Eco-Friendly Methodology for the Formation of Aromatic Carbon–Heteroatom Bonds by Using Green Ionic Liquids

Article abstract of DOI:10.1002/chem.202003827

A new sustainable method is reported for the formation of aromatic carbon–heteroatom bonds under solvent-free and mild conditions (no co-oxidant, no strong acid and no toxic reagents) by using a new type of green ionic liquid. The bromination of methoxy arenes was chosen as a model reaction. The reaction methodology is based on only using natural sodium bromine, which is transformed into an electrophilic brominating reagent within an ionic liquid, easily prepared from the melted salt FeCl₃ hexahydrate. Bromination reactions with this in-situ-generated reagent gave good yields and excellent regioselectivity under simple and environmentally friendly conditions. To understand the unusual bromine polarity reversal of sodium bromine without any strong oxidant, the molecular structure of the reaction medium was characterised by Raman and direct infusion electrospray ionisation mass spectroscopy (ESI-MS). An extensive computational investigation using density functional theory methods was performed to describe a mechanism that suggests indirect oxidation of Br^? through new iron adducts. The versatility of the methodology was successively applied to nitration and thiocyanation of methoxy arenes using KNO₃ and KSCN in melted hexahydrated FeCl₃.

Full text of DOI:10.1002/chem.202003827

Accepted Article
Accepted Article
Title: Eco-friendly methodology for the formation of aromatic carbon-
heteroatom bond using green ionic liquids
Authors: Kenza Richards, Eddy Petit, Yves-Marie Legrand, and Claude
Grison
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To be cited as: Chem. Eur. J. 10.1002/chem.202003827
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01/2020

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Eco-friendly methodology for the formation of aromatic carbon-
heteroatom bond using green ionic liquids
Kenza Richards,a Eddy Petit,b Yves-Marie Legranda and Claude Grison*a
[a]
K. Richards, Y.M. Legrand, Dr. C. Grison
Laboratoire de Chimie Bio-Inspirée et d’Innovations Ecologiques
ChimEco UMR 5021 CNRS-Université de Montpellier
Cap Delta, 1682 Rue de la Valsière, 34790 Grabels, France
E-mail: claude.grison@cnrs.fr
[b]
E. Petit
Institut Européen des Membranes
IEM UMR 5635 CNRS-Université de Montpellier
300 Av. Prof. E. Jeanbrau, 34095 Montpellier, France
Supporting information for this article is given via a link at the end of the document.
Abstract: A new sustainable method is reported for the aromatic
carbon-heteroatom bond formation in solvent-free and mild conditions
(no co-oxidant, no strong acid and no toxic reagents) using a new type
of green ionic liquid. Bromination of methoxy arenes was chosen as a
model reaction. The reaction methodology is based on only using
natural sodium bromine, which is transformed into an electrophilic-
brominating reagent within an ionic liquid, easily prepared from the
melted salt FeCl3 hexahydrate. Bromination reactions, using this in
situ generated reagent, gave good yields and excellent
regioselectivity in simple and environmentally friendly conditions. To
understand the unusual bromine polarity reversal of sodium bromine
without any strong oxidant, the molecular structure of the reaction
medium was characterised by Raman and direct infusion electrospray
ionisation mass spectroscopy (ESI-MS). An extensive computational
investigation using density functional theory methods was performed
to describe a mechanism that suggests indirect oxidation of Br-
through new iron adducts. The versatility of the methodology was
successively applied to nitration and thiocyanation of methoxy arenes
using KNO3 and KSCN in melted hexahydrated FeCl3.
delicate work-up,[6–9] bromodimethylsulfonium bromide,[10]
pyridiniumbromide-perbromide,[11]
cyclohexadien-1-one[12,13]
2,4,4,6-tetrabromo-2,5-
1,3-dibromo-5,5-
or
dimethylhydantoin;[14] necessitating a preparation far from eco-
friendly.
The second strategy is based on biomimetic oxidative
halogenation found in nature. This method consists in the in situ
oxidation of bromide salts,[15] requiring strong oxidants such as
oxone®,[16] ter-butyl
hydroperoxide
(TBHP),[17] nitric
acid,[18] Selectfluor®[19] and ceric ammonium nitrate (CAN),[20]
which are problematic due to their toxicity and the quantity of
organic wastes generated during the treatment.
