Phase transfer catalysis of uncharged species

Abstract

The principal mechanism of quaternary ammonium phase transfer catalysis (PTC) is based on extraction via a stoichiometric anion-exchange process [1,2]. However, a secondary mechanism has been observed in several systems where the extraction is the outcome of nonstoichiometric hydrogen-bonded complex formed between the substrate and the anion of the quaternary ammonium ion pair (equation 16.1). (16.1) Typical substrates are acidic compounds such as hydrogen halides and pseudohalides, carboxylic acids, phenols, water, hydroperoxides (principally hydrogen peroxide) and ammonia. The nature of the complex formed with a given HY, particularly the magnitude of n, is dependent on the nature of the ammonium cation, the type of solvent and, predominantly, the basicity of the anion X-. As could be expected, quaternary ammonium fluoride salts are particularly active in the formation of hydrogen-bonded complexes. These types of complexes were first observed by researchers who were primarily interested in separation of various acids [3,4] and phenols [5] from aqueous solutions. Compounds formed between HCI and tertiary amines such as Et 3 NH+HCI 2-[6] and even Me 3 NH+H 4 CI s-[7] were isolated and characterized. Similarly to the role of the catalyst in normal phase transfer systems, the hydrogen bonding mechanism results in both the extraction and the activation of the substrate, which leads to some interesting catalytic reactions. This chapter is organized according to the nature of the pro tic substrate and outlines the synthetic scope and mechanism of this less well known segment of phase transfer catalysis. 16.2 Water Quaternary ammonium salts, particularly when ion paired with a hydrophilic anion such as hydroxide, fluoride, chloride or acetate, are highly hydrophilic in nature. Their ability to dehydrate inorganic salts was demonstrated in several phase transfer systems [8,9]. This high affinity of quats to water mole-Y. Sasson et al. (eds.), Handbook of Phase Transfer Catalysis