Fluonnated ylides

Whereas access to fluoromethylene ylides is easy, higher homologues are more difficult to prepare because ot the facile (1-elimination of fluorine However hexafluoroisoprapylidene ylides may be prepared in situ from tetrakisftnfluoro-methyl)-l,3-dithietane and tnphenylphosphme [54 (equation 51) or 2,2-dichloro-hexafluoropropane and tnphenylphosphme [55, 56, 57 (equation 52) (Table 21) Pcrfluoro-2-buteiie reacts with tnbutylphosphine to cleanly form a solution of a fluonnated ylide [5/ ] (equation 53)... 

Fluonnated ylides have also been prepared in such a way that fluonne is incorporated at the carhon P to the carbamonic carbon Vanous fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyUithium or hthium dusopropy-lamide did not lead to yhde formation, rather, deprotonation was accomparued by loss of fluonde ion Flowever deprotonation with hydrated potassium carbonate in thoxane was successful and resulted in fluoroolefin yields of45-S0% [59] (equation 54) P-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydnde The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyUithium to form a new yhde, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [6/j such as an acetyhde to form a fluonnated enyne [62] (equation 56)... 

Fluonnated alkynes are readily prepared by treatment of the mtetmediate formed by acylation with a second equivalent of ylide followed by pyrolysis [63, 64, 65] (equation 57)... 

Fluoroolefins may be prepared by the reaction of Wittig reagents and other pho sphorus-contaimng ylides with fluorinated carbonyl compounds. (A discussion of the fluonnated Wittig reagents or other fluonnated phosphorus reagents with nonfluorinated carbonyl compounds is on page 581.) Tnphenylphosphoranes, derived from alkyltriphenyl phosphonium salts, react with 1,1,1-trifluoroacetone [3/] or other trifluoromethyl ketones [32, 33] (equation 26) (Table 10). 

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