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.     

1H and 13C NMR study of α-acetylenic PIII and PIV derivatives. Signs and magnitudes of J(P?C) and J(P?H)

All J(P? H) and J(P? C) values, including signs, have been obtained in acetylenic and propynylic phosphorus derivatives, R₂P(X)? C?C? H and R₂P(X)? C?C? CH₃ (X ? oxygen, lone pair and R ? C₆H₅, N(CH₃)₂, OC₂H₅, N(C₆H₅)₂, Cl) from ¹H and ¹³C NMR spectra. In P^IV derivatives the following signs are obtained: ¹J(P? C)+, ²J(P? C)+, ³J(P? C)+, ³J(P? H)+, ⁴J(P? H)? . Linear relations are observed between ¹J(P? C), ²J(P? C) and ³J(P? C) versus ³J(P? H), indicating that these coupling constants are mainly dependent on the Fermi contact term, though the other terms of the Ramsey theory do not seem to be negligible for ¹J(P? C) and ²J(P? C). In P^III derivatives these signs are: ¹J(P? C)- and +, ²J(P? C)+, ³J(P? C)-, ³J(P? H)-, ⁴J(P? H)+. Only ³J(P? C) and ³J(P? H) reflect a small contribution of the Fermi contact term while in ¹J(P? C) and ²J(P? C) this contribution seems to be negligible relative to the orbital and/or spin dipolar coupling mechanisms.     

An abnormal trend in J(13C?CH) values in some aldehydes

The ²J(¹³C? CH) couplings involving the formyl proton of α-halogeno-aldehydes have been found to decrease with increasing halogen electronegativity. This trend may be rationalised in terms of inductive withdrawal and a conformation dependent hyperconjugative effect. The direct J(¹³CH) couplings are also anomalous.     

Détermination par RMN 1H et 13C des Couplages J(CH) et J(CC) dans des Monomères Precurseurs de Lignine: Vanilline,Férulate d'Ethyle et Alcool Coniférylique Sélectivement Enrichis en 2H et 13C

Some monomer model compounds of lignin have been selectively ²H and ¹³C labelled: vanillin, ethyl ferulate, coniferyl alcohol and ethyl hydrogen malonate. Deuterium isotope effects on the ¹³C chemical shifts in [formyl-²H]vanillin, [5-²H]vanillin and [α,α,5-²H₃]coniferyl alcohol made the unambiguous assignment of the aromatic ¹³C signals possible. Absolute ^1,2,3J(CC) values have been determined on ¹³C spectra of [formyl-¹³C]vanillin, and of ethyl ferulate and coniferyl alcohol in which the vinylic C-γ and C-β carbons were ¹³C enriched. It has been possible to measure ⁴J(C?O, C-4) in vanillin and ⁴J(C-γ, C-4) in ethyl ferulate. The determination of ^1,2,3,4J (CH) absolute values was done by means of gated decoupled ¹³C spectra of the non-labelled compounds. When second order effects made the use of this technique impossible we determined certain J(CH) values and their signs either by analysing the ¹H NMR spectra of ¹³C labelled coniferyl alcohol [²J(C-β, H-γ), ²J(C-β, H-α), ²J(C-γ, H-β), ³J(C-γ, H-α)] or by a double irradiation experiment on the 250 MHz ¹H NMR spectrum of ethyl [β-¹³C] ferulate [for ²J(C-β, H-γ)].     

A 13C- and 195Pt-NMR. Study of the Complexes trans-PtCl2 (amine) (CH2CH2)

¹³C- and ¹⁹⁵Pt-NMR. spectra for the complexes trans-PtCl₂(amine)(CH₂?CH₂) have been measured. For amine = (S)-N-methyl-α-methylbenzylamine the two diastereomers present may be distinguished from the values ³J(Pt,C). The ¹⁹⁵Pt-chemical shift is shown to be sensitive to subtle differences stemming from intramolecular non-bonded interactions.     

Structure and formation of gaseous [C4H6O]+˙ ions: 1—The enolic ions [CH2C(OH)?CHCH2]+˙ and [CH2CH?CHCH(OH)]+˙ and their relationship with their keto counterparts

