A new synthesis of aryl isothiocyanates: Carbon disulfide as a dipolarophile. The reaction of (4,5,6,7-tetrahydro-2H-1,2,3-benzotriazolium-1-yl)arylaminide 1,3-dipoles with carbon disulfide: Synthesis, kinetics, mechanism. Azolium 1,3-dipoles

Article abstract of DOI:10.1039/b006631l

A new synthesis of aryl isothiocyanates in which the aryl nitrogen moiety ultimately comes from an arylhydrazine is described. Treatment of (4,5,6,7-tetrahydro-2H-1,2,3-benzotriazolium-1-yl)arylaminide 1,3-dipoles (derived from cyclohexane-1,2-dione bis(arylhydrazones)) with carbon disulfide in acetone at ambient temperatures gives high yields of aryl isothiocyanates and 2-aryl-4,5,6,7-tetrahydro-2H-1,2,3-benzotriazole as a leaving group. The kinetics and mechanism of the reaction were investigated. The mechanism involves a polar cycloaddition of the triazolium-aminide to the CS₂ generating a partially ring-closed intermediate which fragments to the aryl isothiocyanate. Carbon disulfide is not a kinetic superdipolarophile with (1,2,3-benzotriazolium-1-yl)aminide 1,3-dipoles. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b006631l

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A new synthesis of aryl isothiocyanates: carbon disulfide as a
dipolarophile. The reaction of (4,5,6,7-tetrahydro-2H-1,2,3-
benzotriazolium-1-yl)arylaminide 1,3-dipoles with carbon disulfide:
synthesis, kinetics, mechanism. Azolium 1,3-dipoles
1
Richard N. Butler* and Leonie M. Wallace
Chemistry Department, National University of Ireland, Galway (NUIG), Galway, Ireland
Received 14th August 2000, Accepted 19th September 2000
First published as an Advance Article on the web 23rd November 2000
A new synthesis of aryl isothiocyanates in which the aryl nitrogen moiety ultimately comes from an arylhydrazine
is described. Treatment of (4,5,6,7-tetrahydro-2H-1,2,3-benzotriazolium-1-yl)arylaminide 1,3-dipoles (derived from
cyclohexane-1,2-dione bis(arylhydrazones)) with carbon disulfide in acetone at ambient temperatures gives high
yields of aryl isothiocyanates and 2-aryl-4,5,6,7-tetrahydro-2H-1,2,3-benzotriazole as a leaving group. The kinetics
and mechanism of the reaction were investigated. The mechanism involves a polar cycloaddition of the triazolium-
aminide to the CS2 generating a partially ring-closed intermediate which fragments to the aryl isothiocyanate.
Carbon disulfide is not a kinetic superdipolarophile with (1,2,3-benzotriazolium-1-yl)aminide 1,3-dipoles.
Aryl isothiocyanates are normally synthesized from thermal
Results and discussion
decomposition of monoarylthioureas or more commonly from
the reactions of primary aryl amines with carbon disulfide,
thiophosgene or reagents derived from thiophosgene.1,2 The
necessary dehydrosulfurisation of the thiocarbamic acid inter-
mediates has been achieved with reagents such as hydrogen
peroxide3 or dicyclohexylcarbodiimide in pyridine.4 Treatment
of aryl isocyanides with dibenzoyl disulfide has provided
another route to aryl isothiocyanates.5 New routes are of con-
tinuing interest and recently a new synthesis via arylimino-
1,2,3-dithiazoles has been reported.6 Herein we describe a new,
unexpected route to aryl isothiocyanates using carbon disulfide
where the aryl-nitrogen group comes ultimately from an aryl
hydrazine and no unattractive secondary reagents are involved.
