hammett ρ value of −0.56 was obtained for the para-substituted styrene derivatives. Detailed DFT calculations
show that the reaction proceeds via a two-step process through a doublet spin surface. Finally, using biuret-modified Fe−TAML
as the catalyst and NaOCl as the oxidant under catalytic conditions epoxide was formed with modest yields and turnover
numbers.
tially carry out epoxidation of electron-rich olefins, and Class B
catalysts, such as [Fe(6-Me3-TPA)(OTf)2], which selectively
perform epoxidation of electron-poor olefins like acrylate and
fumarate more rapidly than electron-rich olefins.6 The FeV(O)
intermediate has been proposed to be the active oxidant for
Class A catalysts. In 2009, Talsi et al.7a reported the probable
involvement of FeV(O) during epoxidation by a nonheme iron
catalyst at −70 °C on the basis of only EPR measurements (the
amount was estimated to be around 15%). The epoxidation rate
was estimated by the decay of this S = 1/2 species in EPR
measurements. However, no direct experiments have been
reported yet that characterize the reactivity of FeV(O)
intermediate with olefins with varying electronic and steric
properties. Given that epoxidation is a synthetically useful and
mechanistically important process, such studies would shed
light on the reactivity of nonheme iron complexes that catalyze
the epoxidation reaction.
INTRODUCTION
■
The development of a biomimetic oxidation catalyst containing
cheap, readily available, and nontoxic metals like Fe is being
keenly pursued, but the development of Fe-based complexes
that catalyze epoxidation of olefins with high selectivity using
inexpensive and environmentally friendly terminal oxidants1
remains a challenge. The key to such development lies in
understanding the mechanism of biological and biomimetic
oxidations, especially in identifying metal-based high-valent
iron−oxo intermediates and in mapping their reactivity with
target substrates.2 In biological systems, Fe-containing heme
and nonheme centers have both been shown to catalyze the
epoxidation reaction. For heme-containing cytochrome P450,
the well-established FeIV(O)−porphyrin•+ is the active
oxidant.3 In nonheme enzymes, the reactive intermediate
FeV(O)(OH) has been proposed to catalyze both the
epoxidation and the cis-dihydroxylation reaction.4 These have
been shown to be two closely related transformations, and the
product outcome can be controlled by addition of additives
such as acetic acid.5 In contrast, the related FeIV(O) has been
shown to be weakly active in catalyzing epoxidation reactions.5c
Several nonheme Fe-based complexes catalyze the epoxidation
(and related cis-hydroxylation reaction) and can be divided into
Class A catalysts, such as [Fe(TPA)(OTf)2], which preferen-
We recently reported the stoichiometric formation of
[FeV(O)−(biuret-TAML)]− complex 2 at room temperature
(RT) with NaOCl/m-CPBA as an oxidant.7b The reactivity of
this FeV(O) for activation of C−H bonds and in photochemical
Received: December 22, 2014
© XXXX American Chemical Society
A
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Scheme 1. Catalytic Scheme for the Epoxidation of cis-Cyclooctene by [FeV(O)−(biuret-TAML)]−
water oxidation has been reported.7b,8 In this work, we report
the reactivity of [FeV(O)−(biuret-TAML)]− (2) with a wide
variety of alkenes. The second-order rate constants derived with
alkenes with varying electronic properties indicate the FeV(O)
to be an electrophilic oxidant. Using a combination of
experimental and theoretical studies (DFT), we propose a
detailed mechanism of the epoxidation reaction.
constant value within 10% error. Resulting kobs values correlated
linearly with substrate concentration to give second-order rate
constant k2.
Single-Turnover Reaction. Excess substrate (50 μL, 10−1 M) was
added to a freshly prepared 500 μL solution of 2 [generated by
reaction of 1 (2 × 10−4 M) and NaOCl (2.2 × 10−4 M)] at room
temperature (RT). Reaction mixture was stirred for 20−70 min. After
completion of the reaction (determined by UV−vis spectroscopy), the
products (yield %) were quantified by GC using a calibration curve
obtained with authentic epoxides of respective alkene. The products
were identified by GC-MS and NMR.
EXPERIMENTAL SECTION
■
Materials. Biuret-amide-modified Fe−TAML (1) was synthesized
as reported by us before.9a Aqueous sodium hypochlorite (reagent
grade, Aldrich, available chlorine 4.00−4.99%) was used as received
and quantified by iodiometry. Acetonitrile (HPLC grade, Aldrich) was
used after passing through an activated neutral alumina column. O18-
enriched (98%) water was procured from Shanghai Research Institute
of Chemical Industry, China. Deionized water was used to make all
stock solutions of NaOCl for the reaction and kinetic runs. Styrene, 4-
chlorostyrene, 4-methoxystyrene, and cis-cyclooctene were purchased
from Aldrich and distilled before use. cis-Stilbene, trans-stilbene,
norbornene, methyl trans-cinnamate, and KPF6 were purchased from
Aldrich and used as received. 4-Cyanostyrene was synthesized by the
Wittig reaction,9b and purity was checked by GC. All reactions were
carried out without any special precautions under atmospheric
condition unless mentioned.
