Alkoxy isothiocyanates, RO-N=C=S

Article abstract of DOI:10.1039/a903865e

Methoxy isothiocyanate has been identified as a product of pyrolysis of the silver salt of N-methoxydithiocarbamic acid. It is also formed on FVT of 1-(N-methoxythiocarbamoyl)imidazole and 1-(N-methoxythiocarbamoyl)-1,2,4-triazole. Methoxy isothiocyanate was identified by IR spectroscopy in argon matrices at 10 K supported by good agreement with the ab initio and DFT calculated spectrum. Isopropoxy isothiocyanate was generated from the corresponding silver salt in a similar manner. The oligomerization of MeONCS to trithiolanes and tetrathianes is also described.

Full text of DOI:10.1039/a903865e

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Alkoxy isothiocyanates, RO–N᎐C᎐S†
᎐ ᎐
Allan T. Bech,a Robert Flammang,b Carl Th. Pedersen,*a Ming Wah Wongc and
Curt Wentrup*d
a Department of Chemistry, Odense University, DK-5230 Odense M, Denmark.
E-mail: CThP@chem.ou.dk
b Organic Chemistry Laboratory, University of Mons Hainaut, B-7900 Mons, Belgium
c Department of Chemistry, National University of Singapore, 119260 Kent Ridge, Singapore
d Department of Chemistry, University of Queensland, Brisbane QLD 4072, Australia.
E-mail: wentrup@chemistry.uq.edu.au
Received (in Cambridge, UK) 14th May 1999, Accepted 19th July 1999
Methoxy isothiocyanate has been identified as a product of pyrolysis of the silver salt of N-methoxydithiocarbamic
acid. It is also formed on FVT of 1-(N-methoxythiocarbamoyl)imidazole and 1-(N-methoxythiocarbamoyl)-1,2,4-
triazole. Methoxy isothiocyanate was identified by IR spectroscopy in argon matrices at 10 K supported by good
agreement with the ab initio and DFT calculated spectrum. Isopropoxy isothiocyanate was generated from the
corresponding silver salt in a similar manner. The oligomerization of MeONCS to trithiolanes and tetrathianes is
also described.
hydroxylamines and thiophosgene were also unsuccessful.5
Introduction
The analogous methoxy isocyanate, MeONCO, was generated
by Teles and Maier6 by pyrolysis of N-methoxycarbonyl-O-
methylhydroxylamine. They found that methoxy isocyanate was
stable only below 90 K. It may be expected that alkoxy isothio-
cyanates will also be rather unstable compounds. We have
embarked on an investigation using gas phase mass spectro-
metry and flash vacuum thermolysis (FVT) with matrix
isolation IR spectroscopy in an effort to identify alkoxy isothio-
cyanates. The existence of the neutral molecules in the gas
phase, at least on the microsecond time scale, has been demon-
strated by neutralization–reionization–collisional activation
mass spectrometry.7 Here we report the FVT–matrix IR
spectroscopic study as well as the isolation of oligomers of
MeONCS.
Alkoxy and aryloxy isothiocyanates, RO–N᎐C᎐S, constitute a
᎐ ᎐
hitherto unknown class of compounds, and only a few attempts
at their preparation have been reported in the literature. The
reaction of N-methoxyamine with carbon disulfide was
reported by Traube et al. in 1920.1 The reaction product was
reasonably stable at 0 ЊC but decomposed at higher temper-
atures and was not isolable in a pure state. In analogy with the
well known reaction of carbon disulfide with amines,2 the com-
pound formed was most probably the methoxyammonium salt,
CH3ONHCSSϪ, ϩH3NOCH3. Treatment of this salt with silver
nitrate afforded a yellow compound to which the structure
[CH ON᎐CS ]2Ϫ, 2Agϩ (1) was ascribed on the basis of the
᎐
3
2
elemental analysis.1 It was reported that this silver salt evolved
“pungent smelling fumes” upon heating to temperatures
exceeding 40 ЊC. The nature of these fumes was not investi-
gated. In analogy with the well known synthesis of alkyl
and aryl isothiocyanates by heating metal salts of N-alkyl- or
N-aryldithiocarbamic acids,3 it is reasonable to assume that
the pungent smelling fumes could have been due to methoxy
isothiocyanate (2).
