Bifunctional catalysts hydrogen-bond-donor asymmetric

Cook reported that a 3,3 bis(trifluoromethyl) BINOL catalyzed asymmetric addition of allylindium to hydrazones proceeds in modest to good enantioselec tivities (10 92% ee) [90]. The stoichiometric version of this reaction yields much higher enantioselectivities (84 97% ee). Jacobsen later found that a chiral urea catalyst is effective in catalyzing a similar transformation [96]. The bifunctional catalyst 55 bearing a hydrogen bond donor and a Lewis base that are properly... 

Contrary to the view that applying chiral solvents in synthesis cannot result in appreciable enantioselectivities, Leitner et al. have reported the first example of an asymmetric reaction in which a chiral reaction medium induces a high level of enantioselectivity. Using a specifically designed ionic liquid (312) with a chiral anion as the only source of chirality, up to 84% ee was obtained in the aza-MBH reaction of N-Ts arylaldimines with MVK (Scheme 1.124), which is comparable with values obtained with the best catalysts for the asymmetric aza-MBH reaction in conventional solvents (94% ee, 83% ee ). Possible bifunctional interaction of the zwitterionic intermediate of the aza-MBH reaction with the chiral anion of a CIL-312 containing a hydrogen-bond donor was proposed (Figure 1.6). 

Bifunctional asymmetric catalysis, involving the synergistic activation of both acidic and basic sites in the substrates, has received great attention. The bifunctional catalysts discussed in this section generally contain a hydrogen-bond donor and a Bronsted basic moiety. 

A chiral quinine-derived squaramide catalysed the highly enantioselective Michael addition of malononitrile to chalcones. The reactions take place at a very low catalyst loading (0.5 mol%) and provide the Michael adducts with high yields and good enan-tioselectivities (up to 96% ee). Chiral bifunctional squaramides have also been used as multiple hydrogen-bond donor-acceptor organocatalysts for the asymmetric Michael addition of nitroolefins to 1,3-dicarbonyl compounds. ... 

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