Olefin cis-Dihydroxylation and Aliphatic C-H Bond Oxygenation by a Dioxygen-Derived Electrophilic Iron-Oxygen Oxidant

Article abstract of DOI:10.1002/anie.201502229

Many iron-containing enzymes involve metal-oxygen oxidants to carry out O₂-dependent transformation reactions. However, the selective oxidation of C-H and C-C bonds by biomimetic complexes using O₂ remains a major challenge in bioinspired catalysis. The reactivity of iron-oxygen oxidants generated from an Fe^II-benzilate complex of a facial N₃ ligand were thus investigated. The complex reacted with O₂ to form a nucleophilic oxidant, whereas an electrophilic oxidant, intercepted by external substrates, was generated in the presence of a Lewis acid. Based on the mechanistic studies, a nucleophilic Fe^II-hydroperoxo species is proposed to form from the benzilate complex, which undergoes heterolytic O-O bond cleavage in the presence of a Lewis acid to generate an Fe^IV-oxo-hydroxo oxidant. The electrophilic iron-oxygen oxidant selectively oxidizes sulfides to sulfoxides, alkenes to cis-diols, and it hydroxylates the C-H bonds of alkanes, including that of cyclohexane. Lewis acid mediated O-O bond cleavage: A nucleophilic iron(II)-hydroperoxo oxidant, formed upon oxidative decarboxylation of an iron(II)-α-hydroxy acid complex, undergoes heterolytic O-O bond cleavage in the presence of a Lewis acid to generate an electrophilic iron(IV)-oxo-hydroxo oxidant. The electrophilic oxidant oxidizes sulfides to sulfoxides and alkenes to cis-diols, and it hydroxylates the strong C-H bonds of aliphatic substrates.

Full text of DOI:10.1002/anie.201502229

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International Edition: DOI: 10.1002/anie.201502229
Reactive Intermediates
German Edition:
DOI: 10.1002/ange.201502229
À
Olefin cis-Dihydroxylation and Aliphatic C H Bond Oxygenation by
a Dioxygen-Derived Electrophilic Iron–Oxygen Oxidant**
Sayanti Chatterjee and Tapan Kanti Paine*
18
Abstract: Many iron-containing enzymes involve metal–
ing experiments with H2 O in the dihydroxylation by H O
2 2
oxygen oxidants to carry out O -dependent transformation
support the OÀO bond cleavage prior to cis-dihydroxyla-
2
[14]
reactions. However, the selective oxidation of CÀH and C=C
tion.
bonds by biomimetic complexes using O remains a major
Over the last decades, biomimetic oxidation of alkanes
2
challenge in bioinspired catalysis. The reactivity of iron–
oxygen oxidants generated from an Fe –benzilate complex of
and alkenes by iron complexes has been extensively stud-
II
[15–19]
ied.
The presence of “ready oxidant” H O and substrates
2 2
a facial N ligand were thus investigated. The complex reacted
together allows the complexes to catalyze the CÀH bond
3
with O to form a nucleophilic oxidant, whereas an electro-
hydroxylation and olefin cis-dihydroxylation through putative
2
[17,20–23]
philic oxidant, intercepted by external substrates, was generated
in the presence of a Lewis acid. Based on the mechanistic
high-valent iron–oxo oxidants.
Several iron–oxygen
intermediates such as iron(III)–(hydro)peroxo and iron(IV)–
oxo species have been generated through reduction of
dioxygen by synthetic iron(II) complexes in the presence of
II
studies, a nucleophilic Fe –hydroperoxo species is proposed to
form from the benzilate complex, which undergoes heterolytic
[24–32]
OÀO bond cleavage in the presence of a Lewis acid to generate
electron and proton donors in stoichiometric amounts.
IV
an Fe –oxo–hydroxo oxidant. The electrophilic iron–oxygen
All these studies provide useful mechanistic information on
reductive dioxygen activation by iron(II) complexes. How-
ever, examples of biomimetic iron complexes for oxidation of
oxidant selectively oxidizes sulfides to sulfoxides, alkenes to
cis-diols, and it hydroxylates the CÀH bonds of alkanes,
[33–36]
including that of cyclohexane.
olefins and aliphatic substrates with dioxygen are rare.
