Uncharged Catalysts The Amines

Resin bound quaternary ammonium ions have also been used as catalysts in a variety of reactions conducted in nonpolar media. Such reactions as cyanohydrin formation, cyanoethylation, and the benzoin condensation, were all achieved in the early 1950 s [5, 21] and the method was well established by the 1960 s [22]. In most of the anionic reactions, a resin bound quaternary ammonium cation chloride anion pair was converted to the hydroxide form and then used directly in the reaction. 

One fact about the use of quaternary ions as phase transfer catalysts should be noted. In general, the large, lipophilic quaternary ions are soft in the HSAB sense [25]. As a consequence, the quat tends to pair with the softest anion available in solution. If both iodide ions and hydroxide ions were present, for example, the quat would pair with iodide. If reaction with hydroxide was desired, the catalyst would be poisoned by the presence of iodide. The source of an ion such as iodide could be from the catalyst originally added or it could be the leaving group in the substitution reaction. The choice of reaction conditions should therefore include a consideration of cation, anion, nucleophile and nucleofuge. 

There are several reports in the literature of tertiary amine catalysis of reactions which appear to be of the phase transfer type. The first such example is the reaction of potassium benzoate with benzyl chloride to give benzyl benzoate, reported in a German patent issued in 1913 to be catalyzed by triethylamine [26]. Merker and Scott in 1961 utilized in situ quaternary ammonium carboxylate formation to facilitate the same esterification reaction [27]. Hennis and coworkers rediscovered and clarified amine catalysis in the reaction of benzyl chloride with potassium acetate 

Reeves and coworkers have recently published examples of tertiary amine catalyzed nitrile formation [29], thiocyanate formation [30], alkylation [31], and carbene formation [31 ]. These reactions almost certainly involve catalysis by quaternary ammonium ions generated in situ. 

More recently, Normant and coworkers have reported catalysis of the reaction between potassium acetate and benzyl chloride in acetonitrile by polyamines in a two phase system [28]. It seems likely that the catalytic activity reported by Normant et al. is related to the earlier alkylations discussed above. The authors state in their communication, however, their finding that ... quaternary ammonium salts corresponding to the diamines do not activate anions under the experimental conditions used. . In the reaction referred to here, it seems likely that the diamines are playing a dual role. The diamines are probably assisting in the solubilization of the solid (and relatively insoluble) potassium acetate by chelation of the potassium cation and the homogeneous reaction is then probably catalyzed by the quaternary ion formed in situ. That the catalytic activity of the amine depends on the hardness of the cation (the harder the cation, the less catalytic activity) [28] seems to accord with this interpretation although lattice energy differences cannot be discounted. 

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