Aromatic Finkelstein reaction

SCHEME 7 A copper-catalyzed aromatic Finkelstein reaction [ 146]. 

SCHEME 7.87 An aromatic Finkelstein reaction under continuous flow [147]. 

Halide exchange reactions ( transhalogenation or Finkelstein reaction) have been, until recently, of particular synthetic importance only in the synthesis of alkyl (and strongly activated aryl) fluorides or iodides, where thermodynamic stability (for the fluorides) and solubility differences (for the iodides) shift the reversible exchange processes in the desired direction. With the advent of phase transfer catalysis (PTC) and transition metal catalysis (mainly homogeneous) this important family of reactions has been extended to practically all aromatic and aliphatic halides. 

This reaction was initially reported by Finkelstein in 1910. It is a preparation of alkyl iodide from alkyl bromide or chloride with potassium or sodium iodide in acetone. Therefore, this reaction is generally known as the Finkelstein reaction. Occasionally, it is also referred to as the Finkelstein halide exchange, Finkelstein displacement, or Conant-Finkelstein reaction. Mechanistically, this reaction is a simple nucleophilic substitution (often via Sn2), as iodide is a stronger nucleophile than bromide or chloride. The yield of this reaction is very high and can be quantitative if occurs in DMF. It was found that the trifluoromethyl group retards the displacement of bromide when it presents as an a- or /3-substituent but accelerates the reaction as a substituent in an allylic chloride. Under normal conditions, this type of halide displacement does not occur for aryl halides. For dihalides, unsaturated or cyclic compounds may form via carbocation intermediates, which form transient covalent iodides or are reduced directly by iodide to free radicals. However, the aromatic halide exchange reacts smoothly when 10% Cul is present in the reaction... 

---