Selective ortho-hydroxylation-defluorination of 2-fluorophenolates with a Bis(μ-oxo)dicopper(III) species

Article abstract of DOI:10.1002/anie.201405060

The bis(μ-oxo)dicopper(III) species [Cu^III ₂(μ-O)₂(m-XYL^MeAN)]²⁺ (1) promotes the electrophilic ortho-hydroxylation-defluorination of 2-fluorophenolates to give the corresponding catechols, a reaction that is not accomplishable with a (η²:η²-O₂) dicopper(II) complex. Isotopic labeling studies show that the incoming oxygen atom originates from the bis(μ-oxo) unit. Ortho-hydroxylation-defluorination occurs selectively in intramolecular competition with other ortho-substituents such as chlorine or bromine. O in, F out: [Cu^III₂(μ-O) ₂(m-XYL^MeAN)]²⁺ is a bis(μ-oxo)dicopper(III) species and promotes the electrophilic ortho-hydroxylation-defluorination of 2-fluorophenolates to give the corresponding catechols. Isotopic labeling shows that the incoming oxygen atom originates from the bis(μ-oxo) unit. Ortho-hydroxylation-defluorination occurs selectively in intramolecular competition with other ortho-substituents such as chlorine or bromine.

Full text of DOI:10.1002/anie.201405060

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DOI: 10.1002/anie.201405060
O2 Activation
Selective Ortho-Hydroxylation–Defluorination of 2-Fluorophenolates
with a Bis(m-oxo)dicopper(III) Species**
Joan Serrano-Plana, Isaac Garcia-Bosch, Ryosuke Miyake, Miquel Costas,* and
Anna Company*
Dedicated to Prof. Lawrence Que, Jr. on the occasion of his 65th birthday
Abstract: The bis(m-oxo)dicopper(III) species [CuIII2(m-
O)2(m-XYLMeAN)]2+ (1) promotes the electrophilic ortho-
hydroxylation–defluorination of 2-fluorophenolates to give
the corresponding catechols, a reaction that is not accomplish-
able with a (h2:h2-O2)dicopper(II) complex. Isotopic labeling
studies show that the incoming oxygen atom originates from
the bis(m-oxo) unit. Ortho-hydroxylation–defluorination
occurs selectively in intramolecular competition with other
ortho-substituents such as chlorine or bromine.
cation of anthropogenic organofluorines have increasingly
become subjects of debate due to the toxicity, global warming
potential, ozone depletion, environmental persistence, and
bioaccumulation of these compounds.[4] For this reason, the
degradation of fluorinated organic compounds or activation
À
and transformation of C F bonds into more reactive func-
tional groups is of current interest.[5]
À
In nature, several microbial enzymes can break C F
bonds,[6] and a wide range of fluorinated substrates including
aliphatics (e.g. fluoroacetate, fluoropyruvate) and aromatics
(e.g. fluorobenzoates, fluorophenols) can be defluorinated.
Focusing on aromatics, the defluorination of fluorophenols
usually occurs in aerobic organisms through the mediation of
FAD-containing phenol hydroxylases, which convert 2-fluo-
rophenols into catechols.[7] The oxidative dehalogenation of 4-
fluorophenols has also been reported for cytochrome P450
and chloroperoxidase.[8]
F
luorine forms the strongest single bond to carbon (bond
À1
À
dissociation energy up to 130 kcalmol ) and thus, C F bonds
are considered the most inert organic functionalities. This
arises from the electronegativity of fluorine, which confers
reinforcing ionic forces through the strong polarization of this
bond.[1] Fluorinated organic compounds are renowned for
their unique properties such as thermal stability, enhanced
lipophilicity and ability to suppress metabolic detoxification,
thus increasing the in vivo residence time.[2] Owing to these
properties, fluorinated chemicals represent 20% of all
pharmaceuticals and up to 30% of all agrochemicals.[3] In
spite of their stability, the large-scale production and appli-
Tyrosinase is a ubiquitous dicopper enzyme that catalyzes
the ortho-hydroxylation of phenols to catechols and the
subsequent oxidation to quinones using dioxygen as oxi-
dant,[9] in an overall reaction analogous to that taking place in
FAD-dependent phenol hydroxylases.[7] Tyrosinase operates
via a (h2:h2-peroxo)dicopper(II) species (P), that undergoes
electrophilic attack over the arene. However, in contrast to
