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Synthesis and fungicidal activity of some organic sulfides.

Introduction

The use of elemental sulfur and its compounds as fungicides was known to the ancient Greeks as early as 1000 B.C., however, the first record to suggest the application of sulfur for the control of pest diseases was not made until 1803 by Forsyth [1]. The use of sulfur as an effective remedy for the control of peach mildew was also reported to the London Horticultural Society in 1824 [2,3]. By the middle of the nineteenth century, sulfur had gained a lot of popularity for its fungicidal properties, and its use as a fungicide gradually increased until the fungicidal properties of Bordeaux mixture [4] in the control of mildew of grapes were discovered.

The introduction of dithio carbamates for plant disease control during the 1930's marks the beginning of the use of organic chemicals on an important scale for the control of plant diseases.

Tisdale et al. [5] patented compounds of the general molecular formular X(Y)NCSSZ, where X is hydrogen or alkyl group, Y is hydrogen, alkyl or aryl group and Z is metallic in nature, and thiuram disulfide derivatives, as fungicides, bactericides and microbiocides.

Sulfur and its compounds continue to be important agrochemicals, as shown by the large variety of new sulfur--based crop protection chemicals in current development around the world [6,7,8].

In continuation of our search for suitable organosulfur compounds as fungicides, we prepared compounds 1, 2 and 3 and screened them for biological activity. To our knowledge, no detailed study of the fungicidal activity of these compounds have been reported in the literature.

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Results and Discussion

Using different methods as appropriate, some symmetrical and unsymmetrical disulfides were prepared. Dibenzyl disulfide was prepared by the method of Baroni9 as depicted in Scheme 1

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Scheme 1

Trichloromethanethiosulfenyl chloride was prepared by heating under

reflux trichloromethane sulfenyl chloride, sulfur and catalytic amount of triethyl phosphate, as shown in scheme 2

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Scheme 2

Trichloromethyl phythalimido disulfide was prepared by the reaction of potassium phthalimide with trichloromethane thiosulfenyl chloride as shown in Scheme 3.

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Scheme 3

Analytical and other spectral results (see experimental section) showed that these compounds were pure.

Fungicidal screening

Compounds 1, 2 and 3 were tested for fungicidal activity as described in the experimental section. Also tested for fungicidal activity were guazatine/ imazalil and phenyl mercury acetate, which are well established fungicides so as to compare their activities with those of the synthesized compounds. The fungal organism against which the compounds were tested are Fusarium culmorium([F.sub.1]), Fusarium oxysporium([F.sub.2]) and Gaeumannomyces graminis([F.sub.3]).

Germination of spores were assessed by measuring the diameter of growth of each plate and comparing it with the control. The percentage of inhibition(%I) of growth was calculated by using the following equation

%I = [[dc - d]/dc] x 100/1 %

Where d = diameter of growth for the plate treated with chemical and dc = diameter of growth for the control.

The percentage inhibition of growth were then ranked as given in Table 1. The results of in vitro test are given in Table 2.

The compounds tested 1,2 and 3 showed moderately good activity and were able to control the three fungi investigated even at low concentration. The activity results obtained for these compounds were comparable with those of guazatine/imazalil and phenyl mercury acetate, which are well established fungicides. The moderately good fungicidal activity of these compounds establishes their potential usefulness as good fungicides.

Experimental

Reagents: Most reagents involved in this synthetic work were used without further purification. Diethylether was dried over sodium wire.

Elemental Analysis: All analyses for carbon, hydrogen, nitrogen and sulfur were carried out at Organic II Division, Indian Institute of Chemical Technology, Hyderabad, using a Carlo Erba 1106 elemental analyzer.

Nuclear magnetic resonance spectroscopy: Routine [sup.1]H NMR spectra were obtained using Perkin Elmer R 12B continuous wave spectrometer at 60MHZ and on a Bruker wp 80 instrument at 80 MHZ. Higher field [sup.1]H NMR and [sup.13]C NMR were recorded on a Bruker AM 250 FT spectrometer at 250.13 MHZ and 62.89M[H.sub.Z] respectively. Chemical shifts for 1H NMR and 13C NMR spectra are given relative to internal standard tetramethyl silane (TMS).

