Synthesis of N-nitro-N′-(trimethylsilyl)carbodiimide

Article abstract of DOI:10.1007/s11172-017-1844-2

Nitration of N,N′-bis(trimethylsilyl)carbodiimide with N₂O₅ or (NO₂)₂SiF₆ afforded N-nitro-N′-(trimethylsilyl)carbodiimide, the first representative of N-nitro carbodiimides. Its further nitration led

Full text of DOI:10.1007/s11172-017-1844-2

Russian Chemical Bulletin, International Edition, Vol. 66, No. 6, pp. 991—994, June, 2017
991
Synthesis of N-nitro-N´-(trimethylsilyl)carbodiimide
A. M. Churakov, S. L. Ioffe, A. A. Voronin, and V. A. Tartakovsky
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. E-mail: churakov@ioc.ac.ru
Nitration of N,N´-bis(trimethylsilyl)carbodiimide with N2O5 or (NO2)2SiF6 afforded
N-nitro-N´-(trimethylsilyl)carbodiimide, the first representative of N-nitrocarbodiimides. Its
further nitration led to the release of CO2, which is presumably formed in the course of N,N´-
dinitrocarbodiimide decomposition. The reactions of N-nitro-N´-(trimethylsilyl)carbodiimide
with nucleophiles take place both at the trimethylsilyl group (for example, with NH3) to give
nitrocyanamide salts and at the carbodiimide C atom (for example, with Et2NH) to give the
corresponding nitroguanidines.
Key words: N-(trimethylsilyl)carbodiimides, N-nitrocarbodiimides, N-nitrocyanamides,
nitration of N—SiMe3 group.
Earlier, we have found that nitrocyanamides 1
(R = MeOCO, MeSO2) obtained by the nitration of the
corresponding N-(trimethylsilyl)carbodiimides 2 un-
dergo rapid conversion to the corresponding isocyanates
3 (Scheme 1).1
Results and Discussion
Assuming that dinitrocyanamide 1a and N,N´-di-
nitrocarbodiimide 4a can be labile compounds, we have
chosen for their preparation one of the mildest methods
of nitration, namely, nitration of compounds containing
an N—SiMe3 group with nitronium salts. Substitutive
nitration of this type of compounds have been studied
earlier in a number of works.3,4
The nitration of N-(trimethylsilyl)carbodiimides 2,
as well as the alkylation of nitrocyanamide Ag salt (see
Scheme 1) can a priori lead to both N-substituted nitro-
cyanamides 1 and N-nitrocarbodiimides 4.
We carried out theoretical studies of isomers 1a—c
and 4a—c (Table 1). The optimization of the geometry
and the calculation of the harmonic vibrational freq-
uencies were carried out by the Gaussian 09 program5
using density functional theory (DFT) with the B3LYP
hybrid potential and the 6-311++G(2df,2p) basis. The
calculated frequencies of all the structures optimized in
the gas phase were checked for the absence of imaginary
frequencies.
Later, it was shown2 that alkylnitrocyanamides 1 ob-
tained by the alkylation of nitrocyanamide Ag salt with
alkyl halides also undergo ready conversion to the cor-
responding isocyanates. The mechanism of this process
probably2 includes rearrangement of alkylnitrocyanamides
1 to N-nitrocarbodiimides 4, cyclization of the latter to
cyclic intermediates 5 and their conversion to isocyan-
ates 3 with liberation of N2O (see Scheme 1). Note that
N-nitrocarbodiimides 4 were not isolated or spectrally
detected.
In the present work, a task was set to study a possibil-
ity of the preparation of dinitrocyanamide (1a, R = NO2)
and N,N´-dinitrocarbodiimide (4a, R = NO2). Dinitro-
cyanamide 1a could have been of interest as a high-energy
oxidant, while N,N´-dinitrocarbodiimide as a starting
compound for the preparation of new classes of energetic
compounds.
Scheme 1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 0991—0994, June, 2017.
1066-5285/17/6606-0991© 2017 Springer Science+Business Media, Inc.