Results and discussion
Pyrolysis of the silver salt of N-methoxydithiocarbamic acid 1
On repeating Traube’s pyrolysis of the silver salt 1, we noticed
that when the salt was heated slowly on a Kofler hot bench, it
darkened and decomposed but gave rise to only a small amount
of fumes. However, if the salt was allowed to drop onto the
Kofler bench above 140 ЊC, it deflagrated with evolution of
white fumes with a smell similar to that of methyl isothio-
cyanate.
heat
[CH ON᎐CS ]2Ϫ, 2Agϩ
CH3ONCS ϩ Ag2S
᎐
3
2
1
2
The silver salt 1 was placed in the sublimation oven of an
apparatus for flash vacuum thermolysis (FVT) with the oven set
at a temperature of ca. 100 ЊC to avoid condensation. On heat-
ing the sublimation oven to 140 ЊC as rapidly as possible and
isolating the products in an argon matrix at 10 K, the IR spec-
trum shown in Fig. 1 was obtained. This shows clearly a strong,
broad band at 1888 cmϪ1, which is in the region for ordinary
isothiocyanates. Other major bands that can be ascribed to 2
are at 1075 and 756 cmϪ1. Ab initio and DFT calculations for
2 are in very good agreement with these experimental values
(Table 1). When the product was isolated neat, without Ar, the
same three major bands were observed with some displace-
ments: 1874, 1055 and 750 cmϪ1. On subsequent incremental
heating of the sample on the BaF2 window, the three bands
It is known that 5-alkyl-, 5-aryl-, and the corresponding
5-alkyl(aryl)oxy-1,2,3,4-thiatriazoles decompose thermally and
photochemically to nitriles (cyanates), nitrogen, and sulfur
together with varying amounts of isothiocyanates, R–N᎐C᎐S.4
᎐ ᎐
However, no alkoxy isothiocyanates, RONCS, were detected
during the decomposition of 5-alkoxythiatriazoles to alkyl
cyanates.5 Attempts to prepare such compounds from O-alkyl-
† Calculated frequencies and cartesian coordinates of the computed
structures of 2 and 9 are available as supplementary data. For direct
electronic access see http://www.rsc.org/suppdata/p2/1999/1869, other-
wise available from BLDSC (SUPPL. NO. 57617, pp. 4) or the RSC
Library. See Instructions for Authors available via the RSC web page
(http://www.rsc.org/authors).
J. Chem. Soc., Perkin Trans. 2, 1999, 1869–1873
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Table
1 Calculated IR spectra of MeO–N᎐C᎐S and iso-PrO–
᎐ ᎐
N᎐C᎐Sa,b
᎐
᎐
B3LYP/6-31G*
MeO–N᎐C᎐S (2)
MP2/6-31G*
᎐
᎐
3007
2936
1909
1476
1063
755
(33)
(59)
(553)
(25)
(171)
(117)
(42)
3025
2934
1862
1479
1067
732
(26)
(46)
(511)
(22)
(136)
(152)
(42)
Fig. 2 Collisional activation spectrum (He) of the m/z 210 ions
derived from the trithiolane mixture 3–5. The structure assigned to the
m/z 75 ion is tentative.
515
501
iso-PrO–N᎐C᎐S (9)
᎐
᎐
3014
2938
1917
1325
1138
1107
1023
783
(41)
(14)
3026
2934
1868
1320
1151
1115
1027
764
(24)
(24)
(593)
(39)
(37)
(56)
(86)
(143)
(65)
(637)
(30)
(57)
(47)
(139)
(98)
(61)
535
520
a Frequency in cmϪ1 and intensity (given in parentheses) in km molϪ1
.
