Efficient Cu-catalyzed base-free C-S coupling under conventional and microwave heating. A simple access to S-heterocycles and sulfides

Article abstract of DOI:10.3762/bjoc.9.50

S-aryl thioacetates can be prepared by reaction of inexpensive potassium thioacetate with both electron-rich and electron-poor aryl iodides under a base-free copper/ligand catalytic system. CuI as copper source affords S-aryl thioacetates in good to excellent yields, by using 1,10-phenanthroline as a ligand in toluene at 100°C after 24 h. Under microwave irradiation the time was drastically reduced to 2 h. Both procedures are simple and involve a low-cost catalytic system. This methodology was also applied to the "one-pot" synthesis of target heterocycles, such as 3H-benzo[c][1,2]dithiol-3-one and 2-methylbenzothiazole, alkyl aryl sulfides, diaryl disulfides and asymmetric diaryl sulfides in good yields.

Full text of DOI:10.3762/bjoc.9.50

Efficient Cu-catalyzed base-free C–S coupling under
conventional and microwave heating. A simple
access to S-heterocycles and sulfides
Silvia M. Soria-Castro and Alicia B. Peñéñory*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 467–475.
INFIQC, Departamento de Química Orgánica, Facultad de Ciencias
Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria,
X5000HUA. Córdoba, Argentina, Fax: (+54) 351-4333030
doi:10.3762/bjoc.9.50
Received: 10 December 2012
Accepted: 30 January 2013
Published: 04 March 2013
Email:
Alicia B. Peñéñory* - penenory@fcq.unc.edu.ar
Associate Editor: B. Stoltz
*
Corresponding author
©
2013 Soria-Castro and Peñéñory; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
catalysis; copper; cross-coupling; potassium thioacetate; sulfur
heterocycles
Abstract
S-aryl thioacetates can be prepared by reaction of inexpensive potassium thioacetate with both electron-rich and electron-poor aryl
iodides under a base-free copper/ligand catalytic system. CuI as copper source affords S-aryl thioacetates in good to excellent
yields, by using 1,10-phenanthroline as a ligand in toluene at 100 °C after 24 h. Under microwave irradiation the time was drasti-
cally reduced to 2 h. Both procedures are simple and involve a low-cost catalytic system. This methodology was also applied to the
“
one-pot” synthesis of target heterocycles, such as 3H-benzo[c][1,2]dithiol-3-one and 2-methylbenzothiazole, alkyl aryl sulfides,
diaryl disulfides and asymmetric diaryl sulfides in good yields.
Introduction
Aromatic sulfur compounds including sulfides and hetero- have previously described the reactivity of sulfur anions, such
cycles are relevant in chemical, pharmaceutical and materials as S2− [6], −SCNH(NH2) [7,8], and MeCOS− [9], in photoin-
industries [1-3]. As a consequence, the development of new duced aromatic radical nucleophilic substitution (SRN1 mecha-
methodologies for CAr–S bond formation still represents a chal- nism), as a “one-pot” method for the synthesis of several sulfur
lenge in organic chemistry (Figure 1).
aromatic compounds in moderate to good yields. A common
feature of these reactions concerns the formation of arylthiolate
Nucleophilic substitution of aryl halides by a variety of sulfur anions as intermediates, offering a viable alternative for the
anions, such as PhS−, 1-naphthylS−, S2−, thiourea anion introduction of sulfur functionalities in aryl compounds.
(
−SCNH(NH2)), thioacetate anion (MeCOS−) and thiobenzoate
anion (PhCOS−), can be effectively achieved by photoinduced The formation of C–S bonds can also be accomplished in good
electron transfer reactions to form new C–S bonds [4,5]. We to excellent yields by transition-metal catalysis [10,11], mainly
467

