Umpolung Catalysis in Benzoin-type and Stetter-type Reactions: From Enzymatic Performances to Bio-mimetic Organocatalytic Concepts

The umpolung bezoin-type and Stetter-type reactions are valuable synthetic strategies to furnish an ample number of important compounds and asymmetric building blocks. Biocatalysis is contributing to this field with several thiamine-dependent lyases, enzymes that are able to catalyze these reactions under mild reaction conditions with high yields and enantioselectivities. Furthermore, recent developments in that field have led to an enzymatic process for the asymmetric Stetter reaction. Likewise, taking Nature as inspiration, several bio-mimetic, organocatalytic approaches have been developed for these syntheses as well. Herein one important strategy comprises the development of "holoenzyme" microenvironments for the organocatalyst, what create hydrophobic conditions that may mimic the role of an enzyme (apoenzyme). That has been achieved by several ways, like set-up of micellar systems, anchoring catalysts to long hydrophobic, aliphatic structures, or by immobilizing organocatalysts to several supports like cyclodextrins, polystyrenes, etc. Another important strategy to mimic enzymatic performances is represented by catalyst design. Herein, starting from seminal work dealing with thiazolium salts - bio-mimetic approach of thiamine, cofactor for enzymes -, impressive developments have been achieved with the establishment of triazolium catalysts as powerful organocatalysts for many asymmetric reactions. The present article will provide an overview from biocatalytic concepts to bio-mimetic possibilities in the field of benzoin-type and Stetter-type reactions.