Dimeric phenyl linker

Chapman and Breslow synthesized zinc(II) complexes of monomer and dimers derived from 1,4,7-triazacyclododecane with phenyl 48 and 4,4 -biphenyl linkers 49 (55). They were examined as catalysts for the hydrolysis of 4-nitrophenyl phosphate (NP2 ) and bis(4-nitrophenyl) phosphate (BNP ) in 20% (v/v) DMSO at 55°C. On the basis of the comparison of the pseudo-first-order rate constants, the dinuclear zinc(II) complexes 48 with 1,3-phenyl and 1,4-phenyl linkers are ca. 5 times more efficient than monomer or 49 in the hydrolysis of NP2, leading to the conclusion that the two zinc(II) ions are simultaneously involved in the hydrolysis, as in the enzyme alkaline phosphatase. For the hydrolysis of BNP, a longer dimer 49 is ca. six times more effective than 1,3-phenyl-linked dimer 48 and monomers. 

Dimeric Cinchona-PTCs with Phenyl Linker... 

Figure 4.6 The dimeric cinchona-PTCs with phenyl linker.

During the search for the optimal dimeric PTC for this epoxidation, the Park-Jew group found an interesting result, namely that the functional groups of 9-0 H and 6 -OMe in the cinchona unit, along with 2-F group in the phenyl linker, were critical factors for high enantioselectivity of the reaction (Scheme 4.16). 

Quite recently, one of the most efficient phase-transfer-catalyzed epoxidation methods for chalcone-type enones was developed by the Park-Jew group [11], A series of meta-dimeric cinchona PTCs with modified phenyl linkers were prepared. Among this series, the 2-fluoro substituted catalyst 5, exhibited unprecedented activity and enantioselectivity for the epoxidation of various trans-chalcones in the... 

The first series of the dimeric cinchona-PTCs (3-8) to have a phenyl ring as a linker was designed to examine the primary effect according to the relationship of the attached position (Figure 4.6). One of the two independent cinchona alkaloid units can be located at the ortho-, meta-, or para-position against the other, respectively. The group envisaged that, both chemical yield and enantioselectivity of the asymmetric alkylation of 1 should be affected by the direction of each of the cinchona units. 

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. 

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