Benzylic cations, also

Addition of carbon and fluorine can also be initiated by elettraphilic attack on a/luorinated otefin under strongly acidic conditions [250, 251, 252,253 254 255] Best known are fluoroalkylations ot tetrafluoroethylene by tertiary or highly halogenated allylic or benzylic cations in the presence of antimony pentafluoride (equation 53)... 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

The acetoxy group is introduced exclusively at the benzylic carbon. This is in accord with the intermediate being a weakly bridged species or a benzylic cation. The addition of bromide salts to the reaction mixture diminishes the amount of acetoxy compound formed by shifting the competition for the electrophile in favor of the bromide ion. Chlorination in nucleophilic solvents can also lead to solvent incorporation, as, for example, in the chlorination of 1-phenylpropene in methanol.37... 

Benzyl Alcohols. Benzyl alcohols of nearly all kinds undergo reduction when treated with acid in the presence of organosilicon hydrides. The most obvious exception to this is the behavior of benzyl alcohol itself. It resists reduction by the action of trifluoroacetic acid and triethylsilane, even after extended reaction times.26 Reducing systems consisting of triethylsilane and sulfuric acid/acetic acid or p-toluenesullonic acid/acetic acid mixtures also fail to reduce benzyl alcohol to toluene.134 As previously mentioned, substitution of boron trifluoride for trifluoroacetic acid results in the formation of modest yields of toluene, but only when a very large excess of the silane is used in order to capture the benzyl cation intermediate and suppress Friedel-Crafts oligomerization processes.129,143... 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

Canonical forms can also be drawn for benzylic cations and allylic cations. 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

Likewise, a study on the bromination of these compounds also indicated that the 1,3-addition was 100% syn stercospecific.10 Interestingly, 3-phenylbicyclo[1.1.0]butane-l-carbonitrile, from which a relatively stable benzylic cation can be formed, yielded a mixture of cis- and /ram-products.10 An electron-transfer mechanism has been proposed for these reactions.10 A recent investigation on perchloric acid catalyzed methanol addition to 3-methylbicyclo[1.1.0]butane-1-carbonitrile and methyl 3-mcthylbicyclo[1.1.0]butane-l-carboxylate, however, showed that mixtures of irons- and cA-cyclobutanes were generated, with the m-isomers predominating.11... 

The indirect anodic cleavage of carbon-hydrogen bonds in the benzyl position using triarylamine mediators was also used for mild and selective deblocking of hydroxy, carboxyl, and amino groups. The primarily formed cation radical of the protective group is readily deprotonated in the benzyl position by an added base (Eq. (107)). This benzylic radical is easily further oxidized to the benzyl cation which subsequently is cleaved by attack of a nucleophile, such as water (Eq. (108)). 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

Combining the above values of pAR with the value for the p-methoxybenzyl cation measured by Toteva and Richard158 allows the effect of the three oxygen substituents on the stability of the benzyl cation to be compared in Scheme 22. The values of pAR may also be compared with effects of similar oxygen substitutions at the a-position of the benzyl cation from Table 3, which are also shown in Scheme 22. As expected, the relative magnitudes of the O-, HO, and MeO substituent effects exhibit similar patterns in the a- and... 

As noted above, this correlation and that of Fig. 1 are deficient in not recognizing that the product of the nucleophilic reaction is not the alcohol, as implied by the correlation with pATR, but the protonated alcohol. However, it is reasonable to suppose that variation of the pATas for O-protonation of the alcohols, which are required to correct values of ATR, are small compared with variations in pifR itself (and thus pATH2o) and would not significantly affect the quality of the correlation. It is also true that the correlation is dominated by the large and variable values of pA n.o for aromatic products of deprotonation. These tend to obscure variations in product ratios for tertiary alkyl and secondary benzylic cations which are the focus of a previous discussion of this partitioning by Richard.5... 

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