Electrochemical synthesis of sulfamides

Article abstract of DOI:10.1039/d1cc01428e

Herein we demonstrate the first electrochemical synthesis protocol of symmetrical sulfamides directly from anilines and SO₂mediated by iodide. Sulfamides are an emerging functional group in drug design. Highlights are the direct use of SO₂from a stock solution and no necessity of any supporting electrolyte. Overall, the reaction has been demonstrated for 15 examples with yields up to 93%.

Full text of DOI:10.1039/d1cc01428e

Showcasing research from Waldvogel’s Laboratory/
Department of Chemistry, Johannes Gutenberg-University,
Mainz, Germany.
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Volume 57
Number 39
14 May 2021
Pages 4741–4864
Electrochemical synthesis of sulfamides
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Organic electrosynthesis enables the powerful formation
of symmetrical sulfamides directly from anilines and SO2
in a divided cell mediated by iodide. The direct use of SO2
from a stock solution and no requirement for an additional
supporting electrolyte significantly increases the atom
economy. Sulfamides are an emerging functional group in
medicinal chemistry.
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Electrochemical synthesis of sulfamides†
Stephan P. Blum, Lukas Schaffer, Dieter Schollmeyer and Siegfried R. Waldvogel
*
¨
Cite this: Chem. Commun., 2021,
57, 4775
Received 16th March 2021,
Accepted 12th April 2021
DOI: 10.1039/d1cc01428e
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Herein we demonstrate the first electrochemical synthesis protocol reported the synthesis of symmetrical sulfamides by using stoichio-
of symmetrical sulfamides directly from anilines and SO2 mediated metric amounts of iodine with excess SO2 (B100 equiv.) resulting
by iodide. Sulfamides are an emerging functional group in drug in the in situ formation of highly reactive sulfuryl iodide.12 Alter-
design. Highlights are the direct use of SO2 from a stock solution natively, SO2 surrogates can be utilized in this approach.13,17
and no necessity of any supporting electrolyte. Overall, the reaction
has been demonstrated for 15 examples with yields up to 93%.
Organic electrosynthesis currently experiences a renaissance,18
which is attributed to numerous assets, such as inherent safety,19
limited reagent waste, short cut of many synthetic steps,20–22
Molecules containing sulfamide functionalities slowly found unique reactivity23,24 and the fact that electric current can be
their way into medicinal chemistry, which is attributed to their generated from renewable energy. Moreover, ‘‘green’’ electricity is
versatile biological activities.1,2 Sulfamides are electron-rich, inexpensive in comparison to hazardous and polluting chemical
powerful hydrogen bonding donors/acceptors, and their tetra- redox reagents and therefore electrifying organic synthesis to
hedral configuration is conformationally-rich in regard to their value-added products can pay off.23,25,26 In summary, many
target protein interaction.2,3 They comprise bioisosteric repla- principles of Green Chemistry are applied,27 which is crucial for
cements to sulfamates, sulfonamides, urea, carbamates, and future applications in order to overcome global challenges such as
amides.2,4,5
According to DrugBank database, there are currently four
climate change.28
In this work, the use of light-sensitive iodine can be ele-
approved drugs containing sulfamide groups and 19 in experi- gantly avoided by using electricity to generate I2 from catalytic
mental or investigational phase,6 which is evidence for the emer- amounts of iodide,29 which drastically enhances the atom
ging importance and application of this functionality in drug economy. The in situ generated iodine is considered to catalyze
design. For example, the drug Quinagolide (1, Scheme 1) is used the formation of symmetrical sulfamides directly from anilines
to lower elevated blood levels of prolactin (hyperprolactinaemia).6,7 and SO2. The pollutant and waste product of many industrial
Macitentan (2) is applied to regulate high blood pressure.6,8 processes sulfur dioxide30 is incorporated into the molecule by
Famotidine (3) can medicate heartburn.6,9 The antibiotic Doripe- an atom economical way.21,22 This circumvents the use of
nem (4) is used to treat infectious diseases.6,10
In recent years, methodologies for sulfamide syntheses and
functionalization have been reported increasingly.2,4,5,11–13 Tra-
ditional strategies rely on hazardous and highly corrosive
sulfuryl chloride with anilines (Schemes 2 and 3),4,14 whereas
sulfuryl chloride is chemically derived from sulfur dioxide and
chlorine gas.15 Further drawbacks are rather poor conversion
and complex product mixtures resulting from side reactions,
such as aromatic chlorination.12,16 Rudkevich and coworkers
Department of Chemistry, Johannes Gutenberg University Mainz, Mainz 55128,
Germany. E-mail: waldvogel@uni-mainz.de; Web: https://www.aksw.uni-mainz.de/
† Electronic supplementary information (ESI) available. CCDC 2070594. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
Scheme 1 Approved drugs containing sulfamide functionalities.
