Copper-Catalyzed Difluoroalkylation-Thiolation of Alkenes Promoted by Na2S2O5

Article abstract of DOI:10.1002/ejoc.202100091

A novel and convenient protocol for the difluoroalkylation-thiolation of alkenes catalyzed by Cu/Na₂S₂O₅ system has been developed. This reaction was carried out using readily available starting materials under mild conditions, wherein the C?C and C?S bonds simultaneously were constructed smoothly. The reaction features broad substrate scope of alkenes and disulfides, good functional group tolerance, and good to excellent yields with high selectivity.

Full text of DOI:10.1002/ejoc.202100091

Communications
doi.org/10.1002/ejoc.202100091
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Copper-Catalyzed Difluoroalkylation-Thiolation of Alkenes
Promoted by Na2S2O5
Bao Gao+,[a] Yingjie Ni+,[a] Xiaojun Liu,[a] Tao Jiang,[a] Qian Yan,[a] Ruiting Yang,[a] and
Xiuli Zhang*[a, b]
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tion of alkenes has emerged as a powerful method for the
construction of these compounds, such as difluoroacetylation
with arylation,[8] oxylation,[9] halogenation reactions.[10] More-
over, the visible-light-mediated difluoroacetylation of alkenes
also have been developed as a useful alternative for the
synthesis of α,α-difluoroacetates.[11]
The sulfur-containing molecules are prevalent in a broad
range of materials science, pharmaceuticals, and natural
products, and often display interesting various biological
activities.[12] With this in the background, the simultaneous
construction of CÀ C and CÀ S bonds via the C=C bond has
successfully been disclosed by the groups of Song[13] and Cai.[14]
Very recently, Song has reported commercially available halodi-
fluoroalkyl reagents displaying versatile reactivities, represent
one of the significant synthetic building blocks and key
subunits of pharmaceutically active molecules. Moreover, Cai
et al. have developed a new protocol for difluoroalkylation-
thiolation of alkenes catalyzed by iron facilitated photoredox, in
which the CÀ C and CÀ S bonds simultaneously were constructed
smoothly under mild conditions.
Based on the background above, we assumed if we could
use directly disulfides as a sulfur source to achieve difluoroalky-
lation-thiolation of alkenes, which can efficiently expand the
reaction efficiency from economic performance. In view of this,
it is reasonable to hypothesize that if the alkyl radical generated
by the addition of difluoroalkyl radicals to alkenes is directly
captured by disulfide reagent, which could accomplish a direct
difluoroalkylation-thiolation of alkenes from the perspective of
atomic economy (Scheme 1). Although, in Cai’s work,[14] disul-
fide was utilized as sulfur source to give desired product 3aa in
their control experiment, only 18% yield was obtained. Due to
few reports and great potential in this area, the development of
concise approaches to introduce the difluoroalkyl group
A novel and convenient protocol for the difluoroalkylation-
thiolation of alkenes catalyzed by Cu/Na2S2O5 system has been
developed. This reaction was carried out using readily available
starting materials under mild conditions, wherein the CÀ C and
CÀ S bonds simultaneously were constructed smoothly. The
reaction features broad substrate scope of alkenes and
disulfides, good functional group tolerance, and good to
excellent yields with high selectivity.
Multicomponent reactions, defined as synthetic protocols that
join together three or more coupling partners have emerged as
strategically efficient and synthetically useful methods for
constructing complex carbon frameworks from relatively simple
precursors, thus meeting diverse transformation of easily
available substrates in all fields of organic synthesis.[1] Among
these reactions, transition-metal-catalyzed alkene difunctionali-
zation reactions that involve the installation of two vicinal
functional groups in one step have been studied as powerful
tools for the construction of substituted alkanes, and valuable
building blocks for natural and biologically active compounds
from simple starting materials.[2]
Compared with the common molecules, fluorine-containing
compounds generally show completely different physical and
chemical properties and the introduction of fluorine can
generally enhance the lipophilicity, bioavailability, and metabol-
ic stability of its parent molecules.[3] In view of the ubiquity of
organofluorine groups in materials science and biologically
active molecules,[4] the difunctionalization of unsaturated bonds
is a direct way to construct complex molecular frameworks that
otherwise multistep synthesis is required. Among these fluori-
nated moieties, the bromodifluoroalkyl compounds are ex-
tremely appealing reagents, which can be utilized to make
valuable molecules with a different function, such as the
difluoroalkyl radical,[5] difluorocarbene precursors,[6] and C1 and
C2 synthons.[7] The transition-metal-catalyzed difluoroacetyla-
[a] Dr. B. Gao,+ Y. Ni,+ X. Liu, T. Jiang, Q. Yan, R. Yang, Prof. X. Zhang
Department of Applied Chemistry, Anhui Agricultural University,
Hefei, 230036, People’s Republic of China
E-mail: zhangxiuli@ahau.edu.cn
http://jsxx.ahau.edu.cn/jsxx_show.asp?ID=1983032
[b] Prof. X. Zhang
State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry,
Shanghai 200032, PR China
[+] Authors contributed equally.
