TMSCl-Catalyzed Electrophilic Thiocyano Oxyfunctionalization of Alkenes Using N-Thiocyano-dibenzenesulfonimide

Article abstract of DOI:10.1021/acs.orglett.9b01706

Numerous electrophilic thiocyano oxyfunctionalization reactions of alkenes have been achieved using N-thiocyano-dibenzenesulfonimide, which is a new electrophilic thiocyanation reagent and could be easily prepared in two steps from dibenzenesulfonimide. T

Full text of DOI:10.1021/acs.orglett.9b01706

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
TMSCl-Catalyzed Electrophilic Thiocyano Oxyfunctionalization of
Alkenes Using N‑Thiocyano-dibenzenesulfonimide
Ai-Hui Ye, Ye Zhang, Yu-Yang Xie, Hui-Yun Luo, Jia-Wei Dong, Xiao-Dong Liu, Xu-Feng Song,
Tongmei Ding, and Zhi-Min Chen*
Affiliated Sixth People’s Hospital South Campus and School of Chemistry and Chemical Engineering, Shanghai Key Laboratory for
Molecular Engineering of Chiral Drugs, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: Numerous electrophilic thiocyano oxyfunction-
alization reactions of alkenes have been achieved using N-
thiocyano-dibenzenesulfonimide, which is a new electrophilic
thiocyanation reagent and could be easily prepared in two
steps from dibenzenesulfonimide. This approach provides
efficient, simple, and modular methods for the formation of
SCN-containing heterocycles such as lactones, tetrahydrofur-
ans, dihydrofurans, and dihydrobenzofurans in moderate to
excellent yields. Meanwhile, diverse oxa-quaternary centers
were rapidly constructed. Additionally, this protocol is free of
transition metals and features broad substrate toleraance and
mild reaction conditions.
rganosulfur compounds widely exist in biologically active
natural products, drug molecules, and functional
using an excess amount of oxidant. For instance, radical
thiocyano lactonization of 2-(1-phenylvinyl)benzoic acid and
its derivatives proceeded smoothly and gave excellent yields.
However, hardly any reaction could be observed using 4-
phenylpent-4-enoic acid 2a as the substrate under radical
conditions (Scheme 1a).7b Thus, the development of
complementary and new approaches to the synthesis of
SCN-containing compounds is still highly desirable.
O
materials.1 Among them, thiocyanates not only are present in
bioactive compounds such as HDAC inhibitor psammaplin B
and fasicularin exhibiting cytotoxic activity but also are
versatile and common building blocks due to their easy access
to trifluoromethylthiol compounds, thioethers, thiotetrazole,
etc. (Figure 1).2,3 Consequently, tremendous effort has been
Electrophilic functionalization of alkenes has proven to be an
efficient, widely applicable, and remarkable method for the
preparation of functionalized compounds. During the past
several years, electrophilic halogenation,8 trifluoromethylth-
iolation,9 and sulfenylation10 of alkenes have been widely
investigated. To the best of our knowledge, electrophilic
thiocyano functionalization of alkenes has not been achieved,
although some electrophilic thiocyanations of arenes and β-
keto carbonyl compounds have been reported by the Chen
group and others.11 Herein, we introduce our preliminary
results for a series of electrophilic thiocyano oxyfunctionaliza-
tion reactions of alkenes using a new thiocyanation reagent,
which is part of our ongoing study of the efficient synthesis of
organosulfur compounds (Scheme 1b).12
Our investigation began with the synthesis of N-thiocyano-
dibenzenesulfonimide 1e. As shown in Figure 2, some
electrophilic thiocyanation reagents have been developed; for
example, N-thiocyanatosaccharin (1d) was reported by the Yin
and Chen groups in 2018.11d Compared with electrophilic
SCF3 reagents,13 thiocyanation reagents are relatively scarce. In
Figure 1. Representative examples of SCN-containing bioactive
compounds.
