A new way for synthetizing (E)-methyl methylsulfanyl(phenylamino)methylene carbamates via Beckmann transposition in triflic acid

Article abstract of DOI:10.4314/bcse.v29i2.11

At low temperature in triflic acid, nitroketene S,N-acetals with a tethered phenyl ring react to form the corresponding hydroxynitrilium ions. Quenching with methanol leads to the formation of dications which undergo an unexpected Beckmann transposition a

Full text of DOI:10.4314/bcse.v29i2.11

Bull. Chem. Soc. Ethiop. 2015, 29(2), 291-298.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v29i2.11
ISSN 1011-3924
 2015 Chemical Society of Ethiopia
A NEW WAY FOR SYNTHETIZING (E)-METHYL
METHYLSULFANYL(PHENYLAMINO)METHYLENE CARBAMATES
VIA BECKMANN TRANSPOSITION IN TRIFLIC ACID
Fanté Bamba1,*, Yaya Soro2, Sorho Siaka2, Jérôme Marrot3 and Jean-Marie Coustard4
1Laboratoire de Chimie Organique Structurale, Université Félix Houphouët-Boigny, 22 BP 582
Abidjan 22, Côte d’Ivoire
2Laboratoire des Procédés Industriels de Synthèse, de l’Environnement et des Energies
Nouvelles, Institut National Polytechnique Félix Houphouët-Boigny, BP 991 Yamoussoukro,
Côte d’Ivoire
3Laboratoire ILV-UVSQ - UMR 8180 CNRS, 45, Avenue des États-Unis, F-78035 Versailles
Cedex, France
4Laboratoire IC2MP-UMR 7285 CNRS, 40, Avenue du Recteur Pineau, F-86022 Poitiers
Cedex, France
(Received January 9, 2014; revised December 22, 2014)
ABSTRACT. At low temperature in triflic acid, nitroketene S,N-acetals with a tethered phenyl ring react to
form the corresponding hydroxynitrilium ions. Quenching with methanol leads to the formation of dications which
undergo an unexpected Beckmann transposition affording new (E)-methyl methylsulfanyl(phenylamino)
methylene carbamates.
KEY WORDS: Nitroketene S,N-acetals, Electrophilic aromatic substitution, Superacids, Triflic acid, Beckmann
transposition
INTRODUCTION
Nitroethene compounds are well-known synthons in organic chemistry. For instance, they are
used in the field of drug synthesis [1, 2] and for intramolecular cyclizations involving nitrile
oxides (INOC reactions) [2, 3]. In superacids, non-tethered phenyl ring substrates, such as alkyl
or aryl amines derivatives are polyprotonated to lead to multicharged species that can be trapped
by suitable nucleophiles [4, 5].
We have previously described the behaviour and the reactivity of 1-benzylamino-1-
methylsulfanyl-2-nitroethylene derivatives 1 in triflic acid. It was shown that these compounds
are firstly C,O-diprotonated to give hydroxynitrilium ions 2. By quenching with water, these
ions were transformed into a reactive nitrile oxide 3, which undergoes an intramolecular-1,3-
dipolar cyclization to afford cyclic products 4 (Scheme 1) [6].
R
HO
H
N
NO2
O N
O
N
H
N
R
SMe
NH
CF3SO3H
R
SMe
R
SMe
H
N
SMe
R = H, Me, OMe, F
N
1
2
4
3
Scheme 1. Formation of isoxazolines from 1-benzylamino-1-methylsulfanyl-2-nitroethylenes.
Recently, we have shown that when 1-phenylalkylamino-1-methylthio-2-nitroethenes
derivatives are dissolved with benzene in TFSA, in situ trapping occurs to afford 2-
(hydroxyimino)-2-phenyl-N-(phenylalkyl) acetamides derivatives (Scheme 2) [7].
__________
*Corresponding author. E-mail: fante_bamba1@yahoo.fr

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Fanté Bamba et al.
