Para-substituted fluorobenzenes

In the related paper on 19F screening parameters of para-substituted fluorobenzenes in relation to resonance effects, a few measurements for SOMe and S02Me were recorded but no use was made of them for calculation of years later, Sheppard and Taft113 used these data (carbon tetrachloride solution) to calculate nR values through equation 11 ... 

S02Me and other + R substituents. It was suggested that these differences arise from the much larger substituent-probe interactions in para-substituted fluorobenzenes compared with 6-substituted 2-fluoronaphthalenes. Thus the fluoronaphthalene-based values of aR approximate to aR°. 

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. 

For para-substituted fluorobenzenes containing —R substituents the 19F substituent... 

A plot of the fluorine-19 chemical shift for para-substituted fluorobenzenes versus the carbon-13 chemical shift for the corresponding substituted benzenes also gives a good bilinear relationship. The smaller slope for the n acceptor substituents is in agreement with canonicals of the form ... 

F-shifts in p-substituted fluorobenzenes are supposed to provide a measure of the a- and -electron withdrawing properties of groups para to the F atom181. Bis(p-fluorophenyl)cyclopropenone and l,2-bis(p-fluorophenyl)-4,4-dicyano triafulvene (212/213) were found to exhibit 19F resonances at 61.5 and 65.5 ppm (downfield from C6F6)182. ... 

Recently, Behiman and coworkers discussed the mechanism of the Elbs oxidation reaction and explained why the para product predominates over the ortho product in this oxidation. According to the authors, semiempirical calculations show that the intermediate formed by the reaction between peroxydisulfate anion and the phenolate ion is the species resulting from reaction of the tautomeric carbanion of the latter rather than by the one resulting from the attack by the oxyanion. This is confirmed by the synthesis of the latter intermediate by the reaction between Caro s acid dianion and some nitro-substituted fluorobenzenes. An example of oxidative functionalization of an aromatic compound is the conversion of alkylated aromatic compound 17 to benzyl alcohols 20. The initial step in the mechanism of this reaction is the formation of a radical cation 18, which subsequently undergoes deprotonation. The fate of the resulting benzylic radical 19 depends on the conditions and additives. In aqueous solution, for example, further oxidation and trapping of the cationic intermediate by water lead to the formation of the benzyl alcohols 20 (equation 13) . ... 

Single- or multi-step preparation of [ F]fluoroaryl-type molecular building blocks and some applications A large number of no-carrier-added fluorine-18-labelled aromatic key-intermediates have been synthesised, opening the way to the preparation of more complicated radiopharmaceuticals via multi-step approaches. Scheme 42 non-exhaustively lists a number of para-substituted [ F]fluorobenzene compounds indicating some of their possible chemical interconnections. It also shows some of the precursors for labelling (P1-P7) that have been used for their preparation. 

Cp) of several monosubstituted benzenes are linearly related to the total 7t-electron density at the para position. Furthermore, the corrected shifts (5Cp — are linearly related to/>-carbon 7t-electron density due to resonance interaction with the substituent. These conclusions, based on comparisons with F studies in similar compounds, indicate that both shifts in monosubstituted benzenes and F shifts of />ara-substituted fluorobenzenes are useful parameters for determining the 7t-electron density at the />-carbon atoms. 

Toluene and fluorobenzene undergo mainly -substitution but chlorobenzene gives ortho- and para-substitution products in about equal proportions. Trimethoxybenzene, thiophene, and furfuraldehyde do not react. 

No such restrictions exist as concerns nmr spectra, however, and it was in this direction we turned to find an indicator for w of protic solvents. Earlier studies had indicated that the chemical shift of the para carbon of mono-substituted benzenes relative to internal benzene was suitably sensitive to solvent polarity (160,161), and the problem became one of finding a substituent which was chemically inert so that the solvent effect involved only the mutual dipolar interactions of the solvent and the substituent leading to modification of the substituent-induced polarization of the benzene ring (162). Based on analogous behavior of the F chemical shifts of m- and p-substituted fluorobenzenes (163), benzotrifluoride (43) and chlorobenzene (44) were chosen for study. 

Carbon-fluorine coupling constants of fluorobenzene and selected substituted derivatives are collected in Table 4.60 [402], Benzenoid JCF couplings are about 245+15 Hz. They depend on both type and position of the substituents Electron withdrawing groups increase while electron releasing ones decrease one-bond carbon-fluorine coupling in fluorobenzene, particularly when they are ortho and para to fluorine. These observations can be explained by cannonical resonance formulae which take ( + )- and (-)-M effects into account. The data of fluoroanilines (( + )-M) and fluorobenzaldehydes (( — )-M) provide typical examples (Table 4.60). 

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