Oxygenation of Organoboronic Acids by a Nonheme Iron(II) Complex: Mimicking Boronic Acid Monooxygenase Activity

Article abstract of DOI:10.1021/acs.inorgchem.5b01198

Phenolic compounds are important intermediates in the bacterial biodegradation of aromatic compounds in the soil. An Arthrobacter sp. strain has been shown to exhibit boronic acid monooxygenase activity through the conversion of different substituted phenylboronic acids to the corresponding phenols using dioxygen. While a number of methods have been reported to cleave the C-B bonds of organoboronic acids, there is no report on biomimetic iron complex exhibiting this activity using dioxygen as the oxidant. In that direction, we have investigated the reactivity of a nucleophilic iron-oxygen oxidant, generated upon oxidative decarboxylation of an iron(II)-benzilate complex [(Tp^Ph2)Fe^II(benzilate)] (Tp^Ph2 = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate), toward organoboronic acids. The oxidant converts different aryl/alkylboronic acids to the corresponding oxygenated products with the incorporation of one oxygen atom from dioxygen. This method represents an efficient protocol for the oxygenation of boronic acids with dioxygen as the terminal oxidant.

Full text of DOI:10.1021/acs.inorgchem.5b01198

Article
pubs.acs.org/IC
Oxygenation of Organoboronic Acids by a Nonheme Iron(II)
Complex: Mimicking Boronic Acid Monooxygenase Activity
Sayanti Chatterjee and Tapan Kanti Paine*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur,
Kolkata 700032, India
S
* Supporting Information
ABSTRACT: Phenolic compounds are important intermedi-
ates in the bacterial biodegradation of aromatic compounds in
the soil. An Arthrobacter sp. strain has been shown to exhibit
boronic acid monooxygenase activity through the conversion
of different substituted phenylboronic acids to the correspond-
ing phenols using dioxygen. While a number of methods have
been reported to cleave the C−B bonds of organoboronic
acids, there is no report on biomimetic iron complex exhibiting this activity using dioxygen as the oxidant. In that direction, we
have investigated the reactivity of a nucleophilic iron−oxygen oxidant, generated upon oxidative decarboxylation of an iron(II)−
benzilate complex [(TpPh2)FeII(benzilate)] (TpPh2 = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate), toward organoboronic acids.
The oxidant converts different aryl/alkylboronic acids to the corresponding oxygenated products with the incorporation of one
oxygen atom from dioxygen. This method represents an efficient protocol for the oxygenation of boronic acids with dioxygen as
the terminal oxidant.
Scheme 1. Phenylboronic Acid Monooxygenase Activity by
Arthrobacter sp. Strain PBA
INTRODUCTION
■
Microorganisms play important roles in the aerobic biode-
gradation of aromatic compounds.1,2 At the initial step in the
aerobic biodegradation pathway, aromatic substrates are usually
activated by hydroxylation reactions. A number of metal and/or
organic cofactor-dependent oxygenases carry out hydroxylation
reactions using dioxygen as the oxidant.3−5 One such
hydroxylated product is phenol, an important component in
the bioremediation pathway of aromatics in the soil. Phenolic
compounds are the most important molecules among plant
secondary metabolites and have great significance in plant
development. Similarly, boron is known for its importance in
many biological functions and for the growth of plants.6
Although boron compounds are used in bacterial growth
media,7 studies on the bacterial metabolism of organoboron
compounds are rare. The biological oxygenation of 2-
methylcyclohexylboronic acid by the flavin-dependent cyclo-
hexanone monooxygenase has been reported to displace a
boronic acid substituent from the chiral carbon affording the
corresponding alcohol with retention of absolute configura-
tion.8 In 2003, Wackett and co-workers reported the cleavage of
the C−B bond of arylboronic acids with molecular oxygen by
an Arthrobacter nicotinovorans strain PBA.9 Though little is
known to date about the gene(s) involved in bacterial
phenylboronic acid catabolism,10 Arthrobacter sp. strain PBA
metabolized phenylboronic acids to the corresponding phenols
with the incorporation of one oxygen atom from dioxygen
(Scheme 1). The oxidation of biological arylboronic acid has
been reported to follow a monooxygenase-type mechanism.9
The boronic acid monooxygenase activity of the bacterial strain
by exhibiting the C−B bond cleavage of boronic acid and
concomitant formation of C−O bond without an NIH shift is
an important transformation from both biological and synthetic
viewpoint.
