Alkynylthioimidazolium Salts: Efficient Reagents for the Synthesis of Alkynyl Sulfides by Electrophilic Thioalkynylation

Article abstract of DOI:10.1002/chem.201604760

The efficient synthesis of a series of alkynylthioimidazolium salts through reaction of organozinc compounds with dibromo(imidazolium)sulfuranes is reported. Addition of Grignard reagents to these new species provided a highly modular, clean, and scalable access to a broad variety of alkynyl sulfides in good-to-excellent yields. The utility of this protocol was showcased by the preparation of alkynyl sulfides, which are particularly difficult to obtain or simply unavailable through the existing methodologies. In addition, the synthetic method was extended to the preparation of alkynyl selenides.

Full text of DOI:10.1002/chem.201604760

A Journal of
Accepted Article
Title: Alkynylthioimidazolium salts: Efficient Reagents for the Synthesis
of Alkynyl Sulfides by Electrophilic Thioalkynylation
Authors: Manuel Alcarazo, Javier Pena, Garazi Talavera, and Bernd
Waldecker
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201604760
Link to VoR: http://dx.doi.org/10.1002/chem.201604760
Supported by

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COMMUNICATION
Alkynylthioimidazolium salts: Efficient Reagents for the
Synthesis of Alkynyl Sulfides by Electrophilic Thioalkynylation
Javier Peña, Garazi Talavera, Bernd Waldecker and Manuel Alcarazo* [a]
Abstract:
alkynylthioimidazolium salts by reaction of organozinc compounds
with dibromo(imidazolium)sulfuranes is reported. Addition of
The
efficient
synthesis
of
a
series
of
for its solid X-ray structure). This result suggests an initial
reduction of 1 by the propionyl grignard and concomitant
reaction of the formed thiourea with unreacted 1.
Grignards to these new species provides a highly modular, clean
and scalable access to a broad variety of alkynyl sulfides in good to
excellent yields. The utility of this protocol is showcased by the
preparation of alkynyl sulfides which are particularly difficult to obtain,
or simply unavailable, through the existing methodologies. In
addition, the synthetic method has been extended to the preparation
of alkynyl selenides.
a.
c.
Electrophilic
R'
Dehydrohalogenation
X
alkynylation
-
X
S
H
R
Base
I
O
R
X
=
R
Alkyl,
S
=
X
Cl,
H
=
Aryl
Aryl,
O
R
Alkyl,
=
X
Br, I
R'
ref: 4a-b
b. Sulfur
ref: 4f, 6a-d
R
S
d. Electrophilic
umpolung
thioalkynylation
work
this
The continuously increasing spectrum of applications reached
by sulfur containing compounds expands along different fields,
from organic synthesis and drug discovery to material science.[1]
This demand of organosulfur compounds has stimulated over
decades the development of new methodologies focused on the
efficient introduction of S-based functionalities into organic
substrates, and the subsequent chemoselective transformation
of these primary products.[2]
S
R'
R
X
R'
=
M
M
S
or
R
-MgBr
R-ZnBr
=
=
R
X
Br,
Cl,
Li, Na,
K,
MgBr
Alkyl,
N
N
X
Aryl,
CN, SO3Na,
SR
=
R
Alkenyl
ref: 4c-d, 5
Alkyl, Aryl,
air stable
Alkenyl,
Alkynyl
Scheme 1. Available protocols to access alkynylsulfides.
Among S-containing building blocks the unique
connectivity of 1-alkynylsulfides, constituted by a sulfur atom
directly attached to a chemically very versatile alkyne moiety,
confers an enormous synthetic potential to these fragments;[3]
however, it is also this particular atomic distribution what has
surely prevented their broader use. Note that both, the alkyne
and the sulfide groups are inherently nucleophilic moieties and
the umpolung of one of them is necessary for the construction of
the thioacetylene. This often implies the use of unselective
reagents and/or harsh reaction conditions for their preparation,
which are not always compatible with sensitive functional groups
(Scheme 1a,b).[ 4 ] General routes to alkynylthioethers were
reported only recently;[4f,5,6] however, some limitations regarding
the scope of products still persist.
