Visible Light-Mediated Metal-Free Synthesis of Vinyl Sulfones from Aryl Sulfinates

Article abstract of DOI:10.1002/adsc.201500142

Visible light and eosin Y catalyze the synthesis of vinyl sulfones from aryl sulfinates and alkenes by a photoredox process. The reaction scope is broad in aryl sulfinates and alkenes and the general and simple procedure provides a metal-free alternative for the synthesis of synthetically valuable vinyl sulfones.

Full text of DOI:10.1002/adsc.201500142

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DOI: 10.1002/adsc.201500142
Visible Light-Mediated Metal-Free Synthesis of Vinyl Sulfones
from Aryl Sulfinates
a
a
a
a,
Andreas Uwe Meyer, Stefanie Jäger, Durga Prasad Hari, and Burkhard Kçnig *
a
Institut für Organische Chemie, Universität Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany
Fax: (+49)-941-943-1717; phone: (+49)-941-943-4575; e-mail: Burkhard.Koenig@chemie.uni-regensburg.de
Received: February 11, 2015; Revised: March 22, 2015; Published online: June 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500142.
Abstract: Visible light and eosin Y catalyze the syn-
thesis of vinyl sulfones from aryl sulfinates and al-
kenes by a photoredox process. The reaction scope
is broad in aryl sulfinates and alkenes and the gen-
eral and simple procedure provides a metal-free al-
ternative for the synthesis of synthetically valuable
vinyl sulfones.
transition metal [e.g., Pd, Cu] catalyzed reactions of
[
9]
vinyl tosylates with aryl sulfinates, alkenes or al-
[
10]
kynes with sodium sulfinates (Scheme 1, b), and al-
[
11]
kynes with arylsulfinic acids. In 2011, Taniguchi de-
veloped a copper-catalyzed aerobic system for the
synthesis of alkenyl sulfones from alkenes or alkynes
[
12]
with sodium sulfonates.
Lei et al. reported an
iodide-catalyzed alkenylation reaction with sodium
sulfinates or sulfonyl hydrazides and olefins yielding
Keywords: aryl sulfinates; metal-free conditions;
photocatalysis; vinyl sulfones; visible light
[
13]
vinyl sulfones. In 2013, Yu et al. described a visible-
light photoredox-catalyzed protocol for the synthesis
of b-amidovinyl sulfones from enamides with sulfonyl
[
14]
chlorides.
synthesized exclusively b-amidovinyl sulfones with
Vinyl sulfones are used in synthetic organic transfor- sulfonyl chlorides as sulfonylation reagents.
They used [Ir(ppy) (dtb-bpy)]PF6 and
2
[
1]
mations as intermediates and reagents. A recent ex-
The use of sodium salts of sulfinic acid as sulfonyla-
ample is the photoredox-mediated vinylation by vinyl tion reagents has gained much attention in recent
[2]
[3]
[9,10,12,13,15]
sulfones. They are of biological importance as in- years.
Sodium sulfinates are stable, easy to
hibitors for different enzymes, for example, cysteine handle and readily available from their corresponding
[
4]
[5]
proteases, protein tyrosine phosphatases, and sor- sulfonyl chlorides. A recent example of an ionic reac-
[6]
tases.
tion is the addition of aryl sulfinates to 3-halooxin-
[
16]
Classic vinyl sulfone preparations are based on the doles. Under oxidative conditions sodium and zinc
Knoevenagel condensation (Scheme 1, a), and the sulfinates serve as radical precursors.
Horner–Emmons reaction. Alternative methods are et al. generated sulfinyl radicals via reduction of sulfo-
[
7]
[17]
Stephenson
[
8]
[
18]
nyl chlorides. In 2013, Nicewicz et al. reported the
hydrotrifluoromethylation of styrenes by single elec-
tron oxidation of sodium trifluoromethanesulfinate by
N-methyl-9-mesitylacridinium as the photoredox cata-
[
19]
lyst.
We report here the visible light-mediated,
metal-free synthesis of vinyl sulfones from aryl sulfi-
nates with olefins using green light, the organic dye
eosin Y as photocatalyst and nitrobenzene as the ter-
minal oxidant (Scheme 1, c).
The conditions were optimized by irradiating a reac-
tion mixture of sodium benzenesulfinate (1a), styrene
(
2a), the photocatalyst and nitrobenzene with visible
light. All reactions were performed at 408C to dis-
solve all starting materials and different catalysts, sol-
vents and irradiation times were investigated.