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Russ. Chem. Bull., Int. Ed., Vol. 66, No. 6, June, 2017
Churakov et al.
progress was monitored by 1H NMR, gradually raising the
temperature of the reaction mixtures from –30 to 0 C.
The monitoring showed the disappearance of the signal
for the starting carbodiimide 6 (H = 0.15) and the appear-
ance of the signal for nitrocarbodiimide 4b (H = 0.50).
Table 1. DFT calculation results for pairs of isomers 1 and 4
1 and 4
R
–E´a
–E´´ b
Ec
a.u.
/kcal mol–1
а
b
с
NO2
Me3Si
MeSO2
557.858661 557.865068
4.0
6.1
5.8
3
Fluorotrimethylsilane (7) (H = 0.20, d, JF,H = 7 Hz)
762.029115
941.308137
762.038973
941.317285
and trimethylsilyl nitrate (8) (H = 0.41) were the second
observed reaction products in the nitration with nitronium
hexafluorosilicate and nitric anhydride, respectively. No
other products were observed in the reaction mixtures.
Nitrocarbodiimide 4b was isolated in the individual
state as a white crystalline compound, relatively stable at
0 C. At room temperature, it undergoes slow conversion
to trimethylsilyl isocyanate (9) (H = 0.24) (see Scheme 2).
At 20 C, this conversion reaches 95% within 20 h (moni-
toring by 1H NMR spectroscopy).
The structure of nitrocarbodiimide 4b was confirmed
by spectroscopic methods. In the 14N NMR spectrum,
the signal for the nitro group (N = –20) was found in
the region characteristic of the fragment (C=N—NO2).
In nitrocyanamide, the signal for this group is observed
in the region N –40÷–60. In the 13C NMR spectra, the
signal for the carbon atom (C = 123.8) is in the region
characteristic of the carbodiimide fragment (=C=).2 For
cyanamides, the signal for the carbon atom of the nitrile
group is observed in the region C 100—110 (see Refs 1
and 2).
a
E´ = Etot + ZPE is a total energy of compound 1 including
a zero point energy.
b
E´´ = Etot + ZPE is a total energy of compound 4 including
a zero point energy.
c E is a difference in the total energies for pairs of isomers 1 and 4.
The calculations showed that the carbodiimide struc-
tures 4 are thermodynamically more stable than nitro-
cyanamide ones 1 (see Table 1). Therefore, nitrocyana-
mides 1 (see Scheme 1, R = MeCO2, MeSO2) obtained
by us earlier1 are kinetically controlled reaction products.
Bis(trimethylsilyl)carbodiimide (6) served as the
starting compound in our studies (Scheme 2). It could
have been expected that its nitration with one equiva-
lent of nitrating agent, similarly to the nitration of
N-(trimethylsilyl)carbodiimides 2 (see Scheme 1), would
give N-(trimethylsilyl)nitrocyanamide (1b) as a primary
reaction product. However, N-nitro-N´-(trimethylsilyl)
carbodiimide 4b was found to be the only nitration product
(see Scheme 2). This result can be explained by the easi-
ness of the migration of the SiMe3 group in compound 1b
(cf. Refs 6 and 7). At the same time, the pathway in which
carbodiimide 4b is a primary product of nitration cannot
be excluded either.
Nucleophiles react with nitrocarbodiimide 4b either at
the Si atom, or at the C atom. Ammonia attacks the SiMe3
group, resulting in the formation of the ammonium salt
of nitrocyanamide (10). Conversely, diethylamine reacts
only at the carbodiimide C atom, leading to the silylated
nitroguanidine 11, which after hydrolysis gives nitrogua-
nidine 12 (Scheme 3).
Scheme 2
Scheme 3
We suggested that nitration of silylated nitrocarbo-
diimide 4b with N2O5, similarly to the nitration of
N-(trimethylsilyl)carbodiimides 2 (see Scheme 1), can
give dinitrocyanamide (1a) (Scheme 4). However, carry-
ing out this reaction in both a polar solvent (MeCN) and
a weakly polar one (CH2Cl2) already at –30 C leads to
Nitration of carbodiimide 6 was carried out using
(NO2)2SiF6 or N2O5 in CH2Cl2 as a solvent. The reaction

