1. Hexafluorobenzene synthesis by potassium fluoride influencing hexachlorobenzene in the presence of catalysts
(Continuation).

The presence of two nitro-groups in benzene ring forwards chlorine exchange for fluorine under the influence of KF, though the process passes at high temperature, while the use of Ph₄P^+. Cl^- salt as a catalysts allows lowering the temperature of process [44].

Such salts based on aminophosphazene as 1,1,1,3,3,3-hexapyrrolidino-diphosphazenium chloride chloride and 1,1,1,3,3,3-hexapiperidino- diphosphazenium chloride are effective catalysts for chlorine exchange for fluorine under the influence of alkali metals' fluorides or ammonium fluoride. Thus, the reaction of 2,5-dichloro-2,4,6-trifluoropyridine and KF in the sulpholane -chlorobenzene solvents' system at 215 ^oC produces 3-chloro-2,4,5,6-tetrafluoropyridine with the yield of 75% and pentafluoropyridine with the yield of 24% in the presence of these salts [44]. 2,6-Difluorobenzaldehyde was obtained out of 2-chloro-6-fluorobenzaldehyde by KF influencing in chlorobenzene at 190 ^oC , it's yield was about 88.7 - 88.5 % [44].

The carrying out of chlorine for fluorine exchange process in para-chloronitrobenzene proved to be effective when carrying the process out in bipolar aprotic solvents in the presence of Ph₄PBr and 4 (Table 7).

Table 7. Fluorinating the Benzene Chlorine Derivatives by Potassium Fluoride in the Presence of Catalysts

The presence of two chlorine atoms in benzene ring produces, as a rule, difluoroderivatives, for example in case of 2,6-dichlorobenzaldehyde (table 7) they obtain 2,6-difluorobenzaldehyde, which is an important product for herbicide production [57].

The same result is obtained also for 2,4- dichlorobenzoic acid dichloranhydride (table 6) [57].

We can observe a more complicated picture at fluorinating of tetrachlorobenzotrifluoride 12. In this case depending on the character of catalysts used we see a different degree of fluorination (table 8) [57].

Table 8. Synthesis of Tetrafluorobenzofluoride out of Tetrachlorobenzotrifluoride [57]

Effective catalysts have been found for production of 1,3,5-trifluorobenzene by influencing of potassium fluoride on 1,3,5-trichlorobenzene  (table 9) [57].

Table 9. Obtaining of 1,3,5-trifluorobenzene by Influencing 1,3,5-trichlorine-benzene Using Potassium Fluoride in the Presence of Catalysts [57]

Thus, 2,3,4-trifluoro-5-chloro-1-trifluoromethylbenzene is obtained with the yield of 85% at heating of 2,3,5-trichloro-1-trifluoromethylbenzene in sulpholane  in the autoclave at 210 ^oC and pressure of 2.2 bar in the presence of Et₄PBr [58]. The effectiveness of process is increasing when using fine-loosened anhydrous potassium fluoride and tetrakis(diethylamido)-phosphonium bromide (R2N)4P^{+ .}  Hal^- [59,60].

Taking into account a commercial worth of hexafluorobenzene the majority of information regarding its obtaining has been published as patents, which sometimes hardly reveal the essence of offered technologies. Nevertheless the effectiveness of catalysts use to intensify the known hexafluorobenzene obtaining method is determined. In this case, we manage to soften the tough conditions of synthesis or to refuse the using of solvents. It is stated, that quaternary ammonium salts, guanidine derivatives, quaternary phosphonium salts, krypton derivatives, crown-ethers and polyesters [30,44-48,61,62] evince their most effectiveness as catalysts for intrerphase transfer.

Thus, the authors of work [30] showed, that potassium fluoride applied over calcium or barium fluoride reveals its high activity in the reaction of hexafluorobenzene fluorination. At that, the simultaneous using of such potassium fluoride and quaternary phosphonium salt (for example, tetrakis(diethylamido)-phosphonium bromide) essentially increases the efficiency of chlorine for fluorine exchange process (table 2). As we can see from the table 2, under conditions (230 ^oC, 42.5 h) the using of potassium fluoride dried by dispersion produces the summary yield of fluorination products of only 6%, while when using potassium fluoride, applied over calcium fluoride under analogous conditions produces the mixture of subsequent substitution of chlorine for fluorine products of high yield, though the main product is tetrachlorodifluorobenzene (46 %).

