Fert-Butyl cation

FIGURE 4.10 Potential energy diagram for reaction of fert-butyl cation with chloride anion. 

The transition state for this step involves partial bond fonnation between fert-butyl cation and chloride ion. 

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 fert-butyl 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 weU 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 ... 

Fig. 24. 75.4-MHz 13C MAS spectra showing the formation of the fert-butyl cation and the methylcyclopentyl cation on A1C13 and AlBr3. The methylcyclopentyl cation was synthesized by an intermolecular hydride transfer reaction as shown in the figure.

Preformed or in situ-prepared carbocation salts (tropylium, trityl, etc.) are also active in transforming alkenes to carbocations.119,138,140 Preformed carbocation salts are the simplest initiators in cationic polymerization and ideal if the cation is identical to the one derived from the momomer (e.g., fert-butyl cation in the polymerization of isobutylene). 

Anodic oxidation of 2,6-di-tert-butylphenols (35) in acetonitrile or acetonitrile-perchloric acid leads to 7-tm-butyl-2-methylbenzoxazoles (36). In the absence of nucleophiles, the initially formed cation 37 adds acetonitrile to give 38, which loses a fert-butyl cation, [Eq. (41)].87... 

FIGURE 4.9 fert-Butyl cation, (a) The positively charged carbon is sp2-hybridized. Each methyl group is attached to the positively charged carbon by a same plane. ( >) The sp2-hybridized carbon has an empty 2p orbital, the axis of which is perpendicular to the plane of the carbon atoms. 

FIGURE 4.11 Combi-nation of fert-butyl cation and chloride anion to give tert-butyl chloride. In-phase overlap between a vacant p orbital of (CH3)3C+ and a filled p orbital of Cl- gives a C—Cl a bond. 

Ethers in which one of the groups is secondary begin to show appreciable cleavage at — 30°C. Protonated sec-butyl methyl ether 24 cleaves cleanly at — 30°C to protonated methanol and fert-butyl cation [Eq. (4.12)]. Ethers in which one of the alkyl groups is tertiary cleave rapidly even at —70°C. 

This impressive cascade reaction begins with the formation of a small amount of the fert-butyl cation by reaction of A1C13 with tert-BuCl. The fert-butyl cation abstracts a hydride ion... 

As an example, the fert-butyl cation can be generated by treating 2-methyl-2-propanol (ferf-butyl alcohol) with the superacid FS03H/SbF5 in liquid sulfur dioxide as the solvent. The reaction is shown in the following equation ... 

The Boc group is easily cleaved by brief treatment with trifluoroacetic acid (TFA), CF3COOH. Loss of a relatively stable ferf-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 fert-butyl cation gives isobutylene. 

Then, the fert-butyl cation intermediate can attack a molecule of butene to give the corresponding C8 carbocation. Depending on the particular butene isomer that is alkylated, a different C8 carbocation will be formed (2,2,4-TMP+ from isobutene, 2,2,3-TMP+ from 2-butene, and 2,2-DMH+ from 1-butene) ... 

Finally, these carbenium ions suffer rapid hydride transfer from isobutane, leading to the different octane isomers and regenerating the fert-butyl cation to perpetuate the chain sequence ... 

There is direct evidence, from ir and nmr spectra, that the fert-butyl cation is quantitatively formed when ferf-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 1019) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride and 1-chloroadamantane alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole, but not benzene. It was noted on p. 476 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates. Lewis acids, such as BF3 or AIEta, can also be used to alkylation of aromatic rings with alkene units. 

The isobutyl cation spontaneously rearranges to the fert-butyl cation by a hydride shift ... 

Fig. 22.10(c) shows the following surprising results (i) The predicted energy of adsorption (70 kj mol i at 0 K) is of the same order of magnitude as estimates based on experiments for related molecules (50-63 kJ moh ). (ii) With respect to isobutene in the gas phase separated from the zeolite, the tert-butyl cation is much less stable (-17 kJ moTi) than the isobutoxide (-48 kJ moh ). The reason is that dispersion contributes substantially less to the stabilization of the tert-butyl cation than to the stabilization of the adsorption complex or the isobutoxide. As result, the proton transfer energy increases from 24 kJ moH (DFT) to 59 kJ moh (MP2/DFT) and it seems very unlikely that the fert-butyl cation will be detected in zeolites, even as a short-lived species. 

