β-Ureido acids and dihydrouracils. The kinetics and mechanism of the reversible ring closure of 3-(3'-methylureido)-propanoic acid and 3-(3-phenylureido)-2-methylpropanoic acid in sulfuric acid solutions

Abstract

The pKBH+ values for protonation of the ureido groups of the title acids, and for protonation at the 2-carbonyl groups of the product hexahydropyrimidine-2,4-diones, have been determined by standard UV and NMR methods. A bell-shaped dependence of the observed rate constants on H2SO4 concentration was found for both the ring-closure and ring-opening directions. This medium dependence is satisfactorily reproduced by means of rate equations based on the excess acidity treatment, providing account is taken of unproductive protonation equilibria. The derived slope parameters (m≠) for the ring-closure direction indicate that the slow step involves proton transfer from nitrogen rather than to oxygen. For the ring-opening or hydrolysis reactions, the corresponding slope parameters (m≠m*) are characteristic of similar reactions known to proceed with slow C—N scission concerted with proton transfer. Keywords: mechanism, hydrolysis, cyclization, acid catalysis.