Carbocation rearrangements 1.2- methyl shift

Why does the carbocation intermediate in the hydrolysis of 2 bromo 3 methylbutane rearrange by way of a hydride shift rather than a methyl shift ... 

Carbocation rearrangements can also occur by the shift of an alkyl group with its ejection pair. For example, reaction of 3,3-dimethyJ-l-butene with HCI Leads to an equal mixture of unrearranged 2-chloro-3,3-dimethyTbutane and rearranged 2-chloro-2,3-dimethyibutane. In this instance, a secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group. 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

Clearly, we must be able to predict when to expect a carbocation rearrangement. There are two common ways for a carbocation to rearrange either through a hydride shift or through a methyl shift. Your textbook will have examples of each. Carbocation rearrangements are possible for any reaction that involves an intermediate carbocation (not just for addition of HX across an alkene). Later in this chapter, we will see other addition reactions that also proceed through carbocation intermediates. In those cases, you will be expected to know that there will be a possibility for carbocation rearrangements. 

Perhaps the most spectacular of the natural carbocation rearrangements is the concerted sequence of 1,2-methyl and 1,2-hydride Wagner-Meerwein shifts that occurs during the formation oflanosterol from squalene. Lanosterol is then the precursor of the steroid cholesterol in animals. 

We see that the final product, 1-methylsantene, has a rearranged carbon skeleton corresponding to a methyl shift, and so we consider the rearrangement of the initially formed secondary carbocation to a tertiary ion. 

The secondary carbocation can, as we have seen, rearrange by a methyl shift (Problem 5.16). It can also rearrange by migration of one of the ring bonds. 

Rearrangement by a hydride shift is observed because it converts a secondary carbocation to a more stable tertiary one. A methyl shift gives a secondary carbocation—in this case the same carbocation as the one that existed prior to rearrangement. 

The carbocation then rearranges by a methyl shift, and the rearranged cyclohexadienyl cation loses a proton to form the isomeric product... 

When neopentyl bromide is boiled in ethanol, it gives only a rearranged substitution product. This product results from a methyl shift (represented by the symbol CH ), the migration of a methyl group together with its pair of electrons. Without rearrangement, ionization of neopentyl bromide would give a very unstable primary carbocation. 

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

The dehydration of 3,3-dimethyl-2-butanol illustrates the rearrangement of a 2° to a 3° carbocation by a 1,2-methyl shift, as shown in Mechanism 9.3. The carbocation rearrangement occurs in Step [3] of the four-step mechanism. 

Steps [2] Rearrangement of the 2° carbocation by a 1,2-methyl shift forms a more stable 3° and [3] carbocation. Nucleophilic attack of Br forms the product, a 3 alkyl halide. 

The protosterol carbocation rearranges by a series of 1,2-shifts of either a hydrogen or methyl group to form another 3° carbocation. 

Methyl shift (Section 9.9) Rearrangement of a less stable car-bocation to a more stable carbocation by the shift of a methyl group from one carbon atom to an adjacent carbon atom. 

A1,2-Methyl Shift—Carbocation Rearrangement During Dehydration 332... 

Because 4-6, a tertiary carbocation, is more stable than 4-5, a secondary cation, 4-6 would be expected to be formed preferentially. (However, if the tertiary carbocation did not lead to the product, we would go back to consider the secondary cation.) In addition, the formation of 4-6 appears to lead toward the product, because the carbon bearing the positive charge is number 7 in 4-3. The tertiary carbocation can undergo rearrangement by a methyl shift to give another tertiary carbocation. 

Similarly, carbocation rearrangements can occur by alkyl shifts. For example, Friedel-Crafts alkylation of benzene with l-chIoro-2,2-dimethyl propane yields (l,l-dimethylpropyl)benzene as the sole product. The initially formed primary carbocation rearranges to a tertiary carbocation by shift of a methyl group and its electron pair from C2 to Cl (Figure 26.10). 

A third carbocation rearrangement occurs by shift of a methyl group A second methyl-group shift gives a final carbocation intermediate ... 

The carbocation produced in this El elimination i s 2° and can either eliminate to give the first alkene, or can rearrange by a methyl shift to a 3° carbocation which would produce the last two products. The middle product is major as it is tetrasubstituted versus disubstituted for the last structure and monosubstituted for the first structure. 

Each of these units will still contain two C atoms no isotopic scrambling takes place. This statement is rigorously correct only if no internal rearrangement in the C4 monomers occurs prior to the formation of the Cg dimer. In reality, internal rearrangement is well documented. Its rate has, however, been found to be lower than the rate of isomerization for all catalysts. Randomization of the C label, as observed over sulfated zirconia would require an extremely fast internal atom rearrangement prior to the formation of the Cs intermediate, if only simple methyl shifts inside the dimer took place before p-fission. This model can thus be discarded. It follows that substantial carbon scrambling in the Cs carbocation is required to achieve randomization of the carbon atoms in the ultimate C4 entities. 

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