Addition hexafluorobenzene

Write equations describing the addition-elimination mechanism for the reaction of hexafluorobenzene with sodium methoxide clearly showing the structure of the rate determining intermediate j... 

Other aryl halides that give stabilized anions can undergo nucleophilic aromatic substitution by the addition-elimination mechanism Two exam pies are hexafluorobenzene and 2 chloropyridme... 

Reactions of the halogen iluorides with other unsaturated functional groups are generally reported only with highly fluorinated substrates Hexafluorobenzene and denvatives [118] and octaflvoronaphthalene [119] react with mixtures of bromine and bromine trifluoride by 1,4 addition of fluorine followed by addition of bromine fluoride across a reniatning double bond (equation 22)... 

The stereospecificity of addition suggests a singlet(10) carbene although the ground state of cyclopentadienylidene is known to be a triplet. Attempts to produce a triplet species, which would be expected to react nonstereospecific-ally, in a 4-methyl-cw-2-pentene matrix at 77°K or by dilution of mixtures of the azo compound and olefin with hexafluorobenzene or octafluorocyclo-butane (inert diluents) were unsuccessful. It was concluded that the singlet carbene produced upon photolysis reacts more rapidly with the olefinic... 

The dilution technique makes use of the different concentration dependence of a) S-T-intersystem crossing, and b) [1 -f-2]-cyclo-addition of a carbene to an olefin. The decay of the metastable singlet state is monomolecular, while the stereospecific addition is of the first order with respect to the concentration At high dilution with an inert solvent such as hexafluorobenzene or octafluoro-cyclobutane etc., the same cis-/trans-cyclopropane ratio should be obtained with cis- or trans-olefin as the starting compound. 

In the reaction with hexafluorobenzene at 40"C exclusive 1,4-addition to give 18 is observed. In addition to higher regioselectivity, perfluoro-fcrf-butyl hypofluorite has higher reactivity as a reagent compared to trifluoromethyl hypofluorite (see Section 6.1.1.).14... 

Figure 14.29 shows the (solid + liquid) phase diagram for (benzene + hexafluoro-benzene). A congruently melting solid molecular addition compound with the formula QFU-CeFe ) is evident in this system.26 The rounded top of the freezing curve (solid line) for the addition compound results from almost complete dissociation of the addition compound in the liquid mixture. In other words, benzene and hexafluorobenzene act as independent molecular species in the liquid state and combine together as the addition compound only in the solid state. 

Nucleophilic addition occurs in the rate-determining step at one of the six equivalent carbons of hexafluorobenzene to give the cyclohexadienyl anion intermediate. 

To test this hypothesis, phthalic anhydride, acetylene, and acetylene-d2 were separately reacted with hexafluorobenzene at 690° under the same conditions as those used with acetylene alone. Phthalic anhydride gave tetrafluoronaphthalene, by 1,4-addition, and hexafluorobiphenyl, by insertion of benzyne, in a 1 5 ratio as estimated from the low-voltage mass spectrum and directly-coupled gas chromatography-mass spectrometry ... 

Alternatively, tetrafluoroanthracene could form by the reaction of benzyne with tetrafluoronaphthalene. In that case, however, some difluoroanthracene should also form by 1,4-addition of benzyne to the fluorinated ring. No difluoroanthracene or difluoroanthracene-d8 was found in the products from the reactions of hexafluorobenzene with acetylene and acetylene-d2, respectively. 

This evidence that benzyne is at least one of the intermediates in acetylene pyrolysis has many implications. However, as the ratios of products from acetylene and hexafluorobenzene differ appreciably from those obtained from phthalic anhydride, it might be best at this point to call acetylene a benzynoid precursor. Additional data will be needed and are being accumulated to determine to what extent acetylene reactions proceed at high temperatures through a benzyne intermediate (Fields and Meyerson, 1967a). 

A mechanism in which ketyl radicals play a role was also postulated for the formation of cyclohexylpentafluorobenzene upon irradiation of a cyclohexane solution of hexafluo-robenzene in the presence of benzophenone756. In this case, however, the excited benzophenone abstracts a hydrogen atom from the solvent and the cyclohexyl radical attacks hexafluorobenzene. The resulting radical is transformed further into the substitution product by loss of fluorine and into an addition product by abstraction of a hydrogen atom. Irradiation of a cyclohexane solution of pentafluoropyridine in the presence of benzophenone resulted in the formation of 4-cyclohexyltetrafluoropyridine, while no addition product was observed. 

Hexafluorobenzene and pentafluorotellurium hypofluorite produced, after four days at 25°, a 50% yield of hexafluorocyclo-hexa-2,5-dien-l-yloxy tellurium pentafluoride3. Similar reactions with pentafluorotellurium hypochlorite gave much lower yields of addition products2. 

In view of the relative C—X bond energies, C—F bond activation can be expected to be strongly disfavored. Nevertheless, there is an increasing number of cases where C—F bond addition to electron-rich metal complexes is observed. For example, QF6 oxidatively adds to the Cp Rh(PMe3) fragment, whereas QF5H undergoes only C—H activation.65 Hexafluorobenzene slowly adds to... 

Lead tetraacetate reacts poorly with acyclic alkanes, even 3-methylpentane, although cyclohexane is readily converted to the corresponding acetate at 80 C or with irradiation at room temperature.The yield of acetate is increased 10-fold by the addition of Bu OH, under which conditions BuKD- is thought to act as H-atom abstractor. " The more reactive lead(IV) reagent Pb(OCOCF3)4 has been used to introduce the trifluoroacetate group. Hexafluorobenzene or CF3CO2H are satisfactory solvents and hydrolysis to the alcohol is easily accomplished with NaOH, with an overall yield of ca. 4S%. The secondary C—H bonds are attacked in n-alkanes and arenes also react under these conditions." ... 

Insertion reactions [81], alkene additions [82] and even additions to benzene and hexafluorobenzene [83] have been observed using these sources of carbenes, as shown in Figure 6.53. 

The extremely electrophilic nature of bis(trifluoromethyl)carbene has already been illustrated by the formation of addition products, even with hexafluorobenzene [83] (Section lllB), and the high reactivity results in more side-reactions, such as insertion into C—H bonds, than with difluorocarbene (Figure 6.62). 

In contrast, 2a showed almost no change in nonstereospecific addition when dilution experiments were carried out either with hexafluorobenzene or octafluorocyclobutane 66). 

In cyclohexadienylidene 3g however the stereospecifity in the addition to cfs-butene is lost with increasing concentrations of hexafluorobenzene 68D). The ratio of cfs-adduct/ fflMS-adduct drops from 18.0, 4.5 to 2.0 as the mole % of CeFe is increased from 0, 74 to 90%. Solvents with heavy atoms such as methyl-iodide or phenyl-bromide favor also S T conversions in the case of 3g 68b) ... 

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