Aerobic oxysulfonylation of alkynes in aqueous media: Highly selective access to β-keto sulfones

Article abstract of DOI:10.1016/j.tetlet.2014.03.078

The first application of inexpensive commercially available sulfinate salts to produce β-keto sulfones directly from alkynes via aerobic oxysulfonylation has been developed. It is a highly selective (undesired Glaser-Hay homo-coupling and ATRA process totally suppressed) general method of functionalization of alkynes on water at room temperature involving FeCl ₃/K₂S₂O₈ catalyzed formation of sulfonyl radicals from sulfinate salts.

Full text of DOI:10.1016/j.tetlet.2014.03.078

Tetrahedron Letters 55 (2014) 2845–2848
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Aerobic oxysulfonylation of alkynes in aqueous media: highly
selective access to b-keto sulfones
⇑
Atul K. Singh, Ruchi Chawla, Lal Dhar S. Yadav
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first application of inexpensive commercially available sulfinate salts to produce b-keto sulfones
directly from alkynes via aerobic oxysulfonylation has been developed. It is a highly selective (undesired
Glaser–Hay homo-coupling and ATRA process totally suppressed) general method of functionalization of
alkynes on water at room temperature involving FeCl3/K2S2O8 catalyzed formation of sulfonyl radicals
from sulfinate salts.
Received 13 February 2014
Revised 15 March 2014
Accepted 18 March 2014
Available online 24 March 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Alkynes
Sodium arenesulfinates
FeCl3/K2S2O8 catalysis
Radical aerobic oxysulfonylation
b-Keto sulfones
The transformation of alkynes is a method fundamental to or-
ganic synthesis. Besides the early examples of hydration of alkynes
to ketones1 and addition reactions,2 various novel transition-metal
catalyzed reactions of alkynes were developed which enriched the
basic alkyne chemistry. These reactions also resulted in many
methods of industrial importance, viz. Pd-catalyzed Wacker-type
oxidation,3 click reactions,4 and Sonogashira coupling.5 Conse-
quently, great advances have been made in this field to exploit
the prominent potential of alkynes. In particular, the transition-
metal catalyzed functionalization of terminal alkynes has blos-
somed significantly during the last decade.6 However, only rare
examples of transition-metal catalyzed aerobic oxidation of termi-
nal alkynes have been reported.7,8 The area is extraordinarily chal-
lenging and finding new reactions to broaden it is of great
importance.
Dioxygen, as an oxygen source and oxidant, is an attractive re-
agent in organic synthesis which efficiently complies with the
green chemistry principles.7 Recently, it has been used in a number
of transition-metal catalyzed aerobic oxidative radical reactions of
alkenes to achieve important functionalizations but terminal al-
kynes have seldom been used in this way.7–9 The limited number
of reports with alkynes may be attributed to two main obstacles.
Firstly, it is the well known undesired Glaser–Hay homo-coupling
of terminal alkynes which can hardly be avoided and secondly, it is
the preferential atom transfer radical addition (ATRA) process over
oxygen capture by the in situ generated vinyl radical intermediate
which leads to alkenes in the reaction. Thus, these alkyne-based
reactions are usually not selective generating concurrently a num-
ber of undesired by-products. In the light of above mentioned dif-
ficulties, developing a highly selective aerobic oxidation reaction of
terminal alkynes in which both the homo-coupling and ATRA are
totally suppressed is certainly desired. For overcoming these limi-
tations and in continuation of our work on simple starting materi-
als,10 especially alkynes,10g we report herein a highly efficient
aerobic oxysulfonylation protocol for the synthesis of b-keto sulf-
ones directly from alkynes using dioxygen and arenesulfinates in
aqueous media.
b-Keto sulfones11 are valuable compounds in organic synthesis
and pharmaceutical industry. A number of methods have been re-
ported for their synthesis but b-keto sulfones have rarely been ob-
tained from alkynes.12 To the best of our knowledge, there is only
one example where b-keto sulfones have been prepared from al-
kynes using aerobic oxidative radical reaction.9e All the previous
reports suffer severely from limitations like extremely low yields
in most cases, long reaction time, elevated temperatures, and
incompatibility with green chemistry principles. Consequently,
we report herein an efficient synthesis of b-keto sulfones by aero-
bic oxysulfonylation of alkynes on water at room temperature
(Scheme 1). Besides being catalytic, our protocol involves the use
of cost-effective and environmentally benign catalyst system
(FeCl3/K2S2O8) and solvent (water). Moreover, arenesulfinates have
never been used as sulfonyl radical precursors for ensuing oxygen
capture in conjunction with alkynes.