The triazolium-1-ylarylaminides 2 (Scheme 1) are readily
obtained7,8 by oxidation of the bis(arylhydrazone)s 1 which are
themselves available from condensation of 1,2-diketones with
arylhydrazines. While common 1,2-diaryldiketones, such as
benzil, could be used we found that cyclohexane-1,2-dione
derivatives gave particularly easy reactions and hence we
used the substrates 2. On simple stirring of compounds 2 with
carbon disulfide in acetone at ambient temperatures sur-
prisingly the aryl isothiocyanates 4 were obtained in high
yields (Table 1), along with the 2-aryl-4,5,6,7-tetrahydro-2H-
1,2,3-benzotriazoles 5 and sulfur. The yields of aryl isothio-
cyanates began to decline with the incorporation of electron
Scheme 1
DOI: 10.1039/b006631l
J. Chem. Soc., Perkin Trans. 1, 2000, 4335–4338
This journal is © The Royal Society of Chemistry 2000
4335

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Table 1 Products from the reaction of 2 with CS2
Products
No.
Compound
Mp/ЊC
Yield (%)
δC e (ppm)
νe/cmϪ1
Compound
Mp/ЊC
Yield (%)
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
Oil
89
93
80
94
80
65
25
17
135.0
134.4
134.7
134.9
133.8
136.7
139.7
140.3
2083
2054
2106
2089
2108
2034
2137
2104
5a
5b
5c
5d
5e
5f
92–94
120–121
56–58
Oil
75–77
120–121
175–177
208–210
90
90
84
84
96
61
71
75
25–27a
Oil
Oil
Oil
40–42b
119–120c
109–110d
4g
4h
5g
5h
a Lit.,19 mp 25–26 ЊC. b Lit.,19 mp 42–44 ЊC. c Lit.,19 mp 119–123 ЊC. d Lit.,19 mp 110–112 ЊC. e For –N᎐C᎐S unit.
᎐
᎐
Table 2 Second-order rate constants for the reaction of 2 with CS2
(at λ 400 nm)
withdrawing groups in the aryl ring and with p-CN and
p-NO2 substituents the yields were particularly low (Table 1,
entries 7 and 8). From our previous studies7,8 of the behaviour
of the 1,3-dipoles of type 2 we might expect a pathway involv-
ing a cycloaddition to give the initial cycloadduct 3. Normally
this would rapidly rearrange in situ to the more stable structure
3A and we have isolated structures analogous to this for other
cumene-type dipolarophiles. In principle structures 3 and 3A
could fragment to the products 4, 5 and sulfur but because
of the stability of structures related to 3A where the moiety
k2/10Ϫ3
Solvent
(T/ЊC)
dm3
No. Substrate
molϪ1 sϪ1
1
2
3
4
5
6
7
8
9
2a
EtOAc (27)
EtOAc (37)a
EtOAc (51)
EtOAc (60)
MePh (37)a
MeOPh (37)a
MeAc (37)a
MeCN (37)a
MeAc (37)
MeAc (37)
3.9
8.8
11.1
15.9
2.5
∆E‡ = 32 kJ molϪ1
2a
∆H‡ = 30 kJ molϪ1
∆S‡ = Ϫ192 J KϪ1 molϪ1
2a
2a
2a
S᎐C–N–Ar was present we would expect products 3A to be
᎐
2a
8.0
stable and readily detected. Structures such as 3A were not
encountered in the present reactions. They are easily detectable
because of the quaternary bridgehead carbon-13 signals. In a
number of cases the reactions were pursued at temperatures
down to Ϫ60 ЊC but no intermediates were detected and the
products were rapidly formed in similar high yields even at
Ϫ60 ЊC.
2a
25.1
63.1
10.6
17.5
10.1
0.85
2a
2b (p-Me)
2f (p-Cl)
10
11
12
2h (p-NO2)b MeAc (37)
2e (p-MeO) MeAc (37)
a Data for Fig. 1, Line A, ET(30) values from ref. 16. b Wavelength for
kinetic measurements 460 nm.
(1)
pseudo-first-order conditions with excess carbon disulfide.