The [C₄H₆O]^+˙ ion of structure [CH₂?CHCH?CHOH]^+˙ (a) is generated by loss of C₄H₈ from ionized 6,6-dimethyl-2-cyclohexen-1-ol. The heat of formation ΔH_f of [CH₂?CHCH?CHOH]^+˙ was estimated to be 736 kJ mol^?1. The isomeric ion [CH₂?C(OH)CH?CH₂]^+˙ (b) was shown to have ΔH_f, ? 761 kJ mol^?1, 54 kJ mol^?1 less than that of its keto analogue [CH₃COCH?CH₂]^+˙. Ion [CH₂?C(OH)CH?CH₂]^+˙ may be generated by loss of C₂H₄ from ionized hex-1-en-3-one or by loss of C₄H₈ from ionized 4,4-dimethyl-2-cyclohexen-1-ol. The [C₄H₆O]^+˙ ion generated by loss of C₂H₄ from ionized 2-cyclohexen-1-ol was shown to consist of a mixture of the above enol ions by comparing the metastable ion and collisional activation mass spectra of [CH₂?CHCH?CHOH]^+˙ and [CH₂?C(OH)CH?CH₂]^+˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH₃˙ and CO, [CH₂?CHCH?CHOH]⁺˙ and [CH₂?C(OH)CH?CH₂]^+˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C₄H₆O]^+˙ ion [CH₂?C?CHCH₂OH]^+˙ are also reported.     

Proton and carbon-13 nuclear magnetic resonance studies of substituted pyridines and pyrimidines: VII—The structure of hydroxy- and mercapto- pyridines and pyrimides from J(HH) and J(CH) values

Accurate values of ⁱJ(HH) and ⁱJ(CH) were obtained for a series of hydroxy-and mercaptopyridines and-pyrimidines. It is shown that the ³J(CH) values provide a valuable criterion for differentiating aromatic from quinoidal structures, and is an easy method for determining N-methylation or N-addition sites.     

INDO-FPT calculations of the dihedral angle dependence of 1J(CH) in groups adjacent to a cationic carbon atom

INDO-FPT calculations of ¹J(CH) in the hypothetical ethyl cation are presented in support of a recently derived empirical relationship which describes the effect of a cationic carbon on ¹J(CH) in an adjacent CH_n group.     

Thermal decomposition of di-t-butyl peroxide in the presence of (CH3)2CCH2: Reactions of CH3, (CH3)2ĊCH2CH3, and (CH3)2ĊCH2C(CH3)2CH2CH3 radicals

A study of the reaction initiated by the thermal decomposition of di-t-butyl peroxide (DTBP) in the presence of (CH₃)₂C?CH₂ (B) at 391–444 K has yielded kinetic data on a number of reactions involving CH₃ (M·), (CH₃)₂CCH₂CH₃ (MB·) and (CH₃)₂?CH₂C(CH₃)₂CH₂CH₃ (MBB·) radicals. The cross-combination ratio for M· and MB· radicals, rate constants for the addition to B of M· and MB· radicals relative to those for their recombination reactions, and rate constants for the decomposition of DTBP, have been determined. The values are, respectively, where θ = RT ln 10 and the units are dm^3/2 mol^?1/2 s^?1/2 for k₂/k and k₉/k, s^?1 for k₀, and kJ mol^?1 for E. Various disproportionation-combination ratios involving M·, MB·, and MBB· radicals have been evaluated. The values obtained are: Δ₁(M·, MB·) = 0.79 ± 0.35, Δ₁(MB·, MB·) = 3.0 ± 1.0, Δ₁(MBB·, MB·) = 0.7 ± 0.4, Δ₁(M·, MBB·) = 4.1 ± 1.0, Δ₁(MB·, MBB·) = 6.2 ± 1.4, and Δ₁(MBB·, MBB·) = 3.9 ± 2.3, where Δ₁ refers to H-abstraction from the CH₃ group adjacent to the center of the second radical, yielding a 1-olefin. © 1994 John Wiley & Sons, Inc.     

From Disilene (SiSi) to Phosphasilene (SiP) and Phosphacumulene (PCN)

The generation of heavier double‐bond systems without by‐ or side‐product formation is of considerable importance for their application in synthesis. Peripheral functional groups in such alkene homologues are promising in this regard owing to their inherent mobility. Depending on the steric demand of the N‐alkyl substituent R, the reaction of disilenide Ar₂Si?Si(Ar)Li (Ar=2,4,6‐iPr₃C₆H₂) with ClP(NR₂)₂ either affords the phosphinodisilene Ar₂Si?Si(Ar)P(NR₂)₂ (for R=iPr) or P‐amino functionalized phosphasilenes Ar₂(R₂N)Si? Si(Ar)?P(NR₂) (for R=Et, Me) by 1,3‐migration of one of the amino groups. In case of R=Me, upon addition of one equivalent of tert‐butylisonitrile a second amino group shift occurs to yield the 1‐aza‐3‐phosphaallene Ar₂(R₂N)Si? Si(NR₂)(Ar)? P?C?NtBu with pronounced ylidic character. All new compounds were fully characterized by multinuclear NMR spectroscopy as well as single‐crystal X‐ray diffraction and DFT calculations in selected cases.     

