The base-catalysed cyclisation of phenyl N-(2-hydroxybenzyl)-N-methylcarbamates is concerted.
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: kobs = kc/(1 + [H3O+]/Ka) The values of kc and pKa fit Brønsted equations against the pKa's of the corresponding free phenols but the system does not conform to the reactivity-selectivity hypothesis. The values of the Brønsted parameters beta Y and beta X vary as a function of Y and X according to the equations: beta X = -0.179pKaHY + 0.87 beta Y = -0.179pKaHX + 2.30 The magnitude and sign of the Cordes-Thornton cross-interaction coefficient pXY (-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.
Stĕrba
Vojeslav
V
Department of Organic Chemistry, University of Pardubice, 532 10 Pardubice, Czech Republic. vojeslav.sterba@upce.cz
Hrabík
Oldrich
O
Kaválek
Jaromír
J
Mindl
Jaromír
J
Williams
Andrew
A