An in situ acidic carbon dioxide/glycol system for aerobic oxidative iodination of electron-rich aromatics catalyzed by Fe(NO3)3·9H2O

Article abstract of DOI:10.1039/c4cy00721b

An environmentally benign CO₂/glycol reversible acidic system was developed for the iron(iii)-catalyzed aerobic oxidative iodination of electron-rich aromatics without the need for any conventional acid additive or organic solvent. Notably, moderate to high isolated yields (up to 97%) of the aryl iodides were attained with comparable regioselectivity when ferric nitrate nonahydrate was used as the catalyst with molecular iodine under 1 MPa of CO₂.

Full text of DOI:10.1039/c4cy00721b

Catalysis
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An in situ acidic carbon dioxide/glycol system for
aerobic oxidative iodination of electron-rich
aromatics catalyzed by Fe(NO ) ·9H O†
Cite this: Catal. Sci. Technol., 2014,
4, 4308
3 3
2
Ran Ma, Cheng-Bin Huang, An-Hua Liu, Xue-Dong Li and Liang-Nian He*
Received 3rd June 2014,
Accepted 19th July 2014
An environmentally benign CO /glycol reversible acidic system was developed for the iron(III)-catalyzed
2
aerobic oxidative iodination of electron-rich aromatics without the need for any conventional acid additive
or organic solvent. Notably, moderate to high isolated yields (up to 97%) of the aryl iodides were attained
with comparable regioselectivity when ferric nitrate nonahydrate was used as the catalyst with molecular
DOI: 10.1039/c4cy00721b
www.rsc.org/catalysis
2
iodine under 1 MPa of CO .
promoted by Brønsted acids as solvents or additives, such as
Introduction
1
4b,c,i,j
14f,g
14l
H
2
SO
4
,
AcOH
and TFA, which are generally corro-
Aryl iodides are important synthetic intermediates and
precursors in organic transformation which could undergo
oxidative addition, and thus are subject to a variety of
carbon–carbon and carbon–heteroatom bond formation
sive and require post-reaction neutralization and salt waste
disposal.
The reversible reaction of CO and alcohols could yield
2
alkylcarbonic acids, temporarily generated proton-donating
species which could serve as acid catalysts and be inherently
1
2
3
reactions, such as Negishi, Suzuki, Stille, and Buchwald–
4
Hartwig coupling. Meanwhile, iodoarenes are prevalent
2
neutralized by readily removing CO , thus resulting in simple
building blocks of biologically active molecules, including
neutralization without the need for waste disposal (Scheme 1).
5
drugs, pesticides and fungicides, as well as radiocontrast
Notably, CO is considered as an abundant, non-toxic and
2
6
15
agents for medical X-ray examinations. Due to the significant
easily available C1 resource as well as an ideal reaction
1
6
and versatile properties of iodinated aromatics, great
efforts have been devoted to optimize their preparation
methods across the field of scientific research and industrial
environment for aerobic oxidation which could render such
self-neutralizing in situ acid catalysts sustainable, renewable
and safe. The self-neutralizing acid catalysis derived from
7
manufacture. The well-established Sandmeyer reaction is
2
CO has been applied to many approaches, with profound
among the most efficient methodologies for the construction
of aryl iodides, while electrophilic substitution of aromatics
advantages for both green chemistry and improved
economics.
8
17
with molecular iodine or iodide anions is confronted with the
problem of low reactivity. Although iodination protocols
We have exploited two in situ acidic systems, i.e. scCO
2
/
ethanol and scCO /H O, as reversible acidic media for
2
2
9
using the pre-synthesized iodonium species, such as NIS,
copper-catalyzed aerobic oxybromination of aromatic ethers
1
0
11
13
18
ICl, N-iodosaccharin, IPy
2
BF
4
(ref. 12) and HIO
4
,
could
and sulfides, respectively. Moreover, based on the under-
be possible solutions to this circumstance, its restriction to
highly activated arenes remains a challenge.
standing of reversible acidic catalysis as well as the advances
in catalytic oxidative iodination, we believe that in situ
Oxidative iodination, which requires the combination of
molecular iodine/metal iodide with an appropriate oxidant
and, in many cases, a catalyst, emerges to be a promising
approach that can overcome the limitation caused by
the employment of iodonium reagent. Although it offers
advantages in terms of high efficiency, good selectivity
and wide substrate scope, oxidative iodination is often
2
alkylcarbonic acids derived from CO and ROH could also
promote the iodination reaction as both a proton donor and
a reaction accelerant, thus offering an alternative chemical
methodology for the synthesis of aryl iodides. In this article,
we would like to present ferric nitrate nonahydrate-catalyzed
aerobic oxidative iodination of electron-rich aromatics in a
1
4
2
CO /glycol system with molecular iodine, as shown in
State Key Laboratory and Institute of Elemento-Organic Chemistry,
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Nankai University, Tianjin, 300071, PR China. E-mail: heln@nankai.edu.cn
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00721b
Scheme 1 In situ formation of alkylcarbonic acids.
