A one-pot preparation of cyanamide from dithiocarbamate using molecular iodine

Article abstract of DOI:10.1039/b914283p

An efficient one-pot method for the synthesis of cyanamides from dithiocarbamate salts via a double desulfurization strategy using molecular iodine is disclosed. Dithiocarbamates, by the action of iodine yield isothiocyanates in situ, which on treatment with aqueous NH₃ give thioureas. The thioureas so generated undergo further oxidative desulfurization with I₂ giving corresponding cyanamides in good yields. Environmental benignity, cost effectiveness and high yields are the important attributes of this one pot procedure. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914283p

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COMMUNICATION
www.rsc.org/greenchem | Green Chemistry
A one-pot preparation of cyanamide from dithiocarbamate using
molecular iodine†
Jayashree Nath,a Bhisma K. Patel,*b Latonglila Jamir,c Upasana Bora Sinhac and K. V. V. V. Satyanarayanad
Received 22nd April 2009, Accepted 14th July 2009
First published as an Advance Article on the web 24th July 2009
DOI: 10.1039/b914283p
An efficient one-pot method for the synthesis of cyanamides
from dithiocarbamate salts via a double desulfurization
strategy using molecular iodine is disclosed. Dithiocarba-
mates, by the action of iodine yield isothiocyanates in situ,
which on treatment with aqueous NH3 give thioureas. The
thioureas so generated undergo further oxidative desulfu-
rization with I2 giving corresponding cyanamides in good
yields. Environmental benignity, cost effectiveness and high
yields are the important attributes of this one pot procedure.
rearrangement of amidoximes,15 coupling reactions involving
Pd isocyanides, allyl carbonates and trimethylsilyl azide,16 and
sodium bis(trimethylsilyl)amide as deoxygenating or desulfu-
rizing agents.17 Yet another method for the preparation of
cyanamides involves the reaction of hypervalent iodine (V)
species with N,N ¢-disubstituted glycylamide.18 However, all the
procedures reported so far seem to have severe environmental
concern as they involve direct or indirect use of toxic and corro-
sive reagents, strong alkaline conditions, expensive reagents and
catalysts, high reaction temperatures and tedious purification
procedures.
Introduction
We have been interested to an extent in the synthesis of
isothiocyanates and cyanamides and in this context we recently
developed a method involving hypervalent iodine reagent, diace-
toxyiodobenzene (DIB) as an efficient thiophilic/desulfurizing
agent for the preparation of these compounds from dithiocar-
bamate salts.19 However, this method although efficient, is not
economically viable when applied to large scale reactions. In yet
another related work, we demonstrated that the hypervalency of
iodine is not really essential, particularly for the transformation
of dithiocarbamate to isothiocyanate and molecular iodine
was found to be equally effective.20 In continuation of these
studies, and also while looking at reaction strategies from a
Green chemistry perspective, we thought it would be worthwhile
to investigate an alternative methodology for the synthesis of
cyanamides, involving the use of alkyl or aryl dithiocarbamate
using iodine as a double desulfurizing agent.
Alkyl and aryl cyanamides are an important class of compounds
which are vital intermediates for the synthesis of various
biologically active molecules and can be converted efficiently
into other functionalities by simple chemical reactions. Due to
their unique structure and reactivity, cyanamides have attracted
considerable attention in organic synthesis1 as well as in the
fields of inorganic and material sciences.2 Cyanamides are
key precursors to N-alkyl or N-aryl imides3 and also serve
as useful protecting groups in the synthesis of heterocycles
containing secondary and tertiary amines.4 They are important
precursors in the synthesis of herbicides5 and pharmaceutically
active heterocycles such as tumor inhibitors,6 and a vasodilator
medication called minoxidil,5 known for its ability to reduce hair
loss and promote hair regrowth.
