Fluorine substituent chemical shifts

The evidence for fluorine hyperconjugation derived from nmr measurements available in 1970 was not deflnitive (3,4). Taft and his associates had originally argued that the fluorine substituent chemical shifts (SCS) of p-fluorine atoms in molecules with substituents which are capable of resonance interactions, for example the nitro group, exhibited a special solvent dependence and shifted to lower field as the polar character of the solvent was increased (38,39). The fluorine substituent chemical shift for the aromatic fluorine atom in 4-fluo-robenzotrifluoride, indeed, exhibits a small downfleld shift from 4.95 in 3-methylpentane to 5.75 in nitromethane to 6.05 in 75% aqueous methanol. For comparison, the substituent chemical shift for the fluorine atom in 4-fluoroni-trobenzene exhibits a shift from 9.00 in 3-methylpentane to 10.55 in nitromethane to 11.20 in 75% aqueous methanol (39). Holtz concluded on these grounds that fluorine hyperconjugation, structure XVC, might contribute to the determination of the substituent chemical shift. However, Brownlee, Dayal, and Taft subsequently showed that the effects of dipolar aprotic solvents on the... 

Fluorine Chemical Shift Substituent Chemical Shift (SCS) ... 

Another important component of Table 18 is substituent chemical shift (SCS) datadenvedforeachoftheSlentnes The SCS is simply thedifferencein F-NMR chemical shifts of the substituted compounds and that of unsubstituted fluoroben zene (-113 5 ppm in CDCI3) These values numerically represent the mfluence a substituent has on the shieldmg or deshieldmg of the fluorine nucleus and depend upon substituent position o, m, orp) Fluonne chemical shifts can be predicted for polysubstituted fluorobenzene systems simply by addmg the SCS value of each substituent to a base value of -113 5 ppm... 

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. 

Adcock, W. and Abeywickrema, A.N., Concerning the origin of substituent-induced fluorine-19 chemical shifts in aliphatic fluorides carbon-13 and fluorine-19 nuclear magnetic resonance study of l-fluoro-4-phenylbicyclo[2,2,2]octanes substituted in the arene ring, J. Org. Chem., 47, 2945, 1982. 

TABLE 5. Fluorine-19 NMR substituent chemical shifts (SCS) in acetone0... 

Correlations of fluorine-19 chemical shifts with substituent constants enable the empirical susceptibility of the chemical shift to inductive and resonance effects to be separated. For example, examination of the fluorine-19 chemical shifts of a series of fluorobenzenes gave the following correlation2,20-22 ... 

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 ... 

This is contrary to the normal situation in that electron-withdrawing substituents lead to an upheld shift, and vice versa. Adcock and coworkers have shown, using compounds of the type 1,2 and 317,24, that this reverse effect results from the redistribution of a electrons in the C—F bond resulting from changes in the fluorine % electron density. Compounds such as 1,2 and 3 are such that the relationship between the fluorine and the substituent is well defined in terms of the distance and the number of intervening bonds. The fluorine-19 chemical shifts of compounds of the type 1 follow the correlation ... 

C.A.L. Mahaffy, J.R. Nanney, The prediction of the NMR spectra of fluoro-arenes using statistical substituent chemical shift values, J. Fluorine Chem. 1994,67,67. 

Although the chemical shifts of most commonly encountered orga-nofluorine compounds are upheld of CFC13 and thus have negative values, there are a number of structural situations for fluorine that lead to positive chemical shifts (downfield from CFC13). These include acyl and sulfonyl fluorides as well as the fluorines of SF5 substituents. 

As was the case for proton spectra, the impact of a fluorine substituent on carbon chemical shifts quickly diminishes as one looks at carbons farther away from the carbon bearing the fluorine, with only a relatively small influence being observed for all but the fluorine-bound carbon (Scheme 2.14). 

Chemical shift and coupling constant data for carbons in the vicinity of fluorine substituents will be provided for the various classes of fluo-roorganic compounds discussed in the next four chapters. 

The information and examples presented in this chapter should enable the reader to predict chemical shift and coupling constant values for a single fluorine substituent in virtually any possible environment in which it might be encountered. 

As indicated in Chapter 2, the single fluorine substituent has an extremely broad range of observed chemical shifts, which include sulfonyl fluorides and acyl fluorides absorbing downfield in the region of +40 and +25 ppm, respectively, all the way up to fluoromethyltrimethylsilane, with its signal far upheld at -277 ppm. 

Among the large group of compounds represented as monofluoroal-kanes, primary fluorine substituents are the most shielded, with the rule governing relative chemical shifts being quite simple ... 

Vicinal Fluorine Substituents. The examples provided in Scheme 3.15 exemplify the specific effect of vicinal fluorine substituents on both chemical shift and upon spin-spin coupling constants. As indicated in Section 3.3.1 (Scheme 3.12) each of the fluorines is somewhat shielded by the presence of the other. 

A hydroxyl group one or two carbons further removed, y or 5 to the fluorine substituent, does not influence the fluorine chemical shift significantly (Scheme 3.22). In the case of a secondary system, the fluorine is also unaffected by a y-hydroxy substituent. 

H and 13C NMR Data. The examples in Scheme 3.23 provide characteristic proton and carbon chemical shift and coupling constant data for fluorinated alcohols, ethers, thioethers, sulfoxides, and sul-fones. An ether substituent serves to deshield the carbon of a CH2F by about 20 ppm. This can be compared to the 40-ppm deshielding generally observed in a nonfluorinated ether system. Thus, the fluorine substituent seems to have a damping effect on the usual effects of other substituents. 

Single vinylic fluorine substituents absorb over quite a wide range of chemical shifts, with fluoroallene at the high field end (-169 ppm) and P-fluoroacrylate derivatives at the low field end (-75 ppm) (Scheme 3.36). 

When the fluorine substituent is located at the 2-position or on any alkyl-substituted alkenyl carbon, it experiences the usual deshielding of 30-40 ppm (Scheme 3.38). Note the interesting variation in the chemical shifts and coupling constants for the 1-fluorocycloalkenes. 

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