Copper-Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals for the Synthesis of Sulfinates and Thiosulfonates

Article abstract of DOI:10.1002/adsc.201500785

Copper-catalyzed aerobic coupling of thiols and alcohols affords sulfinates and thiosulfonates. These products are assumed to form via sulfinyl radicals which are not commonly found in oxidative coupling reactions of thiols. A reaction mechanism involving sulfinyl radicals is proposed, and mass and electron paramagnetic resonance (EPR) experimental results are provided.

Full text of DOI:10.1002/adsc.201500785

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DOI: 10.1002/adsc.201500785
Copper-Catalyzed Aerobic Formation of Unstable Sulfinyl
Radicals for the Synthesis of Sulfinates and Thiosulfonates
a
a
a
a,b,
Pranab K. Shyam, Yu Kwon Kim, Chan Lee, and Hye-Young Jang *
a
Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea
Fax: (+82)-31-219-1615; e-mail: hyjang2@ajou.ac.kr
Korea Carbon Capture & Sequestration R&D Center, Deajeon 305-343, Korea
b
Received: August 21, 2015; Revised: October 13, 2015; Published online: December 17, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500785.
Abstract: Copper-catalyzed aerobic coupling of
thiols and alcohols affords sulfinates and thiosulfo-
nates. These products are assumed to form via sulfi-
nyl radicals which are not commonly found in oxi-
dative coupling reactions of thiols. A reaction
mechanism involving sulfinyl radicals is proposed,
and mass and electron paramagnetic resonance
Sulfinates are widely utilized as versatile sulfinylat-
ing agents in the synthesis of a diverse range of orga-
nosulfur compounds including sulfoxides and sulfin-
[6,7,8]
amides,
which justifies the development of various
synthetic protocols for sulfinates. Generally, sulfinates
are synthesized by alkylation of sulfinic acid (or
sodium sulfinate), substitution of sulfinyl chloride
with alcohols, or oxidative coupling of disulfides with
alcohols using stoichiometric amounts of oxidants,
e.g., N-bromosuccinimide (NBS), phenyliodine bis(tri-
(
EPR) experimental results are provided.
Keywords: aerobic oxidation; copper; sulfinates;
sulfinyl radicals; thiosulfonates
[9]
fluoroacetate) (PIB), or Pb(OAc)4. Since the pio-
neering synthesis of sulfinates via sulfinyl chloride
[9a]
was reported by Douglass in 1965, catalytic reaction
conditions for the direct synthesis of sulfinates from
thiols have not been reported. Herein, a copper-cata-
The oxidation of sulfur compounds is one of the most lyzed aerobic coupling of thiols and alcohols to afford
important reactions in synthetic chemistry, biological sulfinates is presented, in which the direct oxidation
processes, and environmental and material indus- of thiols to sulfinyl radicals occurs, followed by SÀO
[
1]
[10]
tries. Sulfinyl radicals are proposed to form during bond formation. In the absence of alcohols, the cat-
the aerobic oxidation of biologically abundant thiols alytic system using copper salts induces the formation
(
e.g., cysteine and glutathione) in vivo, which is asso- of thiosulfonates via sulfinyl radicals (Scheme 1).
ciated with the malfunction of oxidized biomole-
[
2]
cules. Other than these biological reactions, synthet-
ic methods for the generation of sulfinyl radicals have
not been intensively studied. During photo-cleavage
of sulfoxides, thermolysis of sulfinimines, and oxida-
tion of disulfides, vicinal disulfoxides and thiosulfo-
nates are formed, which is considered to be evidence
of the presence of sulfinyl radical intermediates
[
3]
(
Scheme 1).
Copper-catalyzed aerobic oxidation has been ap-
plied to the modification of thiols, due to the environ-
[4]
mentally friendly nature of this catalytic system. Di-
sulfide formation and CÀS/PÀS/NÀS/SÀS bond forma-
[
5]
tion are frequently reported examples. Despite in-
tensive research on copper-catalyzed aerobic coupling
of thiols, the formation of sulfinyl radicals by current-
ly known methods has not been reported, which en-
couraged us to attempt the direct oxidation of thiols
to sulfinyl radicals.
Scheme 1. Generation of sulfinyl radical intermediates.
5
6
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Adv. Synth. Catal. 2016, 358, 56 – 61

COMMUNICATIONS Copper-Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals
Table 1. Optimization of the conversion of 1a to 1b.
