Electrochemically Promoted Nickel-Catalyzed Carbon-Sulfur Bond Formation

Article abstract of DOI:10.1021/acscatal.8b04633

This work describes a nickel-catalyzed Ullmann-type thiolation of aryl iodidesunder mild electrochemical conditions. The simple undivided cell with graphene/nickel foam electrode setups offers excellent substrate tolerance, affording aryl and alkyl sulfides in good chemical yields. Furthermore, the mechanism for this electrochemical cross-coupling reaction has been investigated by cyclic voltammetry.

Full text of DOI:10.1021/acscatal.8b04633

Letter
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2019, 9, 1630−1634
Electrochemically Promoted Nickel-Catalyzed Carbon−Sulfur Bond
Formation
Yang Wang, Lingling Deng, Xiaochen Wang, Zhengguang Wu, Yi Wang,* and Yi Pan
State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023, China
S
* Supporting Information
ABSTRACT: This work describes a nickel-catalyzed Ullmann-
type thiolation of aryl iodidesunder mild electrochemical
conditions. The simple undivided cell with graphene/nickel
foam electrode setups offers excellent substrate tolerance,
affording aryl and alkyl sulfides in good chemical yields.
Furthermore, the mechanism for this electrochemical cross-
coupling reaction has been investigated by cyclic voltammetry.
KEYWORDS: electrochemistry, nickel, Ullmann coupling, C−S bond, metal catalysis, thiolation, arylation
Table 1. Optimization of the Reaction Conditions
he classical Ullmann-type coupling reactions have been
T
developed for over a century for C−N, C−O, and C−S
bond formations.1 The scope of the original copper-catalyzed
biaryl coupling was limited to electron-deficient aryl halides
and requires harsh reaction conditions.2 Modern variants of
the Ullman reaction employing palladium and nickel have
widened the substrate scope and rendered conditions milder,
although these efforts were plagued by several drawbacks The
cross-coupling of thiols with aryl halides generally rely on the
conversion of the thiols to their corresponding thiolates by
means of transition-metal catalysis. The strong coordination of
thiolates to metals often leads to catalyst deactivation and dis-
plays low efficiencies. Thus, Ullmann thiolation is considered
more challenging in contrast to amination and etherification,
which requires high catalyst loading, specific ligand, excessive
heating, and strong base to facilitate this transformation
(Scheme 1).3 Recent development using photoinduced thiol
radicals4 as sulfur source could avoid the problem of catalyst
b
entry [Ni] catalyst ligand solvent
electrolyte
yield
9%
1
2
NiCl2
NiBr2
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L1
L1
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
MeCN
DMA
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
n-Bu4NBF4 (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (4 equiv)
LiBr (1 equiv)
30%
76%
59%
91%
21%
87%
51%
trace
trace
20%
21%
35%
3
NiI2
4
Ni(acac)2
5
6
7
8
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
Scheme 1. Nickel-Catalyzed Ullmann-Coupling of Thiols
with Aryl and Heteroaryl Iodides
9
10
11
12
13
Received: November 18, 2018
Revised: January 22, 2019
© XXXX American Chemical Society
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ACS Catalysis
Letter
Table 1. continued
with X-H (X = N, O, S) donors could offer a complementary
solution to access synthetically and medicinally useful aryl
amines, esters, and thioesters. To our surprise, despite the
potential utility of such transformation, only one example of
electrochemical Buchwald−Hartwig amination and esterifica-
tion has been described previously.12 No electrochemically
enabled Ullmann-type thiolation has been reported (Scheme 1).