Scheme 2. Synthesis of 2-(hydroxyimino)-2-phenyl-N-(phenylalkyl)acetamides derivatives.
In the present paper, we report the synthesis of new (E)-methyl methylsulfanyl
(phenylamino)methylene carbamates via Beckmann transposition in triflic acid from nitroketene
S,N-acetals with a tethered phenyl ring.
RESULTS AND DISCUSSION
Starting compounds
The starting compounds 10a-e were prepared by nucleophilic substitution between aniline 8a or
the corresponding ω-phenylalkylamines 8b-e and 1,1-bis(methylthio)-2-nitroethylene 9 (1.1
molar equivalent) in refluxing 95% ethanol [8] (Scheme 3) in yields varying from 64 to 82%
(Table 1). In all cases, the di-substituted products were formed in a small range.
Table 1. Yields of compounds 10b-e.
Starting compounds
Products
8b
10b
82
8c
10c
74
8d
10d
64
8e
10e
76
Yield (%)
Starting materials have not been described before, except for (E)-N-(1-(methylthio)-2-
nitrovinyl)aniline 10a [8] and 1-methylsulfanyl-2-nitro-N-phenethylethenamine 10b [9].
Scheme 3. Synthesis of starting products 10a-e.
1
Compounds 10a-e were characterized from their H and 13C NMR spectra, with vinylic and
amino protons respectively in the range of δH 6.47-6.58 ppm and 10.32-10.52 ppm. The C-1 and
C-2 carbons resonate at δC 165.0-165.5 ppm and 106.5-106.7 ppm, respectively.
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A new way for synthetizing (E)-methyl methylsulfanyl(phenylamino)methylenecarbamates 293
In organic solvent, compounds 10a-e exist as a sole isomer as shown by a single set of
signals in the 13C NMR spectra. They are probably all (E)-isomers because this conformation
allows formation of an intramolecular hydrogen bond between the N-H and one of the oxygen
atoms of –NO2 group, as previously reported for the 1-arylamino-1-methylthio-2-nitroethenes
10a [8].
Reactions in triflic acid
In the present study, the reactions were carried out in triflic acid at low temperature between -5
to 0 °C and under nitrogen atmosphere. The molar ratio of triflic acid/starting compounds 10a-e
was 50:1. The starting materials were fully transformed within 1 to 5 h of reaction time,
depending on the substituent. The observed reaction, reported in Scheme 4, was generally clean
with yields varying from 18 to 50% (Table 2) except for 10e, because of degradation reactions
leading to the formation of dark polar products which has not been identified by NMR study.
Table 2. Yields of products 11a-d and 16.
Starting compounds
Products
10a
10b
11b
30
10c
11c
18
10d
11d
42
11a
50
16
14
Yield (%)
Scheme 4. Reaction of compounds 10a-e in triflic acid.
The yields of isolated products 11a-d were low, probably because the formed
hydroxynitrilium ions can either react (i) to afford degradation products or (ii) with triflate anion
to form the nucleophilic addition, that easily decomposed during the hydrolysis step of the
reaction medium, as previously observed in situ with 1-amino-2-nitroethylene derivatives [4].
The structure of compounds 11a-d was determined by NMR and HRMS spectroscopy. The
CH3O protons appear in the range of δH 3.64-3.71 ppm and the NH proton as a broad singlet in
the range of δH 9.80-10.07 ppm. Methoxy, imine and carbamate carbons resonate respectively at
δC 52.95-53.00 ppm, δC163.12-163.14 ppm and 174.43-174.50 ppm. The structure of
compounds 11 was corroborated by X-ray crystallographic analysis of 11a [10-12] (Figure 1).
The X-ray analysis indicated the trapping of a methoxy group, the loss of oxime group and the
formation of carbamate derivatives.
Compounds 11a-d are presumably (E)-isomers because this conformation allows formation
of intramolecular hydrogen bond between the N-H and O=C-OMe groups. The formation of
compounds 11 may be explained by the mechanism described in Scheme 5. In this mechanism,
starting materials 10a-e undergo a protonation on the carbon bearing the nitro group and an O-
protonation of the nitro group. This last O-protonation occurred through a fast proton exchange
o
process with the acidic medium, as demonstrated in fluorosulfonic acid at -80 C [13, 14].