In
this endeavor, we have been exploring the dioxygen reactivity
I
ron-containing oxygenases activate dioxygen to catalyze
of biomimetic iron(II)–a-hydroxy acid complexes supported
a variety of biologically important oxidation reactions. In
many oxygenases, high-valent iron–oxo species act as active
oxidants in the oxidation reactions such as hydroxylation of
aliphatic CÀH bonds, cis-dihydroxylation/epoxidation of
by a facial N ligand, hydrotris(3,5-diphenyl-pyrazol-1-yl)bo-
3
Ph2
rate ligand (Tp ). In the complexes, the iron-coordinated a-
hydroxy acid (two-electron sacrificial reductant) anions
provide the necessary electrons and protons for dioxygen
[
1–4]
[37,38]
olefins, oxidation of sulfides, and so on.
For the reduction
reduction.
We have recently reported the reactivity of
of dioxygen and subsequent generation of high-valent iron–
oxo species, the necessary electrons are provided either by the
iron center or by organic cofactors. In the heme enzymes
cytochrome P450, high-valent iron–oxo oxidants oxidize
a
nucleophilic iron–oxygen oxidant derived from
Ph2
II
[(Tp )Fe (benzilate)] (1) toward different substrates
(Scheme 1). The nucleophilic oxidant has been shown to cis-
[5–7]
strong aliphatic CÀH bonds.
For Rieske dioxygenases,
the nonheme enzymes involved in electrophilic cis-dihydrox-
ylation of aromatic compounds, a side-on iron(III)–peroxo
species has been suggested as a key oxidant in the catalytic
[
8–10]
cycle.
However, there is debate as to whether the iron-
(
III)–(hydro)peroxo species performs cis-dihydroxylation or
the OÀO bond is cleaved to form a high-valent iron–oxo–
hydroxo oxidant that carries out the cis-dihydroxylation
Scheme 1. A nucleophilic oxidant generated in the reaction of an
[
8,11,12]
reaction.
Although DFT calculations indicate the
iron(II)–benzilate complex of a monoanionic facial N ligand with O .
3
2
[13]
involvement of an iron(III)–peroxo species, isotope label-
dihydroxylate alkenes with the incorporation of both the
oxygen atoms of molecular oxygen into diols. The oxidant
however did not exchange its oxygen atoms with water.
[
*] S. Chatterjee, Dr. T. K. Paine
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science
[38]
Since high-valent electrophilic iron–oxo intermediates
have been implicated as key oxidants for enzymatic and
biomimetic oxidation reactions, our objectives of this work
were to reverse the philicity of the nucleophilic oxidant and to
evaluate the reactivity of the resulting electrophilic oxidant.
Toward these objectives, we have investigated the reactivity
of the oxidant from 1 in the presence of Lewis acid, because
Lewis acidic metal ions are known to stabilize and modulate
2
A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032 (India)
E-mail: ictkp@iacs.res.in
[
**] T.K.P. acknowledges the Council of Scientific and Industrial
Research (CSIR) and Indian National Science Academy (Project for
Young Scientist Awardee) for financial support. S.C. thanks CSIR for
a fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502229.
9
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[27,30,39–44]
the chemistry of metal–oxygen species.
As an out-
come of our investigation, we report herein the reactivity of
an electrophilic oxidant, generated from 1 in the presence of
a Lewis acid, toward aliphatic and olefinic substrates. The
selective oxidation of strong aliphatic CÀH bonds including
that of cyclohexane and cis-dihydroxylation of olefins by
iron–oxygen oxidants generated in situ are presented.
[
37]
Complex 1 reacts with dioxygen in the presence of an
equimolar amount of Sc(OTf) to undergo oxidative decar-
Scheme 2. Thioanisole-derived products obtained in the reaction of
1 with O and H O in the absence and presence of Sc .