Cyclic Voltammetry. Cyclic voltammetry (CV) was carried out
using a BioLogic potentiostat (model VMP-3) in a conventional three-
electrode electrochemical cell with glassy carbon as the working
electrode, saturated calomel electrode (SCE) as a reference electrode,
and platinum foil as a counter electrode. The measurements were
carried out with a 0.25 mM solution of 2 in acetonitrile (prepared by
addition of 2 equiv of NaOCl into complex 1) with 10 mM KPF6 as
the supporting electrolyte at 0 °C. All peaks were independently
calibrated with respect to an internal ferrocene/ferrocenium (Fc/Fc+)
redox couple under identical condition.
Catalytic Reaction. A 200 equiv amount (with respect to complex
1) of aq NaOCl (at pH 11.4) was added to 1 mL of acetonitrile
solution containing complex 1 (1 × 10−4−1.5 × 10−4 M) and 67−100
equiv of substrate (with respect to complex 1). The reaction was run
with stirring for 10, 40, and 70 min for 4-methoxystyrene, stilbene (cis
and trans), and methyl trans-cinnamate, respectively. At the end of the
reaction, 2 mL of water was added to the reaction mixture and product
was extracted in dichloromethane (five times with 2 mL of
dichloromethane each time). Collected organic phase solvent was
evaporated by purging nitrogen gas, and the volume was adjusted to
500 μL for product quantification by GC.
General Instrumentation. UV−vis spectral studies were carried
out using an Agilent diode array 8453 spectrophotometer attached
with an electrically controlled thermostat. Gas chromatography (GC)
was performed on a PerkinElmer Arnel Clarus 500 instrument
equipped with a hydrogen flame ionization detector and HP-5 (30 m
× 0.32 mm × 0.25 μm) column. Helium was used as carrier gas at a
flow rate of 30 mL min−1. GC-MS were performed on an Agilent
5977A mass-selective detector interfaced with an Agilent 7890B GC in
similar conditions using a HP-5-Ms capillary column (30 m × 0.32
mm × 0.25 μm, J & W Scientific). High-resolution mass spectrometry
(HR-MS) was done in a Thermo Scientific Q-Exactive using an
electron spray ionization source and an Orbitrap as the analyzer and
connected with a C18 column (150 m × 4.6 mm × 8 μm).
O18 Incorporation Experiment. To 200 μL of an acetonitrile
solution of 1 (5 × 10−4 M), 1.2 equiv of NaOCl was added, which
leads to the formation of FeV(O) (2). H2O18 (20 μL) was introduced
into the solvent media of species 2, and the solution was allowed to
stand approximately 3 h at −22 °C. The HRMS showed 30%
incorporation of O18, leading to the formation of [FeV(O18)(biuret-
TAML)]− m/z 431.0763 (calculated m/z 431.0773). Into two separate
sets of the same experiment, 50 μL of a solution of 10−1 M cis-
cyclooctene and methyl trans-cinnamate substrate was added to the
labeled FeV(O18) solution. HR-MS and GC-MS showed the O18-
labeled corresponding epoxides.
Computational Details. To carry out the mechanistic study,
methods based on density functional theory (DFT) have been
employed. All minima and transition states reported in this study were
fully optimized at the UB3LYP/6-31G*, LANL2DZ (Fe) level of
theory11 using the Gaussian 09 suite of quantum-chemical programs.12
The stationary points on the potential energy surface were
characterized by evaluating the vibrational frequencies. The zero-
point vibrational energy corrections and thermal corrections were
applied to the “bottom-of-the-well” values to obtain values for the
Gibbs free energy at 298.15 K. The mechanism was derived
Reactivity of FeV(O) (2) toward Alkene Epoxidation. At least
three kinetic runs were carried out for each experiment; mean values
are reported here. The kinetics were monitored in either the kinetic
mode of the spectrophotometer or the scanning spectral kinetics mode
using a 1.0 cm quartz cell at 396 nm [Isosbestic points of FeIV species
and FeIII] at 25.0 °C.7b,10 All kinetic experiments were carried out in
CH3CN. During kinetic measurements, FeV(O) was prepared by using
1.1 equiv of NaOCl, and substrate was added to it under pseudo-first-
order condition (concentration of 2 was from 2 × 10−5 to 1 × 10−4 M,
depending upon reactivity of the substrate, and different alkenes
concentrations were chosen according to the needs of the experiment,
which were from 2 × 10−4 to 1.8 × 10−1 M, see Figure SI 1,
Supporting Information). The pseudo-first-order rate constant (kobs
)
was calculated at the isosbestic wavelength by nonlinear curve fitting
[At = Aα − (Aα − Ao)e(−kobst)] and had good agreement in rate
B
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Figure 1. (A) UV−vis spectral changes upon reaction of 2 (4 × 10−5 M) with cis-cyclooctene (2.5 × 10−3 M); (B) absorbance vs time plot at 396 nm
for reaction of 2 (4 × 10−5 M) with cis-cyclooctene (2.5 × 10−3 M) (( ) experimental data point; (red line) first-order fit according to the equation
●
[(At = Aα − (Aα − Ao)e(−kobst)]. Reaction was performed at RT in acetonitrile solvent under nitrogen atmosphere.
