Catalyst-free one-pot, four-component approach for the synthesis of di- And tri-substituted: N -sulfonyl formamidines

Article abstract of DOI:10.1039/d1ra00772f

A straightforward one-pot, multicomponent approach was developed to synthesize di- and tri-substituted N-sulfonyl formamidines from sulfonyl chlorides, NaN3, ethyl propiolate, and primary/secondary amines under mild conditions without catalysts or additives. Structural analysis of the di-substituted sulfonyl formamidines indicated formation of the E-syn/anti isomeric form. Tri-substituted analogues only formed E-isomers.

Full text of DOI:10.1039/d1ra00772f

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Catalyst-free one-pot, four-component approach
for the synthesis of di- and tri-substituted N-
sulfonyl formamidines†
Cite this: RSC Adv., 2021, 11, 15161
ab
Ai-Ran Liu,a Lei Zhang,a Jiao Lia and Abudureheman Wusiman
*
A straightforward one-pot, multicomponent approach was developed to synthesize di- and tri-substituted
N-sulfonyl formamidines from sulfonyl chlorides, NaN3, ethyl propiolate, and primary/secondary amines
under mild conditions without catalysts or additives. Structural analysis of the di-substituted sulfonyl
formamidines indicated formation of the E-syn/anti isomeric form. Tri-substituted analogues only
formed E-isomers.
Received 29th January 2021
Accepted 12th April 2021
DOI: 10.1039/d1ra00772f
rsc.li/rsc-advances
Amidines are important nitrogen-containing organic compounds butylisonitrile and N,N-dibromoaryl sulfonamides in the presence
owing to their unique structural and chemical properties.1 N- of a nitrile compound in aqueous media (Scheme 1d), and other
Sulfonyl amidines are a special class of amidines, which serve as groups have applied similar methods.14 However, most protocols
essential intermediates in numerous important organic reactions,2 still typically require special reagents, including transition metal
and can be useful building blocks for synthesizing heterocyclic catalysts or expensive and potentially explosive sulfonyl azides, and
compounds3 or chelating ligands for transition metals.4 In addition, they oen proceed at elevated temperatures. Therefore, the devel-
sulfonyl amidines have wide applications in biopharmaceutical opment of an efficient and practical method for the synthesis of N-
molecular design and the exploration of lead compounds in drug sulfonylamidine derivatives is critical. Although tri-substituted sul-
discovery.5 Over the past decade, research regarding the formation fonyl(form)amidines have been widely explored,6a–r the synthesis of
of N-sulfonyl amidines has led to remarkable progress.6
di-substituted N-sulfonyl formamidines is rare in the literature; to
Recently, multicomponent reaction (MCR) methods have our knowledge, only one research paper (from Jacobson and co-
attracted considerable attention for their potential ability to access workers, 1977)15 has described the synthesis of N,N0-di-substituted
biologically active compounds7 and molecules relevant to drug sulfonyl formamidines using sulfonamide and isocyanides in the
discovery.8 N-Sulfonylamidines t these criteria because they are presence of a copper catalyst.
found in numerous biologically active natural products and
important biopharmaceuticals.5 Various elegant MCR methods
involving the formation of N-sulfonylamidines have already been
developed. For example, a Cu-catalyzed three-component tandem
reaction between (i) sulfonyl azides9/amides,10 (ii) alkynes, and (iii)
primary, secondary, or tertiary amines or ammonium salts has been
described (Scheme 1a). Another recently reported Cu-catalyzed
three-component approach employed sulfonyl chlorides, sodium
azides, and amines (Scheme 1b).11 Additionally, Bi and co-workers12
described a silver-catalyzed, one-pot, four-component reaction
involving terminal alkynes reacting directly with trimethylsilyl azide
(TMSN3), sodium sulnate, and sulfonyl azide (Scheme 1c). Phukan
et al.13 described a metal-free strategy for the synthesis of di-
substituted sulfonyl amidines via a one-pot reaction between tert-
aSchool of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi,
Xinjiang, 830054, P. R China. E-mail: arahman@xjnu.edu.cn
bXinjiang Key Laboratory of Energy Storage and Photoelectrocatalytic Materials,
Urumqi 830054, China
† Electronic supplementary information (ESI) available. CCDC 2055586 and
2055587. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d1ra00772f
Scheme
sulfonylamidines.
