Photocatalytic Oxidative Iodination of Electron-Rich Arenes

Article abstract of DOI:10.1002/adsc.201900298

A visible-light-mediated oxidative iodination of electron-rich arenes has been developed. 2.5 mol% of unsubstituted anthraquinone as photocatalyst were used in combination with elementary iodine, trifluoroacetic acid and oxygen as the terminal oxidant. The iodination proceeds upon irradiation in non- or weakly-electron donating solvents (DCM, DCE and benzene) wherein a spectral window in strongly coloured iodine solutions can be observed at around 400 nm. The method provides good to excellent yields (up to 98%) and shows excellent regioselectivity and good functional group tolerance (triple bonds, ketone, ester, amide). Moreover, the photo-iodination was also upscaled to a 5 mmol scale (1.1 g). Mechanistic investigations by intermediate trapping and competition experiments indicate a photocatalytic arene oxidation and the subsequent reaction with iodine as a likely mechanistic pathway. (Figure presented.).

Full text of DOI:10.1002/adsc.201900298

Title: Photocatalytic Oxidative Iodination of Electron-Rich Arenes
Authors: Rok Narobe, Simon Duesel, Jernej Iskra, and Burkhard
Koenig
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900298
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10.1002/adsc.201900298
Advanced Synthesis & Catalysis
Very Important Publication
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Photocatalytic Oxidative Iodination of Electron-Rich Arenes
Rok Narobe,a,b Simon J. S. Düsel,a Jernej Iskra,b,* and Burkhard Königa,*
a
Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, Germany
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, Slovenia
b
*E-mail: Jernej.Iskra@fkkt.uni-lj.si and Burkhard.Koenig@chemie.uni-regensburg.de
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A visible-light-mediated oxidative iodination of functional group tolerance (triple bonds, ketone, ester,
electron-rich arenes has been developed. 2.5 mol% of amide). Moreover, the photo-iodination was also upscaled to
unsubstituted anthraquinone as photocatalyst were used in a 5 mmol scale (1.1 g). Mechanistic investigations by
combination with elementary iodine, trifluoroacetic acid and intermediate-trapping and competition experiments indicate
oxygen as the terminal oxidant. The iodination proceeds upon a photocatalytic arene-oxidation and the subsequent reaction
irradiation in non- or weakly-electron donating solvents with iodine as a likely mechanistic pathway.
(DCM, DCE and benzene) wherein a spectral window in
strongly coloured iodine solutions can be observed at around
400 nm. The method provides good to excellent yields (up to
98 %) and shows excellent regioselectivity and good
Keywords: Anthraquinone, Arenes; Iodination; Oxidative
Iodination; Photocatalysis
been reported so far.[10] Although catalytic use of
molecular iodine[11] in visible-light photoredox
chemistry is lately being exploited,[12] its use in
combination with irradiation of photocatalysts is
challenging and reports in this area are scarce.[13] The
light absorption of iodine and its complexes often
overlaps with the absorption bands of photocatalysts
and photocatalytic reactions can therefore not be
promoted effectively. Consequently, relatively high
photocatalyst to iodine ratios are required and systems
are limited to low concentrations.[13] In this work, we
show how to circumvent iodine coloration issues and
present an efficient photocatalytic system that utilizes
a stoichiometric amount of iodine to iodinate electron-
rich arenes.
Introduction
Aryl iodides are valuable building blocks in organic
chemistry, as they are widely used as synthetic
intermediates in various C–C couplings,[1] and for the
preparation of hypervalent iodine reagents.[2]
Furthermore,
they
are
also
used
as
radiopharmaceuticals.[3] Consequently, the preparation
of iodoarenes attracts attention within the synthetic
community and the development of new methods for
their synthesis is highly desired. Usually, direct C–H
bond iodofunctionalization is achieved by employing
either iodonium “I+” donating reagents (e.g.
Py2IBF4[4]) or “I+” donating systems, wherein the
reactive “I+” species is formed in situ (e.g.
