Iron-catalyzed C-H hydroxylation and olefin cis-dihydroxylation using a single-electron oxidant and water as the oxygen-atom source

Article abstract of DOI:10.1002/chem.201202147

Bioinspired oxidation: The hydroxylation of alkanes with retention of the configuration and the cis-dihydroxylation of olefins can be accomplished using [Fe^II(CF₃SO₃)₂( ^Me,HPyTACN)] (1) as a catalyst, water as an oxygen-atom source, and cerium ammonium nitrate (CAN) as a single-electron oxidant. The oxidation of C-H bonds and C=C bonds, as well as water oxidation involves a common reaction intermediate formed by the reaction of CAN and [Fe^IV(O)(OH ₂)(^Me,HPyTACN)]⁺.

Full text of DOI:10.1002/chem.201202147

COMMUNICATION
DOI: 10.1002/chem.201202147
À
Iron-Catalyzed C H Hydroxylation and Olefin cis-Dihydroxylation Using a
Single-Electron Oxidant and Water as the Oxygen-Atom Source
Isaac Garcia-Bosch, Zoel Codolꢀ, Irene Prat, Xavi Ribas, Julio Lloret-Fillol,* and
Miquel Costas*[a]
Iron is the dominant metal in oxidative enzymes, and the
chemistry of these metalloproteins has inspired the design
of synthetic oxidation catalysts.[1–2] A selection of non-heme
mononuclear iron complexes can mediate the stereospecific
Scheme 1, bottom) were developed as particularly effective
À
catalysts for C H and C=C oxidation, using H2O2 as an oxi-
dant.[5] Isotopic and product analysis as well as computation-
al methods suggest that highly reactive FeV(O)(OH) species
À
À
hydroxylation of alkyl C H bonds and the cis-dihydroxyla-
are responsible for alkyl C H bond hydroxylation and the
tion of olefins,[1–2] and can be regarded as functional models
of oxygenases. The reaction mechanism of a naphthalene
1,2-dioxygenase, the archetypal Rieske dioxygenase (RD), is
thought to be similar to that of cytochrome P-450,[3] in
which high-valent species may be involved in substrate
attack (Scheme 1, top).[3] In parallel, experimental evidence
epoxidation and cis-dihydroxylation of olefins.[4c,e,5] These
À
species are formed through water-assisted O O bond break-
age in an FeIII
(OOH)
(H2O) intermediate,[6] and have been
characterized by means of variable-temperature mass spec-
trometry, together with isotopic labeling.[4i]
We have also recently reported that 1 in aqueous solution
can be oxidized using CAN (cerium(IV) amonium nitrate)
to
generate
oxoiron(IV)
species
[FeIV(O)
ACHTUNGTRENNUNG(OH2)-
(
Me,HPyTACN)]2+ that can further react with CAN, thus
leading to efficient water oxidation (WO).[7] Mechanistic
studies have led us to propose that oxidizing intermediates
formed in the catalytic cycle of WO mediated by 1 are the
À
same as those involved in alkane C H hydroxylation and
olefin cis-dihydroxylation reactions using H2O2, and there-
À
fore, the iron site in 1 mediates both O O bond cleavage
and formation reactions.[4c,7] Herein, we show that oxidizing
species formed in reactions with CAN can effect stereospe-
À
cific C H hydroxylation, and olefin cis-dihydroxylation, and
that water is the source of oxygen atoms in these reactions.
These reactions clearly indicate enzyme-like metal-mediated
reactivity, that is, freely diffusing radicals are not involved.
