Alkenes radical cations from

Scheme 1 Generation of alkene radical cations from alkenes...

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

A y-lactone was formed in excellent yield by the nucleophilic cyclization of a carboxylic acid onto an alkene radical cation generated from a (i-nilrophosphale under tin hydride conditions (Scheme 21) [139]. Related experiments employing the acetate group and an internal carboxylate nucleophile failed, emphasizing the very rapid collapse of the alkene radical cation/acetate ion pair [127]. 

An extremely interesting feature of these mechanisms is the fact that superoxide and the alkene radical cation are both formed in the reduction (Fig. 20) and also in the Frei oxidation (Fig. 19). In the Frei photo-oxidation, however, they are formed concurrently in a tight ion pair and collapse to product more rapidly than their diffusive separation. In the reduction (Fig. 20), the formation of the radical cation and superoxide occur in independent spatially separated events allowing the unimpeded diffusion of superoxide which precludes back-electron transfer (BET) and formation of oxidized products. The nongeminate formation of these two reactive species provides the time necessary for the radical cation to abstract a hydrogen atom from the solvent on its way to the reduced product. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

Crich D, Huang W (2001) Dynamics of alkene radical cations/phosphate anion pair formation from nucleotide C4 radicals. The DNA/RNA paradox revisited. J Am Chem Soc 123 9239-9245 Das S, Deeble DJ, Schuchmann MN, von Sonntag C (1984) Pulse radiolytic studies on uracil and uracil derivatives. Protonation of their electron adducts at oxygen and carbon. Int J Radiat Biol 46 7-9... 

Photo-NOCAS reactions of p-dicyanobenzene with 2-methylpropene in acetonitrile afforded novel 3,4-dihydroisoquinoline derivatives, as shown in Scheme 132 [482], This photoreaction is initiated by a single electron transfer from olefin to p-dicyanobenzene. Acetonitrile as a nucleophile combined with the alkene radical cation and the resulting radical cation adds to the radical anion of 1,4-di-cyanobenzene. Cyclization to the ortho position of phenyl group followed by loss... 

Reactions of PET-generated alkene radical cations have been one of the important areas of research over the years and several reviews have been written on this subject [2, 5,11], A vivid summary of this topic has been provided by Mattay [10] recently. However, we will discuss here some representative examples of synthetic interest from olefin radical cations. The reactivity profiles of alkene radical cations may be illustrated on the lines of Mattay [10b] as shown below. 

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