Late-Stage Heteroarylation of Hetero(aryl)sulfonium Salts Activated by α-Amino Alkyl Radicals

Article abstract of DOI:10.1002/anie.202103085

We report a late-stage heteroarylation of aryl sulfonium salts through activation with α-amino alkyl radicals in a mechanistically distinct approach from previously reported halogen-atom transfer (XAT). The new mode of activation of aryl sulfonium salts p

Full text of DOI:10.1002/anie.202103085

Angewandte
Communications
Chemie
How to cite:
International Edition: doi.org/10.1002/anie.202103085
German Edition: doi.org/10.1002/ange.202103085
Heteroarylation Hot Paper
Late-Stage Heteroarylation of Hetero(aryl)sulfonium Salts Activated
by a-Amino Alkyl Radicals
Eva Maria Alvarez, Teresa Karl, Florian Berger, Luca Torkowski, and Tobias Ritter*
Abstract: We report a late-stage heteroarylation of aryl
sulfonium salts through activation with a-amino alkyl radicals
in a mechanistically distinct approach from previously
reported halogen-atom transfer (XAT). The new mode of
activation of aryl sulfonium salts proceeds in the absence of
light and photoredox catalysts, engaging a wide range of
hetarenes. Furthermore, we demonstrate the applicability of
this methodology in synthetically useful cross-coupling trans-
formations.
hetarenes. We anticipate that our method will find application
in medicinal chemistry and material science to prepare
heterobiaryls, which are hard to access otherwise.
CÀH arylation of arenes was first reported by Gomberg
[
9]
and Bachmann in 1924, when diazonium salts were treated
with benzene. Although aryl radicals can easily be generated
[
10]
+
from aryl diazonium salts (E(PhN /PhN C) = À0.16 V vs.
2
2
[
11]
SCE), the preparation of the often unstable diazonium salts
requires several steps when starting from an unfunctionalized
arene. More readily accessible aryl halides can be used to
generate aryl radicals in presence of KOtBu at elevated
temperature, however such harsh conditions prevent the use
of functionally more complex small molecules, and only a few
A
ryl sulfonium salts are reactive reagents, which participate
in synthetically useful reactions such as palladium-catalyzed
[
1]
[2]
cross-coupling reactions, photoredox-catalyzed reactions,
sulfur(IV) reductive elimination, and cine-substitution.
[3]
[4]
[12]
examples were reported so far. A more functional-group-
[
13]
The accessibility, high stability, and high redox potential
make aryl sulfonium salts useful precursors for aryl radicals.
Known methods for generating aryl radicals from sulfonium
salts require a photocatalyst, in most cases a transition metal
tolerant method was developed by Kçnig et al. by devel-
oping a photoredox catalyst that becomes sufficiently reduc-
ing through double excitation to reduce aryl halides
À
(E(MeOC H I/ MeOC H I C) = À2.17 V vs. SCE). Alterna-
6
4
6
4
[
5]
complex, and irradiation. Recently, progress was made in
the use of a-amino alkyl radicals as initiators in photoredox
catalysis to generate aryl radicals from aryl halides via
tively, the aryl halide can be activated by halogen atom
[
14]
[15]
abstraction using silyl-,
stannyl-,
or a-amino alkyl
[
6]
radicals, however none of those atom-abstraction processes
are amenable to late-stage functionalization nor known to
enable CÀH arylation of electron-poor hetarenes. Conven-
[6]
halogen-atom transfer. However, little is known about the
role of a-amino alkyl radicals as strong reductants to transfer
electrons to organic molecules and ensuing bond-forming
tional cross-coupling reactions can be a more direct approach
to form the biaryl structures but require the prefunctionali-
zation of both reaction partners, and hetarene halides can be
[
7]
reactions. Herein, we report a mechanistically different
approach for activating sulfonium salts with a-amino alkyl
radicals, which allows for an operationally simple protocol
that proceeds without the need for a catalyst, irradiation, or
inert atmosphere. The methodology reported here generates
a strong reductant (a-amino alkyl radicals) from a weak
reductant (amine) through oxidation and deprotonation,
[16]
difficult to access. Aryl sulfonium salts can be more reactive
than aryl halides, and can be activated in multiple types of
reactions (Scheme 1A), yet they are more thermally stable
and hence safer than diazonium salts, as well as more
accessible for complex small molecules. In addition, they
can be accessed directly in a single step by CÀH functional-
[
8]
a process known as reductant upconversion. We have
applied this new mode of CÀS bond activation for the
ization. The generation of aryl radicals from thianthrenium
+
development of a late-stage heteroarylation of aryl sulfonium
salts. The synergistic cooperation of sodium persulfate and
tributylamine allows for the generation of in situ formed a-
(TT) salts (E(PhTT /PhTTC) = À1.5 V vs. SCE) has been
accomplished by using photoredox catalysts, enabling a vari-
[5b,2a–e]
ety of cross coupling reactions.