Considering the importance of arylboronic acid to phenol
conversion in biological and synthetic chemistry,11 a large
number of oxidation methods by using strong oxidizing agents
such as hydrogen peroxide, oxone, m-chloroperbenzoic acid,
etc. in stoichiometric amounts have been reported.12−19 Efforts
have also been devoted to develop methods with mild and
environmental friendly oxidants for hydroxylation of boronic
acids.20−23 Other mild methods include copper and palladium
catalyzed oxidation,24−27 photocatalytic aerobic oxidative
hydroxylation,28,29 copper mediated electrochemical hydrox-
ylation,30 and oxidation by electrochemically generated super-
oxide anions.31 In spite of several synthetic methodologies
available for the conversion of arylboronic acids to phenols,
biomimetic approaches to arylboronic acid oxygenation using
metal and/or organic cofactors and O2 remain unexplored.
Recently, flavinium catalysts in the presence of a sacrificial
reducing agent have been reported to catalyze the oxidative
hydroxylation of arylboronic acids to phenols with molecular
Received: May 27, 2015
© XXXX American Chemical Society
A
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oxygen.32 Analogous biocatalytic oxidations of C−B bonds with
Baeyer−Villiger oxygenases employing the flavin cofactor have
been reported,33,34 but with the substrate scope focused on
alkylboronates. In flavin-dependent monooxygenases, the
reduced flavin cofactor activates dioxygen to form flavin-
hydroperoxide oxidant.35 Oxygenation reactions catalyzed by
flavinium salt in the presence of sacrificial reducing agents have
been reported to take place via nucleophilic hydroperoxide
species.36−39
Similar to the reactivity observed with organic cofactor-based
nucleophilic hydroperoxide, a metal-based nucleophilic O2-
derived oxidant is expected to oxygenate organoboronic acids.
We have recently reported the two-electron reductive activation
of dioxygen by biomimetic iron(II)-α-hydroxy acid complexes
supported by a facial N3 ligand (TpPh2 = hydrotris(3,5-
diphenyl-pyrazol-1-yl)borate), where the metal-coordinated α-
Figure 1. Optical spectral changes with time during the reaction
between [(TpPh2)FeII(benzilate)] (0.4 mM in benzene) and O2 in the
presence of phenylboronic acid (5 equiv). Inset: X-band EPR
spectrum of the final oxidized solution at 77 K.
hydroxy acid anions acted as the sacrificial reductants.40−42
A
nucleophilic iron−oxygen oxidant thus generated exhibited
oxygen-atom transfer to sulfides and olefin cis-dihydroxylation
reactions. The oxidant was intercepted by external substrates as
probes. On the basis of interception and mechanistic studies,
the oxidant was proposed to be a side-on iron(II)-hydro-
peroxide species (Scheme 2). Since Lewis acidic boronic acids
signal at g = 4.3 in the X-band EPR spectrum typical of a high-
spin iron(III) complex (Figure 1, inset). Thus, the iron(II)-
benzilate complex is converted to an iron(III)-phenolate
complex upon exposure to dioxygen in the presence of
phenylboronic acid.
1
Analysis of the organic product by H NMR spectroscopy
after removal of the metal ion from the oxidized solution
reveals the formation of phenol with 80% yield (Figure S2, SI).
The GC-mass spectrum of phenol displays an ion peak at m/z
= 94 (Figure 2a), which shifts two mass units higher at 96 when
Scheme 2. A Nucleophilic Iron−Oxygen Oxidant
Intercepted in the Reaction between
[(TpPh2)FeII(benzilate)] Complex and Dioxygen
are electrophilic in nature, we have investigated the efficiency of
the O2-derived nucleophilic oxidant from iron(II)-α-hydroxy
acid complexes of the TpPh2 ligand for the oxygenation of aryl/
alkyl boronic acids. In this article, we report the oxygenative
C−B bond cleavage of aryl/alkylboronic acids by the
[(TpPh2)FeII(benzilate)] (1) complex using O2 as the terminal
oxidant. The conversion of different substituted aryl/alkylbor-
onic acids to the corresponding hydroxylated products by the
iron complex and a mechanistic proposal of the oxidative
transformation are reported in this work.
