Vinyl group, alkyl radical stabilization

The substituent effects predicted for vinyl radicals are rather similar to those already observed for alkyl radicals (Table 4). Attachment of alkyl groups or it systems to the radical center stabilize the radical while the introduction of a-acceptors in the a- or / -position are destabilizing. The nature of the... 

One of the reasons for that is the high stability of the O-H bond in the newly formed silanol group (125-130 kcal/mol), and H-r can be not only a hydrocarbon molecule, but also H-OH, H-NH2, etc. The diamagnetic dioxasily-rane groups are also the generators of alkyl radicals (see subsection 4.2). Many intermediates can be obtained as the products of thermal or thermo oxidative transformations or photo transformations of other initial structures. For example, vinoxyl radicals were obtained by the photolysis of peroxide radicals of the vinyl type [119] ... 

They also revealed that primary alkyl-, vinyl-, and aryl-substituted acy radicals generated by treating selenol esters with Bu3SnH can be utilized in intermole-cular alkene addition reactions (Eq. 48) [99,98b]. Acyl radicals exhibit nucleophilic character and react efficiently with alkenes bearing electron-withdrawing or radical-stabilizing groups. 

Styrene and indene derivatives (Scheme 2, Y = Ph) are dimerized to l,4-dimethoxy-l,4-diphenylbutanes or 1,4-diphenylbutadienes (Table 7, numbers 1 and 2) [52]. The product distribution is in some cases strongly dependent on the anode potential and the supporting electrolyte. Dimerization is promoted by a-substituents that stabilize the intermediate radical cation, for example, phenyl, vinyl, alkoxy, dialkylamino groups. IJ-Alkyl substituents strongly decrease the yield of dimers and favor formation of dimethoxy-lated monomers. 

The anti stereochemistry of the sodium-ammonia reduction appears to result from the greater stability of the vinyl radical in the trans configuration, where the alkyl groups are farther apart. An electron is added to the trans radical to give a trans vinyl anion, which is quickly protonated to the trans alkene. 

One of the main advantages of the anionic cyclizations is their regioespecificity and stereoselectivity when compared with radical or other types of reactions leading to cyclic systems. This is usually due to the formation of complexes involving the lithiated alkyl, vinyl or aryl substrate and an unsaturated, double or triple, C—C bond. In some cases, a heteroatom is involved in stabilizing the transition state for the reaction. In other cases, the stereoselectivity of the cyclization is determined by the presence of several functional groups in the substrate. 

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