Substitution, radical substrate effects

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

Although most of the reactions to be considered under this head are net, i.e. overall, displacements or substitutions, this is not commonly achieved directly, cf. Sw2. In some cases a radical is obtained from the substrate by abstraction (usually of H), and this radical then effects displacement on, or addition to, a further species. In some cases, however, the net displacement is achieved by addition/abstraction. 

Recent studies by both Zimmerman and DeCosta and Pincock on the nature of the meta effect have raised concerns about whether heterolytic or homolytic cleavage should be considered the primary photochemical process. According to Pincock, the mechanistic pathway for all substituted arylmethyl substrates begins with homolysis of theC-O ester bond to the substrate followed by a competition between electron transfer to an ion pair or typical ground-state radical reactions (Scheme 7). For those arylmethyl derivatives substituted with a meta electron-donating group, such as methoxy, the electron transfer occurs more rapidly than competing radical processes due to favorable redox properties of the radical pair. 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

The rate of an electron transfer from the reduced catalyst to the substrate is also important. If the rate is excessively high, the electron exchange will occur within the preelectrode space and the catalytic effect will not be achieved. If the rate is excessively low, a very high concentration of the catalyst will be needed. However, at high concentration, the anion-radicals of the catalyst will reduce the phenyl radicals. Naturally, this will be unfavorable for the chain process of the substitution. As catalysts, substances that can be reduced at potentials by 50 mV less negative than those of the substrates should be chosen. The optimal concentration of the catalyst must be an order lower than that of a substrate (Swartz and Stenzel 1984). 

Also, the influence of substituents in the meta and para positions of the benzoyl radical is in accordance with the polar character of the substituent. The rates for 4-cyanoquinoline relative to 4-chloroquinoline with meta- and para-substituted benzoyl radicals were obtained. Plots of log cn/ ci vs. a of the substituents in the benzoyl radical gave a Hammett correlation p was found to be —0.49, implying that the effect of the substituent is small, much smaller than the effect of the substituents in the heteroaromatic substrate. 

---