Transition-metal-free acid-mediated synthesis of aryl sulfides from thiols and thioethers

Article abstract of DOI:10.1021/jo402567b

The preparation of diaryl and alkyl aryl sulfides via acid-mediated coupling of thiols and thioethers with diaryliodonium salts is reported. The scope, limitations, and mechanism of the transformation are discussed.

Full text of DOI:10.1021/jo402567b

Note
pubs.acs.org/joc
Transition-Metal-Free Acid-Mediated Synthesis of Aryl Sulfides from
Thiols and Thioethers
Anna M. Wagner and Melanie S. Sanford*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S
* Supporting Information
ABSTRACT: The preparation of diaryl and alkyl aryl sulfides via acid-mediated coupling of thiols and thioethers with
diaryliodonium salts is reported. The scope, limitations, and mechanism of the transformation are discussed.
Scheme 1. Strategies for Aryl Sulfide Formation
he preparation of aryl sulfides constitutes an active and
Timportant area of synthetic organic chemistry because of
the extensive biological applications of compounds containing
Caryl−S bonds.1,2 For instance, aryl sulfides and aryl sulfoxides
are key components of commercial pharmaceuticals, including
Lansoprazole, Sulindac, Esomeprazole, and Quetiapine. Aryl
sulfides are also present in molecules that are used to treat
cancer, inflammation, asthma, Alzheimer’s disease, Parkinson’s
disease, and HIV.3−8 Finally, aryl sulfides can serve as
precursors to photoacid generators (PAGs), which are widely
used in paints, anticorrosives, microelectronics, and coatings.9,10
One of the most common routes to aryl sulfides involves the
transition-metal-catalyzed cross-coupling of aryl halides or
Initial investigations focused on the reaction of 1 equiv of
thioanisole (PhSMe) with 1 equiv of diphenyliodonium
trifluoroacetate (Ph2ITFA) to generate diphenyl sulfide
(Ph2S). We were pleased to find that with trifluoroacetic acid
(TFA) as the solvent, this reaction provided a 26% yield of the
desired product after 24 h at 100 °C (Table 1, entry 1). We
next examined the reaction in a variety of alternative solvents
using 5 equiv of TFA as an additive. As shown in entries 2−5 of
Table 1, these studies revealed that 1,4-dioxane was the optimal
cosolvent. In dioxane, the reaction proceeded efficiently in the
presence of 5 equiv of either TFA (entry 5) or NaTFA (entry
6). However, overall the yields were more reproducible using
TFA. Notably, replacing TFA with NaOAc or HOAc resulted
in reduced yields (entries 7 and 8). Further optimization of the
phenylation reaction showed that the best yields were obtained
with 8 equiv of TFA for 15 h at 110 °C to provide Ph2S in 77%
yield (entry 9). Finally, because copper catalysts have been used
in similar reactions,21 several Cu(I) and Cu(II) salts were
tested under our optimal conditions. As shown in entry 10, the
addition of 0.1 equiv of copper had a minimal impact on the
overall reaction yield. The reaction time for this transformation
could be reduced from 15 to 1.5 h by using a microwave reactor
(entry 11).
pseudo-halides with thiols under basic conditions.11−16
A
number of base-mediated, transition-metal-free reactions for
aryl sulfide synthesis have also been reported.17−25 While these
represent highly useful transformations, the vast majority of
current synthetic methods for aryl sulfide synthesis require at
least one of the following: (1) the use of strongly basic media,
(2) the use of transition-metal catalysts and/or mediators, (3)
the use of an inert atmosphere, and/or (4) the use of a thiol
substrate. These requirements represent disadvantages with
respect to functional group tolerance, ease of product
purification (particularly for biological applications), and
flexibility of synthetic strategies.
We report herein an orthogonal, metal-free method for the
synthesis of aryl sulfides that involves the reaction of either
RSH or RSR′ with diaryliodonium salts (Scheme 1), versatile
reagents that have found diverse applications.26 These reactions
proceed under acidic conditions, thereby offering the potential
for complementary substrate scope and functional group
tolerance relative to base and/or transition-metal-catalyzed
transformations. This new method offers additional advantages
of compatibility with ambient air and moisture as well as the
flexibility to use either thiols or thioethers as starting materials.
This report describes the optimization, scope, and mechanistic
investigations of this transformation.
