Thioketenes and iminopropadienethiones RN=C=C=C=S from isoxazolones

Article abstract of DOI:10.1071/CH10340

Isoxazolones 6 undergo thermal elimination of propene and isopropylthiol to produce thioketenes 7 at 500-600°C under flash vacuum thermolysis conditions. At 700-900°C further fragmentation occurs to produce iminopropadienethiones, RNCCCS 8. In addition, 3-alkylisoxazolones 6-de rearrange to cyanothioketenes 10-de. Compounds 7, 8, and 10 were characterized by Ar matrix IR spectroscopy and comparison with density functional theory-calculated spectra. Thioketenes 7 reacted with amines to afford thioamides 11. Reaction of aryliminopropadienethiones 8 with amines caused cyclization to 2-aminoquinoline-4(1H)-thiones 16. CSIRO 2010.

Full text of DOI:10.1071/CH10340

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 1694–1702
www.publish.csiro.au/journals/ajc
Thioketenes and Iminopropadienethiones RN5C5C5C5S
from Isoxazolones
A
A B
,
David Kvaskoff and Curt Wentrup
A
School of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, Qld 4072, Australia.
B
Corresponding author. Email: wentrup@uq.edu.au
Isoxazolones 6 undergo thermal elimination of propene and isopropylthiol to produce thioketenes 7 at 500–6008C under
flash vacuum thermolysis conditions. At 700–9008C further fragmentation occurs to produce iminopropadienethiones,
RNCCCS 8. In addition, 3-alkylisoxazolones 6d–e rearrange to cyanothioketenes 10d–e. Compounds 7, 8, and 10 were
characterized by Ar matrix IR spectroscopy and comparison with density functional theory-calculated spectra.
Thioketenes 7 reacted with amines to afford thioamides 11. Reaction of aryliminopropadienethiones 8 with amines
caused cyclization to 2-aminoquinoline-4(1H)-thiones 16.
Manuscript received: 14 September 2010.
Manuscript accepted: 5 October 2010.
Introduction
reactive molecules under the conditions of FVT, including
acetylenes, ynamines, isocyanides, fulminates, C2S2, and the
bis-iminopropadienes (Eqn 1).[6,7]
Iminopropadienones, RN¼C¼C¼C¼O 1, are highly reactive,
theoretically interesting, and in some cases isolable com-
pounds.[1] They can be generated from a variety of precursors,
including isoxazolopyrimidinones 2, Meldrum’s acid deriva-
tives 3, and pyridopyrimidinones 4 and 5 by means of flash
vacuum thermolysis (FVT)[1] or microwave-assisted thermo-
lysis (Scheme 1).[2] Compounds 1 have been employed in
syntheses of a variety of heterocyclic compounds, including
diazepines, diazocines, mesoionic pyridopyrimidinones, and
quinolones.[1–3]
The corresponding iminopropadienethiones, RN¼C¼C¼
C¼S 8, are relatively little-known compounds.[4,5] The
types of precursors used to prepare the iminopropadienones
RNCCCO (Scheme 1) are either unknown or, in the case of 2b,
give only modest yields of 8.[4] 4-Substituted isoxazol-5(4H)-
ones have proved to be excellent precursors for a variety of
SRꢀ
N-R
Ar
Ar
C
NH-R
Ar
ꢁCO2
ෂR
FVT
N
C
C
C
N
R
N
N
O
ꢁRꢀSH
O
O
O
ð1Þ
The formation of the phenyliminopropadienethiones 8a and
8b by FVT of isoxazolone 6a–b was described previously.[4] We
have now discovered that the reaction takes place in two stages,
with initial formation of the thioketenes 7 (Scheme 2), which can
be characterized at low temperature and trapped chemically.
The results are reported herein.
Results and Discussion
The FVT of 6a was carried out over the temperature range
300–10008C and monitored by IR spectroscopy. The products
were isolated in Ar matrices at ,10 K and investigated by IR
O
X
R
R
NH
N
O
O
NH
N
O
N
O
3
2a–X ꢂ O
b–X ꢂ S
SiPr
S
SiPr
R
R
C
RNꢂCꢂCꢂCꢂO
R
ꢁCO2
ෂR
ꢁC3H6
1
N
C C C S
N
N
O
ꢁiPrSH
O
O
O
6
7
8
Me
N
N
N
N
a R ꢂ Ph
b R ꢂ p-MeO-C6H4
c R ꢂ p-NC-C6H4
d R ꢂ isopropyl
e R ꢂ neopentyl
O
NH
O
N
R
H
R
H
4
5
Scheme 1. Routes to iminopropadienones 1.
Scheme 2. Stepwise frgmentation of 4-methylenisoxazolones.
