Electron-transfer oxidation thermal activation

Diacyl peroxides are, however, also electron transfer oxidants, which according to a theoretical analysis should possess standard potentials, °[(ArCOO)2/RCOO RCOO ) of around 0.6 V in water, provided that the electron transfer process is of the dissociative type (50) (Eberson, 1982c). Such a value brings thermal ET steps involving DBPO within reach for redox-active organic molecules, as for example suggested by the so-called CIEEL mechanism of chemiluminescence (Schuster, 1982). 

A THERMAL AND PHOTOCHEMICAL ACTIVATION OF ELECTRON-TRANSFER OXIDATION... 

Since electrochemical methods are described in Volume 7, Chapter 7.1, emphasis will be placed on the thermal and photochemical activation of electron-transfer oxidation. Even with this restriction the scope of electron-transfer oxidation is too extensive to be covered completely in a single chapter. Therefore the approach here is to present those fundamental aspects that allow electron-transfer oxidations to be developed for synthetic transformations. Hopefully this format will encourage the creative chemist to devise myriad oxidative syntheses from a limited number of principles. Fortunately, there are already available a variety of recent monographs with each presenting a restricted coverage to permit the inclusion of detailed and useful examples. For the convenience of the reader these articles are listed as references 17 to 32, with the chapter titles included where appropriate. Taken all together they offer the reader an interesting panoply of electron-transfer oxidations that are intertwined by the principles outlined herein. 

When dte oxidation-reduction equilibria in equation (6a) ate included, the thermal activation of elec-tron-transfn oxidation in equation (3b) follows a course that is akin to charge-transfn activation in equation (5). In both, the A cotiq>lex [RH,A] is the important precursor which is directly converted into the critical contact ion pair [RH, A ]. Such an involvonent of reactive intermediates in common does widen the scqte of electron-transfer oxidations to include both thermal and photochemical pro-... 

The osmylation of arenes (Ar) with osmium tetroxide is a particularly informative system with which to illustrate the close interrelationship between the thermal and photochemical activation of electron-transfer oxidation. For example, a colorless solution of osmium tetroxide in n-hexane or dichlorometbane upon exposure to benzene turns yellow instantaneously. With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic color changes are illustrated in Figure 3 by the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (/F) in the order benzene 9.23 eV naphthalene... 

The variable regiochemistry observed in the collapse of [Ar, Os04 ] to the cycloadduct A1OSO4 underscores the importance of CIP structures in determining the course of electron-transfer oxidation. Since CIP structures are not readily determined as yet, the structural effects induced by qualitative changes in solvent polarity, salts, additives and temperature are reaction variables that must always be optimized in the synthetic utilization of electron-transfer oxidation by either thermal or photochemical activation. 

Protein engineers have developed methods for rational and evolutive protein engineering which empowers them to design enzymes that are tailored for electrochemical applications ( bioelectrozymes ). Bioelectrozymes are usually optimized in specific properties such as stability (e.g., oxidative, thermal) and electron transfer rates (higher activity, mediator acceptance/specificity). An emphasis is often... 

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

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