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

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 kBr/kCl 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 pKa values in water (tentative Brønsted β 0.7). For the reaction of (II) with N-methylmorpholine, kH/kD 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