These reactions are also remarkable because they are funda-
mentally different from canonical oxygenase-like reactions
where high-valent iron–oxo species are formed either by di-
oxygen activation or reaction with oxo donors.[1]
Data from the oxidation of alkane and alkene substrates
(100 mm) in the presence of CAN (100 mm) and by using 1
(1 mm) as catalyst in acetonitrile/water 1:1 are shown in
Scheme 2 and Table 1. Reactions were performed by slow
syringe-pump addition (over 30 min) of an aqueous solution
of CAN to a solution of the catalyst and the substrate at
08C. Following consumption of the oxidant, GC and GC/MS
analysis showed that the conversion of CAN into organic
oxidation products (Scheme 2) ranges from 7% to 38%
when using alkanes, and up to 78% when using alkenes as
substrates (Table 1). Control experiments, in which 1 was
absent, showed no evidence of oxidation products. Alkanes
were converted into mixtures of alcohols and ketones. For
example, the oxidation of cyclohexane gives cyclohexanol
and cyclohexanone in 38% yield overall with respect to
Scheme 1. Proposed mechanism for Rieske dioxygenases (top); iron com-
plexes studied in this work (bottom). Asp=aspartate, His=histidine,
OTf=trifluoromethanesulfonyl
gathered over the last few years indicate that certain syn-
thetic catalysts can react with H2O2, which can be seen as a
two-electron reduced surrogate for O2, to generate highly
electrophilic high-valent oxo–iron species that may resemble
those formed in enzymatic sites.[4]
Using these enzymes as inspiration, the family of com-
ACHTUNGTRENNUNG
plexes [FeII(CF3SO3)2(Me,RPyTACN)], (R=H 1, R=Me 2,
[a] Dr. I. Garcia-Bosch, Z. Codolꢀ, I. Prat, Dr. X. Ribas,
Dr. J. Lloret-Fillol, Dr. M. Costas
Qbis Group, Department of Chemistry
University of Girona
Campus Montilivi, 17071 Girona, Catalonia (Spain)
E-mail: julio-lloret@udg.edu
miquel.costas@udg.edu
Supporting information, which includes experimental details of cata-
lytic reactions and isotopic analysis, for this article is available on the
WWW under http://dx.doi.org/10.1002/chem.201202147.
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freely diffusing carbon-centered radicals and cations are not
involved. When trans-decaline was employed as a substrate,
oxidation takes place preferentially at the most spatially ex-
À
posed secondary C H sites (S[38]/S[28]=1/32), thus imply-
ing that the oxidant exhibits reactivity that is dependent on
À
the steric environment of the C H bonds (see the Support-
ing Information, Table S2). Further support for the steric-
environment dependence arose from the oxidation of 1,1-di-
methylcyclohexane, in which the least sterically hindered C3
and C4 positions were oxidized preferentially. A kinetic iso-
tope effect (KIE) of 4.1 was determined for the oxidation of
a mixture of cyclohexane/ACTHNUTRGNE[NUG D12]-cyclohexane (1:3), a result
that is close to the value that was determined for reactions
in which H2O2 was used as an oxidant (KIE=4.3).[4c] In con-
À
clusion, iron-catalyzed C H oxidations in the presence of
CAN occur stereospecifically, and exhibit selectivity patterns
that are dependent on electronic and steric factors, patterns
that are similar to those found for reactions using H2O2.[5]
Olefins were also employed as substrates. The oxidation
of cis-2-octene, as catalyzed by 1, gave a mixture of products
in 71% yield with respect to CAN (100 mm), and in 29%
yield with respect to the substrate. The mixture of products
was composed of cis-epoxide (E1, 1% yield with respect to
substrate), erythro-vic-diol (D1, 16%), threo-vic-diol (D2,
6%), and 1-hexanal (H1, 6%) (Table 1). Control experi-
ments (see the Supporting Information) indicate that H1
Scheme 2. Catalytic oxidation of alkanes with 1 and CAN. Yields of the
organic oxidation products are based on the conversion of CAN (four
molecules of CAN per molecule of ketone; 2 molecules of CAN per mol-
ecule of alcohol).
Table 1. Catalytic oxidation of olefins with 1–3 and CAN. Product yields
are based on conversion of substrate.
À
arises from oxidative C C bond cleavage of a diol product.
