A facile access to mono-C-alkynylated-o-carboranes from o-carboranes and arylsulfonylacetylenes

Article abstract of DOI:10.1016/j.cclet.2021.05.062

A facile access to mono-C-alkynyl-o-carboranes from o-carboranes and arylsulfonylacetylenes was developed. This facile process tolerates a wide variety of functional groups, occurs at mild conditions in one-pot procedure with short reaction time. The obtained mono-C-alkynyl-o-carboranes can be easily derivatized to synthesize 1,2-difunctionalized o-carboranes. This work provides a useful tool for the functionalization of o-carboranes.

Full text of DOI:10.1016/j.cclet.2021.05.062

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A facile access to mono-C-alkynylated-o-carboranes from
o-carboranes and arylsulfonylacetylenes
Mengyang Bai , Guanyu Tao , Zhenxing Liu , Lili Wang ,
Zheng Duan
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https://doi.org/10.1016/j.cclet.2021.05.062
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Please cite this article as: Mengyang Bai , Guanyu Tao , Zhenxing Liu , Lili Wang , Zheng Duan ,
A facile access to mono-C-alkynylated-o-carboranes from o-carboranes and arylsulfonylacetylenes,
Chinese Chemical Letters (2021), doi: https://doi.org/10.1016/j.cclet.2021.05.062
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Communication
A facile access to mono-C-alkynylated-o-carboranes from o-carboranes and
arylsulfonylacetylenes
Mengyang Bai, Guanyu Tao, Zhenxing Liu, Lili Wang*, Zheng Duan
College of Chemistry, Green Catalysis Center, International Phosphorus Laboratory, International Joint Research Laboratory for Functional
Organophosphorus Materials of Henan Province, Zhengzhou University, Zhengzhou 450001, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A facile access to mono-C-alkynyl-o-carboranes from o-carboranes and arylsulfonylacetylenes
was developed. This facile process tolerates a wide variety of functional groups, occurs at mild
conditions in one-pot procedure with short reaction time. The obtained mono-C-alkynyl-o-
carboranes can be easily derivatized to synthesize 1,2-difunctionalized o-carboranes. This work
provides a useful tool for the functionalization of o-carboranes.
Available online
Keywords:
Alkynylation
o-Carborane
Arylsulfonylacetylene
Transition-metal-free
1,2-Difunctionalized o-carborane
o-Carborane is an electron-deficient icosahedral boron cluster
compound of formula C2B10H12, in which two carbon atoms are
adjacent to each other. o-Carborane has 26 delocalized valence
electrons, exhibits special three-dimensional aromaticity, and
extraordinary chemical and thermal stability [1]. The different
electronegativity between carbon and boron makes the C-H bond
of o-carboranes partially acidic, which can work as a useful
reaction site to obtain functionalized o-carboranes. During the
past few decades, o-carboranes and their derivatives have broad
applications in many fields [2-6]. Among them, some C-alkynyl-
o-carboranes have special luminochromism and can be used in
optoelectronic functional materials as electron-accepting motifs
to tune the LUMO and HOMO energy levels [7,8]. Some of them
have remarkable aggregation-induced emission (AIE) property
and/or stimuli-responsivity and/or environment-sensitivity [9]
with potential for application in novel functional materials. In
addition, alkynyl groups are fundamental structural units in
organic synthesis, and can be easily further derivatized [10].
Thus, the synthesis and performance of C-alkynyl-o-carboranes
have received considerable attention.
and coworkers prepared C-alkynyl-o-carboranes by the reaction
of bromoalkynes with 1-Cu-o-C2B10H11 to give C-alkynyl-o-
carboranes in moderate yields. 1-Cu-o-C2B10H11 was obtained
from the corresponding 1-carboranyllithium and 1.25 equiv. of
CuCl in a THF-ether solution (Scheme 1b) [12]. In 2013, Nie and
co-workers reported the cross-coupling of 1-Cu-o-C2B10H11 and
Cu-C≡C-R to give C-alkynyl-o-carboranes with more excess
amount of n-BuLi and CuCl (4 equiv. respectively, Scheme 1b)
[13]. For a long period of time, C-alkynyl-o-carboranes were
synthesized by these methods, the use of hypertoxic decaborane
and diynes or stochiometric amounts of transition-metal salts
encumbers their broader applications. Very recently, Xie and
coworkers reported a very efficient approach to synthesize C-
alkynyl-o-carboranes by the reaction of iodocarboranes and
terminal alkynes in the presence of base under UV-light (Scheme
1c) [14]. However, this method is more suitable for o-carborane
with substituents such as methyl on the ortho position. Until
now, there are only a few reported mono-C-alkynyl substituted
o-carboranes and the methods for synthesizing those mono-C-
functional carboranes are still very limited. Therefore, it is of
great significance to develop simple and efficient methods to
synthesize mono-C-alkynyl-o-carboranes.
Currently, there are mainly three methods for the synthesis of
C-alkynyl-o-carboranes. In 1964, Dupont and Hawthorne
synthesized C-alkynyl-o-carboranes from decaborane and
corresponding diynes for the first time (Scheme 1a) [11a]. In
1973, Hawthorne modified the method [11b]. In 1976, Zakharkin
Sulfones are important intermediates in synthetic applications
due to their strong electron-withdrawing property. Alkynyl
sulfones have broad applications in the alkynylation and other
 Corresponding authors.
E-mail address: wanglili@zzu.edu.cn (L. Wang); duanzheng@zzu.edu.cn (Z. Duan)