Mass spectrometry: Electron impact (EI) mass spectra were recorded at Organic II Division, Indian Institute of Chemical Technology, Hyderabad, using KRATOS PROFILE high resolution mass spectrometer which is a double-focusing field instrument.

Preparation of dibenzyl disulfide

Benzyl bromide (12.0[cm.sup.3], 0.1mol), sodium thiosulfate pentahydrate (24.82g, 0.1mol) in water (25[cm.sup.3]) and ethanol (20[cm.sup.3]) were mixed and heated under reflux for 23hrs while maintaining the bath temperature at 98-100[degrees]C. Iodine (12.69g, 0.1mol) was added to the hot mixture, followed by the addition of sodium bisulfite to remove excess iodine and the crude product which separated was removed. The aqueous layer was extracted twice with ether (2 x 25[cm.sup.3]) and the ether extracts were combined with the crude product. The total was extracted three times with water (3 x 50[cm.sup.3]), the ethereal layer was separated and dried over anhydrous magnesium sulfate and the ether was removed under pressure to give a solid which was recrystallized from methanol to give dibenzyl sulfide (11.52g, 62%) as a white crystalline solid; (Found: C, 67.89; H, 5.83; S, 25.80. Calc. for [C.sub.14][H.sub.14][S.sub.2]: C, 68.26; H, 5.74; S, 25.99%); m.p. 71-72[degrees]C (lit. m.p. 71[degrees]C; [DELTA]h(CD[Cl.sub.3]) 3.56 (ArC[H.sub.2],S,4H), 7.28 (Ar-H, m, 10H); [DELTA]C (CD[Cl.sub.3]) 43.18 (ArC[H.sub.2]), 127.38 (ArC-1), 128.43 (ArC-3, 5), 129.38 (ArC-2,6), 137.3 (ArC-4); MS: m/z (%), 246 ([M.sup.+], 60), 214 (2), 155 (19), 123 (13), 91 (100).

Preparation of Trichloromethylthiosulfenyl chloride

A mixture of trichloromethylsulfenyl chloride (17.0g, 0.09 mol), sulfur (4.0g, 0.12mol) and triethyl phosphate (0.47[cm.sup.3], 0.003 mol) was heated under reflux for 7hrs at 140-150[degrees]C. The resultant material was filtered and the filterate was fractionally distilled to give trichloromethylthiosulfenyl chloride (8.50g, 50%) as a yellow viscous oil; (Found: C, 5.62; S, 29.34. Calc. for CQ4S2: C, 5.51; S, 39.55%), b.p. 60-65[degrees]C at 3.0mmHg (lit b.p. 63-66[degrees]C at 2.5mmHg); MS: m/z (%) 218 ([M.sup.+], 16) 186 (21), 154 (15).

Synthesis of trichloromethyl phthalimido disulfide

A mixture of potassium phthalimide (2.70g, 0.0146 mol) in water (29[cm.sup.3]) and ice (29g) was stirred vigorously while trichloromethyl thiosulfenyl chloride (3.36g, 0.0154 mol) in hexane (30[cm.sup.3]) was being added over a 15 minutes period. The separated solid was filtered, washed with several portions of water, and air-dried to give trichloromethyl phthalimido disulfide (3.05g, 66%) as a white solid; (Found: C, 32.95; H, 1.27; N, 4.22; S, 19.40. C9H4O3NO2S2 requires: C, 32.89; H, 1.23; N, 4.26; S, 1948; m.p. 129-130[degrees]C; [increment of h] (CD[Cl.sub.3]) 7.31 (Ar-H, m, 4H) [increment of c] (CD[Cl.sub.3]) 132.32 (ArC-3,4), 134.03 (ArC-2,5), 136.87 (ArC-1,6); MS: m/z (%), 316 ([M.sup.+], 48), 284 (11), 252 (19), 146 (26), 106 (27)).