More recently, hydrogen peroxide or oxygen with vanadium and
molybdene catalysts, NH4VO3, V2O5 or H2MoO4•2H2O, were used
for mimicking the vanadium bromoperoxidase to oxidise bromide
salts.[21–28] Even if these oxidants are non-toxic and that water is
the only by-product, the associated catalysts are carcinogenic,
mutagenic, or toxic for reproduction (CMR) compounds. When no
catalyst is used for this reaction, strong acidic conditions, using
HBr for example, and a large excess of H2O2 are necessary.[29–35]
In a sustainable perspective, many efforts are necessary to
develop environmentally friendly reagents and procedures for the
preparation of heteroatom substituted aromatic compounds.
In this work, we propose an unprecedented eco-friendly
bromination methodology using only a green ionic liquid of melted
FeCl3•6H2O, without any strong co-oxidant or toxic reagent. We
describe herein the synthetic potential of the methodology,
characterise the nature of the reactive entities and investigate the
peculiar bromination mechanism by density functional theory
(DFT) calculations. Finally, this new green strategy was extended
to the nitration and thiocyanation of methoxy aromatic substrates.
Introduction
Electrophilic Aromatic Substitutions (SEAr) are very common
synthetic reactions for the introduction of heteroatoms onto
aromatic compounds. These transformations have been widely
investigated for the preparation of aromatic derivatives
functionalized by a halogen-, nitrogen- or sulphur-containing
group. Among well-known examples, bromination of aryl
compounds plays a pivotal role. Aryl bromides are some of the
most common substrates for cross-coupling reactions and for the
preparation of organometallic compounds. They are also key
intermediates in the synthesis of various natural products and for
the industrial production of drugs, pharmaceuticals,
agrochemicals, fire-retardants, dyes and other various
applications.[1,2]
Considering the significant importance of aryl bromides,
numerous synthetic methods have been developed. However,
these methodologies suffer from several drawbacks. They are
based on two main strategies. The first one is the use of
electrophilic-brominating agents such as Br2, a hazardous, toxic,
and corrosive reagent,[3–5] N-bromosuccinimide which requires
Results and Discussion
Bromination of aromatic derivatives using melted FeCl3•6H2O
and NaBr
An alternative and green approach was considered to perform the
bromination of aromatic compounds. Our strategy was based on
the association of non-hazardous and eco-friendly brominating
reagents, in mild conditions, preventing the use of any strong
oxidant. Natural sodium bromine was chosen as the source of
1
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bromine. The challenge remained in oxidising the bromide ion
with a mild oxidant in green conditions.
FeCl3 is solid at 80 °C and in the absence of any support or
solvent the reaction cannot be correctly conducted. Hydrated
FeCl3•6H2O is liquid at 80 °C and seems to behave as the solvent
of the reaction without deactivating solvation effects, as it was
observed when other solvents were added to the reaction (Table
1, entry 4 & 5).
The use of natural sodium bromine to generate an electrophilic-
brominating reagent was investigated using a mild, abundant and
stable Lewis acid, FeCl3, through the bromination of anisole as a
model substrate (Table
1 and electronic supplementary
information - ESI - for details).
Table 2. Melting points of FeCl3 salts.
First, the reaction was conducted solvent-free in supported
conditions using anhydrous FeCl3 (Table 1, entry 1). Silica and
montmorillonite K10 supports were tested but conversions and
yields remained poor. Molecular sieves, ensuring anhydrous
conditions, were added to the reaction but conversion significantly
decreased (Table 1, entry 2). The use of hydrated FeCl3•6H2O
was then tested in supported conditions and gave surprisingly
similar conversion and yield as anhydrous FeCl3 (Table 1, entry
3). However, the reactions using hydrated FeCl3•6H2O in
supported conditions were hardly reproducible because of a poor
homogenization of the reaction medium, despite a strong
mechanical stirring. Therefore, solvents were added or replaced
the supports; but in both cases conversions collapsed (Table 1,
entry 4 & 5). The simplest condition, using hydrated FeCl3•6H2O,
no solvent and no support, was finally tested. After optimisation of
the temperature, duration and NaBr/Fe ratios through a principal
component analysis (PCA) (see ESI), excellent conversion and
yield were obtained (Table 1, entry 6). As a control, anhydrous
FeCl3 in solvent-free and support-free conditions was tested and
no conversion was observed (Table 1, entry 7).
