Alkoxy isothiocyanates as intermediates in the flash vacuum pyrolysis of alkoxythioureas

Article abstract of DOI:10.1071/CH08406

Methoxy isothiocyanate MeONCS 2b was detected by matrix isolation IR spectroscopy following flash vacuum pyrolysis (FVP) of N-methoxythioureas, N-tert-butyl-N′-methoxythiourea 1d being the best precursor. Isothiocyanates 3, amines, and aldehydes are also

Full text of DOI:10.1071/CH08406

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 69–74
Alkoxy Isothiocyanates as Intermediates in the
Flash Vacuum Pyrolysis of Alkoxythioureas
Carl Th. Pedersen,A,D Frank Jensen,B and Robert FlammangC
ADepartment of Physics and Chemistry, University of Southern Denmark – Odense,
DK-5230 Odense M, Denmark.
BDepartment of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark.
CDepartment of Chemistry, University of Mons-Hainaut, B-7000 Mons, Belgium.
DCorresponding author. Email: cthp@ifk.sdu.dk
Methoxy isothiocyanate MeO–NCS 2b was detected by matrix isolation IR spectroscopy following flash vacuum pyrolysis
(FVP) of N-methoxythioureas, N-tert-butyl-Nꢀ-methoxythiourea 1d being the best precursor. Isothiocyanates 3, amines,
and aldehydes are also generated by FVP of several substituted N-alkoxythioureas 1 in the temperature range 400–800◦C.
The formation of these products can be explained either by secondary pyrolysis of initially formed alkoxy isothiocyanates
2, or by an initial cleavage of the O–N bond in 1 via a free-radical mechanism. N-Cyanoamines 4 and/or the tautomeric
carbodiimides 5 are formed by another pathway. The pyrolyses were monitored by IR spectroscopy and online mass
spectrometry or tandem mass spectrometry, and the reaction mechanisms are supported by theoretical calculations.
Manuscript received: 25 September 2008.
Final version: 21 November 2008.
Pathway 1
Introduction
R2O-NCS ꢀ R1NH2
2
NH-OR2
Alkoxy isothiocyanates, RO–N=C=S, constitute a rare and
elusive class of molecules having the characteristics of
reactive intermediates.[1,2] It is known that N-alkyl- and
N-arylthioureas can undergo thermal fragmentation into amines
and isothiocyanates.[3,4] Therefore, the title compounds 1
could be considered as possible sources of alkoxy isothio-
cyanates. Larsen et al. studied the fragmentation of 3-ethoxy-
1-phenylthiourea 1a on heating in the inlet system of a mass
spectrometer but did not obtain any definitive evidence for the
formation of ethoxy isothiocyanate 2a.[5] In previous work,
we have reported the detection of methoxy and isopropoxy
isothiocyanates. Here we report an investigation of various
monosubstituted alkoxythioureas 1 (Scheme 1) by flash vacuum
pyrolysis (FVP) coupled with matrix-isolation IR spectroscopy
and online mass spectrometry.
R1NCS ꢀ R2ONH2
Pathway 2
Pathway 3
S
NHR1
3
R1NꢁCꢁNH
1
R1NH-CN
5
4
a R1 ꢁ Ph, R2 ꢁ Et
b R1 ꢁ Me, R2 ꢁ Me
c R1 ꢁ Et, R2 ꢁ Me
d R1 ꢁ tBu, R2 ꢁ Me
e R1 ꢁ tBu, R2 ꢁ Et
Scheme 1. Thermolysis reactions of thioureas 1.
1-Phenyl-3-ethoxythiourea 1a
The FVP of 1a at 700◦C withAr matrix isolation of the products
at 10 K afforded isothiocyanic acid HNCS as a prominent pro-
duct, readily identified by strong absorptions at 1982 (NSC)
and 3508 (NH) cm−1 in the IR spectrum (Fig. 1).[8] Another
prominent peak at 2255 cm−1 was assigned to PhNHCN 4a.[9] It
was not possible to obtain a pure matrix spectrum of the thiourea
1a, because decomposition giving rise to PhNHCN takes place
below the sublimation temperature (Pathway 3; see above).
