Catalytic Regio- and Stereoselective Alkene Sulfenoamination for 1,4-Benzothiazine Synthesis

Article abstract of DOI:10.1002/chem.201900549

An alkene sulfenoamination reaction with 2-aminothiophenol is developed using iodide catalysis. This reaction renders access to useful 1,4-benzothiazines with good functional group compatibility including both electron-donating and electron-withdrawing substituents. The reaction is proposed to proceed through an inversion of the polarity of the thiol functionality. Our mechanistic studies reveal that both thiiranium and thiyl radical pathways are plausible and that the disulfide reagent can also function as a viable substrate in this reaction.

Full text of DOI:10.1002/chem.201900549

A Journal of
Accepted Article
Title: Catalytic Regio- and Stereoselective Alkene Sulfenoamination for
1,4-Benzothiazine Synthesis
Authors: Nur-E Alom, Navdeep Kaur, Fan Wu, Shannon Jasmine
Saluga, and Wei Li
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To be cited as: Chem. Eur. J. 10.1002/chem.201900549
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Catalytic Regio- and Stereoselective Alkene Sulfenoamination for
1,4-Benzothiazine Synthesis
Nur-E Alom, Navdeep Kaur, Fan Wu, Shannon Jasmine Saluga, and Wei Li*[a]
Abstract: An alkene sulfenoamination reaction with 2-
aminothiophenol is developed via iodide catalysis. This reaction
renders access to useful 1,4-benzothiazines with good functional
group compatibility including both electron-donating and electron-
withdrawing substituents. The reaction is proposed to proceed
through a polarity inversion of the thiol functionality. Our mechanistic
studies reveal that both thiiranium and thiyl radical pathways are
plausible and that the disulfide reagent can also function as viable
substrate in this reaction.
manipulations. Realization of an effective coupling protocol from
unadorned 2-aminothiophenols and alkenes will significantly
expedite the synthetic access to 1,4-benzothiazines for timely
biological studies.
As part of our program to access bioactive heterocycles
from simple chemical feedstock,[12] we are interested in the
utilization of alkenes as building blocks for 1,4-benzothiazine
synthesis. Based on our previous studies, we observed that a
halogen source such as Selectfluor could activate a thiol to an
electrophilic sulfur source, capable of engaging the alkene in
thiiranium formation, and ensuing nitrogen addition to afford the
desired product (Scheme 1b).[13] On the contrary, the use of
bromine afforded a complementary regioisomer proceeding via a
1,2-dibromoalkane intermediate. More importantly here, we
questioned whether an in situ generated catalytic halogen
source would suffice for such a thiol polarity inversion.[14] Herein,
we demonstrate an iodide-catalyzed alkene sulfenoamination to
produce 1,4-benzothiazines, in a straightforward manner, from
alkenes and 2-aminothiophenols.
Alkene difunctionalization is an exceptional strategy for the
rapid assembly of molecular complexity with the simultaneous
installation of two functional groups across a C=C  bond.[1] Of
these, alkene sulfenoamination is
a difficult process to
accomplish yet highly useful to access a number of sulfur (S)-
and nitrogen (N)-containing molecules.[2] A major contributing
cause to this quandary is that the thiophilic nature of many metal
catalysts often precludes the use of transition metal catalysis for
alkene thioaminations.[3] Typically, either the sulfur or nitrogen
functionality needs to be masked as an electrophile for olefin
activation.[4] Subsequent nucleophilic attack from the other
heteroatom can afford the vicinal thioamine product. In this
regard, the Denmark and Shi groups have developed distinct
and elegant strategies to enable intramolecular and
enantioselective
alkene
sulfenoaminations.[5] Alternative
intermolecular coupling processes offering a complementary and
modular entry to a range of S- and N-heterocycles, however,
remain to be exceptionally challenging and rare.[6] In particular,
intermolecular coupling protocols utilizing unfunctionalized sulfur
and nitrogen coupling partners are highly desirable due to the
ease for further synthetic elaborations.[7]
S- and N-containing heterocycles such as 1,4-
benzothiazines, are common motifs in pharmaceuticals and
agrochemicals, possess a wide range of bioactivities.[8] For
example, Rufloxacin is a second-generation fluroquinolone often
prescribed to treat bacterial infection.[9] DHBT-NE-1 has been
known to be relevant in neurodegenerative disease.[10] To
access this class of heterocycle, multistep procedures involving
-halo carbonyls, dielectrophilic precursors, or synthetic
elaborations based on existing core structures, have been
developed.[11] However, these methods are often limited to
specialized starting materials and/or tedious protecting group
[a]
N.-E. Alom, N. Kaur, F. Wu, S. J. Saluga, Prof. Dr. W. Li
Department of Chemistry and Biochemistry and School of Green
Chemistry and Engineering
Scheme 1. Alkene Sulfenoamination Background.
The University of Toledo
Toledo, OH 43606, United States
E-mail: Wei.Li@utoledo.edu
Supporting information for this article is given via a link at the end of
the document.