dedicated to developing practical, simple, and highly efficient
methods for the introduction of the SCN group into organic
motifs.4,5 Traditionally, cyanations of thiols or thioethers are
indirect and limited routes for the construction of SCN-
containing compounds.6 Recently, some radical thiocyano
functionalizations of alkenes have been described, which offer a
direct and attractive method to thiocyanates.7 For example, the
Guo group reported a radical thiocyanooxygenation of alkenes
for the synthesis of SCN-containing heterocycles using K2S2O8
as the oxidant.7a,b The Bolm group documented a photo-
catalytic radical thiocyano functionalization of vinyl arenes
using N-SCN sulfoximines as the thiocyano source.7e Despite
these advances, radical thiocyano functionalizations of alkenes
are still limited in substrate scope and have the disadvantage of
Received: May 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01706
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Organic Letters
Letter
a
Scheme 1. Design of Electrophilic Thiocyano
Oxyfunctionalization of Alkenes
Table 1. Optimization of Conditions
b
entry
solvent
acid
yield (%)
1
2
3
4
5
6
CH2Cl2
MeCN
toluene
dioxane
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
AcCl
AcCl
AcCl
AcCl
TMSCl
MsOH
TMSCl
−
91
85
not determined
not determined
98
87
97
87
c
7
d
8
a
Reaction conditions: 2a (0.2 mmol), 1e (0.24 mmol), acid (0.04
mmol) in solvent (2 mL) stirred at 25 °C for 10−20 min under Ar.
b
c
d
Isolated yield. TEMPO (0.2 mmol) was added. The reaction time
is 5 h.
Using methanesulfonic acid gave an 87% yield (entry 6). Next,
we added radical scavenger TEMPO to the system, and the
desired product was produced without a decrease in yield,
which could exclude the radical process (entry 7). Finally, the
reaction was conducted without an acid catalyst. It was found
that the lactonization process is relatively slow (5 h) and
delivered the desired product in 87% yield (entry 8).
With the optimal reaction conditions in hand (Table 1, entry
5), the scope was tested with various olefinic acids (Scheme 2).
In general, the desired SCN-containing lactones were obtained
in moderate to excellent yields. Compared to the model
substrate (2a), the methyl group at the meta or para position
of the phenyl ring did not have an evident influence on the
yield (3c or 3d, respectively), whereas the ortho substitution
obviously decreased the yield maybe due to steric hindrance
(3b). The electronic effect of substituents was also
investigated. It was found that substrates with electron-poor
or electron-rich aryl rings gave relatively lower yields (3h and
3i). It was found that multisubstituted phenyl acids were also
suitable for this reaction producing the desired lactone
products in good yields (3j and 3k). It should be noted that
thiocyanations of arene and alkene were simultaneously
achieved using 2,4-dimethoxy-substituted substrate 2k, afford-
ing product 3k in 83% yield using 2.0 equiv of 1e.
Furthermore, the heteroaromatic ring and phenylacetylene
substrates also performed well and gave the corresponding
products in moderate yields (3l and 3m). To our satisfaction,
unbiased alkyl-substituted alkene substrates 2n and terminal
alkene 2o were incorporated well and delivered products 3n
and 3o in 85% and 84% yields, respectively. It was found that
2-(1-phenylvinyl)benzoic acid and its derivatives were also
tolerable in this transformation, delivering the corresponding
products in nearly quantitative yield (3p and 3q). Sub-
sequently, 5-phenylhex-5-enoic acid 2r was also subjected to
the reaction to produce SCN-containing δ-valerolactone 3r in
83% yield using AcCl as the catalyst.
Figure 2. Preparation of N-thiocyano-dibenzenesulfonimide.
light of the strong electrophilicity of N-SCF3-dibenzenesulfo-
nimide developed by Shen and Zhao,9f,13 we synthesized N-
thiocyano-dibenzenesulfonimide and examined its reactivity.
To our delight, 1e was easily prepared in two steps from
commercially available dibenzenesulfonimide in 61% total
yield. N-Chloro-dibenzenesulfonimide was prepared according
to previous procedures,13 which was subsequently performed
with AgSCN (1.2 equiv) in CH2Cl2 to produce 1e as a white
solid in 75% yield. It should be noted that the configuration of
1e was confirmed by X-ray crystallography.14
Using 1e developed above, we first selected 4-phenylpent-4-
enoic acid 2a as a model substrate to evaluate thiocyano
lactonization (Table 1). To our delight, 2a was facilely
converted to desired product 3a in 91% yield using acetyl
chloride as an acid catalyst in dichloromethane and at room
temperature (entry 1). Meanwhile, we also tested some other
solvents such as acetonitrile, toluene, and dioxane, but no
better results were obtained (entries 2−4, respectively). Next,
the use of trimethylchlorosilane instead of acetyl chloride
further improved the yield to 98% (entry 5). It should be
noted that a Brønsted acid also can catalyze this reaction.
