Enantiomeric excess statistical

The deviation in the enantiomeric excess (ee) was small, and 90% of all data were within 40 and 48% ee [325]. A kinetic analysis was made by fitting the experimental data to empirical models by parity diagrams (see Figure 4.55). A statistical model with first-order kinetics for hydrogen gave the best fit. 141 from 170 experiments were... 

The study emphasized the enantiomeric excess (ee) as well as the molar yield of (S)-amine. The statistical analysis provided an insight into the various interac-... 

As for the more usual chemical means of resolutions described earher in this book, there are ways around the statistical Hmitation of a kinetic resolution. It is theoretically possible for a process to involve racemization of the exited states of the chiral compound to give a photostationary state - a steady enantiomeric excess. These processes, sometimes caRed photoderacemizations or photoresolutions axe rare, partly because of the need to ensure that the excited state does not undergo some other reaction but instead reverts cleanly to the ground state. This combination... 

Breaking of symmetry has been reported in stirred l crystallization. NaClOs crystallization in an unstirred solution produces a statistically equal number of I- or d-crystals, but crystallization in a stirred achiral solution can produce 99% crystal enantiomeric excess. This is due to a secondary nucleation phenomenon. Dendritic or needle-like structures on the surface of a crystal break off in a stirred solution. The result is an amplification of the corresponding enantiomeric phase. 

Despite the somewhat moderate enantiomeric excess values associated with methylalumination, the development of a few synthetic protocols and the exploitation of the well-known principle of statistical enantiomeric amplification have led to some efficient, selective, and practical processes for the synthesis of... 

Figure 15.6 Statistical analysis of crystallization from stirred and unstirred sodium chlorate solutions. Upper Scatter plots of the percentage of L-crystals for (a) unstirred and (b) stirred experiments. Each point represents a single experiment. Lower Enantiomeric excess of 1- over D-crystals in (c) un.stirred and (d) stirred experiments. (Adapted from Kondepudi el al., 1990.)...

Figure 1 shows the distribution of the CSPs for HPLC used for the determination of enantiomeric excess (ee) that was reported in the Journal of the American Chemical Society in 2005 (a) and 2007 (b) [2, 9]. These statistics show that more than 90% of the ee determinations by chiral HPLC are carried out by the polysaccharide-based CSPs. 

Of course, the probability is small that at any instant, the enantiomeric mixture at equilibrium is exactly equimolar the absence of observable chirality phenomena, such as optical activity, is the result of rapid cancelations of random statistical fluctuations of activity in the time domain of observation. In other words, although, at any instant, the mixture (with a high degree of probability) has an excess of one enantiomer or the other, under measurement conditions, it effectively contains an equal number of enantiomeric molecules. When 10,000,000 dissymmetric [i.e., chiral] molecules are produced under conditions which favor neither enantiomorph, there is an even chance that the product will contain an excess of more than 0.021 % of one enantiomorph or the other. It is practically impossible for the product to be absolutely optically inactive [12],... 

Direct asymmetric autocatalysis amplified the slight excess of one enantiomer, leading to the enantiopure compound by reaction with diisopropylzinc. It is widely accepted that enantiomerically enriched products must form from achiral precursors merely because of statistical fluctuations. Usually, however, enantiomeric enrichment by fluctuations is very low. Thus, an amplification process of enantiomeric enrichment is required. Detailed kinetic analysis revealed that autocatalysis and inhibition are the major players in asymmetric autocatalytic synthesis. It turned out that tetramers serve as catalyst in the Soai reaction. The transition state for the Soai reaction implicates two molecules of pyrimidine alcohols or alcoxides as the dimeric catalysts and one molecule of prochiral aldehyde substrate (Buono and Blackmond 2003). Further kinetic studies using different ratios of substrate and reagent showed that a tetramer template is used. 

---