Electrochemical Oxidative Syntheses of NH-Sulfoximines, NH-Sulfonimidamides and Dibenzothiazines via Anodically Generated Hypervalent Iodine Intermediates

Article abstract of DOI:10.1002/cssc.202101002

Herein, we report a general method for the synthesis of NH-sulfoximines and NH-sulfonimidamides through direct electrochemical oxidative catalysis involving an iodoarene(I)/iodoarene(III) redox couple. In addition, dibenzothiazines can be synthesized from [1,1′-biaryl]-2-sulfides under standard conditions. Notably, only a catalytic amount of iodoarene is required for the generation in situ of an active hypervalent iodine catalyst, which avoids the need for an excess of a hypervalent iodine reagent relative to conventional approaches. Moreover, this protocol features broad substrate scope and wide functional group tolerance, delivering the target compounds with good-to-excellent yields even for a scale of more than 10 g.

Full text of DOI:10.1002/cssc.202101002

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Electrochemical Oxidative Syntheses of NH-Sulfoximines,
NH-Sulfonimidamides and Dibenzothiazines via Anodically
Generated Hypervalent Iodine Intermediates
Xianqiang Kong,*[a, b] Long Lin,[b] Xiaohui Chen,[a] Yiyi Chen,[a] Wei Wang,*[a] and Bo Xu*[b]
Herein, we report a general method for the synthesis of NH-
sulfoximines and NH-sulfonimidamides through direct electro-
chemical oxidative catalysis involving an iodoarene(I)/iodoarene
(III) redox couple. In addition, dibenzothiazines can be synthe-
sized from [1,1’-biaryl]-2-sulfides under standard conditions.
Notably, only a catalytic amount of iodoarene is required for
the generation in situ of an active hypervalent iodine catalyst,
which avoids the need for an excess of a hypervalent iodine
reagent relative to conventional approaches. Moreover, this
protocol features broad substrate scope and wide functional
group tolerance, delivering the target compounds with good-
to-excellent yields even for a scale of more than 10 g.
Introduction
of the mediator is used. To date, there has been only one report
of a genuinely electrocatalytic application of hypervalent iodine
(III) species; thus, the development of electrocatalytic applica-
tions for hypervalent iodine(III) species remains highly
rewarding.[6]
Electrochemical synthesis has been recognized as fascinating
sustainable chemistry due to no need for stoichiometric redox
reagents, thereby avoiding the accompanying waste stream.[1]
In the past ten years, there has been considerable progress in
this research area, and the corresponding methods for con-
structing various types of chemical bonds, synthesizing value-
added organic products, and heterocyclic compounds have
been developed.[2] Among them, hypervalent iodine com-
pounds have a wide range of applications in electrochemical
synthesis.[3] Schmidt and Meinert reported the first electro-
chemical synthesis of (difluoroiodo)benzene(PhIF2).[3d] In 2019,
Elsherbini and Wirth reported an electrochemical generator of
hypervalent iodine reagents in flow reactors.[3e] The iodine(III)
compounds generated at the anode are successfully used as
mediators for different valuable chemical transformations.[4]
Iodine(III)-mediated electrosynthesis is usually carried out in
two steps: (1) generation of a hypervalent iodine compound
and (2) addition of the substrate (ex-cell mediation).[5] Hyper-
valent iodine electrochemistry is used mainly in ex-cell proto-
cols, which defeats the purpose of using electrochemistry.
Therefore, an electrocatalytic one-pot method (in-cell media-
tion) would be more desirable, in which only a catalytic amount
Dibenzothiazines, and NH-sulfoximines and NH-sulfonimida-
mides, the mono-aza analogs of sulfones and sulfonamides,
respectively, are of interest as bioisosteres for drug design.[7]
Recently several research groups have made progress in
simplifying the imination of thioethers[8] and sulfenamides[9] to
afford NH-sulfoximines and NH-sulfonimidamides (Scheme 1a).
