Benzenes, fluorine-substituted

Extensive work on the interaction of aromatic compounds with xenon difluoride has been carried out in order to investigate the reaction mechanism and the scope of the fluorination depending on the substituents electronic nature.26-59 62 It has been found that benzene and substituted aromatics react with xenon difluoride at room temperature in the presence of hydrogen fluoride to form the typical products of electrophilic fluorination contaminated with low quantities of difluoro-substituted molecules. 

Absorption spectra of Pr(M) and Nd(III) complexes with alkyl, aryl, fluorine substituted P-diketones and the associated hypersensitive transitions were studied in detail. Mono, bis, tris and tetrakis complexes in benzene, DMF, DMSO and ethanol were also studied. Spectra both in solids and solution permitted the interpretation of the state of p-diketones based on the trends in the intensity and energy interaction parameters [238,244]. The various complexes were isolated and their IR, 1H NMR, stability constants were studied as well. Octacoordination in the tetrakis complex and tris complex associated with two solvent molecules was noted. 

We have also employed ETS to study the effect of fluorine substitution on the ir orbitals of benzene and ethylene (10). Here we briefly discuss the results for the fluoroethylenes. Fluorine substitution is known to cause only small shifts in tv ionization potentials (IP) of unsaturated hydrocarbons (1 1). For example, the vertical iv IP s of ethylene and perfluoroethylene agree to within 0.1 eV. The reason that has been most often forwarded to explain this is that the electron withdrawing inductive effect, which stabilizes the occupied orbitals, is nearly cancelled by the destabilizing resonance mixing of the fluorine p orbitals with the ir orbitals of the ethylenic double bond. 

The sterically small and inductively strong fluorine atom must play a crucial role at the last step of introducing a sixth substituent on penta-substituted benzenes. Hexa-substituted phenylthiobenzenes(34) [9],vinylthiobenzenes(35) [10],pyridinylbenzenes(36) [ 11 ],phe-noxybenzenes (37) [12], trifluoroethoxybenzenes (38) [12], and pentafluoroethylbenzenes... 

Energetics and Average Product Translational Energies, Fluorine + Substituted Benzenes... 

As in the radical halogenation of alkanes (Section 3-8), the exothermic nature of aromatic halogenation decreases down the periodic table. Huorination is so exothermic that direct reaction of fluorine with benzene is explosive. Chlorination, on the other hand, is controllable but requires the presence of an activating catalyst, such as aluminum chloride or ferric chloride. The mechanism of this reaction is identical with that of bromination. Finally, electrophilic iodination with iodine is endothermic and thus not normally possible. Much like the radical halogenation of alkanes, electrophilic chlorination and bromination of benzene (and substituted benzenes. Chapter 16) introduces functionality that can be utilized in further reactions, in particular C-C bond formations through organometallic reagents (see Problem 54, Section 13-9, and Real Life 13-1). 

Direct arylation of m-l-aryl-2-(2-haloaryl)ethenes produces phenanthrenes. Applying the transformation to dibrominated l,4-bis(styryl)benzene 61 under certain conditions [Pd(OAc)2, PCys HBFq, K2CO3, Ag2C03, DMF, 130 °C] allowed us to synthesize fluorine-substituted [5]helicene 62 in high yield (Scheme 22.15) [31]. The presence of methoxyl groups on bromine-attached benzene rings resulted in the... 

Ffe. 2.17 Existence of polymorphism in fluorine-substituted iV-(2-phenoxyphenyl)- benzene sulfonamides. Reprinted with the permission from Ref. [111]. Cop5uight 2012 American Chemical Society... 

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene we con sider the electronic properties of a trifluoromethyl group Because of their high elec tronegativity the three fluorine atoms polarize the electron distribution m their ct bonds to carbon so that carbon bears a partial positive charge... 

Benzylic carbon-hydrogen bonds in compounds such as methylpentafluoro-benzene, fluoromethylpentafluorobenzene, and difluoromethylpentafluoroben-zene are not capable of metalation by butyllithium Instead nucleophilic substitution of the para fluorines occurs m each example [55] (equation 13)... 

Tetrafluorobenzyne, generally generated by the treatment of pentafluoro-benzene with butyllithium at -78 °C in ether in the presence of the substrate diene, is a versatile dienophile [9, 103, 104], In an interesting study of the use of substituted benzynes to synthesize isoindoles, tetrafluorobenzyne, 4-fluorobenzyne, and 4-(tntIuoromethyl)benzyne were shown to react in moderate yields with A7-(trimethylsiIyl)pyrroles, with the adducts being easily converted to the respective fluorinated isoindoles [705] (equation 87). 

That charge-transfer effects are not involved follows from the fact that the rate of triplet decay in perfluorobenzene is larger than that in benzene. If the benzophenone triplet were to act as acceptor and the benzene derivative as donor in a charge-transfer complex, the substitution of perfluorobenzene for benzene should render this type of process much less probable due to the strongly electron-withdrawing character of the fluorine atoms. 

Table 3.2 provides chemical shift data for various substituted fluoro-benzenes.6 The chemical shifts of para-substituted fluorobenzenes have a reasonable correlation with the cp values of the substituents, the more electron-withdrawing substituents leading to greater deshielding of the p-fluorine. The chemical shifts of ort/zo-substituted fluorobenzenes also exhibit a rough correlation, but there are some significant aberrations. The chemical shifts of meto-substituted fluorobenzenes exhibit no correlation and vary over a much smaller range. 

Cycloaddition of furans followed by a subsequent transformation is still adopted as a useful strategy to prepare fluorine-containing benzene derivatives and isoquinoline compounds <00SL550>. The cycloaddition adduct can also be converted to a trifluoromethyl substituted cyclohexanone compound via hydrogenation and hydrolysis. Examples of these transformations are illustrated below. 

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