Optical yields surface conditions

For the Pt/cinchona catalysts only preliminary adsorption studies have been reported [30]. From the fact that in situ modification is possible and that under preparative conditions a constant optical yield is observed we conclude that in this case there is a dynamic equilibrium between cinchona molecules in solution and adsorbed modifier. This is supported by an interesting experiment by Margitfalvi [63] When cinchonine is added to the reaction solution of ethyl pyruvate and a catalyst pre-modified with cinchonidine, the enantiomeric excess changes within a few minutes from (R)- to (S)-methyl lactate, suggesting that the cinchonidine has been replaced on the platinum surface by the excess cinchonine. 

Asymmetric induction during the reduction of 4-(48) was observed when a surface-modified carbon cathode was used.70 Optical yields were low but the effect of the chiral amino acid bound to the carbon surface was proved to be a true surface phenomenon. Induction of chirality by homogeneous rather than surface-bound agents has also been studied.71 All the isomeric acetylpyridines (48) were reduced in the presence of three different chiral alkaloids. Both carbinol products 2- and 4-(49) were shown to possess induced chirality, but the 3-carbinol (49) had none under any of the conditions tried. More rapid protonation of the intermediate was proposed to account for the lack of induced chirality. Optimization of optical yields was done.72 The pinacols (50) formed along with 49 were found to have no induced chirality. Optical yields have been as high as 50%.73 The role of electroabsorption was found to be important in the reduction of 2-(48).74 Product distributions were noted as a function of surfactant present in the electrolyte, carbinol 49 being favored... 

Solid catalysts can be used, after being impregnated with chiral modifiers such as amines and a-hydroxy acids. Ketones having a second functionality either a or /3 to the carbonyl and able to coordinate to the metal center give best optical yields. This suggests that direct chelation of the substrate to the metal surface occurs. The reaction is used to produce a- and /3-hydroxy acids or esters and to reduce /3-diketones. Rather drastic conditions of P and T limit its scope to simple compounds. 

An interesting phenomenon in water-oil-amphiphile systems is the presence of self-assembled arrays of amphiphiles (surfactants) called micelles. From 1948 to 1950, Philip Alan Winsor reported that upon simple mixing (i.e., without the need for high shear conditions), oil, water, and amphiphiles yielded clear, macro-scopically homogeneous single phases which he termed type IV systems (Winsor, 1948, 1950). The term microemulsion was introduced later by Jack H. Shulman, a Columbia University chemistry professor, to denote these thermodynamically stable optically isotropic, transparent oil-water-amphiphile dispersions (Shulman et al., 1959). Type IV systems contain small droplets of one liquid dispersed within the other, with a self-assembled layer of surfactant molecules (micelles) along the interface between the two phases. The spontaneous self-assembly of the micelle is driven by the thermodynamic tendency to minimize the surface tension between the water and the oil in the presence of the amphiphile (Hoar and Shulman, 1943). 

Lipid monolayers have also been studied by surface rheology, which yields complementary information. The present author is not aware of systematic studies in which optical and rheological investigations have been ceirrled out on one cuid the same sample under the same conditions. 

It has, for example, been found that during the very slow coalescence of Ag inside the cavities of a Nafion membrane, the smallest clusters could be oxidized by the protons H3O+, which are highly concentrated at the surface. In contrast, when, by coalescence, the clusters reach the critical nuclearity for which their potential is higher than °(H30 /H2) = 0 Vnhe, they escape corrosion and are observed by optical absorption. Numerical simulation of the kinetic signal, including the cascade of coalescence reactions (Eq. 11) and of oxidation reactions (Eqs. 35 37), yields the value n = 8 for the upper limit of nuclearity of clusters oxidized by HjO+.t " Therefore, °(Ag,o+/Agio) > °(H30+/H2) = 0 Vnhe (Fig. 11). Note that such a corrosion by H3O+ was not observed under conditions of free diffusion of the clusters, as in solution,because the coalescence enables the clusters to grow much faster up to the supercritical nuclearities. 

The adsorptive selectivity of the gels may be influenced to a certain extent by the preparative conditions. One may, for instance, produce silica gels which adsorb a specific dye of characteristic molecular shape and charge distribution. This is done by dispersing this dye in the silicate solution and the precipitating the gel in the presence of the dye [2, 4, 9]. Similar experiments have been carried out with optically active compounds ([3], see also [1]). The silica gel surface may also be modified in a specific way by adsorbing on it appropriate substances this yields preparations with completely new adsorptive properties [14, 19]. 

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