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(
entries 7–8). The use of the oxidative properties of sodium
tungstate and phosphotungstate failed to deliver comparable
yields of iodoanisole (entries 9–10). As a consequence,
Fe(NO ) ·9H O was selected as the most efficient catalyst
Scheme 2 Aerobic oxidative iodination of electron-rich aromatics.
3
3
2
with the best regioselectivity among the metal salts examined
and was employed for further optimization.
In the second stage, various reaction parameters were sys-
tematically optimized to further enhance the reactivity of
iodination, and the results are depicted in Table 2. Firstly,
Scheme 2. Comparable yields and regioselectivities along
with facilitated post-processing of reaction were achieved by
the introduction of CO and glycol, providing a meaningful
2
example of the beneficial utilization of the greenhouse gas in
the production of fine chemicals. Furthermore, such an
approach becomes especially attractive because iron was
employed as the catalyst with profound environmental and
2
atm of oxygen was found to be adequate to obtain an accept-
able yield of p-iodoanisole (entry 2 vs. 1), and a moderate yield
could be obtained in the case of air as the terminal oxidant
1
9
economic advantages.
(
entry 3). Meanwhile, as the formation of alkylcarbonic acids
had been proven to be vital for the high efficient oxidative
iodination (entry 1 vs. entry 2, Table 1), the influence of
CO2 pressure was then studied. As expected, without the
Results and discussion
As an extension of our interest in the utilization of reversible
alkylcarbonic acids as alternative and benign surrogates for
Brønsted acids, we initially set out to explore the aerobic
oxidative iodination of anisole (1a), which holds two possible
participation of CO , the reaction was restrained (entry 4).
Elevating the pressure of CO from 1 MPa also depressed the
2
2
reactivity of iodination (entries 5–6), in contrast to the
previous reports that the acidity of in situ acids derived from
1
8,20
reaction sites, in the CO
2
/EtOH system. Accordingly,
CO
phenomena might be explained by the dilution effect of
2 atm of O on dense CO
Beyond the influence of CO
2
is enhanced by increasing the pressure of CO
2
.
Such
treatment of anisole with Fe(NO ) ·9H O (10 mol%) gave a
3
3
2
monoiodinated product in 74% yield under the tentative
reaction conditions (1 MPa of O , 0.1 mL of EtOH, 0.55 equiv.
of I ) with the aid of 1 MPa of CO (entry 1, Table 1). Notably,
2
2
.
2
2
, the actual proton donor, i.e.
the alcohols, could also play a crucial role in the formation
of in situ acids. It was found that slightly increasing or
decreasing the amount of ethanol just led to a decrease in
yield (entries 7–8). Consequently, the structure of ROH
was optimized to evaluate its activity toward the generation
of alkylcarbonic acids. While methanol, n-propanol and
even water did not promote the reaction better than ethanol
(entries 9–11), ethylene glycol fulfilled our mission by
improving the total yield of iodoanisole to above 95% (entry
12). The introduction of glycerol as a proton donor failed to
further enhance the reactivity, probably due to its high
2
2
the molar ratio of the p-isomer to the o-isomer reached as
high as 95 : 5. The absence of CO obviously led to a lower
2
oxidative iodination yield (entry 2). Subsequently, further
investigation was carried out to study the catalytic activity for
iodination of anisole. As summarized in Table 1, several iron
catalysts with different anions were screened and none
exhibited a superior reactivity to that of Fe(NO ) ·9H O
3
3
2
(
entries 3–6). Two copper salts that had been demonstrated
to have excellent reactivity toward oxybromination were also
examined, but were less active than their iron counterparts
Table 2 Optimization of various reaction parametersa
2 2
Table 1 Oxidative iodination of anisole in the CO /O
/EtOH systema
b
Yield (%)
CO
(MPa)
2
O
2
(MPa)
c
d
Entry
ROH
p-Isomer
o-Isomer
1
2
3
4
5
6
7
8
9
10
11
12
13
1
1
1
0
2
4
1
1
1
1
1
1
1
1
1
0.2
—
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
70
62
39
45
57
53
61
46
53
39
58
91
49
81
4
3
2
2
3
3
2
2
4
2
3
6
2
5
b
e
Yield (%)
CO
(MPa)
2
c
d
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
p-Isomer
o-Isomer
Entry
Catalyst
1
2
3
4
5
6
7
8
9
Fe(NO
Fe(NO
3
)
)
3
·9H
·9H
2
O
O
1
0
1
1
1
1
1
1
1
1
70
48
59
49
3
18
22
30
13
10
4
3
4
4
f
3
3
2
g
FeCl
FeCl
3
·6H
·4H
2
O
O
2
2
2
H O
Fe(acac)
Fe
CuCl
Cu(NO
3
Trace
Trace
2
MeOH
n-PrOH
Glycol
Glycerol
Glycol
O
3 4
2
·2H
2
O
3
)
2
·3H
2
O
2
h
Na
Na
2
WO
PO
4
·2H
·WO
2
O
Trace
Trace
14
10
2
4
3
a
Reaction conditions: Fe(NO
3
)
3
·9H
2
O (10 mol%), I
2
(0.55 equiv.),
a
b
Reaction conditions: catalyst (10 mol%), I
2
(0.55 equiv.), EtOH
(1 MPa), 100 °C, 10 h. Determined by GC with
biphenyl as the internal standard. Yield of p-iodoanisole. Yield of
o-iodoanisole.
