Olefin radical cations generation

Organic and inorganic radical formation in zeolites can occur spontaneously, on adsorption of molecules into a suitably activated zeolite, or as the result of radiolysis of adsorbed species. Once a radical is formed, EPR spectroscopy can be used to follow its subsequent reactions. For example, Trifunac et al have recently described the use of variable temperature EPR to investigate reactions of olefin radical cations generated in ZSM-5 zeolites. [4]. This work shows clearly the facile rearrangement of radical cations produced by irradiation of... 

Each of these reactants is thought to occur through a common intermediate, the olefin radical cation, but the environment in which this species is generated apparently controls its observed chemistry. 

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. 

Spectroscopic studies with photo-CIDNP techniques revealed the existence of two distinct radical cations generated from hexamethyldewarben-zene, presumably rapidly interconverting. In one of these, the central carbon—carbon bond is significantly stretched and bears the unpaired spin density. In the second, the spin density is confined to one of the olefinic bonds. This example is the first to show conclusively that two different radical ion structures can correspond to a single minimum on the ground-state surface of the neutral (Roth et al., 1984). 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

It is known that radical cations generated by pulse radiolysis of aromatic olefins can add to olefins to give olefin dimer cations (37). In view of the above facts, the isomerization would be accelerated by the participation of olefin molecules. 

Radical cations can be stabilized inside zeolites and on some amorphous metal oxide surfaces. This was recognized early by the EPR observation of radical cations generated spontaneously upon exposure of certain solid catalysts to easily oxidized species such as aromatic hydrocarbons and certain olefins. " These observations reveal the presence of electron acceptor sites and, whether or not... 

Pentacyclosqualene, the symmetrical hydropicene corresponding to squalene, has not been made by acid-induced cation-olefin cyclization of squalene, despite considerable experimental study. A simple, convergent synthesis of pentacyclosqualene using cation-olefin cyclization to generate ring C was developed. The Cjo-framework was constructed by radical coupling to a tetracyclic intermediate that was also used for the synthesis of onoceradiene. 

PET reactions [2] can be considered as versatile methods for generating radical cations from electron-rich olefins and aromatic compounds [3], which then can undergo an intramolecular cationic cyclization. Niwa and coworkers [4] reported on a photochemical reaction of l,l-diphenyl-l, -alkadienes in the presence of phenanthrene (Phen) and 1,4-dicyanobenzene (DCNB) as sensitizer and electron acceptor to construct 5/6/6- and 6/6/6-fused ring systems with high stereoselectivity. 

Intramolecular coupling reactions between nucleophilic olefins have also proven to hold potential as synthetically useful reactions. The first example of this type of reaction was reported by Shono and coworkers who examined the intramolecular coupling reaction of an enol acetate and a monosubstituted olefin (Scheme 41) [50]. This reaction was conducted in an effort to probe the nature of the radical cation intermediate generated from the anodic oxidation of... 

Photoinduced single-electron transfer followed by fragmentation of the radical cation is an efficient method for generating carbon-centered radicals under exceptionally mild conditions. The fate of the thus formed radicals depends primarily on their interaction with the acceptor radical anions. Typically observed reactions are either back-electron transfer or radical coupling, but from the synthetic point of view, another most intriguing possibility is the trapping of the radical with suitable substrates such as olefins (Scheme 16). 

In homogeneous solution, the aminium cation radical may generate an olefin cation radical which reacts in cyclic fashion with another olefin before reaccepting an electron. This pathway relies on rapid bond-forming reactions compared with electron exchange. 

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