Ó CSIRO 2010
10.1071/CH10340
0004-9425/10/121694

RESEARCH FRONT
Thioketenes and Iminopropadienethiones
1695
S
S
C
Ph
C
p-MeO(C6H4)
N
N
O
O
O
O
(a)
(a)
CS2
CS2
CO2
p
CO2
p
*
CO2
*
c
*
n
p
p
p
n
p
p
n
c
CO2
p
*
p
*
p
p
p
n
p
*
(b)
n
*
(b)
*
*
2500 2250 2000 1750 1500 1250 1000 750
ν [cmꢁ1
500
3100 2800 2500 2200 1900 1600 1300 1000 700
ν [cmꢁ1
]
]
Fig. 1. (a) Calculated IR spectrum of thioketene 7a (B3LYP/6–31G**).
(b) Experimental IR spectrum of the product of FVT of 6a at 6008C in Ar,
10 K. Bands [cmꢀ1] due to: CO2 (2344, 663); c, OCS (2050); p, propene
(1650, 1454, 1439, 997, 909, 579); CS2 (1528); thioketene 7a (1810, 1727,
1375, 1020, 879); *, N-phenyliminopropadienethione 8a (2177, 2167, 1593,
1493, 1275, 753 cmꢀ1). Ordinate in arbitrary absorbance units.
Fig. 2. Comparison of (a) calculated IR spectrum (B3LYP/6–31G**) for
thioketene 7b and (b) experimental IR spectrum of the product of FVT of 6b
at 6008C, in Ar at 10 K. Bands [cmꢀ1] due to: p, propene (3090, 2983, 2942,
2923, 2859, 1650, 1454, 1044, 997, 909, 578); c, OCS (2050); CS2 (1528);
CO2 (2340, 663); thioketene 7b (2891, 1819, 1729, 1593, 1469, 1374, 1261,
1115, 1022, 878, 834, 590); n, 4-methoxybenzonitrile (2238, 1612, 1305,
1177); *, N-(4-methoxyphenyl)iminopropadienethione 8b (2164, 1508,
1254, 835). Ordinate in arbitrary absorbance units.
spectroscopy. Below 5008C, most of the starting material was
recovered unchanged. FVT at 500–6008C led to the formation
of a thioketene intermediate, to which structure 7a is assigned
(Scheme 2 and Eqn 2). This compound is characterized by
decomposition due to chemical activation is more likely to occur
in FVT-MS experiments.
its symmetric (1727 cmꢀ1) and anti-symmetric (1810 cmꢀ1
)
SiPr
SiPr
S
R
C
R
S
R
N
S
stretches in the IR spectrum, in excellent agreement with the
calculated bands at 1733 and 1821 cmꢀ1 (Fig. 1). Peaks due to
propene (p) are clearly visible in the spectrum. Isopropylthiol
was isolated in the form of its air oxidation product, diisopropyl
disulfide, after workup of the products of preparative FVT
experiments, but it decomposes in part to propene and hydrogen
sulfide under the reaction conditions. Alternatively, 6 may
eliminate two molecules of propene followed by hydrogen
sulfide. A peak at 2050 cmꢀ1 also seen in several other spectra
is most likely due to OCS[8] rather than the CN radical, which
appears in the same place.[4] Furthermore, evidence for the
structure of 7 was obtained by trapping with diethylamine
in preparative FVT experiments described below. A likely
mechanism for formation of 7 is presented in Eqn 2. Iso-
propylthiol, propene, and H2S were also detected by on-line
mass spectrometry, but curiously, the thioketene 7a was not
detectable in FVT-MS studies.[4] A discord between FVT-IR
and FVT-MS studies is unusual but not unprecedented. Possible
reasons could be: (i) different ionization cross-sections of the
various pyrolysis products; (ii) facile fragmentation of the
product under electron ionisation at 70 eV; and (iii) thermal
instability of the product. It is important to note that FVT-MS
experiments are carried out in the high vacuum of the mass
spectrometer, whereas FVT-IR is carried out in a stream of Ar,
which can act as a collisional deactivator. In other words,
ꢁiPrSH
ꢁC3H6
ð2Þ
N
N
O
OH
O H
O
O
O
7
6
The intensities of the IR bands ascribed to thioketene 7a
diminished at higher temperatures. At 700–9008C, the IR