Received: November 20, 2013
Published: February 19, 2014
© 2014 American Chemical Society
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Note
a
a
Table 1. Optimization of Thioanisole Phenylation
Table 2. Scope of Sulfides and Thioethers
e
entry
solvent
TFA
additive (equiv)
none
yield (%)
1
2
3
4
5
6
7
8
9
10
26
toluene
TFA (5)
36
acetic acid
DMF
TFA (5)
49
TFA (5)
59
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
TFA (5)
65
NaTFA (5)
NaOAc (5)
AcOH (5)
55
3
13
b
TFA (8)
77
c
TFA (8), [Cu] (0.1)
TFA (8)
65−78
83
d
11
a
One equivalent (0.093 mmol) of Ph2ITFA, 1 equiv (0.093 mmol) of
thioanisole, 5 equiv (0.47 mmol) of additive, 0.31 M in solvent at 100
b
°C for 24 h. Eight equivalents (0.74 mmol) of TFA at 110 °C for 15
c
h. [Cu] = Cu(OAc)2, Cu(OTf)2, Cu(II) acetylacetonate, CuCl2,
d
e
CuCN, or CuCl. MW conditions: 1.5 h, 120 °C, 140 W. Yields
determined by GC using hexadecane as the internal standard.
As shown in Table 2, a variety of thiols and thioanisole
derivatives are effective substrates for this transformation. With
alkyl aryl sulfides, the Csp3−S bond is cleaved exclusively,
resulting in diarylsulfide products (entries 2 and 3).
Thioanisoles bearing both electron rich and electron deficient
aryl groups showed good reactivity (entries 4−7). Bromide,
OH, and NH2 substituents were tolerated on the thioanisole
moiety. Importantly, in the latter two cases, no N- or O-
arylation products were detected. However, the aniline
functionality underwent trifluoroacetylation under the reaction
conditions. Primary and secondary alkyl thiols are also viable
substrates, providing alkyl aryl sulfide products in moderate
yields (entries 10 and 11). In contrast, when unsymmetrical
primary dialkyl sulfides [Csp3(primary)−S−Csp3(primary)] were used
as starting materials, mixtures of the two possible alkyl aryl
products were obtained. In the case of mixed secondary and
primary dialkyl sulfides [Csp3(primary)−S−Csp3(secondary)], there
was a preference for formation of the secondary alkyl aryl
sulfide product. However, again these transformations provided
lower yields.27 Finally, pyridine and quinoline-containing thiols
could also be converted to the analogous heterocyclic
thioethers using this method (entries 12−14). These types of
nitrogen heterocycles are often incompatible with transition-
metal-catalyzed C−S coupling reactions because of their strong
coordinating abilities.
The scope of this transformation was next assessed using a
series of different hypervalent iodine coupling partners. A series
of different Ar2ITFA reagents were prepared from the
corresponding aryl iodide and aryl boronic acid, using a
synthesis described by Olofsson and co-workers28 followed by
anion exchange.29 As summarized in Table 3, diaryliodonium
salts bearing both electron-withdrawing and -donating groups
participate in the arylation of thioanisole (entries 2−6). This
reaction also tolerates bromine-substituted diaryliodonium salts
(entries 6−8). For example, bis(4-bromophenyl) sulfide 15,
which represents a useful precursor to photoacid generators,
was obtained in high yield through the coupling of 4-
bromothioanisole with bis(4-bromophenyl)iodonium trifluor-
oacetate (entry 7). Finally, this reaction proceeds well using
diaryliodonium salts with meta substitution (entry 8). In
a
One equivalent (0.5 mmol) of Ph2ITFA, 1 equiv (0.5 mmol) of thiol
or thioether, 8 equiv (4.0 mmol) of TFA, 0.31 M in 1,4-dioxane at 110
b
°C for 15 h. Isolated yields. All results are an average of two runs.
contrast, reactions with ortho-substituted diaryliodonium salts
provided significantly lower yields (entry 9).
We propose that this reaction proceeds via the mechanism
proposed in Scheme 2. This sequence involves (1) initial
oxidation of the thioether or thiol with Ar2ITFA to generate
sulfonium salt A or B followed by (2) nucleophilic substitution
with trifluoroacetate to liberate the aryl sulfide product and
alkyl ester C. Importantly, there is precedent in the literature
that supports the viability of both of these steps. For example, it
has been shown that diaryliodonium salts can react with
substituted sulfides to produce sulfonium salts, albeit in the
presence of a copper catalyst.21,30 Furthermore, Takeuchi and
co-workers have hypothesized that in a related reaction, a
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Note
a
Table 3. Scope of Diaryliodonium Salts
Scheme 3. Reaction Mechanism Probes
to A, where R1 = H, Ar = Ph, and R = butyl, in Scheme 2) was
not detected in situ by HRMS, thus suggesting that it is short-
lived and that oxidation of the sulfide with Ar2ITFA is likely
rate-limiting. We also generated the sulfonium salt butyl
diphenyl sulfonium triflate S1 (Ph2SBuOTf)21 independently
and subjected it to our reaction conditions. This sulfonium salt
underwent rapid conversion to 1 and F3CCOO-nBu (in ≤30
min) under our reaction conditions (Scheme 3b).33,34 These
observations further support our mechanistic proposal.35
In conclusion, this paper demonstrates the arylation of thiols
and thioethers using diaryliodonium salts under acidic
conditions and provides evidence of a mechanism that proceeds
through a sulfonium salt intermediate. This reaction represents
a versatile and robust route to aryl thioether products and is
complementary to existing procedures.