Chem. Commun., 2021, 57, 4775–4778 | 4775
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our reaction optimization by using 4-tert-butylaniline as test
substrate in an 1,1,1,3,3,3-hexfluoroisopropanol (HFIP)/MeCN
(1 : 1) solvent mixture with BDD (boron-doped diamond)
electrodes,35 which resulted in 72% NMR yield (Table 1, entry
2). Higher HFIP content (entry 3) as well as no HFIP (entry 4)
resulted only in traces or no sulfamide formation. Noteworthy,
HFIP is very stable under these electrolytic conditions. It can be
almost quantitatively recovered and reused. Stoichiometric
amounts of HFIP (3.00 equiv., entry 5) significantly improved
the yield to 80% and 1.50 equiv. (entry 6) even resulted in 88%
yield. The application of glassy carbon electrodes showed
similar results (89%, entry 7) and platinum electrodes gave
96% with 89% isolated yield (5a, entry 1). Higher current
density slightly lowered the sulfamide formation (entry 8) and
pyridine instead of DIPEA distinctively reduced the NMR yield
to 52% (entry 9). The use of undivided cells gave 69% (entry 10).
Omitting of the iodide salt (entry 11) as well as the substitution
of NBu4I by NBu4Br (entry 12) or NBu4Cl (entry 13) resulted in
no product formation. An organic base is necessary as omitting
was also rendered in no sulfamide formation (entry 14,
NBu4BF4 was added for better electrical conductivity). No
electricity resulted in no product formation (entry 15). Further
information can be retrieved from the ESI.†
Scheme 2 Synthetic methodologies for symmetrical sulfamide synthesis;
DIPEA = N,N-diisopropylethylamine; DABSO = 1,4-diazabicyclo[2.2.2]octane-
bis(sulfur dioxide) adduct; HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol; py =
pyridine.
Scheme 3 Optimized reaction conditions.
Thereupon, we examined the scope by testing various 4-subs-
tituted anilines as depicted in Scheme 4: halogen-substituted ani-
lines resulted in excellent yields (5b, 90%; 5c, 92%; 5d; 85%),
whereas the less nucleophilic 4-fluoroaniline gave 5e with only
68% isolated yield and 5f gave 73% with a CF3 substituent.
Sulfamide 5g (86%) is derived from Benzocaine, which is a well-
known local anesthetic agent.36 Nitro- and methoxy-substituted
sulfamides gave similar yields (5h, 83%; 5i, 76%). Finally, aniline
derived sulfamide 5j was obtained in 86% yield. Impressively,
3-bromoaniline gave 6 with 93% yield (Scheme 5). It is noteworthy
that sulfamide 7 (86%) bears two different halogen substituents
(bromo and chloro). Anilines with increased steric hindrance are
also eligible for this protocol (8, 73%; 10, 52%), and 3-cyano
substituted aniline gave 9 in 81% yield. A 13-fold scale-up of the
reaction by employing 16.0 mmol aniline substrate gave 2.51 g of 5a
expensive SO2 surrogates with high molecular mass, which
generate waste in course of application.21,22 Additionally,
unprotected anilines are hardly used as substrates in electro-
chemical synthesis due to their low oxidation potentials31
leading to facile anodic oxidation, followed by polymerization
(aniline black).32
Therefore, mostly protection as acylanilides was employed.33
Herein, the issues with over-oxidation and additional protec-
tion/deblocking sequence was avoided by the presence of
iodide salts, as these have even lower oxidation potentials than
the free anilines.
Our electrosynthetic screening and optimization experi-
ments were conducted in a device which is commercially
available34 and allows to obtain all relevant data.25 We began
Table 1 Optimization reactions of the test substrate
Entry
Deviation from the standard conditionsa
Yieldb [%]
1
2
3
4
5
6
7
8
None
96 (89% isolated yield)
BDD electrodes, HFIP/MeCN = 1 : 1
BDD electrodes, HFIP/MeCN = 10 : 1.5
BDD electrodes, no HFIP
BDD electrodes, HFIP (3.00 equiv.)