Frequencies with intensity value less than 22 km molϪ1 at the B3LYP/6-
31G* level are not reported here. b B3LYP and MP2 frequencies were
scaled by 0.9613,16 and 0.9427,17 respectively.
also present. The same products were also observed on direct
monitoring of the FVT reaction by mass spectrometry. The m/z
242 ion gives a prominent fragment peak at m/z 153. The three
peaks with m/z 210 had identical mass spectra, which means
that they are either closely related isomers, or they interconvert
in the mass spectrometer. The identity of the trithiolanes was
further confirmed by the CA mass spectra of the three products
(Fig. 2) and by the 13C NMR spectrum of the mixture. Here,
three narrowly spaced methoxy signals are observed at 63.65,
63.71, and 63.75 ppm. Further, three smaller signals at 153.3,
152.4 and 150.3 ppm correspond to the the imine carbon in
oximes. These main signals are assigned to the three trithiolanes
3–5. Smaller singlets at 63.7 and 154.1 ppm are ascribed to the
tetrathianes 6 or 7, corresponding to the m/z 242 species.
For further proof, the tetrathiane–trithiolane mixture 3–7
was prepared from carbon disulfide, methoxyamine and hydro-
gen peroxide in a manner similar to that described for the
analogous phenylhydrazono derivatives.8 The same mixture of
compounds was obtained, the only difference being that the
trithiolanes were the main products in the pyrolysis experiment,
whereas the tetrathiane was the main product of the oxidative
preparation. It was observed by Gambi8 that, upon reflux in
acetone solution, tetrathianes lose one sulfur atom to give the
corresponding trithiolane; this may explain why the trithiolanes
are the main products in the thermal reactions.
Fig. 1 Ar matrix IR spectrum of MeONCS 2 obtained from silver salt
1. Bands marked A are at 1888, 1075, and 756 cmϪ1
.
remained unchanged until 120 K and then gradually dis-
appeared between 120 and 160 K. If in a similar experiment the
salt was heated slowly, the IR spectrum of the thermolysis
products was much more complicated, and the 1888 cmϪ1 peak
was either absent or very weak in accordance with the Kofler
bench experiments.
The lead salt analogous to 1 was prepared in the same way as
the silver salt. However, this lead salt gave no NCS band upon
pyrolysis and was therefore not investigated further.
It was attempted without success to trap the methoxy iso-
thiocyanate from a preparative thermolysis of the silver salt by
treating the thermolysis products with an amine to give an
N-alkyl-NЈ-alkoxythiourea. Isolation of the neat thermolysis
products on a cold finger at 77 K gave rise to a yellow oil which
did not show any NCS band in the IR spectrum. GC-MS analy-
sis revealed that it consisted of three products with nearly iden-
tical retention times, all of which had the molecular ion at m/z
210, corresponding to a mixture of trithiolanes 3–5. A smaller
peak with m/z 242, corresponding to a tetrathiane (6 or 7) was
When the decomposition of the silver salt 1 was carried
out in an oven mounted in the source housing of a mass
spectrometer,9 the main product observed had m/z 210, corre-
sponding to the trithiolanes, of which at m/z 89 (methoxy
isothiocyanate) is a fragment ion. If the pyrolysis was carried
out directly on the heated probe in the ion source, signals due to
Sn (n = 1–8) appeared first, followed by intense signals at m/z 89
(base peak), 153, 185, 210, and 242. Here, m/z 210 and 242
correspond to the abovementioned trithiolanes and tetrathianes
3–7. The value m/z 185 corresponds to methoxyiminotetra-
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thiolane; a CAMS of this ion gave m/z 89 as the base peak. The
peak m/z 153 is the base peak in the mass spectrum of the
tetrathiane (m/z 242), and a CAMS of m/z 153 again gives
m/z 89 as the base peak. In the mass spectrum of the pyrolysis
product, the signal at m/z 89 remained the base peak at low
voltage (10–12 eV), thus suggesting that this may be due, in
part, to the thermally produced monomer, MeONCS, 2.
