Coupling constants alkenes

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

Table 2.4. Typical HH coupling constants (Hz) of some units in alicycles, alkenes and alkynes ...

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

Hence the compound is nona-2,6-dienal. The relative configuration of both CC double bonds follows from the HH coupling constants of the alkene protons in the H NMR spectrum. The protons of the polarised 2,3-double bond are in trans positions Jhh 5.5 Hz) and those on the 6,7-double bond are in cis positions Jhh = 10.5 Hz). The structure is therefore nom.-2-tmns-6-cis-dienal, D. 

The F—H spin-spin coupling constants of these compounds remain much the same as those of the simple alkenes. 

Proton and Carbon NMR Data. Some representative chemical shift and coupling constant data are provided in Scheme 3.48 for alkenes with vicinal fluorines. 

The chemical shifts for all fluorines, in two representative examples of perfluoro-l-alkenes are given in Scheme 6.32. F—F coupling constants... 

NMR spectra. The use of NMR spectroscopy for distinguishing between the cis and trans isomers is based on the fact that the spin-spin coupling constants of olefine protons in disubstituted alkenes are as a rule different. The spin-spin coupling constant is usually 4-12 cps (7 cps on the average) with cis protons as the double bond and 12-18 cps (15 cps on the average) with the trans isomer and so can be distinguished. 

The presence of electronegative elements directly attached to the same carbon atom as one of the vicinally coupled protons decreases the magnitude of the coupling constant, while the presence of electropositive elements increases it. This effect is small in chains (which are capable of relatively free rotation) but more pronounced in rigid systems such as alkenes. 

The extensive delocalization and aromatic character of pyridones, pyrones, etc. are shown by their chemical shift and coupling constant values (Table 8). By contrast, pyrans and thiins show chemical shifts characteristic of alkenic systems (Table 9). For these and for rings containing only a single endocyclic bond (Table 10), H NMR spectroscopy offers a most useful tool for structure determination. 

The direct Jc 3 H., coupling constants decrease regularly along the series 0>NH>S>Se>Te. The values for 7C 2 H-2> are appreciably larger than the 159 Hz observed for benzene and the 170 Hz for the alkenic protons of cyclopentadiene, while the 7C.3 H.3 coupling constants span this range. 

Reactions of unsaturated esters with electron-rich alkenes have been reported to yield only cyclobutane derivatives. However, NMR examination of the products has indicated the formation of substituted 3,4-dihydro-2H-pyrans. The most informative feature of the spectra is the C-2 proton coupling constants of ca. 3 Hz with the two different protons at... 

Rather less information is available on 4H-chromenes. In the parent (5), the alkenic protons absorb at 5 6.44 and 4.83 (Figure 1). The former has been reported as a doublet (62JA813) and as a sextet (69BSF1715) and the signal from 3-H as a multiplet and a sextet. The coupling constant J2,3 of ca. 6 Hz is smaller than J3A for 2//-chromenes. 

Scheme 4.56). The two-bond, F-F coupling constant observed in such conjugated systems is much smaller than that of the nonconjugated alkene systems. 

The unstable aspect of the ylid is the carbanion phosphonium salts are stable compounds so any substituent that stabilises the anion also stabilises the ylid and this reverses the stereoselectivity to favour the -alkene. Even benzylic ylids give -alkenes as in the reaction9 with the anthracene 37 that gives a good yield of crystalline 38 having a coupling constant between the two marked Hs of 17 Hz. One possible explanation is that the formation of the betaine or oxaphosphetane is reversible if the ylid is stabilised and only the faster of the two eliminations occurs to give the E -alkene. 

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