the slow reaction of alkenes with the FeIV dimer (Figure SI 2,
considering styrene as the representative structure of its derivatives,
such as para-substituted styrenes and stilbene. All energies were
Supporting Information) (separately discussed below). Less
calculated with respect to the infinitely separated reactants. The
than 1% of trans-cyclooctane-1,2-diol was observed as a side
products are also considered to be infinitely separated. All values
product. Quantification of the epoxide was done by GC.
provided in Figure 3 are electronic energies.
Control experiments with only NaOCl in our experimental
conditions showed no formation of cyclooctene oxide.
RESULTS AND DISCUSSION
■
Kinetics of the Epoxidation of Various Alkenes under
Kinetics of cis-Cyclooctene Epoxidation under Single-
Turnover Condition. Compound 2 was prepared at 25 °C
from the parent biuret-TAML activator, (Et4N)2[FeIII(Cl)-
(biuret-TAML] (1), in CH3CN by adding equimolar (1.1
equiv) amounts of NaOCl, as reported before. The
unprecedented high stability of 2 allowed us to perform
extensive kinetic studies in single-turnover condition to
ascertain the mechanism of epoxidation using UV−vis
spectroscopy at RT (Scheme 1). Initial experiments showed
that upon addition of cis-cyclooctene (2.5 × 10−3 M) to a
preformed solution of 2 (4 × 10−5 M), the green color of the
reaction mixture immediately changed to brown. A UV−vis
scanning kinetics study showed that the starting FeV(O) species
changed to the diamagnetic μ-oxo−FeIV dimer species, which
then slowly regenerated the parent FeIII complex 1 (Figure 1A,
peak at 379 nm) quantitatively (for complete characterization
of μ-oxo−FeIV dimer species please see ref 7b). The direct
conversion of 2 to 1 was not observed in initial spectral scans,
since the rate of the comproportionation reaction (1.0 × 105
M−1 s−1) between 1 and 2 (eq 2) was faster than the rate of
epoxidation (29 M−1 s−1; Table 1, entry 8).
Single-Turnover Conditions. To ascertain the mechanism of
epoxidation, single-turnover experiments were performed by
adding preformed 2 (from 2 × 10−5 to 1 × 10−4 M, depending
upon reactivity of the substrate) to various alkenes having
varying electronic and steric properties (Table 1) under
pseudo-first-order conditions. After completion of the reaction,
all products of these single-turnover reactions were identified
by GC-MS and then quantified by GC (Figure SI 3, Supporting
Information). GC-MS showed exclusive formation of epoxide
with moderate yields (>70%) for most alkenes. For the alkenes
trans-stilbene, norbornene, and cis-cyclooctene, the side
products formed were identified as benzaldehyde (<2%),
trans-norbornane-2,3-diol (<1%), and trans-cyclooctane-1,2-
diol (<1%) respectively. When cis-stilbene was used as a
substrate, in addition to the formation of cis-stilbene oxide
(56%), considerable amounts of isomerized products, i.e., trans-
stilbene oxide (21%), were observed (Table 1, entry 6). When
trans-stilbene was used as the substrate, trans-stilbene oxide was
formed exclusively, and no cis-stilbene oxide was found (Table
1, entry 7). Additionally, stereochemical retention was also
observed for the epoxidation of methyl trans-cinnamate. In all
reactions, the starting FeIII complex (1) was quantitatively
regenerated after the single-turnover reaction. To ascertain that
the O atom was incorporated into the alkene upon epoxidation
from the FeV(O), O18 labeling experiments were performed.
