Insight into the chemoselective aromatic: Vs. side-chain hydroxylation of alkylaromatics with H2O2catalyzed by a non-heme imine-based iron complex

Article abstract of DOI:10.1039/d0cy01868f

The oxidation of a series of alkylaromatic compounds with H2O2 catalyzed by an imine-based non-heme iron complex prepared in situ by reaction of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf)2 in a 2?:?2?:?1 ratio leads to a marked chemoselectivity for aromatic ring hydroxylation over side-chain oxidation. This selectivity is herein investigated in detail. Side-chain/ring oxygenated product ratio was found to increase upon decreasing the bond dissociation energy (BDE) of the benzylic C-H bond in line with expectation. Evidence for competitive reactions leading either to aromatic hydroxylation via electrophilic aromatic substitution or side-chain oxidation via benzylic hydrogen atom abstraction, promoted by a metal-based oxidant, has been provided by kinetic isotope effect analysis. This journal is

Full text of DOI:10.1039/d0cy01868f

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Insight into the chemoselective aromatic vs. side-
chain hydroxylation of alkylaromatics with H2O2
catalyzed by a non-heme imine-based iron
complex†
Cite this: DOI: 10.1039/d0cy01868f
a
Barbara Ticconi,a Giorgio Capocasa,a Andrea Cerrato,a Stefano Di Stefano,
Andrea Lapi, a Beatrice Marincioni,a Giorgio Olivo ‡b and Osvaldo Lanzalunga
a
*
The oxidation of a series of alkylaromatic compounds with H2O2 catalyzed by an imine-based non-heme
iron complex prepared in situ by reaction of 2-picolylaldehyde, 2-picolylamine, and Fe(OTf)2 in a 2 : 2 : 1 ra-
tio leads to a marked chemoselectivity for aromatic ring hydroxylation over side-chain oxidation. This se-
lectivity is herein investigated in detail. Side-chain/ring oxygenated product ratio was found to increase
upon decreasing the bond dissociation energy (BDE) of the benzylic C–H bond in line with expectation. Ev-
idence for competitive reactions leading either to aromatic hydroxylation via electrophilic aromatic substi-
tution or side-chain oxidation via benzylic hydrogen atom abstraction, promoted by a metal-based oxidant,
has been provided by kinetic isotope effect analysis.
Received 23rd September 2020,
Accepted 2nd November 2020
DOI: 10.1039/d0cy01868f
rsc.li/catalysis
by heme and non-heme metal complexes which are biomimetic
models of natural oxygenases, although most of them are stoi-
Introduction
The conversion of aromatic compounds to phenolic products
by hydroxylation of aryl rings represents one of the most chal-
lenging transformation in synthetic organic chemistry. The
relevance of phenolic moieties in a variety of natural prod-
ucts, pharmaceutical compounds and fine chemicals obtained
in industrial processes1,2 calls for the development of simple
synthetic protocols for the direct insertion of hydroxyl func-
tions in aromatic rings. Notwithstanding, only a few oxidizing
systems are available for the catalytic conversion of aromatic
substrates to phenols.3–10
Among these catalytic systems, a special attention has been
devoted to those involving the environmentally friendly and sus-
tainable oxidant H2O2 and catalysts based on abundant and low
toxic metals, such as iron and manganese. Along this line, good
results have been obtained in the aromatic oxidation catalyzed
chiometric transformations.11–31
We have recently reported that iminopyridine complex 1,
easily obtained by an in situ self-assembly of 2-picolylamine,
2-picolylaldehyde and iron(II) triflate in a 2 : 2 : 1 ratio in ace-
tonitrile (Scheme 1),32–36 efficiently catalyzes the hydroxyl-
ation of aromatic rings with H2O2 under mild conditions.37
Kinetic isotope effect (KIE) studies, the use of radical scav-
engers, substituent effects on inter- and intramolecular selec-
tivity and rearrangement experiments converged to the opera-
tion of a metal-based electrophilic aromatic substitution
(SEAr) pathway with no involvement of free radical species
(Scheme 2).35,37 Concerning the nature of the active species
(metal-based oxidant), we were not able to isolate the inter-
mediate, formed upon addition of H2O2 to 1, which is compe-
tent for the oxidation.35 Generally, activation of H2O2 by non-
heme iron complexes is due to the formation of FeIII–OOH in-
termediates which undergo either heterolytic O–O bond
a Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza” and,
Istituto CNR per i Sistemi Biologici (ISB-CNR), Sezione Meccanismi di Reazione, c/o
Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”, P.le A.
