Visible-Light-Accelerated C?H Sulfinylation of Heteroarenes

Article abstract of DOI:10.1002/anie.201610210

Heteroaromatic sulfoxides are a frequent structural motif in natural products, drugs, catalysts, and materials. We report a metal-free visible-light-accelerated synthesis of heteroaromatic sulfoxides from sulfinamides and peroxodisulfate. The reaction proceeds at room temperature with blue-light irradiation and allows the C?H sulfinylation of electron-rich heteroarenes, such as pyrroles and indoles. An electrophilic aromatic substitution mechanism is proposed based on the substrate scope, substitution selectivity, and competition experiments with different nucleophiles.

Full text of DOI:10.1002/anie.201610210

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610210
German Edition: DOI: 10.1002/ange.201610210
Synthetic Photochemistry
À
Visible-Light-Accelerated C H Sulfinylation of Heteroarenes
Andreas Uwe Meyer, Alexander Wimmer, and Burkhard Kçnig*
Abstract: Heteroaromatic sulfoxides are a frequent structural
motif in natural products, drugs, catalysts, and materials. We
report a metal-free visible-light-accelerated synthesis of heter-
oaromatic sulfoxides from sulfinamides and peroxodisulfate.
The reaction proceeds at room temperature with blue-light
À
irradiation and allows the C H sulfinylation of electron-rich
heteroarenes, such as pyrroles and indoles. An electrophilic
aromatic substitution mechanism is proposed based on the
substrate scope, substitution selectivity, and competition experi-
ments with different nucleophiles.
À
T
he C H acylation of aromatic compounds is an important
transformation in organic synthesis, and a classic reaction for
the synthesis of aromatic ketones is the electrophilic Friedel–
Crafts acylation. The analogous sulfinylation reaction is,
surprisingly, much less developed and explored despite the
importance of aromatic sulfoxides, which are typical motifs in
natural products,[1] drugs,[2] herbicides,[3] and performance
materials.[4] Several proton-pump inhibitors containing sulf-
oxides (Figure 1) were among the worldwide most sold
pharmaceuticals in 2011[5] and 2013.[6] Sulfoxides also find
applications in organocatalysis,[7] and as ligands in transition-
metal-catalyzed reactions.[8]
Scheme 1. Reactions for the preparation of sulfoxides.
aryl halides and triflates have been developed as an alter-
native strategy by the groups of Poli and Madec,[12] Walsh,[13]
Nolan,[14] and Perrio (Scheme 1c).[15] All of them require
palladium (as the catalyst), ligands, bases, sulfenate anion
precursors, aryl halides or triflates, and often high temper-
atures. A fourth approach involves classical electrophilic
aromatic substitutions, for example, the aluminum chloride
catalyzed arenesulfinylation of benzene and toluene with
sulfinyl chlorides reported by Olah and Nishimura in 1974
(Scheme 1d),[16] the Friedel–Crafts-type reaction of methyl
sulfinates with electron-rich arenes using stoichiometric
amounts of aluminum chloride,[17] and the synthesis of 3-
arylsulfinylindoles from aryl sulfinic acids and indoles.[18]
For the synthesis of many drug motifs, a mild intermo-
lecular sulfinylation of heteroarenes would be useful. There-
Figure 1. Best-selling sulfoxide drugs.
Several routes are described for the synthesis of aromatic
sulfoxides. One is the oxidation of sulfides to yield sulfoxides
(Scheme 1a).[9] Another approach is the nucleophilic substi-
tution of sulfinyl derivatives with organometallic reagents
(Scheme 1b).[10] However, the functional-group tolerance
may be limited by the oxidizing agent or the reactivity of
the organometallic species. In the last decade, many palla-
dium-catalyzed arylation reactions of sulfenate anions[11] with
À
fore, we developed a metal-free C H sulfinylation of hetero-
arenes with sulfinamides and persulfate as a stoichiometric
oxidant. The reaction is accelerated by irradiation with visible
light (Scheme 1e). Sulfinamides are more stable than sulfinyl
chlorides and sulfinic acids, and are easy to handle and readily
available.
The reaction conditions were optimized by irradiating
a mixture of sulfinamide 1, N-Me-pyrrole (2a, 20 equiv), and
ammonium persulfate with visible light at room temperature.
Different sulfinamides 1, solvents, concentrations, amounts of
oxidant and trapping reagent, and irradiation wavelengths
were investigated (Table 1).
[*] M. Sc. A. U. Meyer, M. Sc. A. Wimmer, Prof. Dr. B. Kçnig
University of Regensburg, Faculty of Chemistry and Pharmacy
93040 Regensburg (Germany)
E-mail: Burkhard.Koenig@ur.de
In a typical reaction mixture for the Friedel–Crafts-type
sulfinylation, one equivalent of the sulfinamide 1a, 20 equiv-
alents of N-methyl pyrrole (2a), and one equivalent of
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610210.
