Nickel-catalyzed reductive aryl thiocarbonylation of alkene via thioester group transfer strategy

Article abstract of DOI:10.1021/acs.orglett.0c02091

Herein reported is a nickel-catalyzed reductive aryl thiocarbonylation of alkene via thioester group transfer strategy by using simple and readily available thioesters. In contrast to traditional activation of weaker C(acyl)-S bond, the C(acyl)-C bond of

Full text of DOI:10.1021/acs.orglett.0c02091

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Letter
Nickel-Catalyzed Reductive Aryl Thiocarbonylation of Alkene via
Thioester Group Transfer Strategy
Yunxia Feng, Shimin Yang, Shen Zhao, Dao-Peng Zhang, Xinjin Li, Hui Liu, Yunhui Dong,
and Feng-Gang Sun*
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ABSTRACT: Herein reported is a nickel-catalyzed reductive aryl thiocarbonylation of alkene via thioester group transfer strategy by
using simple and readily available thioesters. In contrast to traditional activation of weaker C(acyl)−S bond, the C(acyl)−C bond of
thioester was selectively cleaved to enable this reaction under mild conditions. Furthermore, this approach features operational
simplicity and broad substrate scope, providing a complementary and practical route for thioester synthesis without requiring toxic
thiol or CO gas.
Scheme 1. Thioester Synthesis
he incorporation of thioester moiety into an organic
Tmolecule is highly attractive, since this structural motif not
only plays a critical role in biological processes such as
metabolism1 and cellular function regulation,2 but also serves
as valuable building blocks3 and versatile intermediates in
organic synthesis.4 In this context, many efforts have been
directed to the construction of thioester through different
strategies. Among all these methods, the most straightforward
methodology for thioester synthesis involves the reaction of a
thiol with an activated carboxylic acid derivative,5 or the
coupling between a thioacid and an electrophile.6 However,
both approaches suffer from significant limitations, because of
the harsh reaction conditions, high oxidation state carbon-based
reactants, and sensitivity of thiols toward oxidation, thereby
restricting the utilities of the reactions. Alternatively, several
synthetically useful methods have been developed, including the
oxidative coupling of aldehydes with thiols7 and the transition-
metal-catalyzed thiocarbonylation of alkenes or organic halides
with carbon monoxide gas.8 Most of these methodologies
require either thiols with a repulsive odor or toxic CO gas, and a
suitable coupling partner is often essential, bearing similar
limitations to conventional syntheses (Scheme 1a). As an
analogue of carboxylic acid, thioester has been intensively
investigated as coupling partner in ketone synthesis via selective
activation of a relatively weak C(O)−S bond.9 Reports on the
direct use of the thiocarbonyl group as a thioester source are
scarce; only a single recent example using photoredox/metal
Received: June 24, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02091
© XXXX American Chemical Society
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A

