Enabling the Use of Alkyl Thianthrenium Salts in Cross-Coupling Reactions by Copper Catalysis

Article abstract of DOI:10.1002/anie.202109723

Alkyl groups are one of the most widely used groups in organic synthesis. Here, a a series of thianthrenium salts have been synthesized that act as reliable alkylation reagents and readily engage in copper-catalyzed Sonogashira reactions to build C(sp³)?C(sp) bonds under mild photochemical conditions. Diverse alkyl thianthrenium salts, including methyl and disubstituted thianthrenium salts, are employed with great functional breadth, since sensitive Cl, Br, and I atoms, which are poorly tolerated in conventional approaches, are compatible. The generality of the developed alkyl reagents has also been demonstrated in copper-catalyzed Kumada reactions.

Full text of DOI:10.1002/anie.202109723

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How to cite:
International Edition: doi.org/10.1002/anie.202109723
German Edition: doi.org/10.1002/ange.202109723
Coupling Reagents
Enabling the Use of Alkyl Thianthrenium Salts in Cross-Coupling
Reactions by Copper Catalysis
Cheng Chen+, Minyan Wang+, Hongjian Lu,* Binlin Zhao, and Zhuangzhi Shi*
Abstract: Alkyl groups are one of the most widely used groups
in organic synthesis. Here, a a series of thianthrenium salts have
been synthesized that act as reliable alkylation reagents and
readily engage in copper-catalyzed Sonogashira reactions to
3
À
build C(sp ) C(sp) bonds under mild photochemical condi-
tions. Diverse alkyl thianthrenium salts, including methyl and
disubstituted thianthrenium salts, are employed with great
functional breadth, since sensitive Cl, Br, and I atoms, which
are poorly tolerated in conventional approaches, are compat-
ible. The generality of the developed alkyl reagents has also
been demonstrated in copper-catalyzed Kumada reactions.
S
ulfonium salts bearing sulfur ions and three organic
functional groups have intrigued chemists for more than
a century because of their high chemical reactivity.[1] In this
regard, these compounds resemble organic halides with their
excellent nucleofugal properties, easy single-electron reduc-
tion, and oxidative addition to transition metals.[2] The
resulting S-(aryl/alkyl) sulfonium salts have been widely
applied as aryl reagents in reactions because of the favorable
2
[3]
À
energetics of the C(sp ) S bond cleavage (Figure 1a).
Moreover, they can also facilely form sulfur ylides by a-
deprotonation.[4] Thus, the generation of alkyl species from
Figure 1. Employment of alkyl thianthrenium salts in Sonogashira
reactions.
3
À
sulfonium salts in cross-coupling reactions through C(sp ) S
bond cleavage is synthetically challenging. Of the sulfonium
salts, thianthrenium salts have recently attracted considerable
attention from chemists.[5] Ritter and co-workers uncovered
the immense synthetic value of aryl thianthrenium salts,
3
À
coupling reactions through selective C(sp ) S bond cleavage
to provide alkyl species, thereby addressing some key short-
comings of conventional sulfonium salts.
À
produced by functionalization of C(aryl) H bonds, in a wide
The Sonogashira reaction has been one of the most widely
used strategies for the synthesis of high-value internal alkynes
from organic halides and terminal alkynes.[7] As a proof of
concept, we report here that a broad range of alkyl
range of reactions (Figure 1b).[6] These results inspired us to
investigate whether thianthrenium salts might undergo cross-
thianthrenium salts can be used in the Sonogashira reaction
[*] C. Chen,[+] Prof. Dr. M. Wang,[+] Prof. Dr. H. Lu, Prof. Dr. Z. Shi
State Key Laboratory of Coordination Chemistry
Chemistry and Biomedicine Innovation Center (ChemBIC)
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing 210093 (China)
E-mail: shiz@nju.edu.cn
3
À
to build C(sp ) C(sp) bonds in the presence of copper
catalysts under irradiation with blue LEDs (Figure 1c).[8]
Other types of cross-coupling reactions, such as the Kumada
3
3
reaction,[9] were also performed to form C(sp ) C(sp ) and
À
3
2
À
C(sp ) C(sp ) bonds by copper catalysts and showed the
excellent generality of these alkyl reagents. In our study, alkyl
thianthrenium salts were conveniently produced from alco-
hols by adding thianthrene (TT) and Tf2O reagents.[10]
Notably, the Wickens group recently reported the electro-
chemical synthesis of 1,2-disubstituted thianthrenium salts
from alkenes and TT. These salts act as the key intermediates
in the production of aziridines (Figure 1d).[11] Using our
method, 1,n-disubstituted thianthrenium salts can also be
generated from the related diols and allow the facile
construction of valuable diynes.
