Phenyl linker

Carbosilane dendrimer [G1] with 4-(hydroxymethyl)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). 

To move the substrate back in place, by shortening the distance from a cyclodextrin to the Mn=0 group, models suggested that the cyclodextrin be attached to the meta rather than para position of the phenyl rings in the catalyst. Thus we synthesized catalyst 139, with such a meta attachment, and indeed it did cleanly convert substrate 136 into the C-9 hydroxy product 137, with no hydroxylation at C-15 [197]. However, as there was no fluorine in the phenyl linkers, the catalyst was destroyed after only 2.5 turnovers. 

This was solved by replacing the phenyl linkers with trifluoropyridine rings in catalyst 140 [197]. The compound was easily made, and it performed the C-9 hydroxylation of substrate 136 to form product 137 with 90 turnovers. 

In 1992, McLendon and coworkers reported results of a study of another series of porphyrin dyads with zinc in one porphyrin and iron in the second. These were related in structure to dyad 3, but the number of phenyl linkers varied from one to three. These workers found an apparent value for y in Eq. 2 of 0.4 A . They explained this result by a theory that attributes the drop in rate constant not to increased distance, but rather to the decrease in conjugation between the porphyrins that occurs at each phenyl ring junction due to the biphenyl twist angle of - 50°. Each phenyl ring results in a drop in rate of approximately six-fold [32]. 

Zinc chlorins are more easily oxidized than their free base counterparts, and this fact has been used in the design of some chlorin-porphyrin-imide triads that demonstrate multistep electron transfer [16]. For example, triad 47 consists of a zinc chlorin linked to a free base porphyrin that also bears a pyromellitimide acceptor moiety. Phenyl linkers join all active constituents. Excitation of the zinc chlorin moiety of 47 in tetrahydrofuran gives CZ-P-Im, which decays with a 20-ps time constant to yield CZ +-P -Im. This initial charge-separated state evolves into CZ +-P-Im with a lifetime of 120 ps. The CZ -P-Im " state is formed with an overall quantum yield of 0.90 and decays biphasically with time constants of 110 ns and 400 ns. 

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... 

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