Electrophilic trapping of enolates in tandem reaction processes and (1,3-diketonato)metal templates for asymmetric catalysis
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The discovery of methods for the catalytic generation of enolates in the presence of suitable electrophilic partners has led to the development of several effective strategies for the tandem formation of multiple bonds. Typically, enones have been utilized as latent enolates in these tandem processes. For instance, catalytic enone hydrometalation in the presence of an aldehyde or ketone partner allows for the formation of reductive aldol products. Similarly, the presence of an α,β-unsaturated carbonyl acceptor has been shown to give products of a reductive Michael reaction. The historical development of these tandem conjugate reduction–electrophilic trapping processes is reviewed herein. Enolate generation through catalytic enone carbometalation has also been employed as a strategy in the development of novel transformations. This approach has been used to devise an efficient protocol for the desymmetrization of enone-dione substrates via rhodium-catalyzed conjugate addition–enolate trapping. Using this methodology, four contiguous stereocenters can be established in a single manipulation, with high levels of both relative and absolute stereocontrol. This technique has provided a concise route to seco-B-ring steroids possessing a 14-hydroxy cis fused C-D ring junction, as found in naturally occurring cardiotonic steroids derived from digitalis purpurea. Numerous stereogenic processes are catalyzed by transition metal complexes of 1,3-diketonates. Despite this fact, the development of enantioselective variants using chiral (1,3-diketonato)metal templates has been slow. Progress toward the development of a novel family of chiral 1,3-diketonate ligands for asymmetric catalysis is described. These ligands, which all arise from acylation of a common pseudo-planar chiral monoketone precursor, have been used to form a variety of (1,3-diketonato)metal complexes. Additionally, the first known examples of C2-symmetric bis(1,3-diketonate) ligands have been obtained through extension of this modular approach to ligand synthesis. Although highly enantioselective catalytic transformations employing these new (1,3-diketonato)metal complexes have not yet been realized, the knowledge gained from the intial studies presented herein establishes a foundation for future development.