Iminium-Ion Formation and Deuterium Exchange by Acetone in the Presence of Pyrrolidine, Pyrazolidine, Isoxazolidine, and Their Acyclic Analogues

Article abstract of DOI:10.1021/ja00525a032

Equilibrium constants for iminium-ion formation in the reaction of acetone in aqueous solution at 35 deg C with pyrazolidinium, isoxazolidinium, O,N-dimethylhydroxylammonium, and N,N'-dimethylhydrazinium ions were found to be 9.33, 8.96, 0.117, and 0.057 M^-1, respectively.The kinetics of hydrolysis of the iminium ions were studied in every case except that of the N-isopropylidene-O,N-dimethylhydroxylammonium ion, whose hydrolysis is too fast to follow by the techniques used with the other iminium ions.The rate of hydrolysis of the N-isopropylidenepyrazolium ion is independent of the pH from about pH 3 to 6; it is hydrogen ion catalyzed at lower pHs and hydroxide ion catalyzed at higher pHs.The rate of hydrolysis of N-isopropylideneisoxazolidinium ions is pH independent from pH 0.5 to about 2, increases until about pH 4, remains pH independent until pH 6.5, and has become too fast to measure above pH 8.Both reactions are general base catalyzed in all the buffers studied.A mechanism is described to fit the kinetics of each of these reactions.The dedeuteration of acetone-d₆ was studied in pyridine buffers in the presence of each of the four hydrazine and hydroxylamine derivatives and also in the presence of the dimethylammonium and pyrrolidinium ion (which was also studied in 3-dimethylaminopropionitrile buffers).All six of these secondary ammonium ions catalyze the dedeuteration by transforming the acetone-d₆ to an iminium ion that is dedeuterated by pyridine more rapidly than the ketone is.The iminium-ion formation is a relatively rapid eqiuilibrium in all cases except that of pyrrolidinium ions, where the intermediate iminum ion loses deuterium and hydrolyzes at comparable rates, and possibly the case of dimethylammonium ions, where the amount of catalysis via iminium-ion formation is too small to reveal mechanistic details.The effect of structure on the efficiency of catalysis of dedeuteration via iminium-ion formation is discussed.