A facile synthesis of 2-imino-4-methylene-1,3-dithiolanes

Article abstract of DOI:10.1002/hlca.201000308

A one-pot synthesis of 2-imino-4-methylidene-1,3-dithiolanes via a three-component reaction of propargyl bromide (=3-bromoprop-1-yne), primary amines, and carbon disulfide (CS₂) is described. Copyright

Full text of DOI:10.1002/hlca.201000308

Helvetica Chimica Acta – Vol. 94 (2011)
831
A Facile Synthesis of 2-Imino-4-methylene-1,3-dithiolanes
by Issa Yavari* and Saeideh Beheshti
Chemistry Department, Tarbiat Modares University, P.O. Box, 14115-175 Tehran, Iran
(phone: þ 98-21-82883465; fax: þ 98-21-82883455; e-mail: yavarisa@modares.ac.ir)
A one-pot synthesis of 2-imino-4-methylidene-1,3-dithiolanes via a three-component reaction of
propargyl bromide (¼ 3-bromoprop-1-yne), primary amines, and carbon disulfide (CS2) is described.
Introduction. – 2-Imino-1,3-dithiolanes are important compounds in crop protec-
tion against herbicides. Common synthetic methods for 2-imino-1,3-dithiolanes are the
reaction of vicinal dithiocyanates in refluxing HCl [1], reaction of vicinal dithiols and
cyanogen chloride (CNCl) [2], acid-catalyzed cyclization of propargyl, allyl, or b-
hydroxyalkyl esters of dithiocarbamic acids [3], photolysis of chlorinated S-allyl
dithiocarbamates [4] and iodocyclization of S-allyl dithiocarbamates [5]. All these
procedures have certain limitations such as tedious process, long reaction time, harsh
reaction conditions, and low yields. As part of our current studies on the development
of new routes in heterocyclic synthesis [6 – 9], we report a facile synthesis of 2-imino-4-
methylidene-1,3-dithiolanes from propargyl bromide, primary amines, and CS2 in good
yields (Scheme 1).
Scheme 1
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Results and Discussion. – The reaction of primary amines 1 and CS2 with propargyl
bromide (¼ 3-bromoprop-1-yne; 2) under solvent-free conditions led to 2-imino-4-
methylidene-1,3-dithiolanes (3) in 80 – 94% yield (Scheme 1). The imino group of
compounds 3 can principally appear in (E)- and (Z)-form due to slow isomerization of
the imino moiety at ambient temperatures. In fact, the NMR spectra of these
compounds are consistent with the presence of the (E)- and (Z)-isomers in nearly
equal amounts. The structures of compounds 3a – 3k were assigned by IR, and 1H- and
13C-NMR spectra. For example, the 1H-NMR spectrum of 3a exhibited four singlets for
MeN (d(H) 1.38 and 1.43) and CH2(5) (d(H) 4.04 and 4.21), along with four doublets
(d(H) 5.24, 5.26, 5.37, and 5.41, 2J ¼ 1.3 Hz), for C(4)¼CH2. The 13C-NMR spectrum of
3a showed ten signals in agreement with the presence of two geometrical isomers. The
1H- and 13C-NMR spectra of 3b – 3k were similar to those of 3a, except for the imino
substituents, which showed characteristic signals in the appropriate regions of the
spectra. The mass spectra of 3a – 3k displayed molecular-ion peaks at the appropriate
m/z values.
Although the mechanistic details of the reaction are not known, a plausible
rationalization is depicted in Scheme 2. Presumably, the intermediate 4 formed from
the reaction of 1 and CS2 is attacked by propargyl bromide (2) to afford 5, which is
converted to product 3 via cyclization reaction and elimination.
Scheme 2
In conclusion, we have developed a simple procedure for the synthesis of 2-imino-
4-methylidene-1,3-dithiolanes by a three-component one-pot reaction of primary
amines, CS2, and propargyl bromide. The present method offers several advantages,
including high yields of products, and environmentally benign and an easy workup
procedure.
Experimental Part
General. Amines, CS2, and propargyl bromide (2) were obtained from Merck, and were used without
1
further purification. IR Spectra: Shimadzu-IR-460 spectrometer; ˜n in cmꢀ1. H- and 13C-NMR spectra:
Bruker DRX-500 Avance instrument; in CDCl3 at 500.1 and 125.7 MHz, resp.; d in ppm rel. to Me4Si as
internal standard, J in Hz. MS: Finnigan-MAT-8430EI-MS mass spectrometer at 70 eV; in m/z (rel. %).
Elemental analyses: Vario EL III CHNOS elemental analyzer.
