N-Hydroxy sulfonamides as new sulfenylating agents for the functionalization of aromatic compounds

Article abstract of DOI:10.1039/c7ob01390f

An unprecedented use of N-hydroxy sulfonamides as sulfenylating agents has been established. In the presence of catalytic amounts of iodine and N-hydroxysuccinimide, N-hydroxy sulfonamides participated in sulfenylation with indoles, 7-azaindole, N-methyl pyrrole, and 2-naphthol to afford structurally diverse thioethers in moderate to excellent yields with very high regioselectivity.

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N-Hydroxy sulfonamides as new sulfenylating agents for the
functionalization of aromatic compounds
Received 00th January 20xx,
Accepted 00th January 20xx
Fu-Xiang Wang, Shao-Da Zhou, Chengming Wang and Shi-Kai Tian*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An unprecedented use of N-hydroxy sulfonamides as sulfenylating agents in a similar manner releasing water and nitric oxide as
agents has been established. In the presence of catalytic amounts byproducts via cleavage of S=O and S−N bonds. We selected
of iodine and N-hydroxysuccinimide, N-hydroxy sulfonamides the sulfenylation of indoles to test our hypothesis since some
participated in sulfenylation with indoles, 7-azaindole, N-methyl indole thioethers serve as potent agents to treat cancer8 and
pyrrole, and 2-naphthol to afford structurally diverse thioethers in allergy.9 Other than sulfonyl hydrazides,6a,7l,m the sulfenylation
moderate to excellent yields with very high regioselectivity.
of indoles was reported to be realized with sulfenylating
agents such as thiols,10 sulfenyl halides,11 disulfides,12 N-
thioimides,13 sulfinates,14 and sulfonyl chlorides.15 Many of
these sulfenylating agents are unstable to air and/or moisture,
are expensive, and/or possess unpleasant odors. Moreover,
many of these reactions require excess sulfenylating agents
and/or additives, suffer from a narrow substrate scope, and/or
yield some byproducts unfriendly to environment.
For a long time much attention has been paid to N-hydroxy
sulfonamides, particularly Piloty’s acid (PhSO2NHOH),1 owing
to their ready accessibility and versatile applications. N-
Hydroxy sulfonamides have been explored as oxygen
nucleophiles,2 nitrogen nucleophiles,3 and sulfonyl sources4
through cleavage of O−H, N−H, and S−N bonds, respecꢀvely
(Scheme 1). Herein we report a conceptually new synthetic
application of N-hydroxy sulfonamides to the sulfenylation of
aromatic compounds through cleavage of S=O and S−N bonds
(Scheme 1). When compared to traditional sulfenylating
agents such as thiols, sulfenyl chlorides, and disulfides,5 N-
hydroxy sulfonamides are much more amenable to handling
because, in general, they are readily accessible solids, free of
unpleasant odor, and compatible with moisture.
Initially, we employed 10 mol% iodine to catalyze the
model reaction of N-hydroxy sulfonamide 1a with indole (2a),
o
which occurred in butanol under nitrogen at 120 C to afford
thioether 3a in 53% yield (Table 1, entry 1). To our surprise,
the yield was enhanced dramatically by addition of an N-
hydroxy imide or an N-hydroxy amide (entries 2-5), and
particularly, the use of 30 mol% N-hydroxysuccinimide (H1
)
gave a 98% yield (entry 2). Replacing iodine with NIS decreased
the yield dramatically and even no desired product was
isolated when using either TBAI or HI as the catalyst (entries 6-
8). A number of organic solvents were examined but no better
yield was achieved (entries 9-14). Finally, a control experiment
was carried out under oxygen, and the desired product was
observed in only a trace amount despite that fact that most of
the N-hydroxy sulfonamide was consumed through oxidation
by the end of the reaction (entry 15).
O
S
O
S
H
O
N
H
N
Ref. 2
R
O
R'
R
R'
Ref. 4
O
O
H
O
Ref. 3
S
R
O
R'
N
O
S
This work
R
OH
R
S
R'
O
Scheme 1 Synthetic applications of N-hydroxy sulfonamides.