hongjianlu@nju.edu.cn
Prof. Dr. Z. Shi
School of Chemistry and Chemical Engineering
Henan Normal University
Xinxiang, Henan 453007 (China)
Prof. Dr. B. Zhao
Department of Chemistry and Materials Science
College of Science, Nanjing Forestry University
Nanjing 210037 (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number for
one of the authors of this article can be found under:
https://doi.org/10.1002/anie.202109723.
An extensive screening of conditions was performed using
thianthrenium salt 1a and alkyne 2a as model substrates
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Table 1: Reaction optimization.[a]
Table 2: Substrate scope of alkyl thianthrenium salts.[a]
Entry
Variation from the standard conditions
Yield of 3aa [%][b]
1
2
3
4
5
6
7
8
9
none
95 (85)[c]
using L2 instead of L1
using L3 instead of L1
using L4 instead of L1
using I instead of 1a
using II instead of 1a
using III instead of 1a
without L1
89
52
18
trace
26
65
26
0
in the dark
10
without Cu(OTf)2
0
[a] Standard conditions: Cu(OTf)2 (10 mol%), L (12 mol%), alkyl
sulfonium salts 1a/I–III (0.20 mmol), 2a (0.24 mmol), K2CO3
(0.6 mmol), CH3CN/MeOH (0.5 mL/0.5 mL), at room temperature, blue
LED, under N2, 3 h. [b] The yields were determined by 1H NMR
spectroscopic analysis of the crude product using CH2Br2 as a standard.
[c] Yield of isolated 3aa.
(Table 1). A catalytic amount of Cu(OTf)2 (10 mol%),
terpyridine L1 (12 mol%), K2CO3 (3.0 equiv) as a base, and
MeCN and MeOH as a cosolvent were reacted at room
temperature under blue-LED irradiation for 3 h, which
afforded the desired alkynylation product 3aa in 95% yield
(entry 1). Other ligands were also screened: terpyridine L2
gave a slightly lower yield (entry 2), while bidentate ligands
such as L3 and L4 exhibited much lower reactivity (entries 3
and 4). Other types of sulfonium salts were screened as well.
[a] Reaction conditions: Cu(OTf)2 (10 mol%), L1 (12 mol%),
1 (0.20 mmol), 2a (0.24 mmol), K2CO3 (0.60 mmol), CH3CN/MeOH
(0.5 mL/0.5 mL), at room temperature, blue LED, under N2, 3 h; yields
are of isolated products. [b] CH3CN/EtOH (0.5 mL/0.5 mL).
nicely showcased by the perfect accommodation of substrates
with both alkyl and aryl halides, including F (1g, 1h), Cl (1i),
Br (1j, 1k), and even I (1l, 1m). This method was applicable
to thianthrenium salts bearing heteroaryl motifs, such as
thiophene (1n) and oxazole (1o). The transformations of
thianthrenium salts bearing alkenyl and alkynyl groups (1p–
1r) led to the formation of several useful enynes that are
extremely laborious to access. In addition, secondary alkyl
thianthrenium salts with both acyclic (1s) and cyclic (1t–1v)
motifs worked very well. Compound 1w, which was derived
from a-linolenic acid with three cis double bonds, could
convert into product 3wa in modest yield with complete
retention of the Z-configuration. Finally, a complex thian-
threnium salt 1x prepared from lithocholic acid also displayed
good reactivity.
Tetrahydrothiophene-derived sulfonium salt
I exhibited
a very low reactivity (entry 5). As expected, the reaction of
diphenyl sulfonium salt II with 2a only generated the desired
product 3aa in 26% yield, along with 1,2-diphenylethyne
(65% yield) as the major product, which indicates that the
2
À
C(sp ) S bond cleavage occurs preferentially over the C-
3
À
(sp ) S bond cleavage (entry 6). In addition, a dibenzothio-
phenium salt III[12] also showed excellent chemoselectivity for
3
À
the C(sp ) S cleavage and provided the desired product 3aa
in a moderate yield (entry 7). Control experiments demon-
strated that a diminished yield was observed without the
ligand (entry 8), and both irradiation with visible light and
a copper catalyst were essential for the reaction (entries 9 and
10).
