Revealing the Structure and Reactivity of the Active Species in the FeCl2-TBHP System: Case Study on Alkene Oxidation

Article abstract of DOI:10.1021/acs.organomet.8b00184

A mechanism involving a quintet ferryl [Fe^IV=O] species has been disclosed for the oxidation of alkene under the FeCl₂-TBHP system. Both theoretical and experimental results suggested that the quintet ferryl species [Fe^IV=O] was produced by the reaction of FeCl₂ with TBHP. A Mulliken atomic spin density distribution on [Fe^IV=O] showed that the O site has strong radical character and could easily react with alkene to form a carbon radical intermediate. This radical could be further oxidized by TBHP to lead to the C=C bond cleavage of alkene.

Full text of DOI:10.1021/acs.organomet.8b00184

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Revealing the Structure and Reactivity of the Active Species in the
FeCl −TBHP System: Case Study on Alkene Oxidation
2
†
‡
§
,†,|
Zhiliang Huang, Xiaotian Qi, Jyh-Fu Lee, and Aiwen Lei*
†The Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,
People’s Republic of China
|
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
3
45 Lingling Road, Shanghai 200032, People’ s Republic of China
‡
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, People’s Republic of China
§National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
S Supporting Information
*
IV
ABSTRACT: A mechanism involving a quintet ferryl [Fe O]
species has been disclosed for the oxidation of alkene under
the FeCl −TBHP system. Both theoretical and experimental
2
IV
results suggested that the quintet ferryl species [Fe O] was
produced by the reaction of FeCl with TBHP. A Mulliken
2
IV
atomic spin density distribution on [Fe O] showed that the
O site has strong radical character and could easily react with
alkene to form a carbon radical intermediate. This radical
could be further oxidized by TBHP to lead to the CC bond
cleavage of alkene.
INTRODUCTION
However, due to the lack of further evidence, debates on the key
intermediates still remain.
Recently, we revealed that 1,1-diphenylethylene (1a) could be
oxidized to afford benzophenone (2a) in 84% yield at room
■
The iron−tert-butyl peroxide (TBHP) system is a well-developed
combination for oxidative reactions.
1a−i
In the past few decades,
it has been widely used toward the construction of C−O and
temperature in the presence of FeCl and TBHP (eq 1). In the
2a,b
2
C−X (X = C, N, etc.) bonds via C−H hydroxylation, cross-
3
4a−c
dehydrogenation coupling, oxidative coupling reactions,
etc.
However, the mechanism of the reaction of the iron salt with
1a,5a,b
TBHP has not yet been settled satisfactorily.
Undoubtedly,
it is of great importance to elucidate the nature of the iron−
TBHP system, since this knowledge will not only be helpful for
the new iron-catalyzed reaction design but also provide some
information for understanding the oxidation mechanism of iron-
meantime, only a trace amount of 2a could be obtained when
FeCl is absent, indicating that FeCl plays a vital role in this
2
2
transformation. To the best of our knowledge, accumulated
6
containing oxygenases.
IV
evidence shows that the ferryl species [Fe O] can be formed
The mechanistic investigation of the reaction of iron with
12a−d
in the FeCl −H O solution (Fenton’s reagent).
In this
7
2
2
2
TBHP has by now a history of decades, and intense discussions
IV
regard, we suspected that a similar [Fe O] species might also
8a−d
always focus on the characterization of the key intermediates.
exist in the FeCl −TBHP system and work as the key
2
To date there are mainly two views: (1) a high-valent iron
species is the key intermediate, as supported by the isolation and
characterization of high-valent non-heme or heme iron oxo
intermediate for the oxidation of the CC bond of 1a. There-
fore, density functional theory (DFT) calculations and experi-
mental characterization have been utilized to study the mecha-
9a−f
or peroxo in the presence of TBHP,
and (2) the correspond-
nism of the oxidation of 1a by FeCl and TBHP. Delightfully, our
2
IV
ing oxy radical, which is produced by O−H or O−O bond
investigation supported the formation of a [Fe O] species in
8b,c,10
homolysis, is the key intermediate for oxidation.
Generally,
the mixture of FeCl and TBHP, and it could react with 1a
2
the well-known high-valent iron intermediates are considered
too active and unstable to live in the extra ligand-free system.
Additionally, up to now, they have not yet been detected in
immediately, exergonic by 37.4 kcal/mol. Herein, we share our
latest discoveries to bring some fresh ideas to the the iron−
TBHP oxidation system.
11a−e
the FeCl −TBHP system even at ultralow temperature. Hence, a
2
great number of people believe the high-valent iron species might
Received: March 29, 2018
5a,8b,c
not exist in the FeCl −TBHP system without extra ligand.
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00184
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Organometallics
Article
calculated by M06 is provided for a discussion of the energy. The
Mulliken atomic spin density of certain atoms was also calculated using
the same method. The optimized structures were displayed using
CYLview.
