Tert-Butyl cation alkylation

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

The most stable of all alkyl cations is the tert-butyl cation. Even the relatively stable tert-pentyl and fen-hexyl cations fragment at higher temperatures to produce the tert-butyl cation, as do all other alkyl cations with four or more carbons so far studied. Methane,ethane, and propane, treated with superacid, also yield ten-butyl cations as the main product (see 2-17). Even paraffin wax and polyethylene give the ten-butyl cation. Solid salts of frrf-butyl and rerf-pentyl cations (e.g., MeaC" SbFg ) have been prepared from superacid solutions and are stable below -20°C. ... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

The assumption that tertiary alkyl cations are not stable in solvents other than super-acids is widespread and was apparently well founded on many experiments by different workers over many years [20, 24]. For this reason the stability of our polymerised solutions was astonishing and it seemed at first unlikely that the cation of the electrolyte could be a simple tertiary ion the tert-butyl cation in the experiment with tert-butyl bromide and the ions 2-4 in the polymerised solutions. This was because we did not know then that Cesca,... 

Flectrophilic addition of polychloroalkanes such as, e.g., chloroform or 1,1,2,2-tetrachloroethane to Cjq with AICI3 in a 100-fold excess gives the monoadduct with a 1,4-addition pattern (Scheme 8.12) [93, 94], The reaction proceeds via a CjqR cation (19, Scheme 8.12) that is stabilized by the coordination of a chlorine atom to the cationic center. The cation is trapped by Cl to give the product 20. The chloroalkyl fullerenes can be readily hydrolyzed to form the corresponding fullerenol 21. This fullerenol can be utilized as a proper precursor for the cation, which is easily obtained by adding triflic acid. The stability of CjqR is similar to tertiary alkyl cations such as the tert-butyl-cation [95],... 

An alternative way of explaining how the cationic charge is spread over the alkyl groups of a tertiary cation, such as the tert-butyl cation, is to write the cation as a hybrid of the following structures ... 

Attack by the tert-butyl cation on another molecule of 2-methylpropene produces an eight-carbon tertiary cation, which then proceeds to another molecule of alkylate ... 

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). 

Alkyl cations like the tert-butyl cation (2) and 2-propyl cation (89) are significantly stabilized by hyperconjugative C-H and C-C a -back donation into the empty carbocationic p-orbitals. Protosolvation involving a -bonds can diminish this hyperconjugative stabilization and thus lead to super-electrophilic carbocationic species. 

Direct n alkylation of isobutane 2 by the tert-butyl cation 4 would yield... 

These common features suggest that carbocations are key intermediates in alcohol dehydration, just as they are in the conversion of alcohols to alkyl halides. Figure 5.6 portrays a three-step mechanism for the sulfuric acid-catalyzed dehydration of ieri-butyl alcohol. Steps 1 and 2 describe the generation of tert-butyl cation by a process similar to that which led to its formation as an intermediate in the reaction of ieri-butyl alcohol with hydrogen chloride. Step 3 in Figure 5.6, however, is new to us and is the step in which the double bond is formed. 

FIGURE 12.5 The mechanism of Friedel-Crafts alkylation. An electrostatic potential map of tert-butyl cation can be viewed on Learning By Modeling. 

Use SpartanView to compare electrostatic potential maps of the ethyl cation, the isopropyl cation, and the tert-butyl cation. How docs the number of alkyl groups attached to the positive carbon change the potential at this atom What does this imply about the direction of electron transfer between an alkyl group and the positive carbon ... 

The NMR spectra -. To calculate the averaged shift of the C and C atoms for the rapid equilibration of epimeric 2-norbomyl cations Olah used an isopropyl ion as a secondary carbocation model. Kramer observes that the signal of the C atom in the tert-butyl cation spectmm is in a lower field than for an isopropyl cation. Kramer means that the positive charge on the secondary cation centre is lower than on the tertiary one this contradicts the fact that tertiary alkyl carbocations are more stable than secondarv ones. Hence he concludes that averaged shifts cannot be calculated from the NMR spectrum of the isopropyl cation since it is not a typical secondary carbocation. 

The main observations can be accounted for by a carbocationic mechanism incorporating intermolecular hydride transfer (77). The multistep transformation illustrated by the reaction between isobutane and 1-alkenes is initiated by the carbocation formed by protonation of the alkene (by the protic acid or the promoted metal halide catalysts) (eq. 48). Intermolecular hydride transfer between isobutane and the cation then generates a new carbocation (tert-butyl cation 13 eq. 49). The cation 13 adds to the alkene to form cation 14 (eq. 50), which then, through intermolecular hydride transfer, forms the product (eq. 51). In this final product-forming step, 13 is regenerated from the isoalkane and then starts a new cycle. Alkane-alkene alkylation, therefore, can be considered a chain reaction with 13 as the chain carrier. 

The exclusive formation of re-butane at short contact time in the ethane-ethylene reaction indicates that a mechanism involving the addition of ethyl cation to ethylene cannot be operative here, since in excess superacid ethylene is completely protonated. Even if this reaction could occur, trivalent butyl cations for conventional acid-catalyzed alkylations cannot participate in this process. The involvement of the trivalent butyl cation should produce isobutane as the main product the 1- and 2-butyl cations would preferentially isomerize to the tert-butyl cation and thus yield isobutane (eq. 52). Since there is no free ethylene present, the ethyl cation will attack ethane via a pentacoordinate (three-center, two-electron) carbocation (15) to produce re-butane (eq. 53). Mechanistic studies with labeled compounds also demonstrated the validity of this mechanism (79). 

Pig. 6. Simplified alkylation cycle including the three main reaction steps (a) alkene addition to a tert-butyl cation, (b) isomerization of the formed trimethylpentyl cation, and (c) hydride transfer from isobutane to the TMP cation. 

Physical organic studies have demonstrated that lerl-butylsulfonyl chloride decomposes cleanly to the tert-butyl cation in water over a pH range 3.5 to 13.0. Clean teri-butyl cation formation is also the only significant reaction in methanol-chloroform. The subsequent product spectrum is a function of the reaction conditions. tert-Butylsulfonyl chloride, is used for the tert-butylation of aromatic compounds in a Friedel-Crafts desulfonylative alkylation in the presence of aluminum chloride-nitromethane as catalyst at 25 °C (eq 1). Alkylation products were obtained free of contamination by the sulfonylation product. 

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