Sonogashira reaction using arylsulfonium salts as cross-coupling partners

Article abstract of DOI:10.1021/acs.orglett.7b02764

Triarylsulfonium, alkyl- and fluoroalkyl(diaryl)-sulfonium, and aryl(dialkyl)sulfonium triflates are successfully used as a new family of cross-coupling participants in the Sonogashira reaction as aryldiazonium, diaryliodonium, and tetraphenylphosphonium salts. It was found that terminal alkynes reacted mildly with triarylsulfonium or (2,2,2-trifluoroethyl)diphenylsulfonium triflate at room temperature under Pdand Cu-cocatalysis to give the corresponding arylalkynes in up to >99% yield. This protocol represents the first use of arylsulfonium salts as cross-coupling partners in the Pd/Cu-catalyzed Sonogashira reaction.

Full text of DOI:10.1021/acs.orglett.7b02764

Letter
pubs.acs.org/OrgLett
Sonogashira Reaction Using Arylsulfonium Salts as Cross-Coupling
Partners
Ze-Yu Tian, Shi-Meng Wang, Su-Jiao Jia, Hai-Xia Song, and Cheng-Pan Zhang*
School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, 205 Luoshi Road, Wuhan 430070,
China
S
* Supporting Information
ABSTRACT: Triarylsulfonium, alkyl- and fluoroalkyl(diaryl)-
sulfonium, and aryl(dialkyl)sulfonium triflates are successfully
used as a new family of cross-coupling participants in the
Sonogashira reaction as aryldiazonium, diaryliodonium, and
tetraphenylphosphonium salts. It was found that terminal
alkynes reacted mildly with triarylsulfonium or (2,2,2-trifluoroethyl)diphenylsulfonium triflate at room temperature under Pd-
and Cu-cocatalysis to give the corresponding arylalkynes in up to >99% yield. This protocol represents the first use of
arylsulfonium salts as cross-coupling partners in the Pd/Cu-catalyzed Sonogashira reaction.
he Sonogashira cross-coupling is a very powerful and
which have been widely applied in the fields of coatings,
adhesives, photoresists, microfabrication, and patterning.10 The
aryl radicals derived from homolytic reduction of triarylsulfo-
nium salts underwent carbon−carbon bond formation with allyl
sulfones and activated olefins.11 Recently, a number of
arylsulfonium salts have been explored as powerful arylation
reagents in the Pd-catalyzed Suzuki−Miyaura cross-couplings
and Mizoroki−Heck reactions.12,13 However, the use of
arylsulfonium salts as the cross-coupling participants in the
Sonogashira reaction has never been known, despite aryl
pseudohalides such as diaryliodonium, aryldiazonium, and
arylphosphonium salts having been extensively exploited in this
type of reaction.6−8 Furthermore, arylsulfonium salts have
featured advantages such as nonvolatility, easy preparation,
nontoxicity, and good thermal stability.14,15 We wonder whether
arylsulfonium salts can be a new sort of the Sonogashira cross-
coupling agents in the presence of Pd- and Cu-catalysts.
T
convenient reaction for the construction of Csp−Csp2
bonds.1 Over the past few decades, this reaction has become
the most important approach to the construction of arylalkynes
and conjugated alkenynes, which are essential building blocks in
the preparation of natural products, macrocycles, pharmaceut-
icals, agrochemicals, and functional materials.2 To date, many
variations and optimizations have been made on the basis of the
original protocol,2−4 in which Heck and Sonogashira first
disclosed a Pd- or Pd/Cu-catalyzed reaction of a terminal alkyne
with an aryl (or vinyl) halide in the presence of a certain base.3 At
present, the substrate scopes of the Sonogashira reaction are
significantly expanded. Aryl pseudohalides, such as aryl tosylates,
diaryliodonium, and aryldiazonium salts, and tetraphenylphos-
phonium chloride as well as the complex alkynes have been
employed in the Sonogashira cross-coupling with encouraging
results (Scheme 1).2c,5−8
Indeed, the Pd(PPh3)4-catalyzed reaction of 1-ethynyl-4-
methoxybenzene 1a with ethyl(diphenyl)sulfonium triflate
2a (1.5 equiv) in DMF in the presence of NEt3 (1 equiv) and
10 mol % CuI at 80 °C for 15 h gave 1-methoxy-4-
(phenylethynyl)benzene (3a) in 36% yield (Table 1, entry 1).
