A highly chemoselective and practical alkynylation of thiols

Article abstract of DOI:10.1021/ja4044196

A thiol-alkynylation procedure utilizing the hypervalent iodine alkyne transfer reagent TIPS-ethynyl-benziodoxolone has been developed. This scalable reaction proceeds in five minutes at room temperature in an open flask using commercially available reagents. The scope of the reaction is broad, with a variety of phenolic, benzylic, heterocyclic, and aliphatic thiols undergoing alkynylation in excellent yield. The method is highly chemoselective as a vast array of functional groups are tolerated. The utility of the thiol-alkynylation in postsynthetic elaboration has been demonstrated through the facile installment of a fluorophore tag on a cysteine-containing peptide.

Full text of DOI:10.1021/ja4044196

Communication
pubs.acs.org/JACS
A Highly Chemoselective and Practical Alkynylation of Thiols
Reto Frei and Jerome Waser*
́
̂
́ ́
Laboratory of Catalysis and Organic Synthesis, Ecole Polytechnique Federale de Lausanne, EPFL SB ISIC LCSO, BCH 4306, 1015
Lausanne, Switzerland
S
* Supporting Information
Scheme 2. Previously Reported Methods for the Synthesis of
Thio-Alkynes (A, B) and Our New Approach (C)
ABSTRACT: A thiol-alkynylation procedure utilizing the
hypervalent iodine alkyne transfer reagent TIPS-ethynyl-
benziodoxolone has been developed. This scalable reaction
proceeds in five minutes at room temperature in an open
flask using commercially available reagents. The scope of
the reaction is broad, with a variety of phenolic, benzylic,
heterocyclic, and aliphatic thiols undergoing alkynylation
in excellent yield. The method is highly chemoselective as
a vast array of functional groups are tolerated. The utility
of the thiol-alkynylation in postsynthetic elaboration has
been demonstrated through the facile installment of a
fluorophore tag on a cysteine-containing peptide.
he development and implementation of highly modular
reactions that provide near quantitative yields over a wide
T
preactivated thiols or disulfide species.3 Other methods utilize
transition-metal catalysts, such as the copper-catalyzed carbon
sulfur coupling between terminal alkynes and disulfides,4 or as in
the elegant study by Yamaguchi, the use of catalytic rhodium to
achieve C-S bond formation by C-H and S-S bond metathesis.5
Alternatively, a range of processes utilize alkenyl6 or alkynyl7
halides bearing leaving groups that undergo elimination under
strongly basic conditions to furnish the desired thio-alkyne
(Scheme 2B). Consequently, the limited functional group
tolerance exhibited by these methods is not surprising as they
require harsh conditions, proceed via highly reactive inter-
mediates, or involve the use of sensitive catalytic systems.
The absence of a broadly applicable thiol-alkynylation process
in current literature is further highlighted by the nonexistence of
cysteine analogues derivatized with a terminal thio-alkyne at the
side chain.8 This motif would generate great interest as it could
serve as a unique point of inception to, for instance, study post-
translational modifications, install fluorophore or biotin tags for
biotechnological purposes and broadly enable bioconjugatio-
n.2a,b In general, thio-alkynes represent an ideal platform for drug
diversification in light of the versatility of existing acetylene
chemistry and, in particular, the extensively used copper-
catalyzed azide-alkyne cycloaddition (CuAAC)9 as well as the
privileged position of organosulfur compounds among top-
selling pharmaceutical drugs.1a Furthermore, the development of
a robust, efficient, and orthogonal thiol-alkynylation reaction is
particularly attractive to the field of material and polymer science
as an alternative tool to the current array of thiol-functionaliza-
tion reactions.2 In this context, we set out to investigate the
feasibility of more facile and experimentally practical methods to
substrate range with perfect chemoselectivity have a transforma-
tional effect in areas extending from material and polymer
science, to chemical biology, drug discovery, and small molecule
organic chemistry. These ideal reactions should also be
experimentally simple to perform, utilize readily available starting
materials, be insensitive to water and oxygen, and should not
require the use of expensive, sensitive, or toxic metal catalysts.