ROH (0.1 mL), 100 °C, 10 h. Determined by GC with biphenyl
as the internal standard. Yield of p-iodoanisole.
b
c
d
(
0.1 mL), O
2
Yield of
g
c
d
e
f
o-iodoanisole. Air as the oxidant. EtOH, 0.075 mL. EtOH,
h
3 3 2
0.125 mL. Fe(NO ) ·9H O (5 mol%).
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Catalysis Science & Technology
Table 3 Substrate scopea
viscosity and low solubility (entry 13). As the attempt to
reduce the catalyst loading turned out to be ineffective (entry
b
Entry
Substrate
Product
Yield (%)
1
4), the reaction conditions were eventually settled as
c
1
97
Fe(NO ) ·9H O 10 mol%, I 0.55 equiv., ethylene glycol
3
3
2
2
0
.1 mL, O 0.2 MPa, CO 1 MPa, 100 °C, 10 h.
2 2
With the optimal reaction conditions in hand, the
2
3
78
generality and feasibility of the present protocol were
demonstrated by performing oxidative iodination with a
broad range of electron-rich aromatics (Table 3). The para
substituent group on anisole caused the reaction to occur at
the ortho position (entries 2–5), and the electron-withdrawing
group suppressed the iodination reactivity (entries 4–5). By
attaching an additional methoxy group to the electron-
deficient aromatic ring, the reaction proceeded a bit more
readily (entry 6 vs. 5). Naphthalene methyl ethers are popular
substrates in the present system as well (entries 7–8). Mono-,
di- and trimethoxy-substituted benzenes underwent oxidative
iodination smoothly with moderate to good yields, but
disubstitution occurred in all cases (entries 9–11). Benzodi-
oxole (12a) was found to decompose under the standard
reaction conditions (entry 12) and, as a result, the yield of its
iodide was far less than that of the analogous benzodioxane
89
72
d
4
e
5
47
40
f
6
7
77
82
(
13a) (entry 13). Other aromatic ethers also proved to be
8
9
competent in the promotion of alkylcarbonic acid formation,
as ethyl-, 2-hydroxyethyl-, phenyl-, benzyl-derived phenyl
ethers reacted smoothly, resulting in good yields of
para-iodinated products (entries 14–17). To our delight,
alkylbenzenes were successfully used as substrates for
iodination, indicating the versatility of the present approach.
Mesitylene (17a) could be transformed into a monoiodinated
product in 84% isolated yield (entry 18), but 1,2,4-trimethyl-
benzene (18a) gave an inseparable mixture of two regio-
isomers (entry 19).
