The relationship between 19F and 13C substituent chemical shifts and electron densities. 2—p-phenylacetyl fluorides

The ¹⁹F substituent chemical shifts (SCS) of a series of para-phenylacetyl fluorides (X? Ph? CH₂? COF) are reported and compared with the related benzoyl fluoride series (X? Ph? COF). A dual substituent parameter analysis of the results for the new series shows that both inductive and resonance effects are reduced by one third when compared with the benzoyl fluorides. ¹³C shifts for the side chain carbonyl were also measured and found to follow a reversed trend in substituent effects, consistent with a pi polarization mechanism. SCS values for the fluorine and its adjacent carbon are not directly related. Ab initio (STO-3G) calculations of the carbon and fluorine electron density for this series have been compared with the appropriate SCS values. From the electron densities and the observation that the fluorine SCS values follow a normal direction, whilst those for electron densities and the observation that the fluorine SCS values follow a normal direction, whilst those for the adjacent carbon are reversed, it is concluded that fluorine SCS values (and Δqπ values) result from polarization of the C? F pi bond and do not merely monitor changes in electron density of the adjacent carbon.     

A comparative study between para‐aminophenyl and ortho‐aminophenyl benzothiazoles using NMR and DFT calculations

Ortho‐substituted and para‐substituted aminophenyl benzothiazoles were synthesised and characterised using NMR spectroscopy. A comparison of the proton chemical shift values reveals significant differences in the observed chemical shift values for the NH protons indicating the presence of a hydrogen bond in all ortho‐substituted compounds as compared to the para compounds. The presence of intramolecular hydrogen bond in the ortho amino substituted aminophenyl benzothiazole forces the molecule to be planar which may be an additional advantage in developing these compounds as Alzheimer's imaging agent because the binding to amyloid fibrils prefers planar compounds. The splitting pattern of the methylene proton next to the amino group also showed significant coupling to the amino proton consistent with the notion of the existence of slow exchange and hydrogen bond in the ortho‐substituted compounds. This is further verified by density functional theory calculations which yielded a near planar low energy conformer for all the o‐aminophenyl benzothiazoles and displayed a hydrogen bond from the amine proton to the nitrogen of the thiazole ring. A detailed analysis of the ¹H, ¹³C and ¹⁵N NMR chemical shifts and density functional theory calculated structures of the compounds are described. Copyright © 2014 John Wiley & Sons, Ltd.     

A unified correlation of substituent effects with carbon,proton and fluorine chemical shifts in aromatic and olefinic systems

At the present time no completely satisfactory quantum mechanical calculations exist for carbon, proton or fluorine chemical shifts in various substituted aromatic or olefinic systems. However, the chemical shifts in such systems have been shown to be well correlated by a linear multiple regression analysis with the Swain and Lupton field and resonance para meters ? and ?, and the semiempirical parameter Q. The utility of Q in testing substituent stereochemistry has been exemplified previously. Here the applications of the complete regression analysis to a wide variety of different systems for the three nuclei are given. The correlation is also shown to apply to selenium in substituted selenophenes. The ¹³C chemical shifts for a series of ortho substituted toluenes are presented and comparisons made with other ortho disubstituted benzenes.     

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-13 NMR spectroscopy of some Strychnos alkaloids. Part 2

¹³C NMR data for a series of natural and semi-synthetic ar-hydroxy- and -methoxy-substituted Strychnos alkaloids are presented and are used to determine substituent-induced chemical shift (SCS) values for the various substitution patterns.     

13C- and 1H-NMR. Spectra of ortho-benzoquinones. On the assignment problem in 13C spectra

¹³C- and ¹H-NMR. spectra of ortho-benzoquinone 1 and its methyl derivatives have been analysed. By means of heteronuclear double resonance experiments it is shown that assignments given in the literature for the olefinic carbon resonances of 1 and of a series of substituted ortho-benzoquinones have to be inverted. ¹³C-carbonyl frequencies of various six-membered cyclic ketones are presented.     

The structure of α-(1,2-dithiole-3-ylidene) ketones and α-(1,2-dithiole-3-ylidene) aldehydes studied by means of n.m.r. spectroscopy

1H and ¹³C n.m.r. studies of a series of twelve 1,2-dithiole-3-ylidene ketones and aldehydes have shown that the geometry of the carbon backbone is the same as found in 1,6,6aλ⁴-trithiapentalenes. No evidence has been found which favours a bicyclic structure for the system. A linear correlation of observed ¹³C chemical shifts with calculated charge densities is found to be valid. The observations are in agreement with a structure which is a hybrid between a true ketonic structure and a true mesoionic structure. By using the difference in the ¹³C chemical shifts of ortho and meta carbon atoms in substituent phenyl groups it is possible to qualify the degree of coplanarity of the phenyl groups with the backbone of the molecule.     