1 Multicomponent reactions for the synthesis of N-
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In recent years, the reaction of enamines and azides has reaction proceeded smoothly in ethanol (EtOH) at room
received increasing interest due to their diversied chemical temperature and afforded di-substituted sulfonylformamidine
reactions.16 More recently, Wan and co-workers developed 5a, in 38% yield (with nearly a 7 : 3 syn/anti isomeric ratio)
methods for the direct synthesis of tri-substituted,6o and NH2- together with N-butyltosyl amide as by-product (entry 1).
featured6t sulfonyl (form)amidines by using N,N-disubstituted Following investigations with other solvents (entries 1–9), it was
enaminoesters and NH2-functionalized enaminone with sulfo- determined that acetonitrile (MeCN) led to the best yield (entry
nylazide. However, NH-functionalized enaminone did not 3) among the tested solvents (i.e., toluene, EtOAc, DMF, THF,
undergo this reaction. In addition these reactions require 5.0 H2O, and MeOH). Closer inspection of the reaction (monitored
and 2.5 equiv. of sulfonyl azides, respectively, and the desired using thin-layer chromatography; TLC) revealed that the inter-
tri-substituted N-sulfonyl formamidine was obtained in a lower mediate TsN3 (generated from 1a and 2), was consumed easily.
yield. We hypothesized that sulfonyl chlorides, NaN3, primary Probably the excess of sulfonyl azide play a crucial role on the
(secondary) amine, and ethyl propiolate might react under same reaction yield.12 Therefore we thought to increase the substrate
conditions to generate N-sulfonyl formamidines via in situ loading of 1a and 2 under the same reaction conditions, when
formation of corresponding NH-mono and N,N-disubstituted both the 1a and 2 dosages were increased to 1.5 equiv. the highest
enaminoesters17 and sulfonyl azides.18 On the basis of our yield (67%) of desired 5a and 24% of N-butyltosyl amide (by-
previous work regarding the synthesis of N-sulfonyl for- product) were obtained, respectively (entry 10). With addition of
mamidine,19 we herein describe a mild and simple one-pot more 1a and 2, the yield did not improve further (entry 11). When
ꢀ
multicomponent method for the synthesis of di- and tri- the reaction temperature was increased to 40 C or decreased to
substituted N-sulfonyl formamidines (Scheme 1e). This 0 ꢀC, the yield was reduced to 60% or 30%, respectively (entries 12
catalyst-free cascade approach avoids any metals and additives, and 13). When, 3-butyn-2-one was used in place of ethyl propiolate
all of the substrates are commercially available, inexpensive and (3), and the same product was isolated in 30% yield (entry 14).
easily handled. In addition these reactions can be carried out in Whereas, under O2 or N2 atmosphere, the yields were no longer
an open atmosphere at room temperature.
improved, and afforded 65% and 68% yields of desired product,
To evaluate the feasibility of this hypothesis, we initially respectively (entries 15 and 16). Therefore, the optimal reaction
considered the formation of di-substituted sulfonyl for- conditions (entry 11) were set as follows: TsCl (1.5 equiv.), NaN3
mamidines, and tosyl chloride (TsCl; 1a), NaN3 (2), ethyl pro- (1.5 equiv.), ethyl propiolate (1.0 equiv.), and BuNH2 (1.0 equiv.) in
piolate (3), and butylamine (4a) were chosen as model MeCN at room temperature.
substrates to optimize the reaction conditions (Table 1). The
Applying the optimized reaction conditions, we explored the
substrate scope of this reaction (Table 2). The aromatic sulfonyl
chlorides were rst investigated using n-butylamine, and it was
observed that electron-donating or electron-withdrawing
groups on the benzene ring (5a–5e) were tolerated well by this
reaction, generating the desired products in moderate to good
yields with 7 : 3 E-syn/anti isomeric ratios. With electron-
Table 1 Optimization of reaction conditionsa
Table 2 Synthesis of di-substituted sulfonyl formamidinesa,b,c
Entry
Solvent
Temp.
t (h)
Yieldb (%)
1
2
3
4
5
6
7
8
EtOH
DCM
MeCN
Toluene
EtOAc
DMF
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
6
12
8
18
18
18
8
8
8
8
8
5
12
5
6
6
38
36
56
12
10
Trace
11
7
18
67d
66
60
30
30
65
68
THF
H2O
9
MeOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
10c
11e
12
13
14f
15g
16h
40 ꢀ
C
C
0 ꢀ
rt
rt
rt
a
a Reactions were performed with 1 (1.5 mmol), 2 (1.5 mmol), 3 (1 mmol), and 4
(1 mmol) in 2.0 mL of solvent at room temperature under open-air conditions,
unless otherwise noted. b Isolated yield aer column chromatography. c The
ratio of the E-syn and E-anti isomers is given in parentheses. d Gram scale
reaction aer 7 h. e 4-Ethoxycarbonyl-1H-1,2,3-triazole was isolated in 70%
yield. f Methylamine aqueous solution was used. g Reaction was performed
in an MeCN/H2O (3 : 1) solvent mixture at 80 ꢀC.
