Hydrogen abstraction benzylic position

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

However, many anomalous results have been reported for these substrates. The benzylic position is not always the most favored. One thing certain is that aromatic hydrogens are seldom abstracted if there are aliphatic ones to compete (note from Table 5.3, that D for Ph—H is higher than that for any alkyl H bond). Several a- scales (similar to the a, ct, and scales discussed in Chapter 9) have been developed for benzylic radicals. ... 

Sometimes it is useful to be able to remove a protecting group by photolysis. 2-Nitrobenzyl carbamates meet this requirement. The photoexcited nitro group abstracts a hydrogen from the benzylic position, which is then converted to a a-hydroxybenzyl carbamate that readily hydrolyzes232... 

In view of the short lifetime ( 10 s), which has been reported for nitrobenzene and might be expected for similar nitroaromatics with low lying (n, 7T ) states, and since intramolecular hydrogen abstractions from benzylic positions take place, lack of oxygen quenching may be inconclusive. 

Bromination occurs exclusively at the benzylic position, i.e. adjacent to the benzene ring. The radical formed at this position is resonance stabilized, whereas no such stabilization is available to the primary radical formed by abstraction of one of the methyl hydrogens. 

The transalkylation of fm-alkylbenzenes follows a different route, since they have no abstractable benzylic hydrogen. They were shown to transalkylate by a dealkylation-transalkylation mechanism with the involvement of free ferf-alkyl cations. The exceptional ability of fcrt-alkyl groups to undergo transalkylation led to the extensive utilization of these groups, especially the fm-butyl group, as positional protective groups in organic synthesis pioneered by Tashiro.223... 

A more detailed investigation using (R)-177 and (S)-l-2Hi-ethylbenzene 178 and the vaulted Fe(III) porphyrin 172 revealed an interesting enantioselecti-vity. It was discovered that the pro-R hydrogen was removed from the benzylic position preferentially to the pro-S by a factor of 2 1. Moreover, the radical formed by pro-R abstraction was trapped with > 90% retention of configura-... 

Nitrobenzyl halides are reduced in a 1 e-process to radical anions, which rapidly lose halide ion to form the neutral nitrobenzyl radical. The rates for this dissociation were calculated from cyclic voltammetry data to be k = 2,5 for m-nitrobenzyl chloride and k = 2 109 secfor o-nitrobenzyl bromide. The nitrobenzyl radicals predominantly dimerize (90%), whereas a small amount yields nitrotoluenes (< 10%) by hydrogen abstraction 4541. From a series of substituted benzyl bromides those with the more positive reduction potential form bibenzyl in 25—74% yield, whereas from the less easily reducible ones dibenzyl-mercury derivatives are obtained (50—60%) 485). Reduction of benzyl chloride at the plateau of the first wave yields dibenzylmercury 4 By reduction of diphenyliodonium hydroxide at -1,6 V 51% diphenylmercury is obtained 488 ... 

Suitable alkylbenzene side chains are oxidized at the benzylic position under the action of mCPBA, air, and NaHC03 to generate the corresponding ketones. The oxygen-centered radical formed from mCPBA, abstracts the benzylic hydrogen atom of the benzylic substrate (68) to form a benzylic radical, and then it reacts with molecular oxygen... 

Alkylbenzenes undergo free-radical halogenation much more easily than alkanes because abstraction of a hydrogen atom at a benzylic position gives a resonance-stabilized benzylic radical. For example, ethylbenzene reacts with chlorine in the presence of light to give cr-chlon >e thy I benzene. Further chlorination can occur to give a dichlorinated product. 

However, the promptness with which the oxidations start and their ease of propagation are not only dependent on the alkyl substituents which are to be oxidized, but also very much on the type and the position of the substituents in the aromatic system. The first, rate-determining step of the alkylarene oxidation - either the formation of a radical cation via an electron-transfer mechanism, or the direct formation of a benzylic radical by a-hydrogen abstraction - is increased with the electron density of the aromatic system supported by the substituents present. 

The mechanism of the Wohl-Ziegler bromination involves bromine radicals (and not imidoyl radicals). The radical initiator is homolytically cleaved upon irradiation with heat or light, and it reacts with Bra (which is always present in small quantities in NBS) to generate the Br- radical, which abstracts a hydrogen atom from the allylic (or benzylic) position. The key to the success of the reaction is to maintain a low concentration of Bra so that the addition across the C=C double bond is avoided. The Bra is regenerated by the ionic reaction of NBS with the HBr by-product. 

Figure 9 drastically simplifies the major reaction paths of alkyl-naphthalene components. Via H-abstraction and successive decomposition reactions, they can easily form, either naphthalenes with unsaturated side chains (vinyl, allyl or alkenyl side chains) or RSR and smaller decomposition products. The preferential radical attack on the alkyl side chain is in the benzyl position due to the weak hydrogen bond. This makes it easy to justify either the formation of RSR or the successive / -decomposition reaction to form vinylnaphthalene. The net result of the successive recombination and condensation reactions of these aromatic species is the formation of PAH of increasing molecular weight with a progressively lower hydrogen to carbon ratio. 

In contrast to photochlorination, the corresponding hydrogen abstraction or addition steps in photobromination are usually reversible, endothermic and more selective.155 The hydrogen abstraction rates in the allylic or benzylic positions can be relatively high, however. Table 6.19 shows the relative reactivities of the benzylic C—H bonds in a series of alkylbenzenes in the photobromination reaction carried out in CCI4.1304 It is apparent that not only electronic but also steric effects control the reaction kinetics. 

The likely pathway of photochemical cleavage is a Norrish type II cleavage reaction (Scheme 17.1). During the photoisomerization, the activated oxygen of the nitro group abstracts a y-hydrogen from the benzylic position to produce a quinone methide-like intermediate, which subsequently rearranges to afford an o-nitrosobenzaldehyde and thereby releases the carboxylic acid product. 

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