The product yield using [Ru(bpy) ]Cl (A) as pho-
tocatalyst strongly depends on the solvent. Dipolar
3
2
Scheme 1. Different vinyl sulfone syntheses.
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Visible Light-Mediated Metal-Free Synthesis of Vinyl Sulfones
Table 1. Optimization of the reaction conditions.
all after 18 h (Table 1, entry 10). Reaction under ex-
clusion of oxygen gave a yield of 55% after 18 h
(
Table 1, entry 11), but the yield in air was significant-
ly higher (73%, Table 1, entry 9), because atmospher-
ic oxygen may serve as an additional terminal oxi-
dant.
Using these conditions, the scope of the reaction of
1,2-dihydronaphthalene (2b) with different sodium
aryl sulfinates 1, eosin Y (C) and ethanol was ex-
plored. As depicted in Table 2 all expected products
3
b–g were obtained in moderate to excellent yields.
The best result of 99% for 3b was obtained for the
[a]
Table 2. Scope of aryl sulfinates.
Entry
Cat
mol%)
hn
[nm]
Solvent
t
Yield
[a]
(
[h]
[%]
1
2
3
4
5
6
7
8
9
A (5)
–
A (5)
–
A (5)
A (5)
B (2)
C (10)
C (10)
–
455
455
–
DMF
DMF
DMF
DMF
6
6
6
6
6
6
6
6
26
6
–
–
–
455
455
455
535
535
535
535
DMF/H O (3:1)
58
59
59
43
2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
[
b]
18
18
18
73
1
0
– [
c]
1
1
C (10)
55
[
[
a]
Determined by GC analysis with naphthalene as internal
standard.
General reaction conditions 1: 1a (0.50 mmol, 3 equiv.),
b]
2
a (0.17 mmol, 1 equiv.), Ph-NO2 (0.17 mmol, 1 equiv.)
and C (10 mol%) in EtOH was irradiated with green
light for 18 h.
N : degassed by three freeze-pump-thaw cycles.
2
[
c]
aprotic solvents, such as DMF, DMSO, NMP and
CH CN led to low or even no yields. The yield in
3
DMF was 26% of 3a (Table 1, entry 1), which did not
increase with more catalyst. The control experiment
in DMF without catalyst led to 6% product formation
by a background reaction. Without light or without
catalyst and light no product is formed. The yields in-
creased in protic solvents. A mixture of DMF/H O
2
(
3:1) led to 58% yield (Table 1, entry 5), but ethanol
was selected as the optimal solvent due to its low tox-
icity and a product yield of 59% (Table 1, entry 6).
Both metal complexes [Ru(bpy) ]Cl (A) and [Ir(p-
3
2
py) (dtb-bpy)]PF (B) (5 and 2 mol%, respectively)
2
6
gave under these conditions identical amounts of 3a
59%, Table 1, entries 6 and 7), while the commercial-
ly available organic dye eosin Y (C) (10 mol%) gave
3% after 6 h irradiation time (Table 1, entry 8).
However, extending the irradiation time to 18 h in-
creased the yield of 3a to 73% (Table 1, entry 9). The
control experiment without catalyst and green light
(
4
[a]
General reaction conditions 1: 1a–f (0.50 mmol, 3 equiv.),
b (0.17 mmol, 1 equiv.), Ph-NO2 (0.17 mmol, 1 equiv.)
2
and C (10 mol%) in EtOH was irradiated with green
light for 18 h.
b]
[
(
535 nm) irradiation showed no product formation at
Isolated yield after purification on SiO2.
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Andreas Uwe Meyer et al.
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[b]
Table 3. Scope of alkenes.
Scheme 2. Formation of the by-product 4 by nucleophilic
attack of the solvent ethanol.
unsubstituted sodium benzenesulfinate (1a) (Table 2,
Entry 1). The structure of product 3b was confirmed
by X-ray single crystal analysis. An electron donating
methyl or an electron-withdrawing bromide substitu-
ent in the para position to the sulfinate and even the
bulky naphthalene group were tolerated with yields
of 74–88% (Table 2, entries 2, 3, 4 and 6). The bro-
mide substituent allows further synthetic modifica-
tions of the products. A lower yield of 51% (Table 2,
entry 5) was obtained from reaction with a methoxy-
substituted aryl sulfinate.