Tota et al. reported the first direct synthesis of NH-sulfoximines
from sulfides using phenyliodine(III) diacetate as the oxidant
and ammonium carbamate as the ammonia source.[8a] Then
Chen and co-workers achieved the synthesis of dibenzothia-
zines from 2-biphenyl sulfides.[8e] Bolm and co-workers devel-
oped a one-pot synthesis of thiophene NH-sulfoximines from
thiophenes.[8c] In 2019, Luisi and co-workers reported a facile
metal-free protocol for the direct synthesis of NH-sulfonimida-
[a] Dr. X. Kong, Dr. X. Chen, Dr. Y. Chen, Dr. W. Wang
School of Chemical Engineering and Materials
Changzhou Institute of Technology
No. 666 Liaohe Road,
Changzhou 213032 (P. R. China)
E-mail: kongxq@czu.cn
wangwei2017@czu.cn
[b] Dr. X. Kong, L. Lin, Prof. Dr. B. Xu
College of Chemistry,
Chemical Engineering and Biotechnology
Donghua University
2999 North Renmin Lu,
Shanghai 201620 (P. R. China)
E-mail: bo.xu@dhu.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202101002
Scheme 1. Literature background.
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mides from sulfenamides.[9] By using excess iodosobenzene as
the oxidant, a highly chemoselective N- and O-group transfer to
sulfenamides was achieved in iPrOH. However, the utility of
these methods is limited by the need to use large excesses of
hypervalent iodine reagents. We speculated that hypervalent
iodine species produced by electrochemical oxidation of aryl
iodides could be used to oxidize sulfides to form NH-
sulfoximines directly. Herein, we report that hypervalent iodine
generated by anodic oxidation of an iodoarene can be directly
used to synthesize NH-sulfoximines and NH-sulfonimidamides
from sulfides and sulfenamides, respectively (Scheme 1d). This
electrocatalytic system employs a catalytic amount of iodoar-
ene. Furthermore, under the reaction conditions, [1,1’-biaryl]-2-
sulfides can be directly transformed to dibenzothiazines.
anode, a Pt plate cathode, and 4-iodoanisole as the catalyst,
desired NH-sulfoximine 2a could be isolated in 84% yield
(entry 1). Variation of the para substituent of the iodoarene
decreased the yield (Table S1). Different anode materials (a Pt
plate [36%] or reticulated vitreous carbon [64%]) were also less
effective (entries 2 and 3). The combination of a C anode and
an Mg cathode yielded none of the desired products (entry 4).
Next, we investigated the effects of solvents: HFIP, MeOH, or
CH3CN alone gave only very low yields of 2a (entries 5–7), and
changing the HFIP/MeOH ratio also decreased the yield
(entries 8 and 9). No product was obtained in the absence of n-
Bu4NOAc (entry 10). When n-Bu4NOAc was replaced by AcOH,
the yield of 2a was only 21% (entry 11). Increasing or
decreasing the current decreased the yield (entries 12 and 13).
Alternative supporting electrolytes (n-Bu4NBF4, n-Bu4NClO4, and
LiClO4) were less effective than n-Bu4NPF6 (entries 14–16).
Screening of other ammonium sources revealed that
NH2CO2NH4 and NH3 ·H2O gave lower yields than AcONH4
(entries 17 and 18); and NH4Cl, NH4Br, and NH4I gave none of
the desired product (entries 19–21). Methyl phenyl sulfone was
obtained in the absence of iodoarenes (entry 22). Finally, in the
absence of electrolyte or electric current, no 2a was obtained
(entries 23 and 24).