However, because the trithiolanes, tetrathiane, and other com-
pounds are also present, it cannot be ascertained that the m/z 89
signal is not, wholly or in part, due to the summation of frag-
ment ion intensities derived from the other products. The CA
spectrum of the m/z 89 ion derived from the pyrolysis mixture
was identical with the one investigated previously under dis-
sociative ionization conditions and ascribed to MeONCSϩ.7
Neutralization–reionization mass spectrometry of the m/z 89
peak showed an abundant recovery signal, thus verifying the
inherent stability of the neutral molecule.
ing to the literature, although it was difficult to obtain the
neutral compound from the primarily formed imidazolium
salt in the yield described.13 No such problem was encountered
in the preparation of the triazole derivative 12.
FVT of both the imidazolide 11 and the triazolide 12 at
400 ЊC with Ar matrix isolation of the products gave rise to the
peaks at 1888, 1076, and 754 cmϪ1 in the IR spectra, thereby
confirming the assignment of MeONCS (2). Signals due to
imidazole (triazole), HNCS (3508, 1982 cmϪ1), CH2O (2864,
2798 cmϪ1), and N-cyanoimidazole14 (2267 cmϪ1) were also
present.
FVT-MS experiments with pyrolysis in the ion source of the
mass spectrometer again afforded the m/z 89 peak. However,
m/z 89 is also formed as a fragment ion from both azolides
upon dissociative ionization.7 In order to probe whether m/z
89 was formed both thermally and dissociatively, the FVP-MS
experiment was carried out at different ionization potentials.
The triazolide showed m/z 89 as the base peak under all con-
ditions (room temperature to 250 ЊC; 10–70 eV). For the imid-
azolide, the intensity of the m/z 89 peak increased from ca. 30%
at room temperature (10–70 eV), to ca. 60% at 250 ЊC (10 eV),
thereby indicating that this peak is due, at least in part, to the
ionization of thermally produced MeONCS (2).
FVT of the tetrathiane–trithiolane mixture did not result in
peaks at 1888 cmϪ1 in the matrix IR spectra. FVT of N-alkoxy-
thioureas also did not produce any such bands but instead gave
rise to phenylcyanamide, PhNHCN.10
Pyrolysis of the silver salt of N-isopropoxydithiocarbamic acid
N-Isopropoxyamine was treated with carbon disulfide in the
same way as N-methoxyamine, and the reaction product was
converted into a silver salt, which was not, however, obtained in
an analytically pure state. When the impure salt was pyrolysed
by rapid heating as above, it gave rise to an IR matrix spectrum
Theoretical calculations
Standard DFT and ab initio calculations15 of the IR spectra
of MeONCS (2) and iPrONCS (9) are presented in Table 1.
The B3LYP/6-31G* structures of the two compounds are also
given in the Table. A complete listing of calculated frequencies
(including weak bands) and the cartesian coordinates of the
computed structures are available as supplementary data.† It is
seen that the bands observed experimentally (vide supra) are
in very good agreement with the strongest calculated bands,
thereby strongly supporting the experimental identifications.
with major absorptions at 1886, 1115, 1028, and 783 cmϪ1
,
again in good agreement with ab initio and DFT calculations
(Table 1). In the absence of Ar, this species was stable until 150
K and then disappeared in the course of further warming to
200 K. In analogy with the results for the methoxy compound,
mass spectrometry of the pyrolysis product showed masses
at m/z 266 and m/z 298 corresponding to trithiolanes 8 and
tetrathianes 9.
Conclusions
Methoxy isothiocyanate 2 is formed from three different pre-
cursors (1, 11, 12) on FVT and identified by its Ar matrix IR
spectrum in conjunction with ab initio and DFT calculations.
The corresponding isopropoxy isothiocyanate 9 is generated
from the silver salt 8. These alkoxy isothiocyanates are stable
until ca. 120 and 150 K, respectively, and oligomerise to
produce mixtures of bis(alkoxyimino)trithiolanes and -tetra-
thianes.
