Enones prochiral. conjugate addition reactions

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

In addition to molecules containing prochiral sites, racemic molecules can participate in catalytic asymmetric transformations. In some transformations, the stereocenter is destroyed in the course of the reaction, and equilibrating prochiral intermediates are formed. An example of such a process is the asymmetric arylation of ketones (Figure 14.8C). In other cases, one of the enantiomers of the substrate reacts with the asymmetric catalyst significantly faster than the other enantiomer. In this case, an enantioenriched product is formed, and the opposite enantiomer of the reactant remains. This last process is called a kinetic resolution and is illustrated for the conjugate reduction of enones via hydrosilylation (Figure 14.8D). In this case, the top and bottom faces of the C-C double bond are diastereotopic because reaction at each face of the enone generates diastereomeric products. This section of the chapter first presents the principles that relate to reactions at prochiral centers of achiral substrates and then presents the principles that relate to reactions of racemic or meso compounds. 

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