Prototropic exchanges and the formal loss of a molecule of water lead to the formation of the
reacting conjugated hydroxynitrilium ion 12. Such hydroxynitrilium cations were previously
observed with 1,1-heterodisubstitud-2-nitroethylene derivatives by NMR experiments [4-6, 15-
17]. They are electrophilic species which can react in situ with aromatic ring [7, 15-20]. In the
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Fanté Bamba et al.
present case, starting compounds 10a-e lead to hydroxynitrilium ions 12 which do not react
through an intramolecular process, probably because of the excessively high activation energy
of the reaction at this low temperature or the length of alkyl chain between the aryl and the NH.
Figure 1. The X-ray analysis of compound 11a.
Scheme 5. Suggested mechanism for the formation of compounds 11a-d and 16.
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A new way for synthetizing (E)-methyl methylsulfanyl(phenylamino)methylenecarbamates 295
In agreement with this hypothesis is the fact that with the 10a, hydroxynitrilium ion formed
reacts with the tethered phenyl ring by way of an electrophilic aromatic substitution mechanism
to give a cyclic subproduct 16 [5]. By quenching with a cold mixture of CH2Cl2/MeOH,
hydroxynitrilium ions 12a-d are trapped with MeOH to give dications13, which undergo an
Beckmann transposition via dications 14, followed by prototropic exchanges [13-20] to afford
carbamic esters compounds 11.
CONCLUSION
New (E)-methyl methylsulfanyl(phenylamino)methylene carbamates have been easily prepared
via Beckmann transposition in triflic acid. The mechanism of the reaction implies the formation
of a stable hydroxynitrilium ion which was trapped by methanol leading to a reactive dication. A
Beckmann transposition of this dication leads to the methyl carbamate derivatives. The present
study constitutes an extension of the transformation of nitroketene S,N-acetals with a tethered
phenyl ring in triflic acid.
EXPERIMENTAL
General remarks
Melting points were determined with a Büchi Melting point B545 apparatus using capillary
o
tubes (temperature rate 2 C/min) and were not corrected. A Bruker DPX 300 spectrometer,
equipped with a low temperature probe, was used for 1H- and 13C-NMR spectra recorded at 300
and 75 MHz, respectively. NMR spectra were recorded at room temperature and chemical shifts
reported relative to Me4Si. The reproducibility of 13C NMR shift was about 0.05 ppm,
depending on cell and concentration. Chemical assignments were made using DEPT-135
techniques and usual chemical shift assignments rules. Electron-impact ionization (70 eV) mass
spectra were obtained with a FinniganIncos 500 Instrument. High resolution mass spectrometry
was performed by the “Centre Régional de Mesures Physiques de l'Ouest - Université de
Rennes, France”. Flash chromatography was achieved on silica gel (20 to 45 m particle size).
Triflic acid was purchased from Acros and 1,1-bis(methylthio)-2-nitroethene from Lancaster
and were used without further purification. No attempt was made to optimize the yields.
(E)-1-Methylthio-2-nitro-N-phenethylethenamine (10b)
1,1-Bis(methylthio)-2-nitroethene (2.2 g, 13.32 mmol) and phenethylamine 8b (1.5 mL, 12.00
mmol) were heated together in refluxing 95% ethanol (75 mL) under nitrogen atmosphere. The
reaction was followed by thin-layer chromatography (CH2Cl2). After disappearance of the
phenethylamine (3 h) and cooling, the mixture was concentrated under reduced pressure and the
resulting oily product was purified by flash chromatography with AcOEt/EP (30/70) and then
crystallized from CH2Cl2/petroleum ether to afford 10b (2.34 g, 82%) as white crystals, m.p.
