Carbocation alkyl shift

The preferred alignment of orbitals for a 1,2-hydride or 1,2-alkyl shift involves coplanarity of the p orbital at the carbocation ion center and the a orbital of the migrating group. 

Strategy A Friedel-Crafts reaction involves initial formation of a carbocation, which can rearrange by either a hydride shift or an alkyl shift to give a more stable carbocation. Draw the initial carbocation, assess its stability, and see if the shift of a hydride ion or an alkyl group from a neighboring carbon will result in increased stability. In the present instance, the initial carbocation is a secondary one that can rearrange to a more stable tertiary one by a hydride shift. 

If a potential carbocation intermediate can undergo a hydride or alkyl shift, this shift occurs in preference to closure of the five-membered ring. 

Rearrangements that involve carbanions are found to be very much less common than formally similar rearrangements that involve car bo cations (p. 109). This becomes more understandable if we compare the T.S. for a 1,2-alkyl shift in a carbocation with that for the same shift in a carbanion ... 

The broken C13-C3 and new C13-C4 bonds suggest a 1,2-alkyl shift of C13 from C3 to a C4 carbo-cation, leaving a carbocation at C3. The broken C9-C11 and new C3-C11 bonds suggest a 1,2-shift of Cl 1 from C9 to a C3 carbocation, leaving a carbocation at C9. Since a shift of Cl 1 from C9 to C3 could only occur after C3 and C9 were connected, this suggests that the C3-C9 bond is formed first. Such a bond would be formed from a C9 carbocation with a C3=C4 n bond. The C9 carbocation could be formed from 6 or 9. Attack of the C3=C4 K bond on C9 puts a carbocation at C4. Then C13 shifts from C3 to C4. That puts a carbocation at C3. Then Cl 1 shifts from C9 to C3. Finally, deprotonation of C8 gives the product. 

In a deep-seated rearrangement like this, it s sometimes easier to work backwards from the product. The n bond at C8=C9 in 12 suggests that the last step is deprotonation of C8 of a carbocation at C9, C. Carbocation C might have been formed from carbocation D by a 1,2-alkyl shift of Cl 1 from C9 to C3. Carbocation D might have been formed from carbocation E by a 1,2-alkyl shift of C13 from C3 to C4. Carbocation E might have been formed from carbocation F by attack of a C3=C4 n bond on a C9... 

The tertiary carbocation contains a strained four-membered ring, and an alkyl shift allows relief of ring strain, generating five-membered rings and a secondary carbocation. It would appear that the relief of ring strain more than compensates for the loss of tertiary character in the carbocation. Thus, it is the secondary carbocation that interacts with a nucleophile. In this case, the nucleophile is water, the major component of the aqueous HCl. The product is thus isoborneol. 

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. 

This rearrangement is favorable because the initial carbocation is secondary while the product is a more stable tertiary carbocation. An example of a rearrangement involving a 1,2-alkyl shift is... 

Having addressed the structure and stability of carbocations, discussions will now be directed to the specific side reactions to which carbocations are subject. Specifically, this section focuses on rearrangements of carbocations known as hydride shifts and alkyl shifts. 

While the hydride shift illustrated in Scheme 5.12 cannot occur as a part of the pinacol rearrangement, the intermediate carbocation is subject to alkyl migrations. As shown in Scheme 5.13, a 1,2-alkyl shift results in transfer of the cation from a tertiary center to a center adjacent to a heteroatom. As the oxygen heteroatom possesses lone electron pairs, these lone pairs serve to stabilize the cation. Thus, the illustrated 1,2-alkyl shift transforms a carbocation into a more stable carbocation. 

Because of 1,2-hydride and alkyl shifts, it is possible to obtain multiple products from SnI reactions. Thus, to induce one product to predominate, we must find a way to stabilize the carbocation. This is done by using highly polar solvents such as acetic acid, dimethyl formamide, and dimethyl sulfoxide. In using this strategy, the lifetime of a carbocation can be extended, allowing the most stable product more time to form. As a result,... 

Like the previous example, this is a solvolysis reaction. Initial protonation of the alcohol followed by water leaving generates a primary carbocation. The bromide can then add to this carbocation generating neopentyl bromide. Since, for this carbocation, 1,2-hydride shifts cannot occur, a 1,2-alkyl shift generates a more stable tertiary carbocation. This new carbocation is not subject to possible 1,2-hydride shifts because any such transformation would generate either a less stable... 

In general, we should expect rearrangements in reactions involving carbocations whenever a hydride shift or an alkyl shift can form a more stable carbocation. Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations. 

Intermediate carbocations can rearrange to more stable carbocations by either a hydride shift or by an alkyl shift. 

Attack of the it electrons of the double bond on H+ yields the carbocation pictured on the far right. A bond shift (alkyl shift) produces the bracketed intermediate, which reacts with Br to yield l-bromo-2-methylcyclobutane. 

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