Base-promoted HF elimination

Eluorine-18 was also used as a tool for the study of the base promoted HF elimination from 4-fluoro-4-(4 -nitrophenyl)butan-2-one [62] (Scheme 3). The F KIEs values were respectively 1.0037,1.0047 and 1.0013 with formate, acetate or imidazole used as the base. The size of the fluorine KIEs corresponds to 5 -15 % of the estimated maximum for complete C-F bond breakage and is consistent with a postulated ElcB-E2 Hke or ElcBjp mechanism. 

Base-promoted HF Elimination from 4-Fluoro-4 (4 -nitrophenyl)butan-2-one... 

Three possible mechanisms of P-elimination (El, E2 and ElcB) could be considered to explain the base-promoted HF elimination from 4-fluoro-4-(4 -nitro-... 

Only by means of primary, secondary and leaving group F KIEs it is not possible to decide whether the base-promoted HF elimination of ketone 1 follows a concerted or a stepwise mechanism. The double-isotopic fractionation method provides strong evidence for a stepwise process, more hkely via an ElcBin mechanism. 

Halide ions, especially F , are weak bases in aqueous solution. However, in dry aprotic solvents Et4N F effectively induces elimination of HBr from 2-bromoethylbenzene (7). The ammonium chloride and bromide only promote halogen exchange. At 80°, the fluoride decomposes into EtjN, HF, and ethylene, thus demonstrating the high protophilicity of the hard fluoride ion. This same property is also shown by its usefulness for generating dihalo-carbenes. Naked fluoride ion (cation sequestered by a crown ether) promotes facile elimination of haloethylenes to form acetylenes and allenes (8). 

M, rearrangement, formal elimination of HF, and aromatization, to yield dipole 248. Yet another possibility is that the isocyanide attacks the anhydride to form the intermediate K, a direct precursor of the dipolar species L. This mode of activation for isocyanides has been investigated by El Kaim, who promoted a series of useful transformations based on this reactivity, leading to products such as trifluoropyruvamides 250 (R = F) [199-201]. 

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