Electron-withdrawing groups nucleophilic additions, carbanion intermediates

These observations suggest that the attachment of electron-withdrawing groups to the double bonds of a,p-unsaturated carbonyl compounds facilitates cyclopropanation by stabilisation of an intermediate carbanion (44) formed during the nucleophilic addition of the anionic centre to the substrate to form the cyclopropane (45) (Scheme 18). 

We noted in this chapter that nucleophilic addition to alkenes occurs when groups capable of electron withdrawal by both induction and resonance are attached to the alkene. The addition of alkoxide anions to 2,2-dichloro-l,l-difluoroethylene occurs on the carbon with the fl uorines. Why does the addition occur to place the negative charge of the carbanion intermediate on the carbon with the chlorines, rather than on the carbon with the more electronegative fluorines ... 

---