Additionally, the MECP location program developed by Harvey and
co-workers was used in this study to gain the structures of MECPs at the
B3LYP/6-31G(d) level of theory. The solvation single-point energies of
the MECPs, which were calculated at the M06/6-311+G(d,p) level in
acetonitrile, have also been summarized. It is also noteworthy that all of
the iron complexes considered in this study are in the high spin state.
The corresponding low spin state structures were determined to be
energetically unfavorable.
COMPUTATIONAL METHODS
■
All of the DFT calculations were carried out with the GAUSSIAN 09
series of programs. The DFT method B3LYP
-311G(d) basis set was used for geometry optimizations. Harmonic
1
3a,b
with a standard
6
vibrational frequency calculations were performed for all of the station-
ary points to confirm if the points were local minima or a transition
structure and to derive the thermochemical corrections for the enthal-
pies and free energies. In this study, the stability of the wave function has
been tested for singlet, triplet, quintet, and sextet state intermediates.
All of the test results confirmed that the wave function is stable under
the perturbations considered. The solvation effects were considered
by single-point calculations on the gas-phase stationary points with
14
15
an SMD continuum solvation model. The M06 functional with
the 6-311+G(d,p) basis set was employed to calculate the solvation
single-point energies in an acetonitrile solvent to provide more accurate
energy information. The Gibbs free energy of each stationary point as
RESULTS AND DISCUSSION
■
Mechanistic Investigation of the Oxidation of Olefin by
the FeCl −TBHP System. Initially, the oxidation of CC
2
bonds of olefins by the FeCl −TBHP system was tested. As
2
shown in Scheme 1, various olefins could be oxidized suc-
cessfully. For example, 1,1-diarylethylenes containing electron-
donating or electron-withdrawing groups are suitable for this
oxidation reaction (2b−d). Halo-substituted 1,1-diarylethylenes
are also tolerated in this transformation to afford the corre-
sponding ketones in good yields (2e−h). 2-(1-Phenylvinyl)-
Scheme 1. Oxidation of CC Bonds of Olefins by the FeCl −
2
a
TBHP System
naphthalene could be oxidized by FeCl and TBHP efficiently as
2
well (2i). (1-Cyclohexylvinyl)benzene and 1-chloro-4-vinyl-
benzene will also be transferred to the desired aldehydes by
this oxidation reaction (2j,k). Hence, these results concluded
that the oxidation of CC bonds of olefins, which has attracted a
16
great deal of attention, is a good model for the mechanistic
investigation.
In the experiment, the addition of FeCl facilitated the oxida-
2
tion of olefin, since only a trace amount of 1a could be oxidized to
2
a at room temperature by TBHP without FeCl . To determine
2
the role of FeCl in this transformation, DFT calculations
2
were performed. The optimal structure of FeCl in acetonitrile
2
(
CH CN) solution was studied first, and computational results
3
showed that the quintet Fe(II) complex A1 (Figure 1) coor-
dinated by two Cl and two CH CN groups is favored over other
3
structures, including the low spin state Fe(II) complexes (see the
Supporting Information for details). The structure of A1 was
characterized by XAS spectroscopy, and experimental results are
shown in Figure S1 in the Supporting Information. The Mulliken
atomic spin density of Fe(II) in A1 is determined to be 3.73
a
The reaction was carried out with FeCl (0.25 mmol, 28.0 mg),
2
alkene (0.25 mmol), and TBHP (1.5 mmol, 5 M in decane) in
CH CN (2 mL) under N at room temperature for 8 h. Isolated yield.
3
2
IV
Figure 1. Free energy profile for the generation of [Fe O] species A6 from quintet Fe(II) species A1 and TBHP.
B
DOI: 10.1021/acs.organomet.8b00184
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Article
could be formed through transition state A5-ts (Figure 2); this
process is endergonic by 12.9 kcal/mol, and the activation free
energy is 25.0 kcal/mol. Meanwhile, another minimum energy
crossing point, 5-MECP, was obtained for this radical substi-
tution process. Optimized structures suggests that the O2−H
bond lengths in 5-MECP and A5-ts are 1.88 and 1.14 Å, respec-
tively, which confirms that 5-MECP occurs earlier before A5-ts.
From an energetic point of view, the relative energies of 4-MECP
and 5-MECP are found to be much lower than those of adjacent
transition states, indicating that spin crossover during the gen-
eration of A6 is a facile process. Other pathways for the reaction
of A1 and TBHP were safely ruled out (see the Supporting
Information for more details). Therefore, the theoretical
IV
calculation results suggested that the [Fe O] species could
be formed in the ligand-free iron-TBHP system. Furthermore, an
Mulliken atomic spin density analysis shows that the spin den-
sities located on Fe(IV) and O in A6 are 3.18 and 0.45, respec-
tively (Figure 2). These data reveal that the oxygen atom would
be very active, as it has strong radical character.