A small doublet corresponding to PhNCS was observed at
2045 and 2051 cm−1. Additional peaks at 2124, 1596, and
752 cm−1 were also assigned to this compound based on com-
parison with previously reported spectra.[10] Further peaks
at 1742, 1727, 1428, 1348, and 1111 cm−1 correspond to
acetaldehyde,[11] as ascertained by comparison with an authen-
ticAr matrix IR spectrum. Acetaldehyde can be formed together
with ammonia by secondary pyrolysis of C2H5ONH2, i.e. the
Results and Discussions
Three fragmentation pathways, 1–3, were considered for the
thermal reactions of the N-alkoxythioureas 1a–e (Scheme 1).
Pathway 1 involves fragmentation to an alkoxy isothiocyanate 2
andanamine. Pathway2generatesanaryloralkylisothiocyanate
3 and an alkoxyamine. Pathway 3 involves the formation of
an N-cyanoamine 4. Several of the N-alkoxythioureas decom-
pose to alkyl- or arylcyanamides and elemental sulfur already at
room temperature.[6] Therefore, in all likelihood, the cyanamides
observed in the present work are formed in the solid phase and
are not the results of FVP reactions. The mechanism of the
solid phase reaction is unknown. Cyanamides can tautomerize to
N-monosubstituted carbodiimides, R1N=C=NH 5,[7] and either
of these tautomers may be observed in the IR and mass spectra.
© CSIRO 2009
10.1071/CH08406
0004-9425/09/010069

70
C. T. Pedersen, F. Jensen, and R. Flammang
98.5
96
94
92
90
88
86
84
82
80
78
76
74
72
70
68
66
64
62.1
C
A
C
C
C
B
C
E
A
A, B
A
A
D
B
B, C
B
E
4000
3600
3200
2800
2400
2000
1800
cmꢂ1
1600
1400
1200
1000
800 700
Fig. 1. IR spectrum (10 K, Ar matrix) of the pyrolysis products of compound 1a at 700◦C. (A) Bands due to PhNCS: 2124, 2051, 2045, 1596,
752 cm−1. (B) Bands due to PhNH2: 1619, 1607, 1503, 1288, 752 cm−1. (C) Bands due to CH3CHO: 1742, 1727, 1348, 1111 cm−1. (D) Band
due to PhNHCN: 2255 cm−1. (E) Bands due to HNCS: 3508, 1982 cm−1
.
R
O
S
The mechanistic alternatives will be addressed below in the
computational section.
N
C
Pathway 4
R-CHO ꢀ HSCN
H
The formation of acetaldehyde (m/z 44), aniline (m/z 93),
ethanol (m/z 46), ammonia (m/z 17), HNCS (m/z 59),
phenyl isothiocyanate (m/z 135), and phenylcyanamide/
phenylcarbodiimide (m/z 118) was confirmed by online FVP-
MS and collisional activation mass spectrometry (CAMS)
experiments on the corresponding molecular ions.
R ꢁ H, CH3 or Ph
HNCS
Scheme 2. Pericyclic formation of aldehydes and HNCS.
amine by-product expected to be formed together with PhNCS
in Pathway 2 (Scheme 1). Authentic C2H5ONH2 was found to
pyrolyze at 500◦C to give a strong IR spectrum due to acetalde-
hyde. An increase of the temperature in the FVP of 1a caused
the phenyl isothiocyanate and N-cyanophenylamine signals to
increase; at 800◦C, their intensities were of the same order.
Direct evidence for Pathway 1 (Scheme 1), the possible for-
mation of ethoxy isothiocyanate EtONCS 2a, remained elusive
in this case. The calculated spectrum of 2a at the B3 LYP/
6–31G∗∗ level has the strongest absorption at 1989 cm−1. Scal-
1-Methyl-3-methoxythiourea 1b
The FVP of 1b proceeded in much the same way as described
for 1a (see Fig. S1, Accessory Publication). However, in this
case, we have the advantage that we know the Ar matrix IR
spectrum of methoxy isothiocyanate 2b as formed by FVP of
1-(N-methoxythiocarbamoyl)-1,2,4-triazole.[1]
Thanks to the lower sublimation temperature of 1b compared
with 1a, we were able to obtain a matrix IR spectrum of unde-
composed 1b. Very little pyrolysis took place at temperatures
below 400◦C, but it was nearly complete at 500◦C. Between 450
and 500◦C, a very weak peak was observed at 1888 cm−1, which
corresponds to the strongest peak in the spectrum of methoxy
isothiocyanate, MeONCS 2b.[1] It has the same shape as pre-
viously observed,[1] and it is absent at pyrolysis temperatures
higher than 500◦C, thus indicating the thermal decomposition to
HNCS and formaldehyde as described above. Yet, the weakness
of this band makes it desirable to find a better precursor.This was
achieved with 1d (see below). Signals due to methylamine[12]
(the potential by-product of Pathway 1) were also present in the
IR spectrum.