With these ideas in mind, we began our study with -
methylstyrene 1 and 2-aminothiophenol 2 as our standard
substrates. Our initial conditions focused on the use of an iodide
source and an oxidant. Gratifyingly, the combination of sodium
iodide (NaI) as the catalyst, potassium persulfate (K2S2O8) as
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the terminal oxidant, and dichloroethane (DCE) as the solvent at
80 °C afforded the desired 1,4-benzothiazine product in 10%
yield (Table 1, entry 1). Examination of solvents and various
oxidants revealed that acetonitrile (MeCN) and K2S2O8 were the
optimal choices (Table 1, entries 2-6). The identity of the iodide
catalyst had marginal effect on the reaction yields (Table 1,
entries 7-9). Lowering the temperature to 50 °C, however, led to
a decrease in reaction yield (Table 1, entry 10). Furthermore,
control reactions without oxidant completely inhibited the
reaction while the absence of the iodide catalyst still afforded the
product, albeit at a significantly lower yield of 12% (Table 1,
entries 11 and 12). Based on these optimization studies, we
chose entry 7 as our optimal conditions for substrate scope
studies.
in the product formation with a reasonable 50% yield and
excellent regioselectivity (Table 2, product 21).
Table 2. Alkene substrate scope.a
Table 1. Reaction Optimization.a
H
NH2
N
Ph
Me
halide (cat)
+
Me
oxidant
solvent
Ph
SH
S
alkene 1
2-aminothiophenol 2
(±)-1,4-benzothiazine
entry
halide catalyst (%)
oxidant (%)
K2S2O8 (100)
solvent
yield (%)b
NaI (10)
NaI (10)
NaI (10)
DCE
DMF
10
29
65
1
2
3
K2S2O8 (100)
K2S2O8 (100)
MeCN
NaI (10)
NaI (10)
NaI (10)
NaI (10)
LiI (10)
Na2S2O8 (100)
oxone (100)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
36
60
4
5
6c
7d
8d
9d
K2S2O8 (100)
K2S2O8 (100)
K2S2O8 (100)
K2S2O8 (100)
78
98(96)
95
KI (10)
94
10d,e
NaI (10)
K2S2O8 (100)
74
11d
12d
K2S2O8 (100)
-
-
MeCN
MeCN
12
0
NaI (10)
aReaction conditions: alkene 1 (0.25 mmol), 2-aminothiophenol 2 (0.375
mmol), oxidant (0.25 mmol), iodide catalyst (10 mol%), solvent (4 mL), 80 ºC,
16 h. bYields were determined by crude 1H NMR using 1,3-benzodioxole as
the internal standard. Yield shown in parenthesis was isolated yield. calkene
(0.375 mmol). dalkene (0.625 mmol). e50 ºC.
With the optimal conditions in hand, we evaluated a range of
alkene substrates.
A number of 1,1-disubstituted styrene
aStandard reaction conditions. bbenzoyl peroxide was used as the oxidant.
cNaI (20 mol%). dNaI (20 mol%), 36 h. ethe product was isolated as an
indistinguishable mixture of diastereomers.
derivatives participated in the reaction with excellent yields
(Table 2, products 3-8).[15] In these cases, highly steric
congested carbon centers were established with great yields.
Cross-coupling ready functional groups such as -bromo, -fluoro,
and -methoxy were tolerated. Similarly, mono-substituted
styrene derivatives worked effectively to afford the desired
products (Table 2, products 9-13). In addition, exceptional
diastereoselectivities were observed for both trans- and cis-1,2-
disubstituted styrene (Table 2, products 14-16). Interestingly, tri-
substituted alkenes also proceeded in this reaction to generate
highly steric congested 1,4-benzothiazine products with
excellent yields and diastereoselectivities (Table 2, products 17
and 18). Furthermore, mono-substituted aliphatic olefins could
afford the desired product with modest regioselectivity (Table 2,
products 19 and 20). Notably, a steroid derived alkene resulted
Next, we wanted to examine the extent of functional group
tolerance of the 2-aminobenzenethiol substrates. In this regard,
we evaluated a range of 2-aminothiophenol possessing a variety
of halogen functional groups. Specifically, versatile bromo-,
chloro-, and fluoro-functionalities all worked with excellent
efficiency under the standard reaction protocol (Table 3,
products 22-25). In addition, the pharmaceutically relevant
trifluoromethyl substituted substrate could also afford the 1,4-
benzothiazine product (Table 3, products 26). Regardless of the
electronic nature of the substitution on the 2-aminothiophenol,
the reaction proceeded with high yields with methoxy-, alkyl-,
and carboxyl-functional groups (Table 3, products 27-29). In the
case of product 29, unprotected carboxylic acid could be
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tolerated in this reaction to deliver the desired product in great
yield. Furthermore, we could also synthesize 1,4-benzooxathiine
30 when a 2-mercaptophenol was used as the substrate (Table
3, product 30). Finally, these substrates all uniformly afforded a
single regioisomeric product in good yields and compatible to a
wide range of functional groups.
to generate the thiyl radical D. Radical addition to the alkene
then affords the radical intermediate E. Single electron transfer
(SET) from the oxidant potassium persulfate produces the
carbocation intermediate C, followed by intramolecular
cyclization to afford the final product and regenerate the catalyst.
