Rearrangements and Isomerization Reactions

It should be noted that Scheme 5.1-44 shows idealized Friedel-Crafts allcylation reactions. In practice, there are a number of problems associated with the reaction. These include polyalkylation reactions, since the products of a Friedel-Crafts alkylation reaction are often more reactive than the starting material. Also, isomerization and rearrangement reactions can occur, and can result in a large number of products [74, 75]. The mechanism of Friedel-Crafts reactions is not straightforward, and it is possible to propose two or more different mechanisms for a given reaction. Examples of the typical processes occurring in a Friedel-Crafts alkylation reaction are given in Scheme 5.1-45 for the reaction between 1-chloropropane and benzene. 

Stutz, A. E., Ed. Glycoscience Epimerization, Isomerization, and Rearrangement Reactions of Carbohydrates, Top. Curr. Chem. 2001, 215. 

After having discussed catalytic reactions involving carbon monoxide, hydrogen, carbon monoxide and hydrogen, as well as carbon monoxide and water, this section is dedicated mainly to isomerization and rearrangement reactions and to carbon-carbon and carbon-nitrogen coupling reactions. 

In addition to the variety of isomerization and rearrangement reactions leading to an abundance of different flavor and fragrance chemicals, a- and P-pinene also undergo carbocationic polymerization to yield polyterpene resins (Figure 3B.10).The reaction can be conducted as a batch or continuous polymerization of P-pinene and a smaller fraction of the less reactive a-pinene in a solvent such as xylene and is catalyzed by Lewis acid metal halides, such as aluminum trichloride (AICI3) in the presence of some parts per million of water to create a strong proton donor (the vast majority of pinene polymerization involves with the use of P-pinene, not a-pinene, mostly due to intrinsic... 

The stability of carbocations increases for alkyl cations with the number of alkyl groups that surround the positive charge and thereby stabilize it by their inductive effects. Thus, a methyl carbocation CH3 is the most unstable and reactive one while the tert-butyl cation [(CH3)3C]+ is the most stable and least reactive. This stability order is also the reason why carbocations frequently undergo isomerization and rearrangement reactions after formation, a reactivity that is very important for all isomerization reactions in refineries (here branched hydrocarbons are highly desired due to their higher octane number - see Chapters 6.9 and 6.10). 

There are relatively few new methodological developments for the synthesis of halodienes. We have chosen to separate the results based on isomerization and rearrangement reactions from the previous part. This section is therefore divided into two parts. 

---