Phosphonium hydroxides, elimination reactions

Studies on the influence of molar ratios between the base and the phosphonium salt have shown696,697 that the substitution reaction is the main reaction when an excess of base is used however, in contrast, the elimination reaction is clearly favoured when an insufficient amount of base is used. These results are not surprising if it is assumed that the substitution mechanism is second order with respect to the hydroxide ion concentration696,697, whereas the elimination reaction EHp is only first order. 

In contrast to the straightforward facile acylation-hydrolysis reaction of the a-fluoro phosphonium ylide, the acylated product from the a-fluoro phosphonate carbanion is cleaved by base in two different ways. When R is a hydrocarbon group, such as CH3 or C5H5CH2, attack at the acyl carbon with bases, such as sodium bicarbonate, sodium carbonate, sodium hydroxide, and potassium silanoate is favored (Path II in equation 13) with resultant elimination of the a-fluorophosphonate anion. Less than 10% of the desired 2-fluoro-3-oxoester is observed. However, when R is a halofluoroalkyl group (CF3, CF2CI, C3F7), attack of the base (aqueous sodium bicarbonate) occurs only at phosphorus (Path I in equation 13) and the 2-fluoro-3-... 

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