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

Vicinal C,H spin coupling (³J_C,H) in substituted alkenes has been investigated systematically. Emphasis is laid on the stereochemical significance (J^trans/J^cis) and on the various structural factors which influence ³J_C,H, such as π-bond order, torsional angle ?, bond angle θ, electronegativity of substituents and steric effects. A new type of γ-effect is observed in ³J^trans_C,H which appears to have the same origin as the γ-shift effect. By comparison of ³J and ³J_H,H, it was found that the relation ³J_C,H ≈? 0·6 ³J_H,H holds for both trans and cis coupling constants. Finally, it is concluded that ³J_C,H constitutes a valuable criterion to distinguish E- and Z-isomers, particularly in trisubstituted alkenes. Applications to natural products are presented.     

The structural dependence of two-bond 13C?13C coupling constants

Two-bond ¹³C? ¹³C coupling constants are discussed on the basis of INDO-SCPT calculations. The dependence of ²J(CC) on bond angle variation and on methyl substitution is evaluated, and it is shown that ²J(CC) depends linearly on the bond orbital s-character product of the terminal carbon atoms, whereas no systematic relationship with the hybridization of the central carbon atom was obtained. Alkyl group substituent effects are found to be additive. The coupling constants of a number of cyclobutane derivatives are discussed on the basis of these structural relationships; it is shown that the experimental findings can be interpreted quite consistently by assuming a dual-pathway coupling mechanism.     

Long-range tin-tin coupling constants: II. Two-bond coupling via carbon

Tin-119 and carbon-13 NMR data for a total of 34 compounds containing the grouping Sn-C-Sn (C is either sp³- or sp²-hybridised) are presented and discussed. In organotin derivatives of alkanes, ²J(Sn-C-Sn) can only be correlated with ¹J(Sn-C₂) if a sign change for the former coupling is assumed. In most of the compounds of this type studied, ¹J(Sn-CH₃) is, due to rehybridisation and in contrast to the usual situation, larger than ¹J(Sn-C²); the same is true in some cases for distannylakenes, the behaviour of which is complicated by changes in the torsional angle about the carbon-carbon double bond. Thus correlation of ²J(Sn-C-Sn) with other spectral parameters is not possible in these cases. The total tin chemical shift range for compounds MeⁿSn(CH₂MME₃)_4-n (M  C, Si, Ge, Sn; n  0–4) is 140 ppm. Incorporation of a ditin fragment in a six-membered ring causes a downfield tin shift of 30 ppm.     

Substituent effects on nuclear spin–spin carbon–carbon coupling constants in derivatives of acetylene

¹J(¹³C?¹³C) nuclear spin–spin coupling constants in derivatives of acetylene have been measured from natural abundance ¹³C NMR spectra and in one case (triethylsilyllithiumacetylene) from the ¹³C NMR spectrum of a ¹³C-enriched sample. It has been found that the magnitude of J(C?C) depends on the electronegativity of the substituents at the triple bond. The equation ¹J(¹³C?¹³C) = 43.38 E_x + 17.33 has been derived for one particular series of the compounds Alk₃SiC?CX, where X denotes Li, R₃Sn, R₃Si, R₃C, I, Br or Cl. The ¹J(C?C) values found in this work cover a range from 56.8 Hz (in Et₃SiC?Li) to 216.0 Hz (in PhC?CCI). However, the ¹J(C?C) vs E_x equation combined with the Egli–von Philipsborn relationship allows the calculation of the coupling constants in Li₂C₂ (32 Hz) and in F₂C₂ (356 Hz). These are probably the lowest and the highest values, respectively, which can be attained for ¹J(CC) across a triple bond. The unusually large changes of the ¹J(C?C) values are explained in terms of substituent effects followed by a re-hybridization of the carbons involved in the triple bond. INDO FPT calculations performed for two series of acetylene derivatives, with substituents varied along the first row of the Periodic Table, corroborate the conclusions drawn from the experimental data.     

Long-range 4J coupling constants and conformation of the four-membered heterocycles

The correlation between the puckering angle of the four-membered cycle and ⁴J has been obtained from analysis of the PMR spectra of some 2-substituted azetidines and oxetanes. The absolute values of ⁴J_cis and ⁴J_trans were found to decrease with decreasing of the puckering angle.     