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Table 1: Optimization of the reaction conditions.
Table 2: Substrate scope.[a]
Entry
Conditions
Yield [%][a]
1
2
3
4
5
6
7
8
9
1a (R2 =nBu), n=1, MeCN, 455 nm
1a (R2 =nBu), n=1, MeCN, no light
1a (R2 =nBu), n=1, MeCN, no light, 558C
1a (R2 =nBu), no oxidant, MeCN, 455 nm
1a (R2 =nBu), n=0.1, MeCN, 455 nm
1a (R2 =nBu), n=1, no solvent, 455 nm
1a (R2 =nBu), n=1, MeCN, 535 nm
1a (R2 =nBu), n=1, MeCN, 400 nm
1a (R2 =nBu), n=1, DCM 455 nm
1a (R2 =nBu), n=1, DCE 455 nm
56
13
27[b]
–
7[b]
23
47
84
44
71
97
10
11
1b (R2 =Ph), n=1, MeCN, 455 nm
[a] Determined by GC analysis with naphthalene as internal standard.
[b] Yield of isolated product. DCM=dichloromethane, DCE=1,2-
dichloroethane.
ammonium persulfate in MeCN with blue-light irradiation
were used to give 3a in 56% yield (Table 1, entry 1). Without
light, the yield decreases to 13% (Table 1, entry 2). The
thermal decomposition of persulfate at 558C is less efficient
than the acceleration by light and led to 27% product yield
(Table 1, entry 3).[19] A control experiment without oxidant
confirmed that persulfate is necessary for product formation
(Table 1, entry 4), and furthermore, a stoichiometric amount
is required (Table 1, entry 5).
[a] Yield of isolated product given in brackets.
various sulfinamides 1, heteroarenes 2 (3–20 equiv), one
equivalent of ammonium persulfate, MeCN or DCE as the
solvent, and blue-light irradiation for 18 h. The results are
summarized in Table 2. Aromatic sulfinamides bearing elec-
tron-donating or electron-withdrawing substituents react to
give moderate to excellent yields with pyrrole (3a–3l, 3n–3q
and 3y–3aa) and indole (3r–3x) derivatives . Bromide and
chloride substituents (3j–3l, 3s, 3z and 3aa) are tolerated and
allow potential further synthetic modifications of the coupling
products. The molecular structures of compounds 3r and 3t
were confirmed by single-crystal X-ray analysis (Table 2). 1-
Methylindole and indole are exclusively sulfinylated at position
3 (3r and 3t). The aliphatic n-propylsulfinamide (1o) reacts
with N-methyl pyrrole to give the corresponding product 3m in
moderate yield. The reaction with a methoxy-substituted
thiophene derivative 2n (product 3ab) and the benzene
derivatives 2o and 2p (products 3ac[21] and 3ad) indicate the
limits of the method towards decreasing nucleophilicity and
increasing aromaticity of the arene component and give only
small product yields. Azulene (2q) led to a high yield of 88%
for 3ae owing to its high nucleophilicity (N 6.66).[22]
Without solvent, the yield was still 23% (Table 1, entry 6).
Green light (535 nm) accelerates the reaction as well (47%
yield, Table 1, entry 7), but not as well as blue light.
Irradiation at 400 nm is even more efficient (84%, Table 1,
entry 8). The optimal solvent varies with the specific sulfina-
mide (see Table S1, entries 1–14 in the Supporting Informa-
tion). Sulfinamide 1a gives the highest product yields of 71%
after 16 h (Table 1, entry 10) and 97% after 18 h in DCE
(Table 2), whereas the yields in MeCN and DCM are lower
(56% and 44% after 16 h, respectively, Table 1, entries 1 and
9). EtOH, DMF, and DMSO are not suitable solvents for the
reaction (Table S1, entries 18–20 in the Supporting Informa-
tion). However, sulfinamide 1b gives a 97% product yield
after 16 h in MeCN upon blue-light irradiation (Table 1,
entry 11). The excess of the heteroarene 2a can be reduced to
10 equivalents with a simultaneous increase of the overall
concentration to 0.2m and a prolonged reaction time of 19 h,
yielding 3a in 52% yield (Table S1, entry 15—compared to
56%, Table 1, entry 1). Only liquid sulfinamides like 1a can
be used at such high concentrations. Sulfinamide 1b is solid
and the yield drops dramatically at higher concentrations
(12%, see Table S1, entry 17—compared to 97%, Table 1,
entry 11). Applying tert-butyl hydroperoxide as the oxidant
did not provide any product with or without blue-light
irradiation (see Table S1, entries 22–25).[20]
Peroxodisulfate is strongly oxidizing with an oxidation
potential of 1.86 V vs. SCE (in MeCN). Its photoinduced
decomposition leads to sulfate radical anions, which are one-
electron oxidants and hydrogen-atom abstraction agents.[19,23]
Sulfinamide 1a has an oxidation potential of 1.73 V vs. SCE
(in MeCN, see Figure S1 in the Supporting Information) and
is readily oxidized (see Scheme S1 in the Supporting Infor-
mation). Hydrogen-atom abstraction from the radical cation
The scope of the reaction was explored using the
optimized reaction conditions (Table 1, entries 10 and 11):
+
1C may give an electrophilic sulfinyl iminum ion that has
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some structural features analogous to the Vilsmeier
Acknowledgments
reagent.[19] Electrophilic aromatic substitution (SEAr) with
electron-rich arenes, such as pyrrole, and subsequent reduc-
tion and amine elimination[24] yields product 3. The amine was
detected in the crude reaction mixture before work up by GC/
MS and TLC and was recovered during product isolation.
