Ferf-Butyl cation stability

The reactivity of aromatic side chains to undergo dealkylation is in line with the stability of the corresponding carbocations. This indicates the possible involvement of carbocations in dealkylation, which was proved to be the case. The intermediacy of the rm-butyl cation in superacid solution was shown by direct spectroscopic observation.228,229 Additional proof was provided by trapping the ferf-butyl cation with carbon monoxide during dealkylation 230... 

The C -CH3 bonds in the cumyl cation are also shortened, but only slightly, by 2.5 pm. Clearly, this falls short of the truncation observed with the C -CH3 bonds of the ferf-butyl cation (Figure 2.19). The bulk of the stabilization of the cation is performed by the phenyl ring via resonance, mitigating the need for extensive stabilization via C-H hyperconjugation. 

Olah found that he could measure the spectrum of the ferf-butyl cation, but he was never able to observe the methyl cation in solution. Why do those extra substituents stabilize the cationic centre ... 

Why is the ferf-butyl cation formed faster To answer this question, we need to look at two things (1) the factors that affect the stability of a carbocation, and (2) how its stability affects the rate at which it is formed. 

Alkyl groups stabilize carbocations because they decrease the concentration of positive charge on the carbon. Notice that the blue area in the following electrostatic potential maps (representing positive charge) is the least intense for the most stable ferf-butyl cation (a tertiary carbocation) and the most intense for the least stable methyl cation. 

The remarkable thermal stability of the di-ferf-butyl-substituted thiepin 6, due to steric screening, has been demonstrated by its synthesis in an analogous manner to the carbocation rearrangement which provides dibenzothiepins. Under acidic conditions, methanesulfonic acid was eliminated from Ihiopyran 5 to give the thiepin via cationic rearrangement.18... 

Neither methyl nor ethyl fluoride gave the corresponding cations when treated with SbFs. At low temperatures, methyl fluoride gave chiefly the methylated sulfur dioxide salt, (CH3OSO) ShF while ethyl fluoride rapidly formed the rert-butyl and ferf-hexyl cations by addition of the initially formed ethyl cation to ethylene molecules also formed ° At room temperature, methyl fluoride also gave the tert-butyl cation. In accord with the stability order, hydride ion is abstracted from alkanes by super acid most readily from tertiary and least readily from primary positions. 

It is interesting that Co4(CO)12 is perfectly stable in the more sterically demanding iso-propyl53 and ferf-butyl alcohols245. This disproportionation reaction is unknown for Rh CQ) and Ir4(CO)i2, probably because of the reduced thermodynamic stability of the cationic species of these noble metals. 

Note Moderately polar solvent, ethereal odor soluble in water and most organic solvents flammable moderately toxic incompatible with strong oxidizers can form potentially explosive peroxides upon long standing in air see the relevant tables in the chapter on laboratory safety commercially, it is often stabilized against peroxidation with 0.5 to 1.0% (mass/mass) p-cresol, 05 to 1.0% (mass/mass) hydroquinone, or 0.01% (mass/mass) 4,4 -thiobis(6-ferf-butyl-m-cresol) can polymerize in the presence of cationic initiators such as Lewis acids or strong proton acids. Synonyms THF, tet-ramethylene oxide, diethylene oxide, 1,4-epoxybutane, oxolane, oxacyclopentane. 

There is direct evidence, from ir and nmr spectra, that the fert-butyl cation is quantitatively formed when ferf-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 1019) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride and 1-chloroadamantane alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole, but not benzene. It was noted on p. 476 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates. Lewis acids, such as BF3 or AIEta, can also be used to alkylation of aromatic rings with alkene units. 

The relative order of cation stability can be related to the ionization potential of the cation, which can be experimentally determined by electron bombardment in a mass spectrometer. As shown in Table 12.1, the ionization potential and relative energies (determined by mass spectrometry) show the relative order of cation stability to be tertiary > secondary > primary > methyl, as expected. In Table 12.1, the lower energy ionization (7.42 eV vs. 8.64 eV for ferf-butyl vs. n-butyl) represents the more stable cation. Similarly, Table 12.2 shows mass spectral data (ionization potential) for a variety of cations, but also shows the energy of cations... 

The addition of silylium ions to CC unsaturated compounds results in the formation of carbocations that are stabilized due their p-silyl substitution. As already mentioned in Sect. 3.3.5 p-silyl-substituted vinyl cations 93 were prepared by intramolecular addition of an incipient silylium ion to a C=C triple bond. A prominent example is ferf.-butyl-substituted vinyl cation 108 that could be isolated in the form of its [B(CgF5)4] salt and was investigated by XRD (Scheme 34). The molecular structures of 108 and related vinyl cations showed the linear coordination at the positively charged carbon atom and indicated the structural consequences of p-sUyl hyperconjugation [83, 84]. While... 

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. 

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