I2/HCl/O2[5]). The most promising systems in terms of
atom and waste economy use elementary iodine or
iodide salts as an iodine atom source in combination
with Lewis acid activation and environmentally
benign oxidants, such as hydrogen peroxide or, even
better, oxygen.[6] Much work has already been done in
this research area,[6-7] but development of new methods,
based on different approaches is an ongoing challenge
as these may offer different selectivity and functional
group tolerance. In contrast to the traditional polar
reactivity manifolds, radical manifolds can be
accessed by the use of visible-light photocatalysis.[8]
Different light-driven halogenation reactions of arenes
were reported before,[9] but to our knowledge no
efficient direct visible-light promoted iodination has
Results and Discussion
Screening and optimization of reaction conditions
An iodination reaction is often accompanied by very
coloured reaction solutions. Iodide salts (e.g. NEt4I), a
colourless iodine atom source, are redox active and get
readily oxidized to iodine (Eox ≈ 0.5 V vs. SHE),[14]
which forms coloured polyiodide ions (I3-) in the
presence of residual iodide ions.[15] The build-up of
polyiodide ions prevents any further photochemically
promoted reactions, as these species strongly absorb
light in an applicable range of the visible spectral
1
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10.1002/adsc.201900298
Advanced Synthesis & Catalysis
by its polarization and thereby trigger electrophilic
iodination.
region (Figure 1a). Another iodine atom source,
molecular iodine (I2), interacts with many commonly
used solvents, causing blueshifting (400-510 nm) of
the molecular iodine absorption band at 520 nm, or
formation of a new charge-transfer (CT) complex band
(240-350 nm).[16] The interaction of iodine with a
solvent molecule reflects the solvent electron donating
ability. In non-interacting solvents (DCM and DCE) or
weakly interacting solvents (benzene), a spectral
window at around 400 nm is observed (Figure 1b).
The spectral window at the edge of the visible-light
region can in principle be used to perform visible-light
promoted reactions.
Table 1. Screening of the reaction conditions.
a.)
0.55 mM I2 in DCE
0.55 mM I3- in DCE
b.)
0.55 mM I2 in MeOH
0.55 mM I2 in MeCN
0.55 mM I2 in DMF
0.55 mM I2 in DCM
0.55 mM I2 in DCE
0.55 mM I2 in benzene
Entry Iodine PC
source
TFA Reaction Yield of
[eq.]
time
2 [%]
1
I2
RFTA
Acr+-Mes
TPP
AQ
-
14 h
1
2
I2
-
14 h
14 h
14 h
0.5 h
0.5 h
2 h
2
SPECTRAL WINDOW
3
I2
-
2
4
I2
-
8
5
I2
AQ
2.0
2.0
2.0
2.0
2.0
2.0
-
64
96
3
Figure 1: a.) UV-VIS spectra of iodine and NEt4I3 in DCE
and b.) UV-VIS absorption of iodine solutions.
6a
7
I2
AQ
Based on these considerations, we attempted to
develop a photoredox-catalyzed iodination system, via
irradiation within the spectral window at around 400
nm (Table 1). We tested different photocatalysts with
relatively high excited state oxidation potentials[17] that
are capable of oxidizing anisole,[18] as well as an iodine
atom source (I-/I3-/I2 to I2/I+).[14] Unsubstituted
anthraquinone as photocatalyst provided the highest
desired product yield among the tested metal-free dyes
(entries 1-4). The oxidation potential of anthraquinone
was further enhanced by the addition of TFA,[19]
resulting in a drastic increase of the reaction rate and
yield of 2 (entry 5). Solvent optimization (Table S1) at
this stage revealed the highest reaction rate in benzene
compared to other tested solvents (vide infra for
mechanistic rationalization). Iodination in benzene
proceeded with 96 % yield of 2 (Entry 6) in only 0.5 h
irradiation time. Addition of TFA, can on the other
hand, also enhance electrophilicity of iodine molecules
I2
TPP
-
8
I2
2 h
2
9b
10b
11
12
I2
-
2 h
0
I2
AQ
2 h
0
NEt4I
NEt4I
AQ
14 h
14 h
0
AQ
2.0
0
General conditions: anisole (0.1 mmol), applied photocatalyst (2.5
mol%), trifluoroacetic acid (2 eq.) or without, indicated amount of
iodine source in 1 mL of DCE. Irradiation with 400 nm LEDs
under oxygen atmosphere. Given yields were determined by GC-
FID analysis, using naphthalene as internal standard. a) Benzene
was used as a solvent. b) No irradiation.