Moro 5, I-00185 Rome, Italy. E-mail: osvaldo.lanzalunga@uniroma1.it
b Institut de Química Computacional i Catàlisi (IQCC) and Departament de
Química, Universitat de Girona, Campus de Montilivi, C/ Maria Aurèlia Capmany
69, 17003 Girona, Spain
† Electronic supplementary information (ESI) available: Instruments and general
methods, selected GC chromatograms and 1HNMR spectra of some oxidation re-
actions. See DOI: 10.1039/d0cy01868f
‡ Current address: Institut de Química Computacional i Catàlisi (IQCC) and
Departament de Química, Universitat de Girona, Campus de Montilivi, 17071 Gi-
rona, Spain.
Scheme 1 Self-assembled imine-based non-heme iron(II) complex 1.
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In particular, the variation of ring/side-chain oxidation
product ratio has been analyzed in terms of substrate ben-
zylic C–H BDE values. Deuteration of the benzylic hydrogen
atoms provided additional insights into the mechanism of al-
iphatic side-chain oxidation.
Scheme 2 Proposed mechanism for aromatic hydroxylation of C–H
bonds promoted by imine-based non-heme iron(II) complex/H2O2.
Results and discussion
cleavage to produce a highly reactive FeVO complex or
homolytic cleavage of the same bond leading to a FeIVO
complex and HO˙.35,38–45 Both intermediates mediate aro-
matic hydroxylation, albeit with different rates, selectivities
and KIEs.11–24,45,46 With 1 and H2O2, we observed a KIE and
a selectivity consistent with those reported in (stoichiometric)
aromatic hydroxylation mediated by putative FeVO spe-
cies.37 On this basis, we proposed the formation of a similar
high-valent iron–oxo complex after the initial oxidation of
Fe(II) complex 1 to Fe(III) followed by detachment of a pyri-
dine arm, and coordination and activation by H2O2.34,37
One of the major challenges in aromatic hydroxylation of
substrates bearing benzylic C–H bonds is the competition of
side-chain oxidation. Most of the oxidation processes and in
particular those involving free radical species such as HO˙
and HOO˙ are characterized by poor selectivity9,13,14,16,47–51
with a certain preference for the aromatic side-chain in view
of the relatively low bond dissociation energies (BDEs) of the
benzylic C–H bonds especially when secondary or tertiary
benzylic C–H bonds are present.13,14,47–49,52,53
Oxidation reactions were carried out by mixing Fe(OTf)2(CH3-
CN)2 (2.5 μmol, TfO− = CF3SO3−), 2-picolylaldehyde and
2-picolylamine (5.0 μmol each) followed by the substrate (250
μmol) in CH3CN at 25 °C. A solution of H2O2 (50 μmol, di-
luted from a 35% w/w H2O2 solution) was then added using a
syringe pump in 30 minutes and the reaction was left under
vigorous stirring for 90 minutes. After addition of the inter-
nal standard, the reaction mixture was filtered over a short
1
pad of SiO2 eluting with AcOEt, and analyzed by GC and H
NMR (see ESI†). Reactions have been carried out under
oxidant-deficient conditions (0.2 eq. with respect to the sub-
strate) in order to minimize the formation of overoxidation
products which may alter the aromatic vs. side-chain selectiv-
ity values.60 Control experiments were performed in the pres-
ence of H2O2 and Fe(OTf)2(CH3CN)2 only, with no ligand
added, to compare the differences in terms of reactivity and
selectivity between the metal-based mechanism catalyzed by
complex
1 and the Fenton-type reactions promoted by
Fe(OTf)2/H2O2.61,62 Products and yields, referring to the
amount of oxidant, are reported in Table 1. With all the sub-
strates, the mass balance was satisfactory (>90%).
