An efficient imidation of thioethers with nitrene in water

Article abstract of DOI:10.1039/d0ob01539c

The first imidation of thioethers with free nitrene in water was realized. N-Cbz sulfilimines are formed via imidation of thioethers with free nitrene generated from α elimination of nosyloxycarbamates. In this work, water is successfully applied as solve

Full text of DOI:10.1039/d0ob01539c

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DOI: 10.1039/D0OB01539C
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An Efficient Imidation of Thioethers with Nitrene in Water
Tao Feng, Zhihui Tang, Xiaoli Luo, and Junming Mo*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The first imidation of thioethers with free nitrene in water was avoiding use of volatile organic solvents, improving reactivity
realized. N-Cbz sulfilimines are formed via imidation of thioethers and simplifying product isolation.16 However, most of
with free nitrene generated from
α
elimination of successful sulfur imidations rely on water sensitive reagents or
nosyloxycarbamates. In this work, water is successfully applied as water sensitive nitrene transfer reactions.12,15 So organic
solvent for free nitrene, and transition metal catalyst is not solvents are needed for most of sulfur imidations, and using
needed.
water as solvent is rare.11b In addition, many of the protecting
groups introduced into sulfoximine groups are sulfonyl
group15a,17 which are difficult to be deprotected. Only a few
preparations of NH sulfoximines were reported.10b,11,15d,15f
Therefore, it is still necessary and attractive to develop safe,
environmentally friendly and efficient synthetic methods of
sulfoximines.
Transition metal catalyzed nitrene transfer reactions are
powerful tools for imidation of thioethers (Scheme 1a, 1b).8
Compared with nitrene transfer, methods for imidation of
thioethers via direct free nitrene addition are limited. Ethyl
In recent years, sulfoximines have received a lot of attention
owing to their interesting biological activities.1 Methionine
sulfoximine (MSO) and buthionine sulfoximine (BSO) can be
employed as inhibitors of glutathione biosynthesis.2
Sulfoximines are also potential drugs as prophylactic
antiasthmatics3 and HIV-1 protease inhibitors.4 In addition, in
the field of organic synthesis, sulfoximines have been widely
used to synthesize pseudopeptides5 and as directing groups for
C-H activation.6 Chiral sulfoximines with stereogenic sulfur
atoms can also be utilized as efficient chiral ligands.7
nosyloxycarbamate is
a
well-developed and versatile
aminating agent for C-N bond formation.18 Nitrene generated
from α elimination of ethyl nosyloxycarbamate is reactive
species that reacts quickly with many substrates via nitrene
addition without transition metal catalysts. As a safe, stable
and low cost nitrene precursor, nosyloxycarbamate is a
suitable candidate of iminating agent for green sulfur
imidation. In recent years several N-OSO2R carbamates have
been applied to imidation of sulfides via different types of
reaction. Acid catalyzed imidation of sulfides with ethyl
trifluoromethanesulfonyloxycarbamate (TfONHCO2Et) via
nucleophilic substitution was disclosed by Tamura.19 Lebel
reported rhodium catalyzed nitrene transfer of chiral N-
mesyloxycarbamates to sulfilimines (Scheme 1c).9d Herein we
disclose an efficient and environmentally friendly thioether
imidation in water (Scheme 1d). N-Cbz sulfilimines are formed
via imidation of thioethers with free nitrene generated from α
elimination of nosyloxycarbamates. As we know, free nitrenes
are sensitive to water and easily hydrolyzed in aqueous media.
Although several examples of catalytic nitrene transfer
reactions in water have been reported,20 reactions of free
nitrenes with substrates in water are still challenging. In the
present study, water is successfully applied as solvent in the
These interesting applications of sulfoximines prompted
chemists to develop various synthesis strategies for the
preparation of sulfoximines. At present, there are two main
synthetic routes for sulfoximines.8 One route is to imidize
sulfides followed by oxidation of sulfilimines.9 The other way is
imidation of sulfoxides which usually come from oxidation of
sulfides.10 Recently, synthesis of NH-sulfoximines from sulfides
by one-pot N- and O-transfers was developed.11 No matter
which route is taken, imidation of sulfur is the key step to
generate sulfoximines. Despite many strategies are available
for imidation of sulfides and sulfoxides, there are still many
challenges. Many imidating reactions of sulfides and sulfoxides
usually need toxic or explosive reagents such as azides,12 t-
BuOCl13 and O-mesitylenesulfonylhydroxylamine (MSH),14 or
require transition metal catalysts with iminoiodinanes
reagents.15 In recent year, the use of water as solvent attracted
attention in organic synthesis because it have benefits on
a. Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, College
of Chemistry and Chemical Engineering, Guangxi University for Nationalities,
Nanning 530006 (China), moandli@163.com.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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addition of free nitrene to thioethers, and transition metal in basic aqueous solution. To obtain more sulfilimine product,
View Article Online
catalyst is not needed.
two equivalent benzyl nosyloxycarbamate
DOI: 10.103w9/aDs0OBa0d15d3e9dC.
