Oxidative generation bond dissociation

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

Four methods are used for generating pools of these onium ions oxidative C-H bond dissociation, oxidative C-Si bond dissociation, oxidative C-S bond dissociation, and oxidative C-C bond dissociation (Scheme 3). In the following sections, we will discuss the principles and synthetic applications of these methods. 

If we consider the generation of alkoxycarbenium ions by C-H bond dissociation, ethers should be of our first choice as precursors of alkoxycarbenium ions by analogy to carbamates. The oxidation potentials of ethers, especially aliphatic ethers, however, are very positive, and therefore, it is rather difficult to oxidize ethers selectively under usual conditions. The regioselectivity is also a problem. Usually a mixture of two regioisomers of products is obtained because two regioisomeric alkoxycarbenium ions are generated. 

Generation of Alkoxycarbenium Ion Pools by Oxidative C-C Bond Dissociation... 

Alkoxycarbenium ion pools can also be generated by oxidative C-C bond dissociation. Oxidative C-C bond dissociation is well known in the literature.38 Thus, the electrochemical oxidation of l,2-dimethoxy-l,2-diphenylethane 32... 

High-valent oxo-complexes, isolated or in situ-generated, interact most often with electron-rich n -systems 1 or suitable C-H bonds with low bond dissociation energy (BDE) in substrates 3 (Fig. 2). These reactions may occur concerted via transition states 1A or 3A leading to epoxides 2 or alcohols 4. On the other hand, a number of epoxidation reactions, such as the Jacobsen-Katsuki epoxidation, is known to proceed by a stepwise pathway via transition state IB to radical intermediate 1C [39]. Similarly, hydrocarbon oxidation to 4 can proceed by a hydrogen abstraction/S ... 

One such application of RSEs, or their constituent bond dissociation energies, is in the study of radical-mediated degradation mechanisms. For example, based on an examination of the relevant C-H and S-H bond dissociation energies in model peptides. Rank et al. postulated a mechanism for generating and propagating oxidative damage via a Met residue of the Ap peptide of Alzheimer s disease or the prion peptide of Creutzfeldt-Jakob disease. In a similar manner, Li et al. used C-H BDE calculations to identify the most vulnerable sites for radical-mediated damage in... 

Scheme 5.18 Regioselective generation of an N-acyliminium ion pool by oxidative C—Si bond dissociation...

Alkoxycarbenium ion pools can also be generated by electrochemical oxidative C—S bond dissociation (Scheme 5.20). " Arylthio (ArS) groups are known to also serve as effective electroauxiliaries for the electrochemical oxidation of heteroatom compounds. " The introduction of an ArS group decreases the oxidation potential and the C—S bond... 

S.S.1.6 Generation of alkoxycarbenium ion pools by oxidative C—C bond dissociation... 

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