Electron-withdrawing groups radicals

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Radical chlorination reactions show a substantial polar effect. Positions substituted by electron-withdrawing groups are relatively unreactive toward chlorination, even though the substituents may be potentially capable of stabilizing the free-radical intermediate " ... 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Mechanistically the rearrangement is formulated to proceed via an intermediate radical-pair or ion-pair. In either case the initial step is the formation of a nitrogen-ylide 2 by deprotonation of the ammonium species with a strong base. The abstraction of a proton from the a-carbon is facilitated by an electron-withdrawing group Z—e.g. an ester, keto or phenyl group ... 

The new reaction appears to be a simple one-step procedure, which is particularly suitable for tertiary alkyl-aryldiazenes for which alternative synthetic routes are less convenient. However, aryl radicals or alkyl radicals in which the carbon-centered radical is bonded to an electron-withdrawing group (COOR, COR, CONR2, CN, S02R, etc.) do not add to diazonium salts or give only poor results (Citterio et al., 1982 c). This indicates that the radical must be a relatively strong nucleophile in order to be able to react with a diazonium ion. 

Similar additions have been successfully carried out with carboxylic acids, anhydrides, acyl halides, carboxylic esters, nitriles, and other types of compounds. These reactions are not successful when the alkene contains electron-withdrawing groups such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

Since the (n-Bu)3Sn radical (R ) is nucleophilic (17), a partial negative charge must be produced at the methine carbon whose chlorine is being abstracted. The rate of this abstraction should clearly be enhanced by electron-withdrawing groups on R due to their stabilization of this charge by inductive effects. As observed, the removal of Cl from EW (or W diad) is expected to be more facile than from EVE (or VE diad) as a result of a -halogen effect in the former structure. 

Halogenation, and particularly chlorination, unlike most radical reactions, is markedly influenced by the presence in the substrate of polar substituents this is because Cl, owing to the electronegativity of chlorine, is markedly electrophilic (c/. p. 314), and will therefore attack preferentially at sites of higher electron density. Chlorination will thus tend to be inhibited by the presence of electron-withdrawing groups, as is seen in the relative amounts of substitution at the four different carbon atoms in 1-chlorobutane (78) on photoehemically initiated chlorination at 35° ... 

A 7r-bond can react with various active species, such as the electrophile oxene and its isoelec-tronic species (nitrenes and carbenes) and radicals. A 7r-bond can also react with a nucleophile, when it is conjugated with an electron-withdrawing group. In these reactions O, N, or C atom(s) are transferred from the active species to the olefins, forming two tr-bonds, such as C—O, C—N, and C—C, at the expense of the 7r-bond. If the 7r-bond is prochiral, chiral center(s) are... 

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