Figure 2. GC-mass spectra of phenol obtained from the reaction of 1
with phenylboronic acid in the presence of (a) 16O2 and (b) 18O2.
the reaction is carried out with 18O2 (Figure 2b). In the reaction
with 1, phenylboronic acid undergoes oxidative C−B bond
dissociation with concomitant formation of a C−O bond to
form phenol as the only product where one oxygen atom from
O2 is incorporated into phenol. In a mixed labeling experiment
with 16O2 and H218O, no incorporation of labeled oxygen is
observed into the product. The oxidized product (phenolate)
remains coordinated to the iron(III) center of the oxidized
complex exhibiting a broad charge-transfer band at 600 nm. As
a result, the extent of intramolecular ligand hydroxylation in the
reaction of 1 with phenylboronic acid could not be quantified
directly by optical spectroscopy. Therefore, with arylboronic
acid substrates, the extent of intraligand hydroxylation was
indirectly calculated by subtracting the percentage of oxy-
genated product from the percentage of intraligand hydrox-
ylation in the absence of any substrate.
RESULTS AND DISCUSSION
■
Complex 1 reacts with dioxygen in benzene to undergo
oxidative decarboxylation of benzilic acid to benzophenone in
quantitative yield over a period of 20 min. In the reaction,
hydroxylation of one of the phenyl rings of TpPh2 occurs to an
extent of 90%.40 The intraligand hydroxylation is inhibited in
the presence of different external substrates, where selective
oxidations of external substrates occur in preference to
intraligand hydroxylation. The reaction of complex 1 with
dioxygen in the presence of phenylboronic acid (5 equiv) in
benzene over a period of 20 min results in the formation of a
deep green solution showing a broad band at around 600 nm
(Figure 1). The ESI-MS of the final oxidized solution exhibits
an ion peak at m/z = 725.2 with the isotope distribution pattern
calculated for [(TpPh2)Fe]+ (Figure S1, Supporting Informa-
tion, SI). The deep green oxidized solution shows a rhombic
Hammett analysis is expected to provide useful information
to understand the nature of the oxidant responsible for the
hydroxylation of boronic acid. However, in the absence of any
observable intermediate during the reaction, no absolute rate of
hydroxylation of boronic acid could be obtained. But the
relative rates can be calculated from product analysis of the
B
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competitive hydroxylation reaction of pairs of arylboronic acids.
Therefore, Hammett analysis was carried out with 1:1 mixtures
of phenylboronic acid and different para-substituted phenyl-
boronic acids (p-X-C6H4B(OH)2, where X = F, Cl, H, tBu,
OMe). A ρ value of +1.001 was obtained from the Hammett
plot of the relative rates (krel) versus σp for p-X-C6H4B(OH)2
(Figure 3). The result strongly indicates that the intermediate
Scheme 3. Oxygenative Conversion of Substituted
Phenylboronic Acids to Phenols by Complex 1 (Bottom:
Percentage Yields of Phenols from Substituted
Phenylboronic Acids)
Table 1. Oxygenation of Organoboronic Acids by Complex
b
1
Figure 3. Hammett plot of log krel vs σρ for p-X-C6H4B(OH)2. The krel
values were calculated by dividing the concentration of product from
para-substitued phenylboronic acids by the concentration of product
from phenylboronic acid.
responsible for hydroxylation of phenylboronic acid has
nucleophilic character. The oxidation of arylboronic acids
using oxone in aqueous acetone buffered with sodium
bicarbonate has been shown to remain unaffected by the
electron density of the aromatic nucleus.13 However, the yields
of phenols from the oxidation of arylboronic acids by
hydroxylamine were low with substrates containing electron-
withdrawing groups on the aromatic ring.22 On the contrary,
oxygenative transformation of phenylboronic acids by complex
1 affords the corresponding phenols in high yields with
electron-withdrawing substituents on the phenyl ring of
boronic acids. Of note, control experiments with iron(II)
perchlorate only or with iron(II)-perchlorate and benzilic acid
do not afford the phenol product from phenylboronic acid. The
results of control experiments with simple iron salts imply the
role of the supporting ligand and of the coligand (benzilate
anion) in affecting biomimetic oxidation reactions by complex
1.