N-Nitro-N´-(trimethylsilyl)carbodiimide
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 6, June, 2017
993
ing temperature below 0 C, a solid residue was evacuated (1 Torr)
for 30 min at 0 C (at temperatures above 10 C, carbodiimide
4b slowly undergoes conversion to trimethylsilyl isocyanate 9
(H = 0.24), for details see below). The yield of carbodiimide 4b
was 3.47 mmol (81%), which was determined by 1H NMR using
cyclohexane as an internal quantitative standard.
Method B. Similar experiment with N2O5 (462 mg,
4.28 mmol) led to carbodiimide 4b in 73% yield. IR (CH2Cl2),
ν/cm–1: 2250, 2300—2380 (=C=); 1260—1280, 1545 (NO2).
1H NMR (CD2Cl2), : 0.50 (s, Me3Si). 13C NMR (–70 C,
CD2Cl2), : –1.59 ((Me3Si); 123.8 (br, =C=). 14N NMR (0 C,
CD2Cl2), : –20.3 (NO2, Δν1/2 = 25 Hz).
the liberation of CO2 (75% yield), which was identified by
the reaction with barite water. This result can be explained
by the fact that the rearrangement of dinitrocyanamide
(1a) to N,N´-dinitrocarbodiimide (4a) proceeds much
readier than in the studied by us earlier nitrocyanamides
1 (R = MeOCO, MeSO2, see Scheme 1), while dinitro-
carbodiimide (4a) decomposes already at –30 C, being
converted initially to N-nitroisocyanate 13 and then to
CO2 following by the mechanism similar to that given in
Scheme 1.
Thermal decomposition of N-nitro-N´-(trimethylsilyl)carbo-
diimide (4b). A solution of carbodiimide 4b (3.74 mmol) in
CH2Cl2 (8 mL) was allowed to stand at 20 C, regularly recording
its 1H NMR spectra. After 20 h, carbodiimide 4b converted to
trimethylsilyl isocyanate (9) by 95%. Isocyanate 9 was found to
be identical to the authentic sample9 in its IR spectrum and 1H
and 13C NMR spectra.10
Scheme 4
Reaction of N-nitro-N´-(trimethylsilyl)carbodiimide (4b) with
ammonia. Carbodiimide 4b obtained by procedure A was dissolved
in CH2Cl2 (10 mL), ammonia was passed through this solution
at –20 C. A precipitate was collected by filtration. The yield of
the ammonium salt of nitrocyanamide 10 was 75% (determined
by UV spectroscopy,  = 265 nm,  = 9000). The ammonium
salt was converted to the potassium one upon treatment with an
ethanolic solution of KOH, which was recrystallized from ethanol
(m.p. 132—134 C, Ref. 11: m.p. 135—136 C). The potassium
salt was found to be identical to an authentic sample12 in its UV,
IR, and 14N NMR spectra.
Reaction of N-nitro-N´-(trimethylsilyl)carbodiimide (4b) with
Et2NH. Diethylamine (0.28 g, 3.88 mmol) was added dropwise to
a solution of carbodiimide 4b (3.88 mmol) obtained by procedure
A in CH2Cl2 (8 mL) at –50 C with stirring. The temperature
was allowed to rise to 0 C over 15 min, the reaction mixture was
maintained at this temperature for 1 h. The solvent was evaporated
at reduced pressure, a solid residue was evacuated (1 Torr) for
1 h at 20 C. The yield of N,N-diethyl-N´-trimethylsilyl-N´´ -
nitroguanidine (11) was 3.67 mmol (95%), which was determined
by 1H NMR spectroscopy in CD2Cl2 using MeCN as an internal
quantitative standard. 1H NMR (CD2Cl2), : 0.30 (s, 9 H,
(CH3)3Si); 1.26 (t, 6 H, 2 CH3CH2, J = 7.0 Hz); 3.50 (q, 4 H,