To the best of our knowledge, a synthetic strategy based on the
electrophilic thioalkynilation of C-based nucleophiles has never
been reported, probably due to the absence of an appropriate
[R-C≡C-S]+ synthetic equivalent. Herein, we introduce
alkynylthioimidazolium salts as a family of air stable reagents
depicting this uncommon reactivity, and describe the
implementation of a highly modular route to alkynylsulfides
making use of these compounds (Scheme 1d).
Me
Me
N
N
iPr
iPr
Y
Br
S
1
Br
S
MgBr4-2
S
a)
b)
iPr
iPr
iPr
iPr
S
N
N
N
N
iPr
iPr
N
N
Me
Me
Me
Me
SbF6
3a-n
Me
Me
2
OMe
3c
CF3
Me
Ph
3b
grams]
SiEt3
3e
3a
3d
[10
OPMB
OTBS
3g
3f
3h
=
Y
N3
OTBDPS
Cl
OBz
O
3i
3j
3k
TESO
PMBO
Ph
N
N
O
3l
O
3m
Boc
OMe
3n
Ph
In a first attempt to prepare alkynylthioimidazolium salts, we
studied the reaction of sulfurane 1 with propionylmagnesium
bromide. Addition of the Grignard reagent caused immediate
decolorization of the reaction mixture already at -78°C, and
precipitation of a bright yellow solid which was probed to be
dicationic disulfide 2 (Scheme 2; see the Supporting Information
Scheme 2. Synthesis of alkynylthioimidazolium salts. Reagents and conditions
(yields): a) 1-propynyl-magnesium bromide, THF, -78°C→ RT; 2 (68%); b)
RZnBr, THF, -78 °C→RT, and then NaSbF6; 3a (98%); 3b, (92%); 3c (91%);
3d (96%); 3e (67%); 3f (84%); 3g, (92%); 3h (82%); 3i (81%); 3j (85%); 3k
(97%),3l (58%); 3m (79%); 3n (89%).
Conversely, switch of the Grignard by the softer
organozinc analogue cleanly provided thioalkyne 3a, which was
isolated as an air stable light yellow powder after anion
exchange with an aqueous solution of NaSbF6. The described
reaction conditions proved to be broadly applicable and, as
shown in Scheme 2, a complete set of alkynylthioimidazolium
salts could be prepared in high yields from the corresponding
alkynylzinc reagents following an identical reaction sequence.
Importantly, many functional groups including ethers, silyl
[a]
Dr. J. Peña, Dr. G. Talavera, B. Waldecker and Prof. Dr. M.
Alcarazo
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstraße 2, 37077-Göttingen, Germany
E-mail: malcara@gwdg.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201604760
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COMMUNICATION
protected alcohols, carbamates, esters, amides, azides, acetals
and halogens were tolerated, opening up numerous
opportunities for further transformation. Compounds 3a-e,n were
isolated as crystalline materials whereas 3f-m, decorated with
longer lateral chains, are amorphous solids or viscous oils.
Indistinctly of their aggregation state, they all are air stable salts
and can be stored during months at room temperature without
evident decomposition. In addition, the synthesis of 3b was
scaled up to 10 grams without compromising the yield of the
product.
by reaction of the corresponding alkynyl thioimidazolium salt
with organozinc reagents; however, even under these softer
reaction conditions, untreatable product mixtures were obtained
when using precursors 3k and 3n as electrophiles.
3
equiv)
(1.0
-
S
R
R
MgBr
THF, -78°C rt, 2h.