Scheme 1. Copper-catalyzed three-component difluoroalkylation-thiolation
of alkenes.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100091
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synchronously with sulfur-containing groups into alkenes
copper catalysts, and a series of copper catalysts were
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remains a meaningful task. Herein, we disclosed an efficient and
practical approach to accomplish difluoroalkylation-thiolation of
alkenes using disulfide reagents as sulfur source and Na2S2O5 as
a reducing agent in the presence of copper catalyst under mild
reaction condition.
At the outset of the project, we examined the feasibility of
the difluoroalkylation-thiolation of styrene with disulfide re-
agent 2a and ethyl (bromodifluoroacetate) BrCF2CO2Et in the
presence of a copper catalyst. First of all, a variety of reductants
were screened with 20 mol% of CuTc as a catalyst, 2 equiv. of
CsF as an additive, and CH3CN as solvent under nitrogen
investigated and the results showed that the reaction per-
formed well with CuCl as catalyst (72% yield) (Table 1, entry 8),
which was better than other copper catalysts. Thus, CuCl was
determined to be the most appropriate catalyst for this trans-
formation. Subsequently, a variety of nitrogen-based ligands
were screened, and the results were shown in Table 1. Among
the ligands examined, the L5 demonstrated the best efficiency
for giving the desired product 3aa in 74% yield in the presence
of CuCl (Table 1, entry 15). Further experiments conducted for
searching proper base found that CsF is still most efficient for
this transformation. Moreover, the amount of CsF and Na2S2O5
were also examined and no further improvements were found
(see Supporting Information). Lastly, 85% yield was furnished
when the reaction time was prolonged to 24 hours (Table 1,
entry 20). A control experiment was also conducted in the
absence of CuBr2 or L5, and showed that no desired product
was obtained.
With the optimized reaction conditions in hand, we
proceeded to evaluate the scope of this three-component
cross-coupling reaction with a variety of substituted alkenes
shown in Table 2. The results revealed that the optimized
procedure could be applied successfully to a range of different
alkenes. The substrates bearing electron-donating and -with-
drawing groups on the aromatic ring were converted to the
corresponding products in the range of 48%–85% isolated
yields. Alkenes bearing electron-donating groups in particular
showed higher levels of reactivity towards this reaction.
However, only 48% yield was obtained for MeO-substituted
substrate 2d and the detailed reason is still not clear. In
addition, the steric hindrance effect was observed. For example,
the reactions of the styrene bearing chlorine-substituent at its
o-position gave the corresponding products in lower yields,
which was lower than its m- and p-position derivatives (Table 2,
3ja–3ia). The substrates 1k and 1l with strong electron-
withdrawing group also had better reactivity, giving corre-
sponding product 3ka and 3la in 67% yield respectively.
Interestingly, naphthyl- and 4-Ph-substituted alkene 1m and 1n
were also compatible with this reaction under the optimized
conditions to give the desired products (65–72% yields)
(Table 2, 3ma, 3na). Interestingly, internal olefin 1o could
participate in this reaction in good yield with high stereo-
selectivity (dr>20:1) (Table 2, 3oa). In the case of alkenes
bearing a substituent group at α-position of double bond
underwent the current difluoroalkylation-thiolation process
smoothly in good yield. In addition to the aromatic alkenes,
aliphatic alkene 1r was also examined under the standard
reaction conditions, furnishing target product 3ra smoothly.
Once we had identified that the alkenes were adequate
substrates for this reaction system, we aimed to expand the
substrate scope to disulfides. As such, substrates containing
methyl, methoxy, fluoride, chloride, and bromide on the
aromatic ring were all competent reaction partners, providing
the corresponding products efficiently in excellent yields
(Table 3, 3aa–3ah). In addition, steric hindrance effect was
observed. For example, the reactions of the disulfides bearing
methyl-substituent at its o, m, p-position gave the correspond-
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°
atmosphere at 60 C for 12 h, which showed that Na2S2O5 was
most efficient for this transformation to furnish product 3aa in
63% yield (Table 1, entry 4).
Encouraged by these preliminary results, we considered
that solvent maybe actually make a difference and then
examined the effects of different other solvents on the reaction.