⇑
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (Lal Dhar S. Yadav).
http://dx.doi.org/10.1016/j.tetlet.2014.03.078
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2846
A. K. Singh et al. / Tetrahedron Letters 55 (2014) 2845–2848
O
O
S
O
O
Ph
Ph
O
O
cat. FeCl3/K2S2O8
H2O, rt
Ts
R1
O2
(air)
R2
Ph
Ph
O2
Ar
S
R2
Ar
Na
Ts
(air)
R1
cat.AgNO3/K2S2O8
H2O, rt
Ph
A catalytic green pathway
Ph
1a
TsNa
Ts
2a
Scheme 1. Aerobic oxysulfonylation of alkynes.
other side products
Scheme 2. Model reaction under AgNO3/K2S2O8 catalysis in aqueous media.
Recently, we have communicated our work on the synthesis of
b-keto sulfones from alkenes and arenesulfinates via aerobic oxy-
sulfonylation using AgNO3/K2S2O8 catalytic system10i and also un-
der transition metal-free conditions.10j Our previous findings led us
to investigate the more challenging reaction of alkynes with sulfi-
nate salts under the same reaction conditions. Our initial trial with
phenylacetylene (1a) and sodium p-toluenesulfinate (2a) in the
presence of 20 mol % of AgNO3 and 20 mol % of K2S2O8 in DMF at
room temperature for 18 h delivered a complex mixture containing
18% of b-keto sulfone 3a (Table 1, entry 1). Our reaction with al-
kenes also worked well under aqueous conditions, so the next trial
was performed in aqueous media which fetched better results (en-
try 2). On using water as a solvent, b-keto sulfone 3a was produced
in 27% yield. The homo-coupled product of 1a and a regioisomeric
mixture of vinyl sulfones were formed in 20% and 24% yield,
respectively along with other side products (Scheme 2). The ex-
pected Glaser–Hay homo-coupling and ATRA process were seri-
ously interfering with our reaction conditions.
completed in 7 h in the presence of FeCl3/K2S2O8 catalyst system
(entry 8). Evaluation of (NH4)2S2O8, TBHP, DTBP, pyridine, and
Et3N as additives in the oxysulfonylation protocol showed that
persulfate salts outperformed the other additives; (NH4)2S2O8
producing a slight drop in yield as compared to K2S2O8 (entries 5,
9–13). In the absence of any additive, the yield of b-keto sulfone
3a was decreased (entry 14). Also, in the absence of any metal
catalyst the reaction became very sluggish (entry 15). The reaction
did not improve in the presence of other solvents (entries 16–21).
The exceptional lower yields in less polar solvents may be due to
insolubility of the reagent/catalyst in them.
With the best catalyst system chosen, we next tried to optimize
the catalyst/additive loading and the substrate-reagent ratio. After
extensive investigation, we established that the maximum yield of
3a (92%) was obtained on using 1.0 equiv of 1a and 1.5 equiv of 2a
in the presence of 20 mol % each of FeCl3 and K2S2O8.
We further tried the reaction using other metal salts in aqueous
media. Copper salts worsen the situation as the side reactions were
facilitated considerably in their presence (entries 3 and 4). Most
delightfully, a dramatic change in the TLC pattern was observed
on using FeCl3 as a catalyst. The side reactions were totally sup-
pressed (reason not very clear) and the yield of b-keto sulfone 3a
rose to 92% (entry 5). Motivated by the results, we tried other iron
salts but FeCl3 produced the best yield (entries 5–7) as well as
accelerated the reaction appreciably. The reaction could be
Once we decided the best set of conditions for our reaction, we
moved onto the substrate-scope of the reaction. The reaction was
tried with various alkynes 1 and sodium arenesulfinates 2 (Table 2).