However, complex product was observed and cannot be
isolated (Table 1, entry 12). The reason may be the
decomposition of 3a via addition of the nitrene to the S=N
bond. Further attempt was to use more thioether to capture
nitrene. Sulfilimine product 3a was obtained in good yield with
two equivalent thioether 1a (75%, Table 1, entry 13). Similarly,
adding 2 in 4 portions (0.25 equiv each time) can also give a
good result (70%, Table 1, entry 14). Higher temperature will
accelerate hydrolysis of nitrene and lead to lower yield (33%,
Table 1, entry 15).
a) imidation of sulfides via nitrene transfer
PhI=NSO2Ar
NSO2Ar
transition metal catalysts
S
R1
R2
S
R1
R2
organic solvents
b) azide as nitrene source
RN3
NR
transition metal catalysts
S
R1
R2
S
R1
R2
organic solvents
c) carbamates as nitrene precursors
Table 1. Optimization of the imidation of thioethers[a]
.
NCO2R
R'SO3NHCO2R
S
R1
R2
S
R1
R2
Cbz
O
base
catalyst
organic solvents
N
S
S
+
O
Bn
Ph
Ns
N
H
2
O
d) this work
S
solvent, rt
Ph
3a
CbzN
1a
NCbz
no catalyst
R1
R2
S
R1
R2
water
entry
1
base/acid
-
solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
yield (%)[b]
0
Scheme 1. Imidation of sulfides via nitrene transfer and nitrene
addition.
2
CF3COOH
Na2CO3
DIPEA
Et3N
0
3
32
To avoid unexpected side reactions in further synthetic
transformations, protected sulfilimines and sulfoximines are
required. Cbz group is a good choice as the protecting group of
sulfilimines and sulfoximines owing to its stability and easy
deprotection.21 In order to introduce Cbz group to sulfilimines,
we started our study by using benzyl nosyloxycarbamate as
imidating agent. Experimentally, we treated thioether 1a with
benzyl nosyloxycarbamate 2 using CH2Cl2 as solvent. Not
surprisingly, no desired sulfilimine product 3a was observed
without any acid catalyst or acid binding agent due to stability
4
<5
5
<5
6
NaOH
CaO
<5
7
47
8
CaO
12
9
CaO
MeCN
toluene
H2O
10
10
11
12
CaO
17
CaO
53
0[c]
CaO
H2O
of benzyl nosyloxycarbamate (Table 1, entry 1). Different from 13
CaO
H2O
75[d]
70[e]
33[f]
acid catalyzed imidation of thioethers with TfONHCO2Et
reported by Tamura,19 acid did not promote the reaction
effectively (Table 1, entry 2). Fortunately, with Na2CO3 as base,
we observed the formation of sulfilimine product 3a with low
but encouraging yield after 12 hours (32%, Table 1, entry 3).
When this proof-of-principle result was established, we next
attempted to optimize the reaction conditions. Among the
bases screening, weaker bases like DIPEA and Et3N, were not
effective in giving the sulfilimine product (Table 1, entry 4-5).
Also, NaOH was not good acid binding agent in this reaction
(Table 1, entry 6). The possible reason was benzyl
nosyloxycarbamate and sulfilimine would decompose quickly
in strong basic solution. Encouraging result emerged when
CaO was used as base. The yield of 3a increased from 32% to
47% (Table 1, entry 7). With CaO as base, effect of some
common solvents in this reaction were studied. THF,
acetonitrile and toluene was not effective in promoting this
reaction (Table 1, entry 8-10). Surprisingly, when water was
used as solvent, the reaction was completed quickly in 40
minutes with 53% yield (Table 1, entry 11). The shorter
reaction time (40 mins) in water may be the reason why the
yield increases. TLC and NMR analysis of the reaction mixture
revealed the benzyl nosyloxycarbamate was consumed quickly
14
15
CaO
H2O
CaO
H2O
[a] Reaction condition: 1a (0.1 mmol, 1.0 equiv), 2 (0.1 mmol, 1.0
equiv), base (1.0 equiv), solvent (1.0 mL), room temperature; entry 1-
10:12 h; entry 11-13: 40 min. [b] Isolated yield based on 2. [c] 2 (2.0
equiv), CaO (2.0 equiv). [d] 1a (2.0 equiv). [e] 1a (1.2 equiv), 2 (1.0
equiv) was divided into 4 portions and added every 10 mins. [f] 1a (2.0
equiv), reaction temperature: 50 oC.