Beilstein J. Org. Chem. 2013, 9, 467–475.
Figure 1: Examples of some pharmaceutically and chemically important derivatives.
using palladium [12], copper [13-16], nickel [17,18] and iron Beaucage reagent and benzothiazole as well as sulfides by using
19,20] salts. Aryl coupling reactions employing different palla- Cu-catalysis to mediate the C–S bond formation. Herein, we
[
dium species as catalysts represent the most extensively studied report the reactions of KSCOMe (1) with aryl iodides under a
approaches to form new CAr–S bonds [21-23]. Nevertheless, Cu/ligand/base-free system to afford S-aryl thioacetates 2 as
copper or iron-mediated coupling reactions have become a con- thiol surrogates in good to excellent yields, under conventional
venient alternative to the expensive Pd/ligand systems due to heating and microwave irradiation as an efficient access to a
the lower cost of the former, its stability, and the ready avail- variety of sulfur compounds.
ability of the ligands. Thus, different copper-salt-based catalytic
systems have been found to be effective for the aryl coupling Results and Discussion
reactions of ArSH [24-29], RSO2Na [30], KSCN [31,32], Initially, we tried PhCOSH with PhI using the catalytic systems
NH2CSNH2 [33,34], MeCSNH2 [35], KSCSOEt [36,37], of CuI/1,10-phenanthroline in toluene according to the method
Na2S·9H2O [38,39] and sulfur [40,41] as well as the synthesis described by Sawada [52]. This methodology proved particu-
of diverse heterocycles containing sulfur atoms by intra- [42- larly sensitive to the nature and concentration of the base used
4
4] and intermolecular [45-47] reactions.
in the deprotonation of the phenyl thiobenzoic acid due to an
undesired hydrolysis of the thioester primary product. Thus, the
Aryl thioesters are versatile intermediates for the synthesis of a reaction with a PhI/PhCOSH/Cs2CO3 ratio of 1:1.5:2 afforded
variety of sulfur compounds including aryl thiols, aryl alkyl Ph2S without traces of the thioester, and with a ratio of 1:1.5:1.5
sulfides, aryl sulfenyl chlorides, aryl sulfonyl chlorides [48] and the PhSCOPh was isolated in only 30% yield. Finally, the for-
sulfonamides [49]. Thioacetate and thiobenzoate derivatives mation of PhSCOPh in our hands increased to 84% (quantified
have been synthesized by the reactions of thioacetate and by GC) by using DIPEA as base with a PhI/PhCOSH/DIPEA
thiobenzoate anions with arenediazonium tetrafluoroborates ratio of 1:1.2:2 (Sawada’s conditions) [52] (Scheme 1).
[
50]. However, this methodology implies moderate overall
yields and the handling of usually unstable diazonium salts.
More recently, S-aryl thioacetates have been obtained by the
catalyst system [Pd2(dba)3-Xantphos] in 1,4-dioxane under
microwave heating at 160 °C in good yields [51]. The synthesis
of arylthiobenzoates by a CuI/1,10-phenanthroline, starting
from thiobenzoic acid and iPr2NEt (DIPEA) in toluene after
2
4 h at 110 °C, has also been reported [52].
As part of our ongoing study on sulfur chemistry, we are inter-
ested in the one-pot synthesis of target heterocycles such as the
Scheme 1: Synthesis of S-phenyl benzothioate.
468

