Explanation of Enantioselectivity

Sutherland et al. developed the Template model based on a geometric explanation of enantioselectivity assuming adsorption of the cinchona alkaloid modifier and methyl pyruvate reactant on the surface of a Pt(III) facet such that the alkaloid mochfier is adsorbed by its quinoline ring in such a way that there is a place for adsorption of the pyruvate molecule to form an intermediate active complex which produces, after hydrogenation of the C=0 bond in pyruvate, either the (/ )-(-)-methyl lactate, in the case of using Cnd as modifier, or the (S)-(+)-methyl lactate using Cn as modifier (see the modifier structures in Scheme 5.20.). 

To date, a fully satisfactory explanation of enantioselective Mizoroki-Heck cyclizations that follow a neutral reaction pathway remains elusive. Direct halide ionization and alkene coordination (4->10) has been excluded as a possible pathway for this transformation, because different results are obtained when the same substrate is cyclized under cationic or neutral conditions. It is proposed that pentacoordinate complex 12 is an intermediate in... 

One example has already been encountered in the form of the binding constants in Table 3.2. These data form one of the first examples of compelling evidence for the involvement of attractive arene -arene interactions in determining the outcome of enantioselective catalysis. These attractive interactions have been frequently invoked as explanations for the observed enantioselectivities of... 

Fig. 35 Explanation of the enantioselectivity by face selective olefin coordination...

You and co-workers have demonstrated a further application of NHCs in the kinetic resolution of formyl p-lactams ( )-265 [103]. Upon treatment with a chiral NHC, the Breslow-type intermediate is formed, followed by ring-opening of the P-lactam moiety, with subsequent trapping of the acylazolium intermediate leading to the enantio-enriched succinimide product 266 and resolved formyl P-lactam (which is reduced to its alcohol 267). The authors note that when R" = H, the products undergo racemisation readily, and this is a possible explanation for the lower levels of enantioselectivity observed in the succinimide products 266 (Scheme 12.60). 

The chapter by Reetz includes many examples and explanations of methods illustrating how directed evolution has been applied to prepare new enzyme catalysts. Directed evolution of enantioselective enzymes has emerged as a fundamentally new approach to asymmetric catalysis. It involves the combination... 

A density functional study of enantioselectivity in the 2-methylproline-catalysed a-alkylation of aldehydes has been reported.14 On the basis of the computed barriers and transition states, an explanation has been provided for the remarkable and unexpected increase in enantioselectivity that is observed when using 2-methylproline instead of... 

The effect of enantioselectivity reversal serves as an additional experimental observation that gives a possible clue for the reaction mechanism. By the proposed additive-product interactions it was predicted that even poor stereoselectivity and discriminating capability of the catalytic additive can give rise to enantioselectivity reversal. This also gives a possible kinetic explanation for the effect of miscellaneous chiral additives in the Soai reaction and their role as potent chiral initiators. 

When the amount of DIOP hgand was reduced to 1 mol equiv. vs. Pd, the opposite enantiomer was formed, but with lower enantioselectivity [45]. No explanation of this unusual finding was given. 

The apolar toluene and the acetic acid were found to be unfavourable both in the hydrogenation of ethyl pyravate and in the hydrogenation of isophorone. It is interesting that adding water to methanol led to increase in the optical purity of dihydro-isophorone, but resulted in the conplete loss of enantioselectivity in the case of etiiyl pyravate. The possible explanation may be that considerable... 

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