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a
dual catalysis to achieve thioester group transfer via an
unprecedented selective C(acyl)−C bond activation has been
disclosed by Hong and co-workers, in which diverse aryl
thioesters could be synthesized in high yields. However, the use
of aliphatic thioester failed as coupling partner in this reaction
(Scheme 1b).10 Given the importance of thioester, the
development of a novel and practical route for their synthesis
is highly desirable.
Table 1. Optimization of Reaction Conditions
b
entry
Ni catalyst
NiCl2
NiCl2
NiCl2
NiCl2
NiCl2
NiCl2
NiCl2
NiCl2
NiCl2·6H2O
Ni(acac)2
NiCl2·DME
Ni(NO3)2·6H2O
Ni(COD)2
NiCl2
NiCl2
NiCl2
NiCl2
NiCl2
ligand
solvent
toluene
CH3CN
dioxane
DMF
yield (%)
In recent years, nickel-catalyzed reductive difunctionalization
of alkene, which allows the installation of two electrophiles
across the CC double bond, has emerged to be a powerful tool
for the preparation of valuable polyfunctionalized compounds.11
In comparison with classical alkene difunctionalization with an
electrophile and a nucleophile, reductive alkene difunctionaliza-
tion avoids the preparation of sensitive organometallics and
allows the reaction to proceed under mild conditions.12,13
Through this strategy, nickel-catalyzed three-component
reductive dicarbofunctionalization of alkenes has been devel-
oped to achieve alkene alkylarylation, diarylation, and
alkylacylation by the groups of Nevado,12a,b Diao,12c,d and
Chu,12e,f respectively. Moreover, Diao,12d Wang,13a−f Peng,13g−i
Kong,13j−l and Shu13m independently described reductive
alkylarylation, dialkylation, diarylation, and arylalkenylation of
organohalide-tethered alkenes. Despite the impressive success
mentioned above, it is highly attractive to develop a novel nickel-
catalyzed difunctionalization protocol to enrich scope of
functional group introduced across the CC bond. In
continuation of our interest in alkene difunctionalization and
to explore another reaction pattern of thioester, we herein
describe a nickel-catalyzed reductive aryl thiocarbonylation of
alkene via the thioester group transfer strategy by using simple
and readily available thioester molecule as a thioester source (see
Scheme 1c).
We commenced our study with acrylamide 1a and S-(p-tolyl)
benzothioate 2a as standard substrates to identify the optimal
conditions, and typical results are summarized in Table 1.
Unfortunately, no desired product of 3aa was observed under
the conditions of NiCl2 (10 mol %), L1 (2,2′-bipyridine, 10
mol %), and zinc powder (0.6 mmol) in toluene at 80 °C for 10 h
(Table 1, entry 1). Optimization of the conditions indicated that
the solvent played an important role on this transformation:
when using dimethyl formamide (DMF) as the solvent, the
targeted product 3aa was isolated in 50% yield (Table 1, entry
4), although full conversion was not reached. In the case of N-
methyl-2-pyrrolidone (NMP), the isolated yield dramatically
increased to 74% with all starting materials consumed; and other
mediate polar solvents such as acetonitrile and 1,4-dioxane
(Table 1, entries 2 and 3) failed to maintain the process of the
coupling reaction. Further attempts made by using mixed
solvents showed that a 1:1 mixture of NMP and dimethyl
sulfoxide (DMSO) was found to be the optimal choice of
solvent, affording 3aa in 85% yield (Table 1, entries 6, 7 and 8).
Conducting reactions with other nickel salts resulted in a slight
decrease in yields (Table 1, entries 9−13), and the use of
Ni(NO3)2·6H2O completely shut down this transformation.
The screening of ligands revealed that 2,2′-bipyridine was the
best choice, and other substituted bipyridine or diazaphenan-
threne ligands (Table 1, entries 14−17) promoted this reaction
with low efficiency. Replacing Zn with Mn as the reductant led to
diminished yield (Table 1, entry 18), but the reaction did not
proceed when B2Pin2 was used (Table 1, entry 19). Finally, we
defined the optimized criteria: NiCl2 (10 mol %), L1 (10
mol %), and zinc powder (0.6 mmol) as the reductant, in a 1:1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L1
L1
NR
0
NR
50
74
85
45
47
74
67
75
0
NMP
NMP/DMSO = 1:1
NMP/CH3CN = 1:1
NMP/dioxane = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
NMP/DMSO = 1:1
74
81
78
77
72
c
56
0
d
NiCl2
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), Ni catalyst (10
mol %), ligand (10 mol %), Zn (0.6 mmol), solvent (2.0 mL), sealed
Schleck tube, N2 atmosphere, 80 °C, 10 h. Isolated yields. NR = No
b
c
d
Reaction. Mn was used. B2Pin2 was used.
mixed solvent of NMP (1 mL) and DMSO (1 mL) at 80 °C for
10 h under a N2 atmosphere.
With the optimized conditions in hand, we then focused on
exploring the substrate scope of this nickel-catalyzed aryl
thiocarbonylation, with respect to an array of thioesters
(Scheme 2). Generally, diverse aryl thioesters were applied for
this reaction with synthetically useful yields for corresponding
products. Aryl thioesters with different alkyl groups including
methyl (3aa, 3ab, 3ai) and t-butyl (3ae) at the ortho-, meta-, and
para-positions were proven to be competent coupling partners,
affording the desired products in decent to moderate yields.
Methoxyl-substituted aryl thioesters (3af−3ah) reacted
smoothly to give yields of 78%−81%, regardless of the positions
on the aromatic rings. Interestingly, reactions with substrates
bearing halogens (3ac−3ad) at the ring of arylthiols were
accomplished readily and delivered products in valuable yields,
which provided functional handles for further modification via
classic coupling reactions. In comparison with unsubstituted aryl
thioester, which afforded alkene difunctionalization product in
83% yield (3aj), the use of 2-naphthyl thioester (3ak) led to an
obvious decrease in yield, presumably because of the electronic
difference between naphthyl and phenyl. However, replacing the
phenyl ring to the 2-thiophenyl group resulted in a moderate
yield of product obtained (3al). Notably, aliphatic thioesters
(3am−3ao), which exhibited no reactivity under photoredox/
nickel dual catalysis in previous Hong’s work,10 were found to be
effective substrates and furnished the corresponding products in
B
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a b
,
process, and the desired thioesters 3ba−3ga were obtained in
moderate to good yields. This approach also exhibited good