Notably, the use of a-
2
2
amino alkyl radicals and enables access to C(sp )–C(sp )
amino alkyl radicals for the activation of sulfonium salts has
not yet been reported before into the multifaceted reactivity
of the sulfonium salts. In combination with the exceptional
site-selectivity of thianthrenium salts, the new activation
mode of sulfonium salts enables the installation of a greater
diversity of hetarenes in a late-stage process, otherwise not
readily accessible. This in turn allows to assemble challenging
linked heteroaryl–(hetero)aryl products. As part of exploiting
the versatility of this new mode of activation of aryl sulfonium
salts, we also expanded the reactivity to other synthetic
cross-coupling of aryl and hetaryl sulfonium salts with
[*] E. M. Alvarez, T. Karl, F. Berger, L. Torkowski, Prof. Dr. T. Ritter
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr (Germany)
E-mail: ritter@mpi-muelheim.mpg.de
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202103085.
ꢁ
2021 The Authors. Angewandte Chemie International Edition
[17b–d]
[17c]
transformations such as borylation,
allylation
iodination
beyond their addition to heterocycles.
In the process of developing a general cine-subtitution
by using the unique reactivity of sulfonium salts as pseudo-
and
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
[
17a]
[4]
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1
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the reaction in the simultaneous presence of both oxidant and
[
18]
reductant. The new mode of activation of sulfonium salts is
operationally simple, and does not require inert conditions.
The reaction works on a broad scope of different coupling
partners (Table 1). The sulfonium salts can be derived from
electron-neutral (e.g. 3, 13, 15, 17), electron-rich (e.g. 4, 5, 12,
1
4) and electron-poor arene derivatives (e.g. 7, 11, 16),
polycyclic hydrocarbons (e.g. 2, 6), or heterocycles (e.g. 20–
6). The sulfonium salt can be a thianthrenium (TT) as well as
2
a dibenzothiophenium (DBT) salt, and may carry substituents
in the para (e.g. 3), meta (e.g. 5), and ortho (e.g. 11, 16)
positions. The hetarene coupling partners can be 5- (e.g. 12,
1
3, 14, 17) or 6-membered heterocycles (e.g. 2, 4, 6). The
reaction can be used to cross-couple hetarene sulfonium salts
to hetarenes to afford compounds such as 20–26, which are
[5c]
challenging to access otherwise. Although the reaction uses
persulfate as a stoichiometric reagent, the functional group
tolerance even towards oxidation-sensitive substrates is high,
presumably due to the presence of excess amine, so that
oxidation-sensitive N-methyl pyrrole derivatives could be
prepared. To highlight the applicability of our method to
complex small molecules, we performed late-stage heteroar-
ylation of biologically relevant molecules such as boscalid (5,
9
), pyriproxyfen (8), bifonazole (10), indomethacin (14), and
Scheme 1. Methods available for the activation of sulfonium salts.
famoxadone (18).
A proposed mechanism of the arylation reaction is shown
in Scheme 2. Single-electron oxidation of an amine produces
an amine radical cation (step I), deprotonation of which yields
an a-aminoalkyl radical (step II), which is a strong reductant
Michael acceptors, we questioned whether nucleophilic
radicals could attack on the electron-poor sulfonium salt. In
line with this rational design, we explored alkyl halides as
alkyl radical precursors through activation with a-amino alkyl
radicals as described recently by Leonori et al. However,
instead, we discovered a homolytic CÀS bond activation of
[
19]
(E = À1.12 V vs. SCE).
from weak reductants is known as reductant upconversion,
which has been used for example for the one-electron
Generation of strong reductants
[8]
[
6a]
[20]
reduction of enones.