Epoxidation was carried out on two different alkenes (methyl
trans-cinnamate, cis-cyclooctene) using a mixture of FeV(O16)
and FeV(O18) (ratio of 68:32) (Figures SI 4 and 5, Supporting
Information). The epoxides obtained after completion of the
reaction showed ∼30% incorporation of O18 into the epoxide,
which confirmed that oxygen incorporated into the epoxide was
transferred from FeV(O) (Figures SI 6 and 7, Supporting
Information).
FeV(O) + alkene → products + FeIII
(1)
The pseudo-first-order rate constant (kobs) correlated linearly
with [cis-cyclooctene] (Figure 1B), and the second-order rate
constant k2 (29 M−1 s−1) was calculated from the slope of the
straight line. After completion of the reaction, product
identification was carried out by GC-MS; the results showed
that cyclooctene oxide was formed as the major product
(∼65%). It should be noted that the fast comproportionation
between 1 and 2 would allow only 50% of 2 to react with
alkenes and yield a maximum of only 50% yield epoxide (with
respect to 2). The higher yields of epoxide observed are due to
The kinetics of the reaction was then studied to determine
the second-order rate constant k2, as described above (Figure SI
1, Supporting Information). The k2 values obtained for all of
C
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a
Table 1. Epoxidation of Different Alkenes by FeV(O) at RT in CH3CN under Air
a
Concentration of FeV(O) used to obtain k2 values was 2 × 10−5 M (for entrie 1 and 10), 4 × 10−5 M (for entries 2−4 and 8), and 1 × 10−4 M (for
b
entries 6, 7, and 9). For yield of epoxides under single-turnover conditions, the concentration of FeV(O) was 2 × 10−4 M. Concentration variation
of substrates used to obtain k2 was in the range of 10−100 equiv (for entries 1−4, 6−8, and 10) and 500−1750 equiv (for entry 9) with respect to
c
FeV(O). For obtaining single-turnover yields the substrate concentration used was 1 × 10−2 M. Yields of epoxides are with respect to FeV(O).
the different alkenes show that the rates of epoxidation of
electron-rich substrates were much faster than those of the
electron-deficient ones. For example, the k2 value for 4-
methoxystyrene (Table 1, entry 1) was determined to be 216
M−1 s−1, while that of methyl trans-cinnamate (Table 1, entry
9) was 0.03 M−1 s−1. This 8000-fold rate difference between
electron-rich and electron-deficient substrates indicates that
FeV(O) has electrophilic character, reminiscent of nonheme
epoxidation catalysts that have been discussed before. The
reactivity of the disubstituted alkenes (e.g., cyclooctene, cis-
stilbene) is much slower than expected, possibly due to the
steric crowding of the disubstituted alkenes. The effect of
electron-donating and -withdrawing groups on alkenes was
explored in more detail by using a series of substituted styrenes.
Kinetic studies revealed that styrene with a para-substituted
electron-donating group was more reactive than styrene having
a para-substituted electron-withdrawing group (Table 1). For
example, k2 for the epoxidation of p-cyanostyrene was
determined to be ∼3 times slower than the rate of p-
methoxystyrene. We were unable to determine the exact k2
value of p-nitrostyrene, since its strong UV absorption in the
300−400 nm region interfered with the FeIII/IV isosbestic point
(Figure SI 8, Supporting Information).
Reactivity of μ-Oxo−FeIV Dimer. The >50% yield of
epoxide formation observed in the single-turnover experiments
is unlikely because the fast comproportionation between 1 and
2 would only allow 50% of 2 to react with alkenes and yield a
maximum of only 50% epoxide (with respect to 2). The higher
yields of epoxide observed are due to the slow reaction of
alkenes with the μ-oxo−FeIV dimer, which was investigated in
D
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the reaction of 4-methoxystyrene and cis-cyclooctene with μ-
oxo−FeIV dimer (Table 2). The μ-oxo−FeIV dimer was
observed at high substrate concentration. However, no
saturation was observed for the kobs with styrene/methoxystyr-
ene with up to 750 equiv of the substrate, implying that the
substrate-mediated disproportionation was probably not
occurring. Further, the reaction of μ-oxo−FeIVdimer with 4-
methoxystyrene in the presence of different amounts of
parental FeIII complex 1 showed reduction in reaction rates
with increasing amounts of 1 into the reaction mixture. The
plot of reaction rate vs amount of FeIII (1) in reaction mixture
shows a good fit into the kinetic model
a
Table 2. Epoxidation of Alkenes by μ-Oxo−FeIV Dimer at
RT in CH3CN Solvent under Air
k1k2[μ‐oxo−FeIV dimer][substrate]
rate =
k−1[FeIII] + k2[substrate]
Using a k2 value of 216 M−1 s−1 (as earlier determined for
methoxystyrene under single-turnover conditions), k1 and k−1
were determined to be 0.024 and 24 620, respectively. (Figure
SI 9, Supporting Information)
Possible Mechanisms of Epoxidation. Three possible
mechanisms for initial attack of “O” into the alkene during the
epoxidation reaction have been proposed:
a
Concentration of μ-oxo−FeIVdimer was 1 × 10−5 M for reaction with
4-methoxystyrene and 2 × 10−5 M for reaction with cis-cyclooctene
b
and styrene. Concentration of substrates was in range from 2.5 ×
10−4 to 4 × 10−3 M.