o
1
95.4 C (CH2Cl2/petroleum ether). H NMR (CDCl3):  = 2.35 (s, 3 H, CH3), 2.90 (t, J = 7.23
Hz, 2H, CH2-Ph), 3.59 (dd, J = 6.99 and 13.00 Hz, 2H, CH2-N<), 6.47 (s, 1H, H-vinylic), 7.14-
7.23 (m, 3H, o-H and p-H, Ar), 7.23-7.28 (m, 2H, m-H, Ar), 10,32 (broad s, NH). 13C NMR
(CDCl3):  = 14.7 (SCH3), 42.6 (CH2-Ph), 46.4 (CH2-N<), 106.6 (=CH-NO2), 127.4 (CH-Ar),
127.7 (2 CH-Ar), 129.2 (2 CH-Ar), 137.8 (ipso-C), 165.3 (-NH-C=). HRMS for C11H14N2O2S
[M]+: calcd. 238.0776, found 238.0788.
(E)-N-(4-Methoxyphenethyl)-1-methylthio-2-nitroethenamine (10c)
From 4-methoxyphenethylamine (1.8 mL, 12.30 mmol) and 1,1-bis(methylthio)-2-nitroethene
o
(2.23 g, 13.52 mmol) was obtained 10c (2.23 g, 74%) as yellow crystals, m.p. 112.4 C
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Fanté Bamba et al.
1
(CH2Cl2/petroleum ether). H NMR (CDCl3):  = 2.42 (s, 3H, SCH3), 2.92 (t, J = 7.0 Hz, 2H,
CH2-Ph), 3.62 (dd, J = 7.0 Hz and 14.2 Hz, 2H, CH2-N<), 3.80 (s, 3H, OCH3), 6.55 (s, 1H, H-
vinylic), 6.85-6.89 (m, 2H, H-Ar), 7.13-7.16 (m, 2H, H-Ar), 10.52 (broad s, NH). 13C NMR
(CDCl3):  = 14.73 (SCH3), 34.96 (CH2-Ph), 46.59 (CH2-N<), 55.66 (OCH3), 106.67 (=CH-
NO2), 114.70 (2 CH-Ar), 129.56 (C-Ar), 130.14 (2 CH-Ar), 159.07 (C-OMe), 165.00 (-NH-C=).
HRMS for C12H16N2O3SNa [M+Na]+: calcd. 291.0779, found 291.0779.
(E)-N-(4-Fluorophenethyl)-1-methylthio-2-nitroethenamine (10d)
From 4-fluorophenethylamine (1.5 mL, 11.43 mmol) and 1,1-bis(methylthio)-2-nitroethene
(2.07 g, 12.58 mmol) was obtained 10d (1.88 g, 64%) as white crystals, m.p. 115.4 oC
1
(CH2Cl2/petroleum ether). H NMR (CDCl3):  = 2.42 (s, 3 H, SCH3), 2.96 (t, J = 7.0 Hz, 2H,
CH2-Ph), 3.65 (dd, J = 7.0 Hz and 13.6 Hz, 2H, CH2-N<), 6.55 (s, 1H, H-vinylic), 6.99-7.05 (m,
2H, H-Ar), 7.16-7.21 (m, 2H, H-Ar), 10.52 (broad s, 1H, NH). 13C NMR (CDCl3):  = 14.72
2
(SCH3), 35.05 (CH2-Ph), 46.31 (CH2-N<), 106.76 (=CH-NO2), 115.95 (d, JCF = 21.4 Hz, m-
CH-Ar), 130.62 (d, 3JCF = 8.2 Hz, o-CH-Ar), 133.41 (d, 4JCF = 3.3 Hz, C-Ar), 162.30 (d, 1JCF
=
244.8 Hz, C-F), 165.05 (-NH-C=). HRMS for C11H13FN2O2SNa [M+Na]+: calcd. 279.0579,
found 279.0580.