Entry Metal Complex Solvent (Temp. [8C]) Yield
[
a]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
CuI
CuI
toluene (100)
toluene (100)
toluene (100)
toluene (100)
toluene (100)
toluene (100)
toluene (100)
toluene (100)
dioxane (100)
THF (65)
74% (0%)
[b]
0%
CuI-TEMPO
CuI-2,2’-bipy
CuI-1,10-phen
CuBr
CuSPh
CuBr2
CuI
CuI
CuI
CuI
CuI
67%
60%
57%
48%
41%
35%
77%
83%
60%
33%
0%
0
1
2
3
4
CH CN (80)
3
DMF (100)
DMSO (100)
THF (65)
–
0%
[
[
a]
The reaction was run under nitrogen atmosphere
No TBD.
b]
Based on our previous work on copper-catalyzed
reactions of benzyl thiols and alcohols, optimization
[
10]
Figure 1. The substrate scope for sulfinate formation.
was performed on the reaction of thiophenol 1a and
benzyl alcohol 1b in the presence of CuI (5 mol%) cohols react with thiophenols to afford the desired
and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, sulfinates 2c and 3c in 90% and 85% yield, respective-
0 mol%) at 1008C under 1 atm of oxygen (see ly. The reaction using electron-rich methoxy-substitut-
1
Table 1). A reaction mixture of 1a and 1b was subject- ed benzyl alcohol shows a decreased yield, compared
ed to the above conditions to provide 1c in 74% yield to halogen- and methyl-substituted alcohols. In the
(
Table 1, entry 1). In the absence of oxygen, the de- case of aliphatic alcohols, the amount of TBD was in-
sired coupling did not occur (Table 1, entry 1). When creased to 1 equivalent. Heptanol and pentanol are
the reaction is conducted without TBD, 1c is not ob- efficiently transformed to 6c and 7c in 71% and 74%
tained (Table 1, entry 2). Other than TBD, the strong yield, respectively. An aliphatic alcohol possessing
organic bases 1,8-diazabicycloundec-7-ene, 1,1,3,3-tet- a phenyl group affords 8c in 66% yield. The use of
ramethylguanidine, and 1,3-diphenylguanidine, afford sterically hindered cyclohexanol results in a decreased
1
c in 13%, 3%, and 1% yield, respectively. The inor- yield (56%). In addition to thiophenol, methyl-,
ganic base KO-t-Bu does not promote this transfor- fluoro-, and methoxy-substituted thiophenols also re-
mation. To modulate the oxidation activity of copper acted with benzyl alcohols to form the desired prod-
salts, catalytic amounts (5 mol%) of 2,2,6,6-tetrame- ucts 10c, 11c, and 12c in 62%, 91%, and 67% yield,
thylpiperidinyl 1-oxy (TEMPO), 2,2’-bipyridine (2,2’- respectively. The reactions of aliphatic thiols with
[10]
bipyr), and 1,10-phenanthroline (1,10-phen) were benzyl alcohols were conducted. As reported by us,
used, but the yield of 1c did not increase (Table 1, en- benzyl mercaptan undergoes the oxidation to form
tries 3–5). Next, copper salts were checked; CuBr, thioaldehydes, which react with benzyl alcohols to
CuSPh, and CuBr catalyzed the reaction to provide afford benzyl benzoates via oxidative esterification
2
1
c in 48%, 41%, and 35% yield, respectively followed by S-O exchange. A less reactive aliphatic
(
Table 1, entries 6–8). The effect of solvents was also thiol, octyl thiol was subjected to the optimized condi-
investigated. Dioxane, THF, CH CN, DMF, and tions to form only dioctyl disulfide. Heteroaromatic
3
DMSO were employed in this reaction (Table 1, en- thiols (2-mercaptobenzimidazole, 2-mercaptopyrimi-
tries 9–13). The reaction in THF shows the highest dine, and 4-mercaptopyridine) were exposed to the
yield even at a lower reaction temperature (Table 1, optimized conditions. 2-Mercaptobenzimidazole did
entry 10). In the absence of copper salts, the forma- not participate in the reaction, to recover starting ma-
tion of 1c does not occur (Table 1, entry 14).
terials. 2-Mercaptopyrimidine and 4-mercaptopyridine
The substrate scope of the reaction was also ex- form thioethers in low yields (see the Supporting In-
plored (see Figure 1). Halogen-substituted benzyl al- formation).