(1) the transfer of oxygen into the alkene by a concerted
process (Scheme 2, path C), as is known for the
epoxidation of alkenes by the enzyme cytochrome
P450,2a,3a,13
synthesized by addition of 0.5 equiv of NaOCl in complex 1,
which has characteristic UV−vis spectra similar to the
previously reported μ-oxo−FeIV dimer (which is formed upon
(2) generation of the acyclic carbocation radical through one-
electron transfer14 (Scheme 2, path B), and
1
equimolar amounts of 1 in 2; characterized by H NMR and
EPR).7b,10 The μ-oxo−FeIV dimer species reacts with alkenes to
form a corresponding epoxide. For example, in the reaction of
μ-oxo−FeIV dimer with 4-methoxystyrene, cis-cyclooctene and
styrene corresponding epoxide were observed with a rate of
reaction more than 50-fold slower than the corresponding
reaction with FeV(O), 2. Although we do not have any
conclusive evidence of the mechanism of this reaction, we
believe that the μ-oxo−FeIV dimer disproportionates into
FeV(O) (2) and FeIII complex (1), after which the FeV(O)
reacts with alkenes to form the epoxide product (Scheme SI 1,
Supporting Information). Two scenarios are possible: (1)
binding of alkene to the μ-oxo−FeIVdimer leads to the
disproportionation reaction, after which the FeV(O) reacts, or
(2) the μ-oxo−FeIV dimer exists in an equilibrium with the
FeV(O) and FeIII complex. The reaction of the alkene with
FeV(O) leads to more disproportionation of the dimer, leading
to the formation of more FeV(O), which reacts with the alkene
until all of it is converted to the starting Fe(III) complex. If the
first hypothesis is correct, a saturation in the kobs values of the
reaction of the μ-oxo−FeIV dimer with the substrate should be
(3) radical intermediate formation via electrophilic attack of
FeV(O) upon alkene with concomitant C−O bond
formation15 (Scheme 2, path A).
Understanding the pathway is important, since the product
selectivity during the epoxidation reaction depends upon the
mechanism through which the reaction proceeds. For example,
both the second and the third pathways have been observed for
synthetic Fe−porphyrin complexes as well as Mn−salen
complexes.16 In fact, it has been shown that the reaction
pathway between 2 and 3 can be manipulated by varying the
axial ligands in synthetic iron porphyrins.17 The other
possibility for the catalytic reaction is the axial coordination
of the second equivalent of NaOCl to the FeV(O), resulting in
the formation of FeV(O)OCl, as observed in Fe−porphyrins.
We exclude this possibility since the addition of excess NaOCl
to complex 2 does not show any change in the UV−vis spectra
of the starting FeV(O). All possible pathways are schematically
shown below (Scheme 2).
Epoxidation by Pathway C. To probe experimentally if
the epoxidation reactions by FeV(O) occurs via pathway C, the
Scheme 2. Detailed Mechanistic Scheme for the Epoxidation of Alkenes by [FeV(O)−(biuret-TAML)]−
E
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substrate cis-stilbene was used as a mechanistic probe, as has
been shown by Bruice et al. in the case of electron-deficient
cytochrome P-450 model complexes (e.g., meso-tetrakis(2,6-
dichlorophenyl)porphinato−oxo−iron(IV), [(Cl8TPP+•)-
FelV(O)].14b,18 If cis-stilbene leads to the exclusive formation
of cis-stilbene oxide, it indicates the predominance of a
concerted mechanism. In contrast, if a mixture of both cis-
and trans-configured epoxide is observed, it would point to the
formation of a radical intermediate that would allow C−C bond
rotation to give the stereoisomers. In the epoxidation of cis-
stilbene with FeV(O), formation of both cis- and trans-stilbene
epoxide is observed (ratio 73:27); hence, the pathways via a
concerted mechanism look unlikely (Path C in Scheme 2).
Further, DFT calculations discussed below also show that the
epoxidation of styrene by FeV(O) complexes are indeed
nonconcerted processes.
precluding the role of FeIV(O) as the active species during
C−O bond formation.
