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

The kinetics of methoxide ion-catalysed solvolysis of 1-acyl-3-(2-halo-5-ni trophenyl)thioureas and cyclization of fluoro derivatives were studied in m ethanol at 25 degreesC. The cyclization involved the substitution of fluori ne 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 slo wer cyclization of the 2-fluoro-5-nitrophenylthiourea anion formed to give 2-amino-5-nitro-1,3-benzothiazole. With the benzoyl derivative, the first s tep involves parallel methanolysis of the benzoyl group and cyclization to 2-benzoylamino-5-nitro-1,3-benzothiazole. At concentrations of sodium metho xide 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 n ot formed and the cyclizations of fluoro derivatives were considerably acce lerated. The slowing of the solvolytic reaction and acceleration of the cyc lization reaction are most probably due to the formation of dianions. Copyr ight (C) 2001 John Wiley & Sons, Ltd.