Replacement Fluoroaromatics

Chromium complexes of fluoroaromatics undergo fluorine replacement more readily and in high yield [24] (equation 14). 

Even the reactivity of the original monochlorotriazinyl dyestuffs can be usefully enhanced by replacing a chlorine by a fluorine atom. It is perhaps ironic that fluorine, often viewed as conferring enhanced stability on molecules, is exploited in fluoroaromatics, whether carbocyclic or heterocyclic, as a method for enhancing reactivity, an aspect of fluorine chemistry clearly demonstrated by its use in reactive dyestuffs. 

In organo-fluorine compounds fluorine atoms can be eliminated by nucleophilic sulfur species to form C —S bonds. In principle, the fluorine to be eliminated can be bonded to aliphatic or araliphatic compounds, as well as to aromatic or heterocyclic compounds however, the replacement proceeds more efficiently the more the fluorine is activated. Therefore, the synthetic usefulness of these reactions is the broadest with fluoroaromatic compounds, including heteroaromatics, with which the reactions often proceed smoothly under mild conditions. The nucleophilic sulfur compound to be reacted is. in most cases, an aliphatic or aromatic thiol or a metal sulfide, but reactions with, for example, thiourea or ammonium thiocyanate have also been described. The sulfur introduced this way can be either oxidized or removed by reduction, opening additional possibilities for modifications of the original fluoro compounds. 

Fluoroaromatic compounds react with nucleophilic sulfur species, resulting in single, multiple, or complete replacement of fluorine, to form C —S bonds. The extent of fluorine substitution depends on (1) the solvent used, (2) the nature of the sulfur reagent, (3) the ratio RS / ArF, (4) the reaction time and temperature, and (5) the nature of the fluoroaromatic compound. 

Both oxygen and nitrogen nucleophiles react more rapidly with fluoroaromatics than corresponding sulphur and carbon nucleophiles, in accordance with Hard-Soft Acid-Base principles [51]. It should be remembered, however, that the situation can become more complex with, for example, base catalysis, where the rate of the second stage becomes important under these rather unusual conditions, an order of replacement Br > Cl > F has been observed [52]. 

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