Single-electron oxidation

Xie Q, Arias F and Echegoyen L 1993 Electrochemically-reversible, single-electron oxidation of Cgg and... 

All bonding or nonbonding orbitals are filled resulting in a stable diamagnetic 18-electron complex. Single-electron oxidation to a ferrocenium cation provides a 17-electron species, in which one electron is unpaired. 

The third primary intermediate in the oxidation chemistry of a-tocopherol, and the central species in this chapter, is the orr/zo-quinone methide 3. In contrast to the other two primary intermediates 2 and 4, it can be formed by quite different ways (Fig. 6.4), which already might be taken as an indication of the importance of this intermediate in vitamin E chemistry. o-QM 3 is formed, as mentioned above, from chromanoxylium cation 4 by proton loss at C-5a, or by a further single-electron oxidation step from radical 2 with concomitant proton loss from C-5a. Its most prominent and most frequently employed formation way is the direct generation from a-tocopherol by two-electron oxidation in inert media. Although in aqueous or protic media, initial... 

L-tryptophan is compulsory, the biosynthetic machinery displays wide latitude in its ability to condense a second auxiliary amino acid—L-alanine in the case of (+)-ll,ll -dideoxyverticillin A (1)—to afford a tryptophan-derived diketopiperazine intermediate 13. Mirroring Woodward and Robinson s biogenetic hypothesis for the calycanthaceous alkaloids, single-electron oxidation of the electron-rich tryptophan residue would likely initiate an oxidative dimerization of the diketopiperazine precursor with concomitant cyclization to yield the octacyclic intermediate 17. Subsequent A-methylation of the amides would then yield an unembellished skeletal core of the dimeric epipolythiodiketopiperazine alkaloids. The first step en route... 

Single electron transfer to dialkylzirconocenes Single-electron oxidation of dialkylzirco-nocenes, e. g. with a ferrocinium [13] or silver salt [14,15], induces the following reaction leading to a cationic zirconocene [13] ... 

The homopolymerization ofl consists of a room-temperature reaction of the monomer dissolved in nitrobenzene in the presence of anhydrous ferric chloride. Polymerizations were carried out under a stream of dry nitrogen. As depicted in Scheme 2, the homopolymerization of 1 to form 6FNE takes place by means of the Scholl reaction. The mechanism of the Scholl reaction was assumed to proceed through a radical-cation intermediate derived from the single-electron oxidation of the monomer and its subsequent electrophilic addition to the nucleophilic monomer. The reaction releases two hydrogens, both as protons, to form the... 

On the one hand, this particular feature makes it more difficult to distinguish between reactions involving radical cations, free radicals or carbenium ions, but on the other hand the chemist acquires an additional tool to control the course of the intended reaction. Some illustrative examples of cyclization reactions that utilize cleavage of the radical cations, primarily generated by single-electron oxidation, will be given in the following sections. 

Since the single-electron oxidation of electron-rich olefins, such as enols, enol ethers, enol acetates, or ketene acetals, is thermodynamically favored compared to simple alkenes, a number of attempts have been made to use... 

Snider and Kwon use either cupric triflate and cuprous oxide or ceric ammonium nitrate and sodium bicarbonate as single-electron oxidants to convert d,s- and ,C-unsaturated enol silyl ethers 9 stereoselectively to the tricyclic ketones 14 in excellent yields [83, 84]. Based on comparison with other experimental data and literature results, the authors try to distinguish between several possible intermediates and propose the following mechanism with a very electrophilic radical cation 10 as the key intermediate. 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

The transformations of SbClj caused by a one-electron transfer from an aromatic compound have been described earlier. If the pure Lewis acid SbClj is used, its reactivity is very difficult to control, and single-electron oxidation as well as chlorination of various aromatic donors can occur readily (Mori et al. 1998). Meanwhile, in the case of EtjO SbClg", the slow release of the active monomer SbClj occurs. In the case of SbClj as such, the 2SbCl5 — C Sb—CI2—SbCl4 dimerization occurs (Cotton and Wilkinson 1988, p. 395). The dimeric form may lead to the following electrophilic chlorination ... 

The catalytic effects and versatility of cerinm(IV) ammonium nitrate as a single-electron oxidant were recently reviewed (Nair and Deepthi 2007, Maulide et al. 2007). However, it should be kept in mind that this oxidative reagent is very powerful and some sensitive fnnctions are not compatible with it. Particularly, some amines are at least as easily oxidizable as the starting molecule bearing the protected group. Anodic oxidation was proposed as a usefnl alternative (De Lamo Martin et al. 2005). 

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