Internal alkene acids were also tested. Desired cyclization
product 3s was obtained in 78% yield using (E)-5-phenylpent-
4-enoic acid 1s as a substrate, whereas desired product 3t was
delivered in only 23% yield using trisubstituted alkene acid 1t.
To demonstrate the practicability of this method, a gram-scale
reaction of 2a was performed under the standard conditions
and desired product 3a was readily obtained in 84% yield.
B
DOI: 10.1021/acs.orglett.9b01706
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Organic Letters
Letter
a
,b
a,b
Scheme 2. Scope of Olefinic Acids
Scheme 3. Scope of Olefinic 1,3-Diketone Compounds
a
Reaction conditions: olefinic 1,3-diketone compound 4 (0.2 mmol),
1e (0.24 mmol), TMSCl (0.04 mmol) in CH2Cl2 (2 mL) stirred at 25
b
°C for 10−20 min under Ar. Isolated yield.
the substrates with a methyl group at different positions as well
as different electronic substituents at the para position gave the
desired products in good yields.
a,b
Scheme 4. Scope of Alkene Alcohols
a
Reaction conditions: alkene alcohol 6 (0.2 mmol), 1e (0.24 mmol),
TMSCl (0.04 mmol) in CH3CN (2 mL) stirred at 25 °C for 10−20
b
min under Ar. Isolated yield.
To further examine the general utility of the reagent,
intermolecular thiocyano oxygenation of alkenes was also
investigated (Scheme 5). First, prop-1-en-2-ylbenzene 8a was
a
Reaction conditions: olefinic acid 2 (0.2 mmol), 1e (0.24 mmol),
TMSCl (0.04 mmol) in CH2Cl2 (2 mL) stirred at 25 °C for 10−20
b
c
d
min under Ar. Isolated yield. With 0.4 mmol of 1e added. AcCl
was used instead of TMSCl.
Scheme 5. Scope of Intermolecular Thiocyano Oxygenation
a,b
of Alkenes
After establishing the lactonization of alkenes using an acid
group as a nucleophile, we wondered whether other groups
such as ketones and alcohols can be used as nucleophiles.
Considering compounds containing the dihydrofuran frame-
work also exhibit many valuable biological activities, some
olefinic 1,3-diketone compounds were subjected to the
standard conditions as the substrates, which can furnish
SCN-containing dihydrofurans (Scheme 3). To our delight,
the reaction proceeded smoothly, forming the desired
dihydrofuran products in moderate to excellent yields.
There is no example of thiocyano etherification of alkene
alcohols, to the best of our knowledge. Inspired by the results
presented above, we sought to develop electrophilic thiocyano
etherification of alkene alcohols for the synthesis of SCN-
containing tetrahydrofurans. It was found that acetonitrile is a
suitable solvent for this transformation. As shown in Scheme 4,
a
Reaction conditions: alkene 8 (0.2 mmol), 1e (0.24 mmol), TMSCl
(0.04 mmol) in R2OH (2 mL) stirred at 25 °C for 10−20 min under
Ar. Isolated yield.
b
C
DOI: 10.1021/acs.orglett.9b01706
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Organic Letters
Letter
chosen as a model substrate with 1e, and methanol was chosen
as the solvent and nucleophile at room temperature.
Fortunately, desired product 9a was obtained in 86% yield.
Subsequently, a number of different 1,1-disubstituted alkenes
were tested using methanol as the solvent and nucleophile,
which always furnished the products in good to excellent
yields. As expected, the yield obviously decreased when the
more sterically hindering isopropanol was used.