Results and Discussion
For optimization of the reaction conditions, we used thioanisole
(1a) as a model sulfide substrate, AcONH4 as an ammonium
source, n-Bu4NPF6 as a supporting electrolyte in an undivided
cell (Table 1). Encouragingly, when the reaction was performed
°
at 25 C in 1:1 (v/v) hexafluoroisopropanol (HFIP)/MeOH at a
constant current of 8 mA with an inexpensive graphite (C)
Having optimized the conditions for this in-cell method, we
proceeded to explore its substrate scope (Scheme 2). Methyl
phenyl sulfides 1 bearing an electron-withdrawing group (Br, Cl,
Table 1. Optimization of reaction conditions.[a]
Entry Variation from standard conditions[a]
Yield of 2a[b] [%]
1
2
none
84
36
Pt anode/Pt cathode
3
4
Reticulated vitreous carbon anode/Pt cathode 64
C anode/Mg cathode
0
5
6
7
8
HFIP instead of 1:1 HFIP/MeOH
MeOH instead of 1:1 HFIP/MeOH
CH3CN instead of 1:1 HFIP/MeOH
2:1HFIP/MeOH instead of 1:1HFIP/MeOH
1:2 HFIP/MeOH instead of 1:1 HFIP/MeOH
No n-Bu4NOAc
AcOH instead of n-Bu4NOAc
10 mA instead of 8 mA
6 mA instead of 8 mA
n-Bu4NBF4 instead of n-Bu4NPF6
n-Bu4NClO4 instead of n-Bu4NPF6
LiClO4 instead of n-Bu4NPF6
NH2CO2NH4 instead of AcONH4
NH3 ·H2O instead of AcONH4
NH4Cl instead of AcONH4
NH4Br instead of AcONH4
NH4I instead of AcONH4
32
12
Trace
76
54
0
21
51
71
71
63
12
76
77
0
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0
No iodoarenes
No electrolyte
No electric current
0[b]
No reaction
No reaction
Scheme 2. Reactions of sulfides 1 to afford NH-sulfoximines 2. Standard
conditions: C anode (40 mm×10 mm×2 mm), Pt cathode
(40 mm×10 mm×2 mm), I=8 mA, 1a (0.2 mmol), AcONH4 (0.4 mmol),
[a] Standard conditions: C anode (40 mm×10 mm×2 mm), Pt cathode
(40 mm×10 mm×2 mm), I=8 mA, 1a (0.2 mmol), AcONH4 (0.4 mmol),
iodoarenes (0.02 mmol), n-Bu4NOAc (0.2 mmol), 1:1 (v/v) HFIP/MeOH
iodoarenes (0.02 mmol), n-Bu4NOAc (0.2 mmol), 1:1 (v/v) HFIP/MeOH
(4 mL), n-Bu4NPF6 (0.1 molLÀ 1), 25 C. [b] Isolated yields are reported.
(4.0 mL), n-Bu4NPF6 (0.1 molLÀ 1), 25 C. 2f: 4-(Methylthio)benzaldehyde was
used as the substrate.
°
°
[c] Methyl phenyl sulfone was isolated in yields of 41%.
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ketone) or electron-donating groups (OMe, Me) at the para
position of the phenyl ring gave the corresponding products in
good yield (2b–2e, 2g). Note that a substrate with a para
aldehyde substituent gave a 90% yield of NH-sulfoximine 2f,
the product of oxidation of the aldehyde group to the
corresponding acid. A substrate with an alcohol functional
group was well tolerated, providing 2h in 80% yield. The
position of the substituents on the benzene ring seemed to
have little influence on the yield. For example, 2,6-dichloro- and
3,5-dichloro-substituted products 2i and 2j were obtained in
similar good yields, and the yield of ortho-bromo compound 2k
was the same as that of para-bromo compound 2c. A 2-
naphthyl derivative gave an excellent yield of 2l (89% yield).
Subsequently, we varied both the R and the R1 groups of the
sulfide. The steric bulk of the groups generally had little effect
on the transformation; substrates bearing an ethyl, allyl, phenyl,
or benzyl group at R1 worked well (2m–2p), as did sulfides with
substituted benzyl groups (2q and 2r) or a phenylpropyl group
(2t). Finally, dialkyl NH-sulfoximines 2u and 2v could also be
obtained in high yields.
To further verify the generality of our electrochemical
approach, we used an [1,1’-biaryl]-2-sulfonamides as the
substrate (Scheme 3). When [1,1’-biaryl]-2-sulfonamides was
used, the product with dibenzothiazines was obtained. The
[1,1’-biaryl]-2-sulfonamides with a substituent (e.g., MeÀ , OCF3,
F, Cl, OMe, CF3) provided the corresponding benzothiazines
(2w–2ab) in moderate yields.
We also used this hypervalent iodine electrocatalysis
method to synthesize NH-sulfonimidamides from sulfenamides
3 (Scheme 4). Initially, we focused on varying the NR1R2 group
of the sulfenamide. Gratifyingly, the reactions were generally
very clean, giving desired unprotected tertiary sulfonimida-
mides 4a–4e in moderate isolated yields. To our delight, a
benzenesulfenamide with a piperidine ring and a t-butyl
sulfenamide with a piperidine ring also reacted smoothly to
give corresponding sulfonimidamides 4f and 4g, respectively,
in 76% and 65% yields, illustrating the broad substrate scope
of this novel strategy. 4-Methylpiperidine-substituted sulfena-
mides 3h–3l, bearing several aromatic, were investigated.
Sulfonimidamides 4h–4j were obtained in yields ranging from
45% to 89%. 2-Naphthyl and disubstituted phenyl substituents
furnished the corresponding NH-sulfonimidamides 4l and 4k.