Apart from different substituted phenylboronic acid, the
dioxygen-derived nucleophilic oxidant from complex 1 also
oxygenates other arylboronic acids (Scheme 3, Table 1). 1-
Naphthylboronic acid and 2-naphthylboronic acid selectively
form 1-naphthol (68%) and 2-naphthol (75%), respectively
(Table 1, and Figures S3 and S4, SI). The oxygenation of
aromatic boronic acid with O2 by complex 1 therefore
functionally mimics the phenylboronic acid monooxygenase
activity of Arthrobacter sp. strain PBA (Scheme 3).
Complex 1 is also capable of converting alkylboronic acids to
the corresponding hydroxylated product (Table 1). Cyclo-
hexylboronic acid is selectively converted to cyclohexanol
(Figure S5, SI). In the reaction, 60% cyclohexanol is formed,
and the intramolecular ligand hydroxylation takes place to an
extent of 30% (Figure S6, SI). Cyclopropylboronic acid forms
cyclopropanol (37%) (Figure S7, SI), where 53% ligand
hydroxylation is observed. Isobutylboronic acid affords 40%
isobutanol (Figure S8, SI) along with 48% ring hydroxylated
a
Calculated by subtracting the % of oxygenated product from the % of
b
intraligand hydroxylation in the absence of any substrate. Reaction
conditions: 0.02 mmol of complex 1 in 10 mL O2-saturated benzene
and 0.1 mmol boronic acids; time: 20 min.
product. Therefore, hydroxylation of aryl/alkylboronic with O2
by complex 1 represents an efficient biomimetic approach for
selective oxyfunctionalization of aliphatic/aromatic substrates.
Further, the oxidations of phenylboronic acids containing
reactive functional groups were studied to evaluate the
chemoselectivity of the nucleophilic oxidant from complex 1
(Scheme 4). When 4-formylphenylboronic acid (5 equiv) is
made to react with complex 1 and dioxygen, 4-hydroxy
benzaldehyde is obtained as the major product (72%) along
with a small amount (10%) of 4-carboxyphenylboronic acid
(Figures S9 and S10, SI). It is therefore evident that the
nucleophilic oxidant preferentially oxidizes the more electro-
C
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vinyphenol (78%), 4-(1,2-dihydroxyethyl)phenylboronic acid
(8%), and 4-formyl phenylboronic acid (10%) (Figures S16 and
S17, SI). A labeling experiment with 18O2 confirms the
incorporation of a labeled oxygen atom into the oxidized
products obtained from 4-vinyl phenylboronic acid. The GC-
MS peak at m/z = 120 for 4-vinyphenol is shifted two mass
units to m/z = 122. The mass fragment corresponding to 4-
(1,2-dihydroxyethyl) phenylboronic acid at m/z = 138 is shifted
by four mass units to m/z = 142. Similarly, the mass fragment
for 4-formyl phenylboronic acid at m/z = 105 is shifted two
units to m/z = 107 (Figure S18, SI). The data from labeling
experiments thus unambiguously prove the involvement of a
metal-based oxidant derived from dioxygen. The oxidant has
preferential selectivity toward the nucleophilic hydroxylation of
boronic acids compared to the oxo atom transfer to sulfides,
dihydroxylation reaction of alkenes, and oxidation of aldehyde
group.
Scheme 4. Reaction of 1 with O2 in the Presence of Different
Arylboronic Acids Containing Reactive Functional Groups
The nucleophilic oxidant from 1 has been proposed as a side-
on iron(II)-hydroperoxide species.40,41 However, there is no
direct experimental evidence for the iron(II)-hydroperoxide
species. The putative iron(II)-hydroperoxide species from 1
may transform to the corresponding iron(III) species under
oxidizing conditions. To test if an iron(III)-hydroperoxo
species is involved in the reaction, the reaction of complex 1
with dioxygen was carried out in the presence of 10 equiv of
thioanisole. The 1H NMR spectrum of the final oxidized
solution displayed paramagnetically shifted proton resonances
typical of high-spin iron(II)-TpPh2 complexes (Figure S19, SI).
