The base-catalysed cyclisation of phenyl N-(2-hydroxybenzyl)-N-methylcarbamates is concerted

Abstract

The kinetics of the cyclisation in aqueous solution of phenyl-(2-hydroxybenzyl)-N-methylcarbamates to 3-methyl-3,4-dihydrobenzo[e][1,3]oxazin-2-ones and phenolate ions fit the rate law:k_obs = k_c/(1 + [H₃O⁺]/K_a)The values of k_c and pK_a fit Brønsted equations against the pK_a's of the corresponding free phenols but the system does not conform to the reactivity–selectivity hypothesis. The values of the Brønsted parameters β_Y and β_X vary as a function of Y and X according to the equations:β_X = −0.179pK_a^HY + 0.87β_Y = −0.179pK_a^HX + 2.30The magnitude and sign of the Cordes–Thornton cross-interaction coefficient p_XY (−0.179) rule out a stepwise mechanism involving a tetrahedral intermediate and is consistent with a concerted displacement mechanism. A similar concerted mechanism is proposed for the base-catalysed cyclisation of phenyl-N-(2-hydroxyphenyl)-N-methylcarbamate esters to benzoxazol-2-ones.