Carbon-13, 1H spin-spin coupling. V—π-benzenechromium tricarbonyl and the π-tropyliumchromium tricarbonyl cation

The ¹³C, ¹H spin-spin coupling constants have been determined for π-benzenechromium tricarbonyl (3) and the π-tropyliumchromium tricarbonyl cation using the ¹³C{²H}double resonance technique described in a previous paper. In addition, conventional analysis of the ¹H-coupled ¹³C NMR spectrum of 3 was carried out and the ¹H NMR spectrum of 3 partially oriented in the nematic phase was analysed. Both treatments also allowed the determination of the ¹H, ¹H coupling constants for this compound. The ⁿJ(CH) results are discussed on the basis of structural data and the theoretical results available. The complexation effects for ¹J(CH) are found to correlate with the C? H overlap population and hybridization changes, and those for ³J(CH) with the CC bond lengths and π-bond orders. The dependence of ²J(CH) on CC bond length as well as on the CCH bond angle is indicated. The liquid crystal results are compared with those of related studies.     

Unexpected behavior of the 3JCH coupling constant in unsaturated compounds

The experimental and theoretical behavior of the (OC) CC H ³J_CH coupling constant is investigated for a series of α,β-unsaturated compounds ( 1 to 8 ), and for some of them, the well-known relationship (³J_CHcis < ³J_CHtrans) was observed. However, for some compounds, close values for ³J_CHcis and ³J_CHtrans couplings were observed, and for aldehydes group containing compounds ( 7 and 8 E), an inversion order is observed (³J_CHcis > ³J_CHtrans). In all cases where the ³J_CHcis < ³J_CHtrans relationship it is not followed, a polar group or electronegative atom oriented in opposite direction (s-trans) to the H CC hydrogen is present, suggesting that conformational preference of such polar group or atoms are important factor on the behavior of ³J_CH couplings. Taking all of these in consideration, a new Karplus-type equation was proposed for ³J_CH couplings in α,β-unsaturated compounds, which can be used for configurational and conformational assignment on trisubstituted double bond derivatives.     

13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho- and peri-interactions involving the carbonyl substituent

Carbon-13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives ¹³C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non-hindered molecules and molecules in which the carbonyl substituent is subject to ortho- or peri-interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range ¹³C,¹³C coupling constants involving the carbonyl carbon, in particular ²J and ³J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. ²J(s-t)>²J(s-c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s-t and s-c refer to the orientation of the C?O group relative to the aromatic bond in question with respect to the connecting single bond). For ketones ³J(t,s-c)<³J(t,s-t), and both of these ³J(t) values decrease with increasing angle of twist, θ, about the single bond, whereas ³J(c,s-c) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three-bond coupling path). Generally it is found that ³J(t)>³J(c) and ³J(t)>²J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.     

Direct and indirect substituent effects on 1J(CH) in vinyl derivatives and related compounds

The variation in the one–bond couplings ¹J(CH) in vinyl derivatives with substituent has been examined. For the geminal proton ¹J correlates very badly with substituent electronegativity but extremely well with σ_I, if conjugating substituents are excluded. In the case of halogen substituents the marked stereospecificity of ¹J(CH) for the cis and trans protons can be rationalised in terms of an intrinsic dependence of π_CH on the dihedral angle between the coupling atoms and the perturbing substituent, with an additional positive increment to the cis coupling due to direct interaction of the substituent non-bonding electrons or to orbital circulation of the substituent electrons. The intrinsic specificity of β-substituent effects on ¹J(CH) is also found in analogous compounds containing C?N and C?O bonds.     

The 13C,H coupling constants in structural and conformational analysis. III.. Long-range carbon–hydroxyl proton couplings as evidence of hydrogen bonding in some acylphloroglucinols

Proton coupled ¹³C NMR spectra have been recorded for some acylphloroglucinol derivatives. Significant couplings over two, three and four bonds were observed between the hydroxyl proton and aromatic carbons for those compounds where the hydroxyl group is hydrogen bonded strongly enough to the carbonyl carbon of the acyl side chain. Typical values were ²J = 4.8 Hz, and ³J = 5.6 Hz or 6.7 Hz corresponding to dihedral angles of c. 0° and c. 180°, respectively; the dihedral angle is defined as the angle between the O—H bond and the plane of the aromatic ring. A stereospecific ⁴J(COH) value of 1.2 Hz for a ‘W’ arrangement of coupled atoms was also found. An interesting example of ‘virtual’ J(CH) coupling was observed in the proton coupled spectrum of 1-butyrylphloroglucinol 2-monomethyl ether in acetone-d₆ caused by the accidentally equal chemical shifts of the two ring protons.     

