Phosphorothioates and phosphorodithioates containing alkylthioalkyl or arylthioalkyl groups

5 Phosphorothioates and phosphorodithioates containing alkylthioalkyl or arylthioalkyl groups 

In the chemical sense, the members of this type belong to the larger group of aliphatic phosphorothioates, but their special insecticidal properties, their historical importance and their wide application justifies their discussion as a separate group. 

The development of the first and still most important members of the group is linked with the name of Schrader. 0,0-Diethyl-0-(2-ethylthioethyI) phosphoro-thioate (demeton-0, 55), introduced under the name Systox , was prepared by the reaction of ethylmercaptane with ethylene oxide, and the reaction of the resulting 2-ethylthio-ethano1 with diethyl phosphorothionochloridate in the presence of an acid binder (Schrader, 1950a, 1950b). 

However, the compound is unstable in this form. It undergoes partial thion-thiol isomerisation, and the commercial product (demeton, Systox ) contains in equilibrium 30% of the thiol isomer (demeton-0,55) and 70% of the thion isomer (demeton-S, 56) (Schrader, 1950c). 

Isomerisation is rapid at high temperatures, proceeding in 3 hours at 130 C. Polar solvents also accelerate isomerisation, probably due to the fact that isomerisation takes place via ionic intermediate products (Fukuto and Metcalf, 1954 Henglein and Schrader, 1955)  

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