spectra were dominated by the very strong cumulenic absorption
at 2167 cmꢀ1 due to Ph-N¼C¼C¼C¼S 8a.[4] Several weaker
peaks due to 8a, which were barely detectable in the 6008C
spectrum (Fig. 1), increased at the same time (1593, 1493, 1355,
1275, 753, and 546 cmꢀ1). The iminopropadienethione 8a was
identified as the carrier of these signals due to the excellent
agreement with the B3LYP/6–31G** calculated IR spectrum
(Fig. S1, Accessory Publication).[4]
The thermolyses of the p-methoxyphenyl derivative 6b were
analogous to those described for 6a above. The thioketene 7b
was obtained at 6008C (Fig. 2). Small amounts of N-(4-methoxy-
phenyl)iminopropadienethione 8b was also formed under these
conditions (Fig. 2), and this compound became the main product
at 700–9008C.[4]
Similar FVT of the p-cyanophenyl derivative 6c at
500–6008C gave rise to thioketene 7c with characteristic bands
at 1733 and 1817 cmꢀ1 (Fig. 3). The calculated frequencies of
7c are 1733 cmꢀ1 (symmetric) and 1824cmꢀ1 (anti-symmetric)

RESEARCH FRONT
1696
D. Kvaskoff and C. Wentrup
S
S
C
p-CN(C6H4)
iPr
C
N
O
O
N
O
O
(a)
CS2
CO2
s
s
CO2
p
CS2
*
s
s
s
s
s
p
*
c
CO2
p
CO2
s
p
p
s
p
p
*
s
*
*
s
(b)
*
c
*
3100 2800 2500 2200 1900 1600 1300 1000 700
ν [cmꢁ1
2500 2250 2000 1750 1500 1250 1000 750
ν [cmꢁ1
500
]
]
Fig. 3. Comparison of (a) calculated IR spectrum (B3LYP/6–31G**) of
thioketene 7c and (b) experimental IR spectrum of the product of FVT of 6c
at 5708C in Ar, 10 K. Bands [cmꢀ1] due to: p, propene (3091, 2984, 2941,
1651, 1453, 997, 909, 578); c, OCS (2050); CS2 (1528); CO2 (2344, 663);
thioketene 7c (2240, 1817, 1733, 1374, 1120, 1022, 894, 847); *, N-(4-
cyanophenyl)iminopropadienethione 8c (2245, 2165, 1362, 1275, 842).
Ordinate in arbitrary absorbance units.
Fig. 5. Top: Calculated IR spectum of 7d. Bottom: IR spectrum of 7d
resulting from FVT of 6d at 5008C in Ar, 10 K. Bands [cmꢀ1] due to: CO2
(2344, 663); CO (2138); s, starting material (1746, 1507, 1465, 1370, 1236,
1171, 1145, 1104, 1055, 916, 903, 861, 780, 742, 595); c, OCS (2050);
CS2 (1528); *, N-(isopropyl)iminopropadienethione 8d (2171).
iPrS
S
SiPr
C
C
R
FVT
R
NC
ꢁC3H6
ꢁiPrSH
N
N
O
O
O
O
6
7
N C C C S
(b)
C
R
R
(a)
C
S
N
C C S
(a)
(a)
N
8
9
*
(b)
a R ꢂ Ph
R
b R ꢂ p-MeO-C6H4
c R ꢂ p-NC-C6H4
d R ꢂ isopropyl
e R ꢂ neopentyl
C
S
NC
10
CO2
Scheme 3. Thioketenes and iminopropadienethiones from isoxazolones.
CS2
*
p
*
CO2
c
p
p
*
*
p
p
(B3LYP/6–31G**). Small amounts of imiopropadienethione 8c
were also formed under these conditions (Fig. 3).
p
p
s
*
(b)
*
A clean IR spectrum of 8c was obtained by FVT of 6c at
7008C (Fig. 4). It features a distinct and prominent band at
2167 cmꢀ1 together with bands at 2245 and 1362 cmꢀ1. The
corresponding calculated frequencies of 8c are 2193, 2256, and
1357 cmꢀ1. This group of experimental bands was bleached
simultaneously upon irradiation with the unfiltered UV light
from a high-pressure Xe/Hg lamp while removing the infrared
radiation with a water filter. A similar IR spectrum was obtained
3100 2800 2500 2200 1900 1600 1300 1000 700
ν [cmꢁ1
]
Fig. 4. IR spectrum resulting of the product of FVT of 6c at 7008C in Ar,
10 K. Bands [cmꢀ1] due to: CO2 (2344, 663); p, propene (3091, 2975–2923,
1650, 1453, 997, 909, 578); c, OCS (2050); CS2 (1528); s, possibly cyano(4-
cyanophenyl)thioketene 10c (1755); *, N-(4-cyanophenyl)iminopropadiene-
thione 8c (2245, 2167, 1982, 1608, 1506, 1362, 1275, 1254, 839).