ing by a factor 0.96 gives an expected absorption at ∼1909 cm−1
.
No peak was observable in this region. This is likely to be due
to the high temperature required to pyrolyze 1a (>700◦C). The
by-product of Pathway 1, aniline, was detected by its IR spec-
trum (1619vs, 1607m, 503s, 1288m, and 752s cm−1). As shown
below, alkoxy isothiocyanates can be observed directly in the
pyrolyses of 1c and 1d, which take place at lower temperatures.
The aforementioned formation of acetaldehyde in the FVP
of 1a could also be due to a pericyclic reaction involving
ethoxy isothiocyanate 2a formed as an intermediate (Pathway 4,
Scheme 2).This process also explains the formation of HNCS, as
the primarily formed thiocyanic acid (HSCN) would tautomerize
to the more stable isothiocyanic acid (Scheme 2).
Similarly to the case of 1a, HNCS, methyl isothiocyanate 3b,
and formaldehyde were formed as evidenced by the correspond-
ing strong signals in the IR and mass spectra. Authentic methyl
The formation of acetaldehyde, aniline, and isothiocyanic
acid could, however, also be a consequence of an ini-
tial homolytic cleavage of the O–N bond in the thiourea.
isothiocyanate3bhasstrongabsorptionsat2125and2237 cm−1
.
The better spectral resolution achieved in a pulsed pyrolysis

Alkoxy Isothiocyanates as Intermediates in the Flash Vacuum Pyrolysis of Alkoxythioureas
71
29.2
28
26
24
22
A
20
18
16
14
12
10
8
D
B
A
C
B, D
C
B
6
D
4
2
B
B
0.1
4000
3600
3200
2800
2400
2000
1800
cmꢂ1
1600
1400
1200
1000
800 700
Fig. 2. IR spectrum (10 K, Ar matrix) of the pyrolysis products of compound 1c at 400◦C, showing (A) bands due to MeONCS 2b at 1887 and
1074 cm−1; (B) bands due to EtNCS at 2237, 2157, 2122, 1347, and 939 cm−1; (C) bands due to HNCS at 3508 and 1982 cm−1; (D) bands due
to MeCHO at 1726, 1430, 1347, and 970 cm−1
.
experiment[13] revealed the presence of a further compound
absorbing at 2232 cm−1 – the typical region for cyanoamines.
The calculated IR spectra of methylcyanoamine CH3NHCN 4b
and the isomeric methylcarbodiimide CH3N=C=NH 5b fea-
ture strong absorption at 2258 and 2173 cm−1, respectively
(B3 LYP/6–31G∗∗; wavenumbers scaled by a factor 0.9613).
The 2232 cm−1 band is ascribed to cyanoamine 4b. A corre-
sponding, prominent peak was observed at m/z 56 in the mass
spectrum of the pyrolysis products.
these bands were not observed. Instead, a series of significant
bands due to 2-methyl-2-propene are noted. It has previously
been observed that N-tert-butyl groups can be eliminated
on FVP.[10] Here, the tert-butyl group may be eliminated as
2-methyl-2-propene from 1d and 1e to form the thiourea 6
(Scheme 3). A strong band at 2129 cm−1 is assigned to tert-
butylcarbodiimide 5d, which was also observed in the FVP-MS
at m/z 98 (Pathway 3).