Driven by the exceptional reactivity of sulfur and its importance
in biology, medicine, and materials science,1 recent research
efforts targeted a series of thiol-based transformations including
thiol alkylations as well as the thiol-ene and thiol-yne reaction
among others (Scheme 1).2
Scheme 1. Thiol Functionalization for the Modification of
Drugs, Biomolecules, and Materials
Unlike the well-established S-Csp3 and S-Csp2 bond forming
processes, existing methods to construct S-Csp bonds are rare in
number and often lack generality or require harsh conditions.
Currently, the most common methods to form thio-alkynes
require a prefunctionalization of the thiol (Scheme 2A). These
methods are generally based on nucleophilic substitutions
between highly reactive lithium acetylide intermediates with
Received: May 3, 2013
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
gain access to thio-alkynes, with a particular focus on silyl
acetylenes as a direct gateway to the most versatile terminal
alkyne (Scheme 2C).
disulfide product. We found that 1,1,3,3-tetramethylguanidine
(TMG) was superior to triethylamine and aqueous cesium
carbonate or sodium hydroxide as base (entries 3−6). Similarly,
we discovered that dimethyl sulfoxide (DMSO) or protic
solvents, such as methanol and ethanol, produced unsatisfactory
levels of the oxidation side reaction, while tetrahydrofuran
(THF) was optimal (entries 6−9).14
Under the optimized conditions, 6 (1.1 equiv) was added in
one portion to a solution consisting of the corresponding thiol
(1.0 equiv), TMG (1.2 equiv), and THF at room temperature
and stirred for 5 min in an open flask to give the alkynylation
product 2 in quantitative yield (Scheme 3).15 The reaction was
not affected by the addition of water, which was required in some
cases for thiol-substrates producing THF insoluble salts with the
guanidine base.16 If desired, the formed 2-iodobenzoic acid
coproduct could also be recovered quantitatively and reused to
synthesize 6.17 The transfer of trimethylsilyl ethyne utilizing
benziodoxolone-derivative 7 was also successful, and alkynyla-
tion product 8 was obtained in 92% yield (Scheme 3A).
Consequently, silyl-protected terminal alkynes with varying
stability can be accessed. Due to its commercial availability, 6 was
chosen to investigate the scope and limitations of this newly
developed thiol-alkynylation reaction (Scheme 3).
Modulating the electronics of the benzylthiol did not change
the reaction outcome as both 9 and 10, bearing an electron-
donating methoxy and an electron-withdrawing chloride groups,
respectively, were obtained in excellent yields (Scheme 3A). A
furan heterocycle was also well tolerated to give product 11 in
97% yield. Furthermore, a highly hydrophobic aliphatic substrate
was alkynylated in quantitative yield (product 12). In addition,
aliphatic thiols bearing a hydroxy or carboxylic acid functionality
underwent the thio-alkynylation reaction with comparable
results (products 13 and 14), demonstrating that the method
was tolerant to nucleophilic oxygen and acidic hydrogens.
