Group transfer reactions defined

Although this model was initially developed for reactions occurring in an adiabatic potential energy surface, it can be extended to weak interaction systems under the assumptions mentioned above, because the reaction path is obtained in terms of independent stretches of the two reactants. This contrasts with the Agmon-Levine approach, which is restricted to atom or group transfer reactions, where reactant and product are connected via a family of parallel curves each defined by the same positive bond order, maintained constant through the compensation between the decrease in the reactant bond order and the increase in the product one. One caution must be observed in our analysis the extensions x are not directly related to the intersection of the reactants curves, although it is convenient to represent them that way. Therefore, we can use the transition state expression to estimate the rates of ET reactions... 

Co within all compounds of the so-called cobalamin (or B12) family. The biological functions of cobalamin cofactors are defined by their axial substituents either a methyl or an adenosyl group. Both cofactors participate in biosynthesis the former in methyl transfer reactions while the latter is a free radical initiator, abstracting H atoms from substrates. Decades after their initial characterization, the fascination with the biological chemistry of cobalamins remains.1109... 

Hydrogen atom transfer implies the transfer of hydrogen atoms from the chain carrier, which is the stereo-determining step in enantioselective hydrogen atom transfer reactions. These reactions are often employed as a functional group interconversion step in the synthesis of many natural products wherein an alkyl iodide or alkyl bromide is converted into an alkane, which, in simple terms, is defined as reduction [ 19,20 ]. Most of these reactions can be classified as diastereoselective in that the selectivity arises from the substrate. Enantioselective H-atom transfer reactions can be performed in two distinct ways (1) by H-atom transfer from an achiral reductant to a radical complexed to a chiral source or alternatively (2) by H-atom transfer from a chiral reductant to a radical. 

Compounds containing a CH2 bonded to two Z groups (as defined on p. 464) can be converted to diazo compounds on treatment with tosyl azide in the presence of a base,164 The use of phase transfer catalysis increases the convenience of the method.165 p-Dodecylbenzenesul-fonyl azide,166 methanesulfonyl azide,167 and p-acetamidobenzenesulfonyl azide168 also give the reaction. The reaction, which is called the diazo transfer reaction, can also be applied to other reactive positions, e.g., the 5 position of cyclopentadiene.169 The mechanism is probably as follows ... 

The heme groups of the cytochromes as well as many other transition metal centers act as carriers of electrons. For example, cytochrome c may accept an electron from reduced cytochrome Cj and pass it to cytochrome oxidase or cytochrome c peroxidase. The electron moves from one heme group to another over distances as great as 2 nm. Similar electron-transfer reactions between defined redox sites are met in photosynthetic reaction centers (Fig. 23-31), in metalloflavo-proteins (Fig. 15-9), and in mitochondrial membranes. 

The process of glycosyl transfer, defined in this mechanistic way, extends to hydrolysis and its reverse (i.e., condensation synthesis from glycoses with expulsion of water) as well as to reactions in which water is not a reactant. However, the terms glycosyl transfer and transglyco-sylation are bound to the group transfer concept and inevitably carry a... 

Figure 13 Potential energy surfaces for electron transfer reactions. Hamionic oscillator potential energy functions for reactants and product are shown, including the nuclear wave functions, which are shaded. The dark shaded region indicates the magnitude of overlap of the nuclear wave functions, which is the Franck-Condon factor, (a) is the normal region, (b) is the activationless region and (c) is the inverted region as defined in the text. (Ref. 72. Reproduced by permission of Namre Publishing Group, www.nature.com)...

Electroactive dendrimers are defined as those that contain functional groups capable of undergoing fast electron transfer reactions [85], The combination of specific electron transfer properties of redox active probes with the unique structural properties of dendrimers offers attractive prospects of their exploitation in electrocatalytic processes of biological and industrial importance [86], Further, the interest in dendrimers containing electroactive units also relies on the fact that electrochemistry is a powerful technique to elucidate the structure and purity of dendrimers, to evaluate the degree of electronic interaction of their chemically and/or topologically equivalent or non- equivalent moieties, and also to study their endo- and exo-receptor capabilities [87],... 

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