Levy has performed NPA, Mulliken, AIM, and CHELPG charge analyses on the / o-propyl, 5cc-butyl, and tert-hvXy cations using MP2/6-31G -level computations. As mentioned briefly in Section 3.4.1, the trivalent carbon atom in fert-butyl cation... 

For both tertiary cations and secondary benzylic carbocation, the ratio of substitution to elimination is quite high. For example, for l-(/i-tolyl)ethyl cation in 50 50 TFE-water, the ratio is 1400."° For the fert-butyl cation, the ratio is about 30 in water" and 60 in 50 50 TFE-water. " These ratios are on the order of 10 if account is taken of the need for solvent reorganization in the substitution process." The generalization is that under solvolysis conditions, tert-alkyl and sec-benzyl carbocations prefer substitution to elimination. 

To answer this question, we need to look at the mechanism of the reaction. Recall that the first step of the reaction—the addition of H" " to an sp carbon to form either the fert-butyl cation or the isobutyl cation—is the rate-determining step (Section 3.7). If there is any difference in the rate of formation of these two carbo-cations, the one that is formed faster will be the preferred product of the first step. Moreover, because carbocation formation is rate determining, the particular carbo-cation that is formed in the first step determines the final product of the reaction. That is, if the ferf-butyl cation is formed, it will react rapidly with CF to form tert-butyl chloride. On the other hand, if the isobutyl cation is formed, it will react rapidly with Cr to form isobutyl chloride. It turns out that the only product of the reaction is ferf-butyl chloride, so we know that the ferf-butyl cation is formed faster than the isobutyl cation. 

The question now is, Why is the fert-butyl cation formed faster than the isobutyl cation To answer this, we need to take a look at the factors that affect the stability of carbocations and, therefore, the ease with which they are formed. 

Why does the stability of a carbocation increase as the number of alkyl substituents bonded to the positively charged carbon increases Alkyl groups decrease the concentration of positive charge on the carbon—and decreasing the concentration of positive charge increases the stability of the carbocation. Notice that the blue— recall that blue represents electron-deficient atoms—is most intense for the least stable methyl cation and is least intense for the most stable fert-butyl cation. 

Common names that have been incorporated into lUPAC nomenclature such as isopropyl, vec-butyl, and so on, are permitted. Thus 1,1-dimethylethyl cation (CHsfsC may be called fert-butyl cation. 

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

The ferf-butyl group is cleaved as the corresponding carbocation. Loss of a proton from fert-butyl cation converts it to 2-methylpropene. Because of the ease with which a tert-butyl group is cleaved as a carbocation, other acidic reagents, such as trifluoroacetic acid,... 

Methanol acts as a nncleophile to captnre fert-butyl cation. 

FIGURE 6.6 (a) A stylized orbital structure of the methyl cation. The bonds are sigma (cr) bonds formed by overlap of the carbon atom s three sp orbitals with the 1 s orbitals of the hydrogen atoms. The p orbital is vacant, (b) A dashed line-wedge representation of the fert-butyl cation. The bonds between carbon atoms are formed by overlap of sp orbitals of the methyl groups with sp orbitals of the central carbon atom. 

In Chapter 5, we study a group of organic cations called carbocations. Following is the structure of one such carbocation, the fert-butyl cation ... 

Methyl and fert-butyl cations. Delocalization of positive charge by the electron-withdrawing inductive effect of the trivalent, positively charged carbon according to molecular orbital calculations. 

In a carbocation, the carbon bearing the positive charge is bonded to three other atoms and, as predicted by VSEPR, the three bonds about the cationic carbon are copla-nar and form bond angles of approximately 120°. According to the orbital hybridization model, the electron-deficient carbon of a carbocation uses sp hybrid orbitals to form a bonds to the three attached groups. The unhybridized 2p orbital lies perpendicular to the a bond framework and contains no electrons. Figure 6.3 shows a Lewis structure, a cartoon, and a calculated structure indicating the vacant 2p orbital for the fert-butyl cation. 

Similarly, in the reaction of HBr with 2-methylpropene, proton transfer to the carbon-carbon double bond might form either the isobutyl cation (a 1° carbocation) or the fert-butyl cation (a 3° carbocation). 

---