RESEARCH FRONT
Thioketenes and Iminopropadienethiones
1697
(a)
iPr
.
.
.
N
S
(b)
iPr
NC
.
S
CS2
~
CO2
~
1.0
0.8
0.6
s
CO
K
p
p
p
0.4
0.2
0.0
CO2
s
p
p
p
p
X
*
p
H2S
p
X
*
a
(c)
3100
2600
2100
1600
1100
600
ν [cmꢁ1
]
Fig. 6. (a) Calculated IR spectrum of isopropyliminopropadienone 8d; (b) calculated IR spectrum of isopropyl(cyano)
thioketene 10d (B3LYP/6–31G**; wavenumbers scaled by a factor 0.9613); the intensities of the calculated iPr-NCCCS
8d are attenuated by a factor of 8 with respect to the thioketene 10d. (c) IR spectrum of product of FVT of 6d at 7008C
isolated in Ar, 10 K. Bands [cmꢀ1] due to: p, propene (3081, 2977, 2941, 2921, 1645, 1453, 1375, 996, 913, 585); s, cyano
(isopropyl)thioketene 10d (2222, 1752); H2S (2569); X, unassigned species (2273, 1722); K (2041) probably due to
N-isopropylketenimine; H2S (2561); CS2 (1527); CO2 (2339); CO (2138); *, N-isopropyliminopropadienethione 8d
(2175/2160, 1957).
in the temperature range 700–9008C. Significant decomposition
occurred above 9008C, leading mainly to terephthalonitrile
(2244 cmꢀ1).
FVT of 6d at 400–6008C produced an intermediate thio-
ketene 7d with characteristic absorptions at 1732 and 1811 cmꢀ1
(Fig. 5) in perfect agreement with the calculated bands (1732
and 1811 cmꢀ1).
In contrast to the aryl derivatives 8a–c, the yields of alkyl-
iminopropadienethiones 8d–e were significantly lower, as a
competing reaction leading to a new series of thioketenes 10
took overhand. This is illustrated in Scheme 3.
We assume that the initial extrusion of CO2 from isox-
azolone 7 leads to the vinylnitrene 9.[4,6,7] Like in other
vinylnitrenes,[9,10] the substituent R can migrate to N (pathway a),
giving the iminopropadienethione 8, or to C (pathway b),
giving a cyanothioketene 10. Pathway (a) has been observed
in other types of arylisoxazolones,[7] but alkyl groups have been
known to migrate to either C or N in vinylnitrenes.[9,10] In the
case of 9d–e, a preference for migration of alkyl groups to C
(pathway b) leads to diminished yields of iminopropadie-
nethiones 8d–e and significant formation of cyanothioketenes
10d–e. Only weak peaks possibly due to cyanothioketenes
10a–c were observed in the aromatic series. It is possible that
the vinylnitrenes 9 may also cyclise to azirenes, which could
exist in thermal equilibrium with the nitrenes;[7,9,10] however,
we have no evidence for the formation of such compounds.

RESEARCH FRONT
1698
D. Kvaskoff and C. Wentrup
(a)
(b)
(c)
CS2
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
CS2
CS2
CO2
CO2
CO2
*
*
CO
p
T
s
p
CO
p
c
c
s
s
p
s
c
p
p
*
*
T
2250 2000 1750 1500
ν [cmꢁ1
2250 2000 1750 1500
ν [cmꢁ1
2250 2000 1750 1500
ν [cmꢁ1
]
]
]
Fig. 7. IR spectra (2400–1400 cmꢀ1; Ar, 10 K) arising from the FVT of 6e at (a) 6008C, (b) 7008C, and (c) 8508C. Bands [cmꢀ1
]
due to: p, propene (1650, 1453); T, thioketene 7e (1819, 1736); s, cyano(neopentyl)thioketene 10e (2236, 1757); c, OCS (2050);
CS2 (1528); CO2 (2340); CO (2138); *, N-neopentyliminopropadienethione 8e (2202, 2012).
FVT of 6d at 7008C with matrix isolation in Ar at 10 K
produces an IR spectrum as shown in Fig. 6. Fragmentation
of the isoxazolone ring resulted in the expected formation of
propene and carbon dioxide, but only weak peaks at 2175 and
2160 cmꢀ1, possibly due to N-isopropyliminopropadienethione
8d were present. Similar spectra were obtained over the tem-
perature range 600–8008C. Given the large calculated extinction
coefficient of the NC3S function (3967 km molꢀ1), the yields
of 8d were very low. The main IR absorptions belong to carbon
disulfide and a compound exhibiting typical thioketene and
nitrile absorptions at 1752 and 2222 cmꢀ1, which are assigned
to cyano(isopropyl)thioketene 10d (Fig. 6). The observed
frequencies agree well with the calculated IR spectrum
(1750, 2236 cmꢀ1). The analogous cyano(tert-butyl)thioketene
nine times stronger than those of 7, which are about twice as
strong as those of 10.
