Evaluation of compounds for insecticidal activity on adult mosquitos*

Bull. Org. mond. Sante) 1970, 42, 377-379Bull. Wld Hlth Org. Evaluation of Compounds for Insecticidal Activity on Adult Mosquitos* 3. Properties of Some N-Methylthiolcarbamates A. B. HADAWAY, F. BARLOW, J. E. H. GROSE, C. R. TURNER & L. S. FLOWER N-methylcarbamates derived from thiophenols possess negligible insecticidal activity against the mosquitos Anopheles stephensi and Aedes aegypti. The probable basic reason for the inactivity is that these compounds are much less efficient inhibitors of cholinesterase than the oxygen analogues. Phenyl N-methylcarbamates containing various groups as substituents on the phenyl ring frequently show outstanding insecticidal properties and several compounds of this kind, such as carbaryl, OMS-33, methiocarb and Zectran, are now established insec- ticides. Investigations on the effect of changes in chemical structure on toxicity have usually dealt with the substituents on the ring or with replace- ment of the hydrogen atom attached to the nitrogen atom by alkyl and acyl groups. Recently, a few compounds have been prepared in which 1 oxygen atom, that joining 2 carbon atoms in the ester linkage, has been replaced by a sulfur atom. These compounds are therefore substituted phenyl N-methylthiolcarbamates. It is of interest to com- pare their biological, physical and chemical properties with those of the corresponding oxygen compounds. PREPARATIONS The compounds were prepared by reaction of methyl isocyanate with phenylthiols; 1 g-2 g of each thiol being dissolved in 1 ml of ethyl ether and 1.14 moles of the isocyanate added. These reactions proceeded more readily than with the corresponding phenols and no catalyst was required. Within a few minutes, the mixtures solidified. The products were recrystallized from various petroleum ether- benzene mixtures. Some physical properties of the compounds are shown in Table 1. * From the Ministry of Overseas Development, Tropical Pesticides Research Unit, Porton Down, England. The nitrogen content of the 4-methyl compound was 7.60% (theoretical value, 7.73 %) and of the 4-chloro compound 6.950% (theoretical value, 6.95 %O). TOXICITIES The toxicity of each compound to mosquitos was determined by topical application of solutions in di-isobutyl ketone to the dorsal surface of the thorax of female Anopheles stephensi and Aedes aegypti mosquitos, 1-2 days old. Mortalities were recorded after 24 hours. No kills were obtained with any of the compounds at a dose of 300 ng per mosquito, which was the maximum which could con- veniently be applied. This dose may be compared with the median lethal doses which were obtained for the oxygen analogues (Table 2). The sulfur-containing compounds were much less toxic than the oxygen compounds. The exact ratios could not, of course, be determined because no kills were obtained with the sulfur compounds but for the most toxic oxygen compounds they were at least 83 and 23 for An. stephensi and Ae. aegypti, respectively. VOLATILITIES The volatility of each substance was determined by a standard test method which involves impregna- tion of glass-fibre filter-papers with the compound at a dosage rate of 1 g/m2 and storage in a room at 25°C and a relative humidity of 50 %-55 %. After varying intervals of time, the amount remaining on 2478 -377- A. B. HADAWAY AND OTHERS TABLE 1 PHYSICAL PROPERTIES OF SOME SUBSTITUTED PHENYL N-METHYLTHIOLCARBAMATES Melting point Yield Melting point (nC)Compound (~~~~~C) (%) ~~carbamate 2-methylphenyl methylthiolcarbamate 90-91 91 101-102 3-methylphenyl methylthiolcarbamate 91-92 87 75-76 4-methylphenyl methylthiolcarbamate 81-83 72 95 4-chlorophenyl methylthiolcarbamate 126-127 91 116-117 2-isopropylphenyl methylthiolcarbamate 121-122 78 96.0-96.5 TABLE 2 MEDIAN LETHAL DOSES FOR FEMALES OF 2 SPECIES OF MOSQUITOS OF THE OXYGEN ANALOGUES OF THE SUBSTITUTED PHENYL N-METHYLTHIOLCARBAMATES UNDER TEST LDso (ng/mosquito) Compound An stephensi Ae. aegypti 2-methylphenyl methylcarbamate 92 190 3-methylphenyl methylcarbamate 17 42 4-methylphenyl methylcarbamate 53 105 4-chlorophenyl methylcarbamate 140 - 2-isopropylphenyl methylcarbamate 3.6 13 the filter-papers was found by extraction with 