Carbocation trifluoromethyl-substituted

The extent of participation of the carbon-carbon double bond in the ionization of anti-7-norbomenyl systems is a function of the substitution at C-7. The placement of an aryl substituent at C-7 diminishes the relative rate acceleration due to participation by the double bond. Evidently, the extent of participation is a function of the stability of the potential carbocation. When an aryl group is present at C-7, the resulting benzyl-type stabilization decreases the importance of participation by the double bond. The degree of stabilization is sensitive to substituents on the phenyl ring. For p-methoxyphenyl, phenyl, and p-trifluoromethylphenyl, the rate factor for the unsaturated relative to the saturated system is 3,40, and 3.5 x 10, respectively. The double bond clearly has a much larger effect on the poorly stabilized p-trifluoromethyl-substituted system. This dependence of the extent of participation on other stabilizing features is a general trend and has been observed with other types of carbocations. ... 

Since these methoxylated and acetoxylated sulfides have an acetal structure, it is expected that Lewis acid catalyzed demethoxylation should generate a carbocation intermediate which is stabilized by the neighboring sulfur atom. In fact, nucleophilic substitution with arenes has been successfully achieved as shown in Scheme 6.7 [43], This procedure is useful for the preparation of trifluoroethyl aromatics. As already mentioned, generation of carbocations bearing an a-trifluoromethyl group is difficult due to the strong electron-withdrawing effect. Therefore, this carbon-carbon bond formation reaction is remarkable from both mechanistic and synthetic aspects. 

A further dependence of the selectivity between different nucleophiles on the stability and reactivity of carbocations was found by Richard and Amyes in a study of reactions of alcohols and carboxylate anions with -substituted a-trifluoromethyl benzyl cations (75, X = Me, OMe, SMe, N(Me)CH2CF3, and NMe2) monitored using the azide clock.305 Apart from the methyl-substituted substrate, for which the reactions approached diffusion control,... 

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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