Table 3. Calculated Values of the Free Energy of Formation
of the Transition State (ΔGa) and Standard Free Energy
(ΔG°) for the One-Electron Transfer in the Transition State
during the Reaction of FeV(O) with Alkene at 25 °C in
a
Acetonitrile
log k2
ΔGa
E1/2 (V,
SCE)
ΔG°
substrate
(M−1 s−1
)
(kJ mol−1
)
(kJ mol−1
)
norbornene
styrene
2.51
2.17
2.06
47.5
49.5
50.1
1.90
1.95
1.76
85.9
90.7
72.4
4-
chlorostyrene
4-cyanostyrene
1.86
1.54
51.3
53.1
2.05
2.03
100.3
98.4
cis-cyclooctene
Epoxidation via Pathways A and B. The loss of
stereochemistry during the epoxidation of cis-stilbene can be
explained by invoking the formation of a radical intermediate
(Scheme 2, Path A or B). To distinguish between pathway A
and pathway B, we looked carefully at the rates of epoxidation
with various substituted alkenes. It has been shown that for
epoxidation reactions operating via pathway B, i.e., the rate-
determining step (RDS) being one-electron oxidation, the
second-order rate constants vary linearly with the ionization
potential (IP) of alkenes.19 In our study of alkene epoxidation
by FeV(O), the plot of k2 vs IP of alkenes does not show any
linearity (Figure SI 10, Supporting Information).
a
ΔGa = −RT ln (k2h/kT); ΔG° = −nF(P1 − P2), where n = 1, P1 =
E1/2 (V, SCE) of substrates oxidation, P2 = E1/2 (V, SCE) of FeV(O)
reduction =1.01, F = 96485 C mol−1, T = 298 K, R = 8.314 J K−1
mol−1, k = 1.38 × 10−23 J K−1, h = 6.6 × 10−34 J s.
To investigate this further, a geometry optimization of the
cationic styrene and the dianionic FeIV(O) complex was
performed in the solvent phase, using the conductor like
polarization continuum model (CPCM) method21 and with
acetonitrile (dielectric constant, ε = 35.688) as the solvent. The
free energy difference between the obtained ionic species and
the rate-determining transition state (RDTS) of pathway A (in
this case, also optimized with solvent corrections for the
purpose of comparison) was calculated to be 13.0 kcal/mol
(Table SI 2, Supporting Information). The calculated free
energy of the reaction leading to the formation of the ionic
intermediates in pathway B was endergonic by 34.4 kcal/mol
(Table SI 2, Supporting Information), which further indicates
its unlikelihood.
DFT studies also point to the fact that pathway B is not
followed during the epoxidation process. The calculations show
that the epoxidation of styrene by FeV(O) complexes 2 is
indeed a nonconcerted process, with C−O bond formation
being the rate-determining step (Figure 3 and Table SI 1,
Supporting Information), followed by a very low barrier ring-
closing step. This corresponds to pathway A in Scheme 2. The
existing literature on analogous systems suggests similar
observations, which were further supported by DFT
studies.3c,18,20 For investigating pathway B (see Scheme 2),
we looked at the correlation diagram of the ionization potential
of the substrates versus the experimentally measured second-
order rate constants (Figure SI 10, Supporting Information).
No correlation between the ionization potential and the rate of
reaction was obtained, which indicates that one-electron
oxidation of the styrene coupled with one-electron reduction
of the monoanionic FeV(O) complex is unlikely. This is in
contrast to the epoxidation catalyzed by Mn and Cr porphyrin
catalysts, in which the second-order rate constants strongly
correlated to the E1/2 of the substrates.18,19 Thermodynamic
analysis of the epoxidation of olefins by 2 precludes the
involvement of the one-electron-reduced FeIV(O) as the active
species during the O-atom transfer leading to formation of the
C−O bond. First, the free energy of activation during the
epoxidation reaction was determined from the second-order
rate constants. This was compared to the standard free energies
required for the formation of a one-electron-oxidized alkene,
i.e., FeV(O) + alkene ⇌ FeIV(O) + alkene•+. (The redox
potential of FeV/FeIV was determined by performing cyclic
voltammetry experiments with chemically synthesized FeV(O);
see the Experimental Section and Figure SI 11, Supporting
Information). The analysis shows that, for several alkenes, the
free energy of the one-electron oxidation of alkenes by FeV(O)
exceeds that of the free energy of the transition state during the
epoxidation of alkenes by FeV(O) (Table 3), thereby
To understand the effect of the electronic environment
associated with the substituent on styrene, different para-
substituted styrene derivatives with electron-donating sub-
stituents displayed enhanced second-order rate constant (k2)
values when compared to electron-withdrawing para sub-
stituents (Table 1). The k2rel (k2rel = k2 /k2 ) value for styrene
and its analogue was used for linear free energy correlation
analysis (Figure 2), and a Hammett ρ value of −0.56 was
