Benzyl cations spectra

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

In a cation such as the (2,4-di-ferf-butyl-6-methyl)benzyl cation 147, a high rotational barrier around the v/r-hybridized atom is observed. The methylene protons are found magnetically nonequivalent in the1H NMR spectrum.356 Recent combined experimental and theoretical studies for the related cation 143 suggest357 that structure 143b is an important resonance contributor. 

The mass spectrum of w-butylbenzene has its base peak at m/z 91, corresponding to cleavage of a benzylic bond. The fragments are a benzyl cation and a propyl radical. The benzyl cation rearranges to the tropylium ion, detected at m/z 91. 

The difficulties encountered in using the analysis of substituent effects in solvolyses as a mechanistic probe mostly arise from the mechanistic involvement of the solvent (Shorter, 1978, 1982 Tsuno and Fujio, 1996). Consequently, the behaviour of benzylic carbocations in the gas phase should be the best model for the behaviour of the solvolysis intermediate in solution (Tsuno and Fujio, 1996). The intrinsic substituent effects on the benzylic cation stabilities in the gas phase have also been analysed by equation (2), and they will be compared here with the substituent effects on the benzylic solvolysis reaction. In our opinion, this provides convincing evidence for the concept of varying resonance demand in solvolysis. Finally, we shall analyse the mechanisms of a series of benzylic solvolysis reactions by using the concept of a continuous spectrum of varying resonance demand. 

Mass Spectrometry The most common mass spectral fragmentation of alkylben-zene derivatives is the cleavage of a benzylic bond to give a resonance-stabilized benzylic cation. For example, in the mass spectrum of -butylbenzene (Figure 16-18),... 

Other types of rearrangements are also known. An example of a rearrangement that is not normally observed in solution chemistry is the rearrangement of a benzyl cation to a tropylium ion. This rearrangement is seen in the mass spectrum of toluene (Figure 28.9). 

The same direction of the equilibrium isotope effect was observed in the nondegenerate 1,2-hydride shift equilibrium of 2-(4 -trifluoromethyl-phenyl)-3-methyl-2-butyl cation [142] with one trideuteriomethyl group at C-2 or C-3 respectively (Forsyth and Pan, 1985). The isotope shifts in the spectrum are much smaller (1.3 ppm-1.45 ppm) than in degenerate cations like [141] because Ky is very much in favour of the benzylic cation structure for [142]. 

Accordingly, loss of the hydrogen atom from the first-formed radical cation is relatively easy, and the largest peak in the spectrum belongs to the ion CyHy", m/2. = 91, the benzyl cation. 

The major ion in the mass spectrum of an alkyl-substituted benzene is often miz 91 (C6H5CH2 ), resulting from cleavage between the first and second carbons of the alkyl chain attached to the ring. The ion presumably originates as a benzylic cation that rearranges to a tropylium cation (CyHy, Section 14.7D). Another ion frequently seen in mass spectra of monoalkylbenzene compounds is m/z 11, corresponding to CeHs. ... 

As part of the same study, the capacity of this novel resin to act as an allyl cation scavenger was demonstrated in a palladium-catalyzed O-alloc deprotection of O-alloc benzyl alcohol (Scheme 7.107) [125], Benzyl alcohol was obtained in high yield with only trace amounts of by-product, thereby eliminating the need for further purification. The resulting C-allylation of the resin was evident from the presence of C-allyl signals in the relevant MAS-probe 1H NMR spectrum. 

One case of n—5 —n delocalization was demonstrated by Stevenson et al. (2006). The potassinm anion-radical salt of l-(9-methyl-9H-fluoren-9-yl)-4-methyl benzyl is characterized by the delocalization of an nnpaired electron within the fluorenyl moiety only. Its ESR spectrnm completely coincides with the spectrnm of the potassium anion-radical salt of the 9,9-dimethyl fluorene anion-radical in THE However, the cesium anion-radical salt of the fluorenyl methylbenzyl derivative produces the ESR spectrum corresponding to the placement of this cation between the fluorenyl and methylbenzyl moiety. The conditions of n—s—n delocalization appear An unpaired electron spends its time within both fluorenyl and methylbenzyl fragments. The situation is explained in Scheme 3.54. 

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. 

Hexakis(benzylthio)benzene acts as a rather efficient electron donor. The ESR spectrum obtained with in situ electrolysis indicates a pattern corresponding to the twelve equivalent protons in the benzyl positions. However, cyclovoltammet-ric measurements reveal that the radical cation exists in a complicated equilibrium with the dication and the parent neutral precursor53. Radical ions generated from C60 have been the subject of several publications and discussions within the last years. Several authors have postulated that C6o + can be observed in solid C60. However it could be demonstrated that this signal has to be ascribed to C6a peroxide or its decomposition products54. 

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