Nickel-Catalyzed Intermolecular Carbosulfonylation of Alkynes via Sulfonyl Radicals

Article abstract of DOI:10.1002/anie.201704862

β,β-Disubstituted vinyl sulfones were obtained with complete regio- and stereocontrol in a multicomponent reaction involving alkynes, organoboronic acids, and sulfonyl chlorides in the presence of a nickel catalyst. The reaction proceeds via sulfonyl radicals generated in situ under mild reaction conditions.

Full text of DOI:10.1002/anie.201704862

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Communications
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International Edition: DOI: 10.1002/anie.201704862
German Edition: DOI: 10.1002/ange.201704862
Synthetic Methods
Nickel-Catalyzed Intermolecular Carbosulfonylation of Alkynes via
Sulfonyl Radicals
Andrꢀs Garcꢁa-Domꢁnguez, Simona Mꢂller, and Cristina Nevado*
Abstract: b,b-Disubstituted vinyl sulfones were obtained with
complete regio- and stereocontrol in a multicomponent reac-
tion involving alkynes, organoboronic acids, and sulfonyl
chlorides in the presence of a nickel catalyst. The reaction
proceeds via sulfonyl radicals generated in situ under mild
reaction conditions.
A
lkyne difunctionalizations represent one of the most
straightforward strategies for the preparation of highly
substituted olefins.[1] In this context, methods to produce
vinylsulfones[2] are very attractive owing to the high relevance
and widespread applicability of these compounds in medicinal
chemistry and organic synthesis.[3,4] Acetylenic sulfones have
typically been utilized as precursors of these highly valuable
synthetic intermediates by means of copper-catalyzed carbo-
metallation reactions with organozinc or Grignard reagents as
carbon donors.[5] However, the need for pre-functionalized
alkynes coupled to the use of sensitive organometallic
reagents has limited the applicability of these transforma-
tions. Difunctionalizations of alkynes that involve the incor-
poration of an SO2-containing group represent a much more
convenient strategy. Thus, sulfonylations of alkynes with
Scheme 1. Intermolecular carbosulfonylations of terminal alkynes.
stereoselective carbosulfonylation of alkynes using sulfonyl
chlorides and boronic acids as reaction partners (Scheme 1B).
The use of mild reaction conditions, in combination with the
stability and ready availability of the reaction partners,
enables the rapid construction of b,b-disubstituted vinyl
sulfones with excellent functional-group tolerance. This
strategy not only complements previous methods, but also
substantially expands the scope with respect to the obtained
carbosulfonylation products by avoiding the use of toxic or
sensitive organometallic reagents.
To find the optimal reaction conditions, 3-fluorophenyla-
cetylene, 4-tert-butylboronic acid, and benzenesulfonyl chlo-
ride were initially selected as starting materials. After initial
screening,[14] NiCl2(Py)4, an air- and moisture-stable precata-
lyst, together with terpyridine-type ligands and K3PO4 as
a base was found to be a suitable combination for a productive
outcome. Specifically, the use of 4,4’,4’’-tri-tert-butyl-2,2’:6’,2’’-
terpyridine (L) delivered the desired vinyl sulfone 1 in 66%
yield when the reaction was carried out in toluene at 808C
(Table 1, entry 1). Remarkably, a single isomer, whose
identity was confirmed by X-ray diffraction analysis, could
be observed in the reaction mixtures.[14] In the absence of
nickel or ligand, no product formation was observed (Table 1,
entries 2,3) whereas the use of the preformed precatalyst
[(L)NiCl2] improved the reaction efficiency, yielding 83% of
the trisubstituted olefin 1 (Table 1, entry 4).
[6]
[7]
[8]
À
À
À
À
concomitant formation of C H, C O, C Se, and C
halide[9] bonds have been extensively explored.[10] In contrast,
carbosulfonylations involving the simultaneous formation of
À
À
C SO2 and C C bonds have been mostly limited to intra-
molecular settings.[11] Intermolecular processes rely on two-
step procedures where halosulfonylation of the alkyne is
followed by a Pd-catalyzed cross-coupling to deliver the
desired product (Scheme 1A).[9] These methods demand two
different catalytic systems and sometimes also harsh reaction
conditions, which limits their synthetic utility. One-step
processes have only recently been developed, as exemplified
by Wu and co-workers with the perfluoroalkylsulfonylation of
terminal alkynes by combining C-centered perfluoroalkyl
radicals, DABCO·2SO2 (DABSO), and hydrazines (Scheme
1B).[12] While these reactions are highly stereoselective, only
sulfonyl hydrazide derivatives containing fluoroalkyl groups
can be produced. As a result, carbosulfonylations of alkynes
that allow the introduction of diverse carbon functionalities in
all reaction partners are still lacking. Given our ongoing
interest in the difunctionalization of p systems,[13] we present
here a general, nickel-catalyzed, three-component regio- and
The nature of the boron partner was key for a successful
outcome, since the use of BF3K or Bpin derivatives consid-
erably reduced the yield of the desired product (Table 1,
entries 5,6). Moreover, other metal precatalysts were ineffi-
cient, thus highlighting the crucial role of nickel in this
transformation (Table 1, entries 7–9).