In the presence of 10 mol% iodine and 30 mol% N-hydroxy
imide H1, a range of N-hydroxy arylsulfonamides smoothly
participated in sulfenylation with indoles to afford structurally
diverse aryl indolyl thioethers in moderate to excellent yields
with very high regioselectivity (Scheme 2, 3a-y). In general, the
sulfenylation reaction occurred at C-3 of the indole ring (3a-o
and 3r-y). When C-3 was occupied by a substituent, C-2 of
theindole ring was the reaction site of choice (3p and 3q). It is
noteworthy that the reaction tolerated a variety of functional
groups such as an alkoxy group, fluoro, chloro, bromo, iodo, a
Inspired by our previous discovery on the sulfenylation
reactions with sulfonyl hydrazides,6,7 we envisioned that N-
hydroxy sulfonamides could serve as alternative sulfenylating
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Table 1 Optimization of the reaction conditionsa
Entry Catalyst
Additive
none
H1
Solvent
Butanol
Butanol
Butanol
Butanol
Butanol
Butanol
Butanol
Butanol
Yieldb (%)
1
I2
53
98
86
90
91
79
0
2
I2
3
I2
H2
4
I2
H3
5
I2
H4
6
NIS
TBAI
HI
I2
H1
7
H1
8
H1
0
9
H1
Cyclohexanol
Glycol
trace
trace
trace
44
10
11
12
13
14
15c
a
I2
H1
I2
H1
Glycerol
DMF
I2
H1
I2
H1
DMSO
0
I2
H1
Toluene
Butanol
80
I2
H1
trace
Reaction conditions: 1a (0.20 mmol), 2a (0.30 mmol), catalyst (10
mol%), additive (30 mol%), solvent (2.0 mL), 120 oC (oil bath), under
nitrogen, 15 h. b Isolated yield. c The reaction was run under oxygen.
Scheme
2
Sulfenylation of aromatic compounds with N-hydroxy
nitro group,
a nitrile group, and an ester group. This
a
sulfonamides. Reaction conditions:
1 (0.20 mmol), 2 (0.30 mmol), I2
regioselective sulfenylation reaction was successfully extended
to some other electron-rich aromatic compounds such as 7-
azaindole (3z), N-methyl pyrrole (3aa), and 2-naphthol (3ab).
Notably, N-methyl pyrrole was very reactive and participated
in oversulfenylation to afford bisthioether 3aa in a good yield
with very high regioselectivity.16 Nevertheless, the
sulfenylation reaction was not applicable to less reactive
aromatic compounds such as furan, thiophene, N,N-
dimethylaniline, phenol, anisole, and 2-methoxynaphthalene.
On the other hand, we failed to observe the sulfenylation
reaction with N-hydroxy alkylsulfonamides such as N-
hydroxyhexadecane-1-sulfonamide.
(10 mol%), H1 (30 mol%), butanol (2.0 mL), 120 oC (oil bath), under
nitrogen, 15 h. b Isolated yields were given. c 1a (2 equiv) was used.
To gain insights into the reaction mechanism, we carried
1
out H NMR spectroscopic analysis of the reaction mixture of
N-hydroxy sulfonamide 1a with indole (2a) in ethanol and
found that the N-hydroxy sulfonamide decomposed to sulfinic
acid 4a, sulfinate ester 5a, and thiosulfonate 6a (Table 2),17
whose structures were further confirmed by high resolution
mass spectrometric analysis.18 While sulfinic acid 4a and
thiosulfonate 6a existed in very small amounts throughout the
process, sulfinate ester 5a was generated in a relatively big
amount but disappeared by the end of the reaction. In
addition, a little more than half of N-hydroxy imide H1
underwent iodine-catalyzed alcoholysis with ethanol to afford
The above conditions were further applied to the
sulfenylation of Fmoc-protected L-tryptophan 2r (Fmoc =
fluorenylmethyloxycarbonyl). To our delight,
component reaction of N-hydroxy sulfonamide 1d, protected
L-tryptophan 2r and butanol (solvent) proceeded via
a
three-
,
sulfenylation and esterification to afford functionalized α-
amino ester 3ac in 91% yield (eqn (1)). Importantly, no
racemization was detected in this case.
diester 7a 19
generated during the reaction.