FVT of N-thiocarbamoyl azolides
It was of importance to find a second precursor for alkoxy
isothiocyanates. It has been shown that thiocarbonyl diimid-
azoles such as 10 react with primary amines to give isothio-
Experimental
General
The FTIR and argon matrix isolation apparatus employing a
quartz thermolysis tube (150 mm length and 8 mm internal
diameter) has been described earlier.18 BaF2 optics were used.
The FVP-MS equipment based on a six-sector tandem
mass spectrometer (Micromass AutoSpec 6F) fitted with a
quartz thermolysis tube (50 mm length, 3 mm inner diameter)
directly connected to the outer ion source was as previously
described.9,19,20 CA spectra were recorded by scanning the
field of the second electric sector and collecting the ions with the
first off-axis photomultiplier detector, the collision gas being
helium. Low ionization voltage mass spectra were recorded on
a Kratos MS25RFA instrument with direct insertion and
pyrolysis on the heatable solid insertion probe tip. Routine EI
cyanates.11 Thermolysis of analogous diimidazoles has been
used for the preparation of N,N-dialkylamino isothiocyanates12
but was not successful for the preparation of benzyloxy
isothiocyanate.11
Methoxyamine reacts readily with both thiocarbonyl diimid-
azole and thiocarbonyl-1,2,4-triazole to give the corresponding
diazoles. The methoxyimino imidazole 11 was prepared accord-
J. Chem. Soc., Perkin Trans. 2, 1999, 1869–1873
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mass spectra were recorded on a Varian MAT 311A spec-
trometer. NMR spectra were recorded on Varian Gemini 300
MHz and Bruker AC 250 MHz spectrometers. HPLC was per-
formed on a Waters Delta Prep 3000 with Waters 600E System
Controller and Waters 484 Tunable Absorbance Detector with
an RP18 column with Delta Pak C18, 300 Å, 15 µ.
C4H6N2O2S4 requires C, 19.82; H, 2.50; N, 11.56; S, 52.92%.
It was not possible by HPLC to obtain it absolutely pure. IR
(KBr neat) ν = 2976, 2936, 2895, 2816, 1529, 1460, 1068, 959,
872, 640 cmϪ1; IR (Ar matrix 10 K) ν = 2991, 2953, 2909, 2828,
1538, 1467, 1442, 1354, 1079, 969, 881 cmϪ1; NMR δH (300
13C
MHz; CDCl3; Me4Si) 4.04 (s, OMe); δ (250 MHz; CDCl3;
Me Si) 63.67 (OCH ), 154.11 (C᎐N–). MS(EI) m/z 242 (Mϩ,
᎐
4
3
88%), 153 (100), 122 (2), 95 (13), 89 (15), 76 (2), 64 (7), 46 (2).
Pyrolysis of the silver salts of N-alkoxydithiocarbamic acid
The silver salt was placed in the sublimation oven of the FVT
apparatus, and the temperature was adjusted to 140 ЊC. The
compound deflagrated. It may be necesesary to place a little
quartz wool between the sublimation oven and the pyrolysis
oven to prevent solid material being deposited on the cryostat
target.
1-(N-Methoxythiocarbamoyl)imidazole 11. 11 was prepared
according to a literature procedure13 from methoxyamine and
N,N-thiocarbonyldiimidazole.
1-(N-methoxythiocarbamoyl)-1,2,4-triazole 12. 12 was pre-
pared according to a literature procedure.22 1,1Ј-Thiocarbonyl-
di(1,2,4-triazole) (1.8 g, 0.01 mol) was dissolved in dry THF
freshly distilled (10 mL) and O-methoxyamine (0.94 g, 0.02
mol) was added under nitrogen during 90 minutes, the colour
changing from orange to yellow. The white compound which
had precipitated was isolated by centrifugation, washed with
dry THF (4 × 10 mL), dried in a desiccator over conc. sulfuric
acid. Yield: (0.5 g, 28%). Mp 126–128 ЊC. Mass spectrum:
m/z 158 (Mϩ, 23%), 128 (4), 89 (100), 59 (12), Mϩ 158.026777,
calc. for C4H6N4OS 158.026233; NMR δH (300 MHz; DMSO;
Me4Si) 3.68 (s, 3H, OCH3), 7.30 (s, 1H, NH), 7.89 (s, 1H,
Pyrolysis of sublimable compounds
The compounds were placed in the sublimation oven and sub-
limed into the pyrolysis oven. The pyrolysis products were
codeposited with ca. 200 mbar of Ar on a BaF2 window in
ca. 20 min.