The wide applications of cyanamides have resulted in the
development of several methods for their synthesis over
the years. Commonest among these is the reaction of cyanogen
chloride/bromide with amines or with imide salts.7 However
this method involves the use of potassium/sodium cyanide
and bromine for the preparation of cyanogen halide (which
is again highly toxic), making the protocol environmentally
unacceptable.
Results and discussion
Our present methodology is based on: (i) formation of isothio-
cyanate from alkyl/aryl dithicarbamate salt by desulfurization
with iodine in the presence of triethylamine as the base in ethy-
lacetate solvent, (ii) treating the in situ generated isothiocyanate
with aqueous NH3 to afford alkyl/aryl thioamides and (iii)
further oxidative desulfurization of thioamides to cyanamide
with iodine in the presence of triethylamine (Scheme 1). Using
aqueous NH3 as the base instead of triethylamine did not work
as expected, due to the competing reactions between ammonia
and molecular iodine forming nitrogen triiodide (NI3), which of
course is well documented in literature.21 The mechanism which
is proposed in the scheme has been authenticated by isolation
and characterization of all the intermediates. Isolation of the
precipitated elemental sulfur further supports the mechanism
proposed.
Literature reports various other methods for preparation of
cyanamide using different synthetic strategies such as cyanation
of amines using CN+ equivalents as synthons,8–14 Tiemann
aCentre for the Environment, Indian Institute of Technology Guwahati,
Guwahati-781 039, Assam, India
bDepartment of Chemistry, Indian Institute of Technology Guwahati,
Guwahati-781 039, Assam, India. E-mail: patel@iitg.ernet.in.
cDepartment of Chemistry, Nagaland University, Lumami Campus-798
601, Nagaland, India
dDepartment of Chemistry, Gitam Institute of Science, Gitam University,
Visakhapatanam, AP-530 045, India
Based on these findings, we thus report herein, a practical,
environmentally benign, high yielding and one-pot preparation
† Electronic supplementary information (ESI) available: General exper-
imental procedures and spectral data. See DOI: 10.1039/b914283p
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(o-, m-, p-) of the substituents attached to the phenyl ring.
Strongly activating (2, 7 and 10), weakly deactivating (3, 4, 5
and 9), moderately deactivating (8) and strongly deactivating
(6) systems all give products with equal ease. The versatility
of the method has been demonstrated by the tolerance of a
number of functional groups such as –NO2 (6), –OMe (2,
7), –COCH3 (8) and –OH (10). Hindered and trisubstituted
dithiocarbamate (11–14) also efficiently give their corresponding
cyanamides (11a–14a) in excellent yields. As shown in Table 1,
dithiocarbamates of napthylamine (15), aliphatic amines (16 and
17) and benzylic amines (18 and 19) give their corresponding
cyanamides in good yields. This method has also been successful
in the preparation of cyanamide (20a) of homoveratryl amine
starting from its dithiocarbamate salt (20).
Scheme 1 Plausible mechanism for the formation of cyanamide.
of cyanamides from dithiacarbamate salts using cheap and non-
toxic reagent molecular iodine (Scheme 1) in an innocuous
solvent ethylacetate.
A variety of substituted aromatic dithiocarbamate salts22
undergo facile transformation to give their corresponding
cyanamides by this methodology (Table 1). This methodology
is equally effective irrespective of the nature and positions
Conclusion
In conclusion, we have developed a general, economical and en-
vironmentally benign method for the preparation of cyanamides
from their corresponding dithiocarbamic acid salts. Although
literature enumerates a number of procedures for the preparation
of cyanamides, the simplicity, environmental acceptability, and
cost effectiveness of this one pot strategy makes it a practical
alternative. Though at first glance the product yields of the
reactions seem to be moderate or maybe just good, but when the
fact that these are actually three step reactions done in a single-
pot is brought to mind, the yields could infact be considered as
very good if not excellent.
