Visible-Light-Induced, Catalyst-Free Radical Arylations of Arenes and Heteroarenes with Aryldiazonium Salts

Article abstract of DOI:10.1002/chem.201703954

In the absence of a photocatalyst and other additives, the radical arylation of diverse arenes and heteroarenes has been achieved with aryldiazonium salts under visible-light irradiation from a blue light-emitting diode (LED). Although the course of some reactions can be rationalized by the formation of strongly light-absorbing charge-transfer (CT) complexes between the diazonium ion and the aromatic substrate, several further examples indicated that the simple presence of an aromatic substrate, showing only weak interactions to the diazonium ion, is fully sufficient to enable product formation.

Full text of DOI:10.1002/chem.201703954

A Journal of
Accepted Article
Title: Visible light induced radical arylations of arenes and
heteroarenes with aryldiazonium salts do not require a
photocatalyst
Authors: Michael C. D. Fürst, Eva Gans, Michael J. Böck, and Markus
Rolf Heinrich
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201703954
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Visible light induced radical arylations of arenes and hetero-
arenes with aryldiazonium salts do not require a photocatalyst
Michael C. D. Fürst, Eva Gans, Michael J. Böck and Markus R. Heinrich*
Abstract: In the absence of a photocatalyst and other additives, the
radical arylation of diverse arenes and heteroarenes has been
achieved with aryldiazonium salts under visible light irradiation from a
blue LED. While the course of some reactions can be rationalized by
the formation of strongly light-absorbing CT complexes between the
diazonium ion and the aromatic substrate, several further examples
indicate that the simple presence of an aromatic substrate – showing
only weak interactions to the diazonium ion – is fully sufficient to
enable product formation.
Additional irradiation allows to conduct BHAS reactions under
milder conditions.[18g-18i]
Biaryls represent an ubiquitous structural motif in many fields of
chemistry.[1] Synthetic methods for the construction of aryl-aryl
bonds are mainly based on transition metal catalysis, including
Suzuki,[2,3] Stille[4] and Ullmann couplings,[5] and biaryls may as
well be obtained through C-H activation reactions,[6] oxidative
couplings[7] and reactions via arynes.[8] Radical-based trans-
formations, which are a further alternative for biaryl synthesis,[9,10]
have recently received attention through new developments in the
field of catalysis,[11] particularly photocatalysis.[12] Such photo-
catalyzed reactions can be conducted with a number of aryl
radical precursors including diazonium[13] and iodonium salts,[14]
diazoanhydrides,[15] carboxylic acids,[16] azo sulfones,[17] and iodo-,
bromo- and chloroarenes.[18] Typically employed photocata-
lysts[19] are complexes of ruthenium[13a,13e,14] or iridium,[16,18g] eosin
Y,[13b,18c] titanium dioxide[13c,d] or strong electron-donors,[18f] and
double excitation of perylene bisimide recently allowed the
conversion of aryl chlorides.[18a]
A generalized mechanism for typical photocatalyzed aryl-aryl
coupling reactions is depicted in Scheme 1a.[12] Activation of the
photocatalyst (PC) by irradiation provides the initiator PC* (step 1),
which triggers the reductive formation of the aryl radical from Ar-
X (step 2). After addition to the substrate (step 3), the electron
resulting from rearomatization (step 4) is taken up by the oxidized
photocatalyst PC+. The key role of the photocatalyst thereby is to
close the radical chain, as only few combinations of Ar-X and
substrate are known (e.g. electron-poor diazonium salts with
electron-rich benzenes),[20,21] for which the electron resulting from
step 4 can directly induce the formation of a new aryl radical from
Ar-X.[22] Alternatively, the chain transfer in radical arylation
reactions can be promoted by strong bases, which is known as
base-promoted homolytic aromatic substitution (BHAS).[23-25]
Scheme 1. Radical aryl-aryl coupling in the presence and absence of a
photocatalyst. ET: electron transfer.
