Electron-transfer oxidation synthetic transformations

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. 

Because the direct electrochemical oxidation of NAD(P)H has to take place at an anode potential of + 900 mV vs NHE or more, only rather oxidation-stable substrates can be transformed without loss of selectivity—thus limiting the applicability of this method. The electron transfer between NADH and the anode may be accellerated by the use of a mediator. At the same time, electrode fouling which is often observed in the anodic oxidation of NADH can be prevented. Synthetic applications have been described for the oxidation of 2-hexene-l-ol and 2-butanol to 2-hexenal and 2-butanone catalyzed by yeast alcohol dehydrogenase (YADH) and the alcohol dehydrogenase from Thermoanaerobium brockii (TBADH) repectively with indirect electrochemical... 

The carbon-carbon bond formation via photoinduced electron transfer has recently attracted considerable attention from both synthetic and mechanistic viewpoints [240-243]. In order to achieve efficient C-C bond formation via photoinduced electron transfer, the choice of an appropriate electron donor is essential. Most importantly, the donor should be sufficiently strong to attain efficient photoinduced electron transfer. Furthermore, the bond cleavage in the donor radical cation produced in the photoinduced electron transfer should occur rapidly in competition with the fast back electron transfer. Organosilanes that have been frequently used as key reagents for many synthetically important transformations [244-247] have been reported to act as good electron donors in photoinduced electron-transfer reactions [248, 249]. The one-electron oxidation potentials of ketene silyl acetals (e.g., E°o relative to the SCE = 0.90 V for Me2C=C(OMe)OSiMe3) [248] are sufficiently low to render the efficient photoinduced electron transfer to Ceo [22], which, after the addition of ketene silyl acetals, yields the fullerene with an ester functionality (Eq. 15) [250, 251]. 

The a-aminoalkyl radicals as well as iminium ions generated as intermediates in electron-transfer reactions of amines can be used for bringing about synthetically useful transformations of amines. The synthetic applications of amine oxidation reactions brought about by thermal, electrochemical and photochemical methods as discussed below. 

The ability of transition metals to bind and activate organic molecules, and to release the transformed organic product with turnover, forms the basis of the vast catalytic chemistry of transition metal complexes. In addition, metal atoms play a key role at the catalytic center of many enzymes. For example, metalloenzymes play key roles in hydrolysis, oxidation, reduction, electron-transfer chemistry, and many other remarkable processes such as nitrogen fixation. The long-term development of synthetic polymers that perform catalytic chemistry in a manner analogous to enzymes, is a goal of profound interest. 

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