Replacement fluonne

Boron tribromide replaces fluonne with bromine on tertiary or secondary carbons however, trifluoromethyl groups are inert in this reaction [70] (equation 57). 

Oxygen nucleophiles (hydroxyl or nitrite) readily replace fluonne mperfluor-inated systems [10, II] (equations 6 and 7). 

Lead tetrafluoride, generated in situ from lead dioxide and hydrogen fluonde, can replace benzylic hydrogen by fluonne [3] Under sirmlar conditions phenol is simultaneously oxidized to 4,4-difluoro-2,5-cyclohexadienone [4 (equations 3 and 4)... 

Vanadium pentafluoride replaces benzylic hydrogen by fluonne but also adds fluonne to the aromatic system, giving fluonnated cyclohexadienes and cyclohexenes [5] (equation 5)... 

Anhydrous hydrogen fluonde, potassium fluonde, and sodium fluonde have all been used to promote the chlorine-fluonne exchangein tetmchloropyrukizine All three fliionne sources replace the chlonne in position 3 first Both sodium and potassium... 

Perfluoropmacol reacts with sulfur tetrafluonde in an unconventional way instead of replacement of the hydroxyl groups by fluorine, the substitution of four fluonne atoms in the sulfur tetrafluonde molecule with oxygen occurs to give the corresponding spirosulfurane [J6J] (equation 79)... 

The reaction of diethyl tartrate with sulfur tetrafluonde at 25 °C results in replacement of one hydroxyl group, whereas at 100 °C, both hydroxyl groups are replaced by fluonne to form a,a -difluorosuccinate [762] The stereochemical outcome of the fluonnation of tartrate esters is retention of configuration at one of the chiral carbon atoms and inversion of configuration at the second chiral center [163,164, 165] Thus, treatment ofdimethyl(+)-L-tartrate with sulfur tetrafluonde gives dimethyl meso-a,a difluorosuccinate as the final product [163, 164], whereas dimethyl meso tartrate is converted into a racemic mixture of D- and L-a,a -difluorosuccmates [765] (equation 80)... 

Numerous examples of application of DAST for replacement of carbonyl oxygen by fluonne have been reported in the field of carbohydrates [193,194,195] and particularly in the field of s/eroidi [141, 142, 183, 196, 197, 198, 199]... 

Functionalized hydrazine groups can be replaced by fluonne. The first example is the replacement ot tnalkylhydrazine groups in bis(l,2,2-trimethylhydrazmo)meth-ane, a hydrazine acetal, by tnchloroacetyl fluonde to fotm 1,1,2- trimethyl-2-fluoromethylhydrazine in 87% yield [5J]... 

Photochemically induced reduction of alkyl perfluoro carboxylates in hexa-methylphosphoramide (FfMPA) can replace one, two, or (in the case of tn-fluoroacetate) three a-fluonne atoms with hydrogen [7, S] (equation 5) Chlorine-containing fluorocarboxylates preferentially lose a-chlorme and may even lose P-chlorinc in preference to a fluonne [9] (equation 6) Cyclic a-fluoroketones are also defluonnated photochemically [10] (equation 7)... 

A nitro group in the artho or para position to fluonne is known to enhance its replacement by hydroxyl [II, 12] Bromine and iodine are much less prone to hydrolysis under similar conditions The effect is much less pronounced with the meta nitro derivative (equation 11) With o-nitro-p-fluoroaniline, it is the amino group ortho to the nitro group, rather than meia fluorine, that is replaced by hydroxyl (equation 12)... 

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 fluonne atom of monofluoroaromatk systems activated toward the S Ar reaction by electron-withdrawmg groups is readily replaced by alkoxide ion [7, 2, 3, 4, 5] (equations 1 and 2),... 

Fluorme replacement m condensedpolyfluoroaromatics can be a complicated process In perfluoroanihracene the fluonne reactivity toward replacement by methoxide foQows the order 2-F > 9-F [14], whereas in perfluoropyrene, the reactivity decreases in the senes 1-F > 3-F > 6-F> 8-F [75] In decafluoro-l,4-di-hydronaphlhalene, the 6-fluorme is most easily replaced [75] (equation 10) All fluonne atoms in perfluoronaphthalene are replaced by alkoxide or aryloxide with Al, 7 -dimethylimidazolidm-2 one (DMI) as the solvent [16] (equation 11)... 

Perfluoropyridine gives the usual replacement of the 4-fluonne on reactions with either aromatic or aliphatic mercaptides [55, 36] The reaction of perfluoropyridine with cesium tnfluoromethyl mercaptide, generated from thiocarbonyl fluoride and cesium fluonde, shows temperature dependence of selectivity in fluonne displacement [55 36] (equation 24)... 

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)... 

The reaction of a wide vanety of pentafluorobenzene denvatives reacts with imidazole m tetrahydrofuran or dimethyl sulfoxide results in the replacement of the 4-fluonne [65] (equation 33)... 

Tns(pentafluorophenyl)phosphine is also replaced when the 4 fluonne reacts with hydrazine [55, 66] (equation 34)... 

When pentafluoromtrobenzene reacts with ortho- or para-ammophenol m dimethylformarmde solution contaimng sodium hydroxide, the 4-fluonne is replaced. Either the amino or the hydroxyl function of the armnophenol can act as the nucleophile dependmg on the reaction conditions [67] (equation 35)... 

In the presence of 18-crown-6, phthahmide ion replaces the 2-, 4-, and 6-fluonnes in perfluoropyndme, whereas only the 4-fluonne is replaced m the absence of the crown ether [77] (equation 40)... 

When position 4 of perfluoropyridine is blocked with a poor leaving group, ammonia replaces the fluonne in position 2 in good yield. Oxidation of the products obtained with hypochlorite, followed by lodme-catalyzed rearrangement, yields interesting fluorodienes [78] (equation 41) Ultraviolet irradiation can be used to assist reactions m which substitution is difficult [79]... 

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)... 

An example of intramolecular replacement of fluonne is found m the thermal rearrangement of syn-S, 16-difluoro[2,2]metacyclophane In the cA-difluoro intermediate, one fluonne atom rearranges while the other is lost as HF in an interesting path to 1-fluoropyrene [85] (equation 47). 

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... 

Reactions of fluorinated dipolarophiles. Electron-deficient unsaturated species generally make better dipolarophiles, therefore, fluonnated alkenes become better dipolarophiles when vinylic fluonnes are replaced by perfluoroalkyl groups For example, perfluoro-2-butene is unreactive with diazomethane, but more highly substituted perfluoroalkenes, such as perfluoro-2-methyl-2-pentene, undergo cycloadditions in high yields [5] (equation 2) Note the regiospecificity that IS observed in this reaction... 

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