Carbocation rearrangements 1.2- shifts

Because S l reactions involve carbocations, rearrangements (1,2 shifts) can occur if they lead to more stable carbocation intermediates. 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Hydride shift (Section 5 13) Migration of a hydrogen with a pair of electrons (H ) from one atom to another Hydnde shifts are most commonly seen in carbocation rearrange ments... 

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. 

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. 

Carbocation rearrangements in SnI reactions through 1,2-hydride shift Carbocations often undergo rearrangements, producing more stable ions. This rearrangement produces a more stable tertiary cation instead of a... 

In understanding these reactions, it is helpful to view the metal-alkene tt complex as an incipient carbocation (just as tt complexes of halogens are incipient carbocations). Alkyl and hydride shifts then bear analogy to carbocation rearrangements. This may be an oversimplification but it makes the chemistry easier to follow. 

The mechanism for this reaction, shown in Figure 22.5, involves a carbocation rearrangement that occurs by an allowed [1,2] sigmatropic shift. The product of this rearrangement, a protonated ketone, is considerably more stable than the initial carbocation, so the migration is quite favorable. Another example of the pinacol rearrangement is provided in the following equation ... 

The primary carbocation rearranges to the more stable secondary carbocation by a hydride shift. 

This is a carbocation rearrangement involving the shift of an alkyl group. The sequence of steps is the same as those seen in Problems 17.37 and 17.38. 

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. 

Less stable carbocations rearrange to more stable carbocations by the shift of a hydrogen atom or an alkyl group. 

The result in Equation [1] is explained by a carbocation rearrangement involving a 1,2-hydride shift the less stable 2° carbocation (formed from the 2° halide) rearranges to a more stable 3° carbocation, as illustrated in Mechanism 18.8. 

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

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

Carbocation rearrangements, 94 Carbocation reactions, 45, 114 Carbonic acid derivatives, 358 Carbonyl group, reduction of, 261 resonance structures, 333 Carboxylic acids, summary of chemistry, 356 Catechol, 432 Center of symmetry, 69 Chair and boat forms, 168 Chemical shift, 2ilff Chiral center, 71... 

These carbocation rearrangements are similar to those that occur dur- ing electrophilic additions to alkenes (Section 6.12). For example, the relatively unstable primary butyl carbocation produced by reaction of 1-chloro-butane with AICI3 rearranges to the more stable secondary butyl carbocation by shift of a hydrogen atom and its electron pair (a hydride ion, H ) from C2 to Cl. 1... 

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). 

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