Adv. Synth. Catal. 2016, 358, 56 – 61
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57

Pranab K. Shyam et al.
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Table 2. Examples of the thiosulfonates.
Entry
R
Yield
1
2
3
4
5
C H5
p-ClC H4
p-FC H4
p-CH C H
85% (1d)
81% (2d)
89% (3d)
76% (4d)
86% (5d)
6
6
6
3
6
4
p-CH OC H
4
3
6
Scheme 3. Reactions of disulfide, thiosulfonate, and thiosul-
finate with benzyl alcohol.
Scheme 2. Reactions of thiophenol and octyl thiol.
During the reaction of thiols and alcohols, we ob-
served thiosulfonates as minor products, which
become the major products in the absence of alcohols.
As shown in Table 2, various aromatic thiols undergo
oxidative dimerization to form thiosulfonates (1d–5d)
in good yields. In addition to dimerization of thiols,
a cross-coupling reaction using thiophenol (1 equiv.)
and octylthiol (1 equiv.) was conducted (Scheme 2).
Scheme 4. Control experiments.
Three different thiosulfonates are formed by homo- ipate in the reaction with alcohols after being dissoci-
dimerization and cross-coupling reaction under the in- ated into the corresponding thiophenol by copper cat-
[12]
dicated conditions. The homo-dimerization of octyl alysts.
thiols to form thiosulfonates in the absence of thio- To explore the reaction mechanism further, 2,2,6,6-
phenols was attempted, but dioctyl disulfides were ob- tetramethylpiperidinyl 1-oxyl (TEMPO) as a radical
served. scavenger was added to the reaction mixture
Because the CuI-catalyzed reaction of thiols pro- (Scheme 4). Instead of forming 1c, diphenyl disulfides
[13] 18
vides sulfinates in the presence of alcohols, and thio- were formed.
O-Labelled benzyl alcohol was used
sulfonates in the absence of alcohols, it is proposed to probe the source of oxygen incorporated into the
that sulfinates might be formed by the reaction of oxi- S=O bonds, to confirm that oxygen from O is incor-
2
dized thiol intermediates with alcohols (Scheme 3). porated into the S=O bond.
First, diphenyl disulfide 1a’ was employed under the
In accordance with the control experimental results,
optimized reaction conditions to form 1c in 36% a reaction mechanism is proposed in Scheme 5. The
yield. Thiosulfonate 1d, which was isolated from the reaction begins with the copper-catalyzed oxidation of
[2,5m]
reaction in the absence of alcohols, was also subjected thiophenol to a sulfinyl radical.
The sulfinyl radi-
to the catalytic reaction and found to show no sulfi- cal may form a sulfinyl-Cu(II) complex. The sulfinyl-
nate formation. Finally, thiosulfinate 1e, which was Cu(II) complex undergoes SÀO bond formation with
synthesized independently, reacts with benzyl alcohol alcohols to form 1c. The sulfinyl radical dissociated
[
11]
1
b to form 1c in 7% yield. According to these ex- from the Cu(II) complexes undergoes dimerization to
periments, sulfinate 1c is not formed from either thio- afford 1,2-disulfoxides. The unstable 1,2-disulfoxide
sulfonates or thiosulfinates. Disulfide 1a’ might partic- isomerizes to 1d. The formation of 1d along with 1c
5
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COMMUNICATIONS Copper-Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals
urements, the copper-catalyzed reaction of 1a and 1b
was quenched after 6 and 10 h, respectively, by cool-
ing the reaction cell in liquid nitrogen. Both EPR
spectra shown in Figure 2 are very similar, except
with respect to the absolute intensity, suggesting
a change in the population of the Cu(II) species only.