With optimized reaction condition in hand, the scope of the
thioether substrate reacting with benzyl nosyloxycarbamate
was examined (Scheme 2). Both electron-donating and
electron-withdrawing substituents installed on the S-phenyl
unit of thioether 1a were tolerated (3b-3k). Substrates bearing
active functional groups (hydroxyl, formyl and acetyl group)
were also obtained in good yields (3l-3n). Replacing methyl
group of thioether 1a with other aliphatic groups, such as ethyl
and benzyl group, did not significantly affect efficiency of the
reaction (3o, 3p). Dialkyl thioether substrates also reacted well
with good yields (3q-3s). When thioether 1a was changed to
bulky substrates like diphenyl thioether and phenyl
cyclopropyl thioether, notable decreases in yields were
2 | J. Name., 2012, 00, 1-3
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observed (3t, 3u). Large scale reaction was also tested. Under thioanisole and tetrahydrothiophene gave goodViteow AertxiccleeOllnelinnet
DOI: 10.1039/D0OB01539C
similar reaction condition, product 3b was obtained in good yields (Scheme 3, 5a-5c, 5f, 5j). KMnO4 was not efficient
yield (2.41 g, 84 %). When Ph3P was used as starting material, oxidant in the case of 5o, 5p and 5u. Interestingly, with mCPBA
useful phosphinimine ylide 4 was also obtained in good yield. as oxidant, substrates 5o, 5p and 5u were obtained with
Imidation of sulfoxides cannot occur under this condition. The higher yield compared with KMnO4 condition. At room
possible reason is weak nucleophilic ability of sulfoxide.
temperature, oxidation of bulky sulfilimine 3p is very
inefficient (yield of 5p < 5%). Raising temperature to 50 oC can
increase yield of 5p to 63%. Bulky diphenyl N-Cbz sulfilimine 3t
resulted in no conversion under both conditions. Starting
material 3t could be recovered from reaction mixture.
Oxidation could be realized to obtain the products 5b in gram
scale with excellent yield (2.71 g, 90 %).
Cbz
R2
O
CaO
N
S
S
O
Bn
+
R1
R2
Ns
N
O
R1
H2O, rt
H
2
1
3
Cbz
Cbz
Cbz
Cbz
N
N
N
N
S
S
S
S
Table 2. Optimization of the oxidation of sulfilimines[a]
.
OMe
3e
45%
Me
MeO
Cbz
OMe
3d
3c
58%
3b
85%
Cbz
Cbz
O
oxidant
solvent
N
S
N
84%[a]
57%
S
Cbz
Cbz
Cbz
Ph
N
S
N
S
N
N
S
Ph
Cl
S
3a
5a
Cl
Cl
entry
oxidant
H2O2 (10 equiv)
solvent
H2O
yield (%)[b]
Cl
3g
Cl
3i
3f
61%
52%
3h
44%
77%
80%
60%
1
0
Cbz
Cbz
Cbz
Cbz
N
S
N
S
N
N
2
NaOCl (5 equiv)
mCPBA (5 equiv)
tBuOOH (5 equiv)
KMnO4 (5 equiv)
H2O2 (10 equiv)
NaOCl (5 equiv)
mCPBA (5 equiv)
tBuOOH (5 equiv)
KMnO4 (5 equiv)
H2O
<5
0
S
S
3
H2O
4
H2O
<5
86
0
OHC
Br
F
HO
3l
3j
3m
76%
3k
68%
5
H2O
Cbz
Cbz
Cbz
N
S
6
EtOH
EtOH
EtOH
EtOH
EtOH
N
N
S
Cbz
Cbz
S
N
S
7
<5
83
0
Ph
Ph
8
O
9
3q
83%
3p
63%
3o
3n
3r
52%
Cbz
73%
Cbz
10
<5
Cbz
N
S
N
S
N
N
S
S
[a] Reaction condition: 3a (0.1 mmol, 1.0 equiv), oxidant, solvent (1.0
mL), room temperature, 24h. [b] Isolated yield based on 3a.
Ph
Ph
3u
27%
3t
30%
50%
3s
66%
Cbz
N
Ph3P
Ph3P
Cbz
R2
Cbz
O
a or b
4
85%
N
S
N
S
R1
R2
R1
N
Scheme 2. Reaction scope with thioethers. Reaction condition: 1 (0.2
mmol, 2.0 equiv), 2 (0.1 mmol, 1.0 equiv), CaO (1.0 equiv), water (1.0
mL), room temperature, 40 min. [a] Gram scale: 1b (20 mmol, 2.0
equiv), 2 (10 mmol, 1.0 equiv), CaO (1.0 equiv), water (20 mL), room
temperature, 40 min.