Fe source
FeCl3
Melting Point
306 °C[36]
37 °C
FeCl3•6H2O
Scope of the method
With the optimal reaction conditions in hand, a substrate scope
was explored and the regioselectivity of bromination was
investigated.
Methoxyarenes led to good conversions and yields with an
excellent para regioselectivity (Table 3, entries 1-5). Interestingly
no dibrominated product was observed; and no 4,4’-
dimethoxybiphenyl was detected, indicating that no coupling
occurred and suggesting that the reaction might not occur by a
radical mechanism when using substrates with methoxy groups.
In details, anisole gave the expected p-bromoanisole with a good
yield (Table 3, entry 1). This regioselective monobromination
reaction is of particular relevance since bromoanisole finds
numerous applications as an intermediate in synthetic chemistry,
such as in the preparation of aryl 1,3-diketones and as substrate
in Suzuki and Heck coupling reactions.[37–41] 1,2- and 1,3-
dimethoxybenzene were brominated with satisfactory yields
(Table 3, entries 2 & 3). Despite the proximity of the two methoxy
groups, no demethylation was observed.[42–44] Methylanisole
substrates gave good yields although the regioselectivity of the
reaction is clearly oriented towards para (Table 3, entries 4 & 5).
Interestingly, no bromination in benzylic position was observed,
as long as there was a methoxy group. Indeed, bromination of
toluene gave poor conversion and yield (Table 3, entry 6), and the
absence of methoxy group allowed a partial radical bromination
in benzylic position (yield of 17 %).
Table 1. Bromination of anisole using anhydrous and hydrated FeCl3 and NaBr.
Entry
Fe source
FeCl3
Conditions
supporta
Conv. (%)
Yield (%)
1
2
33 – 31
21 – 5
The presence of a methoxy group seemed to be a prerequisite for
this reaction. As expected, bromination was limited to activated
substrates since mild electrophilic-brominating reagents are used.
FeCl3 +
molecular sieve
9.5
2
3
4
5
6
7
FeCl3•6H2O
FeCl3•6H2O
FeCl3•6H2O
FeCl3•6H2O
FeCl3
32 – 2.5
20 – 2.5
However, the reaction showed
monobromination with an excellent para regioselectivity, while
respecting the principles of green chemistry.
a
high selectivity for
supporta & solventb
solventb
traces
traces
0
93
0
0
85
0
Table 3. Scope of the method. 1 mmol of FeCl3.6H2O.
-
Reaction conditions: FeCl3•6H2O, NaBr and anisole were put in a sealed tube
a
b
under magnetic stirring at 80 °C for 24 h. SiO2 and MK10 were tested;
cyclohexane, methyl-tetrahydrofuran and diethylcarbonate were tested. Yields
and conversions determined by gas chromatography-mass spectroscopy (GC-
MS) using dodecane as internal standard.
Conv.
(%)
Yield (%)
(Regiosel. (%))
Entry
Substrate
Product
The difference of reactivity between anhydrous and hydrated
FeCl3 in solvent-free and support-free conditions can be
explained by their physical state (Table 2). Indeed, anhydrous
2
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m: medium, vw: very weak, vs: very strong, bd: broad.
85
(97)
1a
93
Raman analysis suggested that the FeCl3•6H2O hydrated salt
could form a Fe(III) ion-pair complex (FeCl2(H2O)4 , FeCl4 ), as
described in the literature (Scheme 1).[36]
+
-
The ionic character of the hydrated salt at a temperature above
its melting point was confirmed by measuring its conductivity. At
80 °C, the conductivity reached 15.5 mS.cm-1, exceeding the limit
of 10 mS.cm-1 corresponding to ionic liquids. Interestingly, the
preparation of this type of ionic liquid is rapid and simple
compared to general cases, that require multiple-steps synthesis
and some treatments, especially for the organic cation.[47–50]
The melted FeCl3•6H2O salt was also studied by direct infusion
electrospray ionization in negative mode after heating at 80 °C
(see ESI). The mass spectrum showed the molecular ion peak of
the expected iron(III) chloride anion, FeCl4- (m/z = 197.91), which
is consistent with the Raman analyses (Scheme 1).
51
(100)
2b
3b
4b
100
51
(100)
97
86
51
(100)
75
(100)
Scheme 1. Effect of temperature on FeCl3•6H2O and the formation of iron(III)
chloride species, observed by Raman and mass spectrometry.
