Monomeric catalysts, cinchona-derive

Table 2.1 Cinchona alkaloid-derived monomeric catalysts and their performance in the phase-transfer-catalyzed alkylation of 1. 

The highest enantioselectivity (up to >99%) yet achieved in the addition of S-nucleophiles to enones was reported in 2002 by Deng et al. [59]. By systematic screening of monomeric and dimeric cinchona alkaloid derivatives they identified the dihydroquinidine-pyrimidine conjugate (DHQD PYR (72, Scheme 4.35) as the most effective catalyst. This material is frequently used in the Sharpless asymmetric dihydroxylation and is commercially available. Screening of several aromatic thiols resulted in the identification of 2-thionaphthol as the nucleophile giving best yields and enantioselectivity. Examples for the (DHQD PYR-catalyzed addition of 2-thionaphthol to enones are summarized in Scheme 4.35. 

Recently, Yashima et al. showed that optically active helical polymers bearing cinchona alkaloid derivatives as the pendant group efficiently catalyze the reaction of nitromethane with electron-poor aromatic aldehydes (up to 87% yield, 94% ee) [14]. Interestingly, these polymers exhibited a higher enantioselectivity than the monomeric chiral units on their own, demonstrating the important role of the induced helical chirality of these polymeric catalysts. 

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