Perfluoroaromatic compounds

In most instances, perfluoroaliphatic magnesium and lithium compounds exhibit less thermal stability than perfluoroaromatic compounds This instability imposes some stringent restrictions on their reactions A prior knowledge ol their stability at various temperatutes is fundamental to their subsequent use as or-... 

Af,Ar,AOV -Tetramethylnaphthalene-l,8-diamine described as Proton Sponge 38 (see Section 1.2) has the ability to act as a fluoride ion donor when its hydrofluoridc is used. Furthermore, it may be used to promote oligomerization reactions of fluorinated alkenes, e.g. 3, and poly-fluoroalkylation reactions with activated perfluoroaromatic compounds, e.g. 4.38... 

Table 9.1 Boiling points of perfluoroaromatic compounds in comparison with hydrocarbon counterparts...

Perfluoroaromatic compounds can be obtained by reductive aromatization of readily accessible perfluorocycloaliphatic precursors [73]. Defluorination can be accomplished by contact with hot (500 °C) iron or iron oxide. After reducing the per-fluoroaliphatic compound the metal surface can be regenerated by passage of hydrogen gas. This method has been scaled up to a continuous flow process for industrial synthesis of a variety of perfluorinated aromatic compounds (Scheme 2.27). 

Even if no —M electron-withdrawing substituents are present, however, aromatic fluorine can be replaced. The ease of this replacement increases with the degree of fluorination. Perfluoroaromatic compounds such as hexafluorobenzene or pentafluoropyridine are especially highly reactive toward a variety of nucleophiles (Scheme 2.40). 

By analogy with the hydrocarbon world , high selectivity is also observed in the regiocheraistry of the second substitution in perfluoroaromatic compounds (Scheme 2.41). For hexafluorobenzene a second nucleophilic replacement always occurs in the position para to the first substituent. A similar, clear preference is also observed for perfluoronaphthalene [95],... 

Systematic exploitation of the different susceptibilities of the different positions of perfluoroaromatic compounds can be used as a tool for combinatorial synthesis of (fluoro)aromatic compounds. The feasibility of this concept was recently demonstrated by R. D. Chambers and coworkers who used the pentafluoropyridine system as an example [99] (Scheme 2.43). Further differentiation of the reactivity toward hard and soft nucleophiles was achieved by partial replacement of fluorine by bromine in this system. 

Hexafluoro-2-butyne adds to activated perfluoroaromatic compounds giving corresponding unsaturated side chains, or with the pyridazine system an interesting cyclization occurs (Eqs. 87 and 88).316... 

FIGURE 9.36 Fluorination of perfluoroaromatic compounds over C0F3. 

Further fluorination of perfluoroaromatic compounds occurs quite smoothly in many cases, using cobalt trifluoride and related reagents (Eq. 94). These reactions are of course much easier to control than fluorinations of the corresponding parent compounds containing hydrogen and the dimeric compound (103), obtained from perfluoropyrimidine, presents some of the best evidence for a radical cation process for this type of reaction (Scheme 29). Even direct fluorination of pentafluoropyridine, with fluorine of low... 

Fig. 6 Molecular structures of the perfluoroaromatic compounds studied by Alkorta et al. [18]...

Low molecular weight fluorine containing polymers form from perfluoroaromatic compounds through a loss of aromaticity when they react with bis(fluoroxy)-difluoromethane [180]. More interesting is formation of poly(a,a,a, a -tetrafluoro-p-xylylene) by a polymerization technique that closely resembles the preparation of poly(p-xylylene) by vacuum pyrolysis of a dimer [181] ... 

After a vigorous initial activity, whereby new fluorinated organometallic compounds were first synthesized, much of the research effort has decreased Some activity has been concerned with the reactions of perfluoroaromatic magnesium compounds... 

Research on perfluoroaromatic lithium compounds, like that of the magnesium compounds, has decreased since their initial discovenes Most of the recent effort IS concerned with reactions of the perfluoroaryllithium compounds with various substrates... 

Although the synthesis of multiple-pyrrolyl compounds can be achieved by SnAt reactions of perfluoroaromatics with pyrrolylsodium at ambient temperature <96JOC9012>, deleterious side reactions are often observed during attempted A-alkylations of the alkali salts of pyrrole. A protocol has therefore been developed for the preparation of N-arylmethylenepyrroles by reduction of the corresponding AT-acyl derivatives by treatment with sodium borohydride/boron trifluoride etherate in a sealed tube <96S457>. ... 

Intramolecular cyclization in perfluoroaromatic systems proves useful for the synthesis of heterocyclic compounds [72] For example, the Fischer indole synthesis, which normally requires the presence of an ortho proton, occurs satisfactorily with an ortho fluorine in theperfluoronaphthalene senes [73] (equation 37)... 

The period from 1960 onwards saw perfluoroaromatic chemistry rapidly unfold, selective methods for fluorination develop, and fluorinated compounds play an increasingly important role in the pharmaceutical and plant-protection industries. Indeed, there have been so many interesting developments in the subject since the original edition [3] that it will be impossible to do justice to this era in one small volume. Remarkably, it has been reported that organofluorine compounds constitute 6-7% of all new compounds recorded in Chemical Abstracts up to 1990 and 7-8% of all chemical patents up to 1997 contain fluorinated compounds. This in itself is an outstanding output for the relatively limited number of workers in the field worldwide and is a tribute to their dedication [59]. 

Either fusion with alkali metals or reaction with aUcali-metal complexes with aromatic hydrocarbons will break down most fluorocarbon systems, due to the high electron affinities of these systems. Such reactions form the basis of some methods of elemental analysis [13], the fluorine being estimated as hydrogen fluoride after ion exchange. Surface defluorination of PTFE occurs with alkali metals and using other techniques [14]. Per-fluorocycloalkanes give aromatic compounds by passage over hot iron and this provides a potential route to a variety of perfluoroaromatic systems (Chapter 9, Section IB). 

Acid-induced processes Although perfluoroaromatic nitrogen heterocyclic compounds are only weak bases, nucleophilic substitution can be induced by protonic or Lewis acids, as outlined in Figure 9.45, and interesting contrasts in orientation can sometimes be achieved because attack ortho to nitrogen is often preferred under these conditions. 

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

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