Using the optimized reaction conditions, we first tested
the scope of the alkyl thianthrenium salts in this cross-
coupling with alkyne 2a (Table 2). A range of primary alkyl
thianthrenium salts was efficiently transformed into the
corresponding products, including those with ether (1c),
cyano (1d), ester (1e), and azetidine (1 f) motifs. Compared
to the traditional Sonogashira reaction using organoha-
lides,[13] the unique chemoselectivity of this reaction was
The substrate scope of the terminal alkynes was then
examined with alkyl thianthrenium salt 1a (Table 3). A wide
range of arylacetylenes with Me (2b, 2c), OMe (2d, 2e),
NMe2 (2 f), F (2g), Cl (2h,i), CO2Me (2j), and CN (2k) groups
at each position of the aryl motif were readily tolerated.
Alkynes bearing mesitylene (2l), naphthalene (2m), and
phenanthrene (2n) could couple with 1a with good efficiency.
Heteroarene-substituted alkynes, including benzo[d]thiazole
2
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Table 3: Substrate scope of terminal alkynes.[a]
Scheme 1. Methylation of alkynes with thianthrenium salt 4.
[a] Reaction conditions: Cu(OTf)2 (10 mol%), L1 (12 mol%), 1a
(0.20 mmol), 2 (0.24 mmol), K2CO3 (0.60 mmol), CH3CN/MeOH
(0.5 mL/0.5 mL), at room temperature, blue LED, under N2, 3 h; yields
are of isolated products. [b] DMF (1.0 mL), 12 h. [c] 12 h. [d] Using 1a
(3.6 equiv) in DMF.
Scheme 2. Investigation of disubstituted thianthrenium salts.
thianthrenium salt 8b bearing a (CH2)4 linker generated diyne
9b, with 10 as a by-product. The formation of 10 may proceed
by a radical-mediated process through cyclization and trap-
ping of the resulting vinyl radical.[16]
Alkyl thianthrenium salts can enable late-stage diversifi-
cation of complex molecules as a range of functional units are
tolerated (Scheme 3).[17] For example, the reaction of d-
glucose derivative 11 with thianthrenium salt 1a produced
(2o), pyridine (2p), and thiophene (2q), also exhibited high
levels of reactivity. The reaction of silylacetylene 2r with
thianthrenium salt 1a led to formation of product 3ar, which
can easily form a terminal alkyne after removal of the silyl
protecting group. Moreover, alkyl-substituted alkynes 2s–2v
were also suitable substrates and produced unsymmetrical
dialkyl alkynes in good yields. In particular, the reaction of 2v,
which contains a reactive hydroxy group for possible O-
alkylation, showed extremely high selectivity for the forma-
À
tion of a C C bond at the terminal alkyne position. In
addition, the triple alkylation of substrate 1w with three
alkynyl groups produced product 3aw in 42% yield.
The S-adenosylmethionine (SAM) superfamily is a family
of sulfonium salts naturally found in the body, which can
methylate complex molecules such as nucleic acids, proteins,
and natural products in an SN2 manner.[14] Inspired by this
biosynthetic procedure, we used MeOTf and TT to prepare
on a large scale thianthrenium salt 4 that bears a methyl group
(Scheme 1).[15] Delightfully, the coupling of terminal alkynes
5a–5c with reagent 4 afforded methylation products 6a–6c in
good to excellent yields.
Instead of using electroorganic synthesis to access 1,2-
[11]
disubstituted thianthrenium salts from alkenes,
disubsti-
tuted thianthrenium salts can be produced from diols. For
example, when diol 7a was allowed to react with TTand Tf2O,
the desired 1,6-disubstituted thianthrenium salt 8a was
generated (Scheme 2). Disubstitution of this reagent with
alkyne 2a led to the corresponding diyne 9a in 91% yield.
Interestingly, the reaction of 2a with a 1,4-disubstituted
Scheme 3. Late-stage modification of complex molecules.
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product 12 in 69% yield. Aspartic acid derivative 13 with
a sensitive NH motif readily formed alkyne 14 without any
racemization. The drug mestranol (15) bearing a terminal
alkyne could be directly transformed with thianthrenium salt
1a into internal alkyne 16 in 62% yield. Finally, the facile late-
stage alkylation of erlotinib (17) by alkyl thianthrenium salts
1a or 4 highlighted the robustness of the developed method.