As evident from the table, aryl alkynes proved to be excellent sub-
strates for the aerobic oxysulfonylation. Both electron-rich (R = Me
or OMe) and electron-poor (R = Br, Cl, F or CN) aryl alkynes were
oxysulfonylated with sulfinate salts 2a (R = Me) and 2b (R = H) in
good yields (3a–o). Not much change in the yield of products
3e–i was observed with the variation of positions (o-, m-, and p-)
Table 2
Substrate scope of the reactiona
Table 1
Optimization of metal salt, additive, and solventa
O
O
S
O
O
O
cat. FeCl3/K2S2O8
H2O, rt, 6-9 h
Ar
S
R2
R1
O2
(air)
Ar
R2
O
O
S
O
Na
catalyst/additive
solvent, rt, 16 h
R1
TsNa
O2
(air)
1
2
3
1a
2a
3a
Entry Ar
R1
R2
Product Time (h) Yiel-db (%)
Entry
Solvent
Metal salt (20 mol %)
Additive (20 mol %)
Yieldb (%)
1
2
3
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4-MeC6H4
4-MeC6H4
4-MeC6H4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3a
3b
3c
3d
3e
3f
7
9
8
7
8
9
8
6
7
9
7
8
8
9
8
7
8
9
6
7
7
8
8
92
85
88
94
82
79
86
93
81
68
90
79
72
65
70
73
77
56
93
82
87
84
89
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
DMF
AgNO3
AgNO3
CuCl
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
(NH4)2S2O8
TBHP
18
27
12
15
92
43
81
92c
84
73
79
70
71
65
17
83
75
70
55
22
23
4-MeC6H4
4-BrC6H4
Ph
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
Water
DMF
4
Cu(OAc)2
FeCl3
Fe2O3
[Fe(Pc)]
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
—
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
5
6
7
3-MeC6H4
2-MeC6H4
4-MeC6H4
3g
3h
3i
3j
3k
3l
8
9
4-MeOC6H4
2-MeOC6H4
4-IC6H4
4-BrC6H4
4-ClC6H4
4-FC6H4
4-CNC6H4
4-PhC6H4
2-naphthyl
2-thienyl
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
DTBP
Pyridine
Et3N
3m
3n
3o
3p
3q
3r
—
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
DMSO
EtOH
CH3CN
DCM
Me Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
4-MeOC6H4 3s
4-BrC6H4
4-ClC6H4
4-FC6H4
3t
3u
3v
3w
Dioxane
Ph
2-naphthyl
a
Reaction conditions: 1a (0.25 mmol), 2a (0.40 mmol), catalyst, additive, and
solvent (3 mL) in a flask open to air.14
a
Reaction conditions: 1 (0.25 mmol), 2 (0.375 mmol), FeCl3 (20 mol %), K2S2O8
b
(20 mol %), and water (3 mL) in a flask open to air.14
Yield of isolated and purified product 3a.
Reaction time 7 h.
c
b
Yield of isolated and purified product 3 and reaction time is given.

A. K. Singh et al. / Tetrahedron Letters 55 (2014) 2845–2848
2847
a) evidence in support of radical pathway:
of alkynes to furnish carbon centered radical 6 which is ultimately
trapped by dioxygen to form b-keto sulfones 3. A study of the de-
tailed mechanism is under investigation.
O
O
S
O
standard
reaction conditions
TEMPO (0.375 mmol)
In summary, a novel aerobic oxysulfonylation protocol to
obtain b-keto sulfones directly from alkynes via radical pathway
has been developed. It involves the use of environmentally benign
and inexpensive catalyst, reagents, and solvent, viz. FeCl3/K2S2O8,
dioxygen, arenesulfinates, and water, respectively. The exceptional
selectivity of the reaction, in which interfering Glaser–Hay
homo-coupling and ATRA process are totally suppressed, is an
additional attribute. Further work to exploit the protocol is
underway in our laboratory.