‘Anomalous’ 2J(Sn?H) coupling in mono– and dichloromethyltin compounds

The linear relationship between the coupling constants ¹J(Sn? ¹³C) and ²J(Sn? H), observed for a number of organotin compounds, does not hold for coupling in the Sn? CH_nCl_3?n group of mono- and dichloromethyltin compounds. A complete determination of all NMR parameters of the compounds Me₃Sn-CH_nCl_3?n (n = 0 to 3) shows no further anomalies, indicating that steric factors must be responsible for the unusually low values of ²J(Sn? H) in the SnCH_nCl_3?n group. Molecular weight measurements support this theory, showing that the chlorine-containing compounds are associated.     

CH,CD, CC and HH coupling constants in isotopically enriched cyclobutene

¹H, ²H and ¹³C NMR studies of cyclobutene and a series of isotopically enriched species have led to a determination of the ¹H? ¹H, ¹³C? ¹H, ¹³C? ²H and ¹³C? ¹³C coupling constants in these compounds. In agreement with general observations, ¹J(CH) is found to depend on the hybridization of the carbon atoms. Likewise, ²J(HH), ²J(CC), ³J(HH) and ³J(CH), but not ²J(CH), depend on the angles between the bonds connecting the coupled nuclei. When comparing cyclobutene with thiete 1,1-dioxide (thiete sulphone) an increase of almost 20 Hz is observed for ¹J(C-2, H-2) in the latter compound. All but one of the observed deuterium isotope effects on chemical shifts are negative. In the case of isotope effects upon the one-bond coupling constants, the obtained values support the results of the theoretical calculations of Sergeev and Solkan.     

Determination Rapide de la Configuration Cis Trans des Alcenes CH?CHCH? Utilisation des Complexes de Terres Rares

In Z? CH? CH?CH? Y compounds (Z or Y being an alkyl group) the ethylenic part of the spectra is often very complex and the ³J(H? C?C? H) coupling constant which is a good tool for determining the configuration, is not easily determined. We have studied such allylic derivatives and many configurations have been assigned through stereospecific synthesis. Except a very few cases, δ CH(Z) of the cis isomer is larger than δ CHZ of the trans isomer. In alcohols RCH?CH? CHOHR′ the stereoisomers behave differently in solutions with europium, praseodymium, holmium and dysprosium complexes. The spectra of the trans isomers remain strongly coupled but ³J(H? C?C? H) becomes easy to measure in the cis compounds.     

High resolution 13C NMR spectroscopy. IV—stereochemical assignments in butenedioic acids and 3-pentene-2-ones

Vicinal ¹³C, H coupling constants ³J(CO, H) for butenedioic acids and ³J(CH₃, H) for 3-pentene-2-ones have been determined and are correlated with the configuration of the corresponding C?C double bond. For both types the relationship ³J(CH) trans > ³J(CH)cis holds; in the case of the CH₃, H couplings, however, the ³J(CH₃, H) trans values are reduced because of steric reasons, so that configurational assignments seem possible only when both isomers are present. Additionally, the coupling constants ³J(COC H₃,H ) and the chemical shifts δ have been evaluated for the pentenones and it is shown that these parameters give information about the predominating conformation of α, β-unsaturated methyl ketones.     

Proton n.m.r. spin-tickling studies of 13C-labeled compounds. The correlation of J(C,H) and J(H,H)

A tabulation has been compiled for twenty ¹³C? H coupling constants of various carboxylic acids and includes ²J(C,H), ³J(C,H) and ⁴J(C,H) values of olefins (both cis and trans); ³J(C,H), ⁴J(C,H) and ⁵J(C,H) values of aromatics; ³J(C,H) and ⁴J(C,H) values of acetylenes; and ²J(C,H) and ³J(C,H) values of rigid aliphatics. This tabulation has been completed in the present study by the spin-tickling proton n.m.r. study of ¹³C-carboxyl-endo-1,2,3,4,7,7-hexachloronorbornene-5-carboxylic acid, which has established that the ²J(C,H) value is negative and the ³J(C,H) values (both cis and trans) are positive in this system. A plot of these twenty J(C,H) values vs the corresponding J(H,H) values of geometrically equivalent model systems (where there is a proton in place of a carboxyl group) gives a correlation coefficient of 0·975 (with a slope of 0·62), indicating that carbon–proton and proton–proton couplings operate by similar mechanisms throughout this broad series of structural types.     

Carbon-proton coupling constants in α-chlorostyrene-α-13C

The two stereochemically distinct two-bond carbon-13- hydrogen coupling constants J(¹³C? CH), for α-chlorostyrene-α-¹³C have been shown to be of similar magnitude but opposite sign (?6.3 and +5.6 Hz). A simple additivity relationship which adequately reproduces all the reported J(¹³C? CH) values for chloroethylenes has been found.     

