Dialkylzincs conjugate additions, cyclic enones

The use of vinyl epoxides as substrates in enantioselective copper-catalyzed reactions, on the other hand, has met with more success. An interesting chiral ligand effect on Cu(OTf)2-catalyzed reactions between cyclic vinyloxiranes and dialkylzinc reagents was noted by Feringa et al. [51]. The 2,2 -binaphthyl phosphorus amidite ligands 32 and 43 (Fig. 8.5), which have been successfully used in copper-catalyzed enantioselective conjugate additions to enones [37], allowed kinetic resolution of racemic cyclic vinyloxiranes (Scheme 8.26). 

High levels of enantioselectivity (94-98% ee) and good chemical yield (72-95%) were observed in the catalytic conjugate addition of dialkylzinc reagents to numerous cyclic enones (eq 1) using a catalyst prepared in situ from Cu(OTf)2 and this chiral phosphoramidite ligand. Here the steric properties of the substrate and the reagent appear to be unimportant. 

The preparation of silyl enol ethers from carbonyl compounds represents one of the major uses of TMSOTf. Recently, the stereochemistry and regiospecificity of such transformation has been addressed for aldehydes and -(iV-alkoxycarbonylamino) ketones, respectively. On the other hand, enantiopure silyl enol ethers can be formed by addition of TMSOTf to zinc enolates, which are obtained from the copper-catalyzed enantioselective conjugate addition of dialkylzinc reagents to cyclic (eq 36) and acyclic enones. ... 

Dual enantioselective control in the Cu-catalysed conjugate addition of dialkylzinc compounds to acyclic and cyclic enones has been realized in the presence of functionalized A -heterocyclic carbene ligands. " For example, a high degree of asymmetric catalysis (up to 96% ee) has been achieved using a (hydroxy amide)-functionalized azolium salt (19). Most likely the selectivity appears because of the facial selection of the enone, which is a result of steric repulsion between the alkyl group on the azolium ring and the alkyl substituent at the / -position of the enone. 

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