The detailed spectral features clearly indicate the
1
presence of a paramagnetic species with S= = , local-
2
ized to a Cu(II) ion with I=3/2. The appearance of
a series of peaks at 315, 296 and 278 mT originates
from a high axial nuclear hyperfine constant (A ) of
z
the nucleus, as well as a large g value. The A and g
z
z
z
values determined from the simulated EPR spectrum
are 585 MHz and 2.254, respectively. The large Az
value is due to the characteristically large hyperfine
coupling along gz of the Cu(II) ion. In addition,
Scheme 5. Reaction mechanisms to form 1c and 1d.
implies the existence of sulfinyl radicals in this aero- a value of g larger than g (or g ) is largely associated
z
x
y
[
3]
2
bic coupling reaction. The mass spectrum of the re- with a stabilization of the d orbital, possibly through
action mixture using 1a also shows the fragments of a lengthening of the bond along the z axis,
sulfinyl species (see the Supporting Information). The known for Cu(H O)6 (g =2.4).
z
[
14]
as is
2+
[15]
The g value is
2
z
z
electron paramagnetic resonance (EPR) spectra strongly dependent on the local bonding environment.
shown in Figure 2 provide evidence of copper(II) spe- Sulfur coordination has been reported to lower the g
z
[5g]
cies possessing a sulfinyl ligand. For the EPR meas- value to 2.16. The measured g value of 2.254 in our
z
case, is quite close to that of sulfur coordinated
Cu(II) complexes, but is definitely higher than the
case of sulfur alone. Thus, it is inferred that the mea-
sured g value is due to the binding of the Cu(II) ion
z
to a sulfinyl radical, which is represented as two reso-
nance forms (Scheme 5).
In conclusion, the direct copper-catalyzed aerobic
oxidation of thiolphenols to generate sulfinyl radicals
was investigated. Compared to thiyl radicals (RSC),
sulfinyl radicals (RSOC) are not often observed during
catalytic oxidative reactions. In this study, the judi-
cious choice of copper catalysts and TBD additives al-
lowed the trapping of sulfinyl radicals to afford sulfi-
nates and thiosulfonates in modest to good yields.
The presence of sulfinyl radicals was supported by
mass spectral and EPR analysis. The involvement of
the sulfinyl-Cu(II) complex during the reaction was
supported by EPR analysis.
Experimental Section
Representative Procedure for the Synthesis of 1c
A mixture of thiophenol (159 mL, 1.5 mmol), benzyl alcohol
(52 mL, 0.50 mmol), CuI (4.8 mg, 0.025 mmol) and 1,5,7-tri-
azabicyclo[4.4.0]dec-5-ene (TBD) (7.1 mg, 0.05 mmol) in
THF (1 mL) was stirred at 658C for 18 h under 1 atmos-
phere of oxygen. The solvent was removed with a rotary
evaporator, and the residue was purified by column chroma-
tography on a silica gel eluting with hexane and ethyl ace-
Figure 2. EPR spectra taken from the reaction mixture after
quenching at 6 and 10 h, respectively. Also shown is the si-
mulated EPR spectrum with S=1/2 and I=3/2 for Cu(II)
tate (hexane: ethyl acetate=98:2) to afforded 1c; yield:
1
96.5 mg (83%). H NMR (CDCl , 400 MHz): d=7.76–7.73
3
(2H, m), 7.55–7.51 (3H, m), 7.34–7.24 (5H, m), 5.05 (1H, d,
[16]
13
species using EasySpin.
J=11.6 Hz), 4.58 (1H, d, J=11.6 Hz); C NMR (CDCl3,
Adv. Synth. Catal. 2016, 358, 56 – 61
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59

Pranab K. Shyam et al.
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1
6
00 MHz): d=144.5, 135.3, 132.2, 129.0, 128.5, 128.4, 125.3,
6.0; HR-MS (EI): m/z=232.0557 ([M] ), calcd. for
Vaidya, G. E. Garrett, D. A. Pratt, Org. Biomol. Chem.
2011, 9, 3320–3330; k) R. M. Mallorquin, G. Vincent, E.
Derat, M. Malacria, J.-P. Goddard, L. Fensterbank,
Aust. J. Chem. 2013, 66, 346–353; l) C. Chatgilialoglu,
in: The Chemistry of Sulphones and Sulphoxides, (Eds.:
S. Patai, Z. Rappoport, C. J. M. Stirling), Wiley & Sons,
Ltd., Chichester, New York, Brisbane, Toronto, Singa-
pore, 1988, pp 1081–1087.
+
C H O S: 232.0558; FT-IR (neat): n=3062, 2945, 1598,
1
1
3
12
2
À1
476, 1132 cm .
Supporting Information
Detailed experimental procedures and spectra of products
are provided in the Supporting Information.
