Cinchona trimeric

Based on the fact that the meta-relationship in catalyst 7 showed good activity in asymmetric alkylation, the same concept was applied to the 1,3,5-trimeric catalyst 36, in which all cinchona units on the phenyl ring were placed in the meta-position to each other hence, it would be expected this relationship might increase or maintain the catalytic efficiency of the meta-dimeric effect. Compared with the result from 7, the effect of trimerization could be regarded as quite similar to that of the metadirecting-dimerization in chemical and optical yields [14]. 

In 2005 and 2006, Siva and coworkers reported several types of new dimeric and trimeric cinchona-PTCs 23-27 and their application to the asymmetric alkylation of 4b (Scheme 6.8). While, generally, high chemical and optical yields were obtained using dimeric 23 and 24, trimeric PTCs 2S-27 showed slightly lower stereoselectivities [19]. 

Jew and Park have also utilized the dimerization effect, as observed in the development of Sharpless asymmetric dihydroxylation, where ligands with two independent cinchona alkaloid units attached to heterocyclic spacers led to a considerable increase in both the enantioselectivity and scope of the substrates, to design dimeric and trimeric cinchona alkaloid-derived phase-transfer catalysts 12 [12] and 13 [13]. These authors investigated the ideal aromatic spacer for optimal dimeric catalysts, and found that the catalyst 14 with a 2,7-bis(bromomethyl) naphthalene spacer and two cinchona alkaloid units exhibited remarkable catalytic and chiral efficiency (Scheme 11.3) [14]. 

Phase-transfer catalysis with cinchona alkaloid derivatives is a very active area within the field of organocatalysis, as indicated in quite recent reviews [98c, 112]. In terms of structure, the quaternary ammonium salts can be varied in a straightforward manner by changing the structure of the benzylic compound used for the alkylation of the nitrogen atom of the quinuclidine moiety. In addition, dimeric and trimeric quaternary ammonium salts of cinchona alkaloids have been prepared [112] and applied successfully in catalysis, as exemplified in Scheme 6.55 for the epoxidation of 2,4-diarylenones [117]. 

Park H, Jeong B, Yoo M, Park M, Huh H, Jew S. Trimeric cinchona alkaloid phase-transfer catalyst a,a, a"-tris[0(9)-allylcinchonidinium]mesitylene tribromide. Tetrahedron Lett. 2001 42(28) 4645 648. 

The meto-dimeric catalyst 25a was found to be more efficient than the other two dimeric catalysts 24 and 26 in the asymmetric alkylation of N-(diphenylmethylene) glycine tert-butyl ester. Soon after, a trimer catalyst 27 was developed by the introduction of three cinchona units on the phenyl spacer, which showed enan-tioselectivity comparable to that with the dimeric catalyst 25a for the asymmetric alkylation even at a lower catalyst loading condition [57]. Further investigation led to the discovery of an efficient and practical 2,7-naphthalene-based dimeric... 

Figure 12.7 Dimeric and trimeric cinchona-derived quaternary ammonium salt phase-transfer catalysts.

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