When applied to the oxidation of alkylaromatics (toluene, eth-
ylbenzene, and cumene), the 1/H2O2 system showed a marked
preference (chemoselectivity) for aromatic ring hydroxylation
over aliphatic side-chain oxidation. Phenolic compounds were
obtained in satisfactory yields accompanied by small amounts of
Oxidation of toluene led to the formation of o-cresol (6%),
a mixture of m- and p-cresols (5%), as the main reaction
products. Remarkably, only a small amount (<1%) of benzal-
dehyde (Table 1, entry 1) deriving from benzylic hydrogen
atom abstraction was detected, indicating a high selectivity of
the 1/H2O2 system for the aromatic hydroxylation. Control ex-
periments in the absence of the imine ligand showed that no
phenolic products are formed with the substrate quantita-
tively recovered.
Ethylbenzene was found to be more reactive than toluene
and the oxidation reaction gave a mixture of o-, m- and
p-ethylphenol (37% of ortho and 45% of meta + para), accom-
panied by the overoxidation product 2-ethyl-p-benzoquinone
(4%) (entry 2). Small amounts of the side-chain oxidation
products, 1-phenylethanol (2%) and acetophenone (3%), were
also observed. Thus, even in the oxidation of ethylbenzene,
which bears weaker secondary benzylic C–H bonds, the 1/
H2O2 system is highly selective for aromatic over aliphatic ox-
idation with a 94 : 6 ring/side-chain chemoselectivity (ring-ox-
ygenated products/side-chain-oxygenated products). Control
experiments, carried out in the absence of imine ligand
(Fenton conditions), showed the exclusive formation of side-
chain oxidation products, with 1-phenylethanol and
acetophenone forming in 8% and 4% yields, respectively.
This result supports the operation of a metal-based oxidant
in the reaction of ethylbenzene with the 1/H2O2 system and
excludes any involvement of HO˙ radicals.49,58,62
side-chain
oxygenated
products.37,54
Remarkably,
the
chemoselectivity for aromatic hydroxylation was significantly
higher than that displayed by other iron catalysts,20,47,55–57
allowing the inclusion of 1/H2O2 system in the restricted family
of the most selective catalytic systems.4,8–10,24
In this context, it is notable that the 1/H2O2 system allowed
the direct conversion of phenylalanine to tyrosine isomers with-
out overoxidation or side-chain functionalization.36
In order to gain more insight into this high
chemoselectivity and define its potential, in this work, we fo-
cused our attention on the oxidation chemoselectivity of poly-
cyclic alkylaromatic substrates containing more reactive ben-
zylic C–H bonds characterized by relatively low BDE values
(75–82 kcal mol−1, Chart 1).59
Chart 1 Benzylic C–H BDE values for some alkylaromatic substrates.
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Table 1 Products and yields of the oxidation of a series of alkylaromatics with H2O2 in CH3CN at room temperature catalyzed by complex 1a
Recovered
Entry
1
Substrate
substrateb
95
Productsc
o-6%
TNd
2.6
m-, p-5%
<1%
2
80
20
m-, p-45%
o-37%
2%
3%
4%
3
4
82
89
14
11
m-, p-42%
1%
o-27%
13%
14%
8%
14%
5
90
9
13%
16%
1%
2%
1%
6
85
13
26%
34%
8%
2%
7
8
88
95
13
4%
5%
5%
4%
29%
9%
5.4
a
Reaction conditions: 20 mol% H2O2, 1 mol% catalyst 1, CH3CN at 25 °C, reaction time: 90 minutes, oxidant added using a syringe pump (30
min). The reported results are the average of at least two runs. Recovered substrates and product yields (%), determined by GC and/or 1H NMR
analysis. Error 5%. Refers to the initial amount of substrate. Yields refer to the amount of oxidant. TN defined as the moles of oxidation
products/mole of Fe. Ketones and aldehydes were considered as the double oxidation products and quinones as the triple oxidation products.