[*] A. Garcꢀa-Domꢀnguez, S. Mꢁller, Prof. Dr. C. Nevado
Department of Chemistry, University of Zurich
Winterthurerstrasse 190, 8057 Zurich (Switzerland)
E-mail: cristina.nevado@chem.uzh.ch
With the optimized conditions in hand (Table 1, entry 4),
we set out to explore the reaction scope. First, several alkynes
Supporting information for this article can be found under:
https://doi.org/10.1002/anie.201704862.
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Table 1: Optimization of reaction conditions.
Table 2: Reaction scope.
Entry
Deviations from above
Yield of 1 [%][a]
1
2
3
4
5
6
7
8
9
None
Without NiCl2(Py)4
Without L
[(L)NiCl2] (10 mol%)
4-t-Bu-C6H4-BF3K (2.0 equiv)
4-t-Bu-C6H4-Bpin (2.0 equiv)
PdCl2/L (10 mol%)
CuI/L (10 mol%)
74 (66)
0
0
85 (83)
18
0
0
3
FeCl2·4H2O/L (10 mol%)
0
[a] Yield determined by 19F-NMR with PhCF3 as an internal standard. In
brackets: yield of isolated product after column chromatography.
were evaluated in combination with 4-tert-butylphenylbor-
onic acid and benzensulfonyl chloride as reaction partners
(Table 2A). Multiple aromatic groups were tolerated, as
demonstrated by the reactions affording products 1–14 in
good to excellent yields. The use of bench-stable and poorly
nucleophilic organometallic reagents translates into high
functional-group compatibility, so that a formyl substituted
arylalkyne could be successfully transformed into 6 in 82%
yield without the use protecting groups. The orthogonality of
this method is also remarkable, since pinacolboryl (Bpin)-
substituted 7 was obtained in 69% yield. Furthermore,
triflates and bromides (8 and 9) turned out to be compatible
with the mild reaction conditions, thus offering the possibility
of further derivatization of the corresponding alkenes
through classical Pd-catalyzed cross-coupling reactions. Addi-
tionally, the use of more hindered ortho-substituted arenes
was also well tolerated, as demonstrated by the reactions
giving 9–11. The method proved useful in the construction of
derivatives containing biologically relevant moieties or fluo-
rophores, as in the case of 13 and 14. Unfortunately, neither
alkyl-substituted terminal alkynes nor internal ones could be
transformed under the standard reaction conditions. The
scope with respect to the organometallic partner was inves-
tigated next using phenylacetylene and benzenesulfonyl
chloride as partners (Table 2B). Phenylboronic acid deriva-
tives containing either electron-donating or electron-with-
drawing groups were efficiently accommodated to yield
alkenes 15–21 in moderate to excellent yields. In addition,
heterocycles participated successfully in the reaction, furnish-
ing products 22–24, whereas vinyl or alkyl boronic acids were
not converted under the reported conditions. The substitution
pattern and the nature of the sulfonyl chloride were also
evaluated in the presence of methyl 4-ethynylbenzoate and 4-
tert-butylphenylboronic acid (Table 2C).
[a] Reaction Conditions: Alkyne (0.15 mmol), R2B(OH)2 (0.30 mmol),
R3SO2Cl (0.30 mmol), K3PO4 (0.4 mmol), [(L)NiCl2] (0.015 mmol),
Toluene (4.0 mL), 808C. [b] At 1008C.
excellent yields, while the presence of electron-deficient
groups led to only moderate yields (29–30). More sterically
demanding sulfonyl chlorides could be successfully engaged
in the reaction to form 31 and 32 in 72% and 59% yield,
respectively. Heteroarenes were also tolerated, as demon-
strated by the synthesis of thiophene and dibenzofuran
derivatives 33 and 34. Remarkably, cyclopropanesulfonyl
chloride could be used as well in this transformation,
delivering the corresponding alkylsulfone 35 in 75%
yield.[15] The reaction is highly selective towards the formation
of trisubstituted vinyl sulfones: in all studied cases, no alkynyl
sulfones could be detected, whereas traces of Suzuki-cou-
pling-type products could be observed only in a few cases in
the crude reaction mixtures (< 5% yield with respect to
boronic acid).