functional group compatibility: various groups in targeted
products, including methyl (3ba, 3ca), methoxy (3da), chlorine
(3ea, 3fa), and fluorine (3ga), were tolerated. In addition, the
effect of substituent at nitrogen on this transformation was also
studied. When replacing N-methyl with N-benzyl (3ha), the
reaction could still proceed smoothly, albeit 65% isolated yield
of product observed, which could be further converted to N−H
oxindole by removal of the benzyl group. Inspired by these
results, we considered functionalizing unactivated alkenes.
However, substrates bearing the N-(2-methylallyl) moiety,
such as 1i and 1j, proved to be challenging coupling partners
with low efficiency, only resulting in a low amount of thioester
products. Subsequently, we investigated the effects of the
electrophile, and we found that aryl bromide was slightly less
reactive than aryl iodide, delivering corresponding product 3aa
in 73% yield. Another aryl bromine substrate containing two
methyl groups was also tested to provide desired 3ka in 77%
yield. Aryl triflate was also a competent substrate, exhibiting
efficiency similar to that of aryl iodide. Other substituted
acrylamides were then examined under standard conditions.
Among them, only the benzyl-substituted substrate was
smoothly converted to the corresponding product in 59%
yield (3la). However, only a complex mixture was formed when
2-phenylacrylamide was used, with all substrates consumed.
A series of control experiments was performed to gain insights
into the mechanism of this nickel-catalyzed reductive alkene
arylthiocarbonylation. When radical scavenger TEMPO was
added to the standard conditions, the reaction still reacted
smoothly with product 3aa being afforded only in 42% isolated
yield. Considering the toxic effect of TEMPO on Ni catalyst, this
finding was not adequate to serve as proper evidence in favor of a
radical-involved process. In contrast, the reactions proceeded
cleanly in the presence of other radical scavengers, such as BHT
and 1,1′-diphenylethylene, and targeted product 3aa was
produced in the yields of 60% and 82%, respectively (Scheme
4a). Moreover, a radical ring closing experiment was performed
by subjecting diallyl ether to the reaction system, and only
alkene difunctionalized product was observed, whereas ring-
closing product was not obtained (Scheme 4b). Taking these
above results into consideration, we could exclude the possibility
Scheme 2. Substrates Scope of Thioesters
a
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), NiCl2 (10
mol %), 2,2′-bipyridine (10 mol %), zinc powder (3.0 equiv), 80 °C,
b
10 h. Isolated yields.
moderate yield. In addition, this transformation could be scaled
up to 1-g quantities (1a, 1.0 g, 3.3 mmol), and targeted
compound 3aa was isolated in 57% yield.
Next, the substrate scope of aryl halides compatible with this
approach was investigated by coupling with (p-tolyl) benzo-
thioate 2a (Scheme 3). A wide range of aryl iodides with
substituents at different positions were found to be amenable to
this nickel-catalyzed reductive alkene difunctionalization
a b
,
Scheme 3. Scope with Respect to Aryl Halides
Scheme 4. Mechanistic Investigations
a
Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), NiCl2 (10 mol
%), 2,2′-bipyridine (10 mol %), zinc powder (3.0 equiv), 80 °C, 10 h.
b
Isolated yields.
C
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of existence of the C-center radical generated by homolytic Ni-
alkyl bond cleavage after the migratory insertion process.
On the basis of the above-mentioned experimental results as
well as previous literature reports from Kong,13i Nevado,12a,b
and Hong,10 a plausible catalytic cycle for this transformation is
proposed as depicted in Scheme 5. Initially, catalytically active
AUTHOR INFORMATION
■
Corresponding Author
Feng-Gang Sun − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China; orcid.org/0000-0001-9366-6087;
Email: fgsun@sdut.edu.cn
Scheme 5. Plausible Catalytic Cycle
Authors
Yunxia Feng − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China
Shimin Yang − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China
Shen Zhao − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China
Dao-Peng Zhang − School of Chemistry and Chemical
Engineering, Shandong University of Technology, Zibo 255049,
People’s Republic of China
Xinjin Li − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China
Hui Liu − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China
Yunhui Dong − School of Chemistry and Chemical Engineering,
Shandong University of Technology, Zibo 255049, People’s
Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02091
Ni(0), formed in situ under reductive conditions, undergoes
oxidative addition with aryl iodide 1 to produce Ni(II) species I.
Next, migratory insertion of intermediate I into double bond
affords a σ-alkyl-Ni(II) complex II. Upon reduction with
stoichiometric Zn(0), a more nucleophilic σ-alkyl-Ni(I)
complex III is furnished, and then the relatively weak C(acyl)-
S bond of thioester 2 preferably oxidative adds to complex III to
give acyl-Ni(III) species IV, which goes through decarbon-
ylation with the formation of intermediate V. Subsequently, CO
migratory insertion into Ni(III)-S bond generates complex VI.
Finally, reductive elimination of complex VI furnishes the
desired product 3, together with a Ni(I) species, which could
undergo hydrogen abstraction from solvent following by
reduction by Zn(0) to regenerate catalytically active Ni(0).
In summary, we developed the first nickel-catalyzed reductive
aryl thiocarbonylation of alkene via the thioester group transfer
strategy by using bench-stable and readily available thioester
molecule as thioester source without the requirements of toxic
thiol or CO gas. This approach demonstrated broad substrate
scopes: aryl, heteroaryl, and even aliphatic thioesters are
compatible. Furthermore, this approach features operational
simplicity and mild conditions, providing complementary and
novel routes for thioester synthesis. The further detail of
mechanism is still under exploration in our laboratory.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the National Natural Science
Foundation of China (No. 21801154) and Natural Science
Foundation of Shandong Province (No. ZR 201709190401).
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