Zinc complexes, support-bound

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

Insight into the role of the serine nucleophile in the catalytic cycle of alkaline phosphatase was gained through studies of the bis(4-nitrophenyl) phosphate reactivity of a mononuclear Zn(II) complex supported by the (S)-1 -(2-hydroxy-2-phe-nylethyl)- 1,4,7,10-tetrazacyclodecane ligand (Scheme 33).226 In aqueous solution, this complex exhibits a pKa value for the zinc-bound alkoxide moiety of 7.30 + 0.02. 

In a related study, the Reinhoudt group examined di- and trinuclear Zn(ll) calix [4]arene complexes. Two or three zinc cations were bound by 2,6-bis (aminomethyl)pyridyl groups at the upper rim in the distal 1,3- or 1,2,3-positions. HPNP transesterification (RNA model) was even more efficient than with the dinuclear Cu(ll) complexes, and occurred at neutral conditions. Mononuclear reference complexes performed much more poorly, and thus supported cooperative effects in the hydrophobic environment of the aromatic macrocycle. Interestingly, a very similar, but rigid calixarene produced a lowered catalytic rate, while the flexible trinuclear complex exhibited weaker substrate binding but superior rate acceleration. In a tentatively proposed mechanism, two zinc cations activate the phosphate ester and another activates the 2-hydroxyl group of the substrate [85, 86]. 

In a related study, Srivastava and Collibee employed polymer-supported triphenyl-phosphine in palladium-catalyzed cyanations [142]. Commercially available resin-bound triphenylphosphine was admixed with palladium(II) acetate in N,N-dimethyl-formamide in order to generate the heterogeneous catalytic system. The mixture was stirred for 2 h under nitrogen atmosphere in a sealed microwave reaction vessel, to achieve complete formation of the active palladium-phosphine complex. The septum was then removed and equimolar amounts of zinc(II) cyanide and the requisite aryl halide were added. After purging with nitrogen and resealing, the vessel was transferred to the microwave reactor and irradiated at 140 °C for 30-50 min... 

The idea of H2C03 as substrate for carbonic anhydrase is strongly supported by inhibitor binding studies158, 159. Small anion or sulfonamide inhibitors are linked to the zinc ion at high pH. The complex picks up a proton and the inhibitor is bound in a neutral form. 

In the zinc(n) porphyrin complex below, the four peripheral CO2H groups are oriented in such a way that 2-dimensional sheets supported by hydrogen bonds assemble. Each Zn ion is octahedraUy bound by a porphyrin and two water ligands ... 

Betts et al. developed a new methodology to purify copper-64 radiopharmaceuticals. Scheme 13.12 illustrates the purification technique based on a commercial DMAP-supported resin. In the approach A, the resin was able to selectively purify a pre-formed solution mixture of copper-64 and zinc-containing metalloradiopharmaceuticals. By the Jahn-Teller effect, the DMAP ligand selectively bound to the zinc-containing complex and the [ Cu]-labeled metalloradiopharmaceuticals remained in solution after filtration of... 

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