Beilstein J. Org. Chem. 2013, 9, 467–475.
Considering that the commercially available salt KSCOMe (1)
is a solid, is easy to handle, is less odorous, and avoids the use
of a relative expensive base with a lesser impact on the environ-
ment, as compared with the liquid PhCOSH, we continued our
study on the synthesis of sulfides by using salt 1. The model
reaction of PhI with KSCOMe was selected to determine the
optimal conditions (solvent and temperature) starting with the
effective CuI and 1,10-phenanthroline catalytic system previ-
ously employed (Scheme 2).
Table 1: Screening of solvents for the CuI-catalyzed reaction of potas-
sium thioacetate (1) with iodobenzene.a
entry solvent
temp. [°C]
time [h] 2a, yield (%)b
1
2
3
4
5
6
7
8
9
DMF
80
24
39
24
48
24
24
24
24
24
0
MeCN
80
<5c
<5c
0
DMSO
THF
100
67
Dioxane
Toluene
Toluene
Toluene
Toluene
100
100
100
100
100
6
34d
0
e
f
0
g
96
aReaction conditions: CuI (5 mol %, 0.025 mmol), 1,10-phenanthroline
10 mol %, 0.05 mmol), 1 (0.75 mmol), PhI (0.5 mmol), solvent (4 mL),
(
conventional heating. bQuantified by GC by the internal standard
method. cTogether with Ph2S and Ph2S2 as main products. dIsolated
yield. eUnder an air atmosphere. fIn the absence of ligand. gCuI
(10 mol %), 1,10-phenanthroline (20 mol %).
Scheme 2: Synthesis of S-phenyl thioacetate.
The use of the polar solvents DMF, MeCN and DMSO gave no the absence of a copper source, no reaction occurred, supporting
conversion at all or yielded only a trace amount of S-phenyl the participation of the copper species in the coupling reaction
thioacetate (2a) together with Ph2S and Ph2S2 from the in situ between thioacetate ion and PhI (Table 2, entry 7).
generation of benzenethiolate anion (Table 1, entries 1–3). In
addition, no reaction in THF occurred after 48 h of heating
Table 2: Screening of copper sources for the copper-catalyzed reac-
under reflux, whereas 2a was formed in low yield when the
tion of potassium thioacetate (1) with iodobenzene.a
reaction was performed in 1,4-dioxane, (Table 1, entries 4 and
entry
[Cu]
2a, yield (%)b
5
). Conversely, better results were obtained in nonpolar toluene
1
2
3
4
5
6
7
CuI
96
93
87
29
37
41
0
at 100 °C affording 34% isolated yield of 2a after 24 h. This
coupling reaction did not occur under air or in the absence of
the ligand and was improved to 96% yield by using 10 and
CuCl
CuCl2·2H2O
Cu(OAc)2
Cu(OTf)2
CuO
2
6
0 mol % of CuI salt and ligand, respectively, (Table 1, entries
–9). Similarly, a series of ligands usually employed in
copper-catalyzed cross-coupling reactions was also screened,
such as L-proline, benzotriazole, tetramethylethylenediamine
—
aReaction conditions: [Cu] (10 mol %, 0.05 mmol); 1,10-phenanthro-
(
TMEDA), dimethylethylenediamine (DMEDA), and acetylace-
line (20 mol %, 0.1 mmol), 1 (0.75 mmol), PhI (0.5 mmol), toluene
(
4 mL), 100 °C (conventional heating), 24 h. bQuantified by GC by the
tone, under the optimized reaction conditions with 1,10-phenan-
throline (Table 1, entry 9). These ligands, with different coordi-
nating and structural properties, gave no positive results, and
PhI was quantitatively recovered.
internal standard method.
A variety of commercially available aryl iodides were subjected
to the optimized reaction conditions (Table 2, entry 1) to deter-
The effect of the Cu(I) and Cu(II) salts on the coupling reaction mine the scope of this catalytic system for the synthesis of
was also evaluated, and the results are summarized in Table 2. S-aryl thioacetates 2, and the results are summarized in Table 3.
CuI and CuCl exhibited similar catalytic activity as did Cu(II)
chloride dihydrate (Table 2, entries 1–3). There is general As described in Table 3, the coupling of thioacetate with aryl
agreement that Cu(I) is the catalytic species in copper-catalyzed iodides bearing electron-withdrawing (EWGs) and electron-
arylation of C, N or O-nucleophiles, which can be generated donating (EDGs) groups proceeded in good to excellent yields
from Cu(II) or Cu(0) precursors by an in situ reduction or oxi- (63–96%). For instance, the para-methoxy and para-acetyl phe-
dation, respectively [53-55]. On the other hand, OTf− or OAc− nyl iodides afforded thioacetates 2c and 2e in 74% and 89%
counter ions decrease catalytic activity in Cu(II) salts, in com- isolated yield, respectively (Table 3, entries 3 and 5). In the
parison to chloride ions (Table 2, entries 4 and 5). Catalysis by absence of the catalytic system CuI/1,10-phenanthroline, the
CuO also affords moderate yield (Table 2, entry 6). Finally, in aryl iodides bearing the para-acetyl, para-nitro and para-cyano
469