5
3
Cbz
Cbz
O
S
Cbz
O
S
O
S
N
N
Cl
O
Me
[a]
5f
: a) 79%
b) 79%
5c
: a) 68%
b) 82%
5b
: a) 92%, 90%
b) 56%
Oxidation of N-Cbz sulfilimines could provide N-Cbz
sulfoximines. To find out efficient oxidative method, a number
of reaction conditions were tested with N-Cbz sulfilimines 3a.
With water as solvent, H2O2, NaOCl, mCPBA and tBuOOH were
not effective in oxidative process (Table 2, entry 1-4). When
KMnO4 was used with water as solvent, an excellent yield was
obtained (Table 2, entry 5). Using ethanol as solvent gave
different results among oxidants screening (Table 2, entry 6-
10). In these cases, KMnO4 was inefficiency, and mCPBA was
the best oxidant with 83% yield (Table 2, entry 8).
Cbz
O
S
Cbz
Cbz
O
S
O
S
N
N
N
Ph
Br
[b]
5p
: a) 28%
b) 63% [b]
5o
: a) 37%
b) 78%
5j
: a) 92%
b) 68%
Cbz
Cbz
O
S
Cbz
O
N
N
O
S
N
S
5u
5s
: a) 31%
b) 76%
: a) 58%
b) 79%
5t
: a) 0%
b) 0%
Once optimal protocols of oxidation were established,
selected N-Cbz sulfilimines were examined with KMnO4
condition and mCPBA condition respectively. Under both
condition, N-Cbz sulfilimines derived from aryl substituted
Scheme 3. Synthesis of N-Cbz sulfoximines via oxidation. Reaction
condition: method a: 3 (0.1 mmol, 1.0 equiv), KMnO4 (0.5mmol, 5.0
This journal is © The Royal Society of Chemistry 20xx
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equiv), water (2.0 mL), room temperature, 24 h. method b: 3 (0.1
mmol, 1.0 equiv), mCPBA (0.5 mmol, 5.0 equiv), EtOH (2.0 mL), room
temperature, 24 h. [a] Gram scale: 3b (10.0 mmol, 1.0 equiv), KMnO4
(50.0 mmol, 5.0 equiv), water (50.0 mL), room temperature, 24 h. [b]
Reaction temperature: 50 oC.
Notes and references
View Article Online
DOI: 10.1039/D0OB01539C
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2
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Two distinct reaction pathways are possible for the
imidation of thioethers with benzyl nosyloxycarbamate
(Scheme 4). The pathway A involves nucleophilic substitution,
and the pathway B involves free nitrene intermediate. In the
pathway A, nucleophilic substitution can occur without any
additive and be promoted by acids as Tamura’s work.19 No
additive condition and several common acid catalysts were
tested in this study, and no desired sulfilimine product was
observed at all (Table 1, entry 1, 2).22 This result indicated that
nucleophilic substitution pathway seemed unlikely. The only
probable pathway was addition of thioether to free nitrene,
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6
7
generated from common
nosyloxycarbamate. Replacing
α
elimination of benzyl
benzyl group of
nosyloxycarbamate 2 to ethyl or tBu group led to no product.23
This result suggested that benzyl group may play a key role in
stabilizing the free nitrene in water.
8
9
V. Bizet, C. M. M. Hendriks, C. Bolm, Chem. Soc. Rev., 2015, 44,
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Ph
pathway A
S
O
O
H+
O
NCbz
S
H
+ NsOH
Bn
O
Bn
Ns
N
H
O
Ph
Ns
N
O
H
nucleophilic substitution
pathway B
O
O
O
O
S
base
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Ph
O
Bn
Bn
NCbz
S
Bn
Ns
N
O
N
O
Ns
N
O
-NsO-
H
Ph
nitrene
 elimination
Scheme 4. Proposed mechanism for the imidation of thioether.
In summary, we have disclosed the first imidation of
thioethers with free nitrene in water. Free nitrene generated
from α elimination of benzyl nosyloxycarbamate reacted with
thioethers to give N-Cbz sulfilimines under aqueous media
without transition metal catalysts. Useful N-Cbz sulfoximines
are accessible via simply oxidation of N-Cbz sulfilimines. We
expect this imidation to offer green, concise, low price and/or
better strategies for the development of new and useful
transformations.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by Guangxi Natural Science
Foundation under Grant No. 2016GXNSFCA380018, the
National Natural Science Foundation of China (No. 21702033),
and Guangxi University for Nationalities (Grant No.
2016MDQD003).
4 | J. Name., 2012, 00, 1-3
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Organic & Biomolecular Chemistry
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Journal Name
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conversion was observed.
23 Reaction condition: 1a (0.2 mmol, 2.0 equiv), nosyloxycarbamate
(0.1 mmol, 1.0 equiv), CaO (1.0 equiv), water (1.0 mL), room
temperature, 40 min.
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