Further investigation showed that Pd[P(t-Bu)3]2 was a better
catalyst than Pd(PPh3)4, Pd(PPh3)2Cl2, Pd(OAc)2, Pd2(dba)3,
and Pd(PCy3)2, which afforded 3a in 81% yield (Table 1, entries
1−6). Remarkably, using K3PO4 instead of NEt3, the Pd[P(t-
Bu)3]2-catalyzed reaction of 1a and 2a in the presence of 10 mol
% of CuI furnished 3a in 96% yield (Table 1, entry 7). Other
inorganic bases such as NaHCO3, K2CO3, and K2HPO4 were also
amenable in the reaction but provided lower yields of 3a (Table
1, entries 8−10). DMF seemed to be the optimal solvent for the
cross-coupling as it performed in other solvents gave 3a in
moderate yields (Table 1, entries 11−13). When the catalyst
Scheme 1. Sonogashira Reactions Using Diaryliodonium,
Aryldiazonium, and Tetraphenylphosphonium Salts as the
Cross-Coupling Participants
On the other hand, arylsulfonium salts are versatile reagents
that have drawn great attention in organic chemistry and
materials science in the past several decades.9 They can be readily
used as efficient photoinitiators to trigger cationic polymer-
ization or as excellent photoacid generators to produce protons
under irradiation through single-electron-transfer processes,
Received: September 5, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02764
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Letter
Table 1. Pd/Cu-Catalyzed Cross-Coupling of 1a and 2a
Scheme 2. Various Phenylsulfonium Salts as the Sonogashira
Cross-Coupling Partners
a
a
entry
Pd catalyst
base
Et3N
solvent
DMF
3a, yield (%)
1
Pd(PPh3)4
36
68
24
32
30
81
2
Pd(PPh3)2Cl2
Pd(OAc)2
Et3N
DMF
3
Et3N
DMF
4
Pd2(dba)3
Et3N
DMF
5
Pd(PCy3)2
Et3N
DMF
6
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Pd[P(t-Bu)3]2
Et3N
DMF
b
7
K3PO4
NaHCO3
K2CO3
K2HPO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
DMF
96 (96 )
8
DMF
83
59
58
22
65
59
94
64
9
DMF
10
11
12
13
DMF
DMSO
1,4-dioxane
toluene
DMF
c
14
a
Reaction conditions: a mixture of 1a (0.2 mmol), phenylsulfonium
d
15
DMF
salt 2 (0.3 mmol), Pd[P(t-Bu)3]2 (10 mol %), CuI (10 mol %), K3PO4
(0.2 mmol), and DMF (2 mL) was reacted in a sealed tube for 15 h.
Yields were determined by HPLC using 3a as an external standard (tR
a
Reaction conditions: a mixture of 1a (0.2 mmol), 2a (0.3 mmol), CuI
(10 mol %), Pd catalyst (10 mol %), K3PO4 (0.2 mmol), and DMF (2
mL) was reacted at 80 °C in a sealed tube for 15 h. Yields were
determined by HPLC using 3a as an external standard (tR = 7.165 min,
λmax = 287 nm, H2O/CH3OH = 10:90). Isolated yield. CuI (7.5 mol
%), Pd catalyst (7.5 mol %). CuI (5 mol %), Pd catalyst (5 mol %).
b
= 7.165 min, λmax = 287 nm, H2O/CH3OH = 10:90). Isolated yield.
b
c
d
accordance with the observations that using these salts in the
Suzuki−Miyaura and Mizoroki−Heck reactions.12
With the optimized reaction conditions in hand (1/2a or 2e
(1.5 equiv)/Pd[P(t-Bu)3]2 (10 mol %)/CuI (10 mol %)/K3PO4
(1.0 equiv)/DMF/80 °C or rt/N2/15 h), the substrate scope of
the reaction was examined (Scheme 3). To our delight, various
terminal arylethynes 1b−p bearing either electron-donating
groups (e.g., methoxy, ethoxy, methyl, and tert-butyl moieties) or
electron-withdrawing groups (e.g., cyano, ester, keto, chloro,
fluoro, nitro, and formyl groups) on the phenyl rings were all
smoothly transformed under the standard conditions to give the
loadings of Pd[P(t-Bu)3]2/CuI were decreased from 10 mol
%/10 mol % to 7.5 mol %/7.5 mol %, a comparable yield (94%)
of 3a was obtained under the same conditions (Table 1, entry
14). Continuously reducing the catalyst loadings of Pd[P(t-
Bu)3]2/CuI from 7.5 mol %/7.5 mol % to 5 mol %/5 mol % led to
64% of 3a (Table 1, entry 15). The absence of either Pd[P(t-
Bu)3]2 or CuI considerably suppressed the reaction (Table S6),
suggesting that an assembly of Pd and Cu catalysts was essential
for the high efficiency of the transformation.
