Electrochemically induced oxidative S-O coupling: synthesis of sulfonates from sulfonyl hydrazides and N-hydroxyimides or N-hydroxybenzotriazoles

Article abstract of DOI:10.1039/C8OB03162B

The process of oxidative S-O coupling under the action of electric current was developed. Aryl, hetaryl and alkyl sulfonyl hydrazides and N-hydroxy compounds (N-hydroxyimides and N-hydroxybenzotriazoles) are applied as starting reagents for the preparation of sulfonates. The reaction is carried out under constant current conditions in an experimentally convenient undivided electrochemical cell equipped with a graphite anode and a stainless steel cathode under a high current density (60 mA cm^?2). NH₄Br in this process acts as a supporting electrolyte and participates in the oxidation of the starting compounds to form a coupling product. The developed strategy represents a quite atom-efficient approach: one partner loses two nitrogen and three hydrogen atoms, while another one loses only one hydrogen atom. Cyclic voltammetry and the control experiment allowed us to propose possible reaction pathways: generated through anodic oxidation molecular bromine or its higher oxidation state derivatives oxidize the starting compounds to form reactive species, which couple to form the S-O bond.

Full text of DOI:10.1039/C8OB03162B

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Journal Name
ARTICLE
Electrochemically induced oxidative S-O coupling: synthesis of
sulfonates from sulfonyl hydrazides and N-hydroxyimides or N-
hydroxybenzotriazoles
Received 00th January 20xx,
Accepted 00th January 20xx
a, c
Alexander O. Terent’ev,*a, b Olga M. Mulina,a Vadim D. Parshin,b Vladimir A. Kokorekin
and
DOI: 10.1039/x0xx00000x
Gennady I. Nikishin a
www.rsc.org/
The process of oxidative S-O coupling under the action of electric current was developed. Aryl, hetaryl and alkyl sulfonyl
hydrazides and N-hydroxy compounds (N-hydroxyimides and N-hydroxybenzotriazoles) are applied as starting reagents for
the preparation of sulfonates. The reaction is carried out under constant current conditions in an experimentally
convenient undivided electrochemical cell equipped with graphite anode and stainless steel cathode under high current
density (60 mA/cm2). NH4Br in this process acts as a supporting electrolyte and participates in oxidation of starting
compounds to form coupling product. Developed strategy represents a quite atom-efficient approach: one partner loses
two nitrogen and three hydrogen atoms, while another one only one hydrogen atom. Cyclic voltammetry and control
experiment allowed to propose possible reaction pathways: generated through anodic oxidation molecular bromine or its
hypervalent derivatives oxidize starting compounds to form reactive species, which couple to form S-O bond.
Here we report electrochemically induced oxidative S-O
coupling process. Sulfonyl hydrazides and N-hydroxy compounds,
such as N-hydroxyimides and N-hydroxybenzotriazoles, are applied
Introduction
For the last decades transition metals-catalyzed cross-coupling has
become one of the most powerful tools in organic synthesis for the
construction of carbon-carbon and carbon-heteroatom bonds.1 The
rise of interest to the oxidative coupling methodology is a logical
continuation of traditional cross-coupling strategy. It doesn’t
require prefunctionalization of starting compounds and as a result
is more ecologically and economically attractive.2 Besides,
application of these transformations in modern organic chemistry
often provides compounds inaccessible through other ways. Among
numerous oxidative coupling processes the reactions with sulfur
and oxygen components are less studied due to their easy
overoxidation and fragmentation.3 Only few examples of oxidative
S-O coupling have already been reported.4
as starting reagents. As a result, various sulfonates are formed.