It is noteworthy that a nickel-catalyzed electrochemical C−H
amination has also been reported by Lutz Ackermann group
rencently.9f
b
entry [Ni] catalyst ligand solvent
electrolyte
yield
14
15
16
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
NiCl2·glyme
L1
L1
L1
L1
DMA
DMA
DMA
DMA
LiBr (2 equiv)
LiBr (3 equiv)
LiBr (4 equiv)
LiBr (3 equiv)
69%
96%
87%
c
d
17
99%
a
Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), solvent (4 mL)
under argon atmosphere for 12 h, GFE = graphite felt electrode,
b
FNE = foamed nickel electrode. 19F NMR yield based on 1a.
c
d
3 V voltage. 92% isolated yield based on 1a.
The proposition of Ullmann thiolation protocol under
electrochemical conditions might be realized through careful
redox manipulation of the nickel catalyst as each electrochem-
ical process seamlessly combines concurrent anodic oxidations
with cathodic reductions.13 Thus, various oxidation states of
nickel complexes could coexist in harmony under electrolytic
conditions and precisely initiated and ceased by fine-tuning
of the cell voltage.14 The use of an undivided cell is rare and
challenging in this situation.15 Graphene/nickel foam electro-
des were chosen in order to enhance the charge exchange.16
Thus, we carried out electrolysis experiments with a survey of
nickel catalysts using p-toluene thiol and p-iodobenzene in
LiBr/DMA solution (Table 1). These studies reveal that nickel
salt and ligand is able to promote the desired cross-coupling
sequence. NiCl2, NiBr2, and NiI2 could afford the thiolation
product in low to moderate yields (entries 1−3). We next
tested Bu4NBF4 as electrolyte, but just 21% yield was afforded
(entry 6). A series of ligands were screened (entries 7−10).
Good to excellent yields were achieved with sterically hindered
groups at para position of the dipyridyl ligands (entries 5 and 7).
Poor conversions were observed using L4−L6 (entries 9−11).
Other solvent such as MeCN provided a very poor result.
Scheme 2. Scope of Thiol Coupling Partner
Scheme 3. Scope of (Hetero)aryl Iodide Coupling Partner
a
Reaction conditions: (hetero)aryl iodides (0.20 mmol), thiols
(0.40 mmol), [Ni] (0.02 mmol), dtbbpy L1 (0.03 mmol), pyridine
b
(0.4 mmol), DMA (4 mL), LiBr (0.60 mmol). Isolated yield based
on iodides.
poison, although restricted substrate scope was displayed and
heteroaryl thiols were not reported. Presumably, strong oxi-
dizing iridium(III) photocatalyst might interfere with the genera-
tion of low-valent nickel species and hindered reductive elimi-
nation step.5
The advances of electrochemical cross-coupling process have
drawn great attention, and many groups have joined this arena.
The redox-efficiency, innate scalability and sustainability6 of
such process prompted the investigation of electrochemical dehy-
drogenative cross-coupling reactions for the C−C,7 C−O,8
C−N,9 and C−S10 bond formations. However, in a majority of
electrochemical dehydrogenative cross-coupling, the substrates
were limited to electron-rich (hetero)arenes with regioselctive
and chemoselective drawbacks.11 We speculated that a elec-
trochemically enabled Ullmann-type cross-coupling of aryl halides
a
Reaction conditions: (hetero)aryl iodides (0.2 mmol), thiols
(0.4 mmol), [Ni] (0.02 mmol), dtbbpy (0.03 mmol), pyridine
(0.4 mmol), DMA (4 mL), LiBr (0.60 mmol). Isolated yield based
on iodides. Made from alkyl bromide.
b
c
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Letter
Figure 1. (A) Gram scale of the reaction; (B) Further elaboration; (C) Radical trapping experiments; (D) Cyclic voltammetry studies;
(E) Proposed reaction mechanism.
The ratio of the added electrolyte has a significant influence
on the yields (entries 13−16). The highest isolated yield was
achieved with 3 equiv of LiBr at Ecell = 3v (entry 17). Under
the optimized conditions, various thiols 2 reacted smoothly with
aryl iodides 1a and 1b to provide sulfides a range of substituted
aryls (3a, 3c, 3f−3q) in good to excellent yields (Scheme 2).
