Impact of sulfur heteroatoms on the activity of quaternary ammonium salts as phase transfer catalysts for nucleophilic displacement reactions

The application of a new class of alkylammonium salts as phase-transfer catalysts was investigated. These salts are tetra(4-thiaalkyl) ammonium bromides, and the key questions of the study focus on how the incorporation of a sulfur atom in the alkyl chains affects the efficacy of the salts as phase-transfer catalysts. Employing the nucleophilic substitution of cyanide for bromide on 1-bromopentane as a model reaction, reaction rate constants and activation energies are evaluated. The kinetic parameters obtained using the tetrathiaallcylammonium salts are compared to those obtained using their tetraalkylammonium analogs. The general trend is that the presence of sulfur in the alkyl chains reduces the reaction rates and increases activation energies. This trend is analyzed both in terms of computational modeling and experimental distribution coefficients to determine the cause of the slower reaction rates. Thiaquats are shown to distribute more into the aqueous phase than traditional quat salts of similar chain length, resulting in lower organic phase concentrations. Quantum calculations indicate stronger ion pairing for the thiaquats, increasing activation energies and slowing reaction rates. Thus, differences in rate enhancements are attributable both to phase distribution and ion pairing effects. (C) 2014 Elsevier B.V. All rights reserved.