2 2
3
1
8
3+
boxylation of benzilic acid to benzophenone in quantitative
yield within 15 min. In the reaction, hydroxylation of one of
Ph2
the phenyl rings of Tp occurs to an extent of 90%. A mixed
1
6
18
labeling experiment with O and H2 O in the absence of
confirms around 29% incorporation of labeled oxygen into
sulfoxide (Supporting Information, Figure S4). Of note, no
oxygen atom from water is incorporated into thioanisole
2
3
+
Sc confirms no incorporation of labeled oxygen into the
[38]
3+
hydroxylated ligand. In the presence of Sc , around 33%
incorporation of labeled oxygen from water into the ligand
takes place, as observed in the ESI-MS of the iron(III)
3
+ [38]
oxide in the absence of Sc . A control experiment with
18
thioanisole oxide, Sc(OTf) , and H2 O in a benzene–aceto-
3
Ph2
)
+
complex [(Tp * Fe] (m/z = 740.2) of hydroxylated ligand
nitrile solvent mixture reveals no exchange of water with
sulfoxide (Experimental Section in the Supporting Informa-
tion). The Lewis acid alone cannot cause the observed labeled
water exchange with sulfoxide. With scandium ions, therefore,
a different oxidant is generated that exchanges its oxygen
atoms with water. To explore the nature of the active oxidant,
Hammett analysis with different para-substituted thioanisoles
was performed. A Hammett 1 value of À0.929 confirms the
electrophilic nature of the iron–oxygen oxidant generated in
the presence of Lewis acid (Figure 2).
Ph2
(
Tp *) (Figure 1).
Figure 1. ESI-mass spectra of the oxidized solution of 1 after the
1
6
3+
16
18
3+
reaction a) with O and Sc and b) with O , H O, and Sc
.
2
2
2
1
8
c) The incorporation of oxygen atom from H O into the phenolate
ring ( O shown as filled O).
2
1
8
When the interception experiment with complex 1 is
3
+
carried out with thioanisole and Sc (1 equiv each), thioani-
sole oxide (35%) is formed as the only product without any
sulfone (Scheme 2; Supporting Information, Figure S1). In
the reaction, 53% intramolecular ligand hydroxylation is
estimated. This result is in contrast to the product obtained in
3
+
+
the absence of Sc , where only sulfone is formed in 34%
Figure 2. Hammett plot of log krel vs s1 for p-X-C H SMe obtained
6
4
[
38]
3+
yield (Scheme 2). With 10 equiv of thisoanisole and one
from the reaction of 1 with one equiv of Sc . The krel values were
calculated by dividing the concentration of product from para-sub-
stitued thioanisole by the concentration of product from thioanisole.
3
+
equiv of Sc , the yield of thioanisole oxide increases to 90%
with almost no ligand hydroxylation. The ratio of sulfone to
sulfoxide depends on the concentration of Sc(OTf) added,
3
3
+
and around one equiv of Sc is sufficient for complete
inhibition of sulfone formation (Supporting Information,
Figure S2). Other Lewis acids also control the sulfone/
sulfoxide selectivity (Supporting Information, Figure S3),
The electrophilic iron–oxygen oxidant is able to cis-
dihydroxylate alkenes to the corresponding cis-diols
(Scheme 3; Supporting Information, Table S1 and Experi-
mental Section). In the presence of 100 equiv of alkenes, the
intramolecular ligand hydroxylation is completely inhibited.
3
+
3+
2+
and unlike Sc , 1.7 equiv of In , around 2.0 equiv of Mg
+
and 3.5 equiv of Na cause complete inhibition of sulfone
formation from thioanisole.
16
18
A labeling experiment with O , H O (60 equiv), and
2
3
2
+
The labeling experiment for thioanisole (10 equiv) oxida-
cyclohexene in the presence of Sc results in the incorpo-
1
6
18
3+
tion with O and H2 O (60 equiv) in the presence of Sc
ration of labeled oxygen atom into the cis-diol product to an
2
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3+
However in the presence of Sc , the electrophilic oxidant is
unable to carry out the oxidation of alkenones.
We have earlier reported that the reaction of 1 with
benzaldehyde (20 equiv) yielded 75% benzoic acid and 40%
benzyl alcohol. Formation of benzyl alcohol and benzoic acid
suggested a Cannizzaro-type mechanism in the reaction.
However, unlike the normal Cannizzaro reaction where
alcohol and acid are formed in 1:1 ratio, the yield of benzoic
acid was found to be higher than that of alcohol. Additional
3
5% benzoic acid was therefore proposed to form via
oxidation of benzaldehyde by the nucleophilic metal-based
[38]
oxidant. On the contrary, the electrophilic oxidant gener-
ated with Sc cannot oxidize benzaldehyde to benzoic acid
Scheme 3. Oxidation of alkenes with O by complex 1 in the presence
3+
2
3
+
of Sc . The values within the brackets indicate the percentage yields
but can participate in a normal Cannizzaro-type reaction to
form benzoic acid and benzyl alcohol in 1:1 ratio with an
overall yield of 80% (Supporting Information, Figure S13).