Bands due to formaldehyde (from 1d) or acetaldehyde (from
1e)werealsoobserved.ThisisinlinewithanintenseHNCSband
(1982 cm−1) and could be due to either Pathway 1 or Pathway 2
(Scheme 3), but Pathway 2 is unlikely in this case because of the
absence of tert-butyl isothiocyanate 3d. Control experiments
showed that tert-butyl isothiocyanate does not decompose to
2-methyl-2-propene at 700◦C. Thus, alkoxy isothiocyanates 2
could be formed by elimination of 2-methyl-2-propene followed
by a Pathway 5 reaction of compound 6 (Scheme 3). Further
fragmentation by the pericyclic Pathway 4 affords acetaldehyde
and HNCS. In summary, methoxy isothiocyanate 2b may be
formed by either Pathway 1 or Pathway 5 (Scheme 3).
1-Ethyl-3-methoxythiourea 1c
The FVP reactions of this compound were similar to those
described for 1b, except that weak bands ascribed to MeONCS
were now clearly observed at 1887 and 1074 cm−1 following
FVP at 400◦C (Fig. 2). Bands due to ethyl isothiocyanate, HNCS
and acetaldehyde were also present.
1-tert-Butyl-3-methoxythiourea 1d and
1-tert-Butyl-3-ethoxythiourea 1e
The formation of methoxy isothiocyanate 2b was confirmed
by FVP-MS. The real-time analysis of the CAMS of the m/z
89 ions (already present without pyrolysis) indeed reveals the
appearance near 450◦C of new peaks at m/z 74, 59, 32, 30,
29 during the pyrolysis of 1d (see Fig. 4). These telltale peaks
were identical with those observed in the CAMS of the m/z 89
ions originating from the FVP of 1-(N-methoxythiocarbamoyl)-
1,2,4-triazole.[2]
The FVP of thiourea 1d finally afforded more substantial
amounts of the elusive methoxy isothiocyanate 2b. Thus, FVP
of 1d at 400◦C gave rise to a significant IR band at 1887 cm−1
,
together with the accompanying bands at 1073 and 755 cm−1, in
excellent agreement with the previously reported spectrum[1] of
methoxy isothiocyanate, MeONCS 2b (Fig. 3). tert-Butylamine,
the by-product of Pathway 1, was also observed in the IR spec-
trum (2966, 1366, 1223, 818 cm−1; Fig. 3). In contrast, FVP of
1e did not afford any band in the 1900 cm−1 region where cal-
culations show EtONCS to have its strongest band. Otherwise,
the reactions of 1d and 1e were analogous.
Theoretical Calculations
One could expect the formation of tert-butyl isothiocyanate
3d as one of the reaction products of the pyrolysis of 1d and
1e via Pathway 2 (Scheme 1). tert-Butyl isothiocyanate has
three significant absorptions at 2068, 2076, and 2084 cm−1, but
Activation energies (ꢀE=) and reaction energies (ꢀEo) for
selected key processes in the FVP of 1b were calculated
at the B3 LYP/6–31G∗∗ and MP2/cc-pVDZ levels of theory
and are indicated in Scheme 4, where values in parentheses

72
C. T. Pedersen, F. Jensen, and R. Flammang
20
19
18
17
16
15
14
13
12
11
10
9
D
A
D
C
*
8
A
7
6
5
C
*
*
4
*
3
B*
*
2
B
*
B*
B
E
1
*
*
*
*
*
0
*
4000
3600
3200
2800
2400
2000
1800
cmꢂ1
1600
1400
1200
1000
800 700
Fig. 3. IR spectrum (10 K, Ar matrix) of the pyrolysis products of compound 1d at 400◦C. (A) Bands due to MeONCS 2b: 1887, 1073,
and 755 cm−1. (B) Bands due to tBuNH2: 2966, 1366, 1223, and 818 cm−1. (C) Band due to HNCS: 1982 cm−1. (D) Bands due to HCHO:
2797 and 1741 cm−1. (ꢁ) Bands due to unpyrolyzed starting material 1d. (E) Band ascribed to tBuNCNH: 2129 cm−1
.