Plots of the k1 values thus obtained against concentration
of CS2 gave second-order rate constants k2 for the reactions
(Table 2). Arrhenius data measured in the range 27 to 60 ЊC
gave ∆E‡ 32 kJ molϪ1, ∆H‡ 30 kJ molϪ1 and ∆S‡ Ϫ192 J KϪ1
molϪ1. The low activation energy reflects an extreme ease
of reaction which is also a synthetic feature of the process.
The negative entropy value is compatible with either a con-
certed or a two-step cycloaddition.15a The influence of solvent
polarity on the rates proved significant (Fig. 1). Concerted
cycloadditions are striking for their insensitivity to solvent
polarity.15b Fig. 1 shows the absence of any influence of
solvent polarity (ET(30) values16) on the rates of the cyclo-
addition of the dipoles such as 2 with acrylonitrile (line B).12
The reaction of the 1,3-dipoles 2 with CS2 herein was sig-
nificantly affected by solvent polarity (line A) (Table 2) and the
process appears to be different to that operating with alkene
and alkyne dipolarophiles. The progressive increase in rate from
toluene to acetonitrile by over a factor of 30 suggests a more
polar transition state in the reaction by comparison with the
rates for acrylonitrile. However for a reaction involving
a dipolar or diradical intermediate such as 9, i.e. a two-step
cycloaddition, the influence of solvent polarity on the rates
might be expected15b to be at least an order of magnitude higher
than that observed. Hence while a highly polar transition state
is indicated a key question is whether the S(8)–C(5) bond is well
developed in the transition state and whether a cycloaddition to
the adduct 3 actually takes place.
The reaction of CS2 with the isoquinolinium-2-ylaminide 6
[reaction (1)] is well known,9–11 and the contrast in its behaviour
with the 1,3-dipoles 6 and the triazolium-aminide 1,3-dipoles 2
is significant and illustrates how different may be the reactions
of azolium and azinium 1,3-dipoles. The cycloadducts 7 are
stable compounds which have been extensively used9–11 as
storage repositories for the dipoles 6. These can be readily
thermally released from the cycloadducts 7 which do not
fragment to aryl isothiocyanates.9–11
Cycloadducts from azolium-N-aminide 1,3-dipoles are
usually unstable and the chemistry of azolium-N-aminide
1,3-dipoles is dominated by secondary reactions, usually
rearrangements, after the initial cycloaddition.7 The marked
difference in the behaviour of the dipoles 2 and 6 with carbon
disulfide and the fact that no cycloadduct or rearranged
cycloadduct was isolated from the reaction with the dipoles 2
made a mechanistic study of this unexpected reaction fully
warranted.
Kinetics and mechanism
(a) Solvent effects and activation parameters. We have previ-
ously reported12–14 detailed kinetic studies of reactions of 1,3-
dipoles of general type 2 [also with Ar groups replacing (CH2)4]
with alkene and alkyne dipolarophiles. The reactions were
found to be stereospecific concerted 1,3-dipolar cycloadditions
involving transition state mixing of the dipole HOMO with
the dipolarophile LUMO. The kinetics of the present reaction
with CS2 were measured using UV spectra by following the
disappearance of the dipoles 2 at 400 nm (2h, 460 nm) under
(b) Substituent effects. Substituents in the N-aryl rings of
dipoles of type 2 show a rare inverted V shaped Hammett plot
in cycloadditions with alkyne and alkene dipolarophiles,12,13
and both electron withdrawing and donating groups inhibit
the rates. This arises because of a special structural effect in the
4336
J. Chem. Soc., Perkin Trans. 1, 2000, 4335–4338

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Fig. 1 Solvent effects for the reaction of 2a with CS2 (line A) and for
a similar 1,3-dipole with acrylonitrile (line B, from reference 12).
