83897-32-9Relevant academic research and scientific papers
Template Effects. 4. Ion Pairing of Aryloxide Ions with Alkali Cations in 99percent Me2SO: Influence on the Rate of Formation of Benzo-18-crown-6 and of Other Williamson-Type Reactions
Illuminati, Gabriello,Mandolini, Luigi,Masci, Bernardo
, p. 555 - 563 (2007/10/02)
The effect of alkali metal ions on the rate of formation of benzo-18-crown-6 in 99percent Me2SO by cyclization of the conjugate base of o-hydroxyphenyl 3,6,9,12-tetraoxa-14-bromotetradecyl ether has been quantitatively accounted for according to a scheme involving separate contributions from free (ki) and cation-paired (kip) phenoxide ion.The study has included several additional intra- and intermolecular alkylations of phenoxide ions as reference reactions to provide a set of 25 equilibrium constants for the association of five different phenoxides with the five alkali cations.Both Coulombic interaction and coordination with the neutral oxygen donors are important in determining ion pair stability, but the order in all cases is dominated by Coulombic interaction.This suggests contact interaction in the phenoxide-cation pairs, which is also consistent with evidence from the UV spectra.Whereas the rate of formation of B18C6 is depressed by Li+ (kip/ki + (kip/ki = 100).In contrast, in the reference reactions the ion pairs with the alkali are either negligibly reactive or much less reactive than the free anions.The association constants of the alkali cations with B18C6 have been determined under the same conditions.A comparative analysis of the extent of interaction of the cations with the reactant, transition state, and reaction product of the crown ether forming reaction shows that the transition state binds cations more strongly than the reactant or reaction product and reveals that cation interaction with both the negative charge and the neutral donors bear significant contribution to the stability of the ion pair transition state.A rationale for the template effect is presented in terms of proximity effects and chemical effects arising from interaction of the cation with the nucleophilic site of the reactant.
Association of Alkali and Alkaline Earth Cations with Benzo-18-crown-6 and Its Neutral and Negatively Charged Acyclic Analogues in Methanol Solution
Ercolani, Gianfranco,Mandolini, Luigi,Masci, Bernardo
, p. 7484 - 7489 (2007/10/02)
The effect of added alkali (Li, Na, K, Rb, and Cs) and alkaline earth (Ca, Sr, and Ba) metal bromides on the acid dissociation of o-hydroxyphenyl 3,6,9,12-tetraoxatridecyl ether (ArOH) in MeOH at 25 deg C has been investigated spectrophotometrically over a wide range of salt concentration.Except for the Li ion, all the investigated salts increased significantly the apparent acidity of ArOH in a way that could be accounted for by the formation of 1:1 addition compounds of the metal cations with both ArOH and its conjugate base ArO-.A proper analysis of the experimental data yielded the equilibrium constants for the formation of the associated species.Complexation of the given cations by benzo-18-crown-6 (C) was also studied spectrophotometrically under the same conditions.The set of equilibrium constants here reported constitute a proper basis for a systematic discussion of the ligation properties of sexadentate ligands with oxygen donors belonging to different structural types.The cyclic ligand C binds cations more strongly than does its open-chain analogue ArOH.The magnitude of the macrocyclic effect is found to be mainly determined by the size of the cation, rather than by its charge.On the other hand, the advantage of the anionic ligand ArO- over ArOH is highly sensitive to the charge of the cation with little or no dependence upon its size.This suggests that direct contact between the oxide ion and the cation is lacking in the ArO-Mz+ species.
