Carbonium ions benzyl

Benzylic carbonium ions Benzyl chloride 7,12-dimethylbenz(a)anthracene... 

Haid Alkyl carbonium ions Benzylic carbonium ions Iminium ions Aldehydes Oxygen atoms of purine yrimidine bases in DNA Endocyclic nitrogens of purine bases in DNA Oxygen atoms of protein serine and threonine residues... 

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

In the next step the hydrated benzylic carbonium ions (3 and 4) react with free ortho and para positions on phenols to form methylene-linked bisphenols, 2,2 (5), 2,4 (6), and 4,4 (7). 

For the oxofluorination reaction, a transition state (29) has been proposed which possesses features of a benzylic carbonium ion. This leads by... 

Acylation of norephedrine (56) with the acid chloride from benzoylglycolic acid leads to the amide (57), Reduction with lithium aluminum hydride serves both to reduce the amide to the amine and to remove the protecting group by reduction (58), Cyclization by means of sulfuric acid (probably via the benzylic carbonium ion) affords phenmetrazine (59), In a related process, alkylation of ephedrine itself (60) with ethylene oxide gives the diol, 61, (The secondary nature of the amine in 60 eliminates the complication of dialkylation and thus the need to go through the amide.) Cyclization as above affords phendimetra-zine (62), - Both these agents show activity related to the parent acyclic molecule that is, the agents are CNS stimulants... 

An early process in the cure reaction is protonation of a methylolphenol, followed by loss of a molecule of water to produce a benzylic carbonium ion (see reaction 4.3). This may be followed by reaction with a second phenol to generate a bridged structure, as illustrated in Reaction 4.4. Alternatively the... 

The factor of seven variation between k2 for the ordinary and benzylic tertiary hydrogen is too small to be associated with carbonium ion formation. The observed degrees of retention and 0 transfer imply a caged radical rapidly reducing Mn(VI) to Mn(V) by accepting oxygen... 

The results of this work are not limited to just S-b-MM and S-b-tBM, but may be extended to include styrene derivatives such as p-methylstyrene and p-t-butylstyrene 1). In addition to t-butyl methacrylate, other alkyl esters capable of stabilizing a carbonium ion, such as benzyl methacrylate and allyl methacrylate, should exhibit similar reactivity toward acidic hydrolysis and TMSI. In contrasting the hydrolysis of tBM blocks with TsOH and their reaction with TMSI, it should be noted that the hydrolysis is reportedly catalytic in nature (7-10), whereas the reaction with TMSI is stoichimetric. Therefore the latter approach may allow one to more easily "dial in" a desired level of methacrylic acid or metal methacrylate. 

For unsubstituted PAH, such as benzo[a]pyrene (BP), pyridinium or acetoxy derivatives are formed by direct attack of pyridine or acetate ion, respectively, on the radical cation at C-6, the position of maximum charge density (Scheme 1). This is followed by a second one-electron oxidation of the resulting radical and loss of a proton to yield the 6-substituted derivative. For methyl-substituted PAH in which the maximum charge density of the radical cation adjacent to the methyl group is appreciable, as in 6-methylbenzo[a]-pyrene (6-methylBP) (Scheme 2), loss of a methyl proton yields a benzylic radical. This reactive species is rapidly oxidized by iodine or MnJ to a benzylic carbonium ion with subsequent trapping by pyridine or acetate ion, respectively. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Methylbenzenes lose a proton from a methyl group to form a benzyl radical. In aqueous M-percbloric acid this reaction is fast with a rate constant in the range 10 lO s and the process is not reversible [24]. The process becomes slower as the number of methyl substituents increases, Hexaethylbenzene radical cation is relatively stable. When the benzyl radical is formed, further reactions lead to the development of a complex esr spectrum. Anodic oxidation of hexamethylbenzene in trifluoroacetic acid at concentrations greater than 1 O M yields the radical-cation I by the process shown in Scheme 6.1 [14], Preparative scale, anodic oxidation of methylbenzenes leads to the benzyl carbonium ion by oxidation of the benzyl radicals formed from the substrate radical-cation. Products isolated result from further reactions of this carbonium ion. 

Kneen showed that a Wagner-Meerwein type rearrangement occurred on reaction of AuBrs(vp) with methanol and ethanol, the substituted product having a 6-membered ring system. Ionisation, rapid rearrangement of the primary carbonium ion to a more stable secondary, benzylic carbonium ion and subsequent attack by solvent can account for this reaction. 

For alkenes, the reactivity is based on the stability of the carbonium ion formed and they follow the order tertiary > secondary > primary. Thus olefins react as follows (CH3)2C=CH2 = (CH3)2C=CHCH3 > CH3CH=CH2 > CH2=CH2. Allylic and benzylic carbonium [19,20] ions are also favored where appropriate. 

When the chromene has a side chain with a double bond in a suitable position, acids induce an intramolecular cyclization. Thus for gainbogic acid74 and cannabichromene211,263 the simplest product (68) comes from attack of the olefin on the benzylic carbonium ion [Eq. (21)]. 

Moat reoently Addy and co-workers1 17 made the notable discovery that the normal reaction ( .e. terminal attack) of benzyl mine with p-sub tituted styrene oxides exhibits a positive p-value in the Hammett plot, while the abnormal reaction (, , bonxylio attack) exhibits a negative p -value. These facts are consistent with a transition state of appreciable carbonium ion character for the latter reaction. 

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