1,3-dipoles 2 where the exocyclic 1-N-aryl moiety is twisted
with the phenyl ring orthogonal to the plane of the triazole ring
due to steric effects from the groups at C-5 and N-2. Hence in
the ground state the structures 2 are not classical 1,3-dipoles
Scheme 2
group from DMAD reduces the rate and for methyl propiolate
we measured a rate constant of 5.4 × 10Ϫ3 dm3 molϪ1 sϪ1. Hence
this reactive alkyne is about five times slower than CS2 and
would be a good candidate to trap any possible intermediates
which might build up in the reaction with CS2. Synthetic
reactions were carried out with the dipole 2a in a competitive
situation with 15 molar excesses of CS2 and methyl propiolate
(Scheme 3) in dry acetone at ambient temperature. No inter-
because both ends of the dipole are not conjugated.13
A
rotation of the 1-N-aryl group to make it co-planar with the
triazole ring is required before the molecules can enter a normal
1,3-dipolar cycloaddition.13 In the ground state the positive
terminus of the 1,3-dipole resides in the triazolium ring and
this is stabilized by electron donating substituents in the 2-N-
aryl group while the negative terminus of the dipole resides
on the exocyclic N(6) atom and this is stabilized by electron
withdrawing substituents in the aryl group. In the twisted
ground state of the molecule electron-donating and electron-
withdrawing groups can separately stablise each end of the
dipole thereby both increasing the activation barrier and giving
a rare phenomenon where electron donating and withdrawing
substituents slow the reaction rate. We have previously dis-
cussed these effects in detail.12,13 The influence of N-aryl
substituents on the rates can therefore indicate whether or not
both ends of the 1,3-dipole are involved in the reaction.
The influences of a number of substituents in the aryl group
on the reaction with CS2 are shown in Table 2. The results were
similar to those for the dipolarophiles dimethyl acetylene-
dicarboxylate13 and acrylonitirile12 and both electron with-
drawing and donating groups inhibited the reaction. Inhibition
of the rates by electron-donating substituents is significant. It
rules out a simple nucleophilic addition to give 9 and indicates
that the C(5)–S(8) bond is significantly developed in the transi-
tion state. The similarity in the substituent effects to those
of alkyne and alkene dipolarophiles where rearranged cyclo-
adducts have been isolated points to the involvement of both
termini of the dipole but the solvent polarity data in Fig. 1
show that the reactions are significantly different. We suggest
a polar transition state 10 (path B, Scheme 2) where the
N–C bond is fully formed while the S–C bond is only partially
formed as the fragmentation to the products sets in. Destabil-
isation of the species 10 by the influence of electron withdrawing
groups at the positive triazolium terminus could cause a more
rapid and less controlled fragmentation leading to decomposi-
tion and lesser yields of aryl isothiocyanates. It seems unlikely
that the adduct 3 is fully formed. If it were, a rearrangement to
3A or a reversal to dipole and CS2, as occurs for compound 7,
would be the most likely outcome.
᎐
Scheme 3 Reagents: (i) CS2 (15 mol); (ii) H–C᎐C–CO2Me (15 mol).
᎐
mediates were trapped by the methyl propiolate. Instead com-
petitive reactions gave rise to a product mixture of compounds
14, 5a and phenyl isothiocyanate as well as some gums and
sulfur (Scheme 3). This was not unexpected as it is unlikely
that the species 10 could be trapped although an intermediate
such as 9 should have been trapped and given rise to different
products. Compound 13 is the product from the reaction of 2a
with DMAD and is formed via a cycloaddition–rearrangement
sequence which we have described previously.7,13 Compound
14, an analogue, can be readily identified from its proton
(c) CS2 versus alkyne dipolarophiles. The rate constant for the
reaction of the dipole 2a with dimethyl acetylenedicarboxylate
(DMAD) in acetone at 37 ЊC is 42 × 10Ϫ3 dm3 molϪ1 sϪ1 while
that for CS2 is 25.1 × 10Ϫ3 dm3 molϪ1 sϪ1 (Table 2). Hence,
unlike thio-ketones17 and thio-esters18 carbon disulfide is not a
superdipolarophile with these 1,3-dipoles. Removing a CO2Me
J. Chem. Soc., Perkin Trans. 1, 2000, 4335–4338
4337

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᎐
and carbon-13 NMR spectra. We will comment on the regio-
chemistry of the reaction in a future work on regiochemistry
with unsymmetrical alkynes and alkenes.
