Electron donor-acceptor complexes catalysis

Catalysis by Electron Donor-Acceptor Complexes Kenzi TAMARU... 

K. Tamaru and M. Ichikama, Catalysis by Electron-Donor-Acceptor Complexes, Wiley, New York, 1975. 

Davydova, 1. R., Kiperman, S. L., Slinkin, A. A., Dulov, A. A. On the Catalytic Activity of Certain Synthetic Organic Polymers Bull. Acad. Sci. USSR (English Transl.) 1964. 1502. 281 Tamaru, Kenzi Catalysis by Electron Donor-Acceptor Complexes. Advan. Catalysis 20,... 

Ichikawa, M., Soma, M., Onishi, T., Tamaru, K. Exchange Reactions of Hydrogen over the Electron Donor-Acceptor Complexes of Various Phthalocyanines with Sodium. J. Catalysis 6, 336 (1966). 

It was accordingly demonstrated that the formation of electron donor-acceptor complexes is associated with the appearance of new catalytic activity and the further extension of the concept of electron donor-acceptor would possibly lead to a fuller insight into the mechanism of heterogeneous catalysis. 

There is a huge body of literature on this topic, e.g.. Comprehensive Treatise of Electrochemistry, Vols. 2 and 7 (J. O M. Bockris, B. E. Conway, E. Yeager, R. E. White, and S. U. M. Khan, eds.). Plenum Press, New York (1981 and 1983) Proc. Symp. on Elec-trocatalysis (W. E. O. Grady, P. N. Ross and F. G. Will, eds.), Electrochem. Soc., Princeton, New Jersey (1982) K. Tamara and M. Ichinawa, Catalysis by Electron Donor-Acceptor Complexes, Kodansha, Tokyo (1975) J. O M. Bockris and S. U. M. Khan, Quantum Electrochemistry, Plenum Press, New York (1979). 

Nucleophilic/electrophilic catalysis substrate activation by Lewis bases via electron pair donor complexes or by Lewis acids via electron pair acceptor complexes... 

The study of these catalysts may sharpen our insight into the phenomenon of catalysis itself however. We have seen that the study of electron donor/ac-ceptor complexes for the activation of molecular hydrogen has shown many correlations between physical properties of the acceptor and catalytic activity. 

Redistribution of electron density in CT complexes results in a modification of the chemical properties of coordinated arenes, and this effect is widely used in organometallic catalysis [2]. To demonstrate the relationship between charge transfer in arene complexes and their reactivity, we focus our attention on carbon-hydrogen bond activation, nucleophilic/ electrophilic umpolung, and the donor/acceptor properties of arenes in a wide variety of organometallic reactions. 

Mechanistically, it was first proposed that the enzyme—metal—substrate (EMS) complex involved first-sphere coordination of substrate to enzyme-bound metal ion. For transition metal ions, 3d orbitals were suggested to facilitate catalysis by promoting delocalization of electrons in the substrate, thereby aiding bond lengthening and breakage, especially for proteases catalyzing peptide bond cleavage . For phosphate transfer enzymes, a transition between rr-levels in the donor-acceptor (substrate-metal) was proposed as the mechanistic role for metal in catalysis . 

Alkali-metal cation catalysis of electron transfer to Acceptor-anions is generally attributed (1) to the formation of stable association complexes (ion pairs) between cation and Acceptor-anion or (2) to formation of stable ternary association complexes that additionally include the Donor as well. ° (These are distinct from the ternary activated complexes formed later along the reaction coordinate.) In case (1), increase in kobs for electron transfer is often attributed to a positive shift in the reduction potential of the acceptor complex upon cation association. Such positive shifts in reduction potential are... 

While Fe(SCys)4, [2Fe-2S], [3Fe-4S] and [4Fe-4S] clusters all function as one-electron donors or acceptors, the more complex double-cubane [8Fe-7S] cluster that is found only in nitrogenases (see Nitrogenase Catalysis Assembly) has the potential to mediate two-electron transfer processes.Three methods have been employed to functionalize Fe-S centers for substrate binding and activation. The first involves having an accessible Fe coordination site as in the mononuclear Fe centers of nitrile hydratase and SOR, and the [4Fe-4S] clusters at the active sites of hydratases/dehydratases and radical-5-adenosylmethionine (SAM) enzymes.Indeed the recent recognition of the importance of the superfamily of radical-SAM enzymes in initiating radical reactions, via cluster-mediated reductive cleavage of SAM to yield a... 

The basis of the catalysis of the splitting of the disulfide is presumably the formation of a charge-transfer complex between the two-electron donor NADPH (equivalent to a hydride anion) and the acceptor flavin combined with proximity effects. Both coenzymes, NADPH and FAD, are bound to the protein by adenosine phosphate-protein interactions, the substrate is loosely bound at the cleft between the units of a protein dimer (Fig. 9.6.12) (Schulz, 1983 Douglas, 1987). 

Metals are crucial components in enzymatic catalysis for largely the same reasons that organometallic complexes make good catalysts. These include the fact that many metal centers can complex to a variety of substrates, undergo facile changes in oxidation state, and provide good electron donors or acceptors. These characteristics can, and often do, work together to accomplish some remarkable chemical feats. 

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