Trapping with Amines
The products of FVT of isoxazolones 6 were trapped in the
gas phase by mixing the pyrolysates with a stream of amine
immediately at the exit of the FVT reactor but before reaching
the cold finger. Alternatively, the products were trapped with
amine on the cold finger; in this case, amine is deposited on the
cold finger before and after the experiment, and the reaction
with the pyrolysate takes place on thawing.
In this way, the thioketenes 7 produced by FVT of 6 at
intermediate temperatures (500–6008C) were trapped to form
the thioamides 11 (Scheme 4). Relatively low isolated yields
(e.g. 42% for 11c) are not unusual for reactive thioketenes,
which tend to oligomerize readily in the condensed phase.[11]
However, the neopentyl-thioketene 7e afforded an acceptable
yield of 83% of 11e.
Trapping of the products formed by FVT of 6a at tempera-
tures of 700–7508C afforded the bis-addition product 17 in
modest yield (18%),[4] and this was achieved only by cold
trapping, i.e. by thawing of the frozen mixture of 8a and
diethylamine on the coldfinger. One reason for the poor yield
of 17 is that another reaction can take place, leading to the
formation of quinoline-4-thione 16a (Scheme 4). The reaction
is formulated in analogy with the corresponding cyclization of
interconverting oxoketenimines and imidoylketenes to quino-
lones.[3] Thus, addition of amine to iminopropadienethiones 8
can take place on either the C¼S or the C¼N group, leading
to ketenimine 12 or thioketene 13. It is unimportant which of
these is formed first, because they are expected to interconvert
with low activation barriers.[12,13] The cyclization of the aryl-
substituted thioketene 14 can now take place easily as a 6p
absorbs at 1753 cmꢀ1 [11a]
.
The IR spectra arising from FVT of 6e at 600–8508C are
shown in Fig. 7. Two compounds are formed initially, a
thioketene assigned as 7e absorbing at 1819 and 1736 cmꢀ1
,
and the neopentyliminopropadienethione 8e absorbing at 2202
and 2012 cmꢀ1 (Fig. 7a). The thioketene 7e disappeared at
7008C, when the absorptions ascribed to 8e reached maximum
intensities (Figs 7b and 8). There is good agreement between the
experimental and calculated spectra of 8e (Fig. 8). 8e is unstable
and disappears upon warming of the solid to 120 K. At an FVT
temperature of 8508C the bands labelled ‘s’, which were already
present at 7008C, correspond to the main species apart from CO2
and CS2 (Fig. 7c). These absorptions, at 2235 and 1756 cmꢀ1
,
are assigned to cyano(neopentyl)thioketene 10e (Fig. 7c).
A summary of the main infrared absorptions of the hetero-
cumulenes produced by FVT of isoxazolones 6a–e is shown
in Table S1 in the Accessory Publication. Overall, an excellent
correlation was found between the observed and calculated
IR absorptions at the B3LYP/6–31G** level (wavenumbners
scaled by a factor 0.9613). The absorptions of 8 are roughly

RESEARCH FRONT
Thioketenes and Iminopropadienethiones
1699
(a)
.
.
.
N
S
(b)
.
S
NC
0.6
0.5
0.4
0.3
0.2
0.1
CS2
CO2
*
p
p
p
CO
c
s
K
p
s
*
p
*
p
p
(c)
0.0
3200
2950
2700
2450
2200
ν [cmꢁ1
1950
1700
1450
1200
950
]
Fig. 8. (a) calculated IR spectrum of 8e; (b) calculated IR spectrum of 10e (B3LYP/6–31G**; wavenumbers scaled by
a factor 0.9613); the intensities of neopentyl-NCCCS 8e are attenuated by a factor of 12.5 with respect to the thioketene
10e. (c) Experimental IR spectrum of the product of the FVT of 6e at 7008C in Ar, 10 K. Bands [cmꢀ1] due to:
CO2 (2340); CO (2142); c, OCS (2050); s, cyano(neopentyl)thioketene 10e (2236, 1757); CS2 (1528); p, propene
(3091, 2981, 2943, 1650, 1454, 1374, 997, 909); *, N-(neopentyl)iminopropadienethione 8e (2970, 2202, 2012, 1306)
(calc. 2227 cmꢀ1). Ordinate in arbitrary absorbance units.
electrocyclic reaction. The resulting non-aromatic product 15
aromatizes to thioquinolone 16 in a series of H-shifts analogous
to quinolone-forming cyclizations.[14]
the 1,3-shift transition states. In other words, these are reactions
with sequential transition states, characterized by a bifurcation
point (valley-ridge inflection) between them.[12,15] When the
rotational barrier becomes larger than the 1,3-shift barrier, there
will necessarily be a 1,3-shift intermediate.[12] Details of our
calculations of the energy surface will be published separately.