Y
Scope with alkyl
a)
Grignards
S
S
Me
3Si
S
Crystals suitable for X-ray analysis were grown by slow
diffusion of diethylether into saturated dichloromethane solution
of 3b, and thus its connectivity could be unambiguously
established (See Figure 1). As expected, 3b adopts and angular
geometry, with a C1-S1-C3 angle of 99.6(2)°. Of particular
relevance in this structure is the key contact measured between
S(1) and one of the fluorine atoms of the hexafluoroantimonate
counteranion (S1-F1, 2.965 Å), which is significantly shorter than
the sum of the van der Waals radii S-F (3.27 Å). Considering the
non-coordinating nature of the SbF6 anion, this interaction is
indicative of high electrophilicity at the sulfur atom in these
compounds.
Et
Ph
Ph
Ph
4
;
5
;
6
97%
;
97%
98%
S
Boc
O
OPMB
S
N
O
S
Ph
7
90%
;
8
98%
;
[b]
9
;
17%
S
(79%)
Cl
Scope with
aryl
b)
Grignards
S
MeO
10
88%
;
SiEt3
Me
11
96%
;
Me
S
S
S
Me
S
Ph
Me
Me
Me
CF3
14
98%
;
12
86%
;
13
89%
S
;
vinyl
Scope with
c)
Grignards
Me
Me
S
S
Me
Me
Me
Ph
98%
Figure 1. Molecular structure of compound 3b in the solid state. Anisotropic
displacement parameter shown at 50% probability level and hydrogens
omitted for clarity. Selected bond lengths [Å] and angles [deg]: C1-S1,
17
;
OMe
OPMB
15
91%
16
92%
;
;
1.695(5); C3-S1, 1.744(5); C1-C2, 1.188(7); S1-F1, 2.965(5); C3-S1-C1,
99.6(2).
S
S
[7]
Ph
Me
Me
N3
OTBS
19
;
18
93%
S
75%
;
With that selection of alkynylthioimidazolium salts in hand,
we set up to preliminary explore their reactivity towards
nucleophiles. Interestingly, all organomagnesium reagents
tested were found to smoothly attack salts 3a-j,l at the sulfur
atom providing the corresponding alkynylthioethers in good to
excellent yields (Scheme 3). This reactivity contrast with the one
Scope with
d)
alkynyl Grignards
Boc
S
S
N
Me
Ph
Ph
Me
SiEt3
21
97%
;
[b]
(77%)
22
46%
;
we
observed
for
imidazolium
salts substituted
thyocianates
or
S
20
99%
;
alkynylthioimidazolium
with electron
withdrawing groups, which suffer the nucleophilic attack at the
sp-hybridized carbon directly bonded to sulphur. [8]
SiEt3
23
CF3
97%
;
The robustness and applicability of the protocol developed is
specifically highlighted by the successful preparation of a
number of structurally differentiated thioacetylenes incorporating
alkyl- 4-9, aryl- 10-14, vinyl- 15-19 or even a second alkynyl- unit
20-23 directly attached to the central sulfur atom. The
preparation of the last two groups of products is remarkable
since their synthesis is not obvious through other routes.[ 9 ]
Although the functional group tolerance is limited by the non-
discriminating reactivity of Grignard reagents, this reaction still
allows the accommodation of valuable functionalities such as
ethers, silanes, silyl ethers, acetals, azides and halogens
substituents on both termini of the thioacetylene. Alkynylsulfides
decorated with carbamates 9, 22 are more efficiently prepared
Scheme 3. Substrate scope for the reaction of 3 with Grignard or organozinc
reagents. [a]All yields are of isolated products; [b] In parenthesis yields
obtained using organozinc reagents obtained from the corresponding Grignard
by transmetallation with ZnBr2.
No drop in the yield of product 4 was observed when the
thioalkynylation is carried out in the presence of TEMPO (50
mol%). Moreover, quenching of the reaction mixtures with
powder Se (2 equiv.) generates selenourea 24 in good isolated
yields (80-85%). Based on this, and considering the electrophilic
character of sulfur in 3, we tentatively propose a substitution
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10.1002/chem.201604760
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mechanism in which the Grignard transfers its negatively
charged R group to the sulfur atom followed by concomitant
elimination of the N-heterocyclic carbene either free or, more
probably, as a Mg complex.