After establishing the scope of sodium aryl sulfi-
nates, the olefin 2 was varied. As vinyl derivatives
yield by-product 4 (see the Supporting Information+)
+
resulting from the reaction of the carbenium ion 3
intermediate and the solvent ethanol, the solvent mix-
ture DMF/H O was used for the reactions of vinyl de-
2
rivatives with 1 (Scheme 2).
Styrene (2a) was reacted with aryl sulfinates 1b–f.
Electron-donating and electron-withdrawing substitu-
ents in the para position, and the bulky naphthalene
moiety were tolerated and afforded moderate to good
product yields of 54–76% (Table 3, entries 1, 2, 3 and
5
(
). The reaction with 1e gave again a lower yield
46%, Table 3, entry 4), while 1-ethenyl-4-methoxy-
benzene (2c) led to product 3m (Table 3, entry 6) in
0%. The methoxy group increases the electron den-
9
sity at the double bond, which may facilitate the
attack of the aryl sulfinate radical. a-Methylstyrene
(
2e) and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroqui-
[20]
noline (EEDQ, 2f) gave only 19% and 11% of the
coupling products 3o and 3p (Table 3, entries 8 and
9), respectively. Analysis of the reaction mixture of 2e
by GC/MS revealed incomplete conversion of the
starting material and the formation of isopropylben-
zene.
The proposed mechanism of the photocatalytic
vinyl sulfone synthesis using eosin Y is shown in
Scheme 3. The cycle is initiated by excitation of the
photocatalyst PC, which is reductively quenched by
the aryl sulfinate 1 giving an oxygen centered radical
[
a]
General reaction conditions 2: 1a–f (0.50 mmol, 3 equiv.),
2
1
a, c–e (0.17 mmol, 1 equiv.), Ph-NO2 (0.17 mmol,
equiv.) and C (10 mol%) in DMF/H O (3:1) was irradi-
2
ated with green light for 18 h.
Isolated yield after purification on SiO2.
Yield based on starting material conversion.
[b]
[
[
c]
d]
Ethanol as solvent instead of DMF/H O (3:1).
2
1
’C resonating with the corresponding sulfur-centered
[21]
aryl sulfinate radical 1C.
Radical 1C attacks the zene reoxidizes the photocatalyst and its radical anion
double bond of 2 to form intermediate 3C. Nitroben- abstracts an H atom of 3C to give the vinyl sulfone 3.
2052
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Visible Light-Mediated Metal-Free Synthesis of Vinyl Sulfones
General Procedure for the Preparation of Sodium
Aryl Sulfinates
Compounds were prepared according to literature proce-
[24]
dures. Sodium sulfite (2.50 g, 0.02 mol, 2 equiv.), sodium
bicarbonate (1.68 g, 0.02 mol, 2 equiv.) and the correspond-
ing aryl sulfonyl chloride (0.01 mmol, 1 equiv.) were dis-
solved in distilled water (9.6 mL). The reaction mixture was
stirred for 4 h at 808C. After cooling to room temperature
the water was removed by lyophilization overnight. The
white residue was extracted with ethanol (25 mL) to obtain
the desired aryl sulfinate as white crystalline powder.
1
Sodium
4-fluorobenzenesulfinate
(1c):
H NMR
(
2
400 MHz, D O): d =7.26 (t, J=8.9 Hz, 2H), 7.63–7.70 (m,
H); C NMR (101 MHz, D O): d =115.7 (d, +), 125.7 (d,
2 C
4 1
2
H
1
3
3
JC,F =9.0 Hz, +), 149.5 (d, J =2.9 Hz, C ), 163.6 (d, J =
C,F
q
C,F
1
9
2
64.4 Hz, C ); F NMR (376 MHz, D O): d =À111.2 (s);
q
2
H
À
MS (ESI): m/z=158.9814 [MÀNa] , calcd. for C H FO S:
6
4
2
1
58.9921.