[
4] For recent review articles on copper-catalyzed aerobic
oxidative coupling reactions, see: a) W. Shi, C. Liu, A.
Lei, Chem. Soc. Rev. 2011, 40, 2761–2776; b) K.
Hirano, M. Miura, Chem. Commun. 2012, 48, 10704–
Acknowledgements
10714; c) S. E. Allen, R. R. Walvoord, R. Padilla-Sali-
This study was supported by the Korea CCS R&D Center
nas, M. C. Kozlowski, Chem. Rev. 2013, 113, 6234–6458.
(
KCRC) grant from the Korea Government (Ministry of
Education, Science and Technology; No.
015M1A8A1048886), C1 gas refinery program through Na-
tional Research Foundation of Korea (NRF) funded by the
Ministry of Science, ICT Future Planning (No.
015M3D3A1065436), and the Human Resources Develop-
[5] For selected articles on copper-catalyzed aerobic oxida-
tive coupling reactions, see: a) T. Hamada, X. Ye, S. S.
Stahl, J. Am. Chem. Soc. 2008, 130, 833–835; b) Y. Guo,
G. Wang, L. Chen, P. Xu, Y. Zhao, Y. Zhou, L.-B. Han,
J. Am. Chem. Soc. 2009, 131, 7956–7957; c) L. Chu, F.-
L. Qing, J. Am. Chem. Soc. 2010, 132, 7262–7263; d) S.
Guo, B. Qoan, Y. Xie, C. Xia, H. Huang, Org. Lett.
2011, 13, 522–525; e) X. Jin, K. Yamaguchi, N. Mizuno,
Chem. Commun. 2012, 48, 4974–4976; f) L. Wang, H.
Huang, D. L. Priebbenow, F.-F. Pan, C. Bolm, Angew.
Chem. 2013, 125, 3562–3564; Angew. Chem. Int. Ed.
2
&
2
ment of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant funded by the Korea govern-
ment Ministry of Trade, Industry
0154010200820).
&
Energy (No.
2
2
013, 52, 3478–3480; g) Y. Yang, W. Dong, Y. Guo,
R. M. Rioux, Green Chem. 2013, 15, 3170–3175; NÀP
References
bond: h) J. Fraser, L. J. Wilson, R. K. Blundell, C. J.
Hayes, Chem. Commun. 2013, 49, 8919–8921; PÀS
[
1] a) A. Attar, W. H. Corcoran, Ind. Eng. Chem. Prod.
Res. Dev. 1976, 17, 102–109; b) F. Zannikos, E. Lois, S.
Stournas, Fuel Process. Technol. 1995, 42, 35–45;
c) D. J. Procter, J. Chem. Soc. Perkin Trans. 1 2001,
bond: i) G. Kumaraswamy, R. Raju, Adv. Synth. Catal.
2
014, 356, 2591–2598; NÀS bond: j) N. Taniguchi, Syn-
lett 2007, 1917–1920; k) N. Taniguchi, Eur. J. Org.
Chem. 2010, 2670–2673; l) X. Tang, L. Huang, C. Qi, X.
Wu, W. Wu, H. Jiang, Chem. Commun. 2013, 49, 6102–
3
7
1
2
35–354; d) D. Y. Sorokin, Microbiology 2003, 72, 725–
39; e) P. De Filippis, M. Scarsella, Energy Fuels 2003,
7, 1452–1455; f) T. Rohwerder, W. Sand, Eng. Life Sci.
007, 7, 301–309; g) E. Wojaczy n´ ska, J. Wojaczy n´ ski,
6104; m) X. Huang, J. Wang, Z. Ni, S. Wang, Y. Pan,
Chem. Commun. 2014, 50, 4582–4584 (copper-mediated
reaction); n) C. Lee, X. Wang, H.-Y. Jang, Org. Lett.
Chem. Rev. 2010, 110, 4303–4356.
2
015, 17, 1130–1133; SÀS bond: o) N. Taniguchi, Eur. J.
[
2] For selected articles on sulfinyl radical formation from
thiol oxidation, see: a) M. D. Sevilla, D. Becker, S.
Swarts, J. Herrington, Biochem. Biophys. Res.
Commun. 1987, 144, 1037–1042; b) C. C. Winterbourn,
D. Metodiewa, Free Radical Biol. Med. 1999, 27, 322–
Org. Chem. 2014, 5691–5694.