We conducted additional experiments to improve the
practicability and operability of this strategy (Scheme 4). The
TT reagent can be efficiently recycled after the reaction
(Scheme 4a). When alkyl thianthrenium salt 1a is used in the
cross-coupling reaction with alkyne 2a, TT can be recovered
in nearly quantitative yield. Furthermore, tandem reactions
can start from alcohols to access alkynylation products
(Scheme 4b). The reaction of alcohol 1aꢀ with TT and Tf2O
forms thianthrenium salt 1a, which can further react with
alkyne 2a to afford 3aa in 57% yield under the standard
conditions.
cycle (Scheme 5b). Using a stoichiometric amount of com-
plex 21 with the terpyridine ligand L1 led to the formation of
by-product 22 by dimerization of 2a. Furthermore, the
addition of the L1 was found to be essential for inhibiting
the formation of the alkyne homocoupling product 23. In
addition, a cyclic voltammetry study proved that the redox
potential (E1/2 = À1.20 V vs. Ag/AgNO3) of the excited
complex formed between copper acetylide 21 and ligand L1
is sufficiently strong to reduce thianthrenium salt 1a (E1/2
=
À0.95 V vs. Ag/AgNO3; see the Supporting Information).
Based on the above results and those of previous studies,[13,18]
it is postulated that the excited copper acetylide species can
reduce the thianthrenium salts to generate alkyl radicals,
3
À
which is followed by formation of the C(sp ) C(sp) bonds.
Alkyl thianthrenium salts can also be cross-coupled with
3
3
3
2
À
À
Grignard reagents to build C(sp ) C(sp ) and C(sp ) C(sp )
bonds using a copper catalyst (Scheme 6). Different types of
Grignard reagents, including alkyl (24a), allyl (24b), phenyl
(24c), and vinyl reagents (24d), were coupled with thian-
threnium salt 1c and provided products 25a–25d in excellent
yields within 15 minutes. These examples demonstrate that
alkyl thianthrenium salts can act as general alkyl sources in
cross-coupling reactions.
Scheme 4. Further optimization of the operation procedures.
Scheme 6. Further application of alkyl sulfonium salts in the Kumada
reaction.
Further experiments were performed to gain insight into
the reaction mechanism (Scheme 5). Adding TEMPO as
a radical scavenger to a mixture of 1a and 2a led to the
amount of product 1c decreasing, with TEMPO-trapped
adduct 20 isolated in 59% yield (Scheme 5a). The use of
a presynthesized copper acetylide 21[13f] as the catalyst for the
reaction of substrates 1a and 2a led to product 3aa being
formed in excellent yield under irradiation with blue LEDs,
thus indicating that it is a visible intermediate in the catalytic
In summary, we identified that alkyl thianthrenium salts
can undergo C(sp ) S bond cleavage and be used as
3
À
alternative alkyl sources in cross-coupling reactions. The
state-of-the-art application of alkyl thianthrenium salts in
Sonogashira reactions is distinguished by its great functional
breadth. We anticipate that alkyl thianthrenium salts will find
immediate application in organic synthesis. The application of
the above-mentioned alkyl reagents in other reactions is
currently underway in our laboratory.
Acknowledgements
We would like to thank the National Natural Science
Foundation of China (Grants 22025104, 21972064 and
21901111), the National Natural Science Foundation of
Jiangsu Province (Grant BK20200765 and BK20170632), the
Excellent Youth Foundation of Jiangsu Scientific Committee
(Grant BK20180007), the “Innovation & Entrepreneurship
Talents Plan” of Jiangsu Province, and the Fundamental
Research Funds for the Central Universities for their
financial support. The project was also supported by the
Scheme 5. Mechanistic experiments.
4
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Conflict of Interest
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The authors declare no conflict of interest.
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Keywords: alkynylation · copper · photoredox reactions ·
radical · sulfonium salts
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Ham, M. B. Plutschack, F. Berger, S. Chabbra, A. Schnegg, C.
Manuscript received: July 20, 2021
Accepted manuscript online: August 11, 2021
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Coupling Reagents
A series of thianthrenium salts have been
3
À
developed that can undergo C(sp ) S
C. Chen, M. Wang, H. Lu,* B. Zhao,
bond cleavage and be used as reliable
alkyl reagents. This ability has been
demonstrated in copper-catalyzed Sono-
gashira and Kumada cross-coupling
reactions under mild conditions.
Z. Shi*
&&&& — &&&&
Enabling the Use of Alkyl Thianthrenium
Salts in Cross-Coupling Reactions by
Copper Catalysis
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!