In addition to 2a, other alkyl(diphenyl)sulfonium salts were
studied (Scheme 2). It was found that methyl(diphenyl)-
sulfonium triflate 2b, diphenyl(propyl)sulfonium triflate 2c,
and butyl(diphenyl)sulfonium triflate 2d reacted with 1a in the
presence of Pd[P(t-Bu)3]2 (10 mol %), CuI (10 mol %), and
K3PO4 (1 equiv) to supply 3a in good yields at 40 or 120 °C
(Scheme 2 and Table S7). Triphenylsulfonium triflate 2e
appeared to be a more powerful phenylation reagent than
alkyl(diphenyl)sulfonium salts, providing >99% of 3a at room
temperature. Dialkyl(phenyl)sulfoniums like 2f, 2g, and 2h were
also suitable reagents for the reaction, which afforded 3a in 26%,
58%, and 72% yield, respectively, at 80 or 120 °C.16
Diphenyl(trifluoromethyl)sulfonium triflate 2i (the Yagupos-
kii−Umemoto’s reagent), a prevalent electrophilic CF3 transfer
source,17 and previously trifluoromethylating terminal alkyne in
the presence of CuI, 2,2′-bipyridine, and K2CO317c yielded 39%
or 44% of 3a in this reaction. Diphenyl(2,2,2-trifluoroethyl)-
sulfonium triflate 2j, a partially fluorinated analogue of 2a, which
was successfully applied in trifluoromethyl cyclopropanation of
olefins,18 afforded >99% or 93% of 3a in the Pd/Cu-catalyzed
reaction with 1a at room temperature or 80 °C. These results
suggested that 2j is a very reactive phenylation reagent
comparable to 2e in this transformation. Nevertheless, treatment
of (perfluoroethyl)diphenylsulfonium triflate 2k or (2,2-
difluoroethyl)diphenylsulfonium 2l with 1a under the same
conditions provided 3a in lower yields. These changes were in
Scheme 3. Sonogashira Reaction of 2a or 2e with Alkynes
a
Reaction conditions: a mixture of 1a (0.2 mmol), phenylsulfonium
salt 2 (0.3 mmol), Pd[P(t-Bu)3]2 (10 mol %), CuI (10 mol %), K3PO4
(0.2 mmol), and DMF (2 mL) was reacted in a sealed tube for 15 h.
b
Isolated yield. 2e was used instead of 2a at room temperature for 15
h. Isolated yield.
B
DOI: 10.1021/acs.orglett.7b02764
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Letter
desired phenylation products 3b−p in good to almost
quantitative yields. The position of substituent on the aryl
segment of arylethyne did not obviously affect the cross-
coupling. For instance, reactions of 1-ethynyl-3-methoxybenzene
1b and 1-ethynyl-2-methoxybenzene 1c with 2a at 80 °C
afforded 3b and 3c in 86−88% yields, which were comparable to
that of 3a from 1a under the same conditions. Alkylalkynes such
as but-3-yn-1-ylbenzene 1q, oct-1-yne 1r, and ethynylcyclopro-
pane 1s reacted with 2a or 2e at 80 °C or room temperature to
form 3q−s in 42−91% yields. Prop-2-yn-1-yl benzoate 1t and
(prop-2-yn-1-yloxy)benzene 1u were also applicable substrates
in the reaction, which gave 3t,u in 63−91% yields at room
temperature using 2e as a phenylation reagent. Notably, the
reaction of unprotected prop-2-yn-1-ol 1v or but-3-yn-2-ol 1w
with 2a at 80 °C furnished 3v in 92% yield or 3w in 76% yield.
Heteroarylethynes like 3-ethynylthiophene 1x and 3-ethynyl-1-
tosyl-1H-indole 1y reacted similarly with 2a or 2e to provide 88%
of 3x and 80% of 3y, indicating good availability of this method.
