Polymeric cinchona-PTCs

Polymeric Cinchona-PTCs with Other Linkers... 

Figure 4.8 Polymeric cinchona-PTCs with other linkers.

Asymmetric Epoxidation with Polymeric Cinchona-PTCs 63... 

From the systematic investigation of the Park and Jew group, several highly efficient and practical polymeric cinchona PTCs were developed (Scheme 6.6). Interestingly, polymeric catalysts with a specific direction of attachment between aromatic linkers (e.g., benzene or naphthalene) and each cinchona unit were found to be effective in the asymmetric alkylation of 4b. The phenyl-based polymeric PTCs with the meta-relationship between cinchona units such as 14, 15, and 18 showed their high catalytic efficiencies. Furthermore, the 2,7-dimethylnaphthalene moiety as in 16 and 17 was ultimately found to be the ideal spacer for dimeric cinchona PTC for this asymmetric alkylation. For example, with 5 mol% of 16, the benzylation of 4b was completed within a short reaction time of 30 min at 0 ° C, affording (S)-5a in 95% yield with 97% ee. Almost optically pure (>99% ee) (S)-5a was obtained at lower reaction temperature (—40 °C) with 16, and moreover, even with a smaller quantity (1 mol%), its high catalytic efficiency in terms of both reactivity and enantioselectivity was well conserved. 

The development of polymeric cinchona-derived PTCs was triggered by the group of Jew and Park in 2001 [8]. The group paid particular attention to the fact that the cinchona alkaloids have demonstrated great utility in the Sharpless asymmetric dihydroxylation. Especially, it was noted that the significant improvements in both stereoselectivity and scope of the asymmetric dihydroxylation were achieved when the dimeric ligands of two independent cinchona alkaloid units attached to heterocyclic spacers were used, such as (DHQ)2-PHAL or (DHQD)2-PYR (Figure 4.4) [9]. 

Cinchona alkaloids are by far the chiral fragments most used for the challenge of achieving chiral PTC using insoluble supports. For this purpose, one of the nitrogen atoms of 71, usually the aliphatic one, is quaternized, either by using this atom to anchor the polymeric backbone (92, Scheme 10.17) or by its appropriate... 

Further useful PTC catalysts were developed by the immobilisation of Cinchona-denved ammonium salts on insoluble polymeric supports. 

Based on the successful introduction of solid-supported catalysts, a novel type of polymeric PTC catalysts derived from the Cinchona alkaloid was... 

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