Vicinal C,H spin coupling in substituted alkenes. Stereochemical significance and structural effects

Abstract

Abstract-Vicinal C,H spin coupling ( 3 J~,~) in substituted alkenes has been investigated systematically. Emphasis is laid on the stereochemical significance (JLranr/Jcii;) and o n the various structural factors which influence 3J(!.H, such as r-bond order, torsional angle 4, bond angle 0, electronegativity of substituents and steric effects. A new type of y-effect is observed in 3J:,2s which appears to have the same origin as the y-shift effect. By comparison of 3JJc_rr,,,l and 3JH,11, it was found that the relation ' J c~,~ = 0.6 %J,,,H holds for both trans and cis coupling constants. Finally, it is concluded that 3 J~, f~ constitutes a valuable criterion to distinguish E-and Z-isomers, particularly in trisubstituted alkenes. Applications to natural products are presented.(1) INTRODUCTION THE STEREOCHEMICAL significance of vicinal H,H spin coupling in alkenes was recognised early and has ever since been utilised in numerous stereochemical assignments. This method, however, is not applicable to trisubstituted alkenes. In such cases, the configuration may be determined by proton NMR using differential chemical shifts,l allylic H,H spin coupling2 or nuclear Overhauser effect^,^ particularly when alkyl substituents are present. Allylic spin coupling though has proven much less reliable2 than vicinal spin coupling and certainly requires availability of both stereoisomers.With the advent of carbon magnetic resonance new criteria for stereochemical assignments became available. The increased sensitivity of the chemical shift parameter to structural and, in particular, to stereochemical changes, allows the assignment of cis-trans (or Z / E ) isomers by the differential shift m e t h~d .~ The two important criteria for assignment are the higher shielding of the olefinic carbons in the Z-isomers and the diamagnetic shift of cis-oriented substituents bearing hydrogen (y-effect5). This method also requires both stereoisomers for a reliable assignment, unless very close model compounds are available.It is the purpose of this paper to show that unambiguous stereochemical assignments in trisubstituted alkenes can be obtained from vicinal C,H spin coupling, i.e. from proton coupled 13C NMR spectra. It has previously been shown6 that in mono-and disubstituted ethylenes 3J6,H > 3J(1 U,H which, in principle, would offer an analogous criterion for cis-trans isomerism as in proton NMR. Applications to trisubstituted alkenes are as yet scarce. An example has been reported for the E-and 2-isomers of methylphosphoenol-u-ketobutyrate.7* Author to whom correspondence should be addressed. (2) ANALYSIS OF SPECTRA Chemical shift assignments are based upon offresonance decoupling experiments, a partial analysis of proton coupled spectra, selective 13C-{1H} decoupling experiments and spectral comparison. In the case of hydrocarbons with gem-dimethyl groups, the y-shift effect was also utilised. For the analysis of the proton coupled carbon spectra, a first order approach was justified in the majority of cases. The spectra of compounds 33, 44...

Identification of a Novel Chlorinated Fatty Acid from the Marine Rhodopirellula baltica

“…This result unambiguously demonstrated a trans relationship between C-8 and H-11, as it is known that the numerical values of the trans coupling constants 3 J CH are higher than those of the cis coupling constants. 4,5,6 The disubstituted alkene between C-4 and C-5 had an E configuration, as confirmed by the 1 H -1 H vicinal coupling constant for the olefinic protons (J = 15.1 Hz). On the basis of these results described above, 1 was determined to be (4E,7E)-7-(chloromethylene)dec-4-enoic acid, which is reported for the first time.…”

Applications of the ene reaction. Synthesis and properties of substituted α-methylene-γ-lactones

“…The cis relationship of the two ester groups is established both from the high-field chemical shift (ca. 6 5.8) of H-1, and the characteristic trans coupling (3JcH -12 Hz) between the ester carbonyl carbon (at C-2) and H-1 (13). The endocyclic double bond of 3 results from isomerization of the initially formed adduct.…”

“…Couplings (3JCd involving a $-unsaturated carbonyl systems depend on the double bond stereochemistry, being significantly larger when the coupled hydrogen is trans to the carbonyl carbon (13). In the case of 2u and 6a the trans-and cis-orientations of the lactone carbonyl carbon and the olefinic hydrogen are reflected by 3JCH values of 12.5 and 6.4 Hz respectively.…”

The action of hypochlorite on m-cresol. Part 2. Muconic acid derivatives

“…The magnitude of the coupling between the carboxyl carbon at 6 168.21 and the methyl hydrogens (3J = 4.0 Hz) indicates that this carboxyl must be geminal with the methyl group, while the magnitude of the doublet coupling (3J = 12.3 Hz) establishes that it is also oriented trans to an olefinic hydrogen (7). This stereochemistry is further confirmed by the characteristic cis coupling (3J = 6.7 Hz) observed between the methyl carbon and the vicinal olefinic proton.…”