To demonstrate the practical utility of this novel electro-
chemical method, we carried out the reactions of 1l and 3b on
a 1 mol scale at a constant current of 12 mA by using a
commercial IKA ElectraSyn 2.0 apparatus. Products 2l and 4b
were isolated in yields of 91% and 71% (Scheme 5). Moreover,
a reaction of 8 mol of 1l gave 2l in 85% yield (13.9 g).
Scheme 3. Reactions of [1,1’-biaryl]-2-sulfonamides 1w–1y to afford diben-
zothiazines 2w–2y. Standard conditions: C anode (40 mm×10 mm×2 mm),
Pt cathode (40 mm×10 mm×2 mm), I=8 mA, 1a (0.2 mmol), AcONH4
(0.4 mmol), 4-iodoanisole (0.02 mmol), n-Bu4NOAc (0.2 mmol), 1:1 (v/v) HFIP/
MeOH (4.0 mL), n-Bu4NPF6 (0.1 molLÀ 1), 25 C, 6 h.
°
To gain insight into the reaction mechanism, we carried out
a series of control experiments involving thioanisole derivatives
(Scheme 6). Under the standard conditions, separate reactions
of methyl phenyl sulfoxide 7 and methyl phenyl sulfilimine 8
showed complete selectivity for the formation of sulfoximine
2a (Scheme 6a,b). Sulfilimine 8 gave 2a even in the absence of
the ammonium source (Scheme 6c). In contrast, the reaction of
sulfoxide 7 in the absence of the ammonium source gave not
2a but methyl phenyl sulfone 9 (Scheme 6d). Finally, the
reaction of sulfide 1a gave a 35% yield of the oxidation product
(sulfoxide 3b) and a trace of methyl phenyl sulfone 9 in the
absence of the ammonium source (Scheme 6e). When thioani-
sole 1a was reacted with 2 equiv. of ammonium acetate
(MeCOONH4) in the presence of 2.5 equiv. of IIII (10) in HFIP/
Scheme 4. Reactions of sulfenamides 3 to afford NH-sulfonimidamides 4.
Standard conditions: C anode (40 mm×10 mm×2 mm), Pt cathode
(40 mm×10 mm×2 mm), I=8 mA, 1a (0.2 mmol), AcONH4 (0.4 mmol),
iodoarenes (0.02 mmol), n-Bu4NOAc (0.2 mmol), 1:1 (v/v) HFIP/MeOH
(4.0 mL), n-Bu4NPF6 (0.1 molLÀ 1), 25 C.
°
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Scheme 5. Gram-scale syntheses of NH-sulfoximine and NH-sulfonimida-
mide.
Figure 1. Cyclic voltammograms recorded at 100 mV/s with a 3 mm diameter
glass carbon electrode as working electrode, Pt wire as counter electrode
and Ag/AgCl as reference electrode in different electrolyte solvents: (1)
background; (2) 10.0 mM 4-iodoanisole; (3) 10.0 mM Thioanisole (1a); (4)
10.0 mM AcONH4/Bu4NOAc/4-iodoanisole; (5) 10 mM AcONH4/Bu4NOAc/4-
iodoanisole /Thioanisole.
enhanced oxidation peaks at around 1.8 V and 2.3 V vs. Ag/AgCl
of curve 5 are observed when in the presence of AcONH4/
Bu4NOAc/4-iodoanisole/thioanisole, indicating the efficiency of
the electrochemical generation of hypervalent iodine reagents
is improved.
A plausible mechanism for the generation of NH-sulfox-
imines and NH-sulfonimidamides by means of our electro-
chemical method is shown in Schemes 7 and 8. Based on
experiments, it is likely that the two mechanisms are simulta-
neously occurring in the reaction system. Hypervalent iodine
catalyst B ((4-methoxyphenyl)-iodanediyl(III) diacetate) is gen-
erated in situ via anodic oxidation of the iodoarene (A) in
acetate-enriched HFIP under constant current electrolysis.[10]
Phenyliodine(III) diacetate directly reacts with NH3 to afford key
Scheme 6. Control experiments.
°
MeOH at 25 C, exclusive formation of the corresponding
sulfoximine 2a was observed (Scheme 6f).
