Rates and Equilibria of the Reaction of 2,4,6-Triphenylthiopyrylium Ion with Piperidine and Morpholine in Me2SO. An Unusual Proton Transfer to a Nitrogen Base

Article abstract of DOI:10.1021/ja00335a036

The complete set of kinetic and equilibrium constants of the reaction of 2,4,6-triphenylthiopyrylium ion (1) with piperidine and morpholine has been obtained in Me2SO at 25 deg C.The reaction involves the formation of both the corresponding 2H- and 4H-thiopyrans, which equilibrate to form only the more stable 2H adduct.The kinetic data are consistent with a two-step process wherein the proton transfer form the protonated 2H- and 4H-thiopyran intermediates to the amine is the rate-controlling step.The thermodynamically favored proton transfer to the neutral adducts by the solvated proton shows a rate below the diffusion limit, whwreas the observed Broensted coefficient would indicate a diffusion-controlled process.This behavior is discussed in terms of the Eigen mechanism.The factors affecting the nucleophilic addition are discussed, and a comparison is made with the previously reported reaction of 1 with primary amines.