Developed strategy represents a quite atom-efficient approach: one
partner loses two nitrogen and three hydrogen atoms, while
another partner loses only one hydrogen atom. Sulfonates
analogous to synthesized structures are known as effective serine
14
protease inhibitors. They are also applied in organic synthesis:15
combination of sulfonyl and N-hydroxy moieties in one molecule
permits to activate both of these groups in order to obtain
sulfonamides,16 arylamides,17 quinazolinedione and urea
derivatives.18 Similar structures with carboxylate fragment instead
of sulfonate, are widely used as precursors of alkyl radicals under
visible light irradiation.19
Among oxidants used for the oxidative coupling processes
electric current is one of the cheapest, most available and
environment-friendly.5 Reactions of oxidative C-C,6 C-N,7 C-O,8 C-S,9
C-P,10 C-Hal,11 S-N12 coupling have already been conducted
electrochemically and this strategy is becoming more and more
popular.13 Organic electrosynthesis in undivided cell is the most
promising, because in this case no special equipment is required.
Results and discussion
Electrosynthesis of sulfonates 3aa-3ma, 3ab-3af from sulfonyl
hydrazides 1a-1m and N-hydroxy compounds 2a-2f was conducted
in H2O-THF, H2O-MeCN, MeOH-THF and MeOH solution with NH4I,
NH4Br, NH4Cl, KBr, NaBr and NH4Br/LiClO4 as supporting
electrolytes (Scheme 1).
p-Toluenesulfonyl hydrazide 1a and N-hydroxysuccinimide 2a
were chosen as starting reagents for the search of optimal
conditions for the electrosynthesis of sulfonate 3aa. The influence
of nature of supporting electrolyte and its molar ratio with starting
compounds, the amount of electricity passed, temperature and
solvent nature on the yield of 3aa was investigated (Table 1).
a. N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47
Leninsky Prospekt, Moscow, 119991, Russian Federation.
b. D.I. Mendeleev University of Chemical Technology of Russia, 9 Miusskaya square,
Moscow, 125047, Russian Federation.
c. Sechenov First Moscow State Medical University, 8-2 Trubetskaya st., Moscow,
119991, Russian Federation.
† Fax: (+7)-499–135–5328; phone: (+7)-499–137–2944; e-mail: terentev@ioc.ac.ru.
Electronic Supplementary Information (ESI) available: general reaction procedures,
characterization data, copies of NMR spectra. See DOI: 10.1039/x0xx00000x
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Electrolysis
Supporting electrolyte
Solvent
DOI: 10.1039/C8OB03162B
O
S
O
O
R
NHNH2
+
HO N
S
N
R
O
Cl
O
O
O
O
Cl
Cl
3aa, 3ba, 3ca, 3da,
3ea, 3fa, 3ga, 3ha,
3ia, 3ja, 3ka, 3la, 3ma,
3ab, 3ac, 3ad, 3ae, 3af
1h: R = 2-naphthyl
1i: R = 2,4,6-Me3C6H2
1j: R = Ph
1a: R = 4-MeC6H4
1b: R = 4-MeOC6H4
1c: R = 4-FC6H4
1d: R = 4-ClC6H4
1e: R = 4-BrC6H4
1f: R = 4-IC6H4
HO
N
HO N
HO
N
O
N
O
O
2a
2b
2e
Cl
1k: R = 5-isothiazolyl
1l: R = 2-thiophenyl
1m: R = Me
2c
O
N
N
N
HO
N
1g: R = 4-AcNHC6H4
N
N
Cl
HO
HO
O
2f
2d
Scheme 1. Electrosynthesis of sulfonates 3aa-3ma, 3ab-3af from sulfonyl hydrazides 1a-1m and N-hydroxy compounds 2a-2f via oxidative
S-O bond formation (in the codification of 3 the first letter index refers to the hydrazide 1 moiety, the second letter index to the N-hydroxy
compound 2 moiety).
Electrode materials and solvents were chosen for their inertness was investigated. Its increase to 7 F/mol led to slight rise in 3aa
under the action of electric current, solvents used should also yield (entry 6), passing of 10 F/mol resulted in target product 3aa in
possess high conductivity. Halogen-containing salts are widely 84% yield (entry 7). Further growth of electricity amount did not
applied as supporting electrolytes in organic electrosynthesis, give better results (entry 8). Usage of less than 3 equivalents of
because they can also act as redox mediators.20 From a formal point NH4Br or application of a mixture of LiClO4 as an inert electrolyte
of view, for the oxidation of nitrogen-containing part of sulfonyl and the catalytic amounts of NH4Br caused a drop of coupling
hydrazides, including 1a, four electrons are required. Thus, in our product yields (entries 9-11). As shown in entries 12-14,
first experiments (Table 1, entries 1-5) theoretically calculated performance of electrosynthesis in the solvents different from H2O-
amount of electricity was passed. Reaction temperature and THF did not result in considerable changes in the reaction efficiency.