Due to the significance of aromatic thiols as important building
blocks in the synthesis of natural products and pharmaceutical
and medicinal compounds,18 a range of functionalized
thiophenols were then subjected to the optimized conditions
(Scheme 3B). The successful formation of products 15−23 in
89−99% yield indicated that substituents, such as halides
(products 16 and 17), methoxy and hydroxy groups (products
18 and 19), and protected and unprotected amines (products 20
and 21) as well as carboxylic acids and esters (products 22 and
23) were well tolerated. This broad functional group tolerance is
unprecedented in the field of thiol-alkynylation. Shifting our
focus to leading scaffolds of heterocyclic chemistry (Scheme 3C),
thiol-substituted thiophene (product 24), benzoxazole (product
25), benzimidazole (product 26), and benzothiazole (product
27) gave the desired thiol-alkynylation products in 85−99%
yields. It is noteworthy that these heterocycles are omnipresent
in medicinally relevant compounds, making their facile
alkynylation a valuable tool to establish a platform for further
diversification.19 Furthermore, polymers with incorporated
thiophene units play a crucial role in organic electronic materials,
making alkynylation product 24 a useful building block.20
As previously stated, a cysteine side chain derivatized with a
terminal thio-alkyne has not yet been reported.8 At the outset, it
was not clear if this was due to insufficient stability or the lack of
methods to access such compounds. Thus, the mild thiol-
alkynylation conditions permitted by 6 appeared very promising
for the functionalization of such challenging and highly
important substrates. In this context, N- and C-protected
cysteine derivatives were examined as starting materials (Scheme
Given the successful exploitation of hypervalent iodine alkyne
transfer reagents by our group and others,10 we speculated that
these electrophilic acetylide equivalents could also be applied to
the alkynylation of thiols. Unlike methods based on nucleophilic
acetylide reagents, electrophilic alkynylation does not require
prefunctionalization of the thiol with an activating group. We
envisaged that such an umpolung strategy would have the
potential to proceed efficiently under mild conditions. Indeed,
the mechanism of reactions involving the addition of
nucleophiles to alkynyliodonium species is known to proceed
via a fast succession of conjugate addition, α-elimination of the
aryliodide followed by a 1,2-shift;11 no long-living anionic or
radical intermediates are formed, which in turn could lead to a
broad functional group tolerance. However, such a thiol-
alkynylation approach has, to the best of our knowledge, only
been reported for the reaction between bis-alkynyl iodonium
salts and a phenyl thiolate anion to give bis-thioalkynes.12 The
absence of additional reports could be attributed to the oxidative
properties of hypervalent iodine reagents, which may lead to the
facile oxidation of thiols, resulting in undesired disulfides.
We began our study with benzylthiol (1) as our model
substrate while screening different alkynylation reagents under
mildly basic conditions (Table 1). Less reactive halogenoacety-
a
Table 1. Optimization of the Alkynylation Reaction
b
b
entry
4−6
base
solvent
yield 2
no reaction
<10%
yield 3
1
2
3
4
5
6
7
8
9
4
5
6
6
6
6
6
6
6
NEt3
NEt3
THF
THF
93%
46%
24%
26%
−
NEt3
THF
51%
74%
71%
>99%
81%
12%
80%
NaOH
Cs2CO3
TMG
TMG
TMG
TMG
THF
THF
THF
DMSO
MeOH
EtOH
15%
85%
17%
a
Benzylthiol (1, 0.40 mmol), alkyne transfer reagent (4−6, 0.44
mmol), base (0.48 mmol), solvent (5.0 mL), 23 °C, 5 min, open flask.
b
Isolated yield of spectroscopically pure product.
lenes, such as triisopropylsilyl ethynyl bromide (4), did not
undergo any reaction under these conditions (entry 1). The
more reactive alkynyliodonium salt 5 furnished the desired thio-
alkynylation (2), as minor product (entry 2). Our initial
reservations were confirmed as disulfide (3), resulting from
oxidation, was identified as a major product. However, to our
delight, we discovered that benziodoxolone-derived hypervalent
iodine reagent 1-[(triisopropylsilyl)ethynyl]-1,2-benziodoxol-
3(1H)-one (TIPS-EBX, 6) furnished 2 as the main product
(entry 3). This commercially available reagent is highly practical
as it is a bench stable solid allowing for simple handling and long-
term storage on the multigram scale.13 While further optimizing
our lead result, we discovered that the right choice of solvent and
base was crucial to minimizing formation of the undesired
B
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Journal of the American Chemical Society
Communication
Scheme 3. Scope of the Thiol-Alkynylation Reaction
3D). We were able to isolate the corresponding alkynylated
products 28 and 29 in 95% and 92% yield, respectively.
Moreover, the reaction is scalable, as 28 could also be obtained in
quantitative yield on the gram scale.16 Selective alkynylation of
the thiol group in the presence of a free amine also proceeded
efficiently to give alkynylation product 30, demonstrating that
protection of the basic aliphatic free N-terminus of peptides is
optional.