Next, we turned our attention to study the extension of this
method to the preparation of selenoacetylenes.[10] Hence, known
selenourea 24[ 11 ] was submitted to bromination conditions
obtaining the corresponding hypervalent selenium compound 25
as a bright orange solid in high yield and analytical purity
(Scheme 4; for the X-ray structure of this compounds see the
Supplementary Information).[ 12 ] Subsequent addition of the
desired zinc acetylide caused the slow disappearance of the
color and the formation of the corresponding imidazolium seleno
alkynes 26a-c. These were isolated as slightly pale yellow solids
that can be stored at room temperature and handle in air for
short times without evident decomposition.
The functionalities present in the products obtained could be
involved into further transformations. Illustrative examples are
described in Scheme 6 taking 17 and 21 as model substrates.
Following standard oxidation procedures, 17 can be selectively
transformed either into sulfoxide 33 or sulphonylacetylene 34.[14]
This last compound is an efficient double Michael acceptor and
reacts with butyl amine, or Na2S to afford dihydrothiazine or
dihydrodithiine derivatives 35 and 36 respectively.[15] Thiophene
37 was also obtained after extensive heating from 17 in the
presence of Na2S. Further studies are still necessary to
determine the complete mechanistic pathway of this
transformation; however, the substitution pattern in 37 supports
a mechanism in which migration of a propenyl cation between
the two sulphur centers takes place after a first hydrothiolation of
the C-C triple bond. Finally 1,4-dithiine 39, which depicts an
asymmetric substitution pattern, was prepared by reaction of
Na2S with di(alkynyl)sulfide 21 (Scheme 6).[16]
Y
Br Se Br
Se
Se
a)
b)
iPr
iPr
iPr
iPr
iPr
iPr
N
N
N
N
N
N
SbF6
Me
Me
Me
Me
26a Y Me
Me
Me
=
25
24
;
=
26b Y Ph
;
=
26c Y
SiEt3
;
Scheme 4. Synthesis of alkynylseleno imidazolium salts. Reagents and
conditions (yields): (a) Br2, CH2Cl2, 0°C→ RT; (68%); b) RZnBr, THF, -
78 °C→RT, and then NaSbF6; 26a (98%); 26b, (94%); 26c (58%).
Figure 2: Molecular structure of compound 26b in the solid state. Anisotropic
displacement parameter shown at 50% probability level and hydrogens
omitted for clarity.[7] Selected bond lengths [Å] and angles [deg]: C1-Se1,
1.839(4); C3-Se1, 1.895(4); C1-C2, 1.192(6); Se1-F1, 3.177 (4); C3-Se1-C1,
96.38(18).
Figure 2 depicts the X-ray structure of 26b. If compared with
that of 3b, it features the structural changes expected when
descending along
a group; namely, longer bond lengths
between the central chalcogen and the flanking carbon atoms
(Se1-C1, 1.839(4)Å; Se1-C3, 1.895(4)Å) and an acuter angle
around Se (C1-Se1-C3; 96.38(18)°), which is consequence of
the decreased tendency to hybridization typical for the heavier
atoms in a group. Shorter contacts than the sum of the Se-F van
der Waals radii (3.37 Å) were again measured between the
chalcogenide and F1 from the hexafluoroantimonate anion (Se1-
F1, 3.177(4)Å).
O
O
O
a)
O
O
c)
b)
Me
S
N
S
S
17
Me
d)
Ph
Ph
Me
Ph
34
35
33
Bu
Ph
Me
Me
O
O
Me
S
Ph
e)
Me
S
S
S
S
S
f)
21
Reaction of 26a-c with Grignards proceeds in an identical
fashion to that observed for their lighter isostere, allowing the
isolation of a series of alkyl-alkynyl 27, aryl-alkynyl 28, 29, vinyl-
alkynyl 30, 31 and even asymmetric dialkynyl selenoethers 32 in
good to excellent yields (Scheme 5).[13]
S
Ph
Ph
Me
37
36
39
38
Scheme 6: Product modification. Reagents and conditions (yields): (a)
mCPBA (1 equiv.), DCM, 0°C, (68%); b) mCPBA (2.5 equiv.), DCM, 0°C,
(86%); c) BuNH2, EtOH, r.t. (26%); d) Na2S·9H2O, MeOH, r.t., (84%); e)
Na2S·9H2O, EtOH, 120°C, 37 (71%), 38 (11%); f) Na2S·9H2O, EtOH, 65°C, 39
(60%).