Most significant is the observation that diols are the main
products of the oxidation of cis-2-octene. Considering the
low pH value of the CAN reaction solution (pH~1), we
propose that threo-diol D2 could originate from acid-medi-
ated ring opening of epoxide E1. Indeed, only diol D2 was
obtained in 43% yield when epoxide E1 (100 mm) was ex-
posed to the reaction conditions. Therefore, we conclude
that D1, which constitutes the main product of the reaction,
arises from a cis-dihydroxlation reaction. Considering that
anti-diol D2 derives from ring opening of epoxide E1, we
can estimate a cis-dihydroxylation/epoxidation (D/E) ratio
of 2.6:1, which is close to the value obtained when H2O2 is
used as the oxidant (D/E=2.2:1). The use of 2 (Scheme 1)
as a catalyst in alkene oxidation reactions in the presence of
H2O2 leads to higher selectivity in favor of the cis-dihydrox-
ylation product (Table 1).[5] Similarly, the use of 2 in the oxi-
dation of cis-2-octene in the presence of CAN also leads to
a higher D/E ratio (6.2:1).
The oxidation of trans-2-octene, as catalyzed by 1, was
also investigated. Again, the corresponding syn-addition diol
(D2 in this case) was obtained as the main product (18%
yield), along with the corresponding syn-epoxide (E2, 5%
yield), anti-diol (D1, 8% yield) and 1-hexanal (H1, 4%
yield). By considering that in this case D1 arose from the
ring opening of epoxide E2, the D/E ratio was calculated as
being of 1.5:1, which is close to the value observed when
H2O2 is used as an oxidant (D/E=2). Further evidence to
indicate that the syn- and anti-diols arise from distinct reac-
tion pathways was obtained from the determination of the
ee values of the two products obtained from the oxidation of
Substrate
cat
E1
[%]
E2
[%]
D1
[%]
D2
[%]
H1
[%]
D/E[a]
cis-2-octene
trans-2-octene
cis-2-octene
trans-2-octene
1
1
2
3
1
–
1
–
tr.
5
tr.
5
16
8
6
6
4
1
4
2.6
1.5
6.2
0.5
18
1
10
11[b]
8[c]
[a] D/E (cis-octene)=D1/
[b] 0% ee. [c] 45% ee. tr.=trace.
ACHUTGTNRENNUG(E1+D2), D/E (trans-octene)=D2/ACHTUTGNREN(NUGN E2+D1).
CAN. The ketone product is presumed to be formed by ini-
tial oxidation to cyclohexanol, followed by subsequent rapid
oxidation to the ketone. To prove this hypothesis, the oxida-
tion of an equimolar mixture of cyclohexane and cycloocta-
nol was carried out; the reaction gave cyclohexanol and cy-
clooctanone as products in a ratio of 1:24, thus indicating
that the alcohol is much more reactive than the alkane.
Indeed, oxidation of cyclooctanol under standard reaction
conditions gives cyclooctanone in 67% yield. Most interest-
À
ing is the observation that the oxidation of tertiary C H
bonds of cis-1,2-dimethylcylohexane (see the Supporting In-
formation) and cis-decaline (Scheme 2) occur without appre-
ciable stereoscrambling. These results suggest that the reac-
tions involve transformations at the metal center, that is,
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Iron-Catalyzed Oxidation Reactions
COMMUNICATION
trans-2-octene when [FeII
ACHTNUTGRENNUG(CF3SO3)2ACHTUGNTREN(NUGN R,R,R)-(bpbpp)] (3,
Scheme 1) was employed as the catalyst. Whereas the syn-
diol was obtained with an ee value of 45%, the anti-diol and
syn-epoxide E2 were obtained in racemic form (Table 1).
À
The source of the oxygen atoms in the C H and C=C oxi-
dation products was determined by isotopic analysis of the
products derived from the reaction using 1 as the catalyst.