Compounds 3: General Procedure. Primary amines 1 (1 mmol), CS2 (0.38 g, 5 mmol), and propargyl
bromide (¼ 3-bromoprop-1-yne; 2; 0.12 g, 1 mmol) were mixed in an agate mortar. After completion of
the reaction (30 min), as indicated by TLC (hexane/AcOEt 4 :1), column chromatography (SiO2;
hexane/AcOEt 5 :1) afforded pure products.

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N-Methyl-4-methylidene-1,3-dithiolan-2-imine (3a)1). Yield: 0.12 g (82%). Brown oil. IR (KBr):
3424, 2925, 2346, 1617, 1462, 1267. 1H-NMR: 1.38, 1.43 (2s, Me); 4.04, 4.21 (2s, CH2S); 5.24, 5.26 (2d, 2J ¼
1.3, ¼CH); 5.37, 5.41 (2d, 2J ¼ 1.3, ¼CH). 13C-NMR: 39.5, 42.5 (Me); 44.1, 42.1 (CH2S); 109.7, 110.6
(¼CH2); 144.1, 145.1 (C¼CH2); 159.9, 161.7 (C¼N). EI-MS: 145 (23, Mþ), 87 (100), 43 (48), 15 (14).
Anal. calc. for C5H7NS2 (145.00): C 41.35, H 4.86, N 9.64, S 44.15; found: C 41.6, H 4.9, N 9.8, S 44.4.
N-Ethyl-4-methylidene-1,3-dithiolan-2-imine (3b). Yield: 0.13 g (80%). Brown oil. IR (KBr): 3418,
1
3
3
3189, 2927, 2671, 1571, 1442, 1184, 1091. H-NMR: 1.26, 1.29 (2t, J ¼ 6.9, Me); 3.28, 3.34 (2q, J ¼ 6.9,
CH2N); 4.01, 4.18 (2s, CH2S); 5.21, 5.24 (2d, J ¼ 1.3, ¼CH); 5.36, 5.39 (2d, J ¼ 1.3, ¼CH). 13C-NMR:
15.3, 15.4 (Me); 39.2, 42.5 (CH2S); 52.3, 53.5 (CH2N); 109.6, 110.5 (¼CH2); 141.1, 143.1 (C¼CH2); 162.3,
163.3 (C¼N). EI-MS: 159 (6, Mþ), 101 (100), 57 (56), 29 (11). Anal. calc. for C6H9NS2 (159.02): C 45.25,
H 5.70, N 8.79, S 40.26; found: C 45.5, H 5.8, N 8.6, S 40.4.
2
2
4-Methylidene-N-propyl-1,3-dithiolan-2-imine (3c). Yield: 0.16 g (92%). Brown oil. IR (KBr): 3197,
1
3
2959, 2925, 2871, 2854, 1604. H-NMR: 0.89, 0.94 (2t, J ¼ 6.8, Me); 1.67, 1.70 (2m, CH2); 3.25, 3.28 (2t,
3J ¼ 7.0, CH2N); 4.01, 4.17 (2s, CH2S); 5.22, 5.23 (2d, 2J ¼ 1.4, ¼CH); 5.35, 5.38 (2d, 2J ¼ 1.4, ¼CH).
13C-NMR: 11.4, 14.1 (Me); 22.6, 23.9 (CH2); 39.2, 42.5 (CH2S); 59.8, 60.9 (CH2N); 109.6, 110.4 (¼CH2);
141.1, 143.1 (C¼CH2); 162.8, 163.4 (C¼N). EI-MS: 173 (6, Mþ), 115 (100), 71 (56), 43 (23), 39. Anal. calc.
for C7H11NS2 (173.03): C 48.51, H 6.40, N 8.08, S 37.01; found: C 48.7, H 6.5, N 7.9, S 36.8.
N-Butyl-4-methylidene-1,3-dithiolan-2-imine (3d). Yield: 0.17 g (90%). Brown oil. IR (KBr): 3439,
1
3
2938, 2345, 1605, 1447, 1266, 753. H-NMR: 0.87, 0.92 (2t, J ¼ 7.0, Me); 1.35, 1.41 (2m, CH2); 1.62, 1.67
(2m, CH2); 3.28, 3.30 (2t, 3J ¼ 6.9, CH2N); 3.99, (2s, CH2S); 5.20, 5.21 (2d, 2J ¼ 1.3, ¼CH); 5.34, 5.37 (2d,
2J ¼ 1.3, ¼CH). 13C-NMR: 13.7, 14.0 (Me); 20.4, 20.5 (CH2); 32.3, 32.5 (CH2); 39.1, 42.4 (CH2S); 57.7, 58.9
(CH2N); 109.5, 110.3 (¼CH2); 141.0, 143.1 (C¼CH2); 162.9, 164.1 (C¼N). EI-MS: 187 (5, Mþ), 129 (100),
85 (58). Anal. calc. for C8H13NS2 (187.05): C 51.29, H 6.99, N 7.48, S 34.23; found: C 51.7, H 6.8, N 7.6, S
34.6.
