Extended Enolates: Versatile Intermediates for Asymmetric C-H Functionalization via Noncovalent Catalysis

Catalyst-controlled functionalization of unmodified carbonyl compounds is a relevant operation in organic synthesis, especially when high levels of site- and stereoselectivity can be attained. This objective is now within reach for some subsets of enolizable substrates using various types of activation mechanisms. Recent contributions to this area include enantioselective transformations that proceed via transiently generated noncovalent di(tri)enolate-catalyst coordination species. While relatively easier to form than simple enolate congeners, di(tri)enolates are ambifunctional in nature and so control of the reaction regioselectivity becomes an issue. This Minireview discusses in some detail this and other problems, and how noncovalent activation approaches based on metallic and metal free catalysts have been developed to advance the field.

Copper(I)-Catalyzed Regioselective Asymmetric Addition of 1,4-Pentadiene to Ketones

By using commercially available 1,4-pentadiene as a pronucleophile, a copper(I)-catalyzed regioselective asymmetric allylation of ketones is achieved. A variety of chiral tertiary alcohols bearing a terminal (Z)-1,3-diene unit are generated in high (Z)/(E) ratio and high enantioselectivity. Both aromatic ketones and aliphatic ketones serve as suitable substrates. Furthermore, the reactions with (E)-C¹(alkyl)-1,4-dienes proceed in moderate yields with acceptable enantioselectivity but with low (Z,E)/others ratio, which demonstrates the partial isomerization of (E)-allylcopper(I) species to (Z)-allylcopper(I) species through 1,3-migration. Subsequent Heck reaction and olefin metathesis compensate for the low efficiency with C¹-1,4-dienes. The synthetic utility of the product is further demonstrated by a copper(I)-catalyzed regioselective borylation of the 1,3-diene group.