Synthesis and hydrolysis of substituted tetrahydropyrimidinium salts. Behaviour of the degradation products on varying pH

Reaction of 1,2-diaryl-3-methyl-1,4,5,6-tetrahydropyrimidinium iodides 1a-i with alkaline solutions afforded N-aroyl-N-aryl-N'-methyltrimethylenediamines 2a-i . Compounds 2 are stable under acid conditions but in neutral or alkaline media spontaneously rearrange giving N-aroyl-N'-aryl-N-methyltrimethylenediamines 3a-i . Treating compounds 3 with concentrated acids reverse reaction takes place. Kinetic studies were performed on this intramolecular N → N' aroyl transfer over the H_o-pH range -0.9 to 2.30. Compounds 3 undergo acyl transfer to give 2 by a mechanism which involves a change in the rate determining step from formation to acid-catalysed decomposition of a six-membered heterocyclic intermediate on going from H_o to pH values. The existence of maxima in the pH rate profile allow to determine apparent pKa values of the hexahydropyrimidine intermediates which gave good correlation with the Swain F substituent constants. Stability of these heterocycles was also predicted by determination of thermodinamic parameters. Comparisons are made with the behaviour of five-membered heterocyclic intermediates (imidazolidine derivatives) which were studied in an earlier paper.     

Detection of a Covalent—Ionic Carbinolamine Intermediate in Aqueous Media by SDTLC on Silica Gel Plates

JPC – Journal of Planar Chromatography – Modern TLC - When the components of a reaction mixture cannot be quantified by UV-visible spectrophotometry, because the λmax values of...     

Kinetic study on the alkaline hydrolysis of 1-aryl-2-phenyl-2-imidazolines. Basicity-rate of hydrolysis and structure relationships

Rate constants (k_obs) of hydrolysis in boiling alkaline ethanolic solution for six 1-aryl-2-phenyl-2-imidazolines were determined. The influence of substituents in the phenyl group at N-1 upon rate of hydrolysis was studied. When the imidazoline ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. It was observed that reaction rates were enhanced by electron-releasing phenyl substituents of N-1 and reduced by electron-withdrawing groups, providing a change in the mechanism of the reaction in the first case that was not observed in the second. Agreement with the Hammett equation allowed comparison between experimental and “calculated” rate constants which are nearly equal. An equation relating the rate constants with the ionization constants of imidazolinium ions is given.     

Synthesis and hydrolysis of substituted imidazolinium salts. Behaviour of the degradation products on varying pH

Reaction of l-alkyl-2,3-diarylimidazolinium iodides 1a-i with alkaline solutions afforded N'-alkyl-N-aroyl-N-arylethylenediamines 2a-i. Compounds 2 are stable under acid conditions but in neutral or alkaline media rearrange giving N-alkyl-N-aroyl-N-arylethylenediamines 3a-i. Treating compounds 3 with concentrated acids reverse reaction 3 → 2 takes place. Kinetic studies were performed on this intramolecular N → N' aroyl transfer over the H_o-pH range ?0.9 to 2.30. Compounds 3 undergo acyl transfer to give 2 by a mechanism which involves a change in the rate determining step from formation to catalysed decomposition of a heterocyclic intermediate I ²⁺ on going from H_o to pH values. The existence of maxima in the pH rate profile allowed to determine apparent pK_a values of the imidazolidine intermediates which gave good correlation with Hammett sigma values. Stability of these heterocycles was also predicted by determination of thermodynamic parameters.     

Simple and efficient microwave assisted N-alkylation of isatin

We present herein the results of microwave promoted N-alkylations of isatin (1)with different alkyl, benzyl and functionalized alkyl halides. Reactions were carried out under different conditions, always employing methodologies compatible with MW assisted chemistry. Generation of isatin anion employing diverse bases and solvents or using the preformed isatin sodium salt was tested. The best results were achieved using K(2)CO(3) or Cs(2)CO(3) and a few drops of N,N-dimethylformamide or N-methyl-2-pyrrolidinone. These reactions present noteworthy advantages over those carried out employing conventional heating.     

Kinetic study on the acid hydrolysis of 1-aryl-2-phenyl-2-imidazolines. Basicity-rate of hydrolysis and structure relationships

Rate constants (k_obs) of hydrolysis in sulfuric acid-water mixtures at 100° for five 1-aryl-2-phenyl-2-imidazolines were determined. The influence of substituents in the phenyl group at N-1 upon the rate of hydrolysis was studied. When the imidazoline ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. In opposition to the behavior in alkaline media it was observed that reaction rates were enhanced by electron-withdrawing phenyl substituents and reduced by electron-releasing groups providing, similarly, a change in the mechanism of the reaction in the second case that was not observed in the first. Agreement with the Hammett equation allowed comparison between experimental and “calculated” rate constants which are fairly close. An equation relating the rate constants with the pK_a values of the imidazolinium ions is given.