2.4 Friedel-Crafts Reactions
Friedel and Crafts reactions are associated with the acylation and alkylation of aromatic systems in presence of Lewis acids (Scheme 1).
Analogous reactions also occur with alkenes, although they are not widely useful.
2.4.1 Alkylation
The simplest example is the reaction of t -butyl chloride with ethylene to afford neohexyl chloride at -10°C in presence of AlCl3 (Scheme 2).
However, several side reactions are usually encountered in alkylation. First, alkene often isomerizes in presence of AlCl3 . Second, alkyl halide may rearrange. In addition, the halide produced may undergo further reaction. These aspects limit their applications.
2.4.2 Acylation
The acylation of alkene can be brought using acid chloride or anhydride as acylating agent in presence of Lewis acid. The electrophilic reagent can be either a complex of the acid chloride and Lewis acid or an acylium cation (Scheme 3).
The reaction may end with the uptake of chloride ion to give a b -chloroketone or by elimination to give β, γ-unsaturated ketone which rearranges using the released acid to yield the more stable conjugated ketone (Scheme 4).
Acylation also suffers due to rearrangement of the alkenes in presence of AlCl3 . Thus, the use of less vigorous Lewis acid such as SnCl4 will be useful to minimize the side reactions.