Selectivity viacatalyst or substrate control in catalytic asymmetric transformations of bifunctional symmetrical substrates

Abstract

Catalytic asymmetric transformations of bifunctional symmetrical substrates pose interesting problems in selectivity. In one extreme, catalyst control leads to identical selectivity at both sites, and amplification of chirality in the product. This improved enantiopurity comes at the expense of reduced yield; the undesired meso byproduct must be separated. Alternatively, substrate control may modify the inherent selectivity of the catalyst. The first transformation may change the substrate to make the second transformation less selective (negative cooperativity), while positive cooperativity can result in selectivity greater than that afforded by simple catalyst control. Similar behavior may occur in polyfunctional substrates with three or more identical reactive sites. Substrate vs. catalyst control and positive vs. negative cooperativity plausibly depends upon the structure of the substrate and the linker between the active sites, but has rarely been investigated systematically. This perspective presents several examples of structure–selectivity relationships with the eventual goal of designing substrates for positive cooperativity and enhanced selectivity in asymmetric catalysis.