current density values were chosen to avoid too many side Increase of reaction temperature to 40 °C provided target product
processes and at the same time carrying out the electrosynthesis 3aa in 95% yield (entry 15), however further heating led to 3aa only
fast enough.
in 62% (entry 16). Thus on the basis of obtained experimental data
The nature of supporting electrolyte was demonstrated to be the reaction best of all goes at 40 °C with 3 equivalents of NH4Br as
quite important for the effectiveness of S-O coupling process (Table a supporting electrolyte, 10 F/mol electricity passed and H2O-THF as
1, entries 1-5). NH4Br turned out to be the best one affording 3aa in a solvent.
68% yield (entry 2). After that effect of amount of electricity passed
Table 1. Optimization of the conditions for the electrochemically induced S-O coupling starting from p-toluenesulfonyl hydrazide 1a and N-
hydroxysuccinimide 2a. a
O
O
O
S
O
O
Electrolysis
S
N
NHNH2
HO N
+
Supporting electrolyte
Solvent
O
O
O
O
1a
3aa
2a
Entry
1
2
3
4
5
6
7
8
Electrolyte (equiv)
NH4I (3)
Electricity passed, F/mol 1a
Temperature, °C
Solvent
Yield 3aa, % b
4
4
4
4
4
7
10
15
10
10
10
10
10
10
10
10
25
25
25
25
25
25
25
25
25
25
25
25
25
25
40
60
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-THF
H2O-MeCN
MeOH-THF
MeOH
45
68
66
31
26
75
84
78
56
60
18
70
65
57
95
62
NH4Br (3)
NH4Cl (3)
KBr (3)
NaBr (3)
NH4Br (3)
NH4Br (3)
NH4Br (3)
NH4Br (2)
9
10
11
12
13
14
15
16
NH4Br (1)
NH4Br (0.2)+LiClO4 (3)
NH4Br (3)
NH4Br (3)
NH4Br (3)
NH4Br (3)
NH4Br (3)
H2O-THF
H2O-THF
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a
General procedure: the solution of p-toluenesulfonyl hydrazide 1a (1 mmol, 186 mg), N-hydroxysuccinimide 2a (1 mmol, 115 mg) and
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DOI: 10.1039/C8OB03162B
supporting electrolyte (0.2-3 mmol, 20-435 mg) in 30 mL of H2O-THF (1:1), H2O-MeCN (1:1), MeOH-THF (1:1) or MeOH was electrolyzed
under constant current conditions (60 mA / cm2 at 25-60 °C) under magnetic stirring with the use of graphite anode and stainless steel
cathode. b Isolated yield.
After that we tested optimal reaction conditions on a wide scope of provided coupling product 3ma with high yield demonstrating the
sulfonyl hydrazides 1a-1m and various N-hydroxy compounds 2a-2f compatibility of this methodology with alkyl substrates too. Not
(Table 2). Arylsulfonyl hydrazides 1a-1i entered the reaction of only N-hydroxysuccinimide 2a can serve as O-component in
oxidative S-O coupling with N-hydroxysuccinimide 2a affording developed process: other N-hydroxyimides 2b-2d and N-
target molecules with yields from moderate to high except 1f and 1i hydroxybenzotriazoles 2e and 2f led to the formation of coupling
cases. Low yield in the case of 3fa product can be explained by a products with moderate yields.
