Radicals radical cation

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

It would be useful to classify reactions as to their type, heterolytic (ionic) or homolytic (free radical), radical cation, or radical anion. Below are a number of examples of these types of reaction. 

It has been suggested that the thermal rearrangement proceeds via a radical pair, whereas the acid-catalyzed process is via a radical-radical cation... 

Iminium Salts. The occurrence of electron transfer as a primary photoprocess is well established in iminium ions (194). With olefins, for example, photolysis of iminium salts produces a radical-radical cation pair, eq. 64 (197) ... 

Reactions of SO /PO -Adduct Radicals, Radical Cations and Heteroatom-Centered Radicals... 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

The electron spin resonance (E.S.R.) spectra of a paramagnetic organic molecule, e.g. free radical, radical cation or radical anion, is directly related to its unpaired electron distribution (spin density). In the region of a magnetic nucleus the hyperfine interaction between the magnetic moments of the nucleus and the electron is a function of the spin density. It has been shown that, for an atom N, a direct correlation exists between its observed hyperfine coupling constant, and [pa—pP), the unpaired electron population of its atomic orbitals 1). 

The molecular ion fragments into cations, radicals, radical cations and neutral molecules of which only the positively charged species are detected. There are several possible fragmentations for each M but the base peak represents the most energetically favoured process with the mjz value of the base peak representing the mass of the most abundant (and therefore most stable) positively charged species. The fragmentation of into the base peak follows the simplified rules outlined in Box 30.2, and for a more detailed interpretation you should consult the correlation tables to be found in the specialist texts referred to at the end of the section. 

J.K. Cha et al. developed a stereocontrolled synthesis of bicyclo[5.3.0]decan-3-ones from readily available acyclic substrates. Acyclic olefin-tethered amides were first subjected to the intramolecular Kulinkovich reaction to prepare bicyclic aminocyclopropanes. This was followed by a tandem ring-expansion-cyclization sequence triggered by aerobic oxidation. The reactive intermediates in this tandem process were aminium radicals (radical cations). The p-anisidine group was chosen to lower the amine oxidation potential. This substituent was crucial for the generation of the aminium radical (if Ar = phenyl, the ring aerobic oxidation is not feasible). 

Rearrangements of Cyclopropyl Radicals, Radical Cations, and Radical Anions... 

Rinderhagen, H., Mattay, J., Synthetic Applications in Radical/Radical Cationic Cascade Reactions, Chem. Eur. J. 2004, 10, 851 874. 

The synthetic potential of the C-Si bond cleavage from allyl- and benzylsilane radical-cations produced by photoreaction of electron-deficient iminium salts has been extensively investigated [25]. When a benzylsilane or allylsilane is irradiated in the presence of an iminium salt such as 27, a one-electron oxidation of the silane to the excited iminium salt produces a radical/radical-cation pair. Subsequent C-Si bond dissociation from the silane radical-cation by the loss of TMS+ leads to an... 

Other physical changes consequent on cross-linking include a lower solubility in solvents and several orders of magnitude higher viscosity. The yield of cross-linking is 0.15 pmol J . It is accompanied by C-H bond rupture with the formation of H2 gas and vinylene (-CH=CH-) unsaturation with yields of G 0.3 and 0.15 pmol respectively. As intermediates alkyl radicals, radical cations, and trapped electrons are suggested (Brede et al. 1989 Spinks and Woods 1990 Shkrob and Trifunac 1995). 

NO is known to be a moderately strong one-electron oxidant with the redox potential ° (NO /NO ") = + 1.04 V). The absolute rate constant [NO + P-carotene] is (1.1 0.1) X 10 M/s. The antioxidant properties of P-carotene not only reflect rates of free-radical scavenging, but also the reactivity of the resultant carotenoid radicals. Radical cations formed are highly resonance-stabilised and therefore relatively unreactive. 

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