Catalyzed Cyclization of 2-((3-Halopropyl)oxy)phenoxide Ions in Zwitterionic and Nonionic Micelles

Abstract

Cyclization of 2-((3-halopropyl)oxy)phenoxide ion (halo = Br, I; PhBr7 and PhI7, respectively) in micelles is a model for SN2 reactions of nucleophilic anions at micelle−water interfaces. Sulfobetaine micelles of dodecyldimethylammoniopropanesulfonate (C12H25N+(CH3)2(CH2)3SO3 -, SB3-12) modestly increase rates of cyclization of PhBr7 and PhI7, and the effect is larger for the latter. The relative rate constants k I/k Br are indicators of medium polarity and their values indicate that the surface polarity of micellized SB3-12 is slightly lower than that of a trimethylammonium micelle and significantly lower than that of water. Changes in 1H chemical shifts of SB3-12 show that the micellar rate effects are not due to changes in micellar structure. Nonionic micelles of polyoxyethylene(23) dodecyl ether (C12H25(OCH2CH2)23OH, Brij 35) modestly speed up cyclization, but reaction appears to be in the water-rich palisade layer of the micelle in conditions such that the distinction between aqueous and micellar pseudophases is uncertain.

Cyclizations of 2-(ω-Bromoalkyloxy)phenoxide Ions in Dicationic Surfactants

“…k obs generally increases monotonically with increasing surfactant concentration and becomes constant when the substrate is fully micellar bound 9a. In the case of high binding constant and at increasing [D], k obs should remain constant provided that the micelle structure does not change, affecting k ‘ m .…”

“…Kinetics. Reactions were followed in deionized, distilled, CO 2 -free water as described 9a. Solutions of 1 were not used until 24 h had passed after the preparation.…”

“…Cyclizations are intramolecular reactions and their rate constants only depend on medium effects because they are first-order processes. In particular, the cyclization reaction of 2-(3-bromopropyloxy)phenoxide (PhBr7), which gives a seven-member cyclic ether, reported in Scheme , has been studied both in homogeneous and in micellar solutions . Being an S N 2 process where the charge in the transition state is dispersed relative to substrate, it is affected by medium polarity; micellar-induced changes in the rate constant of the fully micellar bound substrate can give useful information on the properties of the micellar surface.…”

Molecular Dynamics Simulations of Specific Anion Adsorption on Sulfobetaine (SB3-14) Micelles

“…Zwitterionic-based materials have many applications because they can be designed with unique and tuned properties, they are relatively easy to obtain and synthesize, and they are quite versatile. − Indeed, the presence of cationic and anionic groups with charge densities that can be tuned and separated by controlled distances and spacers provides a very large number of designing possibilities of new zwitterionic materials. Interestingly that in spite of being intrinsically neutral, zwitterionic micelles and surfaces can adsorb ions, preferably anions. − This adsorption generates a negative electrical charge at the surface of the micelles and causes the incorporation of cations in the micellar pseudophase. − The increase of ion concentration has been explored in the catalysis of several chemical reactions and other processes (e.g., acid dissociation) mediated by zwitterionic micelles. − The adsorption of anions is specific in the sense that weakly hydrated anions such as large monovalent species (PF 6 – , ClO 4 – ) are adsorbed preferably to strongly hydrated anions (F – , OH – ). , This ion specific adsorption followed by the incorporation of cations into the micellar pseudophase due to electrostatic effects is called chameleon-like nature of zwitterionic micelles. − …”

Effects of Nonionic Micelles on Dephosphorylation and Aromatic Nucleophilic Substitution

“…However, nonionic surfactants typically have higher molecular weights than ionic surfactants and much longer extended chains. For example, with 0.1 M surfactant and an assumed aggregation number N agg = 100 the centercenter distance is 94Å, and for N agg = 1000 it is 212Å, which is only just outside the sum of the radii of spherical micelles with an extended chain length of approximately 100Å, e.g., for C 12 E 23 , which shows the weakness of distinctions between micellar and aqueous pseudophases in solutions of nonionic surfactants (27).…”