On the possibility that cyclodextrins´ chiral cavities can be available on AOT/ n-heptane reverse micelles. A UV-visible and induced circular dichroism study to characterize this novel nanoreactor.

OSCAR FERNANDO SILVA; JUANA SILBER; RITA HOYOS DE ROSSI; MARIANA ADELA FERNÁNDEZ; NÉSTOR MARIANO CORREA

Los Cocos, Córdoba, Argentina

Conferencia; 9th Latin American Conference on Physical Organic Chemistry; 2007

Latin American Comitee

Several surfactants are able to aggregate in nonaqueous solvents to yield reverse micellar systems (RMs). Small solute particles can be located in three different compartments: (a) the external organic solvent; (b) the micellar interface formed by a surfactant monolayer and (c) the internal water pool. Subsequently, these systems contain aqueous microdroplets entrapped in a film of surfactant and dispersed in a low-polarity bulk solvent. Cyclodextrins (CDs) are a group of well-known cyclic oligosaccharides that are capable of forming reversible noncovalent complexes with a wide variety of guests. The formation of reverse micelles (RMs) of sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) in n-heptane including two different b -CD derivatives (hydroxypropyl-b -CD, hp-b -CD, and decenyl succinyl-b -CD, Mod-b -CD) is reported. Both cyclodextrins can be incorporated into AOT RMs in different zones within the aggregate while b -CD is not. Using UV?Vis and induced circular dichroism spectroscopy (ICD) and different achiral molecular probes (some azo dyes, p-nitroaniline and ferrocene) it was possible to determine that Mod-b -CD is located with its cavity at the oil side of the AOT RM interface while for hp-b -CD the cavity is inside the RMs water pool. Among the molecular probes used, methyl orange (MO) was the only one which gave the ICD signal when was dissolved in the AOT RMs with hp-b -CD, so a detailed study of MO behavior in homogeneous media was also performed to compare with the microheterogeneous media. From the MO absorption spectral changes with the hp-b -CD concentration the association equilibrium constant in pure water (K11W) and inside the RMs (K11RM) were computed. The results show that K11W is almost 10 times larger than the value inside the RMs. The latter can be explained considering that MO resides anchored to the RM interface through hydrogen bond interaction with the hydration bound water. This study shows for the first time that the cyclodextrin chiral cavity is available for a guest in an organic medium such as the RMs, results that seems to us to be quite important, specially to take advantage of this system as nanoreactors in asymmetric syntheses.