Beilstein J. Org. Chem. 2013, 9, 467–475.
Table 3: Copper-catalyzed reaction of aryl iodides with potassium thioacetate (1).a
entry
aryl iodide
product
2 yield (%)b method
A
B
1
2
3
PhI
96 (80)
92 (77)
p-MeC6H4I
p-MeOC6H4I
96 (84)
94 (74)
74 (69)
4
p-MeCONHC6H4I
(72)
5
6
7
8
9
p-MeCOC6H4I
p-NO2C6H4I
p-CNC6H4I
o-MeC6H4I
(89)
(73)
79 (69)
73
64 (58)
84 (71)
o-MeOC6H4I
(76)
(82)
75 (67)
1
0
1
3-C5NH4I
1
1-C10H7I
(75)
470

Beilstein J. Org. Chem. 2013, 9, 467–475.
Table 3: Copper-catalyzed reaction of aryl iodides with potassium thioacetate (1).a (continued)
1
2
3
p-C6H4I2
(63)
—
99 (70)
1
PhBr
—
aReaction conditions: CuI (10 mol %, 0.05 mmol); 1,10-phenanthroline (20 mol %, 0.1 mmol), 1 (0.75 mmol), ArI (0.5 mmol); method A: toluene
4 mL), 100 °C (conventional heating), 24 h; method B: toluene (2 mL), microwave irradiation at 25 W, temperature between 110 °C and 140 °C, 2 h.
(
bQuantified by GC by the internal standard method and isolated yield between parentheses. cArI2:1 ratio of 1:2.
substituents, did not react with potassium thioacetate (1) after conventional and microwave heating (Table 3, entry 13), this
2
4 h in toluene at 110 °C. These observations rule out the possi- Cu-catalyzed C–S coupling reaction being highly chemoselec-
bility of an aromatic nucleophilic substitution to account for the tive for aryl iodides.
obtained results.
In comparison with the already reported procedure for the
Furthermore, sterically hindered ortho-substituted aryl iodides preparation of S-aryl thioacetates by Pd catalysis [51,56], the
afforded good yields of the coupling products 2 (Table 3, methodology herein described has the advantages of using a
entries 8 and 9). For example, the ortho- and para-methoxy aryl lower-cost copper salt and a stable and accessible ligand.
iodides rendered comparable isolated yields of thioacetate Furthermore, the use of a solid air-stable KSCOMe salt instead
derivatives 2c and 2i (Table 3, entries 3 and 9). The reaction of a highly inflammable liquid and corrosive PhCOSH and
between 4-iodoaniline and 1 under CuI/1,10-phenanthroline DIPEA [52] offered environmental advantages, due to the
catalysis afforded quantitatively the corresponding 4-iodoacet- absence of any additional base and the ease of handling.
anilide as the only product. The acetylation of the amino group Moreover the use of microwave irradiation with concomitant
competes effectively with the cross-coupling reaction. decrease of the reaction times to only 2 h is noteworthy.
A similar acetylation reaction was also observed with
2
-iodophenol. In consequence, protected amino and hydroxy Afterwards we applied this procedure to the synthesis of hetero-
groups were required for the copper-mediated C–S bond forma- cycles by the cross-coupling reaction of suitable o-substituted
tion to proceed properly (Table 3, entries 3, 4 and 9).
aryl iodides with KSCOMe. The reduced form of the Beaucage
reagent 6, a sulfurizing reagent used for the synthesis of phos-
This copper-catalyzed coupling reaction was also applied to phorothioate oligonucleotides [57], was synthesized by the reac-
heteroaromatic and other aromatics such as 3-iodopyridine and tion of 2-mercaptobenzoic acid (7) with KSCOMe (1), prob-
1
-naphthyl iodide, affording thioacetate derivatives 3 and 4 in ably through the intermediate 8 (Scheme 3) [58]. We envi-
good isolated yields. Interestingly, the reaction of 1,4- sioned that compound 6 could be easily achieved by a cascade
diiodobenzene with thioacetate rendered the corresponding process starting with the copper-mediated reaction of
disubstituted product 5, a straightforward precursor of 1,4- 2-iodobenzoic acid (9) with salt 1 to yield the thioacetate
benzenedithiol, in high isolated yield. As a result, the simple precursor of 7 (Scheme 4).
methodology here presented could be an attractive alternative
for the synthesis of polysulfides and other sulfur relevant ma-
terials (Table 3, entries 10–12).
In a comparative fashion we have also performed these reac-
tions under microwave irradiation in order to improve conver-
sion times and yields (Table 3, Method B). These reactions
occur in only 2 h with yields comparable to those under conven-
tional heating, for EWGs and EDGs (Table 3, entries 1, 3, 5, 7,
Scheme 3: Synthesis of 6 by reaction between 7 and 1.
9
and 12). Bromobenzene was unreactive towards 1 under both
471