requires C, 60.0; H, 2.5; N, 17.5%); νmax (mull) 2226 (C᎐N),
᎐
2137 cmϪ1 (N᎐C᎐S); δ (CDCl ) 7.24 (2H, d, J 8.8, H ) and
᎐ ᎐
H
3
o
᎐
7.59 (2H, d, H ); δ (CDCl ) 117.9 (C᎐N), 136.1, 126.4, 133.6,
᎐
m
C
3
110.6 (C-1, C-2, C-3, C-4) and 139.7 (N᎐C᎐S); followed by 2-
᎐ ᎐
(p-cyanophenyl)-4,5,6,7-tetrahydro-2H-1,2,3-benzotriazole 5g,
a cream solid (0.08 g, 71%), mp 175–177 ЊC (from acetone)
(Found: C, 69.4; H, 5.6; N, 24.6. C13H12N4 requires C, 69.6; H,
Experimental
Mps were measured on an Electrothermal apparatus. IR
spectra were measured with a Perkin-Elmer Spectrum 1000
FT-IR spectrometer. NMR spectra were measured on a JEOL
LAMBDA 400 MHz instrument with tetramethylsilane as
internal reference and deuteriochloroform as solvent; J values
are given in Hz. Microanalyses were measured on a Perkin-
Elmer model CHN analyser. The substrates 2 were prepared as
previously described.7,8 The following are typical examples for
the preparation of compounds 4 and 5.
Ϫ1
᎐
5.4; N, 25.0%); νmax (mull) 2224 (C᎐N), 1602 cm (C᎐N);
᎐
᎐
δH(CDCl3) 1.9 (4H, m, cyclohexyl methylenes (-CH2-CH2-)),
2.8 (4H, m, cyclohexyl methylenes (-CH -C᎐N-)), 7.71 (2H, d,
᎐
2
J 8.8, Hm) and 8.08 (2H, d, Ho); δC(CDCl3) 21.8, 22.7 (CH2)4,
142.5, 118.1, 133.3, 109.4 (C-1Ј, C-2Ј, C-3Ј, C-4Ј resp. of
2-N-p-C6H4CN) and 147.2 (C-4 and C-5).
Kinetics
(a) A solution of compound 2a (0.29 g, 1 mmol) in dry
acetone (3 ml) was treated with a 15 molar excess of carbon
disulfide (0.9 ml, 15 mmol). The solution was stirred at ambient
temperature for 30 min. The solvent was evaporated under
reduced pressure. The residue in dichloromethane (3 ml) was
placed on a silica gel-60 column (230–400 mesh ASTM). The
products were eluted using a gradient mixture (1:0 to 1:1 v/v)
of petroleum-spirit (bp 40–60 ЊC): CH2Cl2. The first product
off the column in petroleum-spirit was phenyl isothiocyanate 4a
The rate constants were measured by following the disappear-
ance of the dipole to infinity at the wavelengths shown (Table 2)
using a Cary1/Cary3E UV-visible scanning spectrophotometer
equipped with an internal timer and a constant temperature
cell compartment. Temperatures were accurate to 0.5 ЊC. The
accuracy of reproduction on the rate constants was 2.5%.
Rates were measured under pseudo-first-order conditions with
the dipolarophile present in a molar excess of at least 1000.