On the 4JHH long‐range coupling in 2‐bromocyclohexanone: conformational insights

2‐Bromocyclohexanone is a model compound in which a ⁴J_{H2, H6} coupling constant is observed, whereas the corresponding ⁴J_{H2, H4} is absent. The observed long‐range coupling is not only a result of the known W‐type coupling, in the axial conformation, but also because of the less usual diaxial spin–spin coupling in the equatorial conformer. The carbonyl group plays a determining role in describing the coupling pathway, as concluded by natural bond orbital (NBO) analysis; although the and interactions in the axial conformer contribute for transmitting the spin information associated with the W‐type coupling, the strong and hyperconjugations in the equatorial conformer define an enhanced coupling pathway for ⁴J_{H2, H6}, despite the inhibition of this coupling because of interaction and the large carbonyl angle. These findings provide the experimental evidence that orbital interactions contribute for the conformational isomerism of 2‐bromocyclohexanone. Copyright © 2008 John Wiley & Sons, Ltd.     

Vicinal 13CH coupling through olefinic double bonds

Values of long range ¹³CH coupling through the double bond in a number of isopropenyl compounds are reported. There is evidence that substituent dependence of ³J(CH) is not related linearly to that of H? H couplings in vinyl compounds. The ratio of ³J(CH) to ³J(HH) in analogous pairs of compounds appears to increase with decreasing substituent polarity and increasing steric bulk.     

Conformational studies on 2-fluoro-1,2-disubstituted ethanes by NMR spectroscopy. Influence of electronegativity on vicinal proton–proton and fluorine–proton coupling constants

The analysis of the ABKX spectra of thirteen compounds of the series RC(H-K)(F-X)C(H-A)(H-B)X gave the four vicinal proton-proton and fluorine-proton coupling constants. These coupling constants of conformationally mobile structures were used (i) to calculate the populations of the rotational states of the ? CHF? CH₂? bond, (ii) to calculate the vicinal trans proton-proton J(HH)^t and gauche and trans fluorine-proton coupling constants J(FH)^g and J(FH)^t and (iii) to give the unambiguous assignment of protons H-A and H-B. The dependence of the gauche and trans coupling constants with substituent electronegativity is explored. The results extend known correlations towards smaller electronegativity values. More quantitatively, the results and those in the literature, excluding those where deformations of torsional or bond angles occur, give a good fit of the data: a linear fit for J(HH)^t = 15.0-0.77 Σ(ΔE), an exponential fit for J(FH)^g = 15.35 exp [-0.266 Σ (ΔE)] and a linear fit for J(FH)^t = 65.75 - 7.52 Σ (ΔE), where Σ (ΔE) is the sum of the electronegativity difference between hydrogen and the six atoms or groups on the CH? CF fragment.     

Modified Karplus equations and their stereochemical applications

The current status of research on the Karplus equation for the vicinal spin—spin coupling constants (SSCC)³ J _HH, of fundamental significance in applications of NMR spectroscopy for establishing the configuration and conformational features of organic compounds in solutions, is discussed in the review. The data on the analysis of the fundamental molecular factors that affect the³ J _HH SSCC were generalized: the dihedral angle between interacting protons, the electronegativities of - and -substituents, the H-C-C valence angles, the C-C bond length, and effects of "proximity of substituents." The stereochemical applications of different modifications of the Karplus equation proposed in recent years were examined and comparatively analyzed. Possible ways of improving the accuracy of predicting the³ J _HH SSCC by using modern modifications of the Karplus equation and the method of molecular mechanics were discussed.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1483–1506, July, 1992.     

The structural dependence of 13C, 13C coupling constants in cyclopropanes

The smallest one-bond carbon—carbon coupling constants of cyclopropane derivatives have been measured for [1-¹³C]cyclopropane-1,1-dicarboxylic acid and cyclopropanecarboxylic acid. The ¹³C, ¹³C coupling constants of cyclopropylcarbinol have also been determined. The dependence of ¹J(CC) of the three-membered ring on different substituents and hybridization effects is evaluated. The carbon—carbon coupling constant of the unsubstituted cyclopropane is discussed on the basis of self-consistent perturbation theory (SCPT) and s character calculations (localized bond orbitals), and the results are compared with the experimental finding. It is shown that the ¹J (CC) can be interpreted by assuming a dual-pathway coupling mechanism.     

Long range 13C?1H coupling constants IV—methoxy-, amino- and hydroxypyridines

One bond and long range ¹³C? ¹H coupling constants for some methoxy-, amino- and hydroxypyridines are described. An unambiguous assignment of carbon-13 resonances is carried out based on the analysis of the fine splitting caused by long range couplings. J values for compounds other than 2- and 4-hydroxy-pyridines are explainable in terms of the values previouly obtained for cyano- and methylpyridines. The hydroxypyridine-pyridone tautomerism affects ²J(H_α), i.e. ²J associated with the α proton. This effect can be used to differentiate pyridones from hydroxypyridines.     

