Nucleophilic type, racemization process

It should be stressed that Eq. (66) is not the sole representation of kinetics of nucleophile-activated racemization of optically active silanes. Silylonium complexes often appear to be thermodynamically more stable than the substrate in the reaction systems, i.e.,, [Nu] > -l. The steady-state conditions are not met, which usually leads to complex kinetics. On the other hand, the different kinetic laws observed in these processes (287,288) are related to other types of mechanisms, which are discussed later. 

Hence, a reaction of Type I will involve a racemic or achiral/me,t(9 nncleophile which will react enantioselectively with an achiral acyl donor in the presence of a chiral catalyst, while on the other hand, a reaction of Type II will associate an achiral nncleophile and a racemic or udm lmeso acyl donor in the presence of a chiral catalyst. In both cases, when a racemic component is implicated the process constitntes a KR and the maximum theoretical yield of enantiomerically pure product, given perfect enantioselectivity, is 50%. When an achiral/mera component is involved, then the process constitutes either a site-selective asymmetric desymmetrisation (ASD) or, in the case of tt-nucleophiles and reactions involving ketenes, a face-selective addition process, and the maximum theoretical yield of enantiomerically pure product, given perfect enantioselectivity, is 100%. 

This polarimetric method was made even more general by utilizing chiral HPLC techniques. The L-UNCAwas dissolved in the solvent at a concentration of 0.33 M at 20 °C. The tertiary amine (1.5 equiv) was added at time zero. The solution was allowed to stand for an experimentally determined delay time, during which the only process that can occur was epimerization, since there is no nucleophile present. The delay time was determined after carrying out several experiments with different delay times and chosen so as to fall within or just after the first half-life for racemization. At the end of the delay period, benzylamine was added. Benzylamine is a very powerful nucleophile that reacts virtually instantly (regardless of the type of activation) with the resulting mixture of l- and d-UNCAs to form the benzyl amides and quench the epimerization process. Thus, a snapshot of the ratio of l/d activated intermediates at the instant of benzylamine addition was obtained by measurement of the l/d ratio of the benzyl amide products. 

The first strategy involves discrimination between enantiotopic leaving groups (Type A). In the second approach, two enantiomers of a racemic substrate converge into a meso-n-al y complex wherein preferential attack of the nucleophile at one of either allylic termini leads to asymmetric induction, a process that may be referred to as a dynamic kinetic enantioselective transformation (Type B). The third requires differentiation between two enantiotopic transition... 

The importance of fluorinated organic componnds both in medicinal chemistry and biochemistry has resulted in much recent attention towards efficient carbon fluorine bond formation [30]. The reactions developed include a very successful electrophilic asymmetric mono-fluormation of 1,3-dicarbonyl compounds [31]. A nucleophilic variant was also investigated. In this context, the groups of Togni and Mezzetti have established that ruthenium Lewis acids could efficiently catalyze fluorination reactions [32]. In the presence of [Ru(l,2-bis(diphenylphosphino)ethane)2Cl][PF6] (8) (10 mol%), fert-butyl iodide reacted at room temperature with TIF (1.1 equiv.) to yield fert-butyl fluoride (84% yield). This reaction was extended successfully to a range of organic halides (Entries 1-3, Scheme 10.19). The use of the chiral complex [Ru((lS,2S)-N,N bis[2-diphenylphos-phino)benzylidene]diaminocydohexane))Cl][PF6] (9) showed modest chiral induction at the outset of the reaction (Entry 4, Scheme 10.17). The near-racemic mixture obtained at completion points to an SNl-type process in this nucleophilic halide... 

Some nucleophilic substitution reactions that seem to involve a borderline mechanism actually do not. Thus, one of the principal indications that a borderline mechanism is taking place has been the finding of partial racemization and partial inversion. However, Weiner and Sneen have demonstrated that this type of stereochemical behavior is quite consistent with a strictly Sn2 process. These workers studied the reaction of optically active 2-octyl brosylate in 75% aqueous dioxane, under which conditions inverted 2-octanol was obtained in 77% optical purity. When... 

Most of the asymmetric allylic substitution processes start from racemic allylic components rac-l-R (where for 1-R, R designates the general group of the compound, e.g. 1-Me is 1 with R = Me), which in the absence of chiral ligands form meso complexes of the type 2 with palladium(O). Since a nucleophile can attack at either of the two ends of the allylic component, the enantiomers 3-R and ent-3-R are formed. The degree of the enantioselecti-vity of a reaction depends on how well a chiral ligand in 2 can direct the attack of the nucleophile (Nu) to one of the two allylic termini. 

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