Biological screening experiments

In vitro screening of compounds

After a couple of trials that involved cultivation of the fungi Fusarium culmorum, Fusarium oxysporum and Gaeumannomyces graminis in different nutrients, it was found that the most suitable was Saboround Dextrose Agar (SDA). Therefore, these fungi were cultivated in this nutrient for all the screening experiments.

A solution of the chemical to be tested (1000ppm) in SDA medium was prepared by dissolving 0.2g in acetone (20[cm.sup.3]), followed by the addition of distilled water (80[cm.sup.3]) at 60[degrees]C. This mixture was then added to sterilize SDA solution containing agar (13g) in water (100[cm.sup.3]). (Sterilization of the SDA solution had been previously carried out by placing it in an autoclave at 121[degrees]C and leaving it at this temperature for 15 min.). Addition of 20[cm.sup.3] of the 1000 ppm solution to a solution containing SDA (13g) in water (180[cm.sup.3]) gave a 100 ppm solution. Similarly, a 10 ppm solution was prepared by adding 20cm of the 100 ppm solution to a solution containing SDA (13g) in water (200[cm.sup.3]). Also, solutions containing (a) guazatine/imazalil and (b) phenyl mercury acetate at various concentrations of active ingredients (1000ppm, 100ppm and 10ppm)were prepared as standards.

The solutions were poured in Petri dishes and allowed to cool. Each plate was then inoculated with a 5mm agar plug containing actively growing fungus. All plates were kept inside a sterilized incubator, maintained at 25[degrees]C. The growth diameter of the fungal spore was measured every three days until there was complete growth on the control dish, i.e. until all the surface of the plate was covered with fungal spore (approximate diameter, 86[cm.sup.3]).

Acknowledgement

The authors are grateful to Dr. V. Jayathirtha Rao of Indian Institute of Chemical Technology, Hyderabad for useful Suggestions.

References

[1] Forsyth, W., "A treatise on the culture and management of fruit trees" 2nd ed., p.358 Nichols and sons, London. (1803). Cited from "Chemistry of pesticides" edited by Buchel, K.H., John Wiley and sons New York. P.246(1983).

[2] Robertson, J., Trans. Hort.Soc, London 5, 175-188 (1824).

[3] Bergmann, M., Gardeners Chron, 12, 419 (1852).

[4] Millardet, M.P., J. Agr. Prat, 2, 707-710 (1885). Cited from " Agrochemicals--preparation and mode of action", Cremlynz R.J., Wiley and sons London p4 (1991).

[5] Hester, W.F., U.S. Pat, 1, 972, 961 (1934).

[6] Cullen, T.G., Abstr. Div. Agrochem., ACS 199th National Meeting Boston, Apr. 22-27 (1990) paper 10-12.

[7] Young, D.E., U. S. Pat. 4, 476, 113 (1984); 4, 726, 144 (1988).

[8] Schloss, J.V., Accounts of Chem. Res, 21, 348-373 (1988).

S.O. Oladoye and E.T. Ayodele

Department of Pure and Applied Chemistry

Ladoke Akintola University of Technology, Ogbomoso, Nigeria
Table 1: Ranking of the percentage inhibition(%I) of growth.

1 Rank 0 1 2 3 4

%I 10 11-20 21-49 50-79 [greater than or equal to]80

Table 2: In vitro test results for compounds
1, 2, 3; guazatine/imazalil and phenyl mercury acetate.

Compound [F.sub.1](ppm) [F.sub.2] (ppm)

 10 100 1000 10 100 1000

1 2 3 4 2 3 4
2 3 4 4 3 3 4
3 2 3 4 2 3 4
Guazatine/Imazalil 3 4 4 3 3 4
Phenyl mercury acetate 3 4 4 3 3 4

Compound [F.sub.3] (ppm)

 10 100 1000

1 2 3 4
2 3 4 4
3 3 4 4
Guazatine/Imazalil 3 4 4
Phenyl mercury acetate 3 3 4
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Author:Oladoye, S.O.; Ayodele, E.T.
Publication:International Journal of Applied Chemistry
Date:Sep 1, 2011
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