The interconversion of the ketenimine 12 and the thioketene
13 (Schemes 4 and 5) is a very interesting reaction. As in
the analogous oxoketenimine–imidoylketene rearrangement,[12]
the amino groups R2N in 12 or 13 first have to rotate into the
perpendicular orientations required for optimal interaction
between the amine lone pairs and the ketenimine or thioketene
LUMO. These conformations, 17 and 19, are rotational transi-
tion states (Scheme 5).
Conclusion
Thioketenes 7 and 10 are formed by FVT of isoxazolones 6
at medium temperatures (optimally at 500–6008C). Iminopropa-
dienethiones RNCCCS 8a–c are formed cleanly at high FVT
temperatures (optimally at 700–9008C) and characterized by
Having reached the rotational transition state, the 1,3-shifts
proper take place, leading to the zwitterionic intermediate 18 via

RESEARCH FRONT
1700
D. Kvaskoff and C. Wentrup
SiPr
S
S
Rꢀ
SiPr
R
N
R
C
R
FVT
HNRꢀ
N
2
Rꢀ
N
N
O
O
O
O
O
O
7
6
11
a R ꢂ Ph
b R ꢂ p-MeO-C6H4
c R ꢂ p-NC-C6H4
d R ꢂ isopropyl
e R ꢂ neopentyl
FVT
R
N
C C C S
8
HRꢀN
2
HNRꢀ
2
S
N
S
S
C
H
S
C
ෂNRꢀ
R ꢂ Ar
2
X
X
RꢀN
RꢀN
2
2
H
C
R
N
H
N
NRꢀ
NRꢀ
2
2
R
N
12
13
14
15
Et2NH
R ꢂ Ph
S
Ph
N
N
S
X
N
Rꢀ
N
H
N
a X ꢂ H
Rꢀ
b X ꢂ MeO
c X ꢂ NC
17
16
Scheme 4. Chemical trapping of thioketenes 7 and iminopropadienethiones 8.
H
H
H
H
S
C
S
Rot
1,3-TS
1,3-TS
C
R
Rot
RN
RN
C
C
RN
S
RN
S
S
RN
N
N
N
N
N
R
R
R
R
R
R
R
R
R
12
17
18
19
13
Scheme 5. Thioxoketenimine–imidoylthioketene rearrangement.
low temperature IR spectroscopy. Trapping with amines leads to
thioamides 11 or cyclization of transient imidoylthioketenes 13
to quinoline-4-thiones 16. The nature of the thioxoketenimine–
imidoylthioketene rearrangement and the potential for synthesis
of little-known thioquinolones will be investigated further.
Materials
Compounds 6 were prepared as described previously.[4,18]
Matrix Isolation
Matrix isolation experiments were performed using previously
described apparatus.[3,19] In FVT experiments, an internal oven
employed a 10 cm long, 0.7 cm i.d., electrically heated quartz
tube suspended in a vacuum chamber directly flanged to the
cryostat cold head, with a wall-free flight path of ,3 cm
between the exit of the quartz tube and the cold target (CsI for IR
spectroscopy). An alternative external oven consisted of a 20 cm
(0.7 cm i.d.) quartz tube ending in a quartz flange directly
flanged to the cryostat cold head; this tube was heated on a 10 cm
length and had a ,5 cm unheated length connecting it to the
cold head. A small amount of the precursor (5–10 mg) was co-
sublimed with a stream of argon (,10 hPa minꢀ1 from a 2 L
reservoir) and pyrolysed through the FVT oven at the specified
temperature in an operating vacuum of ,10ꢀ5 hPa. The result-
ing mixture was deposited onto the CsI optical window at
7–22 K in the course of 20 to 40 min. The IR bands were
assigned by comparison with authentic samples or literature
data, or, in the case of new compounds, calculated IR data.
Listings of the matrix IR spectra of reference compounds are
provided in the Accessory Publication.
Computational Methods
The Gaussian 03 suite of programs[16] was used to calculate
relative energies and infrared frequencies at the B3LYP/6–31G**
level. Energies were corrected for zero-point vibrational ener-
gies, and all wavenumbers were scaled by a factor of 0.9613.[17]
The temperature used was 298.15 K. Cartesian coordinates,
absolute energies, and vibrational spectral data are provided in
the Accessory Publication.