A subset of cysteine containing dipeptides was also alkynylated
successfully, furnishing alkynylation products 31−33 in 95−98%
yield. These results demonstrated that alkynylation of cysteine
was possible in the presence of tryptophan, tyrosine, and serine,
respectively. Finally, amino acid-derived marketed drug
captopril, an important angiotensin-converting enzyme inhib-
itor,21 was successfully converted to the corresponding thio-
alkyne 34 (Scheme 3E). The reaction proceeded in 90% yield in
the presence of the free carboxylic acid. This example
demonstrated the potential utility of this new thiol-alkynylation
reaction as a facile entry point to carry out drug diversification
studies.
A final competition experiment was carried out to assess the
extent of selectivity exhibited by 6 toward thiols (eq 1). For this
purpose, unprotected L-histidine and L-lysine (amino acids
bearing two of the most nucleophilic substituents present in
biomolecules) along with dipeptide 35 were subjected to the
optimized reaction conditions. The desired thiol-alkynylation
product 31 was still obtained in 97% yield, showcasing the
exceptional chemoselectivity of the developed reaction. It is also
noteworthy that during the reaction scope exploration, we
discovered that the thiol-alkynylation essentially takes place
within 30 s upon addition of 6, which was also the case for the
competition experiment (eq 1).
To highlight the utility of the developed thiol-alkynylation
process in postsynthetic elaboration of modified peptides,
alkynylation product 31 was desilylated using TBAF buffered
with acetic acid, furnishing the corresponding terminal alkyne in
91% yield. We found that the free cysteine-acetylene derivative
was stable as demonstrated by the complete lack of
decomposition observed after stirring a solution of this
compound in the presence of a pH = 7 buffer for several weeks
at room temperature. Furthermore, the introduction of a dansyl
fluorophore was accomplished via a standard CuAAC reaction to
give conjugate 36 in 93% yield.22 To the best of our knowledge,
this is the first example of a terminal thio-alkyne participating in
the powerful CuAAC process.23
C
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Journal of the American Chemical Society
Communication
(6) Examples: (a) Marchueta, I.; Montenegro, E.; Panov, D.; Poch, M.;
Verdaguer, X.; Moyano, A.; Pericas, M. A.; Riera, A. J. Org. Chem. 2001,
66, 6400. (b) Ni, Z.; Wang, S.; Mao, H.; Pan, Y. Tetrahedron Lett. 2012,
53, 3907.
(7) Ziegler, G. R.; Welch, C. A.; Orzech, C. E.; Kikkawa, S.; Miller, S. I.
J. Am. Chem. Soc. 1963, 85, 1648.
In conclusion, we have developed an operationally practical
and highly efficient thio-alkynylation reaction utilizing the
electrophilic alkyne transfer reagent 6. The mild reaction
conditions and high chemoselectivity allow for the alkynylation
of phenolic, benzylic, heterocyclic, aliphatic, and peptidic thiols
while tolerating a vast array of functional groups. Moreover, the
potential utility for postsynthetic elaboration of the obtained
alkynylated products was demonstrated through the first report
of a CuAAC involving a terminal thio-alkyne. The developed
method not only allows a highly efficient functionalization of
thiols but also provides access to unique alkynes with the
potential for further derivatization. Consequently, the thiol-
alkynylation presented herein has the potential to achieve a
privileged position as a novel tool for various applications in
synthetic chemistry, chemical biology, and material science.
Ongoing research efforts are aimed at investigating the
applicability of other electrophilic alkyne transfer reagents and
determining the precise mechanism of this thio-alkynylation
process. Further studies to uncover unprecedented applications
of terminal and silyl protected thio-alkynes are also progressing.
