Ferf-Butyl cation

FIGURE 3.6 Deprotection of functional groups by acidolysis. Protonation followed by carbocation formation during the removal of ferf-butyl-based protectors by hydrogen chloride.8 One mechanism is involved in generating the ferf-butyl cation, which is the precursor of two other molecules. 

The ferf-butyl cation, as its Sb2Fjj salt, has the expected planar central carbon, with relatively short C —Cq, bonds (1.44 A). The structure is in agreement with the C—H hyperconjugation model of Eq. 8. 

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. 

Particularly revealing is the alkylation of isobutane with ferf-butyl cation.35 The product showed, inter alia, the presence of 2,2,3,3-tetramethylbutane (although in low 2% yield). Thus, despite unfavorable steric hindrance in the tertiary-tertiary... 

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... 

Step 1 Protonation of the carbon-carbon double bond to form ferf-butyl cation ... 

The observation of alkyl cations such as the ferf-butyl cation [trimethyl-carbenium ion, (CH3)3C+] 1 and the isopropyl cation [dimethylcarbenium ion, (CH3)2CH+] 2 was a long-standing challenge. The existence of alkyl cations in systems containing alkyl halides and Lewis acids has been inferred from a variety of observations, such as vapor pressure depressions of CH3C1 and C2H5CI in the... 

The 13C NMR shift in the ferf-pentyl cation [C2H5(CH3)2C+] 3 is at 813C 335.4, which is similar to the that of the ferf-butyl cation. The shift difference is much smaller than the 17 ppm found in the case of the related alkanes, although the shift observed is in the same direction. The 13C NMR chemical shifts and coupling constants 7C-h of C3 to Cg alkyl cations 1-13 are shown in Tables 3.2 and 3.3.95... 

Data in Tables 3.2 and 3.3 are characterized by substantial chemical shift deshieldings and coupling constants (/c h) that indicate sp2 hybridization. Also subsequently, Myhre and Yannoni50 have obtained 13C NMR spectrum of ferf-butyl cation 1 in the solid state, which agrees very well with the solution data.95... 

Figure 3.6. Carbon Is photoelectron spectrum of ferf-butyl cation 1.

Alternatively, if the reaction involved trivalent w-butyl cation 468 (from ethylation of ethylene) the ion would inevitably rearrange via 1,2-hydrogen shift to sec-butyl cation 19, which in turn would isomerize into the ferf-butyl cation (1) and thus give isobutane (461) [Eq. (3.125)]. 

It was found possible to measure the kinetics of cleavage of protonated sec-butyl methyl ether 24 by following the disappearance of the methoxy doublet in the NMR spectrum with simultaneous formation of protonated methanol and ferf-butyl cation. The cleavage shows pseudo-first-order kinetics. Presumably, the rate-determining step is the formation of sec-butyl cation followed by rapid rearrangement to the more stable ferf-butyl cation... 

Protonated primary thiols are stable at much higher temperatures. Protonated w-butyl thiol 56 cleaves to ferf-butyl cation only at +25°C [Eq. (4.34)]. 

Both methylcyclopentane and cyclohexane were found to give the methylcyclo-pentyl ion, which is stable at low temperature, in excess superacid.22 When alkanes with seven or more carbon atoms were used, cleavage was observed with formation of the stable ferf-butyl cation 4. Even paraffin wax (see Section 2.2.2.2 on Magic Acid) and polyethylene ultimately gave the ferf-butyl cation 4 after complex fragmentation and ionization processes. 

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). 

As is apparent in the last step, isobutane is not alkylated but transfers a hydride to the Cg+ carbocation before being used up in the middle step as the electrophilic reagent (tert-butyl cation 4). The direct alkylation of isobutane by an incipient tert-butyl cation would yield 2,2,3,3-tetramethylbutane,142 which indeed was observed in small amounts in the reaction of ferf-butyl cation with isobutane under stable ion conditions at low temperatures (vide infra). 

