Algicidol 2-chloro-5-tolyl-1,3,4-thiadiazoles
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Dahle, Norman A.; |
5-Aryl-tetrazoles may be reacted with thiophosgene to yield 2-chloro-5-aryl-1,3,4-thiadiazoles which have unusually selective phytotoxic properties, being particularly useful in combating crabgrass and algae.
It has been discovered that a relatively small class of compounds having the structural formula, ##SPC1##
In which X, Y and Z are selected from the group consisting of hydrogen, chloro and methyl have unusually specific selective herbicidal action, particularly against crabgrass and algae, and may be made by a method which is both novel and convenient. The synthesis and use of these compounds is discussed below, with specific examples presented by way of illustration.
SYNTHESIS PROCEDURE
5-Aryl-tetrazoles
The tetrazole starting material used in this synthesis scheme was prepared by the method of W. G. Finnegan, R. A. Henry and R. Lofquist which utilizes the reaction of an arylnitrile with a mixture of ammonium chloride and sodium azide in dimethyl formamide containing a catalytic amount of lithium chloride:
Finnegan, W. G., Henry, R. A., and Lofquist, R., J. Am. Chem. Soc. 80 3908 (1958) and U.S. Pat. No. 2,977,372.
The following procedure is representative of the novel method of synthesis of the 2-chloro-5-aryl-1,3,4-thiadiazoles.
2-Chloro-5-phenyl-1,3,4-thiadiazole
A stirred mixture of 74.4 g (0.50 mole) of 5-phenyl-2H-tetrazole and 74.0 g (0.65 mole) of thiophosgene in 550 ml of dimethoxyethane was allowed to reflux overnight. The reaction mixture was cooled and filtered. The filtrate was taken to dryness at reduced pressure and the resulting residue leached with two 220-ml portions of hexane. Upon cooling and concentration of the combined hexane portions 40.5 g of the desired product was obtained: mp 82.degree.-84.degree. (lit. 86.degree.-88.degree.).
The reaction with thiophosgene probably produces initially a relatively unstable intermediate product which then undergoes thermal decomposition and rearrangement. Better results are obtained by using an efficient non-reactive solvent to achieve homogenous reaction conditions and heating the mixture to yield the final product, without attempting to isolate an intermediate reaction product.
Use of the 2-Chloro-5-aryl-1,3,4-thiadiazoles as Herbicides
So as to illustrate clearly the selective phytotoxic properties of the herbicides, a group of controlled greenhouse experiments is described below.
1. Post-Emergent Use
An aqueous dispersion of each active compound was prepared by combining 0.4 gram of the compound with about 4 ml. of a solvent-emulsifier mixture (3 parts of a commercial polyoxyethylated vegetable oil emulsifier, one part xylene, one part kerosene) and then adding water, with stirring, to a final volume of 40 ml.
The species of plants on which each compound was to be tested were planted in 4-inch pots in a greenhouse. Ten to 18 days after emergence of the plants, three pots were sprayed with an aqueous dispersion of the active compound prepared as described above, at a rate of 5 lb. of active compound per acre and at a spray volume of 60 gallons per acre. Approximately one week after the spray application the plants were observed and the results rated according to the following schedule:
Type of Action Degree
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C = chlorosis (bleaching)
0 = no effect
N = necrosis 1 = slight effect
G = growth inhibition
2 = moderate effect
F = formative effect (abnormal
3 = severe effect
form of growth)
K = non-emergence 4 = maximum effect
(all plants died)
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The same rating schedule was employed to judge pre-emergent results obtained according to the procedure below.
2. Pre-Emergent Use
A solution of each active compound was prepared by dissolving 200 mg. of the compound to be tested in 200 ml. of acetone. Disposable paper trays about 2 inches deep and half the size of ordinary greenhouse flats were prepared and seeded with a variety of species of plant seeds, then sprayed with the acetone solution at the rate of 10 lb. of active chemical per acre of sprayed area. One flat, which had been seeded with alfalfa, brome, flax, oats, radishes and sugar beets was held at 75.degree.F. day temperature; another seeded with corn, coxcomb, cotton, crabgrass, millet and soybeans was held at 85.degree.F. Twenty-one days after seeding and treatment the flats were examined and herbicidal effect was rated according to the above schedule.
Both post-emergent and pre-emergent results are set forth in the following table.
Herbicidal Activity of 2-Chloro-5-aryl-1,3,4-Thiadiazoles
Structural Formula 1 2 3 4 5 6 7 8 9 10
11
12
13
14
15
and b.p. or m.p.
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Post 0 0 0 0 0 0 0 0 0 0 0 0
m.p. 85-87.degree.C
Pre
K4
G2
0 G2
0 0 0 0 0 0 0 G1
Post N1
N2 N1
N1
G1
G1
N1
0 0 0 G1
0 0 0 0
m.p. 90-93.degree.C
Pre N2 N2
K4
G1
0 G2
0 0 G2
0 0 0 0 G1
Post N1 N3 N2
N2
N2
N1
N2
G1
N4
N4
G3
N1
N1
N4
G2
G2
G1
G1
G2
m.p. 117-119.degree.C
Pre
K4
G2
0 G2
0 G1
N4
0 0 G2
0 N2
G2
Post 0 0 0 0 0 0 0 0 0 0 0 0
m.p. 46-47.degree.C
Pre N2
b.p. 120-130.degree.C/0.35-0.60 mm
K4
G1
0 G2
0 G1
G2
0 0 G1
0 G1
Post N1 N1
N1
N1
N1
N1
G1
0 0 0 G1
G1
G1
0 0 N1
m.p. 74-79.degree.C
Pre K3 N3
K4
G3
0 0 0 0 G3
0 0 0 0 G2
Post N3
N2
N1 N1
N1 N1
N1
G2
G2
G1
0 N1
N1
G1
G1
0 N1
G1
m.p. 71-73.degree.C
Pre N2
K4
G2
0 G2
0 G1
N4
0 0 G1
0 G2
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1 Crabgrass
2 Coxcomb
3 Brome
4 Millet
5 Soybean
6 Cotton
7 Alfalfa
8 Oats
9 Corn
10 Flax
11 Radish
12 Sugar Beet
13 Wheat
14 Grain Sorghum
15 Tomato
The extraordinarily selective nature of the herbicidal action of this class of compounds is clearly evident from the test results. It is this property which makes it possible to combat crabgrass with minimum injury to desirable plant life, particularly turf grasses and grain crops.
Use as Algicides
So as to demonstrate the control of algae with the chemicals, small open-mouthed glass vials containing 15 ml. of a suspension of Chlorella sp. in water were placed beside the potted plants and sprayed at the same time as the plants in the greenhouse tests. The spraying produced a concentration of 8.6 ppm of the test chemical in each vial.
All the compounds exhibited algicidal activity at 8.6 ppm in aqueous media containing growing algae. The following four compounds were also effective at 1 ppm concentration.
2-chloro-5-phenyl-1,3,4-thiadiazole
2-chloro-5-(3-methylphenyl)-1,3,4-thiadiazole
2-chloro-5-(2-chlorophenyl)-1,3,4-thiadiazole
2-chloro-5-(4-chlorophenyl)-1,3,4-thiadiazole
These compounds are preferred for combating algae in cooling water ponds, reservoirs, tanks and irrigation ditches because they may be used at concentrations so low as to be practically harmless to plant and animal life.
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