In this overall context it was a challenging question whether a
photocatalyst is really required to promote radical aryl-aryl
coupling reactions that are unable to maintain a chain mechanism
by themselves. Aryl radical generation from diazonium ions[26]
under UV irradiation is well known,[27] and particular effects, such
as the formation of charge-transfer (CT) complexes between
diazonium ions and the aromatic substrates, might be exploited
as well.[20,28] Herein, we now report that a broad range of radical
aryl-aryl couplings with diazonium salts can be induced through
visible light irradiation, not requiring a photocatalyst or other
additive (Scheme 1b).
Hydroquinone (1), 1,4-dimethoxybenzene (2), 3-hydroxypyridine
(3) and furfurylamine (4) were chosen as substrates for the initial
photoinduced arylation experiments, whereat acidic solvent
systems were used to protect the diazonium ions from ionic side-
reactions.[26],[29] Analysis of the UV-Vis difference spectra[28]
revealed that CT complexes are formed when the diazonium salt
5a is combined with the electron-rich substrates 1, 2 and 4, but
only a very weak absorption was measured for 5a and the 3-
hydroxypyridinium ion 3 at wavelength over 420 nm (Figure 1).
absorption
scaled by
×10
nm
Dipl. Pharm., Apotheker M. C. D. Fürst, E. Gans, M. J. Böck, Prof.
Dr. Markus R. Heinrich
Department of Chemistry and Pharmacy, Pharmaceutical
Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg
Schuhstraße 19, 91052 Erlangen, Germany,
Email: Markus.Heinrich@fau.de
Figure 1. Difference spectra obtained for substrates 1-4 in combination with 4-
chlorophenyldiazonium chloride (5a).
Additional UV measurements confirmed that under the acidic
conditions, none of the substrates 1-4 is protonated at a hydroxy
Supporting information for this article is given via a link at the end of
the document.
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or methoxy group, or at the furan ring system, which could lead to
a particular type of activation.
In the next step, the mixtures of 4-chlorophenyldiazonium chloride
(5a) with each substrate 1-4 were irradiated with three different
light sources (Scheme 2).
When 5a was irradiated with light from the blue LED (450-475 nm),
only partial decomposition was observed with 78% or 67% of 5a
remaining after 24 hours, respectively. In the presence of each of
the substrates 1-4, in contrast, product formation was largely
completed after 6 hours showing the strongly accelerating effect
of the aromatic compounds on the conversion of 5a. The faster
reactions of hydroquinone (1) and 1,4-dimethoxybenzene (2) with
5a to give 6a and 7a thereby correlate with the lower ionization
potentials of 1 and 2 (7.9 and 7.8 eV), compared to the
heterocyclic substrates 3 and 4 (9.5 and 8.4 eV).[30] Although it is
difficult to determine a reaction order from the curves depicted in
Figure 2, their progression nevertheless indicates that an auto-
catalytic (self-accelerating) effect of the formed biaryl compounds
6a-9a is not of importance. A participation of a chain transfer
mechanism could be ruled out for 5a in combination with
substrates 1, 2 and 4 through control experiments initiated by
catalytic amounts of iodide.[20],[29] For 3-hydroxypyridine (3), this
control revealed a weak chain transfer, that is most likely
supported by the excellent aptitude of 3-hydroxypyridinium as an
aryl radical acceptor.[31]
Scheme 2. Arylation under irradiation with three different light sources: UV, UV-
Vis and visible light.
The photoinduced arylations, which had so far been conducted in
NMR tubes (0.1 mmol), were next transferred to a larger reaction
scale (0.5 to 1.0 mmol). The related optimization showed that only
minor differences in yield are observed when the reactions are
conducted under argon or air atmosphere.[29] As the presence of
dioxygen is known to facilitate rearomatization (Scheme 1b, step
3),[32] the lifetime of the preceding cyclohexadienyl radical cannot
be of decisive importance for the reaction course, which further
supports the assumption of a non-chain mechanism.[20] Partici-
pation of the initally formed substrate radical cation (step 1) in the
rearomatization step (step 3) can currently not be ruled out.
The results obtained under optimized conditions, and under
variation of the substitution pattern on the diazonium salt, are
summarized in Scheme 3.