A plausible reaction pathway was proposed for the iron-
catalyzed oxidative iodination of electron-rich aromatics on
the basis of the experimental results of this study and
previous reports. The proposed mechanism is depicted in
Scheme 3. Dissociation of the in situ alkylcarbonic acid
83
81
62
1
1
0
1
1
1
1
2
3
4
17
86
85
produced from CO
2
and ethylene glycol could release
1
1
1
5
6
7
81
80
73
protons, which would be crucial for the reaction [eqn (1)]. I
2
−
+
is oxidized by NO
3
in the presence of H [eqn (2)]. Then, the
f
+
tentative intermediate NO is reoxidized to NO in the
presence of oxygen as presented by eqn (3), which then
oxidizes another I to I [eqn (4)]. However, I , as a kinetically
2
g
+
+
independent species in a strongly acidic media, is difficult to
rationalize in this proposed mechanism. In addition, the
effect of ferric iron needs further exploration and study.
h
i
1
1
a
8
9
84
64
Conclusions
In summary, we have performed efficient iron-catalyzed
oxidative iodination of electron-rich aromatics under aerobic
3 3 2
Reaction conditions: substrate (1 mmol), Fe(NO ) ·9H O (10 mol%),
(0.55 equiv.), ethylene glycol (0.1 mL), CO
00 °C, 10 h. Isolated yield. A mixture of two regioisomers (1b : 1c =
1 : 6). Fe(NO
I
2
2
(1 MPa), O
2
(0.2 MPa),
b
c
1
conditions with the aid of an in situ acidic CO /glycol system.
2
d
e
9
3
)
O
3
·9H
2
O (50 mol%), 15 h. Fe(NO
3
)
3
·9H
2
O (50 mol%).
(30 mol%).
This methodology affords monoiodinated aromatics in up to
f
g
h
Fe(NO
3
)
3
·9H
2
(20 mol%).
Fe(NO
3
)
3
·9H
2
O
I
2
i
9
7% isolated yields without any corrosive acid additive or
3 3 2
(0.75 equiv.). Fe(NO ) ·9H O (30 mol%), a mixture of two regioisomers
(
19b : 19c = 92 : 8).
organic solvent. The present protocol represents an attractive
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residue was purified by column chromatography on silica gel
(
200–300 mesh, eluting with n-hexane/ethyl acetate from pure
n-hexane to 50 : 1) to afford the desired product. The isolated
products were further identified by their NMR and GC-MS
spectra, which are consistent with those reported in the
literature.
Characterization data for the representative products
Scheme 3 The proposed mechanism for oxidative iodination.
2
1
1
1
-Iodo-4-methoxybenzene (1b)
.
H NMR (400 MHz,
CDCl ): δ 3.78 (s, 3 H), 6.61–6.74 (m, 2 H), 7.48–7.65 (m, 2 H).
3
1
3
green approach which offers high iodine atom economy,
simple post-reaction neutralization with no salt waste
disposal and safe operation for aerobic reaction. The reaction
showed a remarkably broad substrate scope including
mesitylene.
3
C NMR (100.6 MHz, CDCl ): δ 55.3, 82.7, 116.3, 138.2, 159.4.
+
GC-MS: m/z (%): 233.95 (100) [M] .
2
2
1
2-Iodo-1-methoxy-4-methylbenzene (2b)
.
H
NMR
(400 MHz, CDCl
3
): δ 2.26 (s, 3 H), 3.85 (s, 3 H), 6.72 (d, J =
8.3 Hz, 1 H), 7.10 (dd, J = 8.3, 1.5 Hz, 1 H), 7.60 (d, J = 1.7 Hz,
1
3
1
1
H). C NMR (100.6 MHz, CDCl ): δ 19.9, 56.4, 85.7, 110.8,
3
+
29.9, 132.0, 139.8, 156.1. GC-MS: m/z (%): 248.00 (100) [M] .
Experimental
Materials and methods
Acknowledgements
The aromatic ethers were purchased from Alfa Aesar China
Co., Ltd and Aladdin Reagent Inc. The other reagents were
obtained commercially from the Tianjin Guangfu Fine
Chemical Research Institute and used without further
purification except for the alcohols, which were distilled
We are grateful to the National Natural Sciences Foundation
of China (no. 21172125, 21121002), the Specialized Research
Fund for the Doctoral Program of Higher Education
(
20130031110013), and the MOE Innovation Team (IRT 13022)
1
according to a documented method prior to use. H NMR
spectra were recorded using a Bruker 400 spectrometer in
of China for financial support.
CDCl
3
; CDCl
3
(7.26 ppm) was used as the internal reference.
1
3
C NMR spectra were recorded at 100.6 MHz in CDCl
3
;
Notes and references
CDCl (77.0 ppm) was used as the internal reference. GC-MS
3
data were collected on a Finnigan HP G1800 A. GC analyses
were performed with a Shimadzu GC-2014 equipped with a
capillary column (RTX-17, 30 m × 0.25 μm) using a flame
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3
3
2
(
40.4 mg, 10 mol%), I
2
(139.6 mg, 0.55 equiv.), and 0.1 mL of
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4312 | Catal. Sci. Technol., 2014, 4, 4308–4312
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