number of side oxidative processes involving iodine or its
Under optimized conditions we also applied sodium sulfinates
derivatives.21 Presumably, steric hindrance of mesitylenesulfonyl 4a, 4d, 4j in our approach. These compounds are widely used in
hydrazide 1i was a key factor in low efficiency of 3ia product various oxidative coupling processes for the formation of carbon-
formation. Reaction of nitro-substituted arylsulfonyl hydrazides sulfur and sulfur-heteroatom bonds.22 As shown in Scheme 2,
resulted in inseparable mixture, probably, of nitro-group reduction sodium sulfinates 4a, 4d, 4j react with N-hydroxysuccinimide 2a less
products. Hetaryl substituted sulfonyl hydrazides 1k and 1l gave efficient than sulfonyl hydrazides 1: target coupling products 3aa,
target products in moderate yields. Methylsulfonyl hydrazide 1m 3da, 3ja were formed in 29-40% yield.
Table 2. Scope of sulfonates 3aa-3ma, 3ab-3af synthesis from sulfonyl hydrazides 1a-1m and N-hydroxy compounds 2a-2f via
electrochemically induced oxidative S-O coupling. a, b
C
Fe
NH4Br
O
S
O
O
+
R
NHNH2
HO
N
N
S
N
H2O-THF
40oC, 10 F
R
O
O
2
3
1
O
O
O
O
O
O
O
O
O
O
O
O
S
S
N
S
N
S
N
O
O
O
O
O
O
O
O
MeO
Cl
F
3aa, 95%
3da, 75%
3ba, 67%
3ca, 73%
O
O
O
O
O
O
O
O
O
O
O
O
S
N
S
N
S
N
S
N
O
O
O
O
O
O
O
O
I
AcHN
Br
3ha, 87%
3fa, 19%
3ga, 45%
3ea, 65%
O
O
O
O
O
O
O
O
O
O
O
O
S
N
S
N
S
N
S
N
O
O
O
O
O
O
O
O
S
N
S
3la, 52%
3ka, 41%
3ia, 17%
3ja, 62%
Cl
Cl
Cl
O
O
O
O
O
O
O
O
Cl
O
O
O
S
N
S
N
S
N
O
S
N
O
O
O
O
O
O
O
O
3ma, 67%
3ad, 55%
3ab, 53%
3ac, 38%
N
N
N
N
O
O
N
O
O
S
S
N
O
O
Cl
3ae, 25%
3af, 53%
a
General procedure: the solution of sulfonyl hydrazide 1a-1m (1 mmol), N-hydroxy compound 2a-2f (1 mmol) and NH4Br (3 mmol,) in 30
ml of H2O-THF (1:1) was electrolyzed under constant current conditions (60 mA / cm2 at 40 °C) under magnetic stirring with the use of
graphite anode and stainless steel cathode. b Isolated yields are given.
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Hypobromites, bromates and perbromates are mainly generated as
a result of the reaction between molecular bromine and hydroxide-
anion forming by the cathodic reduction of H2O.26 Tribromide anion
can be formed through the reaction of molecular bromine with
bromide-anion.27 Starting N-hydroxysuccinimide 2a can be
deprotonated by forming hydroxide-anion with the generation of
anion C. Ionic interaction between p-toluenesulfonyl bromide A as
an electrophile and N-hydroxysuccinimide anion C as a nucleophile
leads to the coupling product 3aa. The reaction between
individually prepared p-toluenesulfonyl bromide A and potassium
salt of N-hydroxy succinimide C’ gave the coupling product 3aa in
68% yield in H2O-THF solution at 40 °C (Scheme 4). This fact
demonstrated the possibility of formation of sulfonate 3aa from
intermediates A and C under reaction conditions.
O
O
C
Fe
NH4Br
SO2Na
O
O
+
HO N
S
N
H2O-THF
40oC, 10 F
O
R
4
O
O
R
3
2a
3aa, 32%
3da, 29%
3ja, 40%
R = Me
R = Cl
R = H
4a: R = Me
4d: R = Cl
4j: R = H
Scheme 2. Electrosynthesis of sulfonates 3aa, 3da, 3ja from sodium
sulfinates 4a, 4d, 4j and N-hydroxysuccinimide 2a (yield based on
NMR; 1,4-dinitrobenzene as internal standard).