2
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10.1002/adsc.201900298
Advanced Synthesis & Catalysis
However, negative control experiments with another
highly oxidative photocatalyst TPP and without any
General conditions: 0.1 mmol of substrate, 2.5 mol% of AQ, 2 equiv. of TFA and 0.55 equiv. of I2 were dissolved in 1 mL of benzene
No side reactions on the benzylic position were
photocatalyst (entries 7 and 8) hint that this ground
and irradiated at 400 nm for 0.5-4 h. Given yields are isolated. Yields in parenthesis are GC yields determined with naphthalene as
observed with methyl substituted arenes and despite
a)
b)
an internal standard. < 4% of ortho isomer was observed by 1H NMR In parenthesis is NMR yield determined with
state activation of iodine by acid is not an important
the possible steric bulk, the method worked well for
hexamethylbenzene as an internal standard. c) 3 equiv. of TFA were used. d) 4 eq. of TFA were used and the reaction was run in DCE.
contribution to the reactivity of the system. Further
both the trimethylbenzene (12) and the tert-butyl
control experiments (entries 9 and 10) confirmed the
substituted arene 13. Notably, no addition of iodine to
essential role of the presence of light and photocatalyst
triple bonds or α-carbonyl position was observed for
for the reaction’s progress. Reactions wherein iodide
arenes 17 and 18, respectively. Despite the acidic
(in form of a soluble tetraalkyl ammonium salt) was
conditions, the acid sensitive ester functionality in
used as iodine atom source did not yield any iodinated
substrate 19 remained untouched, as there are no
product (entries 11 and 12). Other photocatalysts only
strong nucleophiles present in the system. Iodination
provided trace amounts of product.
of the amide derivative required one additional
equivalent of acid to break complexes between amide
Synthetic scope and upscale of the reaction
and iodine (see Figures S8-9) and only then proceeded
well, affording compound 20 in 98% yield. This
approach was applied further for the iodination of
other nitrogen containing compounds: protected
anilines (21-23), phenyl pyrazole (24) and biologically
active pyrazole derivative phenazone (25). The
method was also applicable to the iodination of
benzothiophene yielding 26.
Next, the synthetic scope of the reaction was explored.
First, simple electron-rich iodoarenes (2-14) were
prepared in good to excellent yields. The method
showed high para-regioselectivity for the iodination of
anisole yielding less than 4% of the ortho isomer of 2,
presumably due to a non-thermal activation step. For
all other compounds a single regioisomer was
observed.
Scheme 1: Synthetic scope of the photocatalytic iodination.
3
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10.1002/adsc.201900298
Advanced Synthesis & Catalysis
Table 2: Comparison of different oxidants.
Late-stage iodination of biologically active
gemfibrozil showed that the sterically hindered
carboxylic group (27) can be tolerated in the reaction
conditions. Notably, we also iodofunctionalized a
bioactive naproxen ester derivative that possesses a
chiral benzylic carbon atom. To our delight, we did not
observe
racemization
and
obtained
this
Entry
1
Oxidizing agent
Yield of 2 [%]
pharmacophore derivative 28 in 92% yield. The
method gave lower yields for halogenated anisole
derivatives 15 and 16, as their oxidation potentials are
probably on the limit of the anthraquinone excited
state potential. Unfortunately, the method is not
suitable for more electron-deficient arenes.
O2, AQ (2.5 mol%), 400 nm LED
96
2
3
4
5
30 % H2O2 (5 eq.)
5
7
0
0
30-40 % AcOOH in AcOH (5 eq.)
CH3NO2 (5 eq.)
O2, thioxanthene-9-one
(2.5 mol %), 400 nm LED
In the next step, we performed the reaction on a larger
scale. DCM was used as solvent avoiding benzene on
a larger scale (Scheme 2).