5b
100
23
Characterisation of FeCl3•6H2O in the presence of NaBr
The salt mixture FeCl3•6H2O and NaBr was also studied by direct
infusion electrospray ionization in negative mode after heating at
80 °C (see ESI). The mass spectrum showed new molecular ion
peaks corresponding to iron(III) mixed chloro-bromide anions
Fe(III)ClxBry-, as well as the remaining iron(III) chloride anions
6
6b
(100)
Reaction conditions: 0.5 mmol of NaBr and 0.5 mmol of the substrate were put
in a sealed tube under magnetic stirring at 80 °C for 24 h. Yields and
conversions determined by GC-MS using dodecane as internal standard.
Yields and conversions determined by 1H nuclear magnetic resonance (NMR)
spectroscopy and based on starting compound.
a
b
-
FeCl4 (Scheme 2). The four iron(III) mixed chloro-bromide anions
were detected. In each case, a bromine atom successively
substituted a chlorine atom.
These new anionic species, Fe(III)ClxBry-, were still completely
soluble in the ionic liquid.
According to these results, it seems that sodium bromide was
successfully transformed into an electrophilic-brominating
reagent without the use of any oxidant. Characterisation of the
reaction medium, hydrated FeCl3•6H2O with NaBr, was then
investigated to enlighten this transformation.
The redox potential of Br2/Br- and Fe(III)/Fe(II) are not compatible
-
for the formation of Br2, suggesting that these new Fe(III)ClxBry
anions might constitute the active iron species in the reaction and
behave as the brominating reagents. They might replace FeCl4- in
Spectroscopic analyses of FeCl3•6H2O
+
-
the ion-pair complex (FeCl2(H2O)4 , FeCl4 ) to form a new ion-pair
The FeCl3•6H2O hydrated salt was first characterised by Raman
spectroscopy after heating at 80 °C. The Raman spectrum (see
ESI) showed four characteristic bands, that could be identified by
comparison with previous work of Sharma et al. on diversely
hydrated FeCl3 salts (Table 4).[45,46] The three low frequency
+
-
complexes (FeCl2(H2O)4 , FeClxBry ), as it has already been
observed with several anion species found in association with
+
FeCl2(H2O)4 .[51,52]
-
bands could be assigned to the FeCl4 anion, and the high
frequency and broad band could be assigned to the Fe(H2O)4Cl2
+
cation.
Scheme 2. Transformation of FeCl3•6H2O in the presence of NaBr into new
mixed chloro-bromide anionic species, detected by mass spectroscopy.
Table 4. Comparison of Raman frequencies in cm-1 of melted FeCl3•6H2O
between data from literature and the present study.
Raman frequencies
-
+
FeCl4
Fe(H2O)4Cl2
(cm-1)
Computational studies were then performed to investigate the
reactivity of FeCl3Br-, as a model of the FeClxBry- species, within
the bromination reaction of anisole.
Sharma et al.
present work
115 m
121 m
154 vw
163 vw
337 vs
332 vs
425 m bd
409 m bd
3
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conformations of similar level of free energy were obtained, in
which the aromatic ring of anisole is systematically reoriented to
become parallel to the interacting chlorine-bromine axis (see ESI).
