No abstract

Kinetics of N-acylation of glycine, L-a-alanine, L-valine, L-leucine, and DL-isoleucine with 4-nitrophenyl benzoate in a water32-propanol solvent at various temperatures were studied. The activation energy, enthalpy, and entropy of the process were determined. Correlations of the N-acylation rate constants of the a-amino acids with the composition of the binary solvent at various temperatures were established, and the N-acylation rate constants of the a-amino acids in water were determined.Kinetics of ammonolysis of esters in nonaqueous media have been studied in [1,2]. There have been a number of works devoted to the effect of the structure of the RCO radical in ROX on the reactions of phenyl esters and derivatives of aliphatic carboxylic acids with various amines in dioxane [335]. Over the past years we have studied the kinetics of N-acylation of a-amino acids with benzoyl chloride in water3dioxane[638] and with carboxylic acid esters in water3 2-propanol, water32-methyl-2-propanol, and water3 acetonitrile [9, 10].The present work deals with the kinetics of N-acylation of glycine (Gly), L-a-alanine (L-Ala), L-valine (L-Val), L-leucine (L-Leu), and DL-isoleucine (DL-ILe) with 4-nitrophenyl benzoate (I) in water3 2-propanol.The N-acylation occurs at pH 839.5 by scheme (1).The reaction was performed with a considerable excess of an a-amino acid (~10 !2 M) with respect to ester I (~10 !5 M). The concentration of the anionic form of the a-amino acid was created by adding a specified amount of NaOH into the reaction mixture. We found that at a 1 : 431 : 10 ratio of the anionic (c a ) and zwitter ionic forms (c zi ) of the a-amino acid the rate of hydrolysis of ester I in water32-propanol can be neglected. The rate of reaction (1) follows the firstorder kinetic law (apparent rate constant k ap ). The second-order rate constant k ac of reaction (1) was calculated by Eq. (2). k a = k ap /c a .(2) Table 1 lists the acylation rate constants k ac of the amino acids studied with ester I in water32-propanol. By the ability for acylation, the a-amino acids can be arranged in the following order: Gly > L-Ala > L-Leu > DL-Ile > L-Val. As seen from Table 1, the N-acylation rate increases with increasing water fraction in the binary solvent, which is probably explained by formation of more reactive complexes II and III of the a-amino acids and ester I with the OH groups of solvent components R`OH (R`= H, i-Pr) via hydrogen bonding. The formation of H-complexes II and III should increase the negative charge of the a-amino acid nitrogen and the positive charge on the ester carbonyl carbon, which favors N3C bond formation and accelerates the reaction.

No abstract

No abstract

No abstract