General conditions: 0.1 mmol anisole, 2 eq. TFA, 0.55 eq. I2 and
2.5 mol% of applied photocatalyst or 5 eq. of applied oxidizing
agent in 1 ml benzene. When photocatalyst was used, the solution
was irradiated at 400 nm under oxygen atmosphere. Otherwise, the
solutions were stirred in darkness for 30 min. Given yields were
determined by GC-FID using naphthalene as an internal standard.
To probe whether the photocatalyst oxidizes the arene
in the system, a literature reported trapping reaction
with pyrazole as nucleophile[23] was performed in the
absence of iodine (Scheme 3).[24] The isolated coupled
product 29 strongly indicates in situ formation of an
electrophilic arene radical-cation.
Scheme 2: Photocatalytic iodination in a gram scale
reaction.
Mechanistic investigation
First, our system was compared with commonly used
oxidants to test, if a similar reactivity can also be
achieved by means of classic chemistry. An oxidative
iodination of anisole 1 with excess of nitromethane,
hydrogen peroxide[20] or peracetic acid[21] instead of
oxygen and photocatalysts was investigated (Table 2 ).
Classic oxidants gave only low yields (entries 2-4)
under the given conditions, while the photocatalytic
conditions (entry 1) enabled complete conversion of
the starting material. The experiments show an
advantage of photocatalytic oxidation over the classic
oxidants and suggest that the light-driven iodination
may operate via a different mechanism. Additionally,
thioxanthene-9-one, a non-redox sensitizer with a
similar triplet energy as anthraquinone (265 kJ/mol
and 261 kJ/mol, respectively)[22] was tested (entry 5).
The latter experiment was performed to disprove
singlet oxygen chemistry or other sensitization-driven
reactions, as an important contribution to the reactivity
of the system.
Scheme 3: Trapping of the arene radical-cation with
pyrazole.
We conducted further mechanistic investigations to
prove that the radical cation is an essential
intermediate, through which the iodinated product is
formed. Galli et al. addressed a similar question
before[25] and developed a mechanistic probe that
differentiates between a polar and a radical iodination
mechanism (Scheme 4), based on intermolecular
competition between 1,3-dimethoxybenzene (more
nucleophilic, forms polar product) and 1,4-
dimethoxybenzene (lower oxidation potential, forms
radical product). The two compounds compete for the
substoichiometric amount of iodinating reagent and
the product distribution depends on the prevailing
mechanism of the iodination. The obtained calculated
ratio of the rate constants km-OMe/kp-OMe in our system
is 65±6 (Table S3), while the literature value for
4
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10.1002/adsc.201900298
Advanced Synthesis & Catalysis
classic “I+” iodination is 1420±150[26] and for the only
reported system wherein radical iodination was
proposed is 210±20.[25c] The result indicates that the
photocatalytic iodination might indeed proceed via
iodination of the arene radical cation.
mechanism (Scheme 6) for our photocatalytic
oxidative iodination system, wherein protonated AQ
acts as a photocatalyst whose excited form oxidizes the
arene. The formed arene radical cation subsequently
abstracts iodine atom from iodine molecule and forms
the iodinated product. The coupled by-product of the
reaction is the iodine radical (I·), which can recombine
with another I· to form I2, leading to 100 % iodine
economy of the reaction. The photocatalytic cycle is
concluded with the regeneration of anthraquinone by
oxygen. It should be noted that a polar mechanism
wherein “I+” species attacks an arene (Scheme 4a i)
cannot be fully excluded especially with very electron-
Scheme 4: a.) Possible polar and radical mechanism
of iodination. b.) Competition experiment to
distinguish between the two possible pathways.
The observation of faster reaction rates in benzene
compared to other tested solvents, is also in line with
the obtained mechanistic evidence, supporting a
radical mechanism. Benzene’s interaction with iodine
is unique in a way, as it forms CT complexes, but does
not cause strong blueshifting of the iodine band at 520
nm. This leaves a spectral window at 400 nm, despite
the interaction with the solvent. In the iodine-benzene
CT, benzene donates π-electrons to the iodine and thus
makes the terminal iodine atom of the CT complex
rich arenes.