From these experiments all desired biaryls 6a-9a were obtained
with the highest yields under visible light irradiation from a blue
LED, which provided a total emission range from 425-530 nm,
whereat the core region (> 50% relative spectral intensity)
stretches from 450-475 nm.[29] In contrast to that, the reactions
performed under UV or UV-Vis irradiation led to numerous side-
products in trace amounts.
Firstly, these results demonstrate that CT-based arylations are
feasible for typical diazonium ions, such as the 4-chloro-
phenyldiazonium ion. Particular conditions and highly activating
+
+
+
substitution patterns (F5C6N2 , Cl3F2C6N2 , (O2N)2C6H3N2 ), which
lead to efficient underlying chain reactions, are not required. [20]
Moreover, and even more interesting was the observation made
with 3-hydroxypyridine (3) under visible light, as this experiment
suggests that a strong CT complex is not necessary for a
successful arylation. Due to constant cooling through a stream of
air or a water bath, thermal initiation could be ruled out for all
irradiation experiments.
Further insight was gained by monitoring the reaction course by
1H-NMR spectroscopy in the presence and absence of the radical
acceptor (Figure 2).
%
Scheme 3. Increased reaction scale and variation of the substituent on the
aryldiazonium salt 5. Yields determined after purification by column
chromatography. [a] Yield determined by 1H-NMR spectroscopy.
Among all experiments, limitations were only found for the 4-
methoxyphenyldiazonium ion 5c, which apparently not possess
enough electron affinity to act as oxidant under visible light
irradiation, so that the related biaryls 6c-9c are formed in low
yields. Control reactions with 5c in the absence of an aromatic
substrate confirmed this assumption, as 5c turned out as fully
h
Figure 2. Time course experiments under irradiation with a blue LED (450-475
nm).
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stable under LED irradiation.[29] Acceptor substitution on the
diazonium ion, as exemplified by the cyano, nitro and ester
derivatives 5d-5f did, on the other hand, not generally lead to
increased yields compared to the more or less electron-neutral
diazonium salts 5a and 5b. Only with the electron-rich substrates
hydroquinone (1) and 1,4-dimethoxybenzene (2), above average
yields were obtained for 5d and 5e, which could point to some
underlying radical chain transfer for these particular transform-
ations.[21,22] The photochemical reactivity of the diazonium ions is
in agreement with the half-wave potentials determined by polaro-
graphic reduction, where the 4-methoxy derivative 5c shows a
significantly lower potential (E1/2 = 0.14 V vs. SCE) than all other
diazonium ions incorporated in this study (0.35 V < E1/2 < 0.45 V
vs. SCE).[33] Control reactions under identical conditions, but in
the dark, gave the biaryls 6a-9a only in low yields of 15%, <1%, <
1% and 12%, respectively.[29]
As shown by the biaryls 8a,b,d-f derived from 3-hydroxypyridine
(3), the formation of a strong CT complex between aromatic
substrate and diazonium salt is not required for product formation.
A competition experiment, in which 4-chlorophenyldiazonium
chloride (5a) was irradiated in the presence of equal amounts of
hydroquinone (1) and 3-hydroxypyridine (3), led to yields of 24%
for the hydroquinone-derived biaryl 6a and 42% for the arylated
3-hydroxypyridine 8a.[29] The stronger CT complex between 1 and
5a can thus not direct the reaction towards formation of 6a.
Instead, it appears that a free aryl radical is formed fom 5a, which
chooses among the available substrates according to their
reactivity as radical acceptors.[31] To evaluate expansions of the
scope, further substrates were investigated (Figure 3).
the counterion is likely to occur, so that soluble diazonium
trifluoroacetates are formed in situ. Irradiation of these solutions
with light from the blue LED, albeit at prolonged reaction times of
120 hours, provided the desired products 13a,b,d and 13e in
yields of 47 to 80%. In agreement with earlier observations, the 4-
methoxyphenyldiazonium tetrafluoroborate (5c’) remained largely
stable and gave only a low yield of 13c. A control experiment, in
which diiodomethane was added to the reaction mixture of the
diazonium tetrafluoroborate 5a’, trifluoroacetic acid and benzene,
gave 79% of 4-chloroiodobenzene and no detectable amount of
13a.[29] This result proves the intermediacy of aryl radicals.[26] In
addition, two reactions on a larger 10 mmol scale provided the
biaryls 6e and 8a in yields of 66% and 58%, respectively.