To carry out the Cannizzaro reaction, the oxidant must have
a metal-coordinated hydroxo group. This was further verified
3
+
.
of cis-diols in the absence of Sc
Ph2
II
[46]
by the reaction of [(Tp )Fe (BF)] complex with 20 equiv
of 4-bromobenzaldehyde, where 4-bromobenzoic acid was
not detected. A putative S = 2 iron(IV)–oxo species from
Ph2
II
[
(Tp )Fe (BF)] and O2 has been reported to carry out
[
47]
various oxidation reactions
including the oxidation of
[48]
cyclohexane and n-butane, but the oxidant cannot oxidize
aldehyde to carboxylic acid (Supporting Information, Fig-
ure S14). These results suggest that the nature of electrophilic
Ph2
II
oxidant from 1 is different than that from [(Tp )Fe (BF)].
The nucleophilic oxidant is not efficient to cleave the CÀH
bond of cyclohexane (CÀH bond dissociation energy =
Figure 3. GC-mass spectra of cis-cyclohexane-1,2-diol obtained in the
1
6
18
2
reaction of 1 with cyclohexene, O , and H O a) in the absence of
2
3+
À1
3+
99.5 kcalmol ), but the electrophilic oxidant is powerful
Sc , and b) in the presence of Sc
.
enough to oxygenate the strong CÀH bond of cyclohexane.
The electrophilic oxidant oxidizes cyclohexane to form about
50% cyclohexanol and 5% cyclohexanone. Although the
oxidation of cyclohexane has been reported by synthetic
iron(IV)–oxo complexes, the oxygenation of cyclohexane
with high alcohol/ketone selectivity (A/K = 10) by an O2-
derived oxidant is unprecedented (Scheme 4 and Figure 4a;
extent of 32% (Figure 3; Supporting Information, Table S1).
18
Oxidations of cyclohexene with different amounts of H2
O
[49]
reveal that the percentage of labeled oxygen into cis-diol
18
increases linearly with the amount of added H2 O (up to
0 equiv) and then reaches at a maximum value of 32(3)%
Supporting Information, Figure S5). These results suggest
6
(
18
2
the presence of a pre-equilibrium coordination of H O to
[
45]
the iron center of the active oxidant. Hammett analysis
with various substituted styrenes predicts a negative 1 value
further confirming the electrophilic nature of the oxidant
(
Supporting Information, Figure S6). Thus unlike in the
3+
3+
absence of Sc , the oxidant formed in the presence of Sc
has electrophilic character and can exchange one of its oxygen
atoms with water. The cis-dihydroxylation of cis-2-heptene
with retention of stereochemistry both in the presence and
3
+
absence of Sc confirms that a metal-based oxidant is
involved in the reaction through a concerted mechanism
Scheme 4. Aliphatic CÀH bond oxygenation by complex 1 with O in
2
3+
the presence of Sc . The values within the brackets indicate the
3+
(
Supporting Information, Table S1, Figures S7 and S8). While
percentage yields of oxidation products in the absence of Sc
.
the electron-rich alkenes afford the corresponding cis-diols in
higher yields, electron-deficient alkenes such as tert-butyl
acrylate and dimethyl fumarate yield small percentage of diol
products in the presence of Sc (Supporting Information,
Figures S9 and S10). The oxidant generated in the absence of
any Lewis acid oxidizes the electron-deficient alkene 2-
cyclohexenone (100 equiv) to a mixture of cis-2,3-dihydroxy
cyclohexanone (38%) and 1,4-benzoquinone (17%) (Sup-
porting Information, Table S1 and Figures S11 and S12).