obtained. The relatively small value of ρ indicated the buildup
of partial positive charge on the olefinic carbon in the transition
state. However, previous studies showed that the rate-limiting
carbocation formations via electrophilic additions to substituted
styrenes are associated with much more negative ρ values [i.e.,
−3.58 (hydration) and −4.8 (bromination)].22 When the
barrier heights for pathway A obtained from the DFT
calculations were compared for styrene and p-chlorostyrene,
it was seen that the barrier height for the p-chlorostyrene case
was higher by 1.9 kcal/mol than for styrene (10.5 kcal/mol for
styrene and 12.4 kcal/mol for p-chlorostyrene; Tables SI 1 and
3, Supporting Information). This observation indicates that
pathway A most likely represents the actual epoxidation
mechanism, and therefore, the direction of the flow of electrons
inside the styrene substrates during the reaction occurs from
the phenyl ring to the CH2CH group. To further
investigate the effect of the solvent and the Hammett-type
behavior of the styrene derivatives on the rate-determining
barriers (RDB), the RDB were calculated using the conductor
X
H
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approach of the α carbon of styrene onto the oxo group of the
catalyst, whereas the β pathway involves the attack of the β
carbon onto the catalyst. The complete energy profile suggests
that the β attack is energetically more favorable (see Figure
3B). The reactive species formed by the β attack is more stable
due to the extended conjugation of the phenyl ring.
For the β pathway, there are again two possibilities that need
to be investigated: the reaction could occur on the doublet
surface or on the quartet surface. As Figure 3 indicates, the
reactant FeV(O) complex is more stable as a doublet than a
quartet by 3.0 kcal/mol (Table SI 5, Supporting Information).
This suggests that the species would exist predominantly in the
doublet state, a result that is supported by the experimental
observations (the EPR of FeV(O) shows resonances character-
istic of S = 1/2 species).7b At the intermediate stage, after the
crossing of the first barrier, the calculations show that the
quartet adduct structure is more stable than the doublet, by
10.2 kcal/mol (Table SI 1, Supporting Information). While this
suggests a thermodynamic incentive for the system to convert
from the doublet to the quartet at the intermediate stage, the
barrier for the subsequent reaction is only 3.0 kcal/mol on the
doublet surface, which indicates that the reaction would most
likely proceed in the forward direction on the doublet surface.
It has been noted in the past that while the spin-flip barrier is
expected to be low, it is unlikely that the spin flip would
compete with the forward reaction process for reactions having
forward barriers less than 5.0 kcal/mol23 Hence, the reaction
would proceed to its end on the doublet surface and lead to the
formation of the doublet product (S = 1/2). Subsequent to this,
with no further reaction occurring, this kinetically obtained
product can convert to the quartet product (S = 3/2), which is
12.0 kcal/mol lower in energy (see Table SI 1, Supporting
rel
Figure 2. Hammett plots of log k2 against para-σ+ for the
epoxidation of styrenes and its para-substituted derivatives by
rel
X
FeV(O) at RT in CH3CN. Hammett value ρ is −0.56; k2 = k2 /
k2H, where k2 and k2 are second-order rate constants for para-
X
H
substituted styrene and styrene, respectively.
polarized continuum model (CPCM) in acetonitrile (e =
35.688) for 4-methoxystyrene, styrene, and 4-chlorostyrene
substrates. The obtained result followed the experimental trend,
i.e., the barriers are obtained in the following order: 4-
chlorostyrene (12.7 kcal/mol) > styrene (9.2 kcal/mol) > 4-
methoxystyrene (8.8 kcal/mol, see Table SI 4, Supporting
Information).
Understanding the α and β Pathways during Styrene
Epoxidation. Depending upon the mode of attack on the oxo
group by the substrate styrene, two possible pathways for
epoxidation are possible for pathway A: the α pathway and the
β pathway (Figure 3A and 3B). The α pathway entails the
Figure 3. Gas-phase energy profile with optimized geometries of the intermediates and transition states for the epoxidation of styrene by 2 at the
UB3LYP/6-31G*, LANL2DZ (Fe) level of theory from doublet and quartet surfaces for the (A) α and (B) β pathway (Dis and A denote distance
and angles, respectively; superscripts 2 and 4 represent geometries in the doublet and quartet spin states).
G
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Figure 4. Molecular orbital pictures of the HOMO and LUMO of reactant, intermediate, and product, and orbital occupancy for styrene epoxidation
by FeV(O) at the ROB3LYP/6-31G*, LANL2DZ (Fe) level of theory in the gas phase.
