Base-catalyzed Proton Abstraction from β-(p-Nitrophenoxy)-propiophenone in the Presence of Single-chain, Double-chain and Triple-chain Ammonium Aggregates

Abstract

Proton abstraction from β-(p-nitrophenoxy)propiophenone(β-NPP(5)) catalyzed by hydro- xamate(C₁₂-BHA (6) chol-HA(7)) and imidazole(cholest-Im(8)) anions was studied in the presence of aqueous aggregates of single-chain (CTAB(1)), dpuble-chain(2CnN⁺2C₁(2), n=12-18) and triple-chain (TMAC(3)) ammonium amphiphiles. These three types of ammonium salts gave rise to quite different aggregate morphologies, as "shownin Table 1. The rates of the base-catalyzed proton abstraction were enhanced by 100-4000 foldS in the presence of these hydrophobic aggregates (Figs. 2, 3). The extent of rate-enhancements was drastically changed by the combination of the anionic reagents and the cationic aggregates. The long-chain hydroxamate(C, -BHA): showed proton- abstraction reactivities which reflected the hydrophobic microenvironment of the respective aggregates TMAC(3)>2CnN⁺2C₁>CTAB(1). Cholic acid-derived hydroxamate (chol-HA) showed a small reactivity in the ammonium bilayer membranes, probably because it did not fit in the: bilayer structure (Table 2). The rate of proton abstraction by C₁₂-BHA(6) in 2CnN⁺2C₁, bilayer aggregates was con-siderably influenced by the phase-transition (Table 3) of the bilayer, as shown in Fig. 5. In contrast, the corresponding change was not observed in the case of spontaneous (hydroxide-catalyzed) proton abstraction.