To further confirm the efficiency and practicality of the reaction,
complex molecules like mestranol 1z and erlotinib 1aa were
examined under the standard conditions (Scheme 4). Pleasingly,
Table 2. Symmetric and Asymmetric Arylsulfonium Salts as
the Cross-Coupling Participants
a
b
arylsulfonium salt
3, yield (%)
2m, Ar1 = Ar2 = R = 4-MeC6H4
2n, Ar1 = Ar2 = R = 4-BrC6H4
2o, Ar1 = Ar2 = R = 4-ClC6H4
2p, Ar1 = Ar2 = C6H5, R = 4-MeC6H4
3ab (Ar = 4-MeC6H4), 91
3ac (Ar = 4-BrC6H4), 46
3ad (Ar = 4-ClC6H4), 93
3a/3ab = 6.69:1, >99
2q, Ar1 = C6H5, Ar2 = 4-MeC6H4,
3a/3ab/3ac = 2.13:1:5.57, 76
R = 4-BrC6H4
2r, Ar1 = Ar2 = 4-MeC6H4, R= Et
3ab, 97
2s, Ar1 = Ar2 = 3,5-(Me)2C6H3, R = Et
2t, Ar1 = C6H5, Ar2 = 4-MeC6H4, R = Et
3ae (Ar = 3,5-(Me)2C6H3), 80
3a/3ab = 1.22:1, 87
a
Reaction conditions: a mixture of 1a (0.2 mmol), 2 (0.3 mmol),
Pd[P(t-Bu)3]2 (10 mol %), CuI (10 mol %), K3PO4 (0.2 mmol), and
DMF (2 mL) was reacted at room temperature (for 2m-q) or 80 °C
b
(for 2r-t) in a sealed tube for 15 h. Isolated yield. The molar ratio of
1
the product mixtures were determined by H NMR spectroscopy.
Scheme 4. Sonogashira Reaction with Complex Alkyne or
Sulfonium Salt
sulfonium 2r and bis(3,5-dimethylphenyl)(ethyl)sulfonium 2s
reacted with 1a at 80 °C, supplying, respectively, 3ab in 97% yield
and 3ae in 80% yield. When the asymmetric ethyl(phenyl)(4-
tolyl)sulfonium triflate 2t was treated with 1a, a mixture of 3a and
3ab was obtained in 87% yield with a molar ratio of 3a/3ab =
1.22:1, demonstrating again the easier transformation of the
phenyl group than the 4-tolyl moiety.
In addition, S-(aryl)tetramethylenesulfonium salts have been
successfully utilized as arylation reagents in the Pd-catalyzed
Suzuki−Miyaura reactions and intramolecular C−S/C-H
coupling.13 In our protocol, Sonogashira reaction of ethynyl-
benzene 1e with 1-(1-tosyl-1H-indol-3-yl)tetrahydro-1H-thio-
phen-1-ium 2u at 80 °C in the presence of Pd(OAc)2 (10 mol
%), CuI (10 mol %), X-Phos (10 mol %), and K3PO4 (1.0 equiv)
provided 3y in 82% yield (Scheme 4). The use of a combination
of Pd(OAc)2 and X-Phos instead of Pd[P(t-Bu)3]2 facilitated the
arylation, giving a better yield of 3y. This example affirmed once
again the versatility of arylsulfonium salts as the Sonogashira
cross-coupling partners.
In conclusion, we have verified that triaryl-, alkyl(diaryl)-, and
aryl(dialkyl)sulfonium salts can be used as versatile cross-
coupling participants in the Pd/Cu-catalyzed Sonogashira
reactions. Various terminal arylethynes and alkylalkynes,
including complex bioactive molecules, were smoothly converted
under the standard conditions to form the corresponding
arylation products in good to almost quantitative yields.
Advantages of this reaction include good functional group
tolerance, high reactivity, mild conditions, a wide range of cross-
coupling reagents, and easy access to the arylsulfonium salts.