Stereospecific 5JHortho,OMe couplings in methoxyindoles,methoxycoumarins, and methoxyflavones

Long‐range coupling constants ⁵J_Hortho,OMe were measured in series of methoxyindoles, methoxycoumarins, and methoxyflavones by the modified J doubling in the frequency domain method. The COSY and NOESY spectra revealed the coupling of the –OMe group with a specific proton at the ortho position and its preferred conformation. Homonuclear ¹H–¹H couplings were confirmed by irradiation of the –OMe signal. Density functional theory calculations of ⁵J_Hortho,OMe using the modified aug‐cc‐pVTZ basis set evidenced that the Fermi contact term shows good agreement with the experimental J values. Accurate chemical shift and coupling constant values followed after iterative quantum mechanical spectral analysis using the PERCH software. Copyright © 2014 John Wiley & Sons, Ltd.     

The 13C NMR spectra of nine ortho-substituted phenols

The ¹³C chemical shifts of nine ortho-substituted phenols have been determined in cyclohexane and in dimethyl sulfoxide. All data were acquired in duplicate upon c.w. and FT instrumentation and both sets of data are presented. The chemical shifts at carbons 1, 2 and 3 are correlated with the parameter Q. Q has been defined as P/Ir³ where P is the polarizability of the adjacent C? X bond, I is the first ionization potential of the elements F, Cl, Br, I and H, and r is the C? X bond length. Experimental values of Q are available for other substituents. The field and resonance parameters of Swain and Lupton may be combined with Q to form a three part multiple regression correlation which is more exact than that with Q alone and which applies to all carbons in the aromatic ring. The results of this study suggest that only one value of the Q parameter is needed to characterize the behavior of the nitro group in these solvent systems. This conclusion is contrasted with earlier results of the effect of substituents on proton chemical shifts in these systems.     

Studies of 13C?H coupling constants: III—the conformational dependence of 1J(13CH) for the formyl proton in aromatic aldehydes

¹J(¹³CH) and ²J(¹³CH) involving the aldehydic proton in ortho-alkylbenzaldehydes have been examined for evidence of a hyperconjugative contribution from the aromatic π-electrons. The expected hyperconjugative effect is obscured by changes in the coupling constants resulting from steric inhibition of resonance.     

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.     

Carbon Magnetic Resonance Spectra of α, β, γ, δ- and α, β, α′, β′- Unsaturated Ketones

Carbon-13 spectra of a series of 26 unsaturated ketones (ortho- and para-cyclo-hexadienones and corresponding open-chain analogues) have been measured by Fourier-transform. Pulse spectroscopy. A complete analysis has been achieved by means of double resonance experiments using noise-modulated and coherent off-resonance proton irradiation and with the aid of non-decoupled spectra. Chemical shifts are interpreted in terms of charge distribution in the dienone system and of methyl substituent effects. Carbon chemical shifts were also obtained for O-protonated ortho- and para-cyclohexadienones. One-bond and long-range carbon-proton and carbon-fluorine spin coupling constants are reported for several compounds.     

Hydrogen‐bonding characteristics and unique ring‐opening polymerization behavior of Ortho‐methylol functional benzoxazine

Monofunctional benzoxazine with ortho‐methylol functionality has been synthesized and highly purified. The chemical structure of the synthesized monomer has been confirmed by ¹H and ¹³C nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT‐IR) and elemental analysis. One‐dimensional (1D) ¹H NMR is used with respect to varied concentration of benzoxazines to study the specific nature of hydrogen bonding in both ortho‐methylol functional benzoxazine and its para counterpart. The polymerization behavior of benzoxazine monomer has been also studied by in situ FT‐IR and differential scanning calorimetry, experimentally supporting the polymerization mechanism of ortho‐methylol functional benzoxazine we proposed before. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3635–3642     

Rhodium-Catalyzed ortho-Olefination of Sterically Demanding Benzamides: Application to the Asymmetric Synthesis of Axially Chiral Benzamides

It has been established that an unsubstituted cyclopentadienyl rhodium(III) (CpRh^III) complex is a highly active catalyst for the aerobic oxidative ortho C−H bond olefination of sterically demanding ortho-substituted benzamides with alkenes. This catalysis was successfully applied to the diastereoselective synthesis of axially chiral N,N-dialkylbenzamides. The combination of the ruthenium(II)-catalyzed enantioselective hydrogenation and the CpRh^III-catalyzed diastereoselective ortho C−H bond olefination enabled the asymmetric synthesis of axially chiral N,N-dialkylbenzamide derivatives with high ee values.     

