Finkelstein-type reaction

Before the TPS ether is cleaved with TBAF, secondary alcohol 7 has to be protected as methyl ether. TBAF is a reagent to cleave every silyl ether. Most other functional groups are not affected (see Chapter 2). In the next two steps the conversion of alcohol 33 into mesylate 34, which is a good leaving group, and then into iodide 35 in a Finkelstein type reaction occurs. Acetone is the solvent of choice, because Nal is better soluble in it than NaOMs and consequently reaction equilibrium is forced to the product side. Direct transformation from an alcohol to an iodide is possible with PPha and I2 in an Appel-like reaction, but in some cases this reaction fails. Final procedure is the generation of phosphonium salt 8. 

Vinyl bromides can be converted into vinyl iodides using a Finkelstein-type reaction (Example 7.28) [146]. The process was copper catalyzed and used sodium iodide as a source of nucleophilic iodine. While only a single example was listed, this procedure has the potential to be very attractive to the synthetic community due to operational simplicity. 

An example of reaction type (d) in Table 5-4 is the Finkelstein halide exchange reaction between iodomethane and radioactive labeled iodide ion. The rate constant for this reaction decreases by about 10" on going from less polar acetone to water as solvent [66]. 

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... 

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