FAD-dependent hydroxylases, tyrosinase is incapable of
hydroxylating 2-fluorophenols, which indeed are inhibitors
of this enzyme.[10] Studies with model compounds have shown
that the P species are usually in equilibrium with a highly
electrophilic bis(m-oxo)dicopper(III) species (O), which may
open the door to novel oxidative reactivity hitherto not
attained by the peroxide isomer. Herein, we indeed show that
O species promote the defluorination of 2-fluorophenols to
give the corresponding catechols, thus selectively transform-
[*] J. Serrano-Plana, Dr. I. Garcia-Bosch, Dr. M. Costas, Dr. A. Company
Grup de Quꢀmica Bioinorgꢁnica i Supramolecular (QBIS)
Institut de Quꢀmica Computacional i Catꢁlisi (IQCC)
Departament de Quꢀmica, Universitat de Girona
Campus Montilivi, 17071 Girona (Catalonia) (Spain)
E-mail: anna.company@udg.edu
miquel.costas@udg.edu
Dr. I. Garcia-Bosch
Department of Chemistry, The Johns Hopkins University
Baltimore, MD 21218 (USA)
Dr. R. Miyake
À
À
ing a CAr F into a CAr OH functional group. The reaction
shows a remarkable selectivity, opening a bioinspired alter-
native to multistep transformations involving reactions that
are most commonly associated with highly reactive organo-
metallic compounds.[11]
Department of Chemistry and Biochemistry, Graduate School of
Humanities and Sciences, Ochanomizu University
2-1-1 Otsuka, Bunkyo-Ku, Tokyo 112-8610 (Japan)
[**] Financial support for this work was provided by the European
Commission (FP7-PEOPLE-2011-CIG-303522 to A.C.; ERC-2009-
StG-239910 to M.C., and Marie Curie IOF to I.G.-B.), MINECO
(CTQ2012–37420-C02-01/BQU and CSD2010-00065 to M.C.),
Generalitat de Catalunya (ICREA Academia Award to M.C.), Spanish
Ministry of Science (Ramꢂn y Cajal contract to A.C.), and the
INNPLANTA project INP-2011-0059-PCT-420000-ACT1 (Dr. X.
Ribas). We also thank Dr. Laura Gꢂmez (Serveis Tꢃcnics de Recerca,
Universitat de Girona) for helpful advice in setting up the HR-MS
experiments, and for fruitful discussions.
[CuIII2(m-O)2(m-XYLMeAN)]2+ (1; Scheme 1) has been
described as a thermally unstable compound, which can be
generated at low temperatures (À908C) in acetone or THF by
reaction of the dicopper(I) precursor [CuI2(m-XYLMeAN)]2+
with O2.[12] It has been previously established that this
compound can bind to phenolates and perform their ortho-
hydroxylation, thus mimicking the reactivity exhibited by the
dicopper enzyme tyrosinase.[13] Therefore, the activity of
1 resembles that found for other P and O type dicopper
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405060.
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The ortho-defluorination reaction also occurred when
unsymmetric phenolates bearing only one ortho-fluorine
substituent were used as substrates. Thus, reaction of 1 with
3 equiv of sodium 2-fluorophenolate afforded 31% of 1,2-
dihydroxybenzene along with 8% of 3-fluorocatechol
(Scheme 2). Complex 1 proved to be highly selective in the
defluorination of unsymmetric phenolates bearing other
halogen substituents at the ortho position, such as sodium 2-
chloro-6-fluorophenolate or sodium 2-bromo-6-fluoropheno-
late. Upon reaction with 1, they both afforded exclusively 3-
chlorocatechol and 3-bromocatechol in 62% and 49% yield,
respectively (Scheme 2). Finally, no ortho-hydroxylation
occurred upon reaction of 1 with phenolates bearing sub-
stituents in the ortho position different from fluorine or
hydrogen. Thus, reaction with sodium 2,6-dichlorophenolate
or sodium 2,6-dimethylphenolate only afforded the recovery
of the starting material (Scheme 2). In conclusion, the ortho-
Scheme 1. Ortho-hydroxylation–defluorination reaction of sodium 2,6-
difluorophenolate to give 3-fluorocatechol along with the schematic
representation of Cu2O2 species 1–3.
complexes.[14] The P species in tyrosinase has been shown to
bind fluorophenolates, but the hydroxylation does not occur
in ortho-substituted phenolates.[10] To our surprise, the
reaction of 1 (2 mm) with 3 equiv sodium 2,6-difluoropheno-
late (Na(DFP)) at À908C caused the rapid decay of 1, as
shown by monitoring the reaction by UV/Vis spectroscopy.