Arithmetic manipulation of coupling constants to obtain torsional angles

Various arithmetic combinations of J_trans and J_cis from the AA′BB′ analysis of ? CH₂—CH₂– fragments in 6-membered rings have been examined for their relationship to the internal torsional angle Ψ. The simple ratio R (J_trans/J_cis) appears to offer the most complete removal of substituent electronegativity effects and hence the best values of Ψ. The Karplus equation is very inaccurate when the substituents are highly polar. The simple difference D (J_trans–J_cis) overcompensates for electronegativity effects and thus also gives relatively inaccurate torsional angles. An alternative, artificial linear combination of the coupling constants, such as D′ (J_trans–2J_cis), appears to be able to provide torsional angles that are almost as good as those from the R value, although the Karplus A value must be evaluated. Adjustment of the simple difference D for electronegativity is possible by including a function of the AA′ and BB′ chemical shifts. These results are fairly good, and the parameter A need not be evaluated. This approach may fail, however, with particularly anisotropic substituents, since the chemical shift may depend on factors other than electronegativity. Although all five methods give reasonably good results, the R value appears to offer the most reliable results for the full range of substituents.     

Conformation analysis of d‐glucaric acid in deuterium oxide by NMR based on its JHH and JCH coupling constants

d ‐Glucaric acid (GA) is an aldaric acid and consists of an asymmetric acyclic sugar backbone with a carboxyl group positioned at either end of its structure (i.e., the C1 and C6 positions). The purpose of this study was to conduct a conformation analysis of flexible GA as a solution in deuterium oxide by NMR spectroscopy, based on J‐resolved conformation analysis using proton–proton (³J_HH) and proton–carbon (²J_CH and ³J_CH) coupling constants, as well as nuclear overhauser effect spectroscopy (NOESY). The ²J_CH and ³J_CH coupling constants were measured using the J‐resolved heteronuclear multiple bond correlation (HMBC) NMR technique. NOESY correlation experiments indicated that H2 and H5 were in close proximity, despite the fact that these protons were separated by too large distance in the fully extended form of the chain structure to provide a NOESY correlation. The validities of the three possible conformers along the three different bonds (i.e., C2? C3, C3? C4, and C4? C5) were evaluated sequentially based on the J‐coupling values and the NOESY correlations. The results of these analyses suggested that there were three dominant conformers of GA, including conformer 1 , which was H2H3:gauche, H3H4:anti, and H4H5:gauche; conformer 2 , which was H2H3:gauche, H3H4:anti, and H4H5:anti; and conformer 3 , which was H2H3:gauche, H3H4: gauche, and H4H5:anti. These results also suggested that all three of these conformers exist in equilibrium with each other. Lastly, the results of the current study suggested that the conformational structures of GA in solution were ‘bent’ rather than being fully extended. Copyright © 2016 John Wiley & Sons, Ltd.     

F?F coupling constants in dichlorotetrafluorocyclopropanes

The ¹⁹F NMR spectra of the cis- ( 1 ) and the trans-isomer ( 2 ) of the 1,2-dichlorotetrafluorocyclopropane and that of the 1,1-dichlorotetrafluorocyclopropane ( 3 ) have been investigated at different temperatures and in several solvents. From chemical shift calculations the two geminal fluorines in the cis-isomer ( 1 ) could be assigned and on this basis the two vicinal coupling constants of 1 , J_trans (ca. 140°) and J_cis (ca. 0°), were unequivocally distinguished. By frequency sweep double resonance J_trans has been shown to be of opposite sign to J_gem, whereas for J_cis the situation has been found to be reversed. Therefore J_trans is presumably negative and J_cis positive. Only the N(J_cis + J_trans) value could be extracted from the vicinal coupling constant in the fragment ? CFCl? CFCl? could be evaluated. It has been noted that J_cis is more sensitive to changes in temperature than is J_trans. The variations of J_cis and J_trans induced by solvents are, on the contrary, small and irregular and no correlation with the dielectric constant of the medium has been noted. The different temperature dependence of J_cis and J_trans can be useful for assigning the vicinal F? F coupling constants in cyclopropane derivatives and also for defining their signs. This method was applied to the coupling constants extracted from the ¹³C satellite spectrum of isomer 3 . The coupling constants results were compared with some literature data already known, and some rationalisation and correlations from the trends was attempted.