Kinetics and mechanism of the dehydrohalogenation of 3,5-dichloro-2,4,6-trimethylbenzohydroxamoyl chloride and bromide by tertiary amines and silver nitrate

Abstract

The kinetics of the title reactions in acetonitrile have been studied by using Et₃N, Buⁿ₃N, and N-methylmorpholine at 25 °C and AgNO₃ from 10 to 60 °C. The nitrile oxide produced by the reactions gave 1,3-cycloaddition to the solvent, the rate being determined by independent experiments. The γ-eliminations were found to be second-order processes apart from the reaction of bromide (III) and AgNO₃ with an order higher than one in silver ion. The leaving group effect k_Br/k_Cl was found to be ca. 10³ in the case of AgNO₃ and 1.6 for N-methylmorpholine. The rate of reaction of chloride (II) with amines followed the trend of their pK_a values in water (tentative Brønsted β 0.7). For the reaction of (II) with N-methylmorpholine, k_H/k_D was found to be ca. 2. The results have been interpreted in terms of concerted eliminations, with transition states having strong anionic character with amines and cationic character with AgNO₃. The alternative of a multistep mechanism is presented for the reaction with amines.