Following the complete decay of 1 and subsequent work-up,
1H NMR and HPLC analyses evidenced that the reaction
afforded 3-fluorocatechol in 21% yield (with respect to 1),
indicating that ortho-hydroxylation and defluorination had
taken place (Scheme 2). The yield was unaffected by the use
À
Carene F bond is especially reactive against O, even in the
presence of presumably weaker and/or more nucleophilic
À
ortho-Carene R bonds. This result is especially remarkable,
À
À
because the oxidative dehalogenation of Carene Br and Carene
Cl bonds has been observed upon reacting binuclear cop-
per(I) complexes with O2,[15] whereas defluorination has never
been reported for such systems.
It is well established that O species are usually in a nearly
degenerate equilibrium with their corresponding P isomer.[16]
Thus, it is not clear which one of the two isomers is the real
executor of the phenol ortho-hydroxylation reaction. To
determine the necessity of the bis(m-oxo) core for the ortho-
hydroxylation–defluorination reaction, we studied two
P complexes that are known to promote the ortho-hydroxyl-
ation of phenolates. In particular, we chose the complexes
[CuII (m-h2:h2-O2)(DBED)2]2+ (2)[14b,c] and [CuII (m-h2:h2-O2)-
2
2
(LPy2Bz)2]2+ (3),[14d] which have been previously described by
the groups of Stack and Itoh, respectively (Scheme 1).
Whereas for complex 3 only the P isomer is observed along
the reaction with phenolates,[14d] for complex 2, P converts
into O upon phenolate coordination. In this respect, O is the
real executor of the hydroxylation reaction.[14c] When 2
reacted with Na(DFP) (3 equiv), 3-fluorocatechol was
obtained in 23% yield (with respect to 2). More interestingly,
under analogous reaction conditions, 3 was unable to cleave
Scheme 2. Selective ortho-hydroxylation–defluorination reactions pro-
moted by 1. Reaction conditions: 1) 1, sodium phenolate (3 equiv),
acetone, À908C 2) acidic work-up. See the Supporting Information for
further details.
À
the C F bond and only trace amounts of catechol were
observed (< 3%, Table S1). Control experiments with [CuI-
(MeCN)4]CF3SO3 as the copper source and O2 as oxidant only
afforded trace amounts of the defluorinated product. From
these results it may be concluded that an O species is
necessary to achieve this unprecedented ortho-hydroxyl-
ation–defluorination reaction of phenols by a Cu2O2 species.
A deeper insight into the mechanism of this ortho-
hydroxylation–defluorination reaction was gained by means
of low temperature UV/Vis spectroscopy. Acetone solutions
of 1 at À908C exhibited an intense absorption band at 413 nm
(e = 21000mÀ1 cmÀ1), which remained relatively stable at this
temperature. Addition of 3 equiv of a particular phenolate led
to an immediate color change from yellow to deep purple.
This was readily evident in the UV/Vis spectrum, which
showed the immediate formation of a novel species exhibiting
two new bands in the range of 357–414 nm and 516–639 nm
of higher amounts of substrate (10 equiv) or by lowering the
initial concentration of 1 (0.2 mm) (Table S1).
Isotope labeling experiments were performed by reacting
Na(DFP) with 18O-labeled 1 (generated by initial reaction of
the dicopper(I) precursor with 18O2, subsequent removal of
the excess 18O2, and placing the reaction under N2) affording
3-fluorocatechol with 86% 18O label, thereby demonstrating
that the oxygen atom incorporated into the catechol product
originated from the Cu2O2 unit. Moreover, this dehalogena-
tion reaction was regioselective for the ortho position of the
phenolate, as no defluorination occurred upon reaction of
1 with 4-fluorophenolate (only 4-fluorocatechol was detected
as product).
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a single exponential. The decay rates of complexes 4g–k
,
obtained by reaction of 1 with 10 equiv of the appropriate
para-substituted sodium 2-fluorophenolates, were studied by
UV/Vis spectroscopy and they were fitted to a single expo-
nential function by nonlinear regression methods. Interest-
ingly, plotting the logarithm of the rate of decay (kobs) against
the corresponding Hammett meta-substituent constants (s+)
afforded a linear correlation (R2 = 0.98) that gave a negative
slope (1 = À2.4) (Figure 3) indicating that the reaction
Figure 1. UV/Vis spectra of 1 and 4x in acetone at À908C. Compounds
4x were obtained by reaction of 1 with 3 equiv of the corresponding
sodium phenolate.
depending on the nature of the phenolate (Figure 1 and S1).
Previous rRaman studies on the purple intermediate resulting
from the reaction of 1 with p-chlorophenolate[13] indicate that
this species corresponds to the adduct that results from
binding of the phenolate to the bis(m-oxo) core of 1 (4x,
Figure 1). Cryospray mass spectrometry (CMS) at À908C was
performed to further substantiate the proposed nature of 4a.