(8) Two examples of alkyl substituted thio-alkyne cysteine analogs:
(a) Carson, J. F.; Boggs, L. J. Org. Chem. 1965, 30, 895. (b) Kuligowski,
C.; Bezzenine-Lafollee, S.; Chaume, G.; Mahuteau, J.; Barriere, J. C.;
Bacque, E.; Pancrazi, A.; Ardisson, J. J. Org. Chem. 2002, 67, 4565.
(9) (a) Stang, P. J.; Diederich, F. Modern acetylene chemistry; VCH:
Weinheim, 1995. (b) Diederich, F.; Stang, P. J.; Tykwinski, R. R.
Acetylene chemistry: chemistry, biology, and material science; Wiley-VCH:
Weinheim, 2005. (c) Finn, M. G.; Fokin, V. V. Chem. Soc. Rev. 2010, 39,
1231.
(10) (a) Stang, P. J.; Boehshar, M.; Lin, J. J. Am. Chem. Soc. 1986, 108,
7832. (b) Ochiai, M.; Ito, T.; Takaoka, Y.; Masaki, Y.; Kunishima, M.;
Tani, S.; Nagao, Y. J. Chem. Soc. Chem. Comm. 1990, 118. (c) Brand, J. P.;
Charpentier, J.; Waser, J. Angew. Chem., Int. Ed. 2009, 48, 9346.
(d) Gonzalez, D. F.; Brand, J. P.; Waser, J. Chem.Eur. J. 2010, 16,
9457. (e) Nicolai, S.; Piemontesi, C.; Waser, J. Angew. Chem., Int. Ed.
2011, 50, 4680. (f) Shi, H.; Fang, L. C.; Tan, C. H.; Shi, L. L.; Zhang, W.
B.; Li, C. C.; Luo, T. P.; Yang, Z. J. Am. Chem. Soc. 2011, 133, 14944.
(g) Aubineau, T.; Cossy, J. Chem. Commun. 2013, 49, 3303. Reviews:
(h) Zhdankin, V. V.; Stang, P. J. Tetrahedron 1998, 54, 10927. (i) Brand,
J. P.; Waser, J. Chem. Soc. Rev. 2012, 41, 4165.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and data. This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) (a) Ochiai, M.; Kunishima, M.; Nagao, Y.; Fuji, K.; Shiro, M.;
Fujita, E. J. Am. Chem. Soc. 1986, 108, 8281. A 1,2-sulfur shift has been
proposed in the one-pot synthesis of thiazoles starting from
alkynyliodonium salts: (b) Miyamoto, K.; Nishi, Y.; Ochiai, M. Angew.
Chem., Int. Ed. 2005, 44, 6896.
(12) (a) Stang, P. J.; Zhdankin, V. V. J. Am. Chem. Soc. 1990, 112, 6437.
(b) Stang, P. J.; Zhdankin, V. V. J. Am. Chem. Soc. 1991, 113, 4571.
(13) Commercially available 6 is routinely synthesized in our
laboratory in 30−50 g batches. (a) Zhdankin, V. V.; Kuehl, C. J.;
Krasutsky, A. P.; Bolz, J. T.; Simonsen, A. J. J. Org. Chem. 1996, 61, 6547.
(b) Brand, J. P.; Waser, J. Synthesis 2012, 44, 1155.
AUTHOR INFORMATION
■
Corresponding Author
jerome.waser@epfl.ch
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(14) Under the optimized conditions, it was still not possible to
suppress thiol oxidation when using alkynyliodonium salt 5.
(15) Reaction yields the same outcome under anaerobic conditions
using carefully degassed solvents; details in SI.
■
We thank F. Hoffmann-La Roche Ltd for an unrestricted
research grant and the Swiss State Secretariat for Education,
Research and Innovation for financial support (grant no.
C10.0116 in framework of the COST action CM0804). The
work of R.F. was further supported by a Marie Curie
International Incoming Fellowship (grant no. 331631).
(16) Details in SI.