Control experiments were performed to unravel the
reaction mechanism. The addition of 2,2,6,6-tetramethylpi-
peridine-1-oxyl (TEMPO) or the presence of air completely
suppressed product formation, whereas the use of other
radical inhibitors such as tert-butylhydroxytoluene (BHT) or
1,4-dinitrobenzene partially reduced the yield, thus suggest-
ing the presence of radical species along the reaction course.
The radical nature of the reaction was further supported by
the formation of the cyclic compound 36 when 1-naphthale-
nesulfonyl chloride was used, as a result of intramolecular
Substituents in either ortho or meta positions of the arene
gave the desired carbosulfonylation products 25–28 in good to
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trapping of the arene with a vinyl radical intermediate
(Scheme 2A).[9h] The presence of vinyl chlorides as putative
reaction intermediates resulting from a nickel-mediated
alkyne halosulfonylation was also evaluated (Scheme
Scheme 3. Mechanistic proposal.
In summary, a three-component, regio- and stereoselec-
tive carbosulfonylation of terminal alkynes is presented. The
process uses nickel catalysis to enable the rapid and selective
synthesis of b,b-disubstituted vinyl sulfones through simulta-
neous addition of SO2R and aromatic groups across the
p system. Additionally, the use of stable metallic components
and readily available sulfonyl chlorides confers operational
simplicity, wide scope, and broad functional-group tolerance.
Initial control experiments provide evidence supporting the
involvement of sulfonyl radicals and rule out the presence of
vinyl chlorides as putative reaction intermediates.
Scheme 2. Control experiments.
2B).[16] Experiments in the absence of the boronic partner
did not show conversion even when using a Ni0 precatalyst
(see the Supporting Information for details),[14] thus indicat-
ing that neither simple NiII nor Ni0 complexes are able to
activate the sulfonyl chloride and implicating the boronic
acid, at least, in the initial stages of this transformation. On
the other hand, when the reaction was carried out with
a catalytic amount of organoboronic acid, only trisubstituted
olefin 1 was observed in moderate yields, whereas vinyl
chloride 37 could not be detected in the reaction mixture.
Since vinyl chlorides could have been formed and engaged in
a fast coupling reaction with the boronic acid, 37 was
subjected to the standard reaction conditions. As shown in
Scheme 2B, 37 turned out not be competent in a Suzuki cross-
coupling process, thus ruling out its participation as produc-
tive intermediate in these transformations.
Acknowledgements
We thank the European Research Council (ERC Starting-
grant agreement no.307948) and the Swiss National Science-
Foundation (200020_146853) for financial support and Prof.
Anthony Linden for the X-ray crystal structure determination
of compound 1.
Conflict of interest
Based on these experiments, a reaction mechanism is
proposed that invokes the in situ generation of Aryl-NiI
species A as responsible for the reduction of the sulfonyl
The authors declare no conflict of interest.
chloride to generate sulfonyl radical B (Scheme 3).[17,18]
B
Keywords: carbosulfonylation · nickel · stereoselectivity ·
then adds reversibly to the terminal position of the alkyne,
delivering vinyl radical D,[19] which recombines fast with the
Aryl-NiII species C generated in the previous step (Path I).
The resulting NiIII complex E delivers the desired product
through reductive elimination to give NiI, which regenerates
the active species A by means of a transmetallation with the
organoboronic acid. Alternatively, the desired product could
be formed through a Suzuki cross-coupling reaction from
vinyl chloride F (Path II), but the results summarized in
Scheme 2 seem to rule out this possibility.[20]
sulfonyl radicals · vinyl radicals
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À
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Manuscript received: May 11, 2017
Version of record online: && &&, &&&&
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Synthetic Methods
A. Garcꢁa-Domꢁnguez, S. Mꢂller,
C. Nevado*
&&&& — &&&&
Running mild: Readily available alkynes,
boronic acids, and sulfonyl chlorides can
be combined in the presence of a Ni
catalyst to produce b,b-disubstituted
vinyl sulfones with complete regio- and
stereocontrol. The reaction proceeds via
Nickel-Catalyzed Intermolecular
Carbosulfonylation of Alkynes via Sulfonyl
Radicals
sulfonyl radicals generated in situ under
mild reaction conditions and with high
functional-group tolerance.
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