Alkylation tert-alkyl cation

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 initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

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],... 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

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 ... 

In contrast, compounds such as chlorobenzene and chloroethene, in which the halogen is attached directly to a multiply bonded carbon atom, do not exhibit SNl-type reactions. Evidently then, unsaturated carbon cations such as phenyl or ethenyl are appreciably less stable (more difficult to form) than tert-alkyl cations ... 

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). 

The protolytic condensation of methane in Magic Acid solution at 60°C is evidenced by the formation of higher alkyl cations such as tert-butyl and ferf-hexyl cations23 25 170 (Scheme 5.24). 

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. 

Fig. 5.28. Friedel-Crafts alkylations with tert-alkyl cations as reactive electrophiles. Figure 5.33 exemplifies a way in which compound A can be used.

We have previously mentioned (p. 236) that stable tertiary carbocations can be obtained, in solution, at very low temperatures. The NMR studies have shown that when these solutions are warmed, rapid migrations of hydride and of alkyl groups take place, resulting in an equilibrium mixture of structures. For example, the tert-pentyl cation (5) equilibrates as follows ... 

---