Experimental
All GC/MS data were obtained with an injector port temperature
set at 2008C, with an initial oven temperature of 1008C
increasing at 168C per min until 2708C on a Zebron capillary GC
column ZB-5 (30 m length). High resolution mass spectra were
recorded on Kratos MS25RFA or Finnigan mass spectrometers
in EI mode at 70 eV. TLC was performed on silica gel 60 and/or
neutral alumina plates. Column chromatography used deacti-
vated silica gel 60 and/or neutral alumina 90 with AR grade
solvents. Microanalyses were carried out using a Carlo-Erba
elemental analyser. CDCl3 for NMR spectroscopy was filtered
through basic alumina before use to remove any acidic impu-
rities. Melting points are uncorrected.
Complete listings of the wavenumbers and intensities of
all compounds identified in the matrix IR spectra derived from
FVT of compounds 6a–e at various temperatures (usually
500–10008C) are presented in the Accessory Publication.

RESEARCH FRONT
Thioketenes and Iminopropadienethiones
1701
Preparative FVT
(eluent: ethyl acetate/methanol with the gradient increasing
to 10:1) to yield 3-(4-cyanophenyl)-N,N-diethyl-5-oxo-2,5-
dihydroisoxazole-4-thiocarboxamide 11c (92 mg, 42%, yellow
solid), mp 186–1878C. GCMS: Rt 13.7 min, m/z 257 (56%)
[M ꢀ CO2], 242 (10%), 214 (44%), 185 (5%), 129 (6%), 102
(8%), 57 (10%). IR (KBr) n [cmꢀ1]: 3436(b, NH), 2253
(m, CꢃN), 1642(m), 1477(m), 1269(w), 1026(s, CS), 825(m),
762(m), 628(m). 1H NMR (200 MHz, CDCl3/DMSO-d6) d 1.35
(t, 6H, 3J ¼ 7.3 Hz, CH3), 2.98 (q, 4H, 3J ¼ 7.3 Hz, NCH2), 7.63
(d, 2H, 3J ¼ 8.5 Hz, H-20), 7.74 (d, 2H, 3J ¼ 8.5 Hz, H-30), 9.40
(1H, b. s, NH). 13C NMR (50 MHz, CDCl3/DMSO-d6) d 10.4
(CH3), 41.3 (NCH2), 90.7 (C-4), 110.5 (C-40), 118.0 (CꢃN),
127.4 (C-20), 130.7 (C-30), 136.3 (C-10), 160.0 (C-5), 170.9 (C-3),
190.7 (CS). DEPT 135 (50 MHz, CDCl3/DMSO-d6) d 10.4
(CH3), 41.3 (CH2), 110.5, 127.4, 130.7, 136.3. MS (ESI) m/z 300
(100%) [M ꢀ H]. Anal. Calc. for C15H15N3O2S: C 59.78, H
5.02, N 13.94. Found: C 59.61, H 5.16, N 14.01.
The apparatus consisted of a quartz pyrolysis tube (40 cm long,
19 mm i.d.) inserted into a tubular furnace and connected to a
manifold operated at ,10ꢀ5 hPa. The thermolysate was con-
densed on a cold finger at 77 K.
Two methods were used for trapping with amines: (a) for
trapping on the cold finger at 77 K, the appropriate amine was
deposited on the cold finger before the FVT reaction; further
layers of amine were deposited during and after the FVT
reaction. (b) For trapping in the gas phase the thermolysate
was mixed with a stream of the gaseous reagent immediately at
the exit of the FVT reactor but before reaching the cold finger.
The mixture of thermolysate, reagent, and product was then
condensed on the cold finger at 77 K. The mixture was allowed
to warm to room temperature in an atmosphere of N2.
