Benzyl cations reactivity

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

Poverenov, E. Leitus, G. Milstein, D. Synthesis and reactivity of the methylene arenium form of a benzyl cation, stabilized by complexation. J. Am. Chem. Soc. 2006, 128, 16450-16451. 

The behavior of the isomeric dihydronaphthalenes emphasizes the importance of the relative stabilities of carbocation intermediates in ionic hydrogenations. Treatment of 1,2-dihydronaphthalene with Et3SiH/TFA at 50-60° gives a 90% yield of tetralin after one hour. Under the same conditions, the 1,4-dihydronaphthalene isomer gives less than 5% of tetralin after 70 hours.224 This difference in reactivity is clearly related to the relatively accessible benzylic cation formed upon protonation of the 1,2-isomer compared to the less stable secondary cation formed from the 1,4-isomer.224... 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

Fig. 21. A reaction mechanism proposed by Xiong et al. to rationalize the toluene disproportion reaction on zeolites. The benzyl cations and benzenium ions were proposed as reactive intermediates for this zeolite-catalyzed, high-temperature reaction. (Reprinted with permission from Xiong et al. (122). Copyright 1995 American Chemical Society.)...

This then readily undergoes cleavage to produce benzene and a dialkylbenzene. The initial R+ cation initiating the reaction might arise from some impurity present in the reaction mixture. Consistent with this mechanism is the observed very low reactivity of methylbenzenes due to the necessary involvement of the primary benzyl cations. At the same time 1,1-diarylalkanes undergo cleavage with great ease. 

Acidolysis of benzylic C-O bonds becomes easier with increasing stability of the corresponding benzylic cation. The sensitivity of benzylic linkers towards acids can therefore be fine-tuned by varying the substitution pattern on the arene. Electron-donating substituents will increase the sensitivity towards acids, whereas electron-acceptors will diminish acid-sensitivity. This order of reactivity towards electrophiles... 

Reactivities comparable to allylic halides are found in the nucleophilic displacement reactions of benzylic halides by SN1 and SN2 mechanisms (Table 14-6). The ability of the benzylic halides to undergo SK1 reactions clearly is related to the stability of the resulting benzylic cations, the electrons of which are extensively delocalized. Thus, for phenylmethyl chloride,... 

From the relative solvolysis rates of 213 and 215, Shimizu and coworkers calculated a stabilization by 7 kcalmoD1 of the benzyl cation by the /1-Si—Si bond compared with a /1-C—Si bond. Due to the antagonistic effects of the a- and the /)-silyl groups, the net effect of the disilanyl group is relatively small86. Thus, 213 is only 12 times as reactive as a-methylbenzyl bromide 219, X = Br (Table 7 in 30% acetone the relative rate is decreased to 0.66). This might be compared with the rate acceleration of 1.05 x 105 by... 

It is not intended to extend this discussion of reactions of carbocations with water to consideration of the alcoholic solvents trifluoroethanol (TFE) and hexa-fluoroisopropanol (HFIP), which have been extensively studied and reviewed by McClelland and Steenken.3 However, an important point of interest of these solvents is that their reactivities toward carbocations are greatly reduced compared with water (by up to a factor of 104 in TFE and 108 in HFIP) and that differences in rate constants can be observed between cations which would react indiscriminately at the solvent relaxation limit in water. The following comparisons of rate constants for carbocations with similar pAR values reacting with hexafluoroiso-propanol241,242 reinforces the conclusion that structural variations in the cation lead to changes in intrinsic barrier and, for example, that phenyl substitution is probably associated with such an increase in going from benzyl to benzhydryl (although the benzyl cation itself is not shown). 

A further dependence of the selectivity between different nucleophiles on the stability and reactivity of carbocations was found by Richard and Amyes in a study of reactions of alcohols and carboxylate anions with -substituted a-trifluoromethyl benzyl cations (75, X = Me, OMe, SMe, N(Me)CH2CF3, and NMe2) monitored using the azide clock.305 Apart from the methyl-substituted substrate, for which the reactions approached diffusion control,... 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

Carbocation thermochemical data are usually obtained from measurements of proton transfer or hydride transfer equilibria. In the first case, alkyl cations are produced by protonation of the corresponding olefin. However, some species, like, for example, the benzyl cation, cannot be obtained by this route. Moreover, measurements of proton-transfer equilibria are often complicated by side reactions like addition of the cations to the double bonds. With respect to H transfer, Cl transfer reactions offer the advantage that the position of reactive attack is well defined. Moreover, they are usually much faster than H" transfers and lead to a deeper well in the reaction coordinate because the intermediate adduct [R—Cl—R ]+, a chloronium ion, is more stable than [R—H—R ]+, the corresponding proton bound species4. On the other hand, one disadvantage of the CF transfer measurements is the much greater scarcity of A//°(RC1) data as compared to AH°(RH). 

ECE mechanism (Parker, 1969a see p. 25). In this particular case, a series of benzylic cations were found to show anomalous reactivities, in that the less reactive ones were selective toward the weaker nucleophile, acetonitrile, contrary to predictions based on concepts from homogeneous solution chemistry.1 2 Moreover, in a certain region of water concentrations the competition ratio between the... 

The relative stabilities of substituted benzyl cations [21C ] are correlated by equation (27) with a high resonance demand parameter Tq = 1.29 (Mishima et al., 1987, 1995). The linear correlation for the whole range of substituents down to the 3,5-(Cp3)2 group (Fig. 27), contrasts with the concave Y-T plot (Fig. 7) of the solvolytic reactivities of [21]. Note that the Tq value for the gas-phase stabilities of [21C ] is identical with the r value assigned for the SnI solvolysis of [21] tosylates hence, the r value of 1.29 must be an intrinsic index inherent in [21C ], rather than a correlational artifact of a non-linear relationship for the complex solvolysis mechanism. 

Cross-linking reactions require an intermolecular Friedel-Crafts reaction between tertiary cations and positions of the phenyl group made less reactive by the presence of bulky groups on the indane structure. Electrophilic attack of benzylic cation 29 (Fig. 5) upon unsubstituted positions of the phenyl group of 15 is sterically inhibited. 

The total -electron population for the benzyl cation is also shown on Fig. 3-11, where we see again that the Coulombic term favours reaction at the meta position. Finally, with j/2 and >j/3 as the frontier orbitals, we can see why Z-substituted benzenes are less reactive than benzene ip2 is lower in energy than ip2 in benzene. Furthermore, the Coulombic term must include allowance for the repulsion between the positive charge of the electrophile and any positive charge in the benzene ring. Most of this charge will be in the ortho and para positions, but it will still repel an electrophile even from the meta position. 

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