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

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°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 from 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, whereas the observed Brønsted 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. © 1984 American Chemical Society.