Beilstein J. Org. Chem. 2013, 9, 467–475.
By optimizing the reaction conditions, it was possible to
improve the yield of 6. Hence, the copper-catalyzed reaction
between iodide 9 and 1 in a ratio of 1:2 rendered only hetero-
cycle 6 (53% isolated yield) in a one-pot procedure. (Increasing
the ratio from 1:2 to 1:3 did not increase the isolated yield of 6).
A possible mechanism to account for this involves a cascade of
reactions starting with a cross-coupling and followed by consec-
utive acyl transfers (Scheme 5).
We also explored the possibility of building a benzothiazole
ring by copper-catalyzed reaction of N-(2-iodophenyl)acet-
amide (13) with 1. Thus, together with benzothiazole 14, the
Scheme 4: Reaction of 2-iodobenzoic acid (9) with 1.
When a toluene solution of 9 and 1 in a ratio of 1:1.5 was reaction of 13 and 1 in a 1:1.5 ratio, in the presence of CuI/1,10-
allowed to react in the presence of CuI/1,10-phenanthroline, phenanthroline after 24 h of conventional heating, afforded low
a
3
2
mixture of 2-mercaptobenzoic acid (7) and amounts of the substituted thioacetate derivative S-(2-acetoami-
H-benzo[c][1,2]dithiol-3-one (6) was obtained in 10% and dophenyl)thioacetate (15) and N-(2-mercaptophenyl)acetamide
7% isolated yields, respectively (Scheme 4).
(16) or S-(2-aminophenyl)thioacetate (17) without cyclization,
as detected by GC–MS spectrometry (Scheme 6).
Compounds 7 and 6 derive from the previous C–S coupling
reaction. Thus, the expected 2-(acetylthio)benzoate (10) under- In order to accelerate the ring-closure reaction, a mixture of 13
goes a series of acyl transfers, and therefore, thioanhydride 12, and 1 in a 1:1.5 ratio was heated for 2 h under microwave
and consecutively compound 8, are formed. Irreversible oxi- irradiation, and 2-methylbenzothiazole (14) was isolated from
dation of the latter finally affords heterocycle 6. The free thiol 7 the reaction mixture in 50% as the only reaction product
can be also generated by acyl transfer from 2-mercaptoanhy- (Scheme 7). Increasing the amount of 1 (13:1 in a 1:3 ratio)
dride 11 (Scheme 5).
does not significantly increase the yield of 14 (57%).
Scheme 5: Suggested one-pot synthesis of heterocycle 6.
Scheme 6: Reaction of N-(2-iodophenyl)acetamide (13) with 1.
472

