Preparation of phase transfer catalysts by column ion exchange

Abstract

While the application of quaternary ammonium bromides and iodides in phase transfer catalysis [1] has the disadvantage of catalyst poisoning [2] owing to the lipophilicity of the heavy halogen atoms, the use of onium salts with hydrophilic anions is often limited due to lack of efficient synthetic methods. Most of the current syntheses are based either on quaternization of tertiary amines [3] or on reaction of the substituted ammonium bromides (or iodides) with wet silver oxide followed by neutralization of the resulting hydroxides with the appropriate acids [4]. Since the first method is inapplicable to many chlorides as well as to salts of weak acids, and the latter one is impractical for large scale preparations, we found it imperative to search for a general and inexpensive synthesis of the required phase transfer catalysts. The facile formation of quaternary ammonium bromides and iodides by reaction (1) and the high affinity of these compounds to anion exchange resins [5] suggest the application of an ion exchanger for the process described by eqn. (2).