Cis coupling

In the C NMR spectrum two signals with unusually small shift values [(07/2)2 5c = 7.7 CH 5c = 10.6] and remarkably large CH coupling constants Jch= 161.9 and 160.1 Hz) indicate a mono-substituted cyclopropane ring A. The protons which belong to this structural unit at = 0.41 (AA ), 0.82 BB ) and 1.60 (M) with typical values for cis couplings 8.1 Hz) and trans couplings 4.9 Hz) of the cyclopropane protons can be identified from the CH COSY plot. 

Likewise, the vicinal fluorines in l-chloro-l,2-difluoroethenylben-zene (Scheme 2.3) would appear as doublets due to the three-bond F—F coupling between the two fluorine nuclei. Note that the trans coupling constant is much larger (127 Hz) than the respective cis coupling constant (12 Hz). (Note also that the cis-vicinal fluorines deshield each other significantly, relative to the tram-vicinal pair.)... 

The trans, three-bond F—H coupling constant is large and is usually more than double that of the analogous cis coupling constant. 

Figure 4.10 provides the 19F NMR spectrum of 1,1-difluorobutene. The chemical shifts for its Z- and E-fluorines are -92.8 and -90.8ppm, respectively, with the geminal 2JFF coupling constant being 50 Hz, and the trans 3/HF coupling constant being 25.5 Hz. The cis coupling was too small to be seen in the fluorine spectrum, but was determined to be 2.7 Hz from the proton spectrum shown in Fig. 4.11. The magnitudes of these vicinal F—H coupling constants are considerably diminished as compared to those of monofluoroalkenes.

Vinylic halides or vinyl-metal species couple with a very high degree of stereospecificity, cry-vinylic halides giving cis coupling products and tra/iy-vinylic halides trans coupling products (example 17, Table III). Aromatic halides give homo- or cross-coupling reactions (examples 18 and 19, Table III). 

Ihrig and Smith 62> report similar changes for 3J % of cis-1,2-difluoroethyl-ene and of trans-1,2-difluoroethylene (Table 23). The cis coupling changes... 

It can be seen from Fig. 8 that the minimal basis set on the hydrogen atoms produces larger errors in the OP term and FC term of 1,2-difiuoroethane in the synperiplanar conformation (8) than in the antiperiplanar conformation (6). The same difference was also observed between the trans and cis couplings in the... 

Geometrical factors are also important as shown by Benezra et al. for the cis couplings in hindered compounds. (1966 43 58 1967 100) They proposed a Karplus-like behaviour for these couplings in compounds of the type 75. 

Vicinal H, H coupling constants do not satisfactorily indicate the stereochemistry of substituted thiolanes. However, it has been found during an investigation of a number of 3,4-disubsti-tuted and 2,3,4-trisubstituted thiolanes, e.g., 3 and 4, that the temperature dependence of the trans coupling is much stronger than that of the cis coupling, which is virtually invariant505. 

Compound Solvent 2,3 cis trans 2,4 cis Coupling constants (Hz) 2,5 trans cis trans 3,4 cis trans Ref. 

When the coupling path includes a cyclopropane ring, however, cis coupling is larger, as known from 2,/HH(ds) in proton NMR [129]. 

The observed trans F-F coupling constants are very large (>130 Hz), whereas the analogous cis couplings are much smaller (<15Hz). Both... 

Pertinent JH and 13 C NMR data are included, where available, in Schemes 4.55 and 4.56. As expected, trans F-H coupling is characteristically much larger than the analogous cis coupling. Conjugation does not noticeably affect the carbon chemical shift of the CF2 carbon. 

A modified Karplus correlation can be applied to vicinal coupling in alkenes. The prediction of a larger trans coupling ( = 180°) than cis coupling ( = 0°) is... 

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