Table 1 Preparation of cyanamides from dithiocarbamates and iodinea
Productb
Substrate
Yield (%)c
Experimental
1. R1 = H, R2 = H, R3 = H, R4 = H, R5 = H;
2. R1 = OMe, R2 = H, R3 = H, R4 = H, R5 = H;
3. R1 = Br, R2 = H, R3 = H, R4 = H, R5 = H;
4. R1 = Cl, R2 = H, R3 = H, R4 = H, R5 = H;
5. R1 = F, R2 = H, R3 = H, R4 = H, R5 = H;
6. R1 = H, R2 = NO2, R3 = H, R4 = H, R5 = H;
7. R1 = H, R2 = H, R3 = OMe, R4 = H, R5 = H;
9. R1 = H, R2 = H, R3 = Cl, R4 = H, R5 = H;
10. R1 = H, R2 = H, R3 = OH, R4 = H, R5 = H;
11. R1 = Br, R2 = H, R3 = Me, R4 = H, R5 = H;
12. R1 = F, R2 = H, R3 = F, R4 = H, R5 = H;
13. R1 = I, R2 = H, R3 = Me, R4 = H, R5 = H;
14. R1 = Me, R2 = H, R3 = Me, R4 = H, R5 = H;
1a 80
All the reagents were commercial grade and purified according
to established procedures. Organic extracts were dried over
anhydrous sodium sulfate. Solvents were removed in a rotary
evaporator under reduced pressure. Silica gel (60–120 mesh
size) was used for the column chromatography. Reactions were
monitored by TLC on silica gel 60 F254 (0.25 mm). NMR spectra
were recorded in CDCl3 with tetramethylsilane as the internal
standard for 1H NMR (400 MHz) CDCl3 solvent as the internal
standard for 13C NMR (100 MHz). IR spectra were recorded
in KBr or neat on a Nicolet Impact 410 spectrophotometer.
GC-MS were recorded using a capillary column (30 ¥ 0.25 mm ¥
0.25 mM) in EI mode. Elemental analysis was performed with
a Perkin Elmer 2400 elemental analyzer. Melting points were
recorded on Buchi B-545 melting point apparatus and are
uncorrected.
2a 72
3a 80
4a 70
5a 75
6a 79
7a 78
8. R1 = H, R2 = H, R3 = COMe, R4 = H, R5 = H; 8a 77
9a 65
10a 75
11a 75
12a 81
13a 73
14a 80
15 a-Naphthyl;
16 Cyclohexyl;
17 n-Butyl;
18 Benzyl;
19
15a 80
16a 72
17a 60
18a 82
19a 72
General procedure
To a stirred and ice cooled suspension of dithiocarbamate
1 (540 mg, 2 mmol) in ethylacetate (5 mL), was added tri-
ethylamine (415 ml, 3 mmol). To this was then added iodine
(506 mg, 2 mmol) pinch wise over a period of 10–15 minutes
to yield phenylisothiocyanate. During this period precipitation
of elemental sulfur and triethylammonium iodide salt was
observed. After complete addition of iodine, 25% aqueous
NH3 (2.5 mL) was added drop wise to the stirred reaction
mixture to give 1-phenylthiourea. After stirring for 10 minutes
at room temperature the excess of NH3 was removed in a rotary
20
20a 60
a Reactions were monitored by TLC. b Confirmed by IR, 1H NMR and
13C NMR. c Isolated yield.
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evaporator whereby the solvent ethylacetate was also simultane-
ously removed leaving behind the aqueous layer. To the crude
reaction mixture was then further added ethylacetate (5 mL)
and triethylamine (553 ml, 4 mmol). To the resultant solution,
iodine (506 mg, 2 mmol) was added in small pinches, during
which further precipitation of elemental sulfur was observed.