The pseudo-first-order rate constants k1 were obtained from
straight-line plots of ln (At Ϫ A∞) vs. t; the slopes of which were
Ϫk1. Second-order rate constants k2 were obtained from
plots of these k1 values against the dipolarophile concen-
tration. The rate constants quoted are the mean value of at
least 3 runs. The solvents were purified according to standard
procedures and the substrates were prepared as previously
described7,8 and recrystallised at least twice beforehand.
an oil (0.13 g, 89%); νmax (mull) 2083 cmϪ1 (N᎐C᎐S); δ (CDCl )
᎐ ᎐
H
3
7.21–7.46 (5H, m, Harom); δC(CDCl3) 131.1, 125.7, 129.4, 127.2
(C-1, C-2, C-3, C-4) and 135.0 (N᎐C᎐S); followed by 2-phenyl-
᎐ ᎐
4,5,6,7-tetrahydro-2H-1,2,3-benzotriazole 5a, a beige solid
(0.18 g, 90%), mp 92–94 ЊC (from acetone) (Found: C, 72.4; H,
6.5; N, 21.1. C12H13N3 requires C, 72.3; H, 6.6; N, 21.1%); νmax
(mull) 1593 cmϪ1 (C᎐N); δ (CDCl ) 1.84 (4H, m, cyclohexyl
᎐
H
3
methylenes (-CH2-CH2-)), 2.77 (4H, m, cyclohexyl methylenes
(-CH -C᎐N-)), 7.21–7.43 (3H, m, H , H of 2-N-Ph) and 7.97–
᎐
2
m
p
8.00 (2H, d, Ho of 2-N-Ph); δC(CDCl3) 21.7, 22.9 (CH2)4, 145.4,
118.0, 129.0, 126.3 (C-1Ј, C-2Ј, C-3Ј, C-4Ј resp. of 2-N-Ph) and
139.9 (C-4 and C-5).
References
1 A. K. Mukerjee and R. Ashare, Chem. Rev., 1991, 91, 1.
2 J. Gilmore and P. T. Gallagher, in Comprehensive Organic Functional
Group Transformation, ed. A. R. Katritzky, O. Meth-Cohn and
C. W. Rees, Pergamon, Oxford, 1995, vol. 5, p. 1021.
3 G. Li, H. Tajima and T. Ohtani, J. Org. Chem., 1997, 62, 4539.
4 J. C. Jochims, Chem. Ber., 1968, 101, 1746.
5 S. Tanaka, S. Uemura and M. Okana, Bull. Chem. Soc. Jpn., 1977,
50, 2785.
6 T. Besson, J. Guillard, C. W. Rees and V. Thiery, J. Chem. Soc.,
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7 For a review, cf. R. N. Butler and D. F. O’Shea, Heterocycles, 1994,
37, 571.
8 R. N. Butler, A. M. Gillan, J. P. James and A. M. Evans,
J. Chem. Soc., Perkin Trans. 1, 1989, 159; R. N. Butler and
M. G. Cunningham, J. Chem. Soc., Perkin Trans. 1, 1980, 744.
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and R. Temme, Eur. J. Org. Chem., 1998, 379.
10 R. Huisgen, F. Palacios-Gambra, K. Polborn and D. Boeckh,
Heterocycles, 1999, 50, 353.