(17) With the exception of the production of 2-iodobenzoic acid as
coproduct, we note that the developed thiol-alkynylation method meets
all the standard of the extremely successful “click chemistry” approach:
(a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001,
40, 2004. (b) Wang, Q.; Chan, T. R.; Hilgraf, R.; Fokin, V. V.; Sharpless,
K. B.; Finn, M. G. J. Am. Chem. Soc. 2003, 125, 3192. (c) Tornoe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057. (d) Moses, J.
E.; Moorhouse, A. D. Chem. Soc. Rev. 2007, 36, 1249.
(18) Metzner, P.; Thuillier, A. Sulfur reagents in organic synthesis;
Academic Press: New York, 1994.
(19) The chemistry of heterocyclic compounds, Vol. 44, Wiley
Interscience: New York, 1994.
(20) (a) Murphy, A. R.; Frechet, J. M. Chem. Rev. 2007, 107, 1066.
(b) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew.
Chem., Int. Ed. 2008, 47, 4070. (c) Mishra, A.; Ma, C. Q.; Bauerle, P.
Chem. Rev. 2009, 109, 1141.
(21) Migdalof, B. H.; Antonaccio, M. J.; McKinstry, D. N.; Singhvi, S.
M.; Lan, S. J.; Egli, P.; Kripalani, K. J. Drug Metab. Rev. 1984, 15, 841.
(22) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596.
(23) CuAAC reactions involving sulfur-substituted triple bonds have
been reported only in the case of terminal sulfonyl-alkynes or
pentafluorosulfanyl-alkynes; examples: (a) Gonzalez-Vera, J. A.;
Lukovic, E.; Imperiali, B. J. Org. Chem. 2009, 74, 7309. (b) Ye, C.;
Gard, G. L.; Winter, R. W.; Syvret, R. G.; Twamley, B.; Shreeve, J. M.
Org. Lett. 2007, 9, 3841.
REFERENCES
■
(1) (a) Zambrowicz, B. P.; Sands, A. T. Nat. Rev. Drug Discovery 2003,
2, 38. (b) Paquette, L. A. Sulfur-Containing Reagents; Wiley: Chichester,
2009. (c) Masella, R. Glutathione and Sulfur Amino Acids in Human
Health and Disease; Wiley: Hoboken, 2009. (d) Hoyle, C. E.; Lowe, A.
B.; Bowman, C. N. Chem. Soc. Rev. 2010, 39, 1355.
(2) (a) Lundblad, R. L. Chemical reagents for protein modification, 3rd
ed.; CRC Press: Boca Raton, 2005. (b) Hermanson, G. T. Bioconjugate
techniques, 2nd ed.; Academic Press: San Diego, 2008. (c) Hoyle, C. E.;
Bowman, C. N. Angew. Chem., Int. Ed. 2010, 49, 1540. (d) Hoogenboom,
R. Angew. Chem., Int. Ed. 2010, 49, 3415. (e) Lowe, A. B.; Hoyle, C. E.;
Bowman, C. N. J. Mater. Chem. 2010, 20, 4745. (f) Stephanopoulos, N.;
Francis, M. B. Nat. Chem. Biol. 2011, 7, 876. (g) Stenzel, M. H. ACS
Macro Lett. 2012, 2, 14.
(3) Recent examples: (a) Godoi, B.; Speranca, A.; Back, D. F.; Brandao,
R.; Nogueira, C. W.; Zeni, G. J. Org. Chem. 2009, 74, 3469.
(b) Chandanshive, J. Z.; Bonini, B. F.; Gentili, D.; Fochi, M.;
Bernardi, L.; Franchini, M. C. Eur. J. Org. Chem. 2010, 6440.
(4) Examples: (a) Braga, A. L.; Reckziegel, A.; Menezes, P. H.; Stefani,
H. A. Tetrahedron Lett. 1993, 34, 393. (b) Bieber, L. W.; da Silva, M. F.;
Menezes, P. H. Tetrahedron Lett. 2004, 45, 2735.
(5) Arisawa, M.; Fujimoto, K.; Morinaka, S.; Yamaguchi, M. J. Am.
Chem. Soc. 2005, 127, 12226.
D
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