The conversion of 1-phenylthiourea to phenylcyanamide (1a)
was observed within 5 minutes of the complete addition of
iodine. Completion of the reaction was confirmed by TLC. The
precipitated sulfur was filtered, washed with ethylacetate (2 ¥
5 mL). The organic layer was washed with water (2 ¥ 5 mL)
and dried over anhydrous Na2SO4, concentrated under reduced
pressure and purified over a short column of silica gel eluting
it with hexane: ethylacetate (97:3) to give the pure product 1a
(188 mg, 80%). Oily liquid: 1H NMR (CDCl3, 400 MHz) d (ppm)
7.02–7.07 (m, 3H), 7.28–7.33 (m, 2H), 7.64 (brs, 1H); 13C NMR
(CDCl3, 100 MHz) d (ppm) 112.2, 115.5, 123.6, 129.8, 137.4; IR
(KBr) 3175 (m), 2919 (w), 2227 (s), 1600 (s), 1501 (s), 1249 (m),
748 (s), 689 (m).
132.7, 133.2, 135.0; IR (KBr) 3211 (m), 2923 (w), 2226 (s), 1608
(w), 1509 (s), 1424 (m), 1287 (w), 1038 (w), 863 (w), 804 (m),
743 (w) cm-1; elemental analysis for C8H7BrN2 (211.06): calcd.
C 45.52, H 3.34, N 13.27; found C 45.61, H 3.29, N 13.20.
2,4-Difluoro-phenyl cyanamide (12a). Gummy 1H NMR
(CDCl3, 400 MHz) d (ppm) 6.06 (s, 1H), 6.88–6.96 (m, 2H), 7.20–
7.23 (m, 1H); 13C NMR (CDCl3, 100 MHz) d (ppm) 104.0, 104.2,
104.3, 104.5, 111.27, 111.31, 111.50, 111.54, 111.69, 117.38,
117.41, 117.47, 111.50, 122.9, 123.0, 149.5, 149.7, 152.0, 152.1,
156.2, 156.3, 158.7, 158.8; IR (KBr) 3167 (m), 2927 (w), 2258
(s), 1611 (m), 1524 (s), 1433 (m), 1269 (m), 1215 (m), 1150 (m),
1125 (m), 1087 (m), 962 (m), 852 (m), 804 (m) cm-1; elemental
analysis for C7H4F2N2 (154.11): calcd. C 54.55, H 2.61, N 18.18;
found C 54.61, H 2.58, N 18.20.
2-Iodo-4-methyl-phenyl cyanamide (13a). White solid; Mp:
◦
1
144 C; H NMR (CDCl3, 400 MHz) d (ppm) 2.29 (s, 3H),
6.17 (brs, 1H), 7.17 (m, 2H), 7.56 (s, 1H); 13C NMR (CDCl3,
100 MHz) d (ppm) 20.4, 84.2, 110.7, 115.4, 130.9, 135.4, 139.6;
IR (KBr) 3229 (s), 2919 (w), 2217 (s), 1603 (w), 1502 (s), 1420
(m), 1383 (m), 1283 (w), 1032 (w), 866 (w), 805 (m) cm-1;
elemental analysis for C8H7IN2 (258.06): calcd. C 37.23, H 2.73,
N 10.86; found C 37.28, H 2.68, N 10.80.
Spectral data
Spectral data for reported compounds:
1a–2a, 4a, 6a, 8a–10a, 14a, 16a–19a are reported compounds.19
The spectral data (IR, 1H NMR, 13C NMR, mass and CHNS of
the new compounds are given below.
1-Naphthyl cyanamide (15a). 1H NMR (CDCl3, 400 MHz) d
(ppm) 6.57 (s, 1H), 7.41 (m, 1H), 7.48 (m, 1H), 7.56 (m, 2H), 7.64
(d, 1H, J = 8 Hz), 7.75 (m, 1H), 7.90 (m, 1H); 13C NMR (CDCl3,
100 MHz) d (ppm) 110.9, 112.6, 120.5, 122.7, 123.1, 125.2, 125.5,
125.9, 127.8, 133.5, 133.7; IR (KBr) 3181 (m), 3052 (w), 2944
(w), 2234 (s), 1584 (w), 1530 (m), 1480 (s), 1403 (w), 1344 (w),
1258 (w), 782 (m), 758 (s) cm-1; elemental analysis for C11H8N2
(168.08): calcd. C 78.55, H 4.79, N 16.65; found C 78.49, H 4.78,
N 16.58.