Beilstein J. Org. Chem. 2013, 9, 467–475.
commercial potassium thioacetate with aryl iodides under
microwave irradiation. Electron-donor and electron-acceptor
substituents are tolerated, and polysubstitution can also be
successfully accomplished.
Experimental
Representative experimental procedures for
Scheme 7: Synthesis of 2-methylbenzothiazole (14).
the Cu-catalyzed base-free C–S coupling
Method A: The reactions were carried out in a 10 mL two-
This methodology was finally applied to the one-pot three-step necked Schlenk tube, equipped with a nitrogen gas inlet, a
synthesis of alkyl aryl sulfides, diaryldisulfides and asymmetric condenser and a magnetic stirrer. The tube was dried under
diaryl sulfides. Thus, without isolation, the phenyl thioacetate vacuum, filled with nitrogen and then charged with dried
produced in good yield by the copper-catalyzed reaction of PhI toluene (4.0 mL). ArI (0.5 mmol), CuI (10 mol %), 1,10-
with thioacetate anion 1 was hydrolyzed to the benzene thiolate phenanthroline (20 mol %) and finally potassium thioacetate (1)
anion by the addition of 2 equiv of KOt-Bu. Different chemical (0.75 mmol) were added to the degassed solvent under nitrogen
transformations are possible from the arene thiolate ions and stirred at 100 °C for 24 h. The reaction mixture was cooled
formed; for example, oxidation to the diaryl disulfide, subse- to room temperature. Diethylether (20 mL) and water (20 mL)
quent nucleophilic substitution reaction with alkyl halides to were added and the mixture was stirred. The organic layer was
afford alkyl aryl sulfides, or a second copper-catalyzed reaction separated and the aqueous layer was extracted with diethylether
with a different aryl iodide to yield asymmetric diaryl sulfides. (2 × 20 mL). The combined organic extract was dried over
With this strategy we synthesized different sulfur compounds. Na2SO4, and the products were isolated by radial chromatog-
Accordingly, a solution of benzene thiolate obtained from this raphy from the crude reaction mixture or quantified by GC by
methodology affords bis(phenyl)disulfide (51%) after oxidation using the internal standard method. The identity of all the prod-
by KI/I2, by subsequent substitution reactions with methyl ucts was confirmed by 1H and 13C NMR and MS spectrometry.
iodide and benzyl bromide, methyl phenyl sulfide and benzyl All the thioacetate compounds prepared are known, and their
methyl sulfide in 81% and 66% yields, respectively, and data are in good agreement with those reported.
4
-anisyl phenyl sulfide (4-AnSPh) in 85% yield (quantified by
1
H NMR) by a second copper-catalyzed reaction (Scheme 8).
Method B: The reactions were carried out in a 10 mL glass
tube, filled with nitrogen and then charged with dried toluene
(
(
2 mL). ArI (0.5 mmol), CuI (10 mol %), 1,10-phenanthroline
20 mol %) and finally potassium thioacetate (1) (0.75 mmol)
were added to the degassed solvent under nitrogen. Then, the
tube was sealed with a rubber cap and heated to 110–140 °C for
2
h under microwave irradiation (Fixed Power, 25 W) by using
the SPS method. This method implies irradiation at 25 W to
bring the reaction mixture to 140 °C, then cycling of the power
on and off for the remaining run time (2 h) as the temperature
varies from 110–140 °C. After completion of the experiment,
the vessel was cooled to room temperature before removal from
the microwave cavity, and then opened to the atmosphere. The
work-up of the reaction was similar to that of Method A.
Scheme 8: One-pot three-step synthesis of sulfides. (a) KI/I2;
(b) BrCH2Ph; (c) MeI; (d) 4-AnI, CuI/1,10-phenanthroline (10 and
20 mol %, respectively), 100 °C, 24 h.
Conclusion
We have developed a simple, versatile, efficient and economi-
cally attractive procedure for the synthesis of sulfur hetero-
cycles and a variety of sulfides with good yields. This one-pot
methodology involves a cascade of reactions starting with a
Representative procedure for the one-pot
three-step synthesis of alkyl aryl sulfides,
diaryl disulfides and asymmetric diaryl
C–S bond formation by copper catalysis and followed by sulfides
consecutive acyl transfers, condensation, nucleophilic substitu- The reactions were carried out by using method A. After 24 h at
tion, oxidation or a second copper cross-coupling of the arene 100 °C, KOt-Bu (1 mmol, 2 equiv) was added to the reaction
thiolate anion intermediates from S-aryl thioacetate hydrolysis. mixture and stirred for 10 min. The corresponding alkyl
These thioacetates are obtained by copper-catalyzed reaction of halide (0.75 mmol, 1.5 equiv) or KI/I2 (1.5 mmol/0.51 mmol,
473

Beilstein J. Org. Chem. 2013, 9, 467–475.
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Acknowledgements
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This work was partly supported by Consejo Nacional de Inves-
tigaciones Científicas y Técnicas (CONICET) and Fondo de
Ciencia y Tecnología (FONCYT), Argentina. S. M. S.-C. grate-
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