FVT of 4-[Bis((1-methylethyl)thio)methylene]-3-phenyl-
isoxazol-5(4H)-one 6a
FVT of 4-[Bis-((1-methylethyl)thio)methylene]-
The precursor 6a (500 mg, 1.5 mmol) was thermolysed at 7508C
(10ꢀ4 hPa) in the course of 7 h. The thermolysate was condensed
on a cold finger at 77 K, and the cold finger was layered reg-
ularly with diethylamine (8 ꢁ 0.5 mL) during the course of the
FVT. At the completion of the experiment, the pressure was
equalised with dry nitrogen, and the liquid nitrogen trap was
allowed to evaporate and warm up to RT. A red solution
was collected in the receiver flask, which was magnetically
stirred and kept at 08C for 1 h. The excess amine was evaporated
at 108C (6.7 Pa), and the residue was chromatographed on basic
alumina. The first fraction (n-hexane/diethyl ether 1:1) afforded
diisopropyl disulfide (36 mg, 15%, yellow oil): GCMS:
Rt 2.2 min, m/z 150 [Mþꢂ], 108, 43 (C3H6); 1H NMR (300 MHz,
3-(2,2-dimethyl-propyl)-4H-isoxazol-5-one 6e
The precursor 6e (350 mg, 1.1 mmol) was gently sublimed at
70–958C (10ꢀ4 hPa) and thermolysed at 6008C in the course
of 5 h in a procedure analogous to the FVT of 6c at the same
temperature (vide supra). Flash chromatography on alumina
(eluent: first diethyl ether, then increasing gradient to ethyl
acetate/methanol 10:1) afforded: N,N-diethyl-3-neopentyl-5-
oxo-2,5-dihydroisoxazole-4-thiocarboxamide 11e (250 mg,
1
83%), yellow oil. GCMS: Rt 9.0 min, m/z 226 [M ꢀ CO2]. H
NMR (200 MHz, CDCl3/DMSO-d6) d 0.88 (s, 9H, tBu), 1.21 (t,
6H, 3J ¼ 7 Hz, CH3), 2.92 (s, 2H, CH2), 3.96 (q, 4H, CH2), 8.56
(b. s, 1H, NH). 13C NMR (50 MHz, CDCl3/DMSO-d6) d 11.9
(CH3), 29.3 (CH3), 31.3 (C), 41.6(CH2), 47.3(NCH2), 93.0(C-4),
162.1 (C-5), 171.0 (C-3), 208.5 (C¼S). DEPT-135 d 11.9, 29.3,
41.6 (CH2), 47.3 (CH2). Anal. Calc. for C13H22N2O2S: C 57.75,
H 8.20, N 10.36. Found: C 57.58, H 8.30, N 10.31.
3
CDCl3) d 1.41 (d, 12H, J ¼ 6.7 Hz, CH3), 3.30 (septet, 2H,
3J ¼ 6.7 Hz, CH); 13C NMR (75 MHz, CDCl3) d 22.9 (CH3),
42.7 (CH). Another fraction gave a small amount of benzonitrile
(20 mg, 12%; GCMS Rt 3.1 min, m/z 103 [Mþꢂ]). The ethereal
fraction afforded 3-(diethylamino)-N,N-diethyl-3-(phenyl-
imino)propanethioamide 17 (84 mg, 18%, brown oil), which
was purified further by Kugelrohr distillation (bp 70–808C,
3 ꢁ 10ꢀ6 hPa) and identified by comparison with previously
reported data.[4]
Accessory Publication
The following data is available on the Journal’s website: IR
spectrum of 8a obtained by FVT of 6a at 9008C; listings of
wavenumbers and intensities of all compounds identified in
the matrix IR spectra derived from FVT of compounds 6a–e at
various temperatures; peak listings of reference compounds; and
Cartesian coordinates, energies, and vibrational spectra of cal-
culated molecules.
A polar fraction was then eluted using ethyl actetate/
methanol (9:1), which consisted of 2-(diethylamino)quinoline-
4(1H)-thione 16a (62 mg, 17%, orange solid), mp 210–128C;
GCMS: Rt 12.7min, m/z 232 [Mþꢂ]; 1H NMR (200 MHz, CDCl3/
DMSO-d6) d 1.34 (t, 6H, 3J ¼ 7 Hz, CH3), 2.94 (q, 4H, 3J ¼ 7 Hz,
NCH2), 5.32 (s, 1H, H-3), 7.10–7.43 (m, 2H), 7.41 (d, 3J ¼ 8 Hz,
H-6), 7.74 (d, 1H, 3J ¼ 8 Hz, H-5), NH not observed. 13C NMR
(50 MHz, CDCl3/DMSO-d6) d 11.0 (CH3), 42.2 (NCH2), 119.8
(C-8), 123.4 (C-3), 127.3 (C-5), 128.6 (C-6), 131.7 (C-7), 132.9
(C-4a), 143.5 (C-8a), 156.8 (C-2), 194.6 (C-4). MS (þEI) m/z
232 (30%) [Mþꢂ], 217 (10%), 203 (100%), 189 (15%), 160
(13%), 116 (18%), 100 (86%), 89 (16%), 72 (10%), 58 (12%).
Anal. Calc. for C13H16N2S: C 67.20, H 6.94, N 12.06. Found:
C 67.36, H 6.82, N 12.02.
Acknowledgements
This work was supported by the Australian Research Council and the Centre
for Computational Molecular Science at The University of Queensland. We
thank the late Professor Robert Flammang of the University of Mons-Hainaut,
Belgium, for insightful discussions of FVT-MS.
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