Notably, among the substrates used in Figure 3, only the mixture
of N-Boc pyrrole with diazonium ion 5a shows a strong absorption
at wavelength >420 nm so that the reaction to biaryl 11a can be
explained by excitation of an intermediate CT complex. For
thiophene, 1,4-benzoquinone and benzene, no or at best only
very weak CT absorptions were measured above 420 nm in
combination with 5a/5a’. Along with the previously discussed
observations for 3-hydroxypyridine (3), these results clearly
demonstrate that a strong CT complex is not required for a
successful arylation reaction induced by visible light.
Having found that radical arylations with arenediazonium salts
can be conducted under such simple reaction conditions, we
finally evaluated whether the addition of the typical photocatalyst
31c]
[Ru(bpy)3]Cl2 (2 mol%)[29,
could further improve the reaction
outcome. From eight comparative experiments conducted with 4-
chlorophenyldiazonium chloride (or tetrafluoroborate) 5a,a’ and
the arenes 1-4, thiophene, N-Boc pyrrole, 1,4-benzoquinone and
benzene, only the two reactions to the thiophene- and
benzoquinone-derived biaryls 10a and 12a gave yields increased
by 8% and 11%, respectively, when the photocatalyst was present.
The slow arylation of benzene could be shortened to 24 h through
the addition of [Ru(bpy)3]Cl2, whereat the yield of 13a (79%)
however remained more or less unchanged (c.f. Figure 3).[37]
In summary, these results demonstrate that a broad range of
radical aryl-aryl coupling reactions with arenediazonium salts can
be conducted under simple visible light irradiation, not requiring a
particular catalyst, photocatalyst and other additive such as a
base. A key finding regarding the reaction scope was that photo-
induced transformations are able to proceed independently from
the formation of a strong charge-transfer complex between
diazonium salt and aromatic substrate. The only true limitation
was found to be an insufficient oxidation potential of the
diazonium ion, as it is caused by strong donor substitution.
Further investigations on the photochemical activation and on the
underlying mechanism are currently underway in our laboratory.
Figure 3. Further substrates and large scale reactions. Yields determined after
purification by column chromatography. [a] Yield determined by 1H-NMR
spectroscopy.
While the experiments with thiophene and 1,4-benzoquinone
could be conducted under unchanged conditions (c.f. Scheme 3)
to give 10a and 12a, no trifluoroacetic acid was added in the
reactions converting N-Boc pyrrole to 11a or 11e, thereby
showing that the absence of acid has no deleterious effect. The
arylation of benzene required modified conditions due to the low
solubility of diazonium ions in organic solvents.[34] A practical
solution to this problem could be found in the use of diazonium
tetrafluoroborates in solutions of trifluoroacetic acid and benzene.
Due to the comparable pKA values of trifluoroacetic acid (pKA =
0.23) and tetrafluoroboric acid (pKA = −0.44),[35,36] an exchange of
Acknowledgements
The authors are grateful for the financial support by the
Studienstiftung des Deutschen Volkes (M. C. D. F.). We would
further like to thank Johannes Köckenberger for experimental
assistance and Dominik Sauerbeck (Josef Hofmann Modell- und
Leuchtentechnik) for recording the emission spectra of the LED.
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Keywords: Radical reactions • Arylation • Biaryl •
Photochemistry • Diazonium
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Michael C. D. Fürst, Eva Gans, Michael
J. Böck and Markus R. Heinrich*
Page No. – Page No.
Visible light induced radical
arylations of arenes and heteroarenes
with aryldiazonium salts do not
require a photocatalyst
The radical arylation of diverse arenes and heteroarenes has been achieved with
arenediazonium salts under visible light irradiation, and in the absence of a
photocatalyst or other additives, such as a bases. The simple presence of an
aromatic substrate in the reaction mixture – showing only weak interactions to the
diazonium ion – thus appears to be fully sufficient to enable product formation.
This article is protected by copyright. All rights reserved.