Pathway 1
RCH2ONCS ꢀ (CH3)3CNH2
2b R ꢁ H
2a R ꢁ CH3
NH-OCH2R
NHOCH2R
Pathway 5
S
RCH2ONCS
H2CꢁC(CH3)2 ꢀ S
NHC(CH3)3
NH2
2b R ꢁ H
2a R ꢁ CH
1d R ꢁ H
1e R ꢁ CH3
3
6
Pathway 4
Pathway 2
RCHO ꢀ HNCS
(CH3)3CNCS ꢀ H2NOCH2R
3d
H2CꢁCH(CH3)2
RCHO
ꢀ
ꢀ
NH3
HNCS
Scheme 3. Thermolysis of the N-tert-butyl-Nꢀ-alkoxy thioureas.
correspond to the MP2 results. The fragmentations to MeONCS
2b + MeNH2 (TS1) and to MeNCS 3b + H2NOMe (TS2)
have calculated activation energies of 163 (146) and 171
(156) kJ mol−1, respectively. These estimates of activation bar-
riers suggest that both these processes are readily accessible
under FVP conditions, and the formation of the methoxy iso-
thiocyanate 2b is clearly a favourable process. Fragmentation
reactions leading to formaldehyde, HNCS and MeNCS 3b via
thiourea derivatives 7 and 8 have calculated barriers on the order
of134–191 kJ mol−1;therefore, allthesereactionscantakeplace
at or near the temperature where MeONCS 2b is formed.
The pericyclic decomposition of MeONCS 2b to HSCN and
formaldehyde (Pathway 4) is a favourable process with an acti-
vation barrier of 134 (111) kJ mol−1 (TS8, Scheme 4). Similar
values were calculated for the decomposition of 2a to HSCN
and acetaldehyde (131 (105) kJ mol−1). As expected, the direct
formation of HNCS from 2b has an ∼100 kJ mol−1 higher
calculated barrier (233 kJ mol−1) (TS9, Scheme 4).
The cleavage of the O–N bond in 1b to form the methoxy
radical and thiourea radical 9 (Scheme 5) is predicted to have
a barrier of 196 kJ mol−1 at the B3 LYP/6–31G∗∗ level, and
199 kJ mol−1 at the CCSD(T)/cc-pVDZ level, in excellent agree-
ment with experimental bond dissociation energies of O–N
bonds of ∼200 kJ mol
,
−1 [14] whereas the MP2 method overesti-
mates this bond energy (234 kJ mol−1; Scheme 5). Thus, all the
calculations predict that the free-radical pathway (Scheme 5)
has a significantly higher activation barrier than the molecular
processes in Scheme 4. Nevertheless, some participation of free

Alkoxy Isothiocyanates as Intermediates in the Flash Vacuum Pyrolysis of Alkoxythioureas
73
NH-OMe
55 (/2)
59
NH
NHMe
S
ꢀ
OCH3
S
∆Eo 196 (234)A
NHMe
1b
9
ꢃ
ꢃ
∆E 109 (128)
∆Eo 89 (94)
∆E 139 (135)
HNCS
H2CꢁO
ꢀ H
ꢀ CH3NH
30
Scheme 5. Calculated activation barriers ꢀE= and energies of reaction
29
74
ꢀEo at the B3 LYP/6–31G∗∗ (MP2/cc-pVDZ) levels of theory in kJ mol−1
AꢀEo = 199 kJ mol−1 is obtained at the CCSD(T)/cc-pVDZ level.
.
32
*
*
Similar activation energies have been calculated for the
reactions of thiourea 1a analogous to Schemes 4 and 5. However,
a very notable difference is that the TS1 reaction (ꢀE= = 178
(161) kJ mol−1) leading to EtONCS 2a is now less favourable
than the TS2 reaction (ꢀE= = 167 (153) kJ mol−1) leading to
PhNCS 3a, in agreement with the fact that it is experimentally
very difficult to observe 2a. Changing the R1 group from methyl
to phenyl thus alters the calculated ∼10 kJ mol−1 preference
for reaction via TS1 to a ∼10 kJ mol−1 preference for reaction
via TS2.
m/z
Fig. 4. Collisional activation mass spectrum (CAMS) of m/z 89 ions
(MeONCS 2b) generated after flash vacuum pyrolysis of 1d at a temperature
of ∼450◦C; ꢁ refers to artefact peaks.
