Ferf-Butyl cation intermediate

Figure 24 reports 13C MAS spectra of the ferf-butyl cation (43) and the methylcyclopentyl cation 17 (45) on the solid metal halides A1C13 and AlBr3 the asymmetry parameters, CSAs, and isotropic shifts (Table III) are unambiguous for the species indicated. Repeated attempts in various laboratories to observe the ferf-butyl cation as a persistent species in a zeolite have thus far been unsuccessful. Detailed theoretical work will be required to determine whether or not the ferf-butyl cations are local minima (i.e., true intermediates) on typical reaction pathways in zeolites. The ease with which these cations form in true superacids (liquid or solid) should be contrasted with the history of negative observations in zeolites. 

Because the reaction is catalytic in ferf-butyl cation and the deprotonation/ reprotonation steps are very fast, extensive regioselective deuteriation of the isoalkane is observed at room temperature as shown by GC-MS analysis. The absence of mass 68 (d10-isobutane) and the presence of mass 64 due to S02 formation in the oxidative process are typical features in accord with the oxidative activation of the alkane and the Markovnikov-type addition of deuterons on the intermediate isobutylene (14). However, the exchange process does not take place in the presence of carbon monoxide, which traps the ferf-butyl cation and prevents deprotonation (Scheme 5.7). 

Even the alkylation of isobutane by the ferf-butyl cation 4 despite the highly unfavorable steric interaction has been demonstrated142 by the formation of small amounts of 2,2,3,3-tetramethylbutane 36. This result also indicates that the related five-coordinate carbocationic transition state (or high-lying intermediate) 35 of the degenerate isobutylene-terf-butyl cation hydride transfer reaction is not entirely linear, despite the highly crowded nature of the system (Scheme 5.21). 

Figure 12.5 illustrates attack on the benzene ring by ferf-butyl cation (step 1) and subsequent formation of fert-butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate (step 2). 

Like ferf-butyloxonium ion, ferf-butyl cation is an intermediate along the reaction pathway. It is, however, a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic. Step 2 is the slowest step in the mechanism and has the highest activation energy. Figure 4.7 shows a potential energy diagram for this step. 

Carbocation Positive ion in which the chaige resides on carbon. An example is ferf-butyl cation, (CH3>3C. Carbocations are unstable species that, though they cannot normally be isolated, are believed to be intermediates in certain reactions. 

The alkylation of phenylacetylene with ferf-butyl chloride, benzyl chloride, and diphenylmethyl chloride follows an AdE2 mechanism57-59 with the involvement of the open vinyl cationic intermediate 17 ... 

The conversion of substituted diphenylamines and triphenylamines to carbazoles at platinum anodes in CH3CN-Et4NC104 takes place if the intermediate cation-radical is fairly stable. Thus the anodic oxidation of (V-ethylbis(p-fert-butylphenyl)amine (87) gave 3,6-di-ferf-butyl-Af-ethyl-carbazole (88) in 15% yield152 [Eq. (72)]. 

These common features suggest that carbocations are key intermediates in alcohol dehydrations, just as they are in the reaction of alcohols with hydrogen halides. Figure 5.6 portrays a three-step mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. Steps 1 and 2 describe the generation of terf-butyl cation by a process similar to that which led to its formation as an intermediate in the reaction of ferf-butyl alcohol with hydrogen chloride. 

Step 1 Once generated by the reaction of ferf-butyl chloride and aluminum chloride, tm-butyl cation is attacked by the TT electrons of benzene, and a carbon-carbon bond is formed. (The molecular model depicts the cyclohexadienyl cation intermediate.)... 

Benzene f rf-Butyl cation Step 2 Loss of a proton from the cyclohexadienyl cation intermediate yields ferf-butylbenzene. 

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