Kinetics and mechanism of methanolysis and cyclization of 1-acyl-3-(2-halo-5-nitrophenyl)thioureas

Article abstract of DOI:10.1002/poc.351

The kinetics of methoxide ion-catalysed solvolysis of 1-acyl-3-(2-halo-5-nitrophenyl)thioureas and cyclization of fluoro derivatives were studied in methanol at 25 deg C. The cyclization involved the substitution of fluorine by sulphur anion of thiourea and proceeded in two steps. With the acetyl derivative, the first step is methanolysis and the second step is much slower cyclization of the 2-fluoro-5-nitrophenylthiourea anion formed to give 2-amino-5-nitro-1,3-benzothiazole. With the benzoyl derivative, the first step involves parallel methanolysis of the benzoyl group and cyclization to 2-benzoylamino-5-nitro-1,3-benzothiazole. At concentrations of sodium methoxide higher than ca. 0.01 mol l^-1 the rates of solvolyses of all the acyl halothioureas decreased and at concentrations higher than ca. 0.01 mol l^-1 the rates of solvolyses of all the acyl halothioureas decreased and at concentrations higher than ca. 0.4 mol l^-1 there was an increase in the formation of other product(s) than the product of cyclization. After the addition of 18-crown-6, the side products were not formed and the cyclization of fluoro derivatives were considerably accelerated. The slowing of the solvolytic reaction and acceleration of the cyclization reaction are most probably due to the formation of dianions.