The substitution of chlorine for fluorine in hexachlorobenzene goes notably easier under the influence of KF when using tetrakis(diethylamido)-phosphonium bromide as a catalyst for inter-phase transfer: at 230 ^oC in 42.5 h the reaction products contain 13.5% of chloropentafluorobenzene, 37% of dichlorotetrafluorobenzene and 45% of dichlorotetrafluorobenzene, which is the main product. Hexachlorobenzene fluorination process is going much faster when using potassium fluoride, applied on CaF₂ (Table 2 ). The obtained results can be a consequence of fluorination both on a solid surface and at organic phase and factors increasing the absorption of reagents on a solid surface (pressure, substrates nature) will lead to raising the rate of exchange.

N,N',N''-Hexasubstituted guanidinium chlorides proved as effective ones at obtaining hexafluorobenzene [59,63]. Thus, hexafluorobenzene was obtained out of hexachlorobenzene and KF with the yield of 90% at 160-170 ^oC in the presence of this catalyst.

We used ammonium salts like [(RR'N)₂CNR²R³]^{+ .}  X^- (where R-R³ = alkyl C_1-7, cycloalkyl C_5-8) or hexaethylguanidinium  chloride [64] as catalysts. Carrying out reaction at 160-170 ^oC produces the mixtures, containing 91% of hexafluorobenzene, and 9% of partly  fluorinated fluorochlorobenzenes. The overall yield of hexafluorobenzene is 88.4%. 

The quaternary phosphonium salts are more effective, what caused their wider use and research of exchange processes involving them. Thus, pentafluoro-containing benzene derivatives like C₆F_nX_6-n ( X = F, Cl, CF₃, CN; n = 1-5) are obtained by heating of corresponding haloid containing aromatic compounds with alkali metals fluorides in the liquid phase in the presence of tetrakis(diethylamido)phosphonium bromide in the medium of products of starting substrate partial fluorination with simultaneous selection of target products at 150-200 ^oC [65]. In this case the technological process is becoming greatly simpler and the yield of polyfluoroarimatic compounds increases.

When carrying out a process of hexafluorobenzene fluorination using potassium fluoride in benzonitrile (catalyst amount is 12 g per 115 g of hexachlorobenzene) at 200 ^oC in 5.5 h we obtained a mixture, containing 24.4% of hexafluorobenzene, 39.9% of pentafluorochlorobenzene, 21% of tetrafluorodichlorobenzenes' mixture and 8% of trifluorotrichlorobenzenes [60]. The effectiveness of this catalysts was proven by authors of works [55,66] using example of hexachlorobenzene and partly fluorinated chlorobenzenes.

We should note, that the presence of several chlorine atoms in benzene ring is not an obstacle for exchange reaction taking place [55].

Catalysts can take part in acceleration reaction of fluorodechlorination of chloroaromatic substrates at different process stages. At first, they can increase KF concentration in liquid phase, secondly, lowering the energy of reaction activation because of inter-phase -complex participation in stabilization, they can also lead to increasing of catalytic system activity [59]. Adding to such catalytic system promoters, representing compounds of heterocyclic cycle and aromatic row, ethers, substituted amides of organic acids etc, promotes the rate of chlorine exchange for fluorine and increasing the yield of final products [62]. Under these conditions the compounds with the common formula C₆F₄XY (where X = F, Cl, CF₃, CCl₃, CN, COR; Y = F, H, Cl, CF₃, CCl₃, CN, COR) were obtained with their high yields [62]. The proportion of quaternary tetra-amidophosphonium salt is in the range between 100 : 1 and 5-20 : 1, and proportion of alkali metal fluoride and quaternary tetra-amidophosphonium salt is in the limits ranging from 1000 : 1 to 10 : 1.

This method proved to be so effective, that American company "Albemarle" not only has patented the technology of synthesis of fluorinated aromatic compounds, allowing to raise the yield of final products at lower temperatures and at more moderated figures of pressure compare to usual method and to diminish the reaction period, but it also has built two plants producing several tons of hexafluorobenzene per year [67,68]. Different benzene derivatives are obtained based on it and first of all one of them is pentafluorobromobenzene..