Cyclic voltammograms were carried out at 100 mVsÀ 1 with a
3 mm diameter glassy carbon electrode in related compounds
(Figure 1). When using HFIP/MeOH (1:1) as the solvent, no
obvious oxidative peak is observed (relative to the background
curve 1 in Figure 1). Additionally, there is also no apparent
oxidative peak is observed in the presence of 4-iodoanisole
(Figure 1, curve 2). However, an oxidative peak is observed at
about 1.9 V vs. Ag/AgCl in the presence of thioanisole (Figure 1,
curve 3), which could be ascribed to the oxidation of thioanisole
(1a). Another oxidative peak is observed at about 2.2 V vs. Ag/
AgCl in the presence of AcONH4/Bu4NOAc/4-iodoanisole (Fig-
ure 1, curve 4),[6] which may correspond to the electrochemical
generation of hypervalent iodine reagents. As expected, two
Scheme 7. (Path a) Proposed mechanism for electrochemical synthesis of
NH-sulfoximines and NH-sulfonimidamides.
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complete (witnessed by the disappearance of the 1a), the solvent
was removed under reduced pressure. The residue was poured into
a saturated aqueous solution of NaCl, and the product was then
extracted with Dichloromethane (3×20 mL), dried over Na2SO4, and
concentrated in vacuo. The residue was purified by column
chromatography on silica gel using a mixture of petroleum ether/
EtOAc (v/v=5:1) as eluent to afford the desired pure product 2a.
Acknowledgements
We are grateful to the Natural Science Foundation of Jiangsu
(BK20170661) and the National Science Foundation of China for
financial support (NSFC-21804018).
Conflict of Interest
Scheme 8. (Path b) Proposed mechanism for electrochemical synthesis of
NH-sulfoximines and NH-sulfonimidamides.
The authors declare no conflict of interest.
Keywords: ammonia · electrochemical synthesis · iodine ·
sulfonimidamides · sulfoximines
nitrene intermediate C.[8c] Then C is captured by the sulfide to
afford sulfilimine D, which undergoes nucleophilic attack by an
acetate anion or MeOH to afford sulfanenitrile intermediate E
and regenerate iodoarene A. Intermediate E is attacked by
MeOH to afford sulfoximines 2 (Scheme 7).
Sulfides 1 can be oxidized to sulfones 7 under electro-
chemical conditions (Scheme 8). Intermediate C react directly
with the sulfoxide (7) afford the sulfoximine 2.[11]
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We have developed the first method for the preparation of NH-
sulfoximines, NH-sulfonimidamides, and dibenzothiazines by
means of hypervalent iodoarene-catalyzed reactions using
electricity without any chemical oxidant. Highly selective one-
pot transfers of NH and O were achieved with a simple
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efficient electrochemical method provides access to NH-sulfox-
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(Using synthesis of 2a as an example). An undivided cell was
equipped with a carbon sheet anode (4 cm×10 cm×2 mm) and a
platinum plate (40 mm×10 mm×2 mm) and connected to a DC
regulated power supply. To the cell was added thioanisole 1a
(0.2 mmol), AcONH4 (0.4 mmol), 4-iodoanisole (0.02 mmol), n-
Bu4NOAc (0.2 mmol), n-Bu4NPF6 (0.1 molLÀ 1
) and HFIP/MeOH
(2.0 mL:2.0 mL) were added. The mixture was electrolyzed using
constant current conditions (~8 mAcmÀ 2) at 25 C under magnetic
°
stirring. When TLC analysis indicated that the electrolysis was
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Manuscript received: May 12, 2021
Revised manuscript received: June 9, 2021
Version of record online: ■■■, ■■■■
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FULL PAPERS
Sprinkles of iodine: A method is
reported for the direct electrochemi-
cal oxidative synthesis of NH-sulfoxi-
mines, NH-sulfonimidamides, and di-
benzothiazines, respectively, with
catalysis by an iodoarene(I)/
iodoarene(III) redox couple. The
method requires only a catalytic
amount of iodoarene, and a hyperva-
lent iodine catalyst is generated
in situ from the iodoarene by anod-
ization, which avoids the need for an
excess of a hypervalent iodine
reagent.
Dr. X. Kong*, L. Lin, Dr. X. Chen, Dr. Y.
Chen, Dr. W. Wang*, Prof. Dr. B. Xu*
1 – 7
Electrochemical Oxidative
Syntheses of NH-Sulfoximines, NH-
Sulfonimidamides and Dibenzothia-
zines via Anodically Generated Hy-
pervalent Iodine Intermediates