Remarkably, the strongly reducing
the thianthrenium salt resulting in thianthrene, aryl halide
and hydrodefunctionalized product. Based on these observa-
tions, we envisioned the use of a-amino alkyl radicals to
afford selective aryl radical generation and enable hetero-
arylation of various (hetero)aryl sulfonium salts that has been
challenging to access with conventional modes of reactivity of
a-amino alkyl radical reacts with the thianthrenium salt
(E(ArTT+ / ArTTC) = À1.5 V vs. SCE), rather than the
2
À
2À
À
persulfate (E(S O /SO4
+ SO C) =+ 1.4 V vs. SCE)
2
8
4
despite the much higher reduction potential of the persulfate.
The difference in reaction rate might be a result of the large p-
system of the thianthrenium salt accelerating the rate of its
reduction, as well as the structural reorganization required for
the reduction of persulfate. In addition, the biphasic reaction
mixture as well as the fact that the persulfate oxidant is not
fully dissolved may also rationalize the chemoselective
reduction of the thianthrenium salt in preference to the
persulfate oxidant. Although the reduction of arylthianthre-
nium salts with a-aminoalkyl radicals would be slightly uphill
(+ 0.4 eV), the resulting arylthianthrenyl radical is unstable
toward fast dissociation into thianthrene and aryl radicals
[
5c]
sulfonium salts. The a-amino alkyl radicals are generated in
2
À
/
8
situ by oxidation of an amine with persulfate (E(S O
2
2
À
SO4 ) =+ 2.01 V vs. SCE), making the reaction independent
of light and photoredox catalyst. Despite the success of aryl
[
5b,c]
sulfonium salts as aryl radical precursors,
the previous
scope with respect to hetarenes has been small. Now we show
how, for example, N-methyl imidazole can be installed at
a late-stage in salicin pentaacetate (1) via the selectively
accessible TT salt (Scheme 1B).
À10
[2d]
We optimized thermal conditions for generation of a-
(t1/2 < 10 s). The subsequent addition of aryl radicals to
amino alkyl radicals and found that n-Bu N in combination
(het)arenes (step IV) and the generation of (hetero)biaryls
3
[
21]
with Na S O in DMSO and water are well suited. Control
(step V) have been described in the literature. The CÀS
bond cleavage of thianthrenium salts with a-aminoalkyl
radicals could also proceed through group abstraction,
analogously to the activation of arylhalides with a-aminoalkyl
2
2
8
experiments show that no desired product is observed in the
absence of Na S O or n-Bu N (see the Supporting Informa-
2
2
8
3
tion, Table S1). Although aryl radicals are good H-abstractors
and amines are good H-atom donors, excess amounts of n-
Bu N are needed for the reaction. The n-Bu N is not miscible
[
6]
radicals reported by Leonori et al. However, no reaction of
silyl radicals with thianthrenium salts was observed (see the
supporting information, Table S3), although silyl radicals are
3
3
with the DMSO/ water mixture explaining why it resulted in
smaller amounts of hydrodefunctionalized products com-
pared to other amines. The biphasic reaction mixture enables
[
22]
well known to abstract halogen atoms as well as sulfur-
containing functional groups (e.g. Barton–McCombie deox-
2
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Table 1: Substrate scope of (hetero)arylation of sulfonium salts.
General conditions except where otherwise noted: aryl sulfonium salt (0.2–0.3 mmol), sodium persulfate (3.0 equiv), n-Bu N (10 equiv), hetarene
3
(
50.0 equiv), DMSO:H O (0.1 M), 808C. [a] Sodium persulfate (5.0 equiv) and DIPEA (10.0 equiv) were used. [b] 20 mol% AgNO was added.
2
3
[c] 0.12 mmol scale. [d] The reaction was run with 3.0 equiv of the hetarene. DMSO=dimethyl sulfoxide.
[
23]
ygenation). The observation that the here reported aryla-
single electron reduction (see the Supporting Information,
Table S2). Light is not required for the transformation as it
also proceeds analogously in the dark.
To demonstrate the synthetic potential of this method-
ology, we performed a scale-up synthesis of compound 27
[
5b]
tion reaction is, like the photoredox arylation, halted by the
presence of a weak oxidant like nitrobenzene (E = À1.1 V vs.