,
which suggests that hydroxylamine was
2 | J. Name., 2012, 00, 1-3
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Table 2 1H NMR spectroscopic analysis of the reaction mixture
for the sulfenylation of aromatic compounds with N-hydroxy
sulfonamides (Scheme 3). Initially, oxidation of N-hydroxy
sulfonamide
1
with iodine affords HI and sulfonyl nitroso
homolytic cleavage of which generates
compound 10 4b
,
sulfonyl radical 11 and nitric oxide radical.4b Both radicals
undergo N-hydroxy imide H1-catalyzed hydrogen abstraction
from N-hydroxy sulfonamide
1 to afford sulfinic acid 4 and
HNO.20 Part of HNO is converted to molecular nitrogen and
water through reduction with hydroxylamine,21 which is
generated by iodine-catalyzed alcoholysis of N-hydroxy imide
Entry
Time (h)
1a : 3a : 4a : 5a : 6a
H1 : 7a
H1 with the alcohol solvent. While sulfinic acid
converted to sulfinate ester
under acidic conditions,22 the
reverse hydrolysis reaction occurs readily as the following
reaction progresses.23 Sulfinic acid
is reduced by HI and/or N-
hydroxy sulfonamide which
to afford thiosulfonate 6 6e
participates in iodine-catalyzed sulfenylation with aromatic
compound
to afford thioether 3 6e
4 can be
1
2
3
1
80.8 : 10.9 : 0.7 : 6.8 : 0.8
14.9 : 70.7 : 2.8 : 8.5 : 3.1
0 : 93.6 : 3.5 : 0 : 2.9
65.3 : 34.7
49.0 : 51.0
47.1 : 52.9
5
5
15
4
1
,
Under the standard conditions, sulfinic acid 4a, sulfinate
ester 5b and thiosulfonate 6a each participated in
,
2
.
sulfenylation with indole (2a) to afford thioether 3a (Table 3).
It is noteworthy that trace amounts of sulfinic acid 4a and
thiosulfonate 6a were generated during the reaction of
sulfinate ester 5b according to thin-layer chromatography
(TLC) analysis. Moreover, a series of control experiments
revealed that iodine rather than N-hydroxy imide H1 was
crucial for the sulfenylation of indole
intermediates.
(2a) with these
Table 3 Transformations of intermediates
Scheme 3 Proposed reaction pathways
.
Isolated yield (%)
In summary, we have established a conceptually new
synthetic application of N-hydroxy sulfonamides to the
sulfenylation of aromatic compounds through cleavage of S=O
and S−N bonds. In the presence of 10 mol% iodine and 30
Entry
[S]
Standard
Without I2
Without H1
1
2
3
4a
5b
6a
57
85
88
12
0
63
73
88
mol% N-hydroxysuccinimide,
a
range of N-hydroxy
47
sulfonamides smoothly participated in sulfenylation with
indoles, 7-azaindole, N-methyl pyrrole, and 2-naphthol to
afford structurally diverse thioethers in moderate to excellent
yields with very high regioselectivity. Preliminary mechanistic
studies show that N-hydroxy sulfonamides first decompose to
sulfinic acids and then participate in sulfenylation with
aromatic compounds. This study paves the way for the use of
N-hydroxy sulfonamides as sulfenylating agents in chemical
synthesis.
Addition of one equivalent of 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO), a radical scavenger, to the reaction
mixture of N-hydroxy sulfonamide 1a and indole (2a) almost
inhibited the formation of thioether 3a (eqn (2)). We further
carried out electron-spray ionization (ESI) mass spectrometric
analysis of this reaction mixture and tentatively identified
nitroso-substituted indole 8a and TEMPO-Ts (9a) according to
the high resolution mass data.18 These results suggest that
both nitric oxide radical (NO˙) and sulfonyl radical Ts˙ were
generated through decomposition of N-hydroxy sulfonamide
1a during the sulfenylation reaction.
We are grateful for the financial support from the National
Natural Science Foundation of China (21472178 and
21232007), the National Key Basic Research Program of China
(2014CB931800), and the Strategic Priority Research Program
of the Chinese Academy of Sciences (XDB20000000).
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