BDD electrodes
72
Traces
0
80
88
89
86
52
69
0
0
0
0
0
Glassy carbon electrodes
12.0 mA cmÀ2
9
Pyridine instead of DIPEA
Undivided cell
No iodide source
NBu4Br instead of NBu4I
Glassy carbon electrodes, NBu4Cl instead of NBu4I
No DIPEA, instead: NBu4BF4 (0.10 M)
No electricity
10
11
12
13
14
15
a
b
Standard conditions shown in Scheme 3. Yield of 5a determined by internal NMR standard (1,3,5-trimethoxybenzene).
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Scheme 4 Scope starting from 4-substituted anilines with isolated yields.
Molecular structure of 5a determined by X-ray analysis. Scale-up reaction
of 5a (lower part of the scheme).
Scheme 6 Postulated reaction mechanism.
in situ generated iodonium ion is considered to react with the
amidosulfinate resulting in the formation of sulfamoyl iodide in an
equilibrium reaction. This transformation could possibly be favored
by hydrogen bonding stabilization of the intermediates from stoi-
chiometric amounts of HFIP. Subsequent nucleophilic displacement
with another equivalent of aniline provides the symmetrical aro-
matic sulfamide. As shown in the optimization table (Table 1, entry
2), excess HFIP proved to be unsuitable, which could be due to full
protonation of DIPEA diminishing the reactivity of iodonium ion
generation.
As cathodic reaction, SO2 reduction was identified by cyclic
voltammetry measurements (ESI†). The SO2 anion radical is
considered to dimerize to dithionite during the course of the
Scheme 5 Substrate scope of further anilines with isolated yields.
(6.96 mmol, 87%), which is comparable to the results obtained in electrolysis. Therefore, divided cells give improved yields as the
usual scale with 89% yield by using 1.20 mmol substrate. reduced SO2 species could be reoxidized anodically leading to
Finally, the reaction mechanism was considered (Scheme 6). lower overall current efficiency.
Cyclic voltammetry experiments (ESI†) indicate the initial ano-
In conclusion, we have developed the first electrochemical
dic oxidation of iodide resulting in the formation of iodine, synthesis protocol for symmetrical sulfamides. The use of a SO2
which is supposed to form extraordinarily strong Lewis acid– stock solution significantly increases the atom economy in compar-
base adducts with amine bases, such as DIPEA, triethylamine ison to expensive SO2 surrogates, such as DABSO. Moreover, no
or pyridine. These species have to some extent ionic character additional supporting electrolyte is required, and inexpensive elec-
also leading to the iodonium ion.37–39 Possibly, HFIP could tricity serves as oxidant for the in situ generation of iodine from
further stabilize and promote the formation of those ionic catalytic amounts of iodide. DIPEA and HFIP are crucial for the
species ([R3NI+]IÀ or [(R3N)2I+]IÀ)38,39 via hydrogen bonding success of this reaction due to the formation of the iodonium ion
and therefore explain the necessity of HFIP and DIPEA in this and stabilization of intermediates. Scalability has been proven in a
protocol. Additionally, HFIP is considered to disperse or alter 13-fold scale-up reaction. The broad functional group tolerance in
the charge transfer complex between DIPEA and SO2 in MeCN, combination with excellent yields up to 93% make this protocol a
as a color change of the solution from black to orange was competitive and attractive alternative to the existing methodologies.
observed upon addition of HFIP (ESI†). Rudkevich and coworkers
S. R. W. gratefully acknowledges financial support by the
proposed the formation of sulfuryl iodide as intermediate upon DFG (Wa 1276/17-2). The Carl Zeiss Stiftung is gratefully
reaction of iodine with SO2 by activation with triethylamine or acknowledged for the electrosynthesis network ELYSION. Sup-
pyridine.12 However, we consider the reaction to proceed via port by SusInnoScience in the frame of the State Rhineland-
amidosulfinates generated from anilines, SO2 and DIPEA, which Palatinate is highly appreciated. S. B. thanks Tarik Karakaya for
could also be one of the conductible species in the electrolyte. The practical assistance.
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23 S. R. Waldvogel, S. Lips, M. Selt, B. Riehl and C. J. Kampf,
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There are no conflicts of interest to declare.
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