The oxidation of cis-1,2-dimethylcyclohexane and cis-2-
octene, in the presence of H218O (98% 18O) gave the corre-
sponding tertiary alcohol (96% 18O), and the corresponding
syn-diol (94% 18O), thus demonstrating that water is the
source of the oxygen atoms. In conclusion, 1 mediates the
cis-dihydroxylation of olefins, a reaction that uses water as
the oxygen-atom source and CAN as a single-electron oxi-
dant. In addition, the distribution of products of the reac-
tions are similar to those obtained when H2O2 is used as the
oxidant, thus strongly suggesting that an FeV(O)(OH) inter-
mediate is common to both reactions. However, isotopic
analysis suggests that the oxidizing species in these reactions
are formed through different mechanisms. In the oxidation
reactions using H2O2, the diol product contains one oxygen
atom derived from water and a second oxygen atom derived
from H2O2.[5] This isotopic pattern denotes a water-assisted
[4e,6]
À
O O bond cleavage.
Recently, Nam and co-workers have shown that [FeII-
A
ACHTUNGTRENNUNG
(N4Py)]+ (6) reacts with CAN in aqueous solution
ACHTUNGTRENNUNG
(N4Py)]2+, the oxo ligand of which origi-
Figure 1. Reaction of 1-(O) (prepared by reacting 1 with CAN (6 equiv)
in acetonitrile/water 1:1) with CAN (10 equiv) and cis-2-octene
(100 equiv) at 08C. A) The change in concentration of [CeIV] (black
trace, derived from intensity of peak at lmax =450 nm) and 1-(O) (red
trace, derived from intensity of peak at lmax =776 nm) with t=0 being
the moment of addition of CAN and cis-2-octene. B) UV-Vis and ESI-
MS spectra of 1-(O) just before the addition of a mixture of CAN
(10 equiv) and cis-2-octene (100 equiv) (t=0, top) and 5 s after the addi-
tion (bottom).[10]
nates from water.[9] These species can mediate oxygen-atom
transfer to sulfides, oxidation of ethylbenzene, and oxidative
À
cleavage of cyclohexene; however, neither sterospecific C
H oxidation nor cis-dihydroxylation was observed. Following
these observations, we investigated the use of [FeII
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG ACHTUNTREGG(NNNU CF3SO3)2ACTHUGNTRENNUGN
(Py2TACN)] + (4) and [FeII
in the oxidation of cis-decaline and cis-2-octene under
standard reaction conditions; however, no oxidation prod-
ucts were obtained. We also recently observed that these
complexes do not mediate WO. Indeed, catalysis of WO re-
quires the use of iron complexes that are structurally related
to 1, which contains a tetradentate ligand and two exchange-
able cis sites available for coordination.[7] Therefore, it ap-
pears that there is an obvious correlation between iron com-
plexes that catalyze WO with those that could employ CAN
to mediate stereospecific C-H hydroxylation and olefin cis-
dihydroxylation.[9]
as a mixture of {[FeIV(O)(Me,HPyTACN)]
(CF3SO3)}+ and
ACHTUNGTRENNUNG
{[FeIII(OH)(Me,HPyTACN)](CF3SO3)}+ (see the Supporting
AHCTUNGTRENNUNG
Information). The spectrum also shows relatively intense
peaks at m/z 337.2 and 383.1, which were assigned
to
{[FeIV(O)(OH)(Me,HPyTACN)]}+
and
{[FeIII(O)-
AHCTUNGTRENNUNG
(NO3)(Me,HPyTACN)]}+, respectively. The relatively high in-
tensity of the FeIII OH species does not correspond with its
relative concentration in solution, and therefore may origi-
nate from thermal decomposition of the unstable FeIV spe-
cies in the ESI-MS instrument. The spectra show no evi-
dence to suggest the existence of dinuclear FeIIIFeIV species,
which were previously described by Wieghardt and co-work-
ers.[12]
À
Insights into the reaction mechanism were obtained using
spectroscopic analyses. The reaction of
(6 equiv) in CH3CN/H2O (1:1) at 08C gives [FeIV(O)-
(OH2)(Me,HPyTACN)]2+, 1-(O), which was characterized by
1 with CAN
ACHTUNGTRENNUNG
a low-energy peak in the UV/Vis spectrum (lmax =776 nm,
e=270mÀ1 cmÀ1; Figure 1B), a feature that was previously
described for a solution of this species in acetonitrile.[11] The
intensity of the low-energy band, together with the value of
e, which was previously established using a UV/Vis and
Mossbauer combined experiment,[11] allows us to estimate
that 1-(O) is formed in nearly quantitative yield (>95%).