Experimental support for the proposed SEAr mechanism
is provided by the exclusive sulfinylation of indoles in position
3 (3r and 3t, Table 2), which was confirmed by single-crystal
X-ray analysis. In addition, the limitation of the arene scope
to electron-rich heteroarenes indicates an electrophilic Frie-
del–Crafts-type mechanism: N-Me/Bn/Ph pyrrole (2a–2c),
pyrrole (2e), 2,4-dimethylpyrrole (2 f), 3-ethyl-2,4-dimethyl-
pyrrole (2g), and even 3-acetyl-2,4-dimethylpyrrole (2h) give
the expected products (3a–3c and 3e–3h) in good yield.
A radical mechanism is less likely, since the reaction proceeds
in the presence of 2,2,6,6-tetramethyl-l-piperidinoxyl
(TEMPO). Arenes 2, which are not sufficiently nucleophilic
result in the quantitative recovery of 1 from the reaction
mixture. Mayr et al. have investigated the nucleophilicity (N)
and electrophilicity (E) parameters of a large number of
molecules thoroughly.[25] To support our SEAr mechanism, we
performed competition experiments with two nucleophiles
and one electrophile (Scheme S3). Compound 2g has a high
nucleophilicity (N 11.63).[25b] If 2g reacts with 1a in the
presence of the weaker nucleophiles 2a (N 6.18)[26] or 2i (N
6.9),[26] product 3p is obtained exclusively in 79% and 78%
yield, respectively, a comparable yield to the reaction without
a second nucleophile (81%, Table 2). If two nucleophiles with
similar nucleophilicity are present (2a and 2i), both products
are formed (see Scheme S3, 3a (18%)/3r (72%) = 1:4).
The oxidation of N-methyl pyrrole (oxidation potential
1.20 V vs. SCE in MeCN)[27] by the sulfate radical anion is
thermodynamically feasible and the resulting reactive inter-
mediate could trigger an alternative reaction pathway
(Scheme S2).[27,28] However, the competition experiments
with different nucleophiles and the observed regioselectivity
favor an electrophilic aromatic substitution mechanism.
The influence of light on the reaction was investigated by
steady-state spectroscopy (see the Supporting Information).
The starting materials 1a, 2a, and ammonium persulfate do
not absorb visible light. The addition of peroxodisulfate to 1a
or 2a also did not lead to any absorption in the visible region
(Figures S2,S3). The clear reaction mixture of 1a and 2a does
not absorb visible light either. Only after the addition of
ammonium peroxodisulfate, the reaction mixture turns brown
and absorbs over the whole visible region (Figures S4,S5).
The formation of charge-transfer complexes is likely, but the
exact molecular origin of the absorption remains unclear at
present.
This work was supported by the German Science Foundation
(DFG) (GRK 1626, Chemical Photocatalysis). A.M. thanks
the Fonds der Chemischen Industrie for a scholarship. We
thank Dr. Rudolf Vasold for his assistance in GC-MS
measurements and Regina Hoheisel (both University of
Regensburg) for her assistance with cyclic voltammetry
measurements. We thank Dr. M. Poznik for providing the
graphical abstract.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Friedel–Crafts reaction · metal-free conditions ·
sulfinylation · sulfoxide · visible light
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Manuscript received: October 18, 2016
Final Article published: && &&, &&&&
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Synthetic Photochemistry
A. U. Meyer, A. Wimmer,
B. Kçnig*
&&&& — &&&&
À
Visible-Light-Accelerated C H
Sulfinylation of Heteroarenes
Sulfoxides see the light: Sulfoxides were
obtained from sulfinamides, electron-rich
heteroarenes, and peroxodisulfate
through visible-light-accelerated electro-
philic aromatic substitution. The reaction
proceeds at room temperature with blue-
À
light irradiation and allows the C H
sulfinylation of electron-rich heteroar-
enes, such as pyrroles and indoles.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
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www.angewandte.org
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