EXPERIMENTAL SECTION
■
General. All reactions were conducted on the benchtop without
precautions to exclude air or moisture. All reagents were purchased
and used without further purification. Sulfonium salt S1 was
synthesized using the method described in ref 21. Symmetric
iodonium salts were synthesized using the methods described in refs
28 and 29.
a
One equivalent (0.5 mmol) of Ar1 ITFA, 1 equiv (0.5 mmol) of
2
thioether, 8 equiv (4.0 mmol) of TFA, 0.31 M in 1,4-dioxane at 110
°C for 15 h. Isolated yields. GC yield. All results are an average of
two runs.
b
c
General Procedure for Arylation. To a 20 mL scintillation vial
containing Ar2ITFA (1 equiv, 0.5 mmol) were added a thiol or
thioether (1 equiv, 0.5 mmol) and 1,4-dioxane (1.6 mL, 0.31 M). After
the contents had been mixed, trifluoroacetic acid (8 equiv, 4 mmol)
was added. The vial was sealed with a Teflon-lined cap, and the
reaction mixture was placed into one of the wells of an aluminum
block preheated to 110 °C. After being stirred for 15 h, the reaction
mixture was cooled to room temperature, and water (10 mL) was
added. The solution was extracted with diethyl ether (3 × 5 mL), and
the combined organic layers were washed with water (5 mL). The
organic layer was then dried over sodium sulfate and concentrated in
vacuo. The crude reaction mixture was purified by (1) flash column
chromatography on silica using hexanes or a hexanes/ethyl acetate
mixture or (2) vacuum short path distillation followed by passage
through a silica plug with ether. Yields reported in Tables 1−3
represent an average of two runs.
Scheme 2. Proposed Mechanism for Thioether Arylation
Diphenyl Sulfide (1). The crude product was purified by distillation
(bp = 296 °C at 760 Torr). Brown oil. From thiophenol and Ph2ITFA
(Table 2, entry 1; 77 mg, 83% yield); from thioanisole and Ph2ITFA
(Table 2, entry 2, and Table 3, entry 1; 72 mg, 77% yield); from butyl
phenyl sulfide and Ph2ITFA (Table 2, entry 3; 74 mg, 79% yield).
HRMS EI [M+] calcd for C12H10S, 186.0503; found, 186.0500. 1H and
13C NMR data matched those reported in the literature.36
Phenyl p-Tolyl Sulfide (2). The crude product was purified by
distillation (bp = 308 °C at 760 Torr). Brown oil. From methyl p-tolyl
sulfide and Ph2ITFA (Table 2, entry 4; 82 mg, 82% yield); from
thioanisole and (4-MeC6H4)2ITFA (Table 3, entry 3; 85 mg, 85%
yield). HRMS EI [M+] calcd for C13H12S, 200.0600; found, 200.0605.
1H and 13C NMR data matched those reported in the literature.36
4-Fluorophenyl Phenyl Sulfide (3). The crude product was purified
by column chromatography on a Biotage Isolera Flash Purification
System (Rf = 0.48 in hexanes). Colorless oil. From 4-fluorothioanisole
putative aminosulfonium salt intermediate can undergo
nucleophilic substitution with the conjugate base of a strong
acid.31
To test our mechanistic hypothesis, we prepared n-butyl
phenyl sulfide32 and, after subjecting it to our reaction
conditions, observed n-butyl trifluoroacetate and diphenyl
sulfide by GC−MS (Scheme 3a). Under these reaction
conditions, the putative sulfonium intermediate S1 (analogous
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Note
and Ph2ITFA (Table 2, entry 5; 79 mg, 77% yield); from thioanisole
and (4-FC6H4)2ITFA (Table 3, entry 4; 80 mg, 78% yield). 19F NMR
(471 MHz, CDCl3): δ −113.1. HRMS EI [M+] calcd for C12H9FS,
4-Methoxyphenyl Phenyl Sulfide (13). The crude product was
purified by column chromatography on a Biotage Isolera Flash
Purification System (Rf = 0.50 in a 10% EtOAc/90% hexanes mixture).
