Phase-Transfer Reactions Catalyzed by Lipophilic Cryptands and Dicyclohexano-18-crown-6: Dehydrating Effect of Concentrated Aqueous Alkaline Solutions

Article abstract of DOI:10.1021/ja00322a030

A study of how the concentration of aqueous KOH affects the hydration and hence the reactivity of anions (Cl^-, Br^-, I^-, SCN^-, N3^-) in aliphatic nucleophilic substitutions catalyzed by lipophilic <2.2.2,C10>cryptand (1a) and dicyclohexano-18-crown-6 (DCH18C6) (2) under phase-transfer conditions is reported.A comparison with the same reactions performed in classical liquid-liquid PTC and homogeneous anhydrous conditions is also included.Unlike quaternary onium salts, even at the highest KOH concentrations (53percent; ie., conditions in which a_H2O ca. 0), water in the presence of 1a is not completely removed.Residual hydration depends on the nature of the anion and is the highest for anions with localized and/or less polarizable charge, such as Cl^-, Br^-, and N3^-.As a consequence, rate constants noticeably increase in comparison with those found under conventional PTC conditions but do not reach those of anhydrous solutions.The different behavior of cryptates and quaternary salts is discussed on the basis of the different topology of the two systems.Behavior of crown ethers is in between that of quaternary salts and cryptates, since residual hydration in the presence of 53percent aqueous KOH is lower than that of cryptates, whereas anionic reactivity becomes practically identical with that found under anhydrous conditions.