On the other hand, an ESI-MS spectrum of this mixture ex-
hibits a cluster peak at m/z 469.1, which could be modeled
As determined by UV/Vis monitoring, 1-(O) does not
react with cis-2-octene within the time frame of catalytic ox-
idation reactions using CAN at 08C (see the Supporting In-
formation, Figure S2). However, in the absence of substrate,
the addition of excess CeIV (100 equiv) to a solution of 1-
(O) at 08C causes a hypsochromic shift (Dl=10 nm) in the
UV/Vis band at lmax =776 nm. This shift is strictly associated
with the presence of CeIV, because control experiment in-
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J. Lloret-Fillol, M. Costas et al.
volving lowering the pH and the addition of CeIII did not
show such a shift. The concentration of CeIV could also be
monitored at l=450 nm, and showed a slight drop, which
presumably arises from slow WO (Figure S2). However, the
outcome was different when 1-(O) was added to a mixture
of CAN (10 equiv) and cis-2-octene (100 equiv; Figure 1). In
this case, CeIV was consumed at a substantially faster rate.
Parallel to CeIV consumption, an initial rapid decrease in the
spectral features corresponding to 1-(O) (Figure 1, t=0 s
and t=5 s), followed by a gradual recovery of the initial
concentration of 1-(O) over time was observed.
Parallel ESI/MS monitoring of this reaction shows no
peaks corresponding to 1-(O) immediately after CAN/olefin
addition, and then the appearance of cluster peaks at m/z
383.1 and 448.2 , which were assigned to FeIII species (see
the Supporting Information and Figure 1). The peak at m/z
383.1 was assigned to the hydroxoferric species, {[FeIII(OH)-
quently hydrolyzed, thus releasing the diol, and further oxi-
dized by CeIV to form 1-(O), thus closing the catalytic cycle.
However, the mechanism of the formation of 1V-(O)(OH)
does not involve a simple outer-sphere electron transfer,
and needs special consideration. Spectroscopic monitoring
indicates that at 08C the CeIV oxidant is consumed rapidly
only after addition of the organic substrate. Therefore, the
rate-determining step of the reactions is not the formation
of the oxidant 1V-(O)(OH) but instead, the reaction between
this oxidant and the substrate. The data suggests that 1V-
(O)(OH) is accumulated in the reaction mixture to a certain
level. UV/Vis analysis of the reactions show that the reac-
tion of 1-(O) with CAN only leads to a shift in the band at
lmax =776 nm. Previous kinetic studies on WO catalyzed by
1 showed that these novel spectroscopic features arise from
a FeIV=O–CeIV adduct.[7] Related FeIV=O–ScIII and RuIV=O–
CeIV species have been described in recent reports from the
research groups of Nam and Fukuzumi.[13] Although the
FeIV=O–CeIV adduct can mediate substrate oxidation, it is
unlikely that it is indeed the active species in the reaction.
First, the fact that cis-dihydroxylation is observed is incon-
sistent with the FeIV=O–CeIV adduct being the active oxi-
dant; second, and most importantly, the similarities between
catalytic oxidations using CAN and H2O2 strongly suggests a
common cerium-independent oxidant. This species has been
identified as 1V-(O)(OH), in oxidations using H2O2.[4i] There-
fore, we propose that 1V-(O)(OH) is reversibly formed from
FeIV=O–CeIV.
