Fluoroalkylamine reagents

A possible mechanism for the fluorination reaction of alcohols with fluoroalkylamine reagents is shown for reagent 2. " ... 

Table 8. Fluorination of Secondary Alcohols with Fluoroalkylamine Reagents...

Secondary alcohols give lower yields of fluorinated products (see Table 8) than primary alcohols, which is probably due to side reactions such as elimination and rearrangement. As discussed (vide supra) these undesirable reactions occur with fluoroalkylamine reagents to a greater extent than with DAST, which is therefore preferable in most cases. 

In the aliphatic series there has been little work performed with fluoroalkylamine reagents (FAR). They do not seem to work well, since, for example, methyl 2-hydroxy-2-melhyl-propanoate and the Yarovenko reagent, 2-chloro-/V,/V-diethyl-l,1.2-trinuoroethylamine (1), give the desired methyl 2-fluoro-2-mclhylpropanoate in only 10% yield elimination is the main reaction. Good yields of fluorinated products are obtained from 2-aryl-2-hydroxy-acetates, where elimination cannot occur (sec Table 9). When alkyl substituents arc present in the a-position, elimination products arc again found. 

The fluoroalkylamine reagent (FAR) 2-chloro-l,l,2-trifluorotriethylamine gives alkyl fluorides from alcohols in one step and is known as the Yarovenko reagent." This reagent is less reactive than DAST but was used intensively before the introduction of DAST. Rearrangement and elimination are similar with FAR and DAST (equations 39 and 40).This may be explained by the similar mechanism. Activation of the alcohol by intermediate formation of ester-type derivatives is followed by SN2-type displacement by fluoride ion (Scheme 32). 

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