Scheme 6: Proposed mechanism of the photocatalytic
oxidative iodination reaction.
Conclusion
In conclusion, we reported an efficient light-driven
method for the iodination of electron-rich aromatic
compounds, using iodine, trifluoroacetic acid,
more
nucleophilic
(Scheme
5). Enhanced
nucleophilicity of iodine is then reflected in a faster
reaction with the electrophilic arene radical cations,
leading to the iodinated product. Increased reactivity
of iodine dissolved in interacting solvents (brown
solutions) is a known phenomenon.[27]
inexpensive,
unsubstituted
anthraquinone
photocatalyst, and oxygen as a terminal oxidant. The
reactions proceed best in benzene, but also in DCM.
The method is fast and shows high regioselectivity and
tolerance towards many functional groups. The
developed procedure was used on a gram scale
reaction, as well as for the late-stage functionalization
of bioactive molecules. Mechanistic investigations
suggest that the system operates via the oxidation of
an arene and subsequent reaction with molecular
iodine.
Scheme 5: Iodine-benzene CT complex.
Evidently, the main role of acid is not polarization of
iodine but rather altering the anthraquinone
photoredox properties. A protonation of anthraquinone
leads to enhanced absorption at 400 nm and facilitates
reduction of anthraquinone (see figures S2-6).
The demonstrated approach of irradiation within the
solvent dependent iodine spectral window might be
important in the development of new light promoted
iodination and iodine catalysed methods.
Based on the previously reported mechanism[19] and
Experimental Section
our mechanistic investigation, we propose
a
5
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10.1002/adsc.201900298
Advanced Synthesis & Catalysis
For full experimental data see Supporting Information.
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7, 1164-1177.
General
procedure
for
photocatalytic
iodination of electron-rich arenes.
[9]
The substrate (0.1 mmol) was weighed into a 5 mL
crimp vial. A stirring bar, 1 ml of stock solution (0.5
mg AQ (0.0025 mmol) and 14.0 mg iodine (0.055
mmol) in 1 ml benzene) and 15.4 μL of TFA (0.2
mmol) were added and the vial was sealed with a crimp
cap with a septum. Then an oxygen balloon was
attached to the vial and the atmosphere inside the vial
was purged three times with a 20 ml syringe. The
reaction mixture was shaken briefly, and the vial was
placed approximately 2 cm above a 400 nm LED and
stirred under irradiation at 25 °C. Reaction progress
was followed with visual inspection (colour change
from dark brown to pink due to iodine consumption),
TLC and GC-FID analysis. After completion of the
reaction, the reaction mixture was poured into a 50 mL
round bottom flask and diluted with DCM. Silica was
added, the solvent was evaporated from the suspension
and the residue was used as dry load for column
chromatography on a Biotage® IsoleraTM Spektra.
Petroleum ether and ethyl acetate were used as a
mobile phase in all cases. A 10 g column was
employed with silica gel of type 60 M (40-63 μm, 230-
440 mesh) by Merck as a stationary phase.
[10]
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Acknowledgements
This work was supported by the German Science Foundation
(DFG) (GRK 1626, Chemical Photocatalysis), Slovenian Research
Agency (P1-0134), bilateral project (BI-DE/18-19-013) and an
Erasmus+ scholarship for RN. We thank Dr. Rudolf Vasold
(University of Regensburg) for his assistance in GC-MS
measurements, Dr. Melita Tramšek (Jožef Stefan Institute) for her
assistance in Raman spectroscopy measurements and Regina
Hoheisel (University of Regensburg) for her assistance in CV
measurements.
[13]
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Advanced Synthesis & Catalysis
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7
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Advanced Synthesis & Catalysis
FULL PAPER
Photocatalytic Oxidative Iodination of Electron-
Rich Arenes
Adv. Synth. Catal. 2019, Volume, Page – Page
Rok Narobe, Simon J. S. Düsel, Jernej Iskra,
Burkhard König
8
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