The alkylating ability of methyl and ethyl fluoride-antimony pentafluoride complexes has been investigated by Olah et al.,143,144 who showed the extraordinary reactivity of these systems. Self-condensation was observed as well as alkane alkylation. When CH3F-SbF5 was reacted with excess of CH3F at 0°C, at first only an exchanging complex was observed in the H NMR spectrum. After 0.5 h, the starting material was converted into the ferf-butyl cation 4 (Scheme 5.19). 

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). 

If the ethyl cation would have reacted with excess ethylene, primary 1 -butyl cation would have been formed, which irreversibly would have rearranged to the more stable. sec-butyl and subsequently ferf-butyl cations giving isobutane as the end product. 

Superacid-catalyzed alkylation of adamantane with lower alkenes (ethene, propene, isomeric butenes) has been investigated by Olah et al.151 in triflic acid and triflic acid-B(0S02CF3)3. Only trace amounts of 1 -ferf-butyladamantane (37) were detected in alkylation with 1- and 2-butenes, whereas isobutylene gave consistently relatively good yield of 37. Since isomerization of isomeric 1-butyladamantane under identical conditions did not give even traces of 37, its formation can be accounted for by (r-alkylation, that is, through the insertion of the ferf-butyl cation into the C—H bond (Scheme 5.22). This reaction is similar to that between ferf-butyl cation and isobutane to form 2,2,3,3-tetramethylbutane discussed above (Scheme 5.21). In either case, the pentacoordinate carbocation intermedate, which may also lead to hydride transfer, does not attain a linear geometry, despite the unfavorable steric interactions. 

The products obtained from isobutane and isoalkanes (Table 5.37) are in accord with the above-discussed mechanism. However, the relative rate of formation of the dimethylmethylcarboxonium ion from isobutane is considerably faster than that of the ferf-butyl cation from isobutane in the absence of ozone under the same conditions.642 Indeed, a solution of isobutane in excess Magic Acid-SCECIF solution showed only trace amounts of the ferf-butyl cation after standing for 5 h at —78°C. Passage of a stream of oxygen gas through the solution for 10 times longer a period than in the ozonization experiment showed no effect. It was only when ozone was introduced into the system, that rapid reaction took place. 

On the other hand, ferf-butyl alcohol itself in Magic Acid-S02CIF solution gave the ferf-butyl cation readily and quantitatively, even at —78°C. In the presence of ozone, however, under the same conditions it gave dimethylmethylcarboxonium ion. 

In alkyl cations R3C , n = 3 and m 0. The substituents of the trivalent central atom lie in the same plane as the central atom and form bond angles of 120° with each other (trigonal planar arrangement). This arrangement was confirmed experimentally by means of a crystal structural analysis of the ferf-butyl cation. 

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. 

Figure 3.53 shows an addition of a carboxybc acid to isobutene, which takes place via the tert-butyl cation. This reaction is a method for forming ferf-butyl esters. Because the acid shown in Figure 3.53 is a /i-hydroxycarboxylic acid whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the ferf-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently acidic to protonate the alkene to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here. 

In a few cases, cations other than the proton are eliminated from the sigma complex to reconstitute the aromatic system. The ferf-butyl cation (Figure 5.1, X = tert-Bu) and proto-nated S03 (Figure 5.1, X = SOsH) are suitable for such an elimination. When the latter groups are replaced in an Ar-SE reaction, we have the special case of an ipso substitution. Among other things, ipso substitutions play a role in the few Ar-SE reactions that are reversible (Section 5.1.2). 

The ferf-butyl cations liberated from compounds Ar—tert- Bu, upon ipso reaction of a proton, may also react again with the aromatic compound from which they stemmed. If this course is taken, the te/7-butyl groups are ultimately bound to the aromatic nucleus with a regioselectivity that is dictated by thermodynamic control. Figure 5.4 shows how in this way 1,2,4-tri-ferf-butylbenzene is smoothly isomerized to give 1,3,5-tri-fcrt-buty 1-benzene. 

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