On the basis of our previous experience9a, 12a and literature data we
proposed two different ways for this reaction: ionic and radical
ones (Scheme 3). Some extra experiments including cyclic
voltammetry were conducted. Cyclic voltammetry experiments
were implemented on a working glassy-carbon electrode with H2O-
THF (1:1) system as a solvent and tetrabutylammonium perchlorate
as a supporting electrolyte.
Anyway the initial step of the process is anodic oxidation of
bromide anion resulting in molecular bromine. Cyclic voltammetry
data demonstrated that bromide-anion is oxidized under the
earliest potentials (Figure 1,
participants.
+
1.13 V) among all reaction
O
2 Br
Br
S
NHNH2
O
Br2
OH
[Br]
Br3
H2O
1a
Br2, [Br]
, Br3
H2 + OH
O
S
O
S
Br
-
Br
O
O
A
B
1a
O
O
O
S
N
O
O
3aa
O
O
N
O
D
N
O
O
Br
[Br]
O
- HBr
O
C
N
OH
OH
2a
O
Scheme 3. Proposed reaction pathways.
Figure 1. The CV curves obtained for 3.0 mmol∙L-1 solutions of
NH4Br, p-toluenesulfonyl hydrazide 1a and N-hydroxysuccinimide
2a in 0.1 M Bu4NClO4 in H2O-THF (1:1) on a working glassy-carbon
electrode (d = 2.9 mm) at a scan rate of 100 mV∙s-1.
It is well-known23 that molecular bromine can oxidize p-
toluenesulfonyl hydrazide 1a affording p-toluenesulfonyl bromide
A. In the processes of bromide-anion oxidation and formation of p-
toluenesulfonyl bromide A hypobromites, bromates, perbromates
-
25
([Br]+)24 or tribromide anion Br3
can also participate.
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O
O
Kang, B. He and G. F. Yang, J. Agric. Food. Chem., 2018, 66,
View Article Online
SO2Br
O
O
8914.
DOI: 10.1039/C8OB03162B
H2O-THF
40 o
1 hour
S
N
KO
N
+
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Chem., 2017, 19, 5940.
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K. M. Lancaster and S. Lin, J. Am. Chem. Soc., 2018, 140, 12511;
b) X. Luo, X. Ma, F. Lebreux, I. E. Marko and K. Lam, Chem.
Scheme 4. Synthesis of sulfonate 3aa from individually prepared p-
toluenesulfonyl bromide and potassium salt of N-hydroxy
succinimide C’ (control experiment).
A
Turning back to possible radical pathway, p-toluenesulfonyl
bromide A can decompose with the formation of p-toluenesulfonyl
radical B and bromo radical.28 Radical B can be also formed through
the direct anodic oxidation of sulfonyl hydrazide 1a.29 N-oxyimide
radical D can be formed through different pathways. The first one is
a direct anodic oxidation of N-hydroxysuccinimide 2a (Figure 1,
+1.67 V)30 or N-oxysuccinimide anion C. The second one is the
reaction between 2a and bromo radical or hypobromites, bromates
or perbromates. Recombination of p-toluenesulfonyl radical B and
N-oxyimide radical D provides target product 3aa.
Conclusions
In summary, we have demonstrated atom-efficient
electrochemically induced oxidative S-O coupling, which
permits to avoid usage of stoichiometric amounts of chemical
oxidants. Sulfonyl hydrazides and N-hydroxy compounds were
applied as starting materials. The process is carried out under
constant current conditions in an experimentally convenient
undivided electrochemical cell equipped with graphite anode
and stainless steel cathode. Application of NH4Br as a
supporting electrolyte and a redox catalyst allows oxidizing
starting sulfonyl hydrazides and N-hydroxy compounds
selectively resulting in coupling products in yields from
moderate to high. On the basis of reported literature, cyclic
voltammetry data and control experiment possible reaction
pathways were proposed: molecular bromine, which is
generated through anodic oxidation of NH4Br, or hypobromite,
bromate, perbromate, tribromide anions oxidize starting
compounds to form reactive intermediates, which couple to
form S-O bond.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Russian Science Foundation (Grant
№ 18-13-00027).
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