3a, 0%
b) role of air as the oxidant:
O
O
S
O
standard
reaction conditions
N2 atmosphere
3a, traces
18O
c) 18O-labelling experiment:
O
S
O
standard
reaction conditions
18O2/N2
3a', 85%
Acknowledgments
Scheme 3. Preliminary mechanistic investigations.
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. A.K.S. is thankful to CSIR
for the award of a Junior Research Fellowship (File No. 09/
001(0379)/2013-EMR-I).
of substituents on the phenyl ring of alkynes. The bicyclic
substrate, 2-ethynylnaphthalene could also be employed in the
reaction leading to b-keto sulfone 3p in 73% yield. Likewise, the
representative heterocyclic alkyne, 2-ethynylthiophene, was com-
patible with this protocol (3q). Prop-1-ynylbenzene, an internal al-
kyne, also served as a reaction partner in this protocol albeit in low
yield and the desired product 3r was obtained in 56% yield. Subse-
quently, we explored the oxysulfonylation of phenylacetylene with
a variety of sodium arenesulfinates 2 to define the scope of our cur-
rent reaction system. Aryl sulfinate salts, bearing either an elec-
tron-donating group (R = Me, OMe) or an electron-withdrawing
group (R = Br, Cl, F) furnished the desired keto sulfones (3a, 3s–v)
in good to excellent yields. Even the bulky 2-naphthylsulfinate salt
produced the keto sulfone 3w efficiently (yield 89%) under the
reaction conditions.
In order to gain greater insight into the reaction mechanism, we
conducted a number of experiments (Scheme 3). Inspired by the
previous reports that dioxygen activation based processes mostly
involve radical species,7,9 radical trapping experiment was per-
formed. b-Keto sulfone 3a was not at all formed in the presence of
TEMPO under the standard reaction conditions, elucidating a radical
pathway (Scheme 3a). Studying the role of air in the reaction was
also vital to unfold the intricacies of the mechanism. The reaction
was considerably inhibited under nitrogen (Scheme 3b). We also
performed oxygen labeling experiments. In the presence of
18O2/N2, 18O-labeled product 3a0 was obtained in 85% yield, confirm-
ing that the carbonyl oxygen atom of the b-keto sulfone originated
from dioxygen (Scheme 3c).
References and notes
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Mechanisms, and Structure ; John Wiley & Sons: Hoboken, NJ, 2007; p 1019;
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S. Inorg. Chem. 2007, 46, 1903; (c) Kotov, V.; Scarborough, C. C.; Stahl, S. S. Inorg.
Chem. 2007, 46, 1910.
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(b) Chawla, R.; Kapoor, R.; Singh, A. K.; Yadav, L. D. S. Green Chem. 2012, 14,
1308; (c) Singh, A. K.; Yadav, L. D. S. Synthesis 2012, 591; (d) Chawla, R.; Singh,
A. K.; Yadav, L. D. S. Tetrahedron Lett. 2012, 53, 3382; (e) Singh, A. K.; Chawla, R.;
Yadav, L. D. S. Synthesis 2012, 2353; (f) Chawla, R.; Singh, A. K.; Yadav, L. D. S.
Tetrahedron 2013, 69, 1720; (g) Chawla, R.; Singh, A. K.; Yadav, L. D. S. Synlett
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Chem. 2014, 2032.
A plausible mechanism that is consistent with experimental
data and literature precedents9,13 is depicted in Scheme 4. The
FeCl3/K2S2O8 catalyst system triggers the sulfonyl radical forma-
tion from sulfinate salt. This sulfonyl radical attacks the triple bond
O
R2
S
Ar
O
5
O
S
O
R1
[S2O82-/O2]
FeIII
1
R2SO2
Ar
R2
2
R1
FeII
6
O
O
R2
S
O2
4
11. Markitanov, Y. M.; Timoshenko, V. M.; Shermolovich, Y. G. J. Sulfur Chem. 2014,
35, 188.
