Hydride shift Friedel-Crafts alkylation

An important difference between Friedel-Crafts alkylations and acylations is that acyl cations do not rearrange. The acyl group of the acyl chloride or acid anhydride is transfened to the benzene ring unchanged. The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could conceivably arise from it by a hydride or alkyl group shift. 

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. 

The reaction looks like a simple Friedel-Crafts alkylation, but there is a twist — the leaving group is not on the C which becomes attached to the ring. After formation of the C7 carbocation, a 1,2-hydride shift occurs to give a C6 carbocation. The 1,2-hydride shift is energetically uphill, but the 2° carbocation is then trapped rapidly by the arene to give a 6-6 ring system. 

Figure 7-9 illustrates a typical Friedel-Crafts alkylation. Once formed, the carbocation is a very strong electrophile. A complication that may occur is the rearrangement of the carbocation to a more stable carbocation, as seen in SN1 mechanisms of alkyl halides. These rearrangements may involve a hydride or other shift. 

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... 

Predict the structure of the major product formed in the Friedel-Crafts alkylation of benzene with 1-chlorobutane. Answer rec-Butylbenzene as a result of hydride shift from the C-2 atom of the butyl to give a secondary carbocation. 

Each of the chlorine atoms in the dichloride can interact with AICI3 and initiate a Friedel-Crafts alkylation process. During the first alkylation process, a tertiary benzylic carbocation can form via a hydride shift (Note for a description of how a carbocation... 

Friedel-Crafts reactions. The reaction of osmium-complexed anisoles with electrophiles such as enones is catalyzed by TfOH. Benzylation of arenes by a reductive alkylation with arenecarbaldehyde acetals involves an intramolecular redox process (hydride shift) after protonation of the benzylic ether intermediates. 

The carbocation rearrangements that accompany Friedel-Crafts reactions are like those that accompany electrophilic additions to alkenes (Section 7.10) and occur either hy hydride shift or alkyl shift. For example, the relatively unstable primary hutyl carhocation produced hy reaction of 1-chlorohutane 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. Similarly, alkylation of benzene with l-chloro-2,2-dimethylpropane yields (l,l-dimethylpropyl)benzene. The initially formed primary carbocation rearranges to a tertiary carbocation by shift of a methyl group and its electron pair from C2 to Cl. 

---