Reactions were performed with 1a (1.2 mmol), 2 (1.2 mmol), 3 (1.0
mmol), and 4a (1.0 mmol) in 4 mL of solvent at room temperature
b
under open-air conditions, unless otherwise noted. Isolated yield
aer column chromatography. 1a and
c
d
2 were 1.5 mmol. N-
e
Butyltosyl amide was isolated in 24% yield. 1a and 2 were 1.8 mmol.
f
g
3-Butyn-2-one was used instead of ethyl propiolate (3). Under O2 (1
h
atm) atmosphere. Under N2 atmosphere.
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withdrawing groups, such as NO2, Cl, and Br, the yields were
relatively higher than those obtained with substrates containing
electron-donating groups. The best yield was obtained (5d)
when using 2-chlorobenzenesulfonyl chloride, which revealed
that the substituent on the benzene ring had a signicant effect
on the reaction yield. Then, naphthalene sulfonyl chloride and
3-pyridinyl chloride were employed under the standard condi-
tions, and 79% and 75% yields were isolated, respectively (5f
and 5g). Unfortunately, this methodology did not work with
aliphatic sulfonyl chloride; when n-butyl sulfonyl chloride was
used as a reaction partner, the desired product (5h) was not
detected. Interestingly, the 4-ethoxycarbonyl-1H-1,2,3-triazole
was isolated with 70% yield. In addition, benzyl sulfonyl chlo-
ride was tested, and the same product was observed in 24%
yield under identical reaction conditions. A similar product was
synthesised from enaminone and tosyl azide.16d
Fig. 2 1H NMR spectrum of compounds 5a and 5q.
Next, the scope of linear, branched, and cyclic primary aliphatic
amines was studied, and it was determined that the amine struc-
ture did not appreciably inuence the reaction yield because
moderate to good yields were obtained with similar isomeric ratios
(5i–5o). When tert-butylamines were used as substrates, satisfac-
tory yields of the desired products were observed (5p–5r). Inter-
estingly, the syn/anti rotameric ratios of the products changed to
3 : 7, and this observation was likely attributed to the effect of the
bulky tert-butyl group on the molecular conguration.
Additionally, on the basis of the 1H-NMR spectra, we
observed that the syn/anti rotameric ratios were nearly 7 : 3 for
linear and less branched N-alkyl-N0-sulfonyl formamidines; in
contrast, the N-tert-butyl-N0-sulfonyl formamidines had syn/anti
ratios of 3 : 7 (see the ESI† and Fig. 2). This result was attributed
to the fact that in the syn conguration, the bulky tert-butyl
group introduces more steric hindrance, causing this form to be
relatively less favored (Fig. 1b).
It is worth mentioning that only a trace amount of 5s was
observed when aniline was treated under standard conditions.
However, the same reaction yield (43%) of the desired products
(with a 1 : 1 syn/anti rotameric ratio) was obtained in a MeCN/
H2O solvent mixture at elevated temperature with a longer
reaction time. This special condition is likely required because
of the poor reactivity between aniline and ethyl propiolate.20
To further clarify the structural properties of these sulfonyl
formamidines, the nuclear magnetic resonance (NMR) spectra
and X-ray single crystal structural data were examined. All of the
products had two sets of signals in both their 1H- and 13C-NMR
spectra (see the ESI†). In the 1H-NMR spectra, we observed that
the imide (CH]N) proton appeared as two doublets, with
different intensities and coupling constants (J z 13 Hz and 6.0
Hz). The signals corresponding to the N–H proton also
appeared as two doublets or broad peaks with different inten-
sities. These results conrmed the formation of di-substituted
sulfonyl formamidines as either the Z-syn/anti or E-syn/anti-
isomeric/rotameric forms (Fig. 1a).15
The NMR results are consistent with the X-ray single crystal
data.21 For example, 5c was mainly produced in the E-syn
conguration (Fig. 3), but 5r was predominantly generated in
the E-anti form (Fig. 4). Therefore, we concluded that the linear
and less branched N-alkyl-N0-sulfonyl formamidines exist
mainly as E-syn isomers, and the N-tert-butyl-N0-sulfonyl for-
mamidines exist predominantly in the E-anti isomeric form.
To elucidate the reaction mechanism, a series of control
experiments were carried out (Scheme 2). First, the tosyl
Fig. 3 X-ray single crystal structure of 5c (E-syn), CCDC: 2055587.†
Fig. 1 (a) Geometrical/rotational isomers of N-alkyl-N0-sulfonyl for-
mamidine. (b) The effect of the tert-butyl group on the configuration. Fig. 4 X-ray single crystal structure of 5r (E-anti), CCDC: 2055586.†
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Table 3 Synthesis of tri-substituted sulfonyl formamidinesa,b
Scheme 2 Control experiments.
chloride (1a) and NaN3 (2) were conducted under standard
conditions and isolated 90% yield of tosylazide (A) (Scheme 2a).
