Kinetics and mechanism of methanolysis of benzoyl derivatives of substituted phenylureas and phenylthioureas

Abstract

The methanolysis rate constants and dissociation constants have been measured of benzoyl derivatives of substituted phenylureas and phenylthioureas. The dissociation constants of the thio derivatives are higher by 1 order of magnitude and the rate constants are higher by 2 orders of magnitude than the respective values of the oxygen analogues. Logarithms of the rate and dissociation constants have been correlated with the Hammet σ constant; the ρ constant of the methanolysis of the oxygen derivatives is almost 2x higher than that of the thio derivatives, which is explained by a change in the rate-limiting step. Methylation of the phenyl nitrogen atom increases the acidity by almost 2 orders of magnitude. This effect is due obviously to steric hindrance to the conjugation with the adjacent carbonyl or thiocarbonyl group.

Kinetics and Mechanisms of Reactions of Thiol, Thiono, and Dithio Analogues of Carboxylic Esters with Nucleophiles

“…It was found that these thioureas are 1 order of magnitude more acidic than their oxygen derivatives and that they react faster with methoxide anion by 2 orders of magnitude. The higher reactivity of thioureas compared to ureas was attributed to the superior leaving ability of arylthiourea from the tetrahedral intermediate ( k 2 ) relative to the oxy analogue 6 …”

“…The higher reactivity of thioureas compared to ureas was attributed to the superior leaving ability of arylthiourea from the tetrahedral intermediate (k 2 ) relative to the oxy analogue. 90 The rearrangement of 1-acyl-1-phenylthioureas (RCON(Ph)-CS-NHR 1 , where R is Me or Ph) to 1-acyl-3-phenylthioureas (PhNH-CS-N(R 1 )COR) in methanol is catalyzed by methoxide anion. 91 There is a concurrent nucleophilic attack by this anion at the carbonyl carbon of the substrate (as shown in Scheme 6).…”