11 R. Temme, K. Polborn and R. Huisgen, Tetrahedron, 1998, 54, 9849;
R. Huisgen, Chem. Pharm. Bull., 2000, 48, 757.
(b) A solution of substrate 2e (0.35 g, 1 mmol) in dry acetone
(3 ml) was treated with a 15 molar excess of carbon disulfide
(0.9 ml, 15 mmol). The solution was stirred at ambient tem-
perature for 30 min. The solvent was evaporated under reduced
pressure. The residue in dichloromethane (3 ml) was placed on a
silica gel-60 column (230–400 mesh ASTM). The products were
eluted using a gradient mixture of petroleum-spirit (bp 40–
60 ЊC): CH2Cl2 as described. The first product off the column
was p-methoxyphenyl isothiocyanate 4e, an oil (0.13 g, 79%);
νmax (mull) 2108 cmϪ1 (N᎐C᎐S); δ (CDCl ) 3.77 (3H, s, OCH ),
᎐ ᎐
H
3
3
6.82 (2H, d, J 8.8, Hm) and 7.11 (2H, d, Ho); δC(CDCl3) 55.5
(OCH3), 123.4, 126.9, 114.8, 158.5 (C-1, C-2, C-3, C-4) and
133.8 (N᎐C᎐S); followed by 2-(p-methoxyphenyl)-4,5,6,7-
᎐ ᎐
tetrahydro-2H-1,2,3-benzotriazole 5e, a pale yellow solid (0.22
g, 96%), mp 75–77 ЊC (from acetone) (Found: C, 67.9; H,
6.8; N, 18.5. C12H15N3O requires C, 68.1; H, 6.6; N, 18.3%);
νmax (mull) 1592 cmϪ1 (C᎐N); δ (CDCl ) 1.84 (4H, m, cyclo-
᎐
H
3
12 R. N. Butler, F. A. Lysaght and L. A. Burke, J. Chem. Soc., Perkin
Trans. 2, 1992, 1103.
13 R. N. Butler, D. C. Grogan and L. A. Burke, J. Heterocycl. Chem.,
1997, 34, 1825.
14 R. N. Butler, A. M. Evans, A. M. Gillan, J. P. James, E. M.
McNeala, D. Cunningham and P. McArdle, J. Chem. Soc., Perkin
Trans. 1, 1990, 2537.
hexyl methylenes (-CH2-CH2-)), 2.76 (4H, m, cyclohexyl
methylenes (-CH -C᎐N-)), 3.78 (3H, s, OCH ), 6.93 (2H, d,
᎐
2
3
J 9.0, Hm of 2-N-p-C6H4OCH3) and 7.88 (2H, d, Ho of 2-N-p-
C6H4OCH3); δC(CDCl3) 21.8, 23.0 (CH2)4, 55.4 (p-OCH3),
144.9, 119.5, 114.1, 158.2 (C-1Ј, C-2Ј, C-3Ј, C-4Ј resp. of 2-N-p-
C6H4OCH3) and 133.9 (C-4 and C-5).
15 (a) R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, ed.
A. Padwa, John Wiley & Sons Inc., New York, 1984, vol. 1, p. 93;
(b) R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, ed.
A. Padwa, John Wiley & Sons Inc., New York, 1984, vol. 1, p. 76.
16 C. Reichardt, Chem. Rev., 1994, 94, 2319.
17 L. Fisera, R. Huisgen, H. Giera and R. Sustmann, J. Am. Chem.
Soc., 1995, 117, 9671; L. Fisera, R. Huisgen, I. Kalwinsch,
E. Langhols, X. Li, G. Mloston, K. Polborn, J. Rapp, W. Sicking
and R. Sustmann, Pure Appl. Chem., 1996, 68, 789.
18 R. N. Butler, E. C. McKenna and D. C. Grogan, Chem. Commun.,
1997, 2149.
(c) A solution of compound 2g (0.17 g, 0.5 mmol) in dry
acetone (3 ml) was treated with a 15 molar excess of carbon
disulfide (0.45 ml, 7.5 mmol). The solution was stirred at
ambient temperature for 30 min. The solvent was evaporated
under reduced pressure. The residue in dichloromethane (3 ml)
was placed on a silica gel-60 column (230–400 mesh ASTM).
The products were eluted using a gradient mixture of
petroleum-spirit (bp 40–60 ЊC): CH2Cl2 as described. The first
product off the column was p-cyanophenyl isothiocyanate 4g,
white needles (0.02 g, 25%), mp 119–120 ЊC (from petroleum-
spirit (bp 40–60 ЊC)) (Found: C, 60.0; H, 2.3; N, 17.2. C8H4N2S
19 Catalogue Handbook of Fine Chemicals 1999–2000, Sigma-Aldrich
Co. Ltd., Gillingham, Dorset, UK, 1999.
4338
J. Chem. Soc., Perkin Trans. 1, 2000, 4335–4338