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ence of 1-(O2C8H16) was confirmed by tandem MS–MS anal-
ysis of the assigned peak at m/z 448.2, and comparing the re-
sulting spectrum with
that
of
{[FeIII-
A
ACTHNGUTERNNUG
generated by the reaction of 1, H2O2, and cis-2-octene (see
the Supporting Information). Consistent with the UV/Vis
analysis, the intensity of peaks corresponding to 1-(O2C8H16)
drops over time as the initial concentration of 1-(O) is re-
covered. Finally, when the CeIV species was completely con-
sumed, the low-energy band had returned to its initial inten-
sity.
À
Transition-metal complexes that mediate C H and C=C
oxidation reactions with retention of the configuration,
using single-electron oxidants and water as the oxygen
source, are scarcely precedented in the literature. Such tran-
sition-metal complexes include IrCp complexes, which were
reported by the research group of Crabtree,[14] and [Ru-
These observations can be understood by the set of reac-
tions outlined in Scheme 3. The reaction 1-(O) with CAN
gives the high-valent species [FeV(O)(OH)(Me,HPyTACN)]2+
,
1V-(O)(OH), in a reversible manner; 1V-(O)(OH) can then
mediate cis-dihydroxylation of an olefin, thus forming the
corresponding glycolate species 1-(O2C8H16), which is subse-
ACHTUNGTREN(NUNG Me3TACN)], which was initially reported by Che and co-
workers15a] and was recently shown by Du Bois to mediate
À
C H hydroxylation of complex molecules with retention of
configuration.[15b] Herein, we have shown that iron com-
plexes can also catalyze such oxidation reactions and that
high-valent metal–oxo species similar to those that can oxi-
dize substrates through oxygenase-like mechanisms can be
formed by using single-electron oxidants and water as the
oxygen-atom source. Spectroscopic, computational, and ki-
netic analysis of the reaction intermediates are currently un-
derway. Finally, by optimizing the reactions and identifying
alternative single-electron oxidants, we hope to develop
novel Fe-mediated oxidation reactions of synthetic value.
Acknowledgements
Financial support by MCYT (CTQ2009—08864, and Consolider Ingenio
CSD-2010—00065). European Research Council project ERC-2009-StG-
239910. JLF thanks MICINN for a RyC contract. ICREA Academia
Awards and SGR-239910. We thank RahuCat for a generous gift of trito-
sylTACN, and Severis Tecnics de Recerca of UdG for technical support.
Scheme 3. Proposed catalytic cycle for cis-dihydroxylation. The ligand
has been omitted for clarity. The putative FeIV=O–CeIV adduct is given
within curly brackets.
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Iron-Catalyzed Oxidation Reactions
COMMUNICATION
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À
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Received: June 18, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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5
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ÞÞ
These are not the final page numbers!

J. Lloret-Fillol, M. Costas et al.
Biomimetic Oxidation
I. Garcia-Bosch, Z. Codolꢀ, I. Prat,
X. Ribas, J. Lloret-Fillol,*
M. Costas* .................... &&&& — &&&&
À
Iron-Catalyzed C H Hydroxylation
and Olefin cis-Dihydroxylation Using
a Single-Electron Oxidant and Water
as the Oxygen-Atom Source
Bioinspired oxidation: The hydroxyla-
tion of alkanes with retention of the
configuration and the cis-dihydroxyla-
tion of olefins can be accomplished
(CAN) as a single-electron oxidant.
À
The oxidation of C H bonds and C=C
bonds, as well as water oxidation
involves a common reaction intermedi-
ate formed by the reaction of CAN
ACTHNUTRGNEUNG
using [FeII(CF3SO3)2(Me,HPyTACN)] (1)
as a catalyst, water as an oxygen-atom
source, and cerium ammonium nitrate
ACTHNGUTERNNUG
and [FeIV(O)(OH2)(Me,HPyTACN)]+.
&
6
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www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!