Transition states diastereomeric

This type of procedure is referred to as a kinetic resolution since the enantiomers of the racemic substrate exhibit different rates of reaction with the optically active compound, i.e. the diastereomeric transition states that arise from differences in e.g. non-bonded interactions have different free energies. Horeau and Nouaille (1966) estimate that a rate difference corresponding to A AG of the order of 0 2 cal mol at 25°C could in principle be revealed by this method. 

According to the Eyring equation [In P= — AAH RT + AAS R, where P -A (major diol)/ /c(minor diol)], an asymmetric reaction proceeding through a pair of diastereomeric transition states shows a linear temperature effect on the enantioselectivity. However, if the asymmetric... 

The degree of stereoselectivity is usually not large in these reactions and appears to be due to transition-state rather than initial-state interactions. In other words the diastereomeric transition states derived from the enantiomeric substrates have different free energies in the micelle. To this extent the situation is essentially no different to the stereoselectivity which is often observed in non-micellar reactions involving reactions of enantiomeric substrates with a chiral reagent. In some cases it is possible to identify the noncovalent interactions which are responsible for the stereoselectivity (Brown et al., 1981). 

Diastereoisomeric transition states calculated for propene primary insertion in a model of the Ewen achiral metallocenes are shown in Figure 1.20. The two possible diastereomeric transition states correspond to si (Figure 1.20a) and re (Figure 1.20b) insertions of the monomer for the case of a si chain (i.e., a growing chain in which the last monomeric unit has been obtained by a cis addition of a -coordinated monomer molecule) and are suitable for like (isotactic) and unlike (syndiotactic) propagations, respectively.142,143... 

Cha and co-workers reported that the silver nitrate-mediated heterocyclization of the diastereomerically pure aminoallene 192 gave the desired quinolizidine 193 and 194, both possessing the E double bond geometry, as a 7 1 mixture of diastereomers (Scheme 19.36) [43]. Diastereomeric transition states 197 and 198 were proposed. The quinolizidine 201 was expected to form predominantly from 199. They pointed out that interestingly, the cyclization of a 1 1 mixture of diastereomers of 192 gave a 1 2 mixture of 193 and 194. Compound 193 was successfully converted to the target clavepictine A (195) and B (196). 

Figure 1.15. Molecular models of diastereomeric transition states in (R)-BINAP-Ru-catalyzed hydrogenation of P-keto esters.

Figure 1.19. (5)-TolBINAP/(5,5 )-DPEN-RuH2 species and diastereomeric transition states in the metal-ligand bifunctional catalysis the equatorially oriented phenyl substituents in the DPEN ligands are omitted in the transition states 5 -19A and Re-19A (Ar = 4-CH3C6H4 O = Ru ax = axial, eq = equatorial).

Figure 1.31. Reaction mechanism and diastereomeric transition states of titanocene-catalyzed hydrogenation of imines.

To rationalize the enantioselectivity of the TADDOL-catalyzed HDA reaction between Danishefsky s diene and benzaldehyde, eight possible diastereomeric transition states of different regio- and stereochemistry should in principle be considered for comprehensive analysis. The cycloaddition between the model diene and benzaldehyde can take place along two regio-isomeric meta (C1-06, C4-C5 bond formation) and ortho (C1-C5, C4-06 bond formation) reaction channels. For both of these pathways, an exo- and an endo-approach can be formulated (Scheme 11) [64]. 

The actual product from a cycHzation will be determined as the intermediate car-bene commits to a particular diastereomeric transition state. If these diastereomeric transition states are indeed in full thermal equihbrium [13], then computational modeling of the diastereomeric transition states of 20 could allow us to predict which of them would be favored, and thus which diastereomeric product would be formed. 

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