[
[
6] For reviews on the synthesis and applications of sulfi-
nates, see: a) I. Fernµndez, N. Khiar, Chem. Rev. 2003,
103, 3651–3705; b) M. T. Robak, M. A. Herbage, J. A.
Ellman, Chem. Rev. 2010, 110, 3600–3740.
328; c) M. Benrahmoune, P. ThØrond, Z. Abedinzadeh,
7] For the recent synthetic applications of sulfinate esters,
see: a) F. Yuste, A. H. Linares, V. M. Mastranzo, B.
Ortiz, R. Sµnchez-Obregón, A. Fraile, J. L. G. Ruano, J.
Org. Chem. 2011, 76, 4635–4644; b) J. L. G. Ruano, A.
Parra, L. Marzo, F. Yuste, V. M. Mastranzo, Tetrahe-
dron 2011, 67, 2905–2910.
[8] For recent enantioselective sulfinyl ester syntheses, see:
a) J. W. Evans, M. B. Fierman, S. J. Miller, J. A. Ellman,
J. Am. Chem. Soc. 2004, 126, 8134–8135; b) H. M.
Peltier, J. W. Evans, J. A. Ellman, Org. Lett. 2005, 7,
1733–1736; c) S. Nakamura, M. Tateyama, H. Sugimo-
to, M. Nakagawa, Y. Watanabe, N. Shibata, T. Toru,
Chirality 2005, 17, 85–88; d) M. K. Syed, M. Casey, Eur.
J. Org. Chem. 2011, 7207–7214; e) Y. Zhang, S. Chitale,
N. Goyal, G. Li, Z. S. Han, S. Shen, S. Ma, N. Grinberg,
H. Lee, B. Z. Lu, C. H. Senanayake, J. Org. Chem.
2012, 77, 690–695.
Free Radical Biol. Med. 2000, 29, 775–782; d) B.
Cardey, M. Enescu, ChemPhysChem 2009, 10, 1642–
1648; e) L. Tan, Y. Xia, J. Am. Soc. Mass Spectrom.
2013, 24, 534–542.
[
3] a) F. Freeman, C. N. Angeletakis, W. J. Pietro, W. J.
Hehre, J. Am. Chem. Soc. 1982, 104, 1161–1165; b) F.
Freeman, Chem. Rev. 1984, 84, 117–135; c) Y. Guo,
W. S. Jenks, J. Org. Chem. 1995, 60, 5480–5486; d) Y.
Guo, W. S. Jenks, J. Org. Chem. 1997, 62, 857–864;
e) A. P. Darmanyan, D. D. Gregory, Y. Guo, W. S.
Jenks, J. Phys. Chem. A 1997, 101, 6855–6863; f) D. D.
Gregory, W. S. Jenks, J. Phys. Chem. A 2003, 107, 3414–
3
423; g) R. S. Grainger, B. Patel, B. M. Kariuki, Angew.
Chem. 2009, 121, 4926–4929; Angew. Chem. Int. Ed.
009, 48, 4832–4835; h) A. Latorre, I. López, V. Ramír-
2
ez, S. Rodríguez, J. Izquierdo, F. V. Gonzµlez, C.
Vicent, J. Org. Chem. 2012, 77, 5191–5197; i) J. A.
Souto, W. Lewis, R. A. Stockman, Chem. Commun.
[9] For selected articles on the synthesis of sulfinates, see:
2014, 50, 12630–12632; j) P. T. Lynett, K. Butts, V.
a) I. B. Douglass, J. Org. Chem. 1965, 30, 633–635;
6
0
asc.wiley-vch.de
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2016, 358, 56 – 61

COMMUNICATIONS Copper-Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals
b) I. B. Douglass, F. J. Ward, R. V. Norton, J. Org.
Chem. 1967, 32, 324–326; c) I. B. Douglass, J. Org.
Chem. 1974, 39, 563–564; d) R. B. Boar, A. C. Patel,
Synthesis 1982, 584–586; e) J. M. Klunder, K. B. Sharp-
less, J. Org. Chem. 1987, 52, 2598–2602; f) F. Toda, K.
Tanaka, T. Okada, J. Chem. Soc. Chem. Commun.