The results showed that most common solvents, such as
Toluene, 1,4-dioxane, DMSO, THF et al, were completely
ineffective, which means the CH3CN is crucial to promote this
reaction (see Supporting Information). Next, we focused on the
Table 1. Optimization of reaction conditions.[a]
Entry
Catalyst
Ligand
Reductant
Additive
Yield [%][a]
1
2
3
4
5
6
7
8
CuTc
CuTc
CuTc
CuTc
CuTc
CuO
Cu2O
CuCl
CuCl2 ·2H2O
CuBr
CuBr2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
–
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L5
L5
L5
L5
L5
–
Na2SO3
Na2S2O3
NaHSO3
Na2S2O5
Cu
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
K2CO3
NaOAc
NaHCO3
CsF
CsF
CsF
–
45
18
43
63
trace
trace
29
72
66
62
63
nr
70
27
74
63
51
29
44
85
0
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
9
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20[b]
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23
CuCl
CuCl
0
37
L5
[a] Reaction conditions: styrene 1a (0.2 mmol), 2a (0.24 mmol), BrCF2CO2Et
(0.24 mmol), [Cu] (20 mol%), Liand (20 mol%), Reductant (0.4 mmol),
°
additive (0.4 mmol), CH3CN (2 mL), 60 C, 12 h, isolated yield. [b] 24 h.
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Table 2. Substrate scope of the reaction for alkenes.[a]
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[a] Reaction conditions: alkene 1 (0.2 mmol), 2a (0.24 mmol), BrCF2CO2Et (0.24 mmol), CuCl (20 mol%), L5 (20 mol%), Na2S2O5 (0.4 mmol), CsF (0.4 mmol),
°
CH3CN (2 mL), 60 C, 24 h, isolated yield.
ing products in different yields (Table 3, 3aa–3ac). Moreover, in
the case of disulfide containing a strong electron-withdrawing
group -NO2 was also a suitable substrate, providing product 3ai
in 35% yield. It should be noted that naphthyl and heteroaryl
substrates 2j and 2k could also participate in this reaction in
excellent yield (Table 3, 3aj–3ak).
Difluoroalkylation-thiolation of estrone derivative alkene
bearing a steroid scaffold was also conducted to produce 3sa
in 68% yield with a tolerating ketone group. This trans-
formation indicated that the reaction system is amenable to
functionalization and modification of complex alkenes bearing
the skeleton of natural products, and show high potential
applications in biological evaluation (Scheme 2a). Moreover, the
product 3aa could be reduced to its alcohol derivatives 4 by
NaBH4 in 85% yield and be oxidized to its sulfone derivatives 5
by m-CPBA in 72% yield, which means these two functional
groups are extremely useful and convertible (Scheme 2b).
Control experiments were designed to probe the mecha-
nism of this reaction (Scheme 3, see the SI for details). First, the
radical scavenger TEMPO completely suppressed the model
reaction and 15% yield of the radical coupling adduct 6 was
obtained (Scheme 3a), which demonstrated the presence of
ethyl difluoroacetate radical in the process. Next, when
disulfides 2a was replaced by p-toluenethiol to examine this
reaction and the results show that no desired product 3aa was
provided, which indicated the importance of disulfides as sulfur
source under our reaction condition. Moreover, the product
3aa could be obtained in 37% yield in the absence of Na2S2O5,
which means Na2S2O5 might be accelerating the reduction of
Cu(II) species.[15a]
Based on the above experimental results and previous
literature reports,[15] possible reaction mechanism for the
difluoroalkylation-thiolation of alkenes is depicted in Scheme 4.
Firstly, the initiation of the catalytic cycle occurs with oxidation
of Cu(I) species by BrCF2CO2Et via a single-electron transfer
process, generating reactive difluoroalkyl radical I and Cu(II)
concomitantly. Subsequently, the addition of the radical I to
styrene gives a transient benzyl radical intermediate II, which
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Table 3. Substrate scope of the reaction for disulfides.[a]
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[a] Reaction conditions: styrene 1a (0.2 mmol), 2 (0.24 mmol), BrCF2CO2Et (0.24 mmol), CuCl (20 mol%), L5 (20 mol%), Na2S2O5 (0.4 mmol), CsF (0.4 mmol),
°
CH3CN (2 mL), 60 C, 24 h, isolated yield.
Scheme 2. Synthetic transformations.
reacts directly with disulfides to furnish desired product 3aa.
Finally, recycling Cu(I) species is obtained after the reduction of
Cu(II) species by solvent (CH3CN) or Na2S2O5.
thiolation of alkenes to other substrates for the synthesis of
natural bioactive products are in progress.
In summary, we have developed a new protocol for the
difluoroalkylation-thiolation of alkenes catalyzed by Cu/Na2S2O5 Acknowledgements
system with disulfides as a sulfur source. This reaction allows a
variety of alkenes and disulfides, giving easy access to
difluoroalkylation-thiolation of alkenes. Further investigations
on the mechanism and applications of the difluoroalkylation-
We are grateful for financial support from the National Natural
Science Foundation of China (21702197) and Anhui Agricultural
University (yj2018-45).
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Scheme 3. Control experiments.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Copper · Homogeneous catalysis · Multicomponent
reactions · Synthetic methods
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Manuscript received: January 26, 2021
Revised manuscript received: March 3, 2021
Accepted manuscript online: March 7, 2021
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