Additives electronegativity

The additional electronegative oxygen atom in the sulfoxonium salts stabilizes these ylides considerably, relative to the sulfonium ylides. ... 

A chlorine substituent at the 2-position of a 1,1-difluoroalkene will deshield the fluorines modestly, but as seen in Scheme 4.46, a vicinal alkoxy group shields both fluorines, the trans fluorine more greatly. With the additional electronegative substituent, three-bond F—H coupling constants for such compounds become even smaller. 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

F-H coupling constants also decreases as one accumulates additional electronegative substituents on the carbons bearing the coupling nuclei. 

No additional electronegative atoms stabilize the conjugate base. 

Similar considerations apply to the sulfoxonium and sulfonium ylides, which are formed by deprotonation of the corresponding positively charged sulfur-containing cations. The additional electronegative oxygen atom in the sulfoxonium salts stabilizes these ylides considerably, relative to the sulfonium ylides. ... 

In addition, electronegative fluorines attached to the phosphorus atom would cause a build-up in effective positive charge on the phosphorus nucleus. This, in turn, would result in a contraction of the free electron pair of the phosphorus atom. Because of the contraction in the electron pair, a sigma bond could be formed only by Lewis acids which could approach closely. In BH3 such close approach would be associated with a relatively small increase in deformation energy. On the other hand, in BF3 close approach would require such a large increase in deformation energy that... 

E. As in carbonyls, however, attaching an electronegative atom to P raises the P=0 stretch. The effect of each additional electronegative oxygen is roughly additive. 

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