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fields including building complex organic molecules or naturally
1, entry 1). When the reaction temperature at the second step was
kept at 0 °C, the reaction can be finished within 2 h also, but the
isolated yield of 3a is only 23% (Table 1, entry 2). If 2 equiv. of
2a was used, the reaction can be finished in 2 h also, and the
yield of 3a is up to 57% (Table 1, entry 3), Adding 3 equiv. of 2a
did not improve the yield and the reaction was worse when it was
run at room temperature (Table 1, entries 4 and 5). At last,
solvent effects on the reaction were studied, dimethyl ether
(DME) and Et2O can also give comparable yields (Table 1,
entries 6 and 7).
occurring products [15]. They could be facilely obtained from
varied synthetic methods [15a, 16]. In 2012, García Ruano’s
group reported a strategy that acetylenic sulfones were used as
alkynylating reagents for the construction of the CAr-Csp bonds
[17]. Inspired by these results, we wonder if arylacetylenic
sulfones could be used to synthesize mono-C-alkynyl-o-
carboranes. It should be noted that the carbon of o-carborane is
sp hybridized, and the alkynylation of Csp with acetylenic
sulfones is unknown. Herein, we wish to report our findings
toward the construction of various mono-C-alkynyl-o-carboranes
from o-carboranyllithiums and arylacetylenic sulfones (Scheme
1d).
Table 1
Optimization of the reaction conditions for the synthesis of 3aa.
Entry
2a (equiv.) Solvent T (oC) Time (min) Yield (%)b
1
2
3
4
5
6
7
1
1
2
2
3
2
2
THF
THF
THF
THF
THF
DME
Et2O
-78
0
60
60
60
60
60
60
60
12
23
57
20
56
53
50
0
r.t.
0
0
0
a Reaction conditions: 1a (0.5 mmol), solvent (5.0 mL), the reaction
flask, in nitrogen atmosphere, GC was used to monitor the reaction
process;
b Isolated yields.
After establishing the optimized conditions (Table 1, entry 3),
we examined the substrate scope and limitation of this
alkynylation reaction and the results are summarized in Scheme
2. Gratifyingly, a variety of arylacetylenic sulfones 2a-n were
smoothly coupled to o-carborane 1a, delivering the
corresponding mono-C-alkynyl-o-carboranes 3a-3n in moderate
yields. This reaction tolerated a wide variety of functional groups,
such as Me or Ph, electron-donating groups OMe, NPh2 or
electron-withdrawing groups F, Cl and CF3. Moreover, the
electronic properties of the substituents have no significant effect
on the products yield. The positions of substituents on the Phenyl
ring have no obvious impact on the yield also. In addition, alkyne
with heteroaryl was compatible with this reaction, affording the
product in a relatively lower yield (3n, 32%). The obtained
Scheme 1. Synthesis of C-alkynyl-o-carboranes.
The starting arylacetylenic sulfones were synthesized
according to a modified literature’s procedure [16a]. At the
outset of our studies, o-carborane 1a and phenylethynyl sulfone
2a were chosen as model substrates to optimize the reaction
conditions (Table 1). Initially, n-BuLi (1.2 equiv.) was added
dropwise to o-carborane 1a in THF at 0 °C, and the reaction
mixture was stirred for an hour. Then phenylethynyl sulfone 2a
(1 equiv.) was added and the reaction mixture was stirred for
another hour at -78 °C, the desirable product 3a was obtained
with an isolated yield of 12% and some 1a was recovered (Table
1
products 3a–3n were characterized by H NMR, 13C NMR, 11B
NMR and HRMS (Supporting information).

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Scheme 2. Synthesis of mono-C-alkynyl-o-carboranes.
1,2-Difunctionalized o-carboranes have some unique
photoelectric properties [18]. After successful preparation of a
variety of mono-C-alkynyl-o-carboranes, we turned our attention
to synthesize 1,2-difunctionalized o-carboranes. Unfortunately,
the attempt to synthesize the bisalkynylation compound under
the similar method was failed. But the alkylation and iodination
reactions proceeded smoothly and provided the corresponding
difuncationalized o-carboranes 4a and 4b and C-alkynyl-C'-
iodocarborane 4c in 65%-72% yields (Scheme 3). Even
iodoalkane bearing the hindered isopropyl group proved to be
effective for furnishing the product 4b in 65% yield. The further
derivatization reaction of C-I in compound 4c and applications of
the new mono-C-alkynyl-o-carboranes are currently in progress
in our laboratory.
Scheme 4. Ortho-substituted o-carborane alkynyl functionali-zation.
In summary, a facile synthetic route to mono-C-alkynyl-o-
carboranes from o-carboranes and arylsulfonylacetylenes was
developed. This new method tolerates a wide variety of
functional groups, and the process occurs at mild conditions in
one-pot procedure with short reaction time. The obtained mono-
C-alkynyl-o-carboranes can be further derivatized to synthesize
1,2-difunctionalized o-carboranes. This work provides a very
useful tool for the functionalization and practical applications of
o-carboranes.
Declaration of competing interest
The authors declare that there are no conflicts of interest.
Acknowledgments
Scheme 3. Synthesis of 1,2-difunctionalized o-carboranes.
To investigate the utility of this synthetic method, methyl,
ethyl and isopropyl substituted o-carboranes were synthesized
according the literature [19]. When R1 is methyl or ethyl or
isopropyl, 4d, 4a and 4b were obtained in yields of 51%, 61%
and 67% respectively (Scheme 4). It demonstrates the new
synthetic method is applicable with o-substituted o-carboranes
also.
We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 21672193, 21272218), the
Key Scientific and Technological Project of Henan Province (No.
202102310327), the Ministry of Industry and Information
Technology (No. Z135060009002), the Postdoctoral Research
Grant in Henan Province (No. 001803004), the Programme of
Introducing Talents of Discipline to Universities (111 Project,
No. D20003) and Zhengzhou University of China.
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