Materials
Methoxyamine and isopropoxyamine were prepared according
to literature procedures.21
13C
triazole CH), 8.89 (s, 1H, triazole CH); NMR δ (250 MHz;
DMSO; Me4Si) 60.40 (NOCH3), 143.22 (triazole C), 150.75
(triazole C), 159.83 (CNOCH3).
Silver N-methoxydithiocarbamate 1. 1 was prepared by a
modification of the method used by Traube.1 O-Methoxyamine
(0.47 g, 0.01 mol) in ethanol (10 ml) was cooled to Ϫ10 to
Ϫ15 ЊC with exclusion of water, and carbon disulfide (0.76 g,
0.01 mol) in ethanol (10 ml) was added during 10 min. The
mixture was left with stirring at the same temperature for 1 h,
when silver nitrate 10% (6 ml) was added. A yellow compound
precipitated and was isolated by centrifugation, washed with
water (2 × 10 ml), and then with ethanol (10 ml) and dry ether
(10 ml). The compound was dried in a desiccator over conc.
sulfuric acid in the dark. Yield (0.64 g, 38%). Found: C, 7.47; H,
0.88; N, 4.30; ref. 1: C, 6.56; H, 1.0; N, 4.63. C2H3NOS2Ag2
requires C, 7.13; H, 0.90; N, 4.16%.
N-Alkoxythioureas. These were prepared from alkoxyamines
and isothiocyanates.16,17
Acknowledgements
This work was supported at The University of Queensland
by the Australian Research Council, in Denmark by Statens
Naturvidenskabelige Forskningsraad, in Belgium by the Fonds
National pour la Recherche Scientifique, and in Singapore by
the NUS.
Lead N-methoxydithiocarbamate. This was prepared in the
same way as the silver salt. Yield (0.70 g, 43%). Found: C, 7.35;
H, 0.80; N, 4.13. C2H3NOS2Pb requires C, 7.32; H, 0.92; N,
4.27%.
References
1 W. Traube, H. Ohlendorf and H. Zander, Chem. Ber., 1920, 53,
1477.
2 M. Bögemann, S. Petersen, O. E. Schultz and H. Söll, in Houben-
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Silver N-isopropoxydithiocarbamate 8. This was prepared
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Found: C, 11.33; H, 1.50; N, 4.20. C4H7NOS2Ag2 requires C,
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satisfactory analysis, the compound was used as such in the
pyrolyses.
6 J. H. Teles and G. Maier, Chem. Ber., 1989, 122, 745.
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3,6-Bis(N-methoxyimino)-1,2,4,5-tetrathiane. The method
was analogous to that used for the preparation of the corre-
sponding bis(phenylhydrazono)tetrathiane.8 To O-methoxy-
amine (1.43 g, 0.03 mol) in ethanol (96%; 70 ml) was added
carbon disulfide (6.86 g, 0.09 mol) during 10 min with cooling
in ice under argon. After stirring for a further 2 h at 0 ЊC
(pH = 5–6), hydrogen peroxide 35% (5.83 g, 0.06 mol) was
added during 10 min. The yellow–green solution was stirred for
a further 75 min, when chloroform (200 ml) was added. The
chloroform phase was washed with water (3 × 50 ml) and dried
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spaced peaks and a fourth larger peak corresponding to 3,5-
bis(N-methoxyimino)-1,2,4-trithiolane and 3,6-bis(N-methoxy-
imino)-1,2,4,5-tetrathiane, respectively. These could not be
separated by column chromatography. The tetrathiane was
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1872
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