Activated aromatic fluonne

The replacement of a substituent on an aromauc nng by a nucleophile is termed arylaUon. This chapter considers the replacement by nucleophihc oxygen, sulfur, nitrogen, and carbon of aromatic fluonne atoms, which are often activated by electron-withdrawing groups. 

The replacement of reactive aromatic fluonne by nitrogen nucleophiles is a well-known process for the preparation of aromatic amines The aromatic fluonne IS activated by the presence of electron-withdrawing substituents on the aromatic ring, especially in ortho and para positions [57 38, 39] (equations 25-27)... 

An unactivated aryl fluorine may be activated by complexauon with chro-mium(VI). Replacement of the fluonne in the complexed system occurs readily, and the uncomplexetl aromatic product can be generated by treatment with iodine [84] (equation 46)... 

The mfluoromethyl group activates the fluorine in position 4 ofperfluorotolu ene toward reaction with carbon nucleophiles Examples on che use of perfluoro-toluene as an arylation agent abound, and in all cases, the 4-fluonne atom is replaced predommantly or exclusively [% 87,88,89, 90 (equation 48) In perjluoromesity-lene, the aromatic fluorine atoms are activated toward Ar reaction, and a reaction... 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

---