Kinetic and Theoretical Studies on Alkaline Ethanolysis of 4‐Nitrophenyl Salicylate: Effect of Alkali Metal Ions on Reactivity and Mechanism

Abstract

Pseudo-first-order rate constants (k(obsd)) for reactions of 4-nitrophenyl salicylate (7) with alkali metal ethoxides (EtOM, M = K, Na, and Li) in anhydrous ethanol have been measured spectrophotometrically. Interestingly, the k(obsd) value decreases significantly as the concentration of EtOM increases. Because the phenolic moiety of substrate 7 would be deprotonated and exist as an anionic form (i.e., 7(-)) under kinetic conditions, the ground-state stabilization of 7(-) through formation of a six-membered cyclic complex with M(+) (i.e., 8) is proposed to be responsible for the decreasing k(obsd) trend. The k(obsd) value at a given concentration of EtOK increases steeply upon addition of [18]crown-6 ether (18C6) up to [18C6]/[EtOK] = 1 in the reaction mixture and then remains relatively constant thereafter. In contrast, k(obsd) decreases upon addition of salts (e.g., LiClO(4) or KSCN) to the reaction mixture, which indicates that M(+) ions inhibit the reaction. However, in the presence of 18C6, the k(obsd) value is independent of the concentration of EtOK but remains constant, which indicates that the reaction proceeds through a unimolecular mechanism in the presence of the complexing agent. Although two conceivable unimolecular pathways (formation of ketene 9 and lactone 10) can account for the kinetic results, the reaction has been concluded to proceed via formation of ketene 9 as the reactive intermediate on the basis of theoretical calculations.

Alkali Metal Ion Catalysis and Inhibition in Nucleophilic Substitution Reactions of 3,4-Dinitrophenyl Diphenylphosphinothioate with Alkali Metal Ethoxides in Anhydrous Ethanol: Effect of Changing Electrophilic Center from P=O to P=S

“…11 In contrast, in the absence of complexing agents, we have shown that M + ion inhibits the reaction by forming complex 7M + which prevents the ketene process.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12] It is known that metal ions behave as a Lewis acid catalyst in acyl-group transfer reactions. Since Lewis acidity increases with increasing positive charges, most studies have been focused on multivalent metal ions (e.g ).…”

Kinetic Study on Nucleophilic Substitution Reactions of 4-Nitrophenyl X-Substituted-Benzoates with Potassium Ethoxide: Reaction Mechanism and Role of K+Ion

“…However, we have reported that M + ions strongly inhibit the reaction. 13 More interestingly, the reaction rate has been found to decrease as the concentration of EtOM increases. 13 Thus, the cyclic complex 4M has been suggested to inhibit the reaction of 4 by retarding the subsequent E1cb reaction which yields an -oxoketene.…”

SNAr Reactions of 1‐Halo‐2,4‐dinitrobenzenes with Alkali‐Metal Ethoxides: Differential Stabilization of Ground State and Transition State Determines Alkali‐Metal Ion Catalysis or Inhibition

“…Metal ions have often been reported to act as a Lewis acid catalyst in nucleophilic substitution reactions of various esters . Reactions involved multivalent metal ions (e.g., Mg 2+ , Mn 2+ , Zn 2+ , La 3+ , Eu 3+ , Co 3+ , etc.)…”

“…have intensively been carried out due to their great Lewis acidity . In contrast, studies on reactions involved alkali‐metal ions ( e.g ., Li + , Na + , and K + ) have attracted much less attention, although alkali‐metal ions are ubiquitous in nature and are known to play important roles in biological processes ( e.g ., Na + /K + pump in mammalian cells)…”