b
c
d
In the oxidation of diphenylmethane (entry 3), almost ex-
clusive formation of aromatic oxidation products, o-, m- and
p-benzylphenol (27% of ortho and 42% of meta + para), was
again observed accompanied by only a small amount (1%) of
the benzylic oxidation product benzophenone. Also with this
substrate, a high ring/side-chain chemoselectivity is observed
(>98). The higher chemoselectivity for aromatic hydroxylation
compared to that observed for ethylbenzene is likely a conse-
quence of the higher steric hindrance of the benzylic C–H
bonds of diphenylmethane. As a matter of fact, the oxidizing
species formed upon treatment of 1 with H2O2 has been
found to be rather sensitive to steric effects.33,34 Blank
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experiments with the Fe(OTf)2/H2O2 system showed the exclu-
sive formation of benzophenone (8%).
tion products, 1-hydroxydihydroanthracene (16%) and
2-hydroxydihydroanthracene (8%) (entry 5). As observed with
fluorene, benzylic oxidation competes with the aromatic one
leading to the formation of anthracene (13%), anthrone (1%)
and anthraquinone (2%). Remarkably, the product yields and
selectivity for hydroxylation under argon were almost the
same as those under air, showing that the possibility of par-
ticipation by molecular oxygen in air can be excluded. Again,
aromatic hydroxylation prevails over the benzylic oxidation,
but a lower ring/side-chain chemoselectivity is observed (60 :
40). Control experiments showed that no oxidation of 9,10-di-
hydroanthracene occurs when the reaction is performed ei-
ther in the absence of the oxidant or of the catalyst, while
using Fe(OTf)2 and H2O2 in the absence of the imine ligand,
side-chain oxidation products are exclusively formed in rela-
tively high yields: anthracene (43%), anthrone (30%) and an-
thraquinone (9%). The increase of HAT reactivity of the ben-
zylic positions in the Fenton type reaction can be again
rationalized on the basis of the relatively low benzylic BDEC–H
value of DHA (78 kcal mol−1). Indeed, HAT from 9,10-dihydro-
anthracene to FeIVO complexes is usually facile and fast to
eventually provide anthracene or anthrone.38,39,41,64
In
a
control experiment, the reaction product
p-benzylphenol was used as a substrate in the oxidation with
1/H2O2 system. No products have been detected and the sub-
strate was recovered quantitatively. This result is somewhat
surprising in view of the activation of phenolic compounds
towards electrophilic aromatic substitutions by the electron-
releasing hydroxyl group and may be due to the binding of
the phenolic products on the imine-based iron catalyst which
prevents further oxidation as previously reported for related
systems.11,12,17 The oxidation of fluorene led to the formation
of 2-hydroxyfluorene (13%) and 4-hydroxyfluorene (14%) due
to aromatic oxidation and 9-fluorenone (14%) due to benzylic
oxidation (entry 4). Again, aromatic hydroxylation prevails
over
benzylic
oxidation,
but
the
ring/side-chain
chemoselectivity drops to 66 : 34.
The decrease of selectivity for the aromatic oxidation ob-
served with this substrate can be rationalized on the basis of
a higher HAT reactivity of the benzylic C–H bonds of fluorene
(benzylic BDEC–H = 80 kcal mol−1). As a matter of fact,
9-fluorenone is the exclusive oxidation product observed
when the reaction is carried out in the absence of the imine
ligand in the control experiments as well as in previous re-
ports with iron catalysts14,58,63 or well-defined FeIV–oxo
complexes.38,39,41,64
In the case of 9,10-dihydroanthracene, the competition be-
tween aromatic and side-chain oxidation and the mechanism
of the hydrogen atom transfer from the benzylic C–H bonds
have been investigated in more detail. Formation of the oxi-
dation products of DHA with 1/H2O2 system may be de-
scribed according to the mechanism reported in Scheme 3.