2 CH3CH2, J = 7.0 Hz). The solid product was dissolved in
CH2Cl2 (8 mL), H2O (0.3 mL) was added and this mixture
was stirred for 30 min. The solvent was evaporated at reduced
pressure, the residue was dried in vacuo to obtain N,N-diethyl-
N´´-nitroguanidine (12) (0.58 g, 93%), m.p. 83—88 C, after
recrystallization from water m.p. 93—95 C (Ref. 13: 92—94 C).
The product was identical to the authentic sample in its IR and
1H NMR spectra.
In conclusion, we synthesized N-nitro-N´-(trimethyl-
silyl)carbodiimide (4b), the first representative of a new
class of nitro compounds, N-nitrocarbodiimides, and
studied its reactions with nucleophiles. Attempted syn-
thesis of N,N-dinitrocyanamide (1a) was unsuccessful.
Presumably, this compound readily isomerizes to N,N´-di-
nitrocarbodiimide (4a), which decomposes with the for-
mation of CO2 and N2O already at –30 C.
Experimental
1H, 13C, and 14N NMR spectra were recorded on a Bruker
AM300 spectrometer (300.13, 75.47, and 21.69 MHz, res-
pectively). Chemical shifts are given relative to SiMe4 (1H,
13C) or CH3NO2 14N, external standard, the high-field
(
chemical shifts are negative). IR spectra were recorded on a
Bruker ALPHA-T spectrometer. Reaction progress was moni-
tored by TLC (Merck 60 F254). N,N´-Bis(trimethylsilyl)car-
bodiimide was obtained according to the known procedure.8
Nitration reactions were carried out in anhydrous solvents
under argon.
Reaction of N-nitro-N´-(trimethylsilyl)carbodiimide (4b)
with N2O5. A. A solution of carbodiimide 4b in MeCN (2.5 mL)
was added dropwise from a cooled dropping funnel to a sus-
pension of N2O5 (0.46 g, 4.28 mmol) in MeCN (2.5 mL) at
–30 C. Evolution of a gas was observed. The reaction mixture
was stirred for 20 min at –20 C under argon, trapping a liber-
ated gas with barite water. The yield of BaCO3 was 0.63 g (75%).
The reaction mixture recondensed into a trap cooled to –70 C
at a pressure of 1 Torr (bath temperature Т  0 C). The yield of
trimethylsilyl nitrate in the condensate determined by 1H NMR
was 3.85 mmol (90%).
N-Nitro-N´-(trimethylsilyl)carbodiimide (4b). Method A.
A solution of N,N´-bis(trimethylsilyl)carbodiimide (6) (800 mg,
4.28 mmol) in CH2Cl2 (4 mL) was added to a suspension of
(NO2)2SiF6 (500 mg, 2.14 mmol) in CH2Cl2 (5 mL) with stirring
at –30 C. After a complete dissolution of (NO2)2SiF6 (~20 min),
the temperature was allowed to rise to 0 C. The reaction mixture
was analyzed by 1H NMR: a complete disappearance of the start-
ing compound 6 (H = 0.15) and the appearance of the signals
for N-nitro-N´-(trimethylsilyl)carbodiimide (4b) (H = 0.50)
3
and trimethylfluorosilane (H = 0.20, , JF,H = 7 Hz) were
observed. The reaction mixture was concentrated in vacuo, keep-

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Churakov et al.
B. Similar experiment was carried out using CH2Cl2 as
a solvent. A 75% yield of BaCO3 was reached by maintaining the
reaction mixture for 1 h at –10 C and 2 h at 0 C.
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Received November 30, 2016;
in revised form February 6, 2017