[
24]
SCE),
which is only slightly more oxidizing than the
thianthrenium salt, is in line with a mechanism proceeding via
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Scheme 4. Functional group diversification with a-amino alkyl radicals.
For detailed experimental procedures, see the Supporting Information.
[a] Starting from the O-acetyl protected phenylethanol derived TT salt.
Scheme 2. Mechanism proposal.
(
Scheme 3), resulting in 5.41 g (55%) of two constitutional
isomers that were separated chromatographically. The trans-
formation affords the highest yields when the heteroarene is
used as a co-solvent (20 to 50 equivalents with respect to
substrate), which is appropriate for late-stage functionaliza-
tion reactions with simple yet useful heterocycles such as
those shown in Table 1. The reactions typically proceed best
with 50 equivalents of heterocycle but we have observed that
in some cases, 20 equiv of the heteroarene can be used. It is
noteworthy to point out the high chemoselectivity in the
activation of sulfonium salts given that generation of a-amino
alkyl radicals can activate aryl bromides in a halogen-atom-
In conclusion, we have demonstrated the coupling of aryl
sulfonium salts with electron-poor and electron-rich hetar-
enes through a new mode of activation of sulfonium salts
which relies on a challenging coupling in presence of a strong
reductant, a-amino alkyl radical, which is known to be not
compatible with oxidative conditions. The hetarene coupling
partners are broad in scope, allowing the synthesis of
compounds that are difficult to prepare otherwise, as for
example compound 26. The execution of the reaction is
simple as it does not need any additives, light, catalysts, or
transition metals, and is tolerant of air and moisture. We
believe that this new mode of activation of sulfonium salts will
expand the range of transformations for the late-stage
functionalization of structurally complex compounds under
milder conditions.
[
6]
transfer mechanism.
Acknowledgements
We thank the MPI fꢀr Kohlenforschung for funding. We
thank S. Marcus and D. Kampen (MPI KOFO) for mass
spectrometry analysis and M. Kochius for NMR spectroscopy.
We thank Dr. Fabio Juliꢁ Hernandez for helpful discussion.
Open access funding enabled and organized by Projekt
DEAL.
Scheme 3. Gram-scale synthesis. Reaction conditions: aryl sulfonium
salt (30 mmol), sodium persulfate (3.0 equiv), n-Bu N (10 equiv),
3
hetarene (36.0 equiv), DMSO:H O (0.1 M), 808C, 16 h. DMSO=di-
2
methyl sulfoxide.
Besides arylation of hetarenes, the a-amino alkyl radical
activation of sulfonium salts can also be applied to several Conflict of interest
other transformations, such as borylation, iodination, and
allylation. The addition of aryl radicals to allylchloride can be
used to prepare allylarenes from arylsulfonium salts
A patent application (number EP18204755.5, Germany),
dealing with the use of thianthrene and its derivatives for
(
Scheme 4). The allylation of aryl sulfonium salts further
CÀH functionalization has been filed and F.B. and T.R. may
explores the reactivity of the sulfonium salts in a synthetic
transformation not reported before.
benefit from royalty payments.
[
25]
Furthermore, this
process does not require prior functionalization of the allylic
coupling partner into organometallic reagents to engage in
the conventional transition-metal-catalyzed cross-coupling
Keywords: heteroarylation · late-stage functionalization ·
radical reaction · sulfonium salts · a-amino alkyl radical
[
26]
reactions.
4
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[
2
[
Manuscript received: March 2, 2021
Accepted manuscript online: April 9, 2021
Version of record online: && &&, &&&&
2
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Communications
Heteroarylation
A new mode of activation of sulfonium
salts by a-amino alkyl radicals is reported.
The CÀS bond of the aryl sulfonium salt is
E. M. Alvarez, T. Karl, F. Berger,
L. Torkowski, T. Ritter*
&&&& — &&&&
cleaved by the action of a-amino alkyl
radicals in the absence of light and pho-
toredox catalysis. The methodology ena-
bles a late-stage heteroarylation of
hetero(aryl) sulfonium salts.
Late-Stage Heteroarylation of
Hetero(aryl)sulfonium Salts Activated by
a-Amino Alkyl Radicals
6
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