Four-bond coupling

Coupling Constants. The usual three-bond H—H coupling constant in fluorobenzenes is about 8 Hz, whereas the four-bond coupling constant is between 1 and 3 Hz. Five-bond coupling is usually not observed. Likewise, the three-bond F—H coupling constant is about 8 Hz, the four-bond value 5-6 Hz, and the five-bond coupling constant about 1 Hz. 

There is no noticeable four-bond coupling between the CF3 fluorines and the CH2 group of l,l,l-trifluoro-2-butanone, as can be seen in the H spectrum of this compound given in Fig. 5.7. 

Fluorine NMR data for a number of heptafluoro-n-propyl compounds are given in Scheme 6.19. Little vicinal three-bond F—F coupling is observed for most n-C3F7 compounds (alkyne exception below) rather more prominent is the four-bond coupling (probably significantly through space). 

As was the case for the n-C3F7 group, the most prominent F—F coupling in the n-C4F9 group is usually the four-bond coupling (Scheme 6.21). 

Substantial long-range (four-bond) couplings with the CF3 groups can be observed, whereas little three-bond coupling is evident. 

Hz),88 and the cyclic compound (65 Jpch 12 Hz)89 have been reported. Four-bond couplings of 1 and 3 Hz are reported for the propenylphosphonates (64).88... 

Other Solvent Dependent Three- and Four-Bond Couplings 180... 

Summary of Solvent Effects on Three-and Four-Bond Couplings... 

The equatorial fluorines of these nitrogen-bound SF3 groups are greatly deshielded compared to those in their carbon-bound counterparts in Scheme 7.19. Note also that the four-bond coupling of both the axial and the equatorial fluorines to hydrogen are able to be observed in these compounds. In all likelihood, this coupling is through-space. ... 

Scheme 3.5 Simultaneous four-bond coupling to produce BiBLE ligands.

Let us focus our attention on the para isomer. The signal for Ha should occur (Table 6.5) at 8 7.14 and should be split into a doublet (by F) of doublets (by Hb). The reason that Ha is split by only the Hb ortho to it (and not the Hb para to it) is because of the number of intervening bonds. Similarly, the Hb signal is predicted to appear at 8 8.20, split into a doublet by Ha. The fact that the actual signal appears as a doublet of doublets shows that there is also a long-range (four-bond) coupling between Hb and F (see Chapter 9). 

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