4-Methylidene-N-(2-methylpropyl)-1,3-dithiolan-2-imine (3e). Yield: 0.18 g (94%). Brown oil. IR
(KBr): 3443, 2955, 2924, 2868, 1605, 1275, 1260. 1H-NMR: 0.95, 1.32 (2d, 3J ¼ 6.7, 2 Me); 1.98, 2.03 (2m,
CH); 3.10, 3.12 (2d, 3J ¼ 6.6, CH2N); 4.01, 4.18 (2s, CH2S); 5.23, 5.24 (2d, 2J ¼ 1.3, ¼CH); 5.35, 5.38 (2d,
2J ¼ 1.3, ¼CH). 13C-NMR: 23.7, 23.9 (2 Me); 29.3, 29.6 (CH); 39.1, 42.4 (CH2S); 65.9, 67.1 (CH2N); 109.5,
110.3 (¼CH2); 141.1, 143.5 (C¼CH2); 158.1, 159.1 (C¼N). EI-MS: 187 (9, Mþ), 129 (100), 85 (56), 57
(23), 39 (11). Anal. calc. for C8H13NS2 (187.05): C 51.29, H 6.99, N 7.48, S 34.23; found: C 51.1, H 7.1, N 7.3,
S 34.4.
N-(tert-Butyl)-4-methylidene-1,3-dithiolan-2-imine (3f). Yield: 0.15 g (82%). Brown oil. IR (KBr):
3438, 2930, 2349, 1613, 1525, 1381, 1265, 1198, 752. 1H-NMR: 1.32, 1.34 (2s, t-Bu); 3.93, 4.24 (2s, CH2S);
2
2
5.19, 5.20 (2d, J ¼ 0.8, ¼CH); 5.31, 5.33 (2d, J ¼ 0.8, ¼CH). 13C-NMR: 28.6, 28.7 (Me3C); 38.2, 44.9
(CH2S); 58.2, 58.3 (Me3C); 108.9, 109.6 (¼CH2); 140.7, 145.4 (C¼CH2); 158.2, 160.1 (C¼N). EI-MS: 187
(5, Mþ), 129 (18), 85 (23), 57 (100). Anal. calc. for C8H13NS2 (187.05): C 51.29, H 6.99, N 7.48, S 34.23;
found: C 51.4, H 7.1, N 7.4, S 34.6.
N-Benzyl-4-methylidene-1,3-dithiolan-2-imine (3g). Yield: 0.20 g (89%). Brown oil. IR (KBr): 3036,
2908, 1599, 1433, 1342, 1201, 1007, 896, 742. 1H-NMR: 4.05, 4.22 (2s, CH2N); 4.54, 4.56 (2s, CH2S); 5.26,
5.27 (2d, 2J ¼ 1.3, ¼CH); 5.38, 5.41 (2d, 2J ¼ 1.3, ¼CH); 7.26 – 7.37 (m, Ph). 13C-NMR: 39.4, 42.8 (CH2S);
61.4, 62.6 (CH2N); 109.9, 110.9 (¼CH2); 126.9, 127.1 (CH); 127.8, 127.9 (2 CH); 128.5, 129.0 (2 CH);
138.6, 138.8 (C); 141.1, 143.0 (C¼CH2); 165.1, 166.2 (C¼N). EI-MS: 221 (2, Mþ), 163 (3), 119 (2), 91
(100), 77 (18), 39 (14). Anal. calc. for C11H11NS2 (221.03): C 59.69, H 5.01, N 6.33, S 28.97; found: C 57.9,
H 5.1, N 6.5, S 28.5.
N-(4-Methoxybenzyl)-4-methylidene-1,3-dithiolan-2-imine (3h). Yield: 0.22 g (89%). Brown oil. IR
1
(KBr): 3415, 3184, 2997, 2954, 2928, 2833, 1602, 1511, 1460, 1440. H-NMR: 3.79, 3.81 (2s, MeO); 4.03,
4.21 (2s, CH2N); 4.45, 4.48 (2s, CH2S); 5.24, 5.26 (2d, 2J ¼ 1.3, ¼CH); 5.37, 5.41 (2d, 2J ¼ 1.3, ¼CH); 6.86,
6.88 (2d, 3J ¼ 8.6, 2 CH); 7.25, 7.27 (2d, 3J ¼ 8.6, 2 CH). 13C-NMR: 39.3, 42.7 (CH2S); 55.3, 55.4 (MeO);
60.9, 62.1 (CH2N); 109.9, 110.8 (¼CH2); 113.9, 114.3 (2 CH); 128.4, 129.0 (2 CH); 130.7, 130.9 (C); 141.1,
143.0 (C¼CH2); 158.7, 158.8 (C); 164.5, 166.5 (C¼N). EI-MS: 251 (1, Mþ), 193 (3), 149 (2), 121 (100), 77
1
)
All compounds of type 3 were isolated as 1:1 mixtures of the corresponding (E)- and (Z)-isomers.