To support our mechanistic claim, we conducted several
additional reactions (Scheme 3). First, we installed a radical
clock on the alkene 31. Under the standard conditions, we
isolated 12% yield of the cyclopropyl ring intact product 32,
suggesting an ionic pathway was operative.[18] However, we also
observed a complex mixture of ring opening adducts from
product isolation. Second, we used the disulfide 33 as the
thioamine structure in the reaction, and observed a comparable
70% yield. This result indicates that the active catalytic halogen
source is capable of cleaving the S–S bond to form the sulfur
electrophile A. Our control experiments demonstrated that in the
absence of the oxidant, the iodide salt could also promote
disulfide 33 for product formation to a certain extent (12%). In
the absence of NaI, less than 2% product was observed. Third,
we introduced a radical scavenger such as TEMPO in this
reaction. With 1 equivalent of TEMPO, we did not see complete
inhibition of the reaction. Instead, a decreased yield of 65% was
observed. As the addition TEMPO increased, the yield
decreased further. These data, collectively, suggest that both
pathway A and B were likely to be operative here.
Table 3. Thioamine substrate scope.a
NH2
H
aStandard reaction conditions.
standard
N
+
conditions
SH
Ph
Ph
To gain an understanding of the reaction mechanism, we
first evaluated the regiochemical features for this reaction. For
the styrene-derived substrates, the nitrogen was uniformly
added to the benzylic position whereas the sulfur was attached
to the homobenzylic carbon. This regiochemical outcome was
distinct from our previously developed alkene sulfenoamination
based on halogenation protocols, in which the more nucleophilic
sulfur was usually added to the benzylic position.[12][a] For
aliphatic alkenes, the regioselectivity was lower and favored the
nitrogen atom adding to the terminal position and the sulfur atom
attached to the internal position. With this consideration in mind,
we hypothesize that two potential mechanistic pathways can be
operative here. Based on the control reactions, we reason that
the 2-aminothiophenol can react with the oxidant and the iodide
catalyst to generate the sulfur-based electrophile A (Scheme 2).
S
31
(±)-32, 12% isolated
NH2
Me
H
N
standard
S
+
S
conditions
Me
NH2
S
33
(±)-3, 70%
NH2
SH
Me
H
N
standard
+
conditions
additive
Ph
Me
S
additive: 1 equiv. TEMPO, 65% yield
1.5 equiv. TEMPO, 38% yield
2.0 equiv. TEMPO, 18% yield
Scheme 3. Mechanistic experiments.
From this intermediate, pathway
A
involves the direct
electrophilic attack on the alkene to form the thiiranium ion B.[16]
The thiiranium ion can be in equilibrium with the open
carboncation C, which can then react with the tethered nitrogen
nucleophile to produce the final heterocyclic product.[17] For the
aliphatic alkenes, when the thiiranium is generated, the ring
opening to the carbocation is likely slow. Lewis base addition by
the iodide can occur to open the thiiranium ring at the less
sterically hindered terminal position, followed by intramolecular
displacement to afford the corresponding major regioisomer.
Alternatively, pathway B from intermediate A involves homolysis
In summary, we have disclosed a novel and practical alkene
sulfenoamination reaction using 2-aminothiophenols via iodide
catalysis. A range of 1,4-benzothiazines is produced with good
functional group compatibility and excellent regioselectivity and
stereoselectivity. The substituent electronic nature of the 2-
aminothiophenol has minimal effect on the reaction efficiency.
Furthermore, we demonstrated that disulfide reagents are also
viable substrates. Finally, mechanistic experiments indicate that
both thiiranium activation and radical activation are operative in
this reaction.
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Scheme 2. Potential Reaction Mechanisms.
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Wang, J. Am. Chem. Soc. 2017, 139, 13110-13116.
Acknowledgements
We thank the University of Toledo for a startup grant and a
grant from the Summer Research Awards and Fellowship
Programs. We thank Dr. Yong. W. Kim (University of Toledo)
for NMR assistance.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: alkene sulfenoamination • iodide catalysis • S-
and N-heterocycles • 1,4-benzothiazine • thiiranium
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1
measured the JC-H for the -nitrogen C–H ranges from 139.44 to
142.72 Hz and for the -sulfur C–H ranges from 142.88 to 147.75
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Nur-E Alom, Navdeep Kaur, Fan Wu,
Shannon Jasmine Saluga, and Wei Li*
Page No. – Page No.
Catalytic Regio- and Stereoselective
Alkene Sulfenoamination for 1,4-
Benzothiazine Synthesis
A novel catalytic strategy is introduced to couple alkenes and unfunctionalized
thioamines for the synthesis of useful heterocycles. This catalytic protocol tolerates
a wide range of substrates with great regioselectivty and diastereoselectivity.
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