Stereoselective adsorption

One of the oldest mechanisms of interaction between adsorbed reactant and adsorbed TA has been proposed by Klabunovskii and Petrov [212], They suggested that the reactant adsorbs stere-oselectively onto the modified catalyst surface. The subsequent surface reaction is itself nonstere-ospecific. Therefore, the optically active product is a result of the initial stereoselective adsorption of the reactant, which in turn, is a consequence of the interactions between reactant, modifier, and catalyst. The entities form an intermediate chelate complex where reactant and modifier are bound to the same surface atom (Scheme 14.4). The orientation of the reactant in such a complex is determined by the most stable configuration of the overall complex intermediate. The mechanism predicts that OY only depends on the relative concentrations of keto and enol forms of the reactant,... 

Moreover, it is seen from Table 1.7 that the equilibrium constants at both the nonstereoselective site (fens) and the stereoselective site (fcs) were decreased when the temperature was raised, while concomitantly more nonstereoselective as well as stereoselective adsorption sites became available or accessible at elevated temperatures. The larger number of accessible binding sites at elevated temperatures has been explained by solvation effects The sorbent surface and thus the interaction sites are solvated to a lesser degree so that they are better available for solute interactions [51]. 

It has been known for decades that metal ions can affect the course of organic reactions [lb], A possible role of minerals in the molecular evolution process was critically reviewed by Lahav [2]. The hypothetical involution of minerals in this process encompasses a variety of reactions, from mere adsorption to template-directed synthesis (e. g., of amino acids and sugarlike molecules), stereoselective adsorption of organic molecules, and catalytic condensation of peptides. 

Using labeled Z)Z-Ala HCl these results were confirmed by obtaining a differential adsorption of 12-20% (percent adsorption on chloroform solution was studied using ( LC In this case t/-quartz preferentially adsorbed the i-Ala ester and /-quartz the D-Ala ester with enantiomeric enrichments of up to 1.5-12.4%. Such large differential enrichments are very doubtful and can indicate possible artifacts, although these data were eonfirmed later by Furoyama and coworkers by stereoselective adsorption on quartz According to Furoyama and... 

But Maier and coworkers could not obtain positive results for stereoselective adsorption. Silica gel was imprinted with chiral 2-(4-MeOPh)-3,3-dimethylbutan-l-ol, but after its removal from the matrix the resulting imprinted silica gel was unable to resolve the same racemic mixture of the... 

Heard and Suedee [40] reported the use of cellulose carbamates in stereoselective adsorption and membrane permeation of racemate and individual enantiomers of propranolol. In this study, three carbamates were used including cellulose tris A-(phenyl carbamate) (Ri) cellulose tris A-(3,5-dichlorophenyl carbamate) (R2) and cellulose tris A-(3,5-dimethyl-phenyl carbamate) (R3) (Fig. 4). The S-enantiomer of propranolol was found to be preferentially bound with all the three carbamates used in this study. Further, incubation at 32°C (pH 7.4) increased binding compared to room temperature. The dimethylphenyl carbamate derivative (R3) had the highest sterereoselective binding compared to the dichlorophenyl (R2) and phenyl (Ri) forms. The ratio R to S enantiomer flux values for Ri, R2, and R3 were 2.65, 2.31, and 1.35, respectively. In the control experiment with no carbamates, however, no stereoselective adsorption was observed. 

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Chemical relaxation methods can be used to determine mechanisms of reactions of ions at the mineral/water interface. In this paper, a review of chemical relaxation studies of adsorption/desorption kinetics of inorganic ions at the metal oxide/aqueous interface is presented. Plausible mechanisms based on the triple layer surface complexation model are discussed. Relaxation kinetic studies of the intercalation/ deintercalation of organic and inorganic ions in layered, cage-structured, and channel-structured minerals are also reviewed. In the intercalation studies, plausible mechanisms based on ion-exchange and adsorption/desorption reactions are presented steric and chemical properties of the solute and interlayered compounds are shown to influence the reaction rates. We also discuss the elementary reaction steps which are important in the stereoselective and reactive properties of interlayered compounds. 

In Equation 1.15, q represents the adsorbed amount of solute, ns and qs are the saturation capacities (number of accessible binding sites) for site 1 (nonstereoselect-ive, subscript ns) and site 2 (stereoselective, subscript s), and fens and bs are the equilibrium constants for adsorption at the respective sites [54]. It is obvious that only the second term in this equation is supposed to be different for two enantiomers. Expressed in terms of linear chromatography conditions (under infinite dilution where the retention factor is independent of the loaded amount of solute) it follows that the retention factor k is composed of at least two distinct major binding increments corresponding to nonstereoselective and stereoselective sites according to the following... 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

Liu, H. B. and Hamers, R. J. Stereoselectivity in molecule-surface reactions Adsorption of ethylene on the silicon(OOl) surface. Journal of the American Chemical Society 119, 7593 (1997). 

This stereoselectivity is not a result of adsorption on the electrode but stems from the nature of the benzylic cation formed as an intermediate, because the same oxazolines are formed in a similar ratio in the homogeneous reaction of 54 with Mn(OAc)3. 

The stereochemistry of acetoxy group addition to tram-4, 4 -dimethoxystil-bene 52 is analogous. However, the recent finding 101 that the analogous reaction, anodic addition of two benzoyloxy groups across a double bond, produces the same mixture of products from both cis- and frans-stilbene (stereochemistry the same as in equation (31) ), would seem to make the adsorption requirement unnecessary. The stereoselectivity is then explained on the basis of formation of the thermodynamically most stable acetoxonium ion in a stepwise oxidation mechanism1011. 

The use of cyclodextrins as the mobile phase components which impart stereoselectivity to reversed phase high performance liquid chromatography (RP-HPLC) systems are surveyed. The exemplary separations of structural and geometrical isomers are presented as well as the resolution of some enantiomeric compounds. A simplified scheme of the separation process occurring in RP-HPLC system modified by cyclodextrin is discussed and equations which relate the capacity factors of solutes to cyclodextrin concentration are given. The results are considered in the light of two phenomena influencing separation processes adsorption of inclusion complexes on stationary phase and complexation of solutes in the bulk mobile phase solution. 

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