Supporting Information, Figure S15). The electrophilic oxi-
dant exchanges one of its oxygen atoms with water present in
the reaction medium, which is evident from the incorporation
3+
18
of around 27% of labeled oxygen atom from H2 O into
cyclohexanol (Figure 4b). Substrate with relatively weak CÀ
H bond such as methylcyclohexane (CÀH bond dissociation
À1
energy = 94.3 kcalmol ) can be oxygenated in the presence
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complex 1 is able to cis-dihydroxylate alkenes to diols with
partial incorporation of oxygen atom from water and can
participate in a Cannizzaro reaction, the oxidant is proposed
to be an iron(IV)–oxo–hydroxo species (II) with the oxo and
hydroxo groups disposed cis to each other. The high-valent
iron–oxo species can exchange its oxygen atoms with water
and carries out the oxidation of external substrates
(
Scheme 5). The heterolytic OÀO bond cleavage reaction of
peroxide to generate iron(IV)–oxo species with the present
[52,53]
system is reminiscent of isopenicilin N synthase (IPNS).
Figure 4. GC-mass spectra of cyclohexanol formed in the reaction of
1
6
3+
16
18
Example of such intermediate in enzymatic or model systems
is rare. An iron(II)–(alkyl)hydroperoxo species, obtained by
one-electron reduction of the iron(III)–peroxo precursor, has
1
with a) O in the presence of Sc and b) O and H O in the
2 2 2
+
3
presence of Sc
.
been reported to undergo heterolytic OÀO bond cleavage to
3
+
[54]
and in the absence of Sc . The nucleophilic oxidant oxidizes
methylcyclohexane, affording 1-methylcyclohexanol (12%)
and 2-methylcyclohexanol (3%) with only 15% conversion.
The electrophilic oxidant, in contrast, preferentially activates
the tertiary CÀH bond of methylcyclohexane to form 1-
generate an iron(IV)–oxo species.
The iron(II) complex
reported here reductively activates dioxygen to form an iron–
oxo oxidant via Lewis acid mediated OÀO heterolysis of an
iron(II)–hydroperoxide species.
In conclusion, a nucleophilic iron–oxygen oxidant is
proposed to form in the reaction of an iron(II)–benzilate
complex with dioxygen, which undergoes heterolytic OÀO
methylcyclohexanol (45%) along with a small amount (13%)
of 2-methylcyclohexanol (Scheme 4; Supporting Information,
Figure S16). Similarly, the CÀH bond of adamantane can be
bond cleavage in the presence of a Lewis acid to generate an
electrophilic iron–oxygen oxidant. The reversal of philicity of
the nucleophilic oxidant mediated by Lewis acids is a unique
result which sheds light on the nature of key oxidants involved
activated by the nucleophilic as well as by the electrophilic
oxidant, but the extent of oxidation of adamantane by the
electrophilic oxidant is higher where selectively 1-adamanta-
nol (47%) and 2-adamantanol (12%) are formed (Scheme 4;
Supporting Information, Figures S17 and S18). The normal-
ized 38/28 CÀH bond selectivity is a clear indication of
in O -dependent transformation reactions. The electrophilic
2
iron–oxygen oxidant from the complex, yet unobserved for
spectroscopic characterization, was intercepted by external
substrates as probes that were intermolecularly oxidized. The
electrophilic oxidant performs cis-dihydroxylation of alkenes
and hydroxylation of strong CÀH bonds of aliphatic sub-
a metal-based oxidation reaction.
Based on the experimental results, we propose that
a nucleophilic iron(II)–hydroperoxo species (I) is formed
upon two-electron reductive activation of dioxygen and
concomitant decarboxylation of benzilic acid to benzophe-
none (Scheme 5). DFT calculations earlier predicted a lower
strates including that of cyclohexane with high alcohol/ketone
selectivity. The oxidant exchanges its oxygen atoms with
water. On the basis of interception and mechanistic studies, an
iron(IV)–oxo–hydroxo species is proposed as the active
oxidant that exhibits electrophilic behavior. The versatile
reactivity of the oxidant presented here would provide useful
information toward the development of bioinspired oxidation
catalyst for olefin cis-dihydroxylation and aliphatic CÀH
cleavage reaction using dioxygen. Further studies in that
direction are being performed in our laboratory.
Keywords: dioxygen · electrophilic oxidants · iron · Lewis acids ·
oxidation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9338–9342
Angew. Chem. 2015, 127, 9470–9474
Scheme 5. Proposed mechanism for the formation of O -derived iron–
oxygen oxidants in the reductive activation of dioxygen by an iron(II)–
benzilate complex.
2
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