While the ion pair complex remained stable (Br-Cl distance at 2.86
Å), multiple interaction points between the bromine atom and
several carbon atoms of the aromatic ring of anisole can be
noticed (
Figure 2 A.). Indeed, Electrostatic Surface Potential of the most
stable conformer shows the bromine-chlorine dipole and the
superposition of the anisole ring with the bromine atom,
generating a π-enyl interaction (
Computational studies
Molecular structure of the (FeCl2(H2O)4 , FeCl3Br-) ion-pair
+
complex. The molecular structure of the (FeCl2(H2O)4 , FeCl3Br-)
+
ion-pair complex was determined by density functional theory –
Hartree-Fock (DFT-HF) calculations using Gaussian 16 package
(See Section 3.1 of ESI). In the solid state, FeCl2(H2O)4+ has been
found to crystallise in two forms, in which the chlorine atoms are
either in trans or in cis position from the central iron atom, leading
to trans-dichloride or cis-dichloride tetrahydrate cations.[52–54] The
+
-
+
-
(trans-FeCl2(H2O)4 , FeCl4 ) and (cis-FeCl2(H2O)4 , FeCl4 )
Figure 2 B.). Additionally, the partial atomic charge of the bromine
atom was calculated and remained positive in every calculation
complexes were constructed from these crystallographic data. A
-
chlorine atom was replaced by a bromine atom in FeCl4
+
generating the starting geometry structures of (trans-FeCl2(H2O)4 ,
+
FeCl3Br-) and (cis-FeCl2(H2O)4 , FeCl3Br-) (see ESI). The
distance between the cation and the anion was increased by 2 Å
allowing the system to orientate freely during the geometry
optimisation. As expected, all optimised conformers showed a
complex between the FeCl2(H2O)4+ cation and the FeCl3Br- anion,
since they carry opposite charges. However, this interaction was
made through the halogen atoms, which normally bear negative
charges. Interestingly, for the most stable conformers of the cis-
and trans-complexes, the halogen atoms concerned are a
chlorine atom of the FeCl2(H2O)4+ cation and the bromine atom of
the FeCl3Br- anion, with a distance of 2.84 - 2.99 Å (Figure 1 A. &
B.). The partial atomic charges of these singular geometries were
calculated using different models (Mulliken, Hirschfield, CM5 and
APT, see ESI for details) and all showed a null to a positive charge,
up to +0.30, of the bromine atom, while the charges of the chlorine
atoms ranged between -0.34 and -0.71.
model; up to +0.31 in APT charge model, which is even higher
than the one without anisole.
Figure 2. A. Optimised geometry and B. Electrostatic Surface Potential of the
most stable conformer of (trans-FeCl2(H2O)4+, FeCl3Br-) in the presence of
anisole.
These computational results clearly suggest a polarity inversion
of the bromine atom, illustrated by the positive partial charge in
the ion pair complexes. The presence of anisole does not alter the
positive partial charge of the bromine atom, and even seems to
increase it.
The Umpolung behaviour of the bromine atom in the reaction
mixture of melted FeCl3•6H2O and anisole, makes it an
electrophile donating bromine reagent and allows the nucleophilic
attack of the anisole (Scheme 3). The nucleophilic attack leads to
the reduction of the two iron atoms within the ion pair complex:
+
-
FeCl2(H2O)4 into FeCl2(H2O)4 and FeCl3Br- into FeCl3 . The re-
aromatisation of the anisole ring occurs classically as in SEAr,
and leads to the formation of HFeCl3.
Figure 1. Optimised geometry of the most stable conformers of A. (cis-
FeCl2(H2O)4
showing the partial charges and interatomic distances of Cl and Br.
+
+
, ,
FeCl3Br-) and B. (trans-FeCl2(H2O)4 FeCl3Br-) complexes
These results highlight the fact that, after the central iron, the
bromine atom is the most electron-deficient site in the ion pair
complexes, making it an electrophilic site. The electrophilic
behaviour of the bromine atom was then investigated in the
reaction mixture, i.e. in the presence of anisole.
Exploration of the reactivity of FeCl3Br- in the bromination
reaction. Among the most stable conformers of (cis- and trans-
+
FeCl2(H2O)4 , FeCl3Br-) complexes presenting similar Gibbs free
+
energy, only the (trans-FeCl2(H2O)4 , FeCl3Br-) complex was
considered for exploring the mechanism of the bromination
reaction. Three constructions with different orientations of anisole
were tested to explore different spatial approaches. The aromatic
ring of anisole was initially presented parallel (0°), oblique (45°)
and perpendicular (90°) to the iron dichloride axis of the trans-
FeCl2(H2O)4+ cation (see ESI). After geometry optimisation, three
Scheme 3. Exploration of the bromination mechanism on anisole with the ion-
pair complex (FeCl2(H2O)4+, FeBr3Cl-).
4
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A new methodology for the bromination of methoxy arenes was
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from melted FeCl3•6H2O and a natural salt, NaBr, without the use
of any additive. The absence of a strong oxidant, toxic solvent and
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10.1002/chem.202003827
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Polarity reversal: new methodology for C-heteroatom bond
formation. The reaction is performed in a green ionic liquid easily
prepared from melted FeCl3•6H2O and natural salts NaBr, KSCN
or KNO3, under unprecedented conditions of sustainability. DFT
calculations revealed a Fe-anionic species formation inducing the
challenging transformation of the anionic substituent into an
electrophilic reagent.
6
This article is protected by copyright. All rights reserved.