Study of ring closure reaction of substituted phenyl N-(2-thiocarbamoylphenyl)carbamates catalysed by methoxide ion

Studies were made of the kinetics of methoxide ion-catalysed reactions of seven substituted phenyl N-(2-thiocarbamoylphenyl)carbamates, 4-methoxyphenyl N-(2-thiocarbamoylphenyl)-N-(methyl)carbamate and five substituted phenyl N-(4-thiocarbamoylphenyl)carbamates, leading to the respective cyclisation products (i.e. 4-thioxo-1H,3H-quinazolin-2-one or 1-methyl-4-thioxo-1H,3H-quinazolin-2-one) and/or methanolysis product, i.e. methyl N-(4-thiocarbamoylphenyl)carbamate. The comparison of the rate constants, beta(1g) and p constants of the 2-thiocarbamoyl derivatives (beta(1g) = -1.15, p = 3.1 +/- 0.1) and 4-thiocarbamoyl derivatives (p = 4.6 +/- 0.2, beta(1g) = -1.55) shows that the ring closure reaction proceeds by the B(Ac)2 mechanism with the splitting off of phenoxide anion being the rate-limiting step, while the methanolysis follows the E1cB mechanism. The ring closure reaction of 4-methoxyphenyl N-(2-thiocarbamoyl)-N-(methyl)carbamate proceeds kinetically in two steps, the respective rate constants differing by one order of magnitude. The NMR spectrum, spectral record and computational calculations of the ring closure reaction indicate that the process involves parallel reactions of two rotamers formed due to hindered rotation. Copyright (c) 2005 John Wiley & Sons, Ltd.