Radicals continued fragmentation reactions

Free radicals are the atom groups or molecule fragments having unpaired electrons. Most of them are unstable with high reactivity. Interacting between themselves and with other molecules they produce new compounds that continue chemical reactions based on chain principle - similar to neutrons in chain nuclear reactions. In many cases such processes are the main reason of pathologic condition of living systems [1]. 

The formed radical anion fragments into a radical and the anion of the leaving group (equation 2). Coupling between the radical and the nucleophile present in the reaction media forms the radical anion of the substitution product (equation 3), which by an ET to the substrate regenerates a radical anion responsible for continuing the propagation cycle of the process (equation 4). 

The aromatic Sj l reaction is a chain process (Scheme 10.1). In the initiation step, the radical anion of the substrate ArX" is formed this radical anion fragments to afford the radical Ar" and the X ion. In some system, the ET and fragmentation occur simultaneously (dissociative ET (DET)) [2]. The radical thus formed can react with the Nu to give the radical anion ArNu ", which by ET to the substrate affords the substitution product ArNu and the radical anion ArX " required to continue the propagation cycle. 

While this reaction with solvent continues to provide free radicals, these may be less reactive species than the original initiator fragments. We shall have more to say about the transfer of free-radical functionality to solvent in Sec. 6.8. 

Despite great advances in our understanding of electron transfer (ET) and subsequent reactions of radical ions over the last 20 years, work in this area continues to provide challenges and surprises. While it was recognized many years ago that the addition or removal of an electron can be used to activate molecules towards addition or fragmentation it is only recently that the kinetics of a number of these reactions have been quantified systematically to allow comparison with and improvement of ET theories. Normally one... 

The driving force for the fragmentation is formation of the C=0 double bond. If R reacts with Bu3SnH, a tributyltin radical is produced which continues the chain. Carried out in this manner this reaction is called the Barton deoxygenation of alcohols, since alcohols are precursors for the thiono esters. 

The photocatalytic radical generation was also used in additions to methyl cyanoformate 143 [240]. The product distribution was dependent on the reaction temperature. At room temperature ot-imino esters 144 were formed in 51-99% yield, while at 90 °C nitriles 145 were isolated in 59-76% yield. At low temperature, the iminyl radical 140C generated by the radical addition, abstracts a hydrogen atom from the substrate to continue a chain reaction, while its fragmentation to the nitrile prevails at high temperature, generating carbon dioxide and a methyl radical 146, which acts as a chain carrier. 

Reactions of PhCH2Cl and m-02NC6H4CH2Cl with 9 and 10 failed to produce the a-alkylated ketones under similar reaction conditions. The failure of a chain reaction with PhCH2Cl reflects the increased reduction potential of the alkyl halide while, in the case of the latter, the radical anion formed by the facile reduction does not readily undergo the fragmentation step required to continue the chain process. 

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