1-(Methylthio)-2-nitro-1-(3-phenylpropylamino)ethene (10e)
From 3-phenylpropylamine (1.4 mL, 9.85 mmol) and 1,1-bis(methylthio)-2-nitroethene (1.79 g,
10.83 mmol) was obtained 10e (1.89 g, 76%) as yellow crystals, m.p. 84 oC (CH2Cl2/petroleum
1
ether). H NMR (CDCl3):  = 2.02 (dd, J = 7.49 and 14.66 Hz, 2H, CH2), 2.40 (s, 3H, CH3),
2.71 (t, J = 7.49 Hz, , 2H, CH2-Ph), 3.40 (dd, J = 7.49 and 13.11 Hz, 2H, CH2-N<), 6.58 (s, 1H,
vinylic H), 7.15-7.23 (m, 3H, aromatic o-H and p-H), 7.26-7.33 (m, 2H, aromatic m-H), 10.32
(broad s, NH). 13C NMR (CDCl3):  = 14.6 (SCH3), 30.9 (CH2), 33.1 (CH2-Ph), 44.2 (CH2-N<),
106.5 (=CH-NO2), 126.7 (aromatic CH), 128.8 (2 aromatics CH), 129.0 (2 aromatics CH), 140.7
(ipso-C), 165.5 (-NH-C=). HRMS for C12H16N2O2S [M]+: calcd. 252.0932, found 252.0916.
Methylmethylsulfanylphenethylaminomethylenecarbamate (11b)
1-Methylsulfanyl-2-nitro-1-(2-phenethylamino)ethylene 10b (238 mg, 1 mmol) was dissolved
o
with stirring in triflic acid (4.4 mL, 50 mmol) at -5 to 0 C for 1 h under nitrogen atmosphere.
After the disappearance of the starting compound, the acidic solution was poured into a mixture
o
80 mL of CH2Cl2/MeOH (87:13) at -60 to -40 C and let to warm at room temperature. When
temperature reached about -15 oC, brine (10 mL) and anhydrous Na2CO3 (6 g) were added. The
extraction was carried out promptly with dichloromethane (4 x 20 mL). The organic phase was
dried over MgSO4 and the solvent evaporated under reduced pressure. The resulting oil was
purified by flash chromatography (eluent petroleum ether/ethylacetate: 90/10) to yield 11b (75
mg, 30%) as viscous oil. 1H NMR (CDCl3):  = 2.45 (s, 3H, SCH3), 2.93 (t, J = 7.6 Hz, 2H, Ph-
CH2), 3.57 (dd, J = 7.6 Hz and 13.7 Hz, 2H, CH2), 3.71 (s, 3H, OMe), 7.20–7.28 (m, 3H, p- and
o-H-Ar), 7.30-7.36 (m, 2H, m-H-Ar), 9.80 (broad s, 1H, NH). 13C NMR (CDCl3):  = 14.07
(SCH3), 36.01 (Ph-CH2), 45.68 (CH2), 52.97 (OCH3), 127.28 (CH-Ar), 129.09 (2 CH-Ar),
129.17 (2 CH-Ar), 138.08 (C-Ar), 163.12 (>C=N), 174.50 (>C=O).HRMS for C12H16N2O2S
[M]+: calcd. 252.0932, found 252.0919.
(E)-methyl ((methylthio)(phenylamino)methylene)carbamate (11a)[21]
From (E)-N-(1-(methylthio)-2-nitrovinyl)aniline (1 mmol) was obtained 11a (0.5 mmol, 50%),
o
1
m.p. 66 C. H NMR (CDCl3):  = 2.40 (s, 3H, SCH3), 3.77 (s, 3H, OCH3), 7.20- 7.50 (m, 5H,
H-Ar), 11.33 (broad s, 1H, NH). 13C NMR (CDCl3):  = 13.87 (SCH3), 52.55 (OCH3, 126.29,
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A new way for synthetizing (E)-methyl methylsulfanyl(phenylamino)methylenecarbamates 297
127.60, 129.14, 136.56, 162.84 (>C=N), 173.95 (>C=O). HRMS for C10H12N2O2S [M]+: calcd.