Since triphenyl-, ethyl(diphenyl)-, and diethyl(phenyl)-
sulfonium triflates were all effectively transformed in the reaction
with alkyne (Scheme 2), each phenyl group of 2e and 2a could be
utilized by isolating the thioether byproducts (diphenyl sulfide
from 2e and ethylphenyl sulfide from 2a), transferring them to
the corresponding arylsulfonium salts (e.g., 2a and 2g), and again
phenylating the alkyne. This proposal would improve the atom
utilization of triaryl- and diarylsulfonium salts. Moreover,
diphenyl(fluoroalkyl)sulfonium triflates are also reliable phe-
nyl-transfer reagents, with 2j being the most powerful one. The
asymmetric triarylsulfonium and alkyl(diaryl)sulfonium triflates
reacted with alkyne, yielding mixtures of products in which the
reaction of 1z or 1aa with 2a in DMF in the presence of Pd[P(t-
Bu)3]2 (10 mol %), CuI (10 mol %), and K3PO4 (1.0 equiv) at 80
°C for 15 h afforded 3z in 83% yield or 3aa in 75% yield,
respectively.
Furthermore, symmetric and asymmetric arylsulfonium salts
were explored in the Pd/Cu-catalyzed Sonogashira reaction with
alkynes. As shown in Table 2, tris(4-tolyl)sulfonium, tris(4-
bromophenyl)sulfonium, and tris(4-chlorophenyl)sulfonium
triflates 2m−o reacted with 1a at room temperature to give
91% of 3ab, 46% of 3ac, and 93% of 3ad. The C−Br bond of 2n
was partially retained, and the C−Cl bond of 2o was completely
tolerated. Reaction of diphenyl(4-tolyl)sulfonium triflate 2p and
1a under the standard conditions provided a mixture of 3a and
3ab in >99% yield with a molar ratio of 3a/3ab = 6.69:1, which
1
was determined by H NMR spectroscopy. This observation
hints at a slower transfer of the electron-rich 4-tolyl group than
the phenyl moiety in 2p. Similarly, treatment of (4-
bromophenyl)(phenyl)(4′-tolyl)sulfonium 2q with 1a afforded
a mixture of 3a, 3ab, and 3ac in 76% yield with a molar ratio of
3a/3ab/3ac = 2:13:1:5.57, implying that the electron-deficient 4-
bromophenyl motif was transformed faster than the phenyl
group in 2q. Alkyl(diaryl)sulfoniums such as ethyl(di-4-tolyl)-
C
DOI: 10.1021/acs.orglett.7b02764
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products with the relatively electron-deficient aryl groups were
predominantly formed. This protocol is the first report on the use
of arylsulfonium salts as cross-coupling partners in the Pd/Cu-
catalyzed Sonogashira reaction with alkynes.19 Application of
arylsulfonium salts as promising arylation reagents in other
transformations is currently underway in our laboratory.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02764.
Experimental information, reaction condition optimiza-
tion, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
* E-mail: cpzhang@whut.edu.cn, zhangchengpan1982@
hotmail.com.
ORCID
Cheng-Pan Zhang: 0000-0002-2803-4611
Notes
The authors declare no competing financial interest.
(11) Donck, S.; Baroudi, A.; Fensterbank, L.; Goddard, J.-P.; Ollivier,
C. Adv. Synth. Catal. 2013, 355, 1477.
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Eur. J. 2016, 22, 6542. (b) Wang, S.-M.; Song, H.-X.; Wang, X.-Y.; Liu,
N.; Qin, H.-L.; Zhang, C.-P. Chem. Commun. 2016, 52, 11893. (c) Wang,
X.-Y.; Song, H.-X.; Wang, S.-M.; Yang, J.; Qin, H.-L.; Jiang, X.; Zhang,
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131, 10015. It was reported that the one-electron reduction of
aryl(dialkyl)sulfonium salts never gave S−Caryl bond splitting, whereas
reduction of diaryl(alkyl)sulfonium salts preferentially gave S−Caryl
bond cleavage. In this Sonogashira reaction, both types of arylsulfonium
salts were transformed by S−Caryl bond breakage.
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ACKNOWLEDGMENTS
■
C.-P.Z. thanks the Wuhan University of Technology, the
Fundamental Research Funds for the Central Universities, the
National Natural Science Foundation of China (21602165), the
“Chutian Scholar” Program from Department of Education of
Hubei Province (China), the “Hundred Talent” Program of
Hubei Province, and the Wuhan Youth Chen-Guang Project
(2016070204010113) for financial support.
DEDICATION
■
This work is dedicated to Professor John A. Gladysz at Texas
A&M University on the occasion of his 65th birthday.
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D
DOI: 10.1021/acs.orglett.7b02764
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