Moreover the oxidized solution did not show any signal in the
X-band EPR spectrum. If an iron(III)-hydroperoxide species
was involved in the oxidation of substrate, the final solution
after two-electron oxidation of thioanisole would show EPR
signals of iron(III) products. All these results rule out the
possibility of an iron(III)-hydroperoxide species as the active
oxidant for the hydroxylation reaction.
From the results discussed above, a mechanism is proposed
for the oxidative hydroxylation of organoboronic acids. Initially,
an iron(II)-hydroperoxide species (A) is generated upon
reductive activation of dioxygen by the iron benzilate complex
(1). The nucleophilic iron(II)-hydroperoxide species is
proposed to attack the electrophilic boron center of organo-
boronic acid to form an iron(II)-peroxoborate (B) species
(Scheme 5). The peroxo intermediate (B) then undergoes
Baeyer−Villiger type rearrangement where migration of the aryl
(or alkyl) moiety of boronic acid to the oxygen atom bonded to
the iron center occurs to form the corresponding hydroxylated
product (C), concerted with the heterolytic O−O bond
cleavage of the hydroperoxide. Hydrolysis of the phenolatobo-
rate in C results in the formation of phenol and boric acid. The
iron(II) species (D) with coordinated hydroxylated product
(alkoxide/phenolate) reacts with oxygen to get one-electron
oxidized to form the corresponding iron(III) complex (E). For
arylboronic acid, the green color of the final solution arises due
to the phenolate coordinated iron(III) species.
philic boron center compared to the aldehydic center. The
oxidant generated from 1 is known to react with benzaldehyde
to afford benzoic acid and also takes part in Cannizzaro type
reaction to form a mixture of benzyl alcohol and benzoic acid.41
The metal-based oxidant oxidizes benzaldehyde to benzoic acid,
and the resulting iron(II)-hydroxo species participates in a
Cannizzaro reaction to convert benzaldehyde into benzyl
alcohol and benzoic acid.41 Here, the boron center is oxidized
in preference to the aldehydic group, and no Cannizzaro type
mechanism operates. Similarly, reaction of 1 with 1 equiv of 2-
(methylthio)phenylboronic acid affords a mixture of 2-
(methylthio)phenol (30%) and 2-(methylsufinyl)phenol
(5%), indicating that the boron center as well as the sulfide
moiety get oxidized (Figures S11 and S12, SI). With increasing
concentration of the substrate, the oxo transfer reactivity to the
sulfide moiety decreases (Figure S13, SI). In the presence of 10
equiv of substrate, 2-(methylthio)phenol is formed as the major
product (70%) along with a minor product, 2-(methylsufinyl)-
phenylboronic acid (10%) (Scheme 4, and Figures S14 and
S15, SI). With an intermediate concentration (3−5 equiv) of
the substrate, 2-(methylsufinyl)phenol and 2-(methylsufinyl)-
phenylboronic acid are observed as minor products in addition
to the major product, 2-(methylthio)phenol (Figure S13, SI).
The reaction of complex 1 with thioanisole (10 equiv) has been
reported to form 80% thioanisole oxide and 10% methyl phenyl
sulfone. The ratio of sulfone/sulfoxide was found to be
dependent on the amount of added sulfide. With 1 equiv of
thioanisole, 34% sulfone was observed as the only product with
no sulfoxide.41 The results obtained with 2-(methylthio)-
phenylboronic acid mirror the concentration dependent oxo
transfer reactivity to sulfides by complex 1, and the boronic acid
moiety is more prone to oxidation compared to the oxo transfer
reaction to a sulfide group.