Pathway 1
MeONCS 2b ꢀ MeNH2
TS1
NH-OMe
NHMe
Free radical reactions involving initial dissociation of either
of the N–H bonds in 1a and 2a have very high calculated barriers
ꢃ
S
∆E 163 (146)
∆Eo 110 (115)
(NH bond dissociation energies 344–387 (358–493) kJ mol−1
and are therefore not likely to be involved.
)
Pathway 2
1b
MeNCS 3b ꢀ MeONH2
TS2
Conclusions
ꢃ
∆E 171 (156)
Methoxy isothiocyanate MeONCS 2b has been identified as
a product of FVP of the corresponding N-alkoxythiourea 1d
by means of its Ar matrix IR spectrum. Weaker and therefore
less securely identified signals due to MeONCS 2b were also
observed in the FVP of 1b and 1c. Theoretical calculations sup-
port the formation of 2b from thiourea 1b with an activation
barrier of 140–160 kJ mol−1. Ethoxy isothiocyanate 2a is pos-
sibly formed in the FVP reactions of thioureas 1a and 1e but
was not directly observable, either because it was formed in too
low concentration, or because it decomposed under the high-
temperature FVP conditions. The formation of aldehydes and
isothiocyanic acid, HNCS, as secondary fragmentation products
may take place via pericyclic reactions of the alkoxy isothio-
cyanates or by free radical reactions involving primary cleavage
of the O–N bond of the alkoxythioureas.
∆Eo 56 (55)
NH
O
TS3
ꢀ H2CꢁO
HS
H
NH
ꢃ
S
∆E 148 (134)
NHMe
∆Eo ꢂ40 (ꢂ68)
NHMe
7
1b
TS5
TS4
ꢃ
ꢃ
∆E 92 (95)
∆E 143 (139)
∆Eo 26 (32)
∆Eo ꢂ75 (ꢂ59)
TS6
HNCS
NH2
ꢀ MeHN2
ꢃ
S
∆E 191 (172)
∆Eo 101 (92)
NHMe
8
ꢃ
∆E 187 (169)
∆Eo 76 (63)
TS7
MeNCS 3b
ꢀ NH3
Computational Methods
TS8
Pathway 4
O
S
Calculated IR spectra were obtained at the B3 LYP level of
theory[15] with the 6–31G∗∗ basis set.[16] Energies of ground and
transition structures were obtained from fully optimized
geometries at the B3 LYP/6–31G∗∗ and MP2/cc-pVDZ levels of
theory. All calculations were performed using the Gaussian 03
program package.[17] Transition structures were located using
the default optimization algorithm in the Gaussian 03 program,
and verified by frequency calculations and tracing the intrinsic
reaction coordinate in cases where the normal mode associated
with the imaginary frequency did not clearly connect the two
minima.
N
C
CH2O ꢀ HSCN
CH2O ꢀ HNCS
ꢃ
∆E 134 (111)
H
∆Eo ꢂ65 (ꢂ123)
S
C
TS9
O
ꢃ
N
∆E 233
H
∆Eo ꢂ124 (ꢂ151)
Scheme 4. Calculated activation barriers ꢀE= and energies of reaction
ꢀEo at the B3 LYP/6–31G∗∗ (MP2/cc-pVDZ) levels of theory in kJ mol−1
.
radical processes cannot be excluded. If Scheme 5 were fol-
lowed, then formaldehyde could be formed from the methoxy
radical CH3O• with a barrier of 139 (135) kJ mol−1, and HNCS
and methylamine could be formed from the thiourea radical 9
with an activation barrier of ∼109 (128) kJ mol−1 (Scheme 5).
Experimental
General Methods
The Fourier-transform (FT)-IR and argon matrix isolation appa-
ratus employing a quartz thermolysis tube (150 mm length and

74
C. T. Pedersen, F. Jensen, and R. Flammang
8 mm internal diameter) has been described earlier.[18] BaF2
optics were used. For some of the pyrolyses, e.g. 1d and 1e, a
modified oven was used, in which the hot tube extends to within a
fewmmfromthecoldBaF2 disk.TheFVP-MSequipment, based
on a six-sector tandem mass spectrometer (MicromassAutoSpec
6F) fitted with a quartz thermolysis tube (50 mm length, 3 mm
inner diameter) directly connected to the outer ion source was
previously described.[19]
[4] R. H. Shapiro, J. W. Serum, A. M. Duffield, J. Org. Chem. 1968, 33,
243. doi:10.1021/JO01265A048
[5] C. Larsen, U. Anthoni, C. Christophersen, P. H. Nielsen, Acta
Chem. Scand. 1969, 23, 324. doi:10.3891/ACTA.CHEM.SCAND.