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Helvetica Chimica Acta – Vol. 94 (2011)
(17), 39 (12). Anal. calc. for C12H13NOS2 (251.04): C 57.34, H 5.21, N 5.57, S 25.51; found: C 57.2, H 5.4, N
5.7, S 25.7.
4-Methylidene-N-phenyl-1,3-dithiolan-2-imine (3i). Yield: 0.17 g (84%). Brown oil. IR (KBr): 3431,
2925, 1587, 1475, 1266, 1196, 934, 753. 1H-NMR: 4.14, 4.19 (2s, CH2S); 5.18, 5.31 (2d, 2J ¼ 1.3, ¼CH); 5.39,
2
5.42 (2d, J ¼ 1.3, ¼CH); 6.70 – 7.37 (m, Ph). 13C-NMR: 39.8, 42.7 (CH2S); 110.3, 111.0 (¼CH2); 120.1,
120.2 (2 CH); 124.8, 124.9 (C); 128.8, 129.1 (2 CH); 129.2, 129.4 (C); 141.1, 143.1 (C¼CH2); 150.9, 151.7
(C¼N). EI-MS: 207 (8, Mþ), 149 (100), 105 (58), 77 (29). Anal. calc. for C10H9NS2 (207.02): C 57.93, H
4.38, N 6.76, S 30.93; found: C 58.3, H 4.5, N 6.9, S 31.2.
N-(4-Methoxyphenyl)-4-methylidene-1,3-dithiolan-2-imine (3j). Yield: 0.20 g (85%). Brown oil. IR
1
(KBr): 3297, 3046, 3000, 2956, 2834, 1711, 1596, 1502, 1463, 1440. H-NMR: 3.80, 3.81 (2s, MeO); 4.10,
4.17 (2s, CH2S); 5.17, 5.28 (2d, 2J ¼ 1.4, ¼CH); 5.37, 5.39 (2d, 2J ¼ 1.4, ¼CH); 6.87, 6.89 (2d, 3J ¼ 8.3,
2 CH); 6.93, 6.95 (2d, 3J ¼ 8.3, 2 CH). 13C-NMR: 39.5, 42.7 (CH2S); 55.3, 55.4 (MeO); 110.1, 110.8
(¼CH2); 114.2, 114.5 (2 CH); 121.1, 121.5 (2 CH); 140.9, 143.0 (C); 144.0, 144.9 (C¼CH2); 156.8, 156.9
(C); 166.8, 167.5 (C¼N). EI-MS: 237 (2, Mþ), 179 (6), 135 (4), 107 (100), 77 (11). Anal. calc. for
C11H11NOS2 (237.03): C 55.67, H 4.67, N 5.90, S 27.02; found: C 55.9, H 4.7, N 5.8, S 27.4.
N-(3-Chlorophenyl)-4-methylidene-1,3-dithiolan-2-imine (3k). Yield: 0.22 g (90%). Brown oil. IR
(KBr): 2919, 2357, 2090, 1600, 1440, 1342, 1268, 754. 1H-NMR: 4.05, 4.22 (2s, CH2N); 4.58, 4.60 (2s,
2
2
CH2S); 5.26, 5.27 (2d, J ¼ 1.3, ¼CH); 5.38, 5.41 (2d, J ¼ 1.3, ¼CH); 7.18 – 7.49 (m, 4 CH). 13C-NMR:
39.5, 42.9 (CH2S); 58.4, 59.5 (CH2N); 110.2, 111.1 (¼CH2); 129.4, 129.6 (CH); 129.7, 129.8 (CH); 128.2,
128.3 (CH); 126.9, 127.1 (CH); 133.0, 133.1 (C); 136.2, 136.4 (C); 141.0, 142.8 (C¼CH2); 166.3, 167.4
(C¼N). EI-MS: 241 (4, Mþ), 183 (12), 139 (7), 111 (100), 77 (11). Anal. calc. for C10H8ClNS2 (240.98): C
49.68, H 3.34, N 5.79, S 26.53; found: C 49.5, H 3.4, N 5.9, S 26.6.
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Received August 16, 2010