[11] For selected articles regarding oxidation of disulfides to
thiosulfinates, see: a) D. A. Cogan, G. Liu, K. Kim,
B. J. Backes, J. A. Ellman, J. Am. Chem. Soc. 1998, 120,
8011–8019; b) D. J. Weix, J. A. Ellman, Org. Lett. 2003,
5, 1317–1320; c) M.-T. Cai, G.-S. Lv, J.-X. Chen, W.-X.
Gao, J.-C. Ding, H.-Y. Wu, Chem. Lett. 2010, 39, 368–
369; d) M. Kirihara, S. Naito, Y. Ishizuka, H. Hanai, T.
1
1
995, 639–640; g) M. Xia, Z.-C. Chen, Synth. Commun.
997, 27, 1321–1326; h) T. J. Connolly, T. Durst, Tetra-
Noguchi, Tetrahedron Lett. 2011, 52, 3086–3089; e) K.
Bahrami, M. M. Khodaei, D. Khaledian, Tetrahedron
Lett. 2012, 53, 354–358; f) J. Su, Y. Qiu, K. Ma, Y. Yao,
Z. Wang, X. Li, D. Zhang, Z. Tu, S. Jiang, Tetrahedron
hedron Lett. 1997, 38, 1337–1340; i) S. M. Bonesi, M.
Fagnoni, A. Albini, J. Org. Chem. 2004, 69, 928–935;
j) S. Turcaud, T. Martens, E. Sierecki, J. PØrard-Viret, J.
Royer, Tetrahedron Lett. 2005, 46, 5131–5134; k) A. R.
Hajipur, A. R. Falahati, A. E. Ruoho, Tetrahedron Lett.
2
014, 70, 7763–7769.
[
12] For selected articles on metal-mediated disulfide bond
cleavage, see: a) R. D. Bach, S. J. Rajan, H. B. Vardhan,
T. J. Lang, N. G. Albrecht, J. Am. Chem. Soc. 1981, 103,
2
006, 47, 2717–2719; l) J. Coulomb, V. Certal, L. Fen-
sterbank, E. Lacôte, M. Malacria, Angew. Chem. 2006,
18, 649–653; Angew. Chem. Int. Ed. 2006, 45, 633–637;
7727–7734; b) A. Ichimura, D. L. Nosco, E. Deutsch, J.
1
Am. Chem. Soc. 1983, 105, 844–850.
13] Although adducts of TEMPO and sulfinyl radicals
m) D. Balcells, G. Ujaque, I. Fernandez, N. Khiar, F.
Maseras, J. Org. Chem. 2006, 71, 6388–6396; n) J. Dra-
bowicz, M. Kwiatkowska, P. Kiełbasi n´ ski, Synthesis
[
were formed in the absence of catalysts and O in the
2
following reference, the reaction of thiophenol and
TEMPO in the presence of copper catalysts and O2
provided diphenyl disulfides: P. Carloni, E. Damiani,
M. Iacussi, L. Greci, P. Stipa, Tetrahedron 1995, 51,
2008, 3563–3564; o) J. Coulomb, V. Certal, M.-H. Lar-
raufie, C. Ollivier, J.-P. Corbet, G. Mignani, L. Fenster-
bank, E. Lacôte, M. Malacria, Chem. Eur. J. 2009, 15,
10225–10232; p) R.-H. Zhu, X.-X. Shi, Tetrahedron:
12445–12452.
Asymmetry 2011, 22, 387–393; q) M. R. Maldonado, F.
Sehgelmeble, F. Bjarnemark, M. Svensson, J. Šhman,
P. I. Arvidsson, Tetrahedron 2012, 68, 7456–7462.
10] In our previous publication, the copper-catalyzed aero-
bic coupling of benzyl thiols and alcohols provided
esters, see: S. Lim, M. Ji, X. Wang, C. Lee, H.-Y. Jang,
Eur. J. Org. Chem. 2015, 591–595.
[
14] a) M. Valko, P. Pelikan, S. biskupic, M. Mazur, Chem.
Pap. 1990, 44, 805–813; b) B. J. Hathaway, D. E. Billing,
Coord. Chem. Rev. 1970, 5, 143–207.
[
[
[
15] W. B. Lewis, M. Alei, L. O. Morgan, J. Chem. Phys.
1
966, 45, 4003–4013.
16] S. Stoll, A. Schweiger, J. Magn. Reson. 2006, 178, 42–
5.
5
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