Atom-transfer-chain catalysis

Besides ETC catalysis, atom-transfer chain (ATC) catalysis has been reviewed [2]. The initiation involves radicals and the redox propagation step exchanges an... 

S.3 Atom-Transfer-Chain (ATC) Catalysis 3.2.8.3.1 Halogen-ATC Catalysis... 

Keywords Bond dissociation energies Catalysis Chain reactions Hydrogen atom transfer Metal-hydrogen bonds Radicals... 

One of the most striking features of CCT is the exceptionally fast rate at which it takes place. The molecular weight of a polymer can be reduced from tens of thousands to several hundred utilizing concentrations of cobalt catalyst as low as 100—300 ppm or 10 3 mol/L. The efficiency of catalysis can be measured as the ratio between the chain-transfer coefficients of the catalyzed reaction versus the noncatalyzed reaction. The chain-transfer constant to monomer, Cm, in MMA polymerization is believed to be approximately 2 x 10 5.29 The chain-transfer constant to catalyst, Cc, is as high as 103 for porphyrins and 104 for cobaloximes. Hence, improved efficiency of the catalyzed relative to the uncatalyzed reaction, CJCu, is 104/10 5 or 109. This value for the catalyst efficiency is comparable to many enzymatically catalyzed reactions whose efficiencies are in the range of 109—1011.18 The rate of hydrogen atom transfer for cobaloximes, the most active class of CCT catalysts to date, is so high that it is considered to be controlled by diffusion.5-30 32 Indeed, kc in this case is comparable to the termination rate constant.33... 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

The nucleophilic substitution on poly(vinyl chloroformate) with phenol under phase transfer catalysis conditions has been studied. The 13c-NMR spectra of partly modified polymers have been examined in detail in the region of the tertiary carbon atoms of the main chain. The results have shown that the substitution reaction proceeds without degradation of the polymer and selectively with the chloroformate functions belonging to the different triads, isotactic sequences being the most reactive ones. 

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