When using tetraethylene glycol dimethyl ether and 18-crown-6 the catalysis effectiveness depends on substrate origin - the isomeric composition of trifluortrichlorobenzenes, forming at using these catalysts and without their using in sulpholane, at the same degrees of conversion is approximately the same [69] and in turn is close to isomeric composition at fluorination of trifluorotrichlorobenzenes using potassium fluoride at 350 ^oC. The authors consider, that speeding up of chlorine substitution for fluorine mainly occurs because of potassium fluoride nucleophilic reactivity rising.

Fluorodechlorination of tetrafluorodichlorobenzene by KF goes slowly (only traces of pentafluorochlorobenzene can be seen in 6 hours) in the presence of catalytic amounts of diglyme, tetraglyme or 18-crown-6 ester. [59]. The combination of hexaethylguanidinium chloride with polyesters results in noticeable growth of activity of catalytic system (conversion of terafluorodichlorobenzene doubles). We should notice, that such a growth of activity was mentioned by the authors of work [30] when adding polyester to KF, applied on inert supporter (CaF₂, BaF₂) at fluorinating of hexafluorobenzene. 

When using polyesters as catalysts the catalytic effect is explained from the point of view of increasing of "active" ion -fluoride current concentration, which leads to speeding up of fluorodechlorination [59]. When using other catalysts the catalytic effect has some other origin and is connected not only with increasing of concentration of "active" ion- fluoride, but also with great participation of these catalysts in stabilization of anionic -complexes, that results in decreasing of activation fluorodechlorination reaction energy probably because of more effective stabilization of inter-phase-complex [59].

Fig.1. The depending of conversion degree on reaction period (mole proportion C6Cl2F4 : KF : catalyst, 1:1:0.05) [70]

1 - hexaethylguanidinium chloride

2 -  tetra(diethylamido)phosphonium bromide

3 - 18-crown-6

4 - tetraethylene glycol dimethyl ether

5 - sulpholane (mass ratio C₆Cl₃F₃ : sulpholane , 1:1).

At the same time in case of catalysts of tetra(diethylamido)phosphonium bromide and hexaethylguanidinium chloride the difference of isomeric compositions compare to non-catalytic variant becomes noticeable as soon as conversion reaches 8.5%, and mainly developing at conversion of 20%. The appearance of catalytic effect of these catalysts depends to a great extent on the origin of substrate. It can be connected with the scheme differences, taking place in every case of the processes [64].

According to up-to-date conceptions, the process of chlorine-aromatic compounds fluorodechlorination using potassium fluoride goes both on the surface of solid phase and in -phase, directly bordering to potassium fluoride [16]. In both cases, the catalytic effect is connected with increase of reactivity of potassium fluoride. In first case, as they think, it occurs because of more effective coordination of substrate on the surface of potassium fluoride, and in the second one the reason of it is forming of incoherent ionic pares between catalyst and ion-fluoride [16].

There are estimations, that poly-fluorochlorobenzenes' fluorodechlorination goes according to the addition - eliminating scheme with intermediate forming of anionic -complex [71]. At that it is considered, that the distinction of fluorination rate in catalytic and non-catalytic reactions can be not only due to catalyst's influence on increase of "active" ion-fluoride concentration but also to stabilization of corresponding -complex. At the same time if a participation of catalyst in speeding up of fluorodechlorination is limited by increasing of current concentration of "active" ion-fluoride, then catalytic effect will slightly depend on the substrate's nature. On the contrary, if a catalyst is taking part in stabilization of intermediate -complex, we can expect, that in this case fluorodechlorination of more active substrates will accelerate more strongly, than of less active ones. The authors of work [72] have studied the influence of different catalysts on the fluorodechlorination process of trifluorbenzenes and demonstrated, that maximum rate of trifluorotrichlorobenzenes' conversion is observed when using tetra(diethylamido)phosphonium bromide and hexaethylguanidinium chloride as catalysts, and in sulpholane  when there is no catalyst at all the reaction goes in a least effective way (fig. 1). Besides that, the activity is decreasing in a row of isomers: 1,2,4-F₃C₆Cl₃ > 1,2,3-F₃C₆Cl₃ > 1,3,5-F₃C₆Cl₃ , i.e. in accordance with the influence of fluorine and chlorine atoms on stabilization of anionic -complex [71,72]. 

The information cited here allow us to establish the effect of applying catalysts at obtaining fluoroaromatic compounds by alkali metals fluorides influencing chlorine containing aromatic compounds.