224.0619, found 224.0619.
Methyl-(4-methoxyphenethylamino)methylsulfanylmethylenecarbamate (11c)
From (E)-N-(4-methoxyphenethyl)-1-methylthio-2-nitroethenamine (1 mmol) was obtained 11c
(0.18 mmol, 18%) as viscous oil. 1H NMR (CDCl3):  = 2.45 (s, 3H, SCH3), 2.86 (t, J = 7.1 Hz,
2H, Ph-CH2), 3.52 (dd, J = 7.1 Hz and 13.3 Hz, 2H, CH2), 3.71 (s, 3H, OMe), 3.80 (s, 3H, Ph-
OCH3), 6.85–6.88 (m, 2H, H-Ar), 7.12-7.15 (m, 2H, H-Ar), 10.07 (broad s, 1H, NH). 13C NMR
(CDCl3):  = 14.07 (SCH3), 35.11 (Ph-CH2), 45.90 (CH2), 52.95 (OCH3), 55.65 (Ph-OCH3),
114.56 (2 CH-Ar), 130.07 (2 CH-Ar), 152.77 (C-Ar), 158.90 (>C-OMe), 163.12 (>C=N),
174.44 (>C=O). HRMS for C13H18N2O3SNa [M+Na]+: calcd. 305.0936, found 305.0936.
Methyl-(4-fluorophenethylamino)methylsulfanylmethylenecarbamate (11d)
From (E)-N-(4-fluorophenethyl)-1-methylthio-2-nitroethenamine (1 mmol) was obtained 11d
1
(42.20 mmol, 42%) as viscous oil. H NMR (CDCl3):  = 2.38 (s, 3H, SCH3), 2.82 (t, J = 7.0
Hz, 2H, Ph-CH2), 3.47 (m, 2H, CH2), 3.64 (s, 3H, OCH3), 6.91-6.97 (m, 2H, H-Ar); 7.08-7.12
(m, 2H, H-Ar), 9.80 (broad s, 1H, NH). 13C NMR (CDCl3):  = 14.06 (SCH3), 35.21 (Ph-CH2),
2
3
45.66 (CH2), 53.00 (OCH3), 115.87 (d, JCF = 21.4 Hz, m-CH- Ar), 130.51 (d, JCF = 7.7 Hz,
4
1
o-CH-Ar), 133.72 (d, JCF = 3.3 Hz, C-Ar), 162.22 (d, JCF = 244.8 Hz, C-F), 163.14 (>C=N),
174.43 (>C=O). HRMS for C12H15FN2O2SNa [M+Na]+: calcd. 293.0736, found 293.0736.
(E)-2-(methylthio)-3H-indol-3-one oxime (16)
From (E)-N-(1-(methylthio)-2-nitrovinyl)aniline was obtained sub product 16 (0.14 mmol,
14%). m.p. 198 oC. 1H NMR (CDCl3/DMSO-d6):  = 2.60 (s, 3H, SCH3), 7.11-7.18 (m, 1H, H-
Ar), 7.29-7.41 (m, 2H, H-Ar), 7.93 (d, J = 7.0 Hz, 1H, H-Ar), 13.00 (broad s, 1H, OH). 13C
NMR (CDCl3/DMSO-d6):  = 12.05 (SCH3), 118.29, 121.89, 125.01, 126.12, 131.53, 152.72,
155.47 (>C-N=OH), 170.57 (>C=N). HRMS for C9H8N2OS [M]+: calcd. 192.0357, found
193.0357.
ACKNOWLEDGEMENTS
We acknowledge CNRS/France for financial support, the ‘Ministère des Affaires Etrangères’
France for fellowship funds (Y.S. and S.S.) and the ‘Ministère de l’Enseignement Supérieur’
Côte d’Ivoire for fellowship funds (B.F.).
DEDICATION
This publication is dedicated in memory of Professor Jean-Marie Coustard who supervised this
work but passed away at the end of August 2013. Rest in peace dear Professor.
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