Fluorobenzenes protonation

Table 20 [56] hsts fluonne chemical shifts for the 12 fluorobenzenes in the senes Complete proton, carbon, and fluonne spectral analyses of this... 

Evidence has been presented from F-nmr shielding effects in m- and p-substituted fluorobenzenes (10) that solvent interaction modifies the O/ values of a number of the substituents of Table V (specifically NMc2, NH2, NHCOMe, OMe, CH3SO, SF3, CF3SO, CF3, SO3ME, CN, CO2R, NO2 and MeCO). The indicated changes in Oj between hydrocarbon and weakly protonic solvents are... 

No o-isomer is ever obtained, and (94) and (95) are found not to be interconvertible under the conditions of the reaction. This, coupled with the fact that eNH2 is known to be able to remove protons (deuterons) from a benzene ring [it removes proton (deuteron) 106 times faster from fluorobenzene with an o-deuterium substituent than from deuteriobenzene itself],... 

The acetylation over protonic zeolites of aromatic substrates with acetic anhydride was widely investigated. Essentially HFAU, HBEA, and HMFI were used as catalysts, most of the reactions being carried out in batch reactors, often in the presence of solvent. Owing to the deactivation effect of the acetyl group, acetylation is limited to monoacetylated products. As could be expected in electrophilic substitution, the reactivity of the aromatic substrates is strongly influenced by the substituents, for example, anisole > m-xylene > toluene > fluorobenzene. Moreover, with the poorly activated substrates (m-xylene, toluene, and fluoroben-zene) there is a quasi-immediate inhibition of the reaction. It is not the case with activated substrates such as anisole and more generally aromatic ethers. It is why we have chosen the acetylation of anisole and 2-methoxynaphtalene as an example. 

Fluorine acidifies an ortho proton rather more than chlorine or bromine, whose weak directing effects are more or less comparable . Deprotonation of fluorobenzene is feasible in THF, and at temperatures below — 50°C the lithiated species 162 is stable and does not collapse to a benzyne (Scheme 81) ... 

Typically, superbases care little for coordination effects, and simply remove the most acidic proton on offer this provides useful alternative selectivities in the lithiation of aromatic rings, for example. With groups that direct principally by acidification, orthomet-allation occurs, and treatment with BuLi—KOBu-f is the most efficient way of orthofunc-tionalizing fluorobenzene or trifluoromethylbenzene (Scheme 241). ... 

An early report (Briscese and Riveros, 1975) revealed that in the gas phase, alkoxide ions can displace fluoride from fluorobenzene (91). Hydroxide ion fails to react because C6H5F is more acidic than H20 and thus proton transfer becomes the most important channel. Similar reactions with other monohalobenzenes are complicated because these substrates usually generate halide ions directly by dissociative electron attachment. 

Table 20 [56] lists fluonne chemical shifts for the 12 fluorobenzenes in the senes CgHg. Complete proton, carbon, and fluonne spectral analyses of this senes have been published [131, 132 133,134] Approximate coupling constants are = 10 Hz, 4JHF = 5 Hz, = 1 Hz, 37FF = 20 Hz, 4JFF = 5 Hz, and 57FF = 15 Hz... 

Protonation of fluorobenzene in the gas phase has been studied by infrared photodissociation (IRPD) spectroscopy by Solca and Dopfer.351 F-protonated fluorobenzene was formed in significant amount when protonation was carried out with CH5+. It was found to be the most stable isomer in the gas phase by quantum mechanical calculations [B3LYP/6-311G(2df,2pd) level] separated by a large energy barrier from the four Wheland intermediates. F-protonated fluorobenzene is best described as a weakly bound ion-dipole complex between the phenyl cation and HF. 

The results obtained in the gas-phase isopropylation of various aromatic hydrocarbons with isopropyl chloride over Nafion-H catalyst showed only a relatively small variation of reactivity in going from fluorobenzene to xylenes.235 Therefore, it has been assumed that the reaction rate is controlled by the formation of a reactive electrophilic intermediate (possibly, protonated alkyl halide 61, or some form of incipient alkyl cation) rather than by cr-complex formation between the electrophile and the aromatic nucleus [Eq. (5.89)]. 

Fluorobenzene with ammonia leads only to the formation of aniline+. However, the TOF mass spectra also exhibit signals due to protonated and unprotonated ammonia clusters which must be produced by dissociative electron transfer (dET). In this case, direct evidence for a 1-2 precursor for the aniline+ product with three competing channels is provided ... 

From these data it can be pointed out that for a given size of the clusters the proton affinity of water is smaller than for the other solvents consequently, for fluorobenzene/methanol or para-difluorobenzene/water systems, a proton affinity of 205/215 kcal mol-1 seems to be the limit of the reaction process (it is reached for two molecules of methanol and three molecules of water). 

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

FIGURE 6.7 The proton-decoupled and proton-coupled 19F NMR spectrum (282.4 MHz) of fluorobenzene in CDC13. There is long range coupling in the H and 13C spectra (see text for explanation). 

The site of protonation of a large number of substituted fluorobenzenes under Cl (CH4 or N2/H2) was inferred on the basis of the entropy difference (A S°,/2) between the protonated and unprotonated species158. The A S°[Pg.221]

Proton abstraction from fluorobenzene yields the o-fluorophenyl anion (139) which, under CA conditions in a flowing afterglow instrument, gives the 2,3-dehydrophenyl anion (140) via HF loss317. The expected products from this reaction would be F and benzyne, since benzyne is less acidic than HF (the AH°.dCld values are 372 and 366 kcal mol-1, respectively) and 140 should readily undergo proton transfer from HF. The possibility of a concerted loss of HF was tested by analyzing the reaction of the D-labeled anion 141. The nearly equal amount of HF and DF eliminated from this precursor imply the involvement of the species 142. 

For some of these compounds, the protonic hyperfine coupling constants are known. The isotopic A /Ap ratio is 1.21 for radicals produced by addition to C6H6 and to the ortho position of C6H5CH3 and 1.15-1.18 for several fluorobenzenes (133). Quantum chemical calculations that include averaging over 33 vibrational modes in CeH7-C6H6Mu have shown that the dynamics account quantitatively (A /Ap = 1.16) for the... 

With this in mind, how would you expect fluorobenzene to react Most election density is removed first from the ortho positions by induction, then from the meta positions, and then from the para position. Any conjugation of the lone pairs on fluorine with the tc system would increase the electron density in the ortho and para positions. Both effects favour the para position and this is where most substitution occurs. But is the ring more or less reactive than benzene This is hard to say and the honest answer is that sometimes fluorobenzene is more reactive in the para position than benzene (for example, in proton exchange and in acetylation—see later) and sometimes it is less reactive than benzene (for example, in nitration). In all cases, fluorobenzene is significantly more reactive than the other halobenzenes. We appreciate that this is a rather surprising conclusion, but the evidence supports it. 

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