26
equiv)
(1.0
Se
R-
R
MgBr
rt
THF, -78°C
Se
Y
In conclusion, we describe herein a new synthetic strategy
for the preparation of a broad spectrum of alkynylsulfides and
selenides based on an unprecedented electrophilic thio- or
selenoalkynylation. The method developed is operationally
simple, scalable, high yielding, and uses inexpensive starting
materials. In addition, a broad range of functionalities were well
tolerated and the products could be further transformed into
useful building blocks. Considering the continuously increasing
number of applications reached by sulphur containing
compounds, we expect our method to find a wide range of
application, particularly in the synthesis of organic functional
materials. Efforts along these lines are currently underway in our
laboratory.
Ph
Se
Et
S
Ph
Ph
MeO
Me
Se
27
98%
28
91%
;
;
29
65%
Se
;
Me
Se
Se
Ph
30
Me
Ph
Me
SiEt3
32
31
;
88%
;
92%
89%
;
Scheme 5: Substrate scope for the reaction of 26 with Grignard reagents.
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COMMUNICATION
Acknowledgements
S. Synth. Commun. 1992, 22, 1473-1479; (c) Kauffmann, T.; Ahlers, H.,
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D., Org. Biomol. Chem. 2015, 13, 5859-5861.
Financial support from the European Commission (ERC Starting
Grant to M.A.) is gratefully acknowledged. We also thank the X-
ray crystallography department at the MPI-Kohlenforschung for
assistance. G. T. thanks the regional government of the Basque
Country (Spain) for support.
[5]
[6]
Reeves, J. T.; Camara, K.; Han, Z. S.; Xu, Y.; Lee, H.; Busacca, C. A.;
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Keywords: electrophilic thioalkynylation • Grignard reagents •
alkynylthioimidazolium salts • alkynylthioethers •
alkynylselenoethers
[7]
[8]
[9]
CCDC 1505415-1505418 contain the supplementary crystallographic
data for this paper.
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J.; Ragogna, P. J. Inorg. Chem. 2007, 46, 8594-8602.
[ 3 ] (a) Acetylene Chemistry: Chemistry, Biology and Material Science;
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Scopelliti, R.; Waser, J. Chem. Eur. J. 2016, 22, 11997-12001.
[13] For the synthesis of symmetric dialkynyl selenoethers see: Zhu, Y. X.;
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2015, 26, 35-41.
[14] (a) Zhong, P.; Guo, M. P.; Huang, N. P. Synth. Commun. 2002, 32,
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Org. Chem. 1980, 45, 5015-5017.
[15] Verboom, W.; Sukhai, R. S.; Meijer, J. Synthesis 1979, 47-48.
[16] Meijer, J.; Vermeer, P.; Verkruijsse, H. D.; Brandsma, L. Recl. Trav.
Chim. Pays-Bas 1973, 92, 1326-1330.
[4]
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This article is protected by copyright. All rights reserved.

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COMMUNICATION
Javier Peña, Garazi Talavera, Bernd
Waldecker and Manuel Alcarazo*
Page No. – Page No.
Alkynylthioimidazolium salts:
Efficient Reagents for the Synthesis
of Alkynyl Sulfides by Electrophilic
Thioalkynylation
Novel Thioalkynylation reagents: Alkynylthioimidazolium salts are easy to
prepare, air stable compounds that reacts with Grignards affording alkynylthioethers
of diverse architecture. Thus, they are very convenient synthetic equivalents of the
[R-C≡C-S]+ cation.
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