CONCLUSIONS
The oxidant from 1 has been reported to carry out the cis-
dihydroxylation reaction of alkenes. Unlike other alkenes,
styrene affords a mixture of 1-phenylethane-1,2-diol (20%),
benzaldehyde (50%), and benzoic acid (30%).41 To compare
the selectivity of the oxidant toward dihydroxylation vs boronic
acid oxygenation, 4-vinyl phenylboronic acid was used as a
substrate (Scheme 4). With 5 equiv of 4-vinyl phenylboronic
acid, complex 1 reacts with oxygen to form a mixture of 4-
■
In conclusion, we have developed an efficient protocol for
selective oxygenative transformation of aryl/alkylboronic acids
to the corresponding hydroxylated products by an iron(II)-
benzilate complex using dioxygen as the oxidant. A dioxygen-
derived nucleophilic iron−oxygen oxidant is implicated to carry
out the chemoselective oxidative C−B bond cleavage of
boronic acids. The reactivity of the nucleophilic iron−oxygen
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solution for 2 min. The reaction solution was allowed to stir at room
temperature for 20 min. After the reaction, the iron complex was
decomposed by the addition of 10 mL of 3 M HCl solution. Then,
organic products were extracted either with diethyl ether or with
chloroform (3 × 15 mL), and the organic layer was dried over
anhydrous sodium sulfate. After removal of the solvent, organic
products were analyzed by GC-MS and 1H NMR spectroscopy.
Quantification of the oxidized organic products were performed by 1H
NMR spectroscopy by comparing the peak area of four aromatic ortho
protons (d, 7.81 ppm) of benzophenone. For GC analyses,
naphthalene was used as an internal standard, and the products
were identified by comparison of their GC retention times and GC-
MS with those of authentic compounds. All the products were
quantified by 1H NMR spectroscopy. Cyclohexanol, isobutanol,
cyclopropanol, 2-(methylthio)phenol, 4-vinylphenol, 1-naphthol, and
2-naphthol were quantified by GC-MS. For Hammett analyses, the
relative yields were quantified by GC-MS.
Scheme 5. Proposed Mechanism for Oxidative
Hydroxylation of Boronic Acids with Dioxygen by
Biomimetic Iron(II)−Benzilate Complex
1H NMR (500 MHz, CDCl3, 295 K) data of the oxidized products
are provided below. The spectra were compared with those of
authentic compounds or with those reported in the literature.
Benzophenone: δ 7.80 (d, 4H), 7.59 (t, 2H), 7.50 (t, 4H). Phenol: δ
7.25 (t, 2H), 6.94 (t, 1H), 6.83 (d, 2H). 4-Fluoro phenol: δ 6.77 (m,
2H), 7.60 (m, 2H). 4-Chloro phenol: δ 6.77 (m, 2H) 7.19 (m, 2H). 4-
tert-Butyl phenol: δ 7.25 (d, 2H), 6.75 (d, 2H), 2.25 (s, 9H). 4-
Methoxy phenol: δ 6.78 (m, 4H), 4.47 (s, 1H), 3.77 (s, 3H). 4-
Hydroxy benzaldehyde: δ 9.88 (s, 1H), 7.81 (d, 2H), 6.95 (d, 2H). 4-
Carboxyphenylboronic acid: δ 8.14 (d, 2H), 7.12 (d, 2H). 2-
(Methylthio)phenol: δ 6.98 (t, 1H), 7.01 (d, 1H), 7.48−7.45 (m,
2H), 2.34 (s, 3H). 2-(Methylsufinyl)phenylboronic acid: δ 7.99 (d,
1H), 7.49 (d, 1H) 7.45 (t, 1H), 7.15 (t, 1H), 2.76 (t, 3H). 2-
(Methylsufinyl)phenol: δ 7.48 (d, 1H), 7.25 (m, 2H), 7.15 (d, 1H),
2.97 (3H, s). 4-Vinyphenol: δ 5.02 (d, 1H), 5.68−5.45 (m, 1H), 6.62
(dd, 1H), 6.73 (d, 2H), 7.28−7.26 (m, 2H). 4-(1,2-Dihydroxyethyl)-
phenylboronic acid: δ 7.57 (d, 2H), 7.35 (d, 2H), 4.85 (m, 1H), 3.83
(m, 1H), 3.74 (m, 1H). 4-Formylphenylboronic acid: δ 7.86 (d, 2H),
8.11 (d, 2H), 10.06 (s, 1H).
oxidant has thus been harnessed in the oxidative hydroxylation
of boronic acids. The reaction functionally mimics boronic acid
monooxygenase activity. The oxygenative C−B bond cleavage
of arylboronic acids by the iron(II) complex presented here
would provide insights into the mechanism of bacterial
phenylboronic acid catabolism. The relevance of this work
with Flavin-dependent monooxygenases and Baeyer−Villiger
monooxygenases highlights the promising aspects of nucleo-
philic nonheme metal-based hydroperoxides in various
oxidative transformation reactions.