Computer‐Assisted 3D Structure Elucidation (CASE‐3D): The Structural Value of 2JCH in Addition to 3JCH Coupling Constants

We present a method to use long‐range CH coupling constants to derive the correct diastereoisomer from the molecular constitution of small molecules. A set of 79 ²J_CH and ³J_CH values collected from a single HSQMBC experiment on a sample of strychnine were used in the CASE‐3D (computer‐assisted 3D structure elucidation) protocol. In addition to the most commonly used ³J_CH coupling constants, the subset of 32 ²J_CH values alone showed an excellent degree of configuration selection. The study is mainly based on comparison of DFT‐calculated ^2,3J_CH values with experimental ones, critical for the case of ²J_CH. But the configuration selection also works well using ³J_CH values predicted from a semi‐empirical Karplus‐based equation limited to H?C?C?C fragments. The robustness, shown using strychnine as a proof of concept, makes the J‐based CASE‐3D analysis a viable option for the application in fields such as peptide and carbohydrate research, organic synthesis, natural‐product identification and analysis, as well as medicinal chemistry.     

Formation and structure of diruthenium complexes Ru2(CO)6−n (PPh3) n {μ-C(CH=CHPh)C(Ph)C(CH=CHPh)C(Ph)} (n=1, 2)

Ruthenium carbonyl triphenylphosphine complexes Ru₂(CO)_6−n (PPh₃) _n {μ-C(CH=CHPh)C(Ph)C(CH=CHPh)C(Ph)} (n=1, 2) were obtained by the reaction of complex Ru₂(CO)₆{μ-C(CH=CHPh)C(Ph)C(CH=CHPh)C(Ph)} containing the ruthenacyclopentadiene moiety with PPh₃ in refluxing toluene. The complexes were characterized by IR and by¹H,¹³C, and³¹P NMR spectroscopy, and by X-ray analysis. The monophosphine derivative is identical to the complex formed by fragmentation of the Ru₃(CO)₈(PPh₃){μ-C(CH=CHPh)C(Ph)C(CH=CHPh)C(Ph)} cluster and contains the PPh₃ ligand at the ruthenium atom of the ruthenacyclopentadiene moiety. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1836–1843, September, 1998     

Structural studies on aryl‐substituted enaminoketones and their thio analogues. Part II. Experimental and DFT calculated carbon–carbon coupling constants,nJ(CC)'s (n = 1–3)

Spin–spin carbon–carbon coupling constants across one, two and three bonds, J(CC), have been measured for a series of aryl‐substituted Z‐s‐Z‐s‐E enaminoketones and their thio analogues. As a result, a large set, altogether 178, of J(CC)s has been obtained. It consists of 82 couplings across one bond, 31 couplings across two bonds and 65 couplings across three bonds. Independently, the DFT calculations at the B3PW91/6‐311++G(d,p)//B3PW91/6‐311++G(d,p) level yielded a set of theoretical J(CC) values. A comparison of these two sets of data gave an excellent linear correlation with parameters a and b close to ideal; a = 0.9978 which is not far from unity and b = 0.22 Hz which is close to zero. The ¹J(CC) couplings determined for the crucial fragment of the molecules, i.e. ? C?C? C?O (or ? C?C? C?S), are: ¹J(C?C) ≈ 68 Hz (67 Hz) and ¹J(C? C) = 60.5 Hz (60.0 Hz). The corresponding couplings found for the Z‐s‐Z‐s‐E isomer of the parent enaminoketone, 4‐methylamino‐but‐3‐en‐2‐one are 64.1 and 59.3 Hz, respectively. The most sensitive towards substitution of the oxygen atom by sulfur are two‐bond couplings between the α‐vinylic and aromatic C_ipso carbon atoms, which attain 12 Hz in the enaminoketone derivatives and decrease to 5 Hz in their thio analogues. Copyright © 2009 John Wiley & Sons, Ltd.     

A new technique for differentiating between 2J(C,H) and 3/4J(C,H) connectivities

We present a new pulse sequence that yields two simultaneously detected types of long‐range correlation spectra. The one spectrum is to show all ⁿJ(C,H) connectivities and the other is to show exclusively ²J(C,H) connectivities. The method is demonstrated by using strychnine as a test sample. A comparison with HMBC shows that the ²J(C,H)/ⁿJ(C,H) experiment supplies a ⁿJ(C,H) spectrum that is of equal standard with regard to sensitivity and spectral information. The additional ²J(C,H) spectrum allows the disentanglement of ²J(C,H) and ⁿJ(C,H) signals (n > 2) in HMBC type spectra, which greatly simplifies signal assignment and structure elucidation in general. Copyright © 2003 John Wiley & Sons, Ltd.