Product analysis of the oxidation of 9,10-dihydro-
anthracene (DHA) showed formation of the aromatic oxida-
Scheme 3 Proposed mechanism for DHA oxidation by the 1/H2O2 system.
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In the metal-based electrophilic aromatic substitution (path
a), the first step involves the electrophilic attack of the puta-
tive iron(V)–oxo active species on the aromatic ring to give the
Wheland complex. In the next step, the aromaticity is re-
stored by deprotonation, leading to the formation of 1- and
2-hydroxydihydroanthracene and the iron(III) catalyst.
Concerning the side-chain oxidation process, in principle,
two possible mechanistic pathways are feasible: a metal-
based HAT process promoted by a high-valent iron–oxo spe-
cies (path b) or a HAT process promoted by oxygen-centered
radicals (HO˙) formed through Fenton-type reactions of the
iron complex with H2O2 (path c).65
DHA and DHA-d4, respectively. Under the reasonable assump-
tion that the kAr values are the same for both DHA and DHA-
d4, from the ratios (%Hbenz)/(%HAr) and (%Dbenz)/(%DAr),
it is possible to determine the kH/kD value for the benzylic
HAT process. From the product yields of the oxidation of
DHA and DHA-d4 reported in Table 1, entries 5 and 6, respec-
tively, a KIE (kH/kD) value of 3.6 0.7 is obtained.
This result rules out a Fenton type HAT process but sup-
ports the involvement of a selective metal-based oxidant that
is able to discriminate between the C−H and C−D bonds.
This KIE value is also in accordance with that previously ob-
served in cyclohexane hydroxylation catalyzed by complex 1
(KIE = 3.3).33 In fact, reactions initiated by highly reactive
oxygen-centered radicals, such as hydroxyl radicals, are gener-
In the HAT metal-based process, initial hydrogen atom ab-
straction from the substrate to the iron(V)–oxo complex leads
to the formation of an Fe(IV)–OH complex and a benzylic radi-
ally characterized by kH/kD values between 1 and 2.35,61,71–73
A
cal which, in turn, can produce i) anthracene by
a
value of 3.6 for kH/kD is instead in line with those obtained
with other non-heme amine-based iron complexes reported
in the literature (KIE values in the range of 3–4) for which
desaturation process or ii) 9-hydroxy-9,10-dihydroanthracene
by reaction with the Fe(IV)–OH complex (oxygen rebound) or
by oxidation and reaction with a water molecule in a non-
rebound process.66,67 9-Hydroxy-9,10-dihydroanthracene can
be further oxidized to anthrone and anthraquinone. Alterna-
tively, a Fenton-type reaction promoted by the highly reactive
and rather unselective HO˙ (or HOO˙) can occur.51,62,68–70 This
species can abstract a hydrogen atom from the benzylic C–H
bond of the substrate leading to the formation of a benzylic
radical, which can be easily oxidized to a cation. The latter
can react with H2O leading to 9-hydroxy-9,10-dihydro-
anthracene which then follows the pathways described above.
In order to gain more insight into this reaction mecha-
nism, a product isotope effect study has been carried out.
Product analysis of the oxidation of DHA and its deuterated
counterpart DHA-d4 by 1/H2O2 system has been compared.
While the aromatic oxidation occurs with the same rate con-
stants kAr, benzylic oxidation is characterized by a rate con-
stant, kHBenz or kDBenz, for DHA or DHA-d4, respectively, as
shown in Scheme 4.