Information, and Figure 3B). Hence, the calculations predict a
reaction occurring on the doublet surface and leading to the
formation of a final quartet product: a result that matches the
experimental observation that the final product is a quartet.
Molecular Mechanism of Epoxidation by FeV(O).
Figure 4 shows the frontier orbitals obtained from the DFT
calculations for the styrene substrate. As mentioned in the
previous section, the reaction would occur on the doublet
surface, with the quartet finally being obtained as the
thermodynamic product at the end of the reaction. Parts R
and I of Figure 4 show the MOs corresponding to the reactant
and the intermediate Fe complexes on the doublet surface. The
HOMO in the intermediate doublet geometry is seen to be
localized on the styrene. Part P shows the MOs obtained for
the kinetic doublet product as well as for the thermodynamic
quartet product. As also indicated in Figure SI 12, Supporting
Information, the energy of the HOMO in the quartet structure
is 3.9 kcal/mol lower than the energy of the HOMO in the
doublet. According to the “HOMO rule”, the more stable of
two structures of a given species is the one whose HOMO is at
a lower energy.24 This explains why the quartet structure is
more stable than the doublet and, therefore, why the final
product of the reaction is a quartet.
Catalytic Activity. Quantitative regeneration of FeIII (1) after
the single-turnover reaction gave us an opportunity to study the
epoxidation reaction in the catalytic condition. The catalytic
epoxidation at RT in CH3CN under air was performed with
excess NaOCl (adjusted to pH 11.4). Different olefins (such as
4-methoxystyrene, trans-methyl cinnamate, and trans-stilbene)
were used for the catalytic condition. To optimize reaction
conditions, the influence of catalyst concentration was studied
in the epoxidation of cis-stilbene, trans-stilbene, and methyl
trans-cinnamate in the presence of catalyst 1. It was observed
that the increase in catalyst loading (1−1.5%) leads to faster
conversion and greater formation of epoxide due to the increase
in the reaction rate (see Figure SI 13, Supporting Information).
When the catalyst loading was <1%, less conversion was
observed. If the reactions were performed under exclusion of
dioxygen, the conversion and yield of epoxide were unaffected;
this indicates that molecular oxygen did not have any role in the
formation of epoxide. The products formed under optimized
reaction conditions are shown in Table 4. Under our reaction
condition, side products included trans-diol and aldehydes, but
the amounts were less than 5% of the epoxide. PhCH2C(O)Ph
was also identified as a side product during the epoxidation of
the cis/trans-stilbene reactions. In the case of 4-methoxystyrene,
H
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a
Table 4. Summary of the Catalytic Epoxidation of Different
Alkenes by 1 at RT in CH3CN under Air
ASSOCIATED CONTENT
* Supporting Information
Detailed kinetic data, gas chromatography data for product
formation, HRMS of FeV(O); details of DFT calculations are
also provided. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
ic503053q.
■
S
c
conversion
(%)
yield (%)/epoxide (reaction
time)
alkene
a
4-methoxystyrene
74
97
99
95
79 (10 min)
86 (40 min)
89 (40 min)
90 (70 min)
b
cis-stilbene
a
trans-stilbene
b
methyl trans-cinnamate
AUTHOR INFORMATION
Corresponding Authors
*E-mail: k.vanka@ncl.res.in.
*E-mail: ss.sengupta@ncl.res.in.
a
Catalyst 1 (1 × 10−4 M):NaOCl (2 × 10−2 M):Substrates (1 × 10−2
■
b
M) = 1:200:100. Catalyst 1 (1.5 × 10−4 M):NaOCl (3 × 10−2
c
M):Substrates (1 × 10−2 M) = 1.5:300:67. Yield of epoxides is with
respect to alkenes consumed during the reaction (5% error).
Author Contributions
All authors have contributed to the writing of the manuscript
and approved its final version.
the increase in reaction time leds to increased conversion of
alkene but with reduced yield of epoxide. Several unidentified
side products were observed during this reaction.
Notes
The authors declare no competing financial interest.
CONCLUSION
■
A mechanism has been proposed for the epoxidation of olefins
ACKNOWLEDGMENTS
■
by FeV(O) (see Scheme 3). Using a combination of
S.S.G. acknowledges DST, New Delhi (SERB, EMR/2014/
000106) for funding. K.K.S. and M.K.T. acknowledge CSIR
(Delhi) for a fellowship. B.B.D. also acknowledges CSIR for a
SRA position.
Scheme 3. Proposed Catalytic Cycle for Epoxidation with
Complex 1 and NaOCl
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