Kinetics and mechanism of aminolysis of (Z)-4-arylidene-2-phenyl-5(4H)oxazolones

Abstract

SHARIFA S. ALKAABI and AHMAD S. SHAWALI Can. J. Chem. 70, 2515 (1992). The kinetics of the reactions of a series of (Z)-4-arylidene-2-phenyl-5(4H)oxazolones 1 with n-butylamine and piperidine were studied spectrophotometrically in dioxane, ethanol, and cyclohexane under pseudo-first-order conditions and at different temperatures. The relation k,(obs) = kz[amine] + k3[aminelZ was found applicable for all reactions studied in either dioxane or ethanol. However, in cyclohexane the n-butylaminolysis of 1 followed only third-order kinetics k,(obs) = k 3 [ n -~u~~, l Z .The kinetics of the reaction of 1 with n-butylamine in the presence of catalytic amounts of tri-. The rate constants kZ and k3 correlated well with the Hammett equation and the corresponding activation parameters were determined. The results were interpreted in terms of a mechanism involving solvent-and amine-catalyzed processes. Recently, in a study designed to determine the steric effect of remote substituents on peptide bond formation, it was reported that the reaction of (Z)-4-benzylidene-2-phenyl-5(4H)-oxazolone with benzylamine in acetonitrile followed second-order kinetics (10). This result seems surprising since the reaction of compounds 1, being cyclic esters, with nitrogen nucleophiles in aprotic solvent would be expected to exhibit a kinetic pattern similar to that reported for aminolysis of acyclic esters. Extensive kinetic studies, in our laboratory and others, on the aminolysis of acyclic esters in aprotic solvents established that such reactions are subject to general base catalysis (1 1-23). Thus, under pseudo-firstorder conditions the reactions of esters with primary amine (A) and in the presence of a tertiary amine (B) were found to follow the rate law: k,(obs.In a few cases the kZ term was not observed (16,21,22). Since compounds 1 are versatile intermediates in the syntheses of peptides and didehydroamino acids (24), it was thought important to investigate in some detail the kinetics of their aminolysis to shed light on the mechanism of their ring cleavage by amines in both aprotic and protic solvents. For this purpose we studied the kinetics of the reactions of a series of (Z)-4-arylidene-2-phenyl-5-(4H)-oxazolones lai with 11-butylamine in dioxane, ethanol, and cyclohexane at '~u t h o r to whom correspondence may be addressed. Present address: Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt. different temperatures under pseudo-first-order conditions. The other objective of the present study was to determine both solvent and substituent effects on the aminolytic reactivity of 1.Results and discussion (Z)-4-Arylidene-2-phenyl-5(4H)-oxazolones In-i were prepared from hippuric acid and the appropriate substituted benzaldehyde in the presence of acetic anhydride and anhydrous sodium acetate according to the Erlenmeyer method (25). The Z configuration indicated was based on the fact that the latter method of synthesis of 1 was reported to give the thermodynamically more stable Z isomer (6,(26)(27)(28). The...