23-0324
[6] Attempts to prepare 1-methyl-3-phenoxy thiourea from methyl isoth-
iocyanate and phenoxyamine result in quantitative formation of ele-
mentary sulfur.The reaction of CS2 with methoxyamine also produces
sulfur. Already Schiff noted in 1876 that the reaction between hydrox-
ylamine and isothiocyanate yielded sulfur and cyanamides: R. Schiff,
Ber. Dtsch. Chem. Ges. 1876, 9, 574. The formation of cyanamides
via Pathway 3 formally corresponds to the loss of S + EtOH. The for-
mation of EtOH is indicated in the presence of a band at 3655 cm−1 in
the IR spectra of the pyrolysis products from compounds 2a and 2e.
The OH frequency is reported for ETOH at 3656 cm−1: W. A. P. Luck,
O. Schrems, J. Mol. Spectr. 1980, 60, 333. This is further supported by
the presence of m/z 46 in the mass spectrum of 2a.−A1 corresponding
IR band is observed in 2b, 2e, and 2d at 3666 cm corresponding
to the OH frequency in MeOH reported at 3667 cm−1: A. Serrallach,
R. Meyer, H. H. Günthard, J. Mol. Spectrosc. 1974, 52, 94. Bands cor-
responding to the O–C vibration are also observed; however, owing
to the presence of unpyrolyzed starting materials, these bands cannot
be unambiguously assigned to the alcohols.
The pulsed pyrolysis apparatus[13] employed a solenoid valve
(General Valve Corp. Series 9) operating at a frequency of four
pulses per min and a pulse duration of 10 s. The sample was
evaporated at 100◦C in a stream ofAr passed through the sample
chamber at a stagnation pressure of 100 Pa. The sample chamber
was attached directly to the valve, and both were held at the
same temperature. The exit aperture of the valve led into an
electrically heated quartz capillary (60 mm long, inner diameter
0.9 mm). The pyrolysis temperature in the oven was measured
by optical pyrometry through a quartz window. Temperatures
between 1000◦ and 1200◦C were used.
Materials
[7] W. Jabs, M. Winnewisser, S. P. Belov, F. Lewen, F. Maiwald,
G. Winnewisser, Mol. Phys. 1999, 97, 213. doi:10.1080/
002689799164072
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Matrix IR Spectra of Authentic Samples
Ar matrix, 10 K, ν/cm−1: Benzaldehyde 3063(w), 2825(m),
2743(m), 1716(v), 1654(m), 1597(s), 1584(s), 1455(m),
1392(m), 1311(m), 1205(s), 1166(m), 1073(w), 828(s),
748(s).
Aniline 3498(w), 3411(w), 3048(w), 1702(vw), 1619(v),
1503(m), 1281(m), 1176(w), 1090(w), 875(w), 755(s).
Formaldehyde 2863(w), 2797(m), 1741(s), 1498(w), 1245(vw).
Methyl Isothiocyanate 2237(m), 2156(m, sh), 2125(v), 1592(w),
1527(m), 1452(vw), 1420(m), 1105(vw).
Ethyl Isothiocyanate 2228(v), 2203(v), 2155(v), 2132(v),
1453(m), 1347(s), 1269(m), 1064(m), 939(m).
Acetaldehyde 2750(w), 2737(w), 1749(m), 1726(s), 1431(m),
1348(m), 872(w).
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Accessory Publication
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Electronic supplementary information showing computational
data (Cartesian coordinates, absolute energies, and IR spectra)
for calculated species is available from the journal’s website.
Acknowledgements
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The authors thank Professor Curt Wentrup, the University of Queensland,
Australia, for valuable discussions and suggestions. The support of the
Danish National Science Research Council and the Danish Centre for Sci-
entific Computing is acknowledged. The Mons Laboratory thanks the Fond
National de la Research Scientifique for its contribution towards purchase
of the Micromass AutoSpec 6F mass spectrometer.
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