12. (a) Lai, C.; Xi, C.; Jiang, Y.; Hua, R. Tetrahedron Lett. 2005, 46, 513; (b)
Sreedhar, B.; Rawat, V. Synlett 2012, 413; (c) Qian, H.; Huang, X. Synthesis
2006, 1934.
OH
O
S
O
OO O
O
O
S
O
S
R2
Ar
R2
Ar
R2
Ar
O
R1
R1
R1
3
13. (a) Taniguchi, T.; Zaimoku, H.; Ishibashi, H. Chem. Eur. J. 2011, 17, 4307; (b)
Taniguchi, T.; Idota, A.; Ishibashi, H. Org. Biomol. Chem. 2011, 9, 3151.
Scheme 4. A plausible mechanism for the formation of b-keto sulfones 3.

2848
A. K. Singh et al. / Tetrahedron Letters 55 (2014) 2845–2848
14. General procedure for the one-pot synthesis of b-keto sulfones 3a–w: A mixture of
alkyne (0.25 mmol), sodium sulfinate (0.375 mmol), FeCl3 (20 mol %),
S, 11.46.
1
2
Compound 3r: Solid; yield 56%. 1H NMR (400 MHz, CDCl3) d: 7.99–7.94 (m, 2H),
7.85–7.79 (m, 2H), 7.63 (tt, J = 7.2, 1.4 Hz, 1H), 7.54 (tt, J = 7.6, 1.4 Hz, 1H), 7.48
(tt, J = 8.2, 1.4 Hz, 2H), 7.43 (tt, J = 7.6, 1.8 Hz, 2H), 5.26 (q, J = 2.6 Hz, 1H), 1.55
(d, J = 7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3) d: 192.69, 136.33, 136.19, 134.37,
134.22, 129.96, 129.30, 129.10, 128.90, 65.17, 13.40. EI-MS (m/z): 274 (M+).
Anal. Calcd for C15H14O3S: C, 65.67; H, 5.14; S, 11.69. Found: C, 65.57; H, 5.42;
S, 11.53.
K2S2O8 (20 mol %), and water (3 mL) was stirred at rt in an open flask for 6–9 h
(Table 2). After completion of the reaction (monitored by TLC), the mixture was
extracted with EtOAc (3 ꢀ 5 mL). The combined organic phases were dried over
anhyd. Na2SO4, filtered, and concentrated under reduced pressure. The
resulting crude product was purified by silica gel column chromatography
using a mixture of EtOAc-n-hexane (1:4) as eluent to afford an analytically
pure sample of b-keto sulfones 3 (Table 2).
Characterization data of representative compounds. Compound 3a: Solid; yield
92%. 1H NMR (400 MHz, CDCl3) d: 7.91 (d, J = 7.6 Hz, 2H), 7.81 (t, J = 8.6, 2H),
7.60 (t, J = 7.4 Hz, 1H), 7.46 (t, J = 7.8 Hz, 2H), 7.39 (d, J = 7.8 Hz, 2H), 4.68 (s,
2H), 2.44 (s, 3H); 13C NMR (100 MHz, CDCl3) d: 188.30, 145.51, 135.89, 135.84,
134.48, 129.93, 129.42, 128.98, 128.75, 63.90, 21.82. EI-MS (m/z): 274 (M+).
Anal. Calcd for C15H14O3S: C, 65.67; H, 5.14; S, 11.69. Found: C, 65.95; H, 5.26;
Compound 3t: Solid; yield 82%. 1H NMR (400 MHz, CDCl3) d: 7.93 (dd, J = 8.2,
1.2 Hz, 2H), 7.81 (d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.4 Hz, 2H), 7.59 (t, J = 7.4 Hz,
1H), 7.44 (t, J = 7.6 Hz, 2H), 4.78 (s, 2H); 13C NMR (100 MHz, CDCl3) d: 188.01,
137.79, 135.64, 134.61, 132.60, 130.35, 129.94, 129.30, 129.04, 63.54. EI-MS
(m/z): 337, 339 (M+, M++2). Anal. Calcd for C14H11BrO3S: C, 49.57; H, 3.27; S,
9.45. Found: C, 49.60; H, 3.42; S, 9.67.