General Reaction Conditions for the Photocatalytic
Vinyl Sulfone Synthesis
A 5-mL crimp-cap vial was charged with the aryl sulfinate
1
1
(0.50 mmol, 3 equiv.), the respective olefin 2 (0.17 mmol,
equiv.), nitrobenzene (17.1 mL, 0.17 mmol, 1 equiv.), photo-
Scheme 3. Proposed mechanism for the visible light-mediat-
ed metal-free synthesis of vinyl sulfones 3 and TEMPO-
trapped reaction intermediate 5.
catalyst PC (5 or 10 mol%) and a stirring bar. Solvent
2 mL) was added via syringe and the vessel was capped to
(
prevent evaporation. The reaction mixture was stirred and
irradiated using blue LEDs (455 nm) or green LEDs
TEMPO addition to the reaction mixture of 1a, 2b,
eosin Y and nitrobenzene stops the vinyl sulfone syn-
thesis and the TEMPO addition product 5 (Scheme 3)
was detected, supporting that the photoreaction pro-
(
535 nm) for 6 or 18 h at 408C. The progress could be moni-
tored by GC analysis and GC/MS analysis using naphtha-
lene as internal standard.
The reaction mixture was diluted with water (5 mL) and
ceeds via a radical pathway (see the Supporting Infor- extracted with EtOAc (3”5 mL). The combined organic
mation). The oxidation potential of sodium benzene- layers were dried over MgSO , and the solvents were re-
4
sulfinate (1a) was determined by cyclic voltammetry moved under reduced pressure. Evaporation of volatiles led
to the crude product. Purification of the crude product was
as À0.37 V vs. SCE (see the Supporting Informa-
[22]
performed by automated flash column chromategraphy (PE/
EtOAc, 25% EtOAc) yielding the corresponding vinyl sul-
fone 3 as yellowish oil.
tion). Therefore, 1a is easily oxidized and able to
donate an electron to the excited photocatalyst. In
the case of eosin Y the oxidative quenching cycle is
thermodynamically possible and a mechanistic alter-
native in which nitrobenzene acts as an oxidant for
3
-(Benzenesulfonyl)-1,2-dihydronaphthalene
(3b):
1
H NMR (400 MHz, CDCl ): d=2.50 (dt, J=8.3, 1.3 Hz,
3
2
H), 2.86 (t, J=8.3 Hz, 2H), 7.10–7.16 (m, 1H), 7.20–7.31
[
23]
the photocatalyst. A more detailed mechanistic dis-
cussion is provided in the Supporting Information.
(
m, 3H), 7.51–7.57 (m, 2H), 7.58–7.64 (m, 2H), 7.91–7.96
1
3
(m, 2H); C NMR (101 MHz, CDCl ): d =21.8 (À), 27.6
3
C
In conclusion, vinyl sulfones were obtained from (À), 127.2 (+), 127.9 (+), 128.0 (+), 129.1 (+), 129.3 (+),
aryl sulfinates and alkenes by metal-free photoredox 130.6 (+), 131.0 (C ), 133.4 (+), 135.3 (+), 135.6 (C ), 138.3
C ), 139.7 (C ); HR-MS (ESI): m/z=271.0790 [M+H] ,
q
q
+
(
catalysis with green light. The reaction uses the organ-
ic dye eosin Y as photoredox catalyst and allows the
conversion of a variety of aryl sulfinates and alkenes
with functional group tolerance. A plausible reaction
mechanism was proposed and is supported by spectro-
scopic and electrochemical investigations, and
TEMPO-trapped intermediates.
q
q
calcd. for C H O S: 271.0787.
16
15
2
TEMPO Trapping of Radical Reaction Intermediates
To
.50 mmol, 3 equiv.), 2b (21.8 mL, 0.17 mmol, 1 equiv.), nitro-
benzene (17.1 mL, 0.17 mmol, 1 equiv.), (108 mg,
00 mol%), TEMPO (32.6 mg, 0.21 mmol, 1.25 equiv.) and
a 5-mL crimp-cap vial were added 1a (82.1 mg,
0
C
1
a stirring bar. EtOH (2 mL) was added via syringe and the
vessel was capped to prevent evaporation. The reaction mix-
ture was stirred and irradiated using green LEDs (535 nm)
for 18 h at 408C. After the irradiation the reaction mixture
was submitted to mass spectrometry (LC-MS) without any
Experimental Section
For full experimental data see the Supporting Information.
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Andreas Uwe Meyer et al.
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further work-up. TEMPO adduct 5. MS (ESI): m/z=
[5] S. Liu, B. Zhou, H. Yang, Y. He, Z.-X. Jiang, S. Kumar,
+
4
28.2262 [M+H] , calcd. for C H NO S:428.2254.
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