Kr Ks ratio

Equation (81)), while the other two C=C double bonds in the structure are intact. Under the same reaction conditions, the racemic carvone is also resolved kinetically with a KR/KS ratio of 33 1. Asymmetric hydrogenation of a,/Tacetylenic ketones to chiral propargylic alcohols is still unavailable. 

With an increase of conversion, the enantiopurity of unreacted (S)-substrate increases and the diastereoselectivity of the product decreases. Using Ru-((S)-binap)(OAc)2, unreacted (S)-substrate was obtained in more than 99% ee and a 49 1 mixture of anti-product (37% ee (2R,iR)) at 76% conversion with a higher kR ks ratio of 16 1 [46]. In the case of a racemic cyclic allyl alcohol 24, high enantiopurity of the unreacted alcohol was obtained using Ru-binap catalyst with a high kR ks ratio of more than 70 1 [Eq. (16)] [46]. In these two cases, the transition state structure is considered to be different since the sense of dia-stereoface selection with the (S)- or the (R)-catalysts is opposite if a similar OH/ C=C bond spatial relationship is assumed. 

In asymmetric synthesis, a chiral compound is synthesized from an achiral precursor in such a way that the formation of one enantiomer predominates over the other.23 The asymmetry of the reaction is induced by the presence of a diastereomeric complex and is a result of the formation of two distinct diastereomeric transition states separated in energy by the amount AAG >0. The ratio of the rate constants for the formation of the two enantiomers, kR and ks, is related to AAG according to equation (2.1.1 ).25 Assuming a kinetically controlled reaction, the kR/ks ratio will be reflected in the relative amount of each enantiomer formed. 

Scheme 4.1 Enantioselective kinetic resolution of a racemate. = rate constants for the individual enantiomers of the substrate, E = enantiomeric ratio, i.e., the ratio between the specificity constants kat/Km for the fast and slow reacting enantiomer. If a racemate is used as substrate, then these concentrations are equal at the start (i.e. zero conversion), and hence E = kR/ks.

Over-all Coefficients. In most practical situations, the ratio kR/ks is not known. Ordinary sampling of the liquids and analysis will give the coordinates Ce and cr on Fig. 5.9, but it is ordinarily impossible to approach the... 

In Case 1 both the ring closure steps (with rate constants ks and kR) are faster than dissociation of the 7r-complexes to reform the alkenol and N-X+ species. Here the [S]/R] product ratio is determined only by the difference in the activation energy leading to the 7is- and rcR-complexes because immediately after these are formed the cyclization occurs. Thus, the chirality is set by the approach of the alkene to the halonium ion. 

Identifying Ks with KM, KT approximately equals the ratio (kuncat KM)/kcat, which is the inverse of termed the proficiency" which has been determined experimentally for a number of systems (see Section 2.3.4 below). If the Kurz equation captured the whole essence of enzyme catalysis, Kr should be proportional to KM)/kcat. 

When light hydrocarbons terminate predominantly as paraffins (kh>>ko), or when a-olefins are rapidly hydrogenated in secondary reactions (ks>kr), we should obtain a light product distribution with a low and constant value of a. We describe below two such systems. A Fe-based catalyst (a-Fe2C>3) at very high H2/CO ratios (-9) gives only C to C5 paraffins with a constant chain growth... 

The synthesis of amino acid esters can be carried out enantioselectively when optically active EBTHI zirconaaziridines are used. After diastereomeric zir-conaaziridines are generated and allowed to equilibrate (recall Scheme 3), the stereochemistry of the chiral carbon center in the insertion product is determined by competition between the rate constants kSSR and ksss for the epimerization of zirconaaziridine diastereomers and the rate constants [EC] and ks[EC] for ethylene carbonate (EC) insertion (Eq. 31) [43]. When kR[EC] and ks[EC] are much greater than kSSR and ksss> the product ratio reflects the equilibrium ratio as shown in Eq. 32. However, the opposite limit, where epimerization is much faster than insertion, is a Curtin-Hammett kinetic situation [65] where the product ratio is given by Eq. 33. 

In Fig. 11, at high concentrations of ethylene carbonate, the rate constants ks[EC] and kR[EC] for insertion into the EBTHI zirconaaziridine 17q are much greater than kSSR and ksss and insertion occurs more rapidly than the equilibrium can be maintained. The product ratio reflects the equilibrium of 17q, where Keq is 17.2 (Eq. 32) [21]. Beak has called this limit a dynamic thermodynamic resolution pathway [66]. In contrast, at the lowest concentration of ethylene carbonate in Fig. 10, the first-order rate constants kSSR and ksss for diastereomer interconversion are comparable to the effective first-order rate constants for insertion. As Keq is known to be 17.2, ks/kR can be calculated the 53% ee of (S)-amino acid ester 19q (Scheme 9) implies that kslkR<0.19 (Eq. 33) and that the rate constant for insertion kR[EC] into the minor diastereomer is at least five times faster than ks[EC] into the major diastereomer. 

On the other hand if the acid is the interesting part then the same two types of reaction can be carried out to produce enantiomerically enriched esters or acids as required. Both these reactions are equilibria. Both are also kinetic resolutions (chapter 28) and it is not usually possible to get both products in high ee. The key statistic is the rate ratio ( E ) of hydrolysis or esterification of the two enantiomers. Since the molecules we shall be using are not natural substrates of lipases, the E value will vary considerably but, as you know from chapter 28, it does not need to be very large for acceptable results. This E is often called ks/kR in chemical kinetic resolution and called the s value in chapter 28. 

An analogous study involving 4-chloronitrobenzene (32) provided a nice picture of how subtly the balance between substitution and reduction is determined by the extent of ion association phenomena. In Figure 2, plots are shown of the dependence of the rate constants for the substitution (ks) and the reduction (kr) processes on the [18-crown-6] [2-PrOK] ratio. As is evident, ks and kr follow opposite trends. 

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