Benzylic radical intermediate

Reaction occurs exclusively at the benzylic position because the benzylic radical intermediate is stabilized by resonance. Figure 16.20 shows how the benzyl radical is stabilized by overlap of its p orbital with the ring 77 electron system. 

The oxidation generates highly delocalized phenoxy radicals (PhO, Scheme 2.21), which may initiate (i) a radical polymerization process, trapping the reactant (CF) to give a benzyl radical intermediate (QMR), or it may (ii) follow a radical coupling to produce the p-QM p-O-QM, which being a reactive electrophile could undergo cationic polymerization. 

Various saccharin derivatives 260 have been prepared by chromium (VI) oxide catalyzed H5IO6 oxidation of substituted ort/ro-toluenesulfonamides 259 <06T7902>. The reaction presumably proceeds through a benzylic radical intermediate 261 generated from the... 

A plausible reaction pathway is shown in Scheme 3.146. Alkyl radicals, formed by electron transfer to alkyl bromides from a reduced titanocene complex (step 1), add to the terminal carbon of styrene yielding benzyl radical intermediates (step 2). Recombination of the benzyl radicals with a titanocene complex gives rise to benzyl-Ti intermediates (step 3) which fhen undergo transmetalation with "BuMgCI to afford benzylmagnesium chlorides (step 4). The dialkylated products are formed by reaction of benzylmagnesium chlorides with fhe alkyl halides (step 5). 

The photochemical and photophysical behavior of the N,N-dimethylaminoalkyDstyrenes 37-42, in which the amino group is attached to the styrene ci- or p-carbon by an ethyl, propyl, or butyl polymethylene linker, has been investigated in the author s laboratory. The preparative results are summarized in Scheme 7 and display remarkable dependence upon the chain length and point of attachment to styrene. Whereas formation of the intramolecular adduct 43 upon irradiation of 37 requires a-C- H transfer from a Al-methyl to the styrene P-carbon, formation of adduct 45 upon irradiation of 40 requires a-C-H transfer from a A -methyl to the styrene a-carbon. The latter process results in formation of a secondary alkyl radical rather than the more stable benzyl radical intermediate. The (aminopropyl)sty-renes 38 and 41 both fail to undergo intramolecular photoaddition. The (aminobutyl)styrenes 39 and 42 form adducts which result from a-C—H transfer from a Al-methylene to the styrene P-carbon, the former reaction leading to a mixture of dias-tereomeric aminocyclopentanes 44 and the later reaction to ami-nocyclohexanes 46. 

Benzylic hydrogens are even more reactive than allylic hydrogens in radical substitution reactions due to the additional delocalization that is possible for a benzylic radical intermediate (see Practice Problem 10.12). 

We have already seen in this chapter that we can substitute bromine and chlorine for hydrogen atoms on the benzene ring of toluene and other alkylaromatic compounds using electrophilic aromatic substitution reactions. We can also substitute bromine and chlorine for hydrogen atoms on the benzylic carbons of alkyl side chains by radical reactions in the presence of heat, light, or a radical initiator like a peroxide, as we first saw in Chapter 10, (Section 10.9). This is made possible by the special stability of the benzylic radical intermediate (Section 15.12A). For example, benzylic chlorination of toluene takes place in the gas phase at 400-600 °C or in the presence of UV light, as shown here. Multiple substitutions occur with an excess of chlorine. 

Halogenation of a larger alkyl side chain is highly regioselective, as illustrated by the halogenation of ethylbenzene. When treated with NBS, the only monobromo organic product formed is 1-bromo-l-phenylethane.This regioselectivity is dictated by the resonance stabilization of the benzylic radical intermediate. The mechanism of radical bro-mination at a benzylic position is identical to that for allylic bromination (Section 8.6A). 

Atom-transfer addition of primary and secondary bromide oxazolidinones to alkenes in the presence of Lewis acids has been investigated and the effects of solvent, temperature, and catalyst were determined. The best Lewis acids were found to be Sc(OTf)3 and Yb(OTf)3 and control was possible using chiral auxiliary oxazolidinones. Tertiary bromides did not react (Scheme 37). Stereochemistry of reduction of the cw-mesityl-alkene (53) with BusSnH proceeds to give the ( )-alkene (54) as the major product ( Z = 9 1). Theoretical calculations at the BLYP/6-31G level were undertaken to rationalize the stereochemistry. Asymmetric hydroxylation of the benzylic position of a range of substrates can be achieved by using a chiral dioxomthenium(VI) porphyrin (55). The oxidation proceeds via a rate-limiting H-abstraction to produce a benzylic radical intermediate. ... 

Cp atom than at the Ca atom of styrene, consistent with a non-concerted mechanism. The rate-limiting generation of a benzylic radical intermediate has been proposed and it has been suggested that the spin delocalization effect is more important than the polar effect in the epoxidation reactions. The head-on approach model rather than a side-on approach model is implicated in the epoxidation.The dioxoruthenium(VI) complex (5) (Figure 2) catalyses the enantioselective epoxidation of alkenes with enantiomeric excess (ee) up to 11%. Kinetic studies on the epoxidation of para-substituted styrenes suggest the formation of a radical intermediate. ... 

The observed by-products (tert-butyl ether and the compound resulting from the dimerization of the diphenylmethane) seem to support a radical mechanism (Scheme 4.18). Firstly, the benzylic radical intermediate and the BuO radical 18-A, generated via an SET process, undergo an electrophilic addition to styryl acetate leading to an iron-coordinated radical intermediate 18-B. The latter, after decomposition of the intermediate 18-C, leads to the desired product, AcO Bu, and re-generates the iron catalytic species. 

The DCNB-sensitized addition reactions of 1,1-diarylethylenes with ammonia or primary amines yield the a ri-Markovnikov adducts. The mechanism is analogous to that shown in Scheme 7 for addition to sthbene. The regioselectivity is determined by nucleophilic attack of the amine on the alkene cation radical to yield the more stable benzyl radical intermediate. The mechanism and dynamics of the reactions of p-methoxystyryl radical cations with amines have been investigated. Anihne and EtjN are found to react as electron donors with rate constants near the diffusional Hmit. Primary amines react as nucleophiles, with somewhat slower rate constants. 

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