Control Experiments. A mixture of Fe(ClO4)2·6H2O (7.3 mg,
0.02 mmol) and phenylboronic acid (12.2 mg, 0.10 mmol) was
dissolved in an oxygen saturated acetonitrile−benzene (1:5) solvent
mixture (12 mL), and the solution was stirred for 20 min. In another
control experiment, Fe(ClO4)2·6H2O (7.3 mg, 0.02 mmol) was mixed
with an equimolar amount of sodium benzilate (4.6 mg, 0.02 mmol) in
an oxygen saturated acetonitrile−benzene (1:5) solvent mixture (12
mL) in the presence of 5 equiv of phenylboronic acid (12.2 mg, 0.10
mmol), and the resulting solution was stirred for 20 min. After the
reaction, 10 mL of 3 M HCl solution was added to decompose the
metal complex. The organic compounds were extracted with diethyl
ether (3 × 15 mL), and the organic layer was dried over anhydrous
sodium sulfate. In both of the experiments, no phenol formation was
EXPERIMENTAL SECTION
■
All chemicals and reagents were obtained from commercial sources
and were used without further purification unless otherwise stated.
Solvents were distilled and dried before use. Preparation and handling
of air-sensitive materials were carried out under an inert atmosphere in
a glovebox. The ligand KTpPh2 was prepared according to a procedure
reported in the literature.43 The iron(II)-benzilate complex [(TpPh2)-
FeII(benzilate)] (1) was synthesized according to the literature
procedure.40
Electro-spray ionization (ESI) mass spectra were recorded with a
Waters QTOF Micro YA263 instrument. Solution electronic spectra
(single and time-dependent) were measured on an Agilent 8453 diode
array spectrophotometer. X-band EPR measurements were performed
on a JEOL JES-FA 200 instrument. All room temperature NMR
spectra were collected on a Bruker Avance 500 MHz spectrometer.
GC-MS measurements were carried out with a PerkinElmer Clarus
600 using Elite 5 MS (30 m × 0.25 mm × 0.25 μm) column with a
maximum temperature 300 °C. Labeling experiments were carried out
with 18O2 gas (99 atom %) or H2O18 (98 atom %) purchased from
Icon Services Inc., USA.
Reaction of Complex 1 with O2. The iron(II) complex (19 mg,
0.02 mmol) was dissolved in dioxygen-saturated benzene (10 mL).
The solution was allowed to stir at room temperature for 20 min. After
the reaction, the oxidized solution was treated with 10 mL of 3 M
hydrochloric acid (HCl) solution. The organic products were then
extracted with diethyl ether (3 × 15 mL), and the organic layer was
dried over anhydrous sodium sulfate. After removal of the solvent, the
colorless residue was analyzed by GC-MS and 1H NMR spectroscopy.
Reactions of Complex 1 with O2 in the Presence of
Organoboronic Acids. The iron(II) benzilate complex (19 mg,
0.02 mmol) was dissolved in benzene (10 mL) under a nitrogen
atmosphere. To the solution was added the required equivalent of
aryl/alkylboronic acid, and pure O2 gas was purged through the
1
detected, as confirmed by GC-MS and H NMR spectroscopy.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01198.
Spectroscopic data of oxygenated compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +91-33-2473-4971. Fax: +91-33-2473-2805. E-mail:
ictkp@iacs.res.in.
Notes
The authors declare no competing financial interest.
E
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ACKNOWLEDGMENTS
■
T.K.P. acknowledges the Council of Scientific and Industrial
Research (CSIR), India for the financial support (Project File
No. 01(2776)/14/EMR-II). S.C. thanks CSIR for a research
fellowship.
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DOI: 10.1021/acs.inorgchem.5b01198
Inorg. Chem. XXXX, XXX, XXX−XXX