Electrophilic Aromatic Substitutions via Sigma Complexes Ar-SE Reactions

1 Electrophilic Aromatic Substitutions via Sigma Complexes ( Ar-SE Reactions ) 

The electrophilic aromatic substitution via sigma (Wheland) complexes, or the Ar-SE reaction, is the classical method for functionalizing aromatic compounds. In this section, we will focus on the mechanistic foundations as well as the preparative possibilities of this process. 

Bruckner R (author), Harmata M (editor) In Organic Mechanisms — Reactions, Stereochemistry and Synthesis Chapter DOI 10.1007/978-3-642-03651-4 5, Springer-Verlag Berlin Heidelberg 2010 

Strictly speaking there is only indirect evidence for the occurrence of sigma complexes as Side Note 5.1. short-lived intermediates of Ar-SE reactions. Formed in the rate-determining step, sigma com- Stable Cyclohexadienyl plexes completely explain both the reactivity and regioselectivity of most Ar-SE reactions (cf. Cations Section 5.1.3). In addition, the viability of short-lived sigma complexes in Ar-SE chemistry is supported by the fact that stable sigma complexes could be isolated in certain instances. 

---