we were able to obtain excellent yields of arylisothiocyanates
(8a–12a) having various di- and tri- substitutions. Aliphatic
amines (15 and 16) including benzylamines (13 and 14) yielded
thia-Michael adducts as the major product along with traces
of isothiocyanate (<5%). This may be due to the higher basic
character of aliphatic amines (13–16) compared to aromatic
amines (1–12) listed in Table 3. Due to the higher basic character of
these amines, they prefer path III and not the deprotonative path
IV proposed for aryl amines. The isolated thia-Michael adduct of
these dithiocarbamate salts, when treated with three equivalents of
NaOH in dioxane gave corresponding isothiocyanates 13a, 14a,
15a and 16a respectively in good yields (Table 3). Thus, from
synthetic utility, reactions of aliphatic dithiocarbamate salts 13–16
were carried out in dioxane solvent and upon formation of thia-
Michael adduct (0.5 h) was treated with 3 equivalents of NaOH
to give isothiocyanate in good yields (4.5 h).
General experimental procedure
General procedure for the preparation of phenyl isothiocyanate
from its dithiocarbamate triethylammonium salt. To a solu-
tion/suspension of the phenyl dithiocarbamate triethylammo-
nium salt 1 (5 mmol) in water (10 mL) was added methyl acrylate
(8 mmol). The heterogeneous reaction mixture was stirred at
room temperature for 1.5 h. The resultant isothiocyanate (1a)
was extracted with hexane (2 ¥ 10 mL) and the hexane layer
was dried over anhydrous Na2SO4. The crude isothiocyanate so
obtained was purified over a short column of silica gel using
1
100% hexane as the eluent to give product 1a in 82%. Oily; H
NMR (400 MHz, CDCl3):d (ppm) 7.21–7.37 (m, 5H); 13C NMR
(100 MHz, CDCl3):d (ppm) 125.8, 127.4, 129.6, 131.3, 135.3; IR
(KBr): 3064, 2164, 2063, 1591, 1489, 1474, 1451, 1070, 927, 905,
749, 684 cm-1.
Acknowledgements
Conclusion
We are grateful to the DST (SR/S1/OC-15/2006) and CSIR
[01(2270)/08/EMR-II] for funding. LJ and HG thank the CSIR
for fellowships. Thanks are also due to CIF, IIT Guwahati for
Mass and NMR spectra.
In summary, an aryldithiocarbamate salt having triethylammo-
nium as its counter cation, when treated with a Michael acceptor,
methyl acrylate, in an aqueous alkaline medium (above 8.5) the
major product obtained is an arylisothiocyanate. However for the
same reaction when the counter cation is a proton (H+), the only
product obtained is thia-Michael adduct, irrespective of the pH
(6–10) of the medium. The thiocarbonyl sulfur (=S) atom having
large orbital-coefficient is softer and attacks on the b-carbon of
the Michael acceptor compared to the thiol/thiolate sulfur (-
SH/S-) which is/was generally believed to be the nucleophile in
a dithiocarbamate salt for these kind of reactions. The difference
in product formation with change in the counter cation can be
accounted by two different kinds of mechanisms. This method
gives an easy access to the preparation of various isothiocyanates
and is perhaps one of the simplest and cost effective method.
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Experimental
General remarks
Unless otherwise stated, all reagents were purchased from com-
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progress was monitored by TLC using Merck silica gel 60 F254
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