Benzyl system aromatics

SN1 reactions of benzylic systems. Perhaps the most interesting system to be examined using the variable oxygen probe is a series of derivatives [108] of 1-phenylethanol (Edwards et al., 1986b). By varying substituents in the aromatic ring of the R group of R-OX it was possible to monitor... 

Among the halides that react through this process are unactivated aromatic and heteroaromatic halides, vinyl halides, activated alkyl halides [nitroalkyl, nitroallyl, nitro-benzyl and other benzylic halides substituted with electron-withdrawing groups (EWG) as well as the heterocyclic analogues of these benzylic systems] and non-activated alkyl halides that have proved to be unreactive or poorly reactive towards polar mechanisms (bicycloalkyl, neopentyl and cycloalkyl halides and perfluoroalkyl iodides). 

Fig. 7. Schematic representation of the stereoelectronic factors in the cieavage of radical ions. Case (a) involves a benzylic system, where the scissile bond can strongly overlap with the (generic) n orbitals of the aromatic system bearing the unpaired electron. Case (b) shows an aromatic system which is orthogonal to the scissile bond...

Introduction of substituents into the aromatic ring makes the benzylic system prone to further deblocking methods. Thus, o-nitrobenzyl ethers, available from o-nitrobenzyl bromide, are cleaved photolyti-caiiyi38 and the mono- and di-methoxybenzyl groups (Mpm and Dmpm) are removable by oxidation... 

For the aromatic substrates (toluene and mesitylene) competition is observed between benzylic and aromatic C-H activation. Thus, toluene affords the meta-(402) and para (403) tolyl-complexes, alongside the benzyl system 401. Upon heating, the benzyl complex is lost, and a 2 1 thermodynamic mixture of 402 403 obtained. Similarly, mesitylene affords the 3,5-dimethylbenzyl (405) and mesityl (404) complexes (3 1), though comprehensive characterisation of the latter was precluded by its failure to convert cleanly to the corresponding chloride, the dichloride 373 instead being obtained. 

The reaction involves two intermolecular H shifts. The biradical [A] decays with an initial [l,5]-sigmatropic H shift to give another biradical [B]. This biradical undergoes a second [l,5]-sigmatropic shift to afford the aromatic bicy-clic adduct. The mechanism gets support by the synthesis of [C] from deuter-ated dienyne-allene, where deuterium from one of the deuterated gem-dimethyl groups migrates to benzylic system (Scheme 24.25) [9]. 

The combination of a secondary benzyl alcohol with Hf(OTf)4 in nitromethane was a highly effective secondary benzylation system. Secondary benzylation of carbon (aromatic compounds, olefins, an enol acetate), nitrogen (amide derivatives), and oxygen (alcohols) nucleophiles was carried out with a secondary benzyl alcohol and 1 mol % of Hf(OTf)4 in the presence of water. Secondary benzyl alcohols and nucleophiles bearing acid-sensitive functional groups (e.g.,icri-butyldimethylsilyloxy and acetoxy groups and methyl/benzyl esters) could be used for alkylation. Hf(OTf)4 was the most active catalyst for this alkylation, and trifluoromethanesulfonic acid (triflic acid, HOTf) also proved to be a good catalyst. In such cases, the catal)fiic activity of metal triflates and HOTf increased in the order La(OTf)3 [Pg.346]

The phenylacetic acid derivative 469 is produced by the carbonylation of the aromatic aldehyde 468 having electron-donating groups[jl26]. The reaction proceeds at 110 C under 50-100 atm of CO with the catalytic system Pd-Ph3P-HCl. The reaction is explained by the successive dicarbonylation of the benzylic chlorides 470 and 471 formed in situ by the addition of HCl to aldehyde to form the malonate 472, followed by decarboxylation. As supporting evidence, mandelic acid is converted into phenylacetic acid under the same reaction conditions[327]. 

FIGURE 11 10 The lowest energy tt molecular orbital of benzyl radical shows the interaction of the 2p orbital of the benzylic carbon with the TT system of the aromatic ring... 

Vanadium pentafluoride replaces benzylic hydrogen by fluonne but also adds fluonne to the aromatic system, giving fluonnated cyclohexadienes and cyclohexenes [5] (equation 5)... 

The dehydration reaction leads by an Ea process to 8 and is promoted by the tertiary, benzylic nature of the OH group at Ce and its antiperiplanar trans relationship to the H atom at Csg. Furthermore, one of the cannonical forms of the enolizable 0-dicarbonyl system present at Cn and Cia has a double bond in the C ring. Thus, dehydration leads to aromatization of the C ring, and this factor must provide some of the driving force for the reaction. 

Directed lithiation of aromatic compounds is a reaction of broad scope and considerable synthetic utility. The metalation of arenesulfonyl systems was first observed by Gilman and Webb and by Truce and Amos who reported that diphenyl sulfone is easily metalated at an orf/io-position by butyllithium. Subsequently, in 1958, Truce and coworkers discovered that metalation of mesityl phenyl sulfone (110) occurred entirely at an orf/io-methyl group and not at a ring carbon, as expected. Furthermore, refluxing an ether solution of the lithiated species resulted in a novel and unusual variation of the Smiles rearrangement and formation of 2-benzyl-4,6-dimethyl-benzenesulfinic acid (111) in almost quatitative yield (equation 78). Several other o-methyl diaryl sulfones have also been shown to rearrange to o-benzylbenzenesulfinic acids when heated in ether solution with... 

Sections D through H of Scheme 3.2 involve oxygen nucleophiles. The hydrolysis reactions in Entries 12 and 13 both involve benzylic positions. The reaction site in Entry 13 is further activated by the ERG substituents on the ring. Entries 14 to 17 are examples of base-catalyzed ether formation. The selectivity of the reaction in Entry 17 for the meta-hydroxy group is an example of a fairly common observation in aromatic systems. The ortho-hydroxy group is more acidic and probably also stabilized by chelation, making it less reactive. 

The catalytic system employing (2 - Fur)3P as ligand was applied to the coupling of methyl vinyl ketone and ethyl vinyl ketone to aromatic, aliphatic, acetylenic, and olefinic aldehydes (Scheme 23) [37]. Despite the hydrogenation conditions, alkyne and alkene moieties, as well as benzylic ether and nitro functional groups all remained intact. Furthermore, extremely high lev-... 

Zinc chloride was used as a catalyst in the Friedel Crafts benzylation of benzenes in the presence of polar solvents, such as primary alcohols, ketones, and water.639 Friedel-Crafts catalysis has also been carried out using a supported zinc chloride reagent. Mesoporous silicas with zinc chloride incorporated have been synthesized with a high level of available catalyst. Variation in reaction conditions and relation of catalytic activity to pore size and volume were studied.640 Other supported catalytic systems include a zinc bromide catalyst that is fast, efficient, selective, and reusable in the /wa-bromination of aromatic substrates.641... 

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