Colorless oil (40 mg, 37% yield). HRMS EI [M+] calcd for C13H12OS,
1
204.0409; found, 204.0405. H and 13C NMR data matched those
reported in the literature.36
216.0609; found, 216.0607. H and 13C NMR data matched those
1
4-Bromophenyl Phenyl Sulfide (4). The crude product was purified
by distillation (bp = 355 °C at 760 Torr). Brown oil. From 4-
bromothioanisole and Ph2ITFA (Table 2, entry 6; 76 mg, 57% yield);
from thioanisole and (4-BrC6H4)2ITFA (Table 3, entry 6; 94 mg, 70%
yield). HRMS EI [M+] calcd for C12H9BrS, 263.9608; found,
263.9611. 1H and 13C NMR data matched those reported in the
literature.36
reported in the literature.36
Phenyl 4-Trifluoromethylphenyl Sulfide (14). The crude product
was purified by column chromatography on a Biotage Isolera Flash
Purification System (Rf = 0.53 in hexanes). Colorless oil (72 mg, 57%
yield). 19F NMR (377 MHz, CDCl3): δ −62.3. HRMS EI [M+] calcd
for C13H9F3S, 254.0371; found, 254.0383. 1H and 13C NMR data
matched those reported in the literature.40
Bis(4-bromophenyl) Sulfide (15). The crude product was purified
by distillation (bp = 411 °C at 760 Torr). Clear oil (100 mg, 58%
yield). HRMS EI [M+] calcd for C12H8Br2S, 341.8713; found,
341.8721. 1H and 13C NMR data matched those reported in the
literature.41
3-Bromophenyl Phenyl Sulfide (5). The crude product was purified
by distillation (bp = 352 °C at 760 Torr). Brown oil. From 3-
bromothioanisole and Ph2ITFA (Table 2, entry 7; 71 mg, 54% yield);
from thioanisole and (3-BrC6H4)2ITFA (Table 3, entry 8; 73 mg, 55%
yield). HRMS EI [M+] calcd for C12H9BrS, 263.9608; found,
263.9611. 1H and 13C NMR data matched those reported in the
literature.37
ASSOCIATED CONTENT
■
4-Hydroxyphenyl Phenyl Sulfide (6). The crude product was
purified by column chromatography on a Biotage Isolera Flash
Purification System (Rf = 0.55 in a 25% EtOAc/75% hexanes mixture).
Brown oil (91 mg, 90% yield). HRMS EI [M+] calcd for C12H10OS,
S
* Supporting Information
Copies of NMR spectra of all compounds and experiments
used to assess the possibility of a radical mechanism. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
202.0452; found, 202.0449. H and 13C NMR data matched those
reported in the literature.38
2,2,2-Trifluoro-N-[4-(phenylthio)phenyl]acetamide (7). The crude
product was purified by flash column chromatography (Rf = 0.23 in a
10% EtOAc/90% hexanes mixture). Colorless oil (57 mg, 38% yield).
1H NMR (700 MHz, CDCl3): δ 9.51 (1H, br s), 7.50−7.51 (m, 2H),
7.25−7.35 (multiple peaks, 7H). 13C NMR (175 MHz, CDCl3): δ
154.7 (q, JC−F = 37 Hz), 135.0, 134.2, 133.9, 131.6 (q, JC−F = 26 Hz),
129.3, 127.6, 127.5, 121.1, 115.6 (q, JC−F = 290 Hz). 19F NMR (471
MHz, CDCl3): δ −75.7. HRMS electrospray (m/z): [M + H] calcd for
C14H11F3NOS, 298.0508; found, 298.0503.
AUTHOR INFORMATION
■
Corresponding Author
*mssanfor@umich.edu
Notes
The authors declare no competing financial interest.
Cyclohexyl Phenyl Sulfide (8). The crude product was purified by
column chromatography on a Biotage Isolera Flash Purification
System (Rf = 0.52 in hexanes). Colorless oil (54 mg, 56% yield).
HRMS EI [M+] calcd for C12H16S, 192.0973; found, 192.0971. 1H and
13C NMR data matched those reported in the literature.36
Octyl Phenyl Sulfide (9). The crude product was purified by flash
column chromatography (Rf = 0.43 in hexanes). Colorless oil (71 mg,
64% yield). HRMS EI [M+] calcd for C14H22S, 222.1442; found,
222.1439. 1H and 13C NMR data matched those reported in the
literature.24
Phenyl 2-Pyridyl Sulfide (10). Before extraction with diethyl ether,
2 M NaOH was added until the solution was basic. The crude product
was purified by distillation (bp = 321 °C at 760 Torr). Brown oil (52
mg, 56% yield). HRMS electrospray (m/z): [M + H] calcd for
C11H10NS, 188.0528; found, 188.0527. 1H and 13C NMR data
matched those reported in the literature.24
ACKNOWLEDGMENTS
■
This material is based upon work supported by U.S.
Department of Energy (Office of Basic Energy Sciences)
Grant DE-FG02-08ER 15997. A.M.W. thanks the National
Science Foundation for a predoctoral fellowship.
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(35) Experiments using the radical traps (2,2,6,6-tetramethylpiper-
idin-1-yl)oxidanyl (TEMPO) and butylhydroxytoluene (BHT) were
also performed. The results, which show that there is no inhibition by
these reagents, are included in the Supporting Information.
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