the involvement of
a
FeVO species has been pro-
posed,40,44,45 but lower than that reported for characterized
FeIVO species.38,39,41,64 Furthermore, when the oxidation of
DHA was performed under argon, no significant variation of
the reaction selectivity was observed. The poor sensitivity to
the presence/absence of O2 also does not support the opera-
tion of a free radical-based oxidation since the presence of O2
is known to affect both the efficiency and the selectivity of ox-
idative processes with Fenton-type systems.35,58,72,74 Thus,
the results obtained with the non-heme imine-based iron
complex clearly indicate that both aromatic and aliphatic
side-chain oxidations are performed by the same high valent
iron–oxo species, similar to that observed in non-heme PDP
iron complexes.30,45
Eventually, xanthene oxidation was considered. This sub-
strate has the lowest benzylic BDEC–H value (75 kcal mol−1) in
the series of Table 1. The oxidation of xanthene led to the for-
mation of 2-hydroxyxanthene (4%) as the only phenolic prod-
uct, accompanied by larger amounts of 9-hydroxyxanthene
(5%) and xanthone (29%), which are the products of benzylic
oxidation (entry 7). The ring/side-chain chemoselectivity ob-
served for this substrate (33 : 64) is in accordance with the
most reactive benzylic C–H bond. Blank experiments in the
absence of the imine ligand show the exclusive formation of
benzylic oxidation products, 9-hydroxyxanthene (1%) and
9-xanthone (65%).
According to Scheme 4, the ratio of the overall yields of
benzylic (%Benz) and aromatic (%Ar) oxidation products,
(%Benz)/(%Ar), can be considered equal to the ratio of the
corresponding rate constants, kHBenz/kAr and kDBenz/kAr, for
In order to confirm that the HAT from the benzylic C–H
bonds occurs via the metal-based process, also with this sub-
strate, a KIE study has been carried out (see Scheme 5). Prod-
ucts and yields of the oxidation of xanthene and xanthene-d2
are reported in Table 1, entries 7 and 8, respectively. From
the product yields, a KIE value of 3.3
0.7 is obtained
confirming that the benzylic HAT process from xanthene is
promoted by a metal-based oxidant.
Table
2
summarizes the aromatic vs. side-chain
chemoselectivity together with the benzylic BDEC–H values of
the alkylaromatic substrates. The data reported clearly indi-
cate that a high selectivity for aromatic hydroxylation is
Scheme 4 Aromatic and benzylic product distribution in the oxidation
of DHA and DHA-d4 by the 1/H2O2 system.
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Scheme 5 Aromatic and benzylic product distribution in the oxidation
of xanthene and xanthene-d2 by the 1/H2O2 system.
Table 2 Correlation between benzylic BDEC–H of alkylaromatics and aro-
matic vs. side-chain chemoselectivity in the oxidation promoted by the 1/
H2O2 system
Fig. 1 Percentage of aromatic oxidation as a function of benzylic
BDEC–H of alkylaromatics in the oxidation promoted by the 1/H2O2
system.
BDE
Ring/side-chain
chemoselectivity
Substrate
(kcal mol−1
)
substrates, the side-chain/ring oxygenated product ratio regu-
larly increases on decreasing bond dissociation energy (BDE)
of the benzylic C–H bond in line with expectation. An inver-
sion of chemoselectivity is found with xanthene where ben-
zylic oxidation products are more abundant than phenolic
ones. Clear evidence for metal-based competitive reactions
leading either to aromatic hydroxylation (electrophilic aro-
matic substitution) or side-chain oxidation (benzylic hydro-
gen atom abstraction) has been provided by the analysis of
deuterium kinetic isotope effects and the poor sensitivity of
the product yields to the presence/absence of O2.
Toluene
88
84
82
80
78
75
>98 : 2
94 : 6
98 : 2
66 : 34
60 : 40
33 : 67
Ethylbenzene
Diphenylmethane
Fluorene
9,10-Dihydroanthracene
Xanthene
observed for most alkylaromatic substrates with benzylic
BDEC–H higher than 82 kcal mol−1. This implies that the oxi-
dation will selectively occur on the aromatic ring for most
benzylic substrates. A fair correlation between the selectivity
data and the strength of the benzylic C–H bonds is outlined
by the ring/side-chain chemoselectivity, which regularly de-
creases on decreasing benzylic BDEC–H values (Fig. 1). With Experimental
xanthene, the low benzylic BDEC–H value (75 kcal mol−1) de-
termines an inversion of selectivity with benzylic oxidation
Oxidation procedure
prevailing over the aromatic one.