Selected spectral data
◦
2-Bromo-phenyl cyanamide (3a). White solid; Mp: 94.5 C;
1H NMR (CDCl3, 400 MHz) d (ppm) 6.36 (brs, 1H), 6.96–
7.01 (m, 1H), 7.25–7.39 (m, 2H) 7.52–7.53 (m, 1H); 13C NMR
(CDCl3, 100 MHz) d (ppm) 109.91, 109.95, 116.1, 124.9, 129.2,
133.0, 135.3; IR (KBr) 3150, 2237 (m), 1602 (m), 1504 (m), 1425
(w), 1286 (w), 1026 (w), 738 (m) cm-1; elemental analysis for
C7H5BrN2 (197.03): calcd. C 42.67, H 2.55, N 14.21; found C
42.61, H 2.62, N 14.11.
3,4-Dimethoxyphenylethylcyanamide (20a). Gummy; 1H
NMR (CDCl3, 400 MHz) d (ppm) 2.84 (t, 2H, J = 7.2 Hz),
3.28 (q, 2H, J = 7.2 Hz), 3.83 (s, 3H), 3.84 (s, 3H), 4.37 (brs,
1H), 6.72–6.82 (m, 3H). 13C NMR (CDCl3, 100 MHz) d (ppm) d
35.5, 47.5, 55.92, 55.95, 111.4, 111.9, 116.5, 120.9, 130.0, 147.8,
148.9. IR (KBr) 3274 (w), 2937 (w), 2219 (s), 1592 (w), 1517 (s),
1464 (m), 1262 (s), 1236 (m), 1156 (m), 1142 (m), 1026 (m), 913
(w) cm-1; elemental analysis for C11H14N2O2 (206.24): calcd. C
64.06, H 6.84, N 13.58; found C 64.12, H 6.80, N 13.54.
2-Fluoro-phenyl cyanamide (5a). White solid; M.p: 95 ◦C; 1H
NMR (CDCl3, 400 MHz) d (ppm) 6.68 (brs, 1H, NH), 6.90–7.45
(m, 4H). 13C NMR (CDCl3, 100 MHz) d (ppm) 110.9, 115.7,
115.9, 116.8, 124.0, 124.1 125.09, 125.12, 125.6, 125.8, 150.1,
152.5. IR (KBr) 3339 (m), 3069 (m), 2233 (s), 1679 (m), 1621
(s), 1503 (s), 1455 (s), 1378 (m), 1266 (m), 1198 (m), 1109 (m),
1061 (m), 756 (s) cm-1; elemental analysis for C7H5FN2 (136.13):
calcd. C 61.76, H 3.70, N 20.58; found C 61.80, H 3.73, N 23.53.
Acknowledgements
B. K. P acknowledge the support of this research from DST New
Delhi (SR/S1/OC-15/2006) and CSIR 01(1688)/00/EMR-
II. H.G to CSIR for fellowships and RY to the institute. Thanks
are due to CIF IIT Guwahati for NMR spectra.
4-Methoxy-phenyl cyanamide (7a). White Solid; Mp: 86–
89 ◦C; 1H NMR (CDCl3, 400 MHz) d (ppm) 3.78 (s, 3H), 6.87
(d, 2H, J = 8.8 Hz), 6.95 (d, 2H, J = 8.8 Hz); 13C NMR (CDCl3,
100 MHz) d (ppm) 55.8, 112.8, 115.2, 117.0, 130.6, 156.1; IR
(KBr) 3180 (m), 2926 (m), 2218 (s), 1509 (s), 1295 (m), 1238 (s),
1105 (m), 1037 (m), 826 (m); elemental analysis for C8H8N2O
(148.17): calcd. C 64.85, H 5.44, N 18.91; found C 64.91, H 5.40,
N 18.93.
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