IV
In order to stabilize the [Fe O] species for spectroscopic
Figure 2. Optimized structures of several key intermediates and transition
states. The values in parentheses are Mulliken atomic spin densities of
certain atoms, and the other values are bond lengths, given in Å.
characterization, an electron-rich ligand TMC (1,4,8,11-
tetramethyl-1,4,8,11-tetraazacyclotetradecene) was employed
11e
in this system. As shown in Figure 3, the XANES (X-ray
absorption near-edge structure) spectrum of Fe(OTf) (TMC)
(
Figure 2), which confirms that the spin state of A1 is a quintet.
2
displays a small peak at 7112.3 eV, which is assigned to a 1s → 3d
Thus, A1 is regarded as the reaction initiator and set as the
relative zero free energy point in following calculations. In addi-
tion, the spin states of other structures were also studied. Unless
otherwise noted, the high spin state is more favored in this
reaction system (see the Supporting Information for details).
The subsequent reaction of A1 with TBHP is shown in Figure 1.
In the beginning, a ligand exchange between A1 and TBHP
occurs to afford A2 with a free energy increase of 5.9 kcal/mol.
Then, homolytic cleavage of the O−O bond leads to the sextet
Fe(III) intermediate A4 and tert-butoxyl radical via transition
state A3-ts with an activation free energy of 12.8 kcal/mol. More-
over, the minimum energy crossing point (MECP) 4-MECP was
also located between A3-ts and A4. Structural analysis shows that
the O−O bond lengths in A3-ts and 4-MECP are 1.75 and 2.99 Å
17
Fe(II) transition. After the addition of TBHP, Fe(II) is oxi-
dized, as evidenced by a right shift of edge energy. The pre-edge
energy and edge energy of the oxidized iron complex are at
7113.8 and 7125.2 eV, respectively, consistent with values previ-
18
ously reported for oxoiron(IV) species. In addition, XANES
spectra exhibiting a large pre-edge area is also a feature of
oxoiron(IV) species, which is caused by its low-symmetry
17
IV
distorted structure. Hence, the [Fe O] species could be
formed and spectroscopically characterized in the reaction of
ferrous salt with TBHP by introducing TMC as the ligand, which
coincides well with our theoretical calculations.
IV
Since the quintet [Fe O] species A6 is extremely active, it
could react with 1a immediately to afford the carbon radical
species A7. As shown in Figure 4, this process is highly exergonic
by 37.4 kcal/mol. Subsequently, the radical substitution between
A7 and TBHP occurs through transition state A8-ts, forming
(
Figure 2), respectively, which clearly revealed the activation
process of the O−O bond in TBHP. After hydrogen abstraction
by the tert-butoxyl radical, the quintet [Fe O] species A6
IV
Figure 3. XANES spectra of Fe(OTf) (TMC) and the mixture of Fe(OTf) (TMC) and TBHP at −40 °C in CH CN.
2
2
3
C
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Article
IV
Figure 4. Free energy profile for the generation of product 2a from the quintet [Fe O] species A6 and 1a.
Figure 5. Structural information on several key intermediates and transition states. The values in parentheses are Mulliken atomic spin densities of
certain atoms, and the other values are bond lengths, given in Å.
the sextet Fe(III) intermediate A9 with a free energy decrease
of 33.6 kcal/mol. The activation free energy of this step is
tert-butoxyl radical forms intermediate A10 with a free energy
increase of 7.1 kcal/mol. Then, A10 undergoes a hydrogen
abstraction process to generate the key quintet Fe(IV) species A12
via transition state A11-ts with an activation energy of 18.7 kcal/mol.
Another minimum energy crossing point, 10-MECP, was also
obtained after A10. The structural information shown in Figure 5
25.0 kcal/mol. As the spin state of A9 is a sextet and A7 is a
quintet, 9-MECP was located between A8-ts and A9. The rela-
tive energy is found to be −74.7 kcal/mol, which is only 2.2 kcal/mol
higher than that of A9. Hydrogen bonding between A9 and a
D
DOI: 10.1021/acs.organomet.8b00184
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Article
suggests that 10-MECP occurs before A11-ts. Moreover, the
relative energy of 10-MECP is determined to be 14.4 kcal/mol
lower than that of A11-ts. Finally, the oxidation product 2a and
intermediate A15 were generated through C−C bond cleavage
transition state A13-ts from A12; the activation barrier is
the National Synchrotron Radiation Research Center. The Program
of Introducing Talents of Discipline to Universities of China
(111 Program) is also appreciated.
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(
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.8b00184.
■
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*
̈
ORCID
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Aiwen Lei: 0000-0001-8417-3061
Notes
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1
(
This paper is dedicated to Professor Xiyan Lu on the occasion of
his 90th birthday. This work was supported by the National
Natural Science Foundation of China (21390402, 21520102003,
9
(
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