R-Butyl cation

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

Subsequently it was found that the same cations could also be generated from alcohols in super acid-S02 at - 60°C 1 and from alkenes by the addition of a proton from super acid or HF-SbF5 in S02 or SO20IF at low temperatures.12 Even alkanes give carbocations in super acid by loss of H. For example,13 isobutane gives the r-butyl cation... 

The most stable of all alkyl cations is the t-butyl cation. Even the relatively stable f-pentyl and f-hexyl cations fragment at higher temperatures to produce the t-butyl cation, as do all other alkyl cations with four or more carbons so far studied.21 Methane,22 ethane, and propane, treated with super acid, also yield r-butyl cations as the main product (see 2-18). Even paraffin wax and polyethylene give f-butyl cation. Solid salts of t-butyl and f-pentyl cations, e.g., Me3C SbF4, have been prepared from super-acid solutions and are stable below -20°CP... 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

More recently, salts of the triphenylmethyl cation Ph3C+ have been shown to be effective in converting the diones into pyrylium salts (61BSF538). The cation may be generated in situ, for example from triphenylmethyl chloride and antimony(V) chloride (69T1209). The r-butyl cation has been used for the same purpose (67T4001). Earlier workers considered that the synthesis involved dehydration of the diketone to a 4//-pyran followed by rapid oxidation to the pyrylium salt. In view of the successful application of Ph3C, the reaction... 

We have a choice of reagents for the -butyl cation a halide with Lewis acid catalysis, and -butanol or isobutene with protic acid catalysis. The least wasteful is the alkene as nothing is lost. Protonation gives the r-butyl cation and two r-butyl groups are added in one operation.3... 

In summary, all of the other isomeric butyl carbocations rapidly rearrange to the most stable to r-butyl cation and cannot be detected under these conditions. 

CF3COOH, PhSH, 20°, 1 h, 100% yield.18 Thiophenol is used to scavenge the liberated r-butyl cations, thus preventing alkylation of methionine or tryptophan. Other scavengers such as anisole, thioanisole, thiocresol, cre-sol, and dimethyl sulfide have also been used.19... 

While the formation of multiadducts in the above reactions clearly demonstrates the difficulties confronted in terms of controlling the reaction, the issue of whether Ceo and C70 undergo addition by carbon electrophiles is of great interest, because such a reaction would provide a useful method for carbon-carbon bond formation for the derivatization of fiillerenes. Initial attempts to test the possibility of electrophilic alkylation of Ceo with tert-hutyl chloride and AICI3 gave only polymeric products, probably formed via isobutene, indicating the insufficient reactivity of Cgo towards / r/-butyl cation. 

Kramer and coworkers recently showed that AlBr3 forms 2 1 salts with f-BuCl and f-BuBr in halogenated hydrocarbons at low temperatures. These ionogenic interactions were followed by C-NMR and clearly proved that homoconjugated anions of the type Al2BrgX accompanied the r-butyl cation. 

CH3)2CH+> H2C==CH—CH+ CH3CH+ > H2C=CH > Ph > CHj. The stabilities of various carbocations can be determined by reference to the order of stability for alkyl carbocations, 3 > 2° > 1 > CH3. The acetyl cation has a stability similar to that of the r-butyl cation. Secondary carbocations, primary benzylic cations, and primary allylic cations are all more stable than primary alkyl cations. Vinyl, phenyl, and methyl carbocations are less stable than primary alkyl cations. 

Differences in stability between carbonium ions are much larger than between free radicals. The /er/-butyl free radical, for example, is 13 kcal more stable than the methyl free radical the / r/-butyl cation is 71 kcal more stable than the methyl cation. 

Kebarle and coworkers have addressed the question of stability of the r-butylbenzenium ion obtained by gas-phase reaction of benzene with r-butyl cation. 

