Host-guest Interactions of Cucurbituril with N-quaternized-4-(p-substituted Benzoyl)pyridinium Cations and Control of the Ketone to Gem-diol Equilibrium
N-Methyl-4-(p-substituted benzoyl) pyridinium cations show affinity towards cucurbituril (CB) and they form either endo or exo complexes. The mode of complexation is controlled by solvent polarity and also by the chemical identity of the group that is attached on the positively charged nitrogen atom. For example, the N-methyl derivative prefers the endo form in water and the exo form in DMSO. The N-benzyl analogue prefers the exo form in both water and DMSO. Interestingly, in water, the N-methyl-4-(p-substituted benzoyl) pyridinium cations exist in equilibrium of their keto and gem-diol forms, whereas the ratio of the gem-diol versus the keto form is controlled by the p-substituent: with electron withdrawing substituents (e.g., nitro, formyl) the gem-diol form dominates. However, in the presence of CB, the stabilization realized by having the keto group inside the hydrophobic interior of CB is greater than the stabilization realized via hydrogen bonding of the two hydroxyl groups of the gem-diol with water, shifting the equilibrium towards the keto form
A. Thangavel et al., "Host-guest Interactions of Cucurbituril with N-quaternized-4-(p-substituted Benzoyl)pyridinium Cations and Control of the Ketone to Gem-diol Equilibrium," Abstracts of Papers of the American Chemical Society, vol. 235, American Chemical Society (ACS), Apr 2008.
235th ACS National Meeting (2008: Apr. 6-10, New Orleans, LA)
Article - Conference proceedings
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