Notably, alkyl (3d, 3r−3v), heteroaryls (3w and 3x), and ester
(3e) bearing thiols, which have proven to be challenging sub-
strates using other cross-coupling procedures, also underwent
highly selective reactions under the present conditions.
The scope of the aryl iodide coupling partner was also explored.
Using conditions identical to those employed in Scheme 2, a
range of aryl iodides 1 underwent radical thiolation with p-methyl
or methoxyl thiophenol to provide sulfides 4 (Scheme 3). Aryls
bearing ketone (4e), ester (4f), and borane (4g) functionalities
were tolerated in this electrochemically enabled radical process.
Heteroarenes including thiophene (4l), pyridine (4m), furan
(4n), carbazole (4o), and benzopyrazole pyridine (4m), furan
(4n), carbazole (4o), and benzopyrazole (4p) substituted
iodides also furnished the corresponding products in high
efficiency. Notably, linear ester could also be well-adopted in
this nickel-catalytic system (4q).
in 63% yield (Figure 1B). In the radical-scavenging experiment,
by adding 3 equiv of TEMPO, the cross-coupling product was
completely suppressed, and the homocoupling product 7 was
observed (Figure C-i). Further trapping of thiol radical with
diphenylacetylene 8 under the same conditions furnished the
thiolated olefin 9 in 52% yield, which verified the radical
pathway of this process (Figure C-ii). No evidence indicated
the presence of bromine radicals in the reaction mixture (see
Supporting Information). Therefore, it is likely that thiols are
directly oxidized to thiol radicals at the anode. To elucidate the
reaction mechanism, cyclic voltammetry experiments were per-
formed at the concentration of 10−4 M in MeCN with n-Bu4NPF6
as electrolyte (0.2 M) using glassy carbon working electrode,
Pt wire counter electrode, and SCE reference electrode with
scan rate at 0.2 V·s−1. First, we compared the redox potentials of
all the possible reactive intermediates. As shown in Figure 1D-i,
iodide 1b and Ni(II) catalyst displayed relatively high oxidative
potentials (2.17 and 1.03 V vs SCE, respectively). Thiol 2b
exhibited multiple irreversible oxidative waves from 0.88 V vs SCE.
By adding pyridine to 2b, only one oxidative wave showed at
1.04 V, indicating that pyridine could stabilize the oxidation
process of thiols. These anodic events substantiated the initi-
ation of catalytic cycle was more likely to be oxidation of 2b to
its radical G in assistance of base. In a separate voltammetric
study, we measured the redox potentials of nickel complex
species (Figure 1D-ii). An oxidative wave at 1.21 V was
observed with the preformed Ni(0)-L, which shifted to 0.74 V
on the addition of iodide 1b to the Ni(0) complex. These
events strongly suggested that Ni(0) was the actual reactive
Further elaboration of this electrochemical coupling sequence
is presented in Figure 1. The scalability of this reaction was
demonstrated through the cross-coupling of p-iodobenzonitrile
and 4-methoxybenzenethiol on gram scale under the standard
conditions with 100 mL undivided cell setup (Figure 1A).
Electrochemical cross-coupling can also derivatize thiol motifs
in bioactive molecules such as iodinated estrone 5, affording 6
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Letter
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species and oxidative insertion of aryl iodide resulted in the for-
mation of Ni(II) intermediate. Based on the above evidence,
a plausible mechanism for this Ni-catalytic electrochemical
thiolation is proposed. As illustrated in Figure 1E, a single
electron transfer (SET) oxidation of the thiol on anode pro-
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.8b04633.
■
S
Synthetic procedures and charaterization data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: yiwang@nju.edu.cn.
ORCID
Yang Wang: 0000-0002-0222-2083
Yi Wang: 0000-0002-8700-7621
Author Contributions
All authors have given approval to the final version of the
manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21472082, 21402088, and 21772085)
and the Fundamental Research Funds for the Central Universities
(No. 020514380148).
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