Due to the ease of dealkylation of r-alkylbenzenes, such as t-butylbenzene, some interesting applications of this process in both the syndietic and mechanistic areas have resulted. The intermediacy of the r-butyl cation was hinted to account for the observed products. Thus, Knight et al obtained excellent yields of pivalic acid by dealkylating r-butylbenzene and 1,3-dimethyl-S-t-butylbenzene in the presence of carbon monoxide and BFs-HaO as the catalyst (equation 120). They also found that H3PO4, H2SO4 and MeSOsH were ineffective for the dealkylation-carbonylation of t-butylbenzene. 

Brouwer and Olah et obtained direct evidence for the r-butyl cation intermediate by H NMR studies of r-butylbenzene in superacid solutions at low temperature. F rcasiu and Schlosberg demonstrated that, in the presence of carbon monoxide, the r-alkyl cation generated in situ from the dealkylation of a tertiary alkyl arene resulted in acylation of the dealkylated arene, as shown in Scheme IS. 

The 13+C shift in the r-butyl cation (CH3)313+C 1 in S02ClF-SbF5 solution at —20 ° is at S13c 335.2 (all CMR shifts are from 13C TMS) with a long-range coupling to the methyl protons of 3.6 Hertz. 

Substitution of the methyl group in the r-butyl cation by hydrogen thus causes an upfield shift of 10.4 ppm. Although the CMR shift of the carbocation center of the r-butyl cation is more deshielded than that of the isopropyl cation (by about 10 ppm), this can be explained by the methyl substituent effect, which may amount to 22 ppm. The tertiary butyl cation thus is more delocalized and stable than the secondary isopropyl. 

The 13C+ shift in the r-amyl cation C2H5C+(CH3)2 3 is at 613C 335.4 which is similar to that of r-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 Jc-h of C3-C8 alkyl cations 1—13 are shown in Tables 2 and 332. ... 

Di-f-butyl carbonate cleaves immediately at —80 0 with alkyl-oxygen fission, giving the r-butyl cation l and protonated carbonic acid. The structure of the latter has been established from the 13C NMR spectrum of the central carbon atom which shows a 4.5 hertz quartet, being coupled to three equivalent hydroxyl protons168. ... 

Infrared and Raman spectra of the alkyl cations were also recorded46 and are in agreement with the carbenium structure of these ions. Raman spectra provide strong evidence that the r-butyl cation possesses a planar carbon skeleton with one hydrogen atom of each CH3 group above the plane of the carbon atoms (C3v point group symmetry). 

Tertiary C—H bonds show the highest reactivity. However, protolytic cleavage of tertiary and secondary nitroalkanes is a major side reaction, and can lead to the formation of a variety of byproducts. Protolytic denitration was demonstrated by reacting 2-nitro-2-methylpropane with FSOjsH-SbFi, HF-SbFs, and HF-PF5 at -80 C. The protolytic clevage reaaion yields r-butyl cation and nitrous acid (or subsequently, nitrosonium ion). 

The transition state for this step involves partial bond formation between r butyl cation and chloride ion. 

Here, the electrophile is r -butyl cation formed by a hydride migration that accompanies ionization of the carbon-chlorine bond. 

Because of its resonance stabilization, the (primary) allyl cation is about as stable as a simple secondary carbocation, such as the isopropyl cation. Substituted allylic cations generally have at least one secondary carbon atom bearing part of the positive charge they are about as stable as simple tertiary carbocations such as the r-butyl cation. 

The Boc group is easily cleaved by brief treatment with trifluoroacetic acid (TFA), CF3COOH. Loss of a relatively stable r-butyl cation from the protonated ester gives an unstable carbamic acid. Decarboxylation of the carbamic acid gives the deprotected amino group of the amino acid. Loss of a proton from the r-butyl cation gives isobutylene. 

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

If we determine the rate of this process by measuring the disappearance of r/-butyl iodide over time, we can ultimately find the height of transition state 1. The rates of the faster captures of the carbocation do not affect the rate of ionization of tert-butyl iodide. The carbocation is gobbled up by the nucleophiles at a rate faster than that at which it is formed. In the limit, each molecule of the tert-butyl cation is captured before another is produced from er -butyl iodide. The rate of formation of product depends on how fast the r/-butyl cation is produced in the slow, rate-determining step but not on the rate of the fast capture by a nucleophile. 

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