Mechanisms of acid decomposition of dithiocarbamates. 5. Piperidyl dithiocarbamate and analogues

In this work, the acid cleavage at 25 degrees C in 20% v/v aqueous ethanol of a series of analogues of piperidine dithiocarbamate X(C2H4)2NCS2(-) (X = CH2, CHCH3, NH, NCH3, S, O) was studied. The pH-rate profiles were obtained in the range of H(o)-5 and pH 5. They all presented a dumbell shaped curve with a plateau from which the pH-independent first-order rate constant k(o) (or the specific acid catalysis k(H)) was calculated, in addition to the acid dissociation constant of the free (pKa) and conjugate acid (pK(+)) species of the DTC. LFERs of the kinetically determined pKa and pK(+) versus pKN (pKa of parent amine) were used to characterize the reactive species and the structure of the transition state of the rate-determining step. For X = CH2, CH3CH the values of k(H) agree with those of alkDTCs in the strong base region of the Br?nsted plot of log k(H) versus pKN where the transition state is close to a zwitterion formed by intramolecular water-catalyzed S-to-N proton transfer of the dithiocarbamic acid. However, when X = NH, CH 3N, O, S, the reactive species is the DTC anion, which is as reactive as an arylDTC, and similarly, the pK(+) values correspond to a parent amine that is about 3-4 pK units more basic. The solvent isotope effect indicated that the acid decomposition of these dithiocarbamate anions is specifically catalyzed by a Hydron anchimerically assisted by the heteroatom through a boat conformation.