Asymmetric Ionic Systems

Figure 4 shows simulated binodal curves for asymmetric ionic systems. Coexistence curves are shown for X = 1,0.75,0.50, and 0.25, where X = cr+/cr is the ratio of the radius of cations and anions and therefore provides a measure of the asymmetry of the system. The results of our simulations indicate that as X decreases from 1 (the restricted model) to 0.05 (the most asymmetric system considered in our simulations), both T and p decrease. For nearly symmetric electrolytes (e.g., X = 0.75), the effect of 7 is relatively... 

Scheme 22.13 Proline/polyelectrolyte and proline/modified polystyrene ionic systems and their use as organocatalysts in asymmetric aldol reactions.

The first reported use of ionic chiral auxiliaries in asymmetric synthesis was the work of Gudmundsdottir, Scheffer, and Trotter on the di-Tt-methane photorearrangement of the dibenzobarrelene system 33 [31]. Of a number of chiral auxiliaries tested,best results were obtained with prohne fert-butyl ester, which led to dibenzosemibullvalene derivative 34 in an enantiomeric excess of 95% at 40% conversion following diazomethane workup. By determining the ab-... 

On the basis of encouraging work in the development of L-proline-DMSO and L-proline-ionic liquid systems for practical asymmetric aldol reactions, an aldolase antibody 38C2 was evaluated in the ionic liquid [BMIM]PF6 as a reusable aldolase-ionic liquid catalytic system for the aldol synthesis of oc-chloro- 3-hydroxy compounds (288). The biocatalytic process was followed by chemical catalysis using Et3N in the ionic liquid [BMIM]TfO at room temperature, which transformed the oc-chloro-(3-hydroxy compounds to the optically active (70% ee) oc, (3-epoxy carbonyl compounds. The aldolase antibody 38C2-ionic liquid system was also shown to be reusable for Michael additions and the reaction of fluoromethylated imines. 

Instead of an aqueous alkaline base, organic-soluble, non-ionic phosphazene bases such as BEMP and BTPP have been used with the third-generation catalyst 4d to realize a homogeneous system for the asymmetric alkylation of 1 (Scheme 2.6), which is sometimes advantageous from a practical viewpoint [24]. 

It should be noted that not only the Lewis base but also typical Lewis acid roles can be emulated by organocatalytic systems. The proton is arguably the most common Lewis acid found in Nature, and these exist in two forms classified by the nature of the hydrogen bond polar covalent (RX-H) and polar ionic (RX+H-Y ). In the former case, in asymmetric transformations the chiral information is dictated by the chiral anion, whilst in the latter case the anion is non-chiral and the enantioselectivity is introduced by a chiral ligand (usually an amine base), which complexates the proton. This activation is discussed more extensively in Chapter 7. 

As mentioned earlier, one can have a range of parent compounds which can form such mixed systems which show a wide range of concentration and they have well-defined phase diagram as a function of concentration.24 Depending upon the ionic radii of the parent compounds, their FE or AFE transition temperature and structural details, the phase diagram (Figure 1) may be symmetric (isostructural) or asymmetric (otherwise). 

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