Aer that the reaction of ethyl propiolate (3) and butylamine
(4a) gave 93% yield of enaminoester (B) under same conditions
(Scheme 2b). This indicates that established reaction condi-
tions are suitable for the formation of two intermediates. Next
the tosylazide (A) was treated with enaminoester (B) and iso-
lated 73% of desired 5a together with 17% of N-butyltosyl amide
and 15% of 4-ethoxycarbonyl-1H-1,2,3-triazole, respectively (see
the ESI, S6†). This result revealed that the 1,2,3-triazole and N-
substituted sulfonamide were eliminated aer formation of two
intermediates.
On the basis of the experimental results obtained in the
present study and a literature survey,16 a potential mechanism
for the MCR was proposed (Scheme 3). First, the ethylpropiolate
and amine reacted to generate enaminoester (B),17 while sulfo-
nylazide (A)18 was simultaneously formed from sulfonyl chloride
and NaN3 under the same conditions. Then, the triazoline
intermediate (C) was produced from active components A and B
through a 1,3-dipolar cycloaddition reaction.22 The subsequent
cycloreversion of the intermediate C and release of one mole-
cule of ethyldiazoacetate afforded the desired product 5 (route,
a).23 The elimination of corresponding sulfonamide from C
yields 4-ethoxycarbonyl-1H-1,2,3-triazole (route, b).24
a
Reactions were performed with 1 (1.5 mmol), 2 (1.5 mmol), 3 (1 mmol),
and 6 (1 mmol) in 4 mL of solvent at room temperature under open-air
b
conditions, unless otherwise noted. Isolated yield aer column
chromatography. c Dimethylamine aqueous solution was used.
presented in Table 3. First, we investigated the reactivity of
various sulfonyl chlorides under the optimized conditions.
Arylsulfonyl chlorides bearing electron-donating or electron-
withdrawing groups on their benzene rings were well toler-
ated by this reaction process, and generated the desired prod-
ucts in good yields (7a–7d). Moreover, with naphthalene
sulfonyl chloride, the reaction proceeded smoothly, and the
desired product 7e was obtained in a satisfactory yield. Hetero
aromatic sulfonyl chlorides were also tolerated by this reaction,
and the best yield (7g) was obtained using 3,5-
dimethylisoxazole-4-sulfonyl chloride. Notably, the aliphatic
sulfonyl chlorides, i.e., benzylsulfonyl chloride and butylsul-
fonyl chloride, afforded the products 7h and 7i in 73% and 89%
isolated yields, respectively. Interestingly, when aliphatic
sulfonyl chlorides and secondary amine were tested, the 4-
ethoxycarbonyl-1H-1,2,3-triazole was not observed. This indi-
cates that the N,N-disubstituted triazoline intermediate (see the
Scheme 3) preferentially undergoes cycloreversion and elimi-
nate ethyldiazoacetate to form sulfonyl amidine.
Aer a thorough examination of the synthesis of di-
substituted N-sulfonyl formamidines, the developed MCR was
further extended to the synthesis of tri-substituted sulfonyl
formamidines by using secondary amines. First, the reaction
between TsCl (1a), NaN3 (2), ethyl propiolate (3), and diethyl-
amine (6a) was optimized (for details, see ESI,† p. S3), and the
desired tri-substituted sulfonyl formamidine (7a) was isolated
in 91% yield under the optimized conditions. The substrate
scopes of the reactions were explored, and the results are
Next, dimethylamine was tested, and the desired product, 7j,
was isolated in good yield (85%). When the cyclic and hetero-
cyclic secondary amines, such as pyrrolidine, piperidine, and
morpholine were used as the starting materials, the corre-
sponding sulfonylamidines (7k–7o) were obtained with yields
between 59% and 86%. Finally, N-methylaniline reacted slowly
and led to a 67% isolated yield (7p).
On the basis of the 1H and 13C-NMR spectra of the tri-
substituted N-sulfonyl formamidines (one set of signals in
both types of spectra, see the ESI†), it was determined that these
reactions produced only the E-isomer; this result is consistent
with earlier reports.25
Finally, aqueous ammonia was used as a reaction partner,
but unfortunately, the expected mono-substituted N-sulfonyl
formamidine was not detected.
Scheme 3 Proposed reaction mechanism.
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Conclusions
In summary, a straightforward, one-pot, multicomponent
method was developed for the synthesis of di- and tri-
substituted N-sulfonyl formamidines using readily accessible
substrates under very mild conditions free of catalysts or other
additives. This protocol is inexpensive to carry out and step-
economic, so it provides a potential route for the construction
of diverse N-sulfonyl formamidines in moderate to high yields.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21762044), and Innovation Training
Project of Xinjiang Undergraduate Education (No:
S202010762017).
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