The CMS spectrum of a freshly prepared sample of 4a is
dominated by two intense peaks at m/z 783.2571 and m/z
763.3194 (Figure 2). Mass values and isotopic patterns of
Figure 3. Hammett plot for the thermal decay of 4g–k at À908C in
acetone.
occurred through an electrophilic attack on the aromatic
ring of the phenolate. Remarkably, the hydroxylation reaction
of nonfluorinated phenolates catalyzed by tyrosinase and
some model compounds is also of electrophilic nature, and
exhibits 1 values that are similar to the value found for this
ortho-hydroxylation–defluorination reaction.[13,14,17] This data
provides strong experimental support that the reaction of
1 with Na(DFP) entails an initial binding of the phenolate
moiety to one of the copper centers, and then proceeds
through an electrophilic attack of one of the oxo ligands at the
À
ortho-Carene F site, akin to that occurring in the hydroxylation
of phenolates.
At this point, we aimed to improve the reaction yield,
which remained moderate for most substrates (Table S1,
Scheme 2). By definition, the observed ortho-hydroxylation–
defluorination reaction does not involve a neat gain or loss of
electrons, but experimentally it is observed that the oxidation
state of copper changes from + 3 to + 2 at the end of the
reaction. We speculated that the source of the electrons,
which reduce the metal center, could originate from a reaction
with a second Cu2O2 unit releasing O2, from an oxidative
degradation of the ligand, or from a combination of both.
Since the m-XYLMeAN ligand was recovered intact after the
complete decay of 4x, substoichiometric reactions most likely
originate from an intermolecular reaction with a second
Cu2O2 unit. Following this reasoning, the addition of a reduc-
ing agent was envisioned to provide the necessary electrons
from an external source, thus facilitating the reaction and
increasing the yields. Indeed this strategy has been previously
successfully applied for the intramolecular oxidative aromatic
dechlorination by a P species.[15b] Unfortunately, in the
present case, the addition of [CuI(MeCN)4]CF3SO3, sodium
ascorbate, or zinc dust as reducing agent caused the rapid
Figure 2. Spectra of cryospray mass spectrometry (CMS) experiments
at À908C corresponding to the reaction of 1 with 3 equiv of Na(DFP)
in acetone.
these peaks are consistent with their formulation as {[CuIII2(m-
O)2(m-XYLMeAN)](OTf)}+ and {[CuIII2(m-O)2(C6H3F2O)(m-
XYLMeAN)]}+. In agreement with this interpretation, colli-
sion-induced dissociation experiments conducted over the m/
z 763.3 species produced a new peak at m/z 634.3 correspond-
ing to the loss of a phenolate ligand (m/z 129.0; Figure S3).
This experiment further confirmed the nature of 4a as
a phenolate-bound O species.
Kinetic analysis indicated that the formation of 4x was too
fast to be studied by conventional benchtop UV/Vis spectro-
scopic techniques but its decay was substantially slower
followinga first-order process that could be adjusted to
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decomposition of 1, which was previously reported for other
O species.[14e] However, we observed that 2 (which is a P spe-
cies in the absence of a phenolate substrate) could be
generated in the presence of sodium ascorbate. Most remark-
ably, whereas the reaction of 2 in acetone at À908C with
sodium 2-chloro-6-fluorophenolate and sodium 2-bromo-6-
fluorophenolate afforded 41% of 3-chlorocatechol and 34%
of 3-bromocatechol, respectively, the presence of sodium
ascorbate (2 equiv with respect to 2) raised the yields to 83%
in both cases.
with oxygen as a sacrificial oxidant by means of a synthetic
metal complex. This reaction finds precedent in FAD-
dependent hydroxylase enzymes in biological systems. It
constitutes a fascinating transformation from a basic chemical
À
point of view not only because of the strength of the C F
bond but also because of the rather unusual selectivity
properties it exhibits.[19] Even though product yields are still
far from practical for synthetic purposes, this work points
towards the involvement of O species in the ortho-hydroxyl-
À
ation of 2-fluorophenolates by cleaving the C F bond, thus
From all these results a mechanistic picture of the ortho-
hydroxylation–defluorination can be drawn. 2-fluoropheno-
lates bind rapidly to one of the copper(III) centers in 1 to
afford 4x (step a, Scheme 3), which evolves through a first-
obtaining the corresponding catechol. Moreover, this reaction
is also interesting from the perspective of environmental
À
science, because it enables the transformation of the inert C
F bonds of common persistent pollutants into a more reactive
À
C OH unit. Studies regarding the catalytic version of this
transformation are currently underway in our lab.
Received: May 7, 2014
Published online: July 9, 2014
Keywords: aromatic defluorination · copper · dioxygen ·
.
hydroxylation
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À
the presence of competing substituents with weaker C X
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À
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