Electron-transfer oxidation photochemical activation

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. 

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. 

Excited molecular complexes of the donor-acceptor type are called excimers if formed from identical molecules and exiplexes if originated from different molecules. From the theory, it is concluded that photochemical influence will more readily accelerate electron transfer in a weak donor-acceptor pair than in a strong pair (Juillard and Chanon 1983). An organic molecule in an electron-excited state is a more active oxidant or stronger reducer than the same molecule in a ground state. 

This article will illustrate several methods by which back electron transfer can be obviated and hence by which organic transformations can be accomplished. Because this field has been so active, a comprehensive review of all work accomplished toward these objectives would be impossible. The coverage of this article is therefore restricted to recent, rather arbitrarily chosen, experiments which exemplify the basic principles governing both electron exchange between excited organic molecules and appropriate redox partners and the subsequent chemical reactivity of the reduced and oxidized species formed in the photochemical step. 

An interesting variation of this system involves embedding surface active zinc and manganese(III) porphyrins in lecithin membranes. Provided that the porphyrin is negatively charged, and with zwitterionic PSV in the bulk aqueous phase, the Mn111 can be photochemically oxidized to Mn,v. No electron transfer from PSV" to MuIV occurs, perhaps because the MnIV is deeply embedded in the membrane.310... 

The photosynthetic process involves photochemical reactions followed by sequential dark chemical transformations (Fig. 3). The photochemical processes occur in two photoactive sites, photosystem I and photosystem II (PS-I and PS-II, respectively), where chlorophyll a and chlorophyll b act as light-active compounds [6, 8]. Photoinduced excitation of photosystem I results in an electron transfer (ET) process to ferredoxin, acting as primary electron acceptor. This ET process converts light energy to chemical potential stored in the reduced ferredoxin and oxidized chlorophyll. Photoexcitation of PS-II results in a similar ET process where plastoquinone acts as electron acceptor. The reduced photoproduct generated in PS-II transfers the electron across a chain of acceptors to the oxidized chlorophyll of PS-I and, consequently, the light harnessing component of PS-I is recycled. Reduced ferredoxin formed in PS-I induces a series of ET processes,... 

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