Oxidations were carried out by mixing Fe(CF3SO3)2(CH3CN)2
This result is in qualitative accordance with the general
correlation between the amount of side-chain oxygenated
products and the strength of aliphatic C–H bonds reported in
several iron-catalyzed alkylbenzene oxidations.11–24,39,64 How-
(1.09 mg, 2.50 μmol), 2-picolylamine (50 μL of a 0.1 M solu-
tion in CH3CN, 5.0 μmol) and 2-picolylaldehyde (50 μL of a
0.1 M solution in CH3CN, 5.0 μmol) in a vial at 25 °C. Sub-
strate (250 μmol) and CH3CN were then added up to a total
volume of 1 mL. A solution of H2O2 in CH3CN (1.74 M di-
luted from a 35% w/w H2O2 solution) was added over 30 mi-
nutes using a syringe pump under vigorous stirring and left
reacting for additional 1 hour. Finally, an internal standard
was added and the reaction mixture was filtered over a short
pad of SiO2 with 10 mL of AcOEt and analyzed by GC, GC–MS
ever, the 1/H2O2 system shows
a
remarkably high
chemoselectivity for aromatic hydroxylation, higher than that
found with most of the catalytic systems reported in related
studies.20,47,55–57
Conclusions
1
and H NMR. Oxidation products were identified by compar-
Imine-based iron complex 1 prepared in situ by reaction of
2-picolylaldehyde, 2-picolylamine, and Fe(OTf)2 efficiently cat-
alyzes the oxidation of alkylaromatic compounds with H2O2
ing with authentic specimens (o-cresol, m-cresol, p-cresol,
benzaldehyde, 2-ethylphenol, 3-ethylphenol, 4-ethylphenol,
1-phenylethanol,
acetophenone,
2-benzylphenol,
as oxidant under mild conditions.
chemoselectivity for the aromatic oxidation is observed with
monocyclic aromatic systems. With more activated polycyclic
A
remarkable
3-benzylphenol, 4-benzylphenol, benzophenone, 2-hydroxy-
fluorene, 9-fluorenone, anthracene, anthrone, anthraquinone,
and 9-xanthone) or by comparing of their spectral data with
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those reported in the literature (2-ethyl-p-benzoquinone,75
17 O. V. Makhlynets and E. V. Rybak-Akimova, Chem. – Eur. J.,
2010, 16, 13995–14006.
4-hydroxyfluorene,76 2-xanthenol,9 9-hydroxyxanthene77).
1-Hydroxydihydroanthracene
dihydroanthracene, were isolated from the crude reaction
mixture of an oxidation reaction carried out on
semipreparative scale by column chromatography (see ESI†).
and
2-hydroxy-
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19 S. P. de Visser, K. Oh, A.-R. Han and W. Nam, Inorg. Chem.,
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a
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Conflicts of interest
There are no conflicts to declare.
22 P. Liu, Y. Liu, E. L.-M. Wong, S. Xiang and C.-M. Che, Chem.
Sci., 2011, 2, 2187–2195.
Acknowledgements
23 N. V. Tkachenko, R. V. Ottenbacher, O. Y. Lyakin, A. M.
Zima, D. G. Samsonenko, E. P. Talsi and K. P. Bryliakov,
ChemCatChem, 2018, 10, 4052–4057.
24 O. Y. Lyakin, A. M. Zima, N. V. Tkachenko, E. P. Talsi and
K. P. Bryliakov, ACS Catal., 2018, 8, 5255–5260.
25 R. V. Ottenbacher, E. P. Talsi and K. P. Bryliakov, Appl.
Organomet. Chem., 2020, 34, e5900.
26 J.-F. Bartoli, V. Mouries-Mansuy, K. Le Barch-Ozette, M.
Palacio, P. Battioni and D. Mansuy, Chem. Commun.,
2000, 827–828.
We thank the Ministero dell'Università e della Ricerca for fi-
nancial support. This work was also partially supported by
the Università di Roma “La Sapienza” (Progetti di Ricerca
2018). We also thank the CIRCC, Interuniversity Consortium
of Chemical Catalysis and Reactivity.
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