95% ethanol and determination of the compound in the extracts. Each of the thiolcarbamates possessed an absorption maximum in the ultraviolet region around 247 nm-249 nm and optical density measurements at the maximum were used for their determination. Under the conditions of this test, almost all com- pounds which do not decompose show a constant rate of loss in terms of mass/unit area/unit time and the values in Table 3 therefore refer to g/m2/day. There is little difference between sulfur and oxygen analogues for the three methyl isomers but the sulfur compound is more volatile than the oxygen com- pound for the 4-chloro- and 2-isopropyl- substituents. SOLUBILITY IN n-HEXANE Solubility in n-hexane can be taken as an indica- tion of the ability of a particular compound to dis- solve in the wax layer of insect cuticle. Values of TABLE 3 COMPARISON OF VOLATILITIES OF SUBSTITUTED PHENYL N-METHYLTHIOLCARBAMATES AND THEIR OXYGEN ANALOGUES Rate of volatilization Substituent (g/m2/day) on phenyl ring Sulfur compounds IOxygen compounds 2-methyl 0.33 0.34 3-methyl 0.42 0.50 4-methyl 0.35 0.32 4-chloro 0.38 0.11 2-isopropyl 0.17 0.11 this property were therefore obtained for some of the thiolcarbamates and are shown in Table 4. The differences between the oxygen and sulfur analogues vary according to the substituent and are not so great as to provide an explanation of the toxicity differences. TABLE 4 COMPARISON OF SOLUBILITIES IN n-HEXANE OF SUBSTITUTED PHENYL N-METHYLTHIOLCARBAMATES AND THEIR OXYGEN ANALOGUES Solubility Substituent (g/100 ml of saturated solution at 25°C) on phenyl ring Sulfur compounds lOxygen compounds 2-methyl 0.140 0.134 3-methyl 0.135 0.261 2-isopropyl 0.083 0.190 378 EVALUATION OF INSECTICIDAL COMPOUNDS AGAINST ADULT MOSQUITOS. 3 BASE-CATALYSED HYDROLYSIS The mechanism of the reaction When an ester undergoes hydrolysis, the bond which is broken could be one of two alternatives and either acyl- or alkyl-oxygen fission may occur. When thiolcarbamates are the esters, it is possible to determine the mechanism by identification of the products. Acyl-oxygen fission will yield a thiophenol, alkyl-oxygen fission a phenol. The thiophenol and phenol can be readily distinguished by their ultra- violet absorption characteristics and by their res- ponses to such reagents as 4-amino antipyrine. The thiolcarbamates did, in fact, hydrolyse with acyl- oxygen fission because the thiophenols were formed. Rates of hydrolysis The thiolcarbamates hydrolysed much more rap- idly than the corresponding carbamates. Thus, while the half-life of 2-isopropylphenyl N-methyl- carbamate in 40% ethanol: pH 12 buffer is 13 min- utes, the corresponding sulfur compound has a half-life of 7-8 minutes in pH 9 buffer and even at pH 8 the rate of hydrolysis is appreciable. INHIBITION OF ESTERASES Carbamates are reversible inhibitors and must possess an adequate stability in aqueous solution if the enzyme is to remain inhibited. As the thiol- carbamates are so much less stable than the corre- sponding oxygen compounds, it is perhaps not surprising that they did not appreciably inhibit either bovine erythrocyte cholinesterase or the cor- responding enzyme in An. stephensi homogenates. KUME tVALUATION DE L'ACTIVITE INSECTICIDE DE DIVERS COMPOSJES SUR DES MOUSTIQUES ADULTES: 3. PROPRIETE-S DE QUELQUES N-METHYLTHIOCARBAMATES On a recemment prepare un certain nombre de N-m&e thylthiocarbamates et compar6 leurs proprietes biolo- giques, physiques et chimiques avec celles des N-m6thyl- carbamates correspondants. En application topique sur des moustiques femelles (Anopheles stephensi et Aedes aegypti), les composes sulfures se sont revel6s quasi depourvus d'activit6 insec- ticide et beaucoup moins toxiques que leurs analogues oxygen6s. On n'a pas not6 de differences marqu6es entre les deux types de compos6s sous le rapport de la volatilite; par contre les composes sulfures se sont montres beaucoup moins stables que les N-methylcarbamates. Les N-methylthiocarbamates n'ont eu aucune action inhibitrice appr6ciable sur la cholinest6rase d'erythrocytes de bovins et sur l'enzyme correspondante contenue dans des homogenats d'An. stephensi, ce qui explique proba- blement leur manque de proprietes insecticides. 379