Scheme 8. Synthetic Applications
We also carried out the reaction with silyl enol ethers for the
construction of α-SCN ketones. To our satisfaction, the linear
substrates reacted very well, producing the products in
quantitative yield. However, cyclic silyl enol ether 10c gave
an only 56% yield (Scheme 6). It is worth mentioning that an
acid initiator is not integral in this system.
a,b
Scheme 6. Scope of Silyl Enol Ethers
A plausible mechanistic pathway for the thiocyano
lactonization is proposed in Scheme 9. In the presence of an
Scheme 9. Proposed Mechanism for Electrophilic
Thiocyano Lactonization of Alkene Acid
a
Reaction conditions: alkene 8 (0.2 mmol), 1e (0.24 mmol), in
b
CH2Cl2 (2 mL) stirred at 25 °C for 10−20 min under Ar. Isolated
yield.
To further extend the application range of 1e and synthesize
diverse SCN-containing heterocycles, we studied the cycliza-
tion of olefinic phenols with 1e, which could enable the rapid
preparation of useful dihydrobenzofuran frameworks (Scheme
7). Considering that electrophilic thiocyanation may occur at
acid catalyst, a thiiranium ion intermediate 17 is easily formed.
Subsequently, intermediate 17 is attacked by a nucleophilic
carboxylic acid group to generate desired lactone 3. Due to the
strong electrophilicity of 1d, the thiiranium ion intermediate
could be formed with activated alkenes.
Scheme 7. Preliminary Studies of Cyclization of Olefinic
a,b
Phenol
In summary, N-thiocyano-dibenzenesulfonimide, a new
electrophilic thiocyanation reagent, has been designed and
readily prepared. A variety of practical and efficient electro-
philic thiocyano oxyfunctionalizations of alkenes have been
successfully developed by using N-thiocyano-dibenzenesulfo-
nimide. These transformations are valuable because various
SCN-containing compounds were obtained in moderate to
excellent yields and an oxa-quaternary center was efficiently
constructed under simple and mild conditions. Further studies
of the development of asymmetric electrophilic thiocyanations
of alkenes are underway.
a
Reaction condition A: 12a (0.2 mmol), 1e (0.24 mmol), TMSCl
(0.04 mmol), in CH2Cl2 (2 mL) stirred at 25 °C for 10−20 min
under Ar. Reaction condition B: 12a (0.2 mmol), 1e (0.24 mmol),
AcCl (0.04 mmol), in CH3CN (2 mL) stirred at 25 °C for 10−20 min
b
under Ar. Isolated yield.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01706.
■
the para position of phenol, compound 12a with a 4-Me-
phenol motif was used as a model substrate. It was found that
both TMSCl and AcCl can be used as acid initiators, and
product 13a was obtained in a higher yield (88%) using AcCl
as the acid promoter.
S
Ultimately, the synthetic utilities of products 7a and 9a have
been achieved. As indicated in Scheme 8, treatment of 7a with
acetylenylmagnesium chloride and LiCl in THF delivered
thioalkyne 14 in moderate yield. Next, nucleophilic sub-
stitution of diphenylphosphine oxide on compound 9a gave
broad potential phosphonothioate 15 in 72% yield. Addition-
ally, [3+2] cycloaddition of 9a with sodium azide was also
performed, affording desired thiotetrazole 16 in 62% yield.
Experimental details, X-ray crystallography structure of
compound 1e, analytical data for new compounds, and
NMR spectra of new compounds (PDF)
Accession Codes
CCDC 1909782 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
D
DOI: 10.1021/acs.orglett.9b01706
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Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chenzhimin221@sjtu.edu.cn.
ORCID
Zhi-Min Chen: 0000-0002-6988-8955
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for financial support provided by the
National Natural Science Foundation of China (21871178 and
21702135) and the Innovation Fund (2017B009) from the
Joint Research Center for Precision Medicine of Shanghai Jiao
Tong University.
(12) (a) Xi, C.-C.; Chen, Z.-M.; Zhang, S.-Y.; Tu, Y.-Q. Org. Lett.
2018, 20, 4227. (b) Kang, J.-C.; Tu, Y.-Q.; Dong, J.-W.; Chen, C.;
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2019, 21, 2536.
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(14) CCDC 1909782 (1e) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
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