Two‐Phase Oxidations with Aqueous Hydrogen Peroxide Catalyzed by Amphiphilic Pyridinium and Diazinium Salts

Abstract

AbstractAmphiphilic pyridinium and diazinium salts were shown to be effective catalysts in two‐phase (water/chloroform or water/dichloromethane) sulfoxidations and N‐oxidations with hydrogen peroxide under mild conditions. This unprecedented oxidation method utilizes covalent bonding of hydrogen peroxide to a simple pyridinium or diazinium nucleus to increase the lipophilicity of the hydroperoxide species and to subsequently activate it for oxidations in a non‐polar medium. The catalytic efficiency was found to depend on the type of heteroarenium core and on the lipophilicity of the catalyst. Five series of heteroarenium catalysts were prepared and investigated: 1‐Alkyl‐3,5‐dicyanopyridinium, 1‐alkyl‐3,5‐dinitropyridinium, 1‐alkyl‐3‐cyanopyrazinium, 1‐alkyl‐4‐cyanopyrimidinium and 1‐alkyl‐4‐(trifluoromethyl)pyrimidinium triflates (alkyl=butyl, hexyl, octyl, decyl, dodecyl and hexadecyl). Among them, the 1‐octyl‐3,5‐dinitropyridinium and 1‐decyl‐4‐(trifluoromethyl)pyrimidinium triflates were found to be superior catalysts, showing the best stability and the highest catalytic activity, achieving acceleration by a factor of 350 relative to the non‐catalyzed reaction. In contrast to other organocatalytic two‐phase oxidations that use hydrogen peroxide, the presented method is characterized by high chemoselectivity and low catalyst loading (5 mol%) and with the reactions being performed under mild conditions, that is, at 25 °C using diluted hydrogen peroxide and a non‐basic aqueous phase. The catalysts have simple structures and are readily available from commercial materials. Practical applications are demonstratedviathe oxidation of several types of sulfides and amines.magnified image