13C?13C coupling constants in diacetylene (1,3-butadiyne) and its bis(triethylsilyl) derivative

Completely ¹³C-labelled diacetylene and its bis(triethylsilyl) derivative have been synthesized and all the possible spin–spin couplings between the acetylenic carbon nuclei have been determined from their Fourier transform ¹³C NMR spectra. J(CC) values in diacetylene have been also computed by means of the finite perturbation INDO method. Carbon–proton coupling constants in diacetylene have been determined from the spectrum at natural abundance. It has been established that J(CC) values across the triple bond in diacetylene and bis(triethylsilyl)diacetylene are greater than in acetylene and triethylsilylacetylene, respectively. The increase is interpreted in terms of σ- and π-electronic changes which occur with the coupling of two isolated triple bonds into a dimer-like system. All CC coupling constants are greater in diacetylene than in bis(triethylsilyl)diacetylene, which indicates that strong (p→d)π interaction takes place in the latter compound.     

Simulation of13C NMR chemical shifts for polychlorinated and polybrominated oxybenzenes with two-particle increment scheme

A two-particle system of OY-Cl and OY-Br mixed increments for predicting¹³C NMR chemical shifts of polyhalogenated polyoxybenzenes has been developed. It has been found that only theortho- and para-interactions of the OY and Hal substituents contribute significantly to the¹³C chemical shifts and that theortho-effects of the OY located between Ha1 and H and those of the OY located between two Ha1 atoms are different. Additional effects are due to solvating solvents. The increment scheme is predictive over the whole class of compounds under consideration and may be realized on personal computers.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 617–624, April, 1994.     

Additivity of proton and carbon nmr chemical shifts in protonated benzene carboxylic acids

The additivity of the proton and carbon chemical shift increments due to structural changes in a series of conjugate acids derived from benzene polycarboxylic acids is reported. In the acids, two sets of increments had been applied, one to the ortho diacids and one to acids which do not bear ortho substituents. In the conjugate acids only one set of increments is required. The disappearance of the ‘ortho’ effect may be the outcome of properties inherent to these conjugate acids.     

The proton magnetic resonance of heterocyclic diketones

A full analysis of the 60 MHz ¹H NMR spectra of heterocyclic analogues of 1,3-indandione [N-(2-bromoethyl)-phthalimide, phthalic anhydride and thiophthalic anhydride] and of 1,2-indandione (N-methylisatin, coumarandione and 2,3-dihydrothionapthene-2,3-dione) has been carried out. The protons of the symmetric compounds resonate at lower fields than those of the asymmetric ones and the proton ortho to the CO function is more deshielded than the proton ortho to the heteroatom. Coupled Hartree-Fock perturbation theory has been applied to estimate the ring current contributions to the proton chemical shifts and to the magnetic susceptibilities. MO calculations based on the finite perturbation theory at the CNINDO levels provide information on the relative importance of σ and π pathways for the various coupling constants.     

13C NMR spectroscopy of natural substances: IV—13C NMR studies of trachylobane diterpenes: Complete carbon assignment

The diterpene trachylobane and a series of derivatives have been completely analysed by FT ¹³C NMR spectroscopy. All ¹³C frequencies for trachylobanol have been unambiguously assigned by experimental techniques, i.e. by proton single-frequency selective decoupling and shift reagent experiments. Shift calculations have been performed for the parent compound by using known methods. The results are discussed in comparison with the experimental data. Satisfactory agreement between predicted and experimental values has been obtained.     

The NMR spectra of porphyrins—12: 13C and proton NMR spectra of the zinc(ii)coproporphyrin isomers2

The ¹³C and proton NMR spectra of the zinc(II) complexes of the tetramethyl esters of the four coproporphyrin type isomers are reported and assigned. Effects of aggregation phenomena on these shifts are discussed and a method involving addition of a slight excess of pyrrolidine is proposed for measurement of the spectra of the “monomeric” species; spectra obtained under these conditions are capable of simple, straight-forward interpretation and assignment in terms of molecular symmetry. Thus, a facile distinction between the type isomers is obtained.The “monomer” chemical shifts so derived allow consistent SCS parameters to be derived. The C_β-Me SCS are shown to be related to the bond order of the C_β-C_β bond in the porphyrin ring, and are thus quite different from the corresponding SCS in pyrroles.Aggregation shifts in the ¹³C and proton spectra are shown to be consistent with the presence of “stacked” aggregates with the ring current of one molecule affecting the other, together with an additional effect on the chemical shifts of the meso carbons, which is probably steric in origin.