Monomeric PTCs

From an evaluation of the catalytic efficiency of 3-5 using standard catalytic phase-transfer benzylation of 1, it was found that all compounds had the ability to catalyze this phase-transfer benzylation, and in all cases the (S)-isomer of the benzylated imine 2a was formed in excess (Scheme 4.3). The 1,3-phenyl-linked dimeric PTC 4 showed the highest enantioselectivity among the three dimeric PTCs. The order of enantioselectivity of the three PTCs, along with the monomeric PTC M, was as follows 1,3-dimeric PTC 4 > 1,4-dimeric PTC 5 = monomeric PTC M 1,2-dimeric... 

The lack of any difference in enantioselectivity between the 1,4-phenyl-dimeric PTC S and the monomeric PTC M implies that the two cinchona alkaloid units of the 1,4-phenyl-dimeric PTC do not sterically affect each other. In the case of 1,2-phenyl-dimeric PTC 3, the severe steric repulsion between the two cinchona alkaloid units may lead to an unfavorable conformation, thereby affording poor enantioselectivity. 

PMHS corresponding polymeric reagent (PMHS, polymethylhydrosiloxane), a substantial rate increase was observed over the monomeric model (complete reduction of acetophenone in less than 1 min with PMHS vs. only 60% conversion in 1 h with (EtO)2SiHMe) [55]. The related hydrosilylation of 86 by chiral PTC uses an interesting ep/iedra-derivedhalometallated catalyst 25 (Scheme 10.12) [56],... 

Despite such numerous and fruitful studies having been conducted on the asymmetric phase-transfer catalytic reactions, the development of cinchona-PTCs has, until the start of the 21st century, been mainly focused on the monomeric cinchona-PTCs (Figure 4.3). 

As (—)-cinchonidine-derived ammonium salts have been mainly used as chiral PTCs in monomeric cinchona-PTCs via the asymmetric alkylation of 1, and have generally shown better results than those of others [e.g., derived from (+)-cinchonine, (—(-quinine, and (+)-quinidine], the Park-Jew group primarily prepared (—) -cinchonidine derivatives to identify both the optimal linker and best relationship of attachment for the two cinchona units, and to compare catalytic efficiency with that of monomeric cinchona-PTCs. 

Investigations carried out in our laboratories since 1981 have demonstrated the effectiveness of PTC methodology using the 1-oxypyridinyl group in monomeric (pyridine 1-oxide, 3a 3-trimethylsilylpyridine 1-oxide, 3b and 4-trimethylsilylpyridine 1-oxide, 3c), dimeric (l,3-bis[l-oxypyridin-3-yl]-1,1,3,3-tetramethyldisiloxane, 4), and polymeric (poly[methyl[l-oxypyridin-... 

Very recently, the Siva group successfully reported a chiral multisite phase-transfer catalytic Michael addition by the use of 2,4,6-(triscincho-niummethyl)phenyl-l,3,5-triazines as new polymeric chiral quaternary ammonium PTC catalysts. Catalyst 12c showed a higher catalytic efficiency than the corresponding monomeric catalyst 8q in terms of chemical yield and enantioselectivity due to the trimeric reaction sites of 12c, which can promote efficient ion-pair interactions between the a-carbon of the diethyl-malonate and trimeric catalysts due to steric factors (Scheme 16.22). ... 

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