Carbon-13 NMR studies on some 5-substituted quinoxalines

Eleven 5-substituted quinoxalines (NO₂, NH₂, COOH, OCH₃, CH₃, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their ¹³C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities. The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines. The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl₃, CD₃COCD₃, CD₃CN, CD₃OD, C₆D₆ and CD₃COOD. In all solvents an abnormally low ²J(CF) (～ 12 Hz) was found for C-4a and no C? F spin-spin splitting could be detected for the three-bond coupling of C-8a. Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide. There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6. A linear relationship also exists between the chemical shift of C-8 (‘para’ position) and the Hammett σ_p parameter of the substituent.     

Application of the Seth-Paul-Van Duyse equation—I : A simple relation defining new X+(R) substituent constants on the basis of electrophilic σ+ constants

The Seth-Paul-Van Duyse equation (SPVDE) correlating the CO stretching frequencies of a great number of R₁COR₂ molecules with X (R) substituent constants has been reinvestigated. A simple empirical relation defining new X⁺(R) constants on the basis of electrophilic σ constants has been derived: X⁺ (R) = 0·238 σ⁺ + 1·077. The three earlier reported relationships between the X(R) constants and Hammett σ values have been replaced by this single relationships and a set of new X⁺(R) constants has been calculated. The new X⁺(R) constants applied to 287 R₁COR₂ compounds containing the para- or meta-substituted benzene rings fit very well the SPVDE. It has been possible to extend the SPVDE to various aromatic systems.     

Substituent chemical shift correlations. Hammett σp+ values and shifts for exocyclic protons in para-substituted benzene derivatives

The methyl ¹H NMR shifts for series of para-substituted N,N-dimethylanilines as their conjugate acids in trifluoroacetic acid, and series of para-substituted N,N,N-trimethylphenyl-ammonium iodides in acetonitrile and in deuterium oxide, and the methylene shifts for series of para-substituted N,N-diethylanilines as their conjugate acids in deuteriosulfuric acid, are shown to be linearly related to the Hammett σ_p+ parameter. It is proposed that this dependence reflects a response of the chemical shift of the proton of the probe moiety to the electron density at the point of attachment of the probe to the aromatic ring and that this response is determined by the electric field effect of the charge at the point of attachment. Literature data are cited to indicate that Hammett σ_p+–¹H NMR shift relationships may be general for probe moieties lacking a through-resonance mechanism for interaction between the probe and the aromatic ring.     

Mass spectrometry of homoadamantane derivatives

Homoadamantane derivatives can be divided into two groups according to their mass spectra. To the first group belong compounds with electron attracting substituents (COOH, CI, COOCH₃, Br); compounds with electron releasing substituents (OCH₃, OH, NH₃, NHCOCH₃) constitute the second group. The most characteristic feature of the first group compounds is the splitting off of the substituent. The hydrocarbon fragment [C₁₁H₁₇]⁺ thus formed then loses olefin molecules with the formation of corresponding ionic species C_11?nH_17?2n. The 3-substituted compounds of this group undergo thermal Wagner-Meerwein type rearrangements into adamantane derivatives, resulting in the [C₁₀H₁₅]⁺ (m/e 135) ion formation; this is the main difference between 1- and 3-substituted homoadamantanes. The series of [C_nH_2n?6X]⁺ ions (where X = OCH₃, OH, NH₂, NHCOCH₃, n = 6 to 10) are characteristic of the mass spectra of the second group compounds, the ion [C₆H₆X]⁺, [M ? C₅H₁₁]⁺ being the most abundant. The intensity ratio of [M ? C₅H₁₁]⁺ to [M ? C₄H₉]⁺ ions is 10:1 for 1-substituted and 3:1 for 3-substituted compounds of this group, allowing the location of the substituent. Some individual features of the spectra are also reported.     

1H magnetic resonance spectroscopy of some substituted acetophenones

Measurements are reported on the nuclear magnetic resonance spectra of the acetyl protons of a series of substituted acetophenones. Although the extreme values of the chemical shifts, δ for the meta- and para- substituted compounds differ only by 0·2 units, the values themselves are linearly related to the Hammett substituent constants. No such relationship exists for the ortho-substituted compounds. The magnitude of the chemical shift is independent of substrate concentration over a five-fold variation.     

Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para-Phenylsubstituenten auf die Rotationsbarriere um die N?B-Bindung

The electronic influence of substituents on the free enthalpy of rotation around the N? B bond in aminoboranes was investigated in two series of compounds: (a) (CH₃)₂N?BCl (phenyl-p-X), containing the para-phenyl substituent at the boron atom, and (b) (p-X-phenyl)CH₃N?B(CH₃)₂, containing the para-phenyl substituent at the nitrogen atom of the N? B linkage (X = ? NR₂, ? OCH₃, ? C(CH₃)₃, ? Si(CH₃)₃, ? H, ? F, ? Cl, ? Br, ? I, ? CF₃ and ? NO₂). By comparing the rotational barriers in corresponding compounds of both series, a reverse effect of the substituents could be observed. Electron-withdrawing substituents in the para position of the phenyl ring increase the ΔG_c^≠ if the phenyl group is attached to the boron atom; on the other hand, a lower ΔG_c^≠ is observed if the phenyl ring is bonded to the nitrogen atom of the N? B system. Substitution of the phenyl ring with electron-donating substituents in the paraposition exerts the opposite effect. Within each series of compounds, the differences of ΔG_c^≠ values [δ(ΔG_c^≠) = ΔG_c^≠ (X) ? ΔG_c^≠ (X = H)] between substituted and unsubstituted compounds can be explained in terms of inductive and mesomeric effects of the ring substituents and can be correlated with the Hammett σ constant of each substituent. A comparison of the slopes of the plotted lines shows that the influence of the ring substituents is more pronounced in compounds with N-phenyl-p-X than in those with B-phenyl-p-X.     

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

13C NMR spectra of p- and m-substituted phenyl N-methyl- and phenyl N,N-dimethyl-carbamates

¹³C NMR spectra of p- and m-substituted phenyl N-methylcarbamates, phenyl N,N-dimethylcarbamates and p- and m-substituted phenyl propionates were recorded, and their para ¹³C SCS (substituent chemical shifts) were analysed by DSP (dual substituent parameters) and DSP-NLR (non-linear resonance) equations. It was found that the fixed substituent Y, ? OCONHCH₃, ? OCON(CH₃)₂ and ? OCOC₂H₅, were all mild in the sense that DSP analysis gave a good correlation, leaving little room for improvement by the DSP-NLR treatment. Further, the three series of compounds gave similar ρ_I and ρ_R values (para derivatives, 3.2–3.3 and 17.7–18.0; meta derivatives, 5.1–5.2 and 21.8–22.0). Examination of the corresponding analyses of similar compounds indicated that the ρ_I and ρ_R values and, hence, their ratio ρ_R/ρ_R = λ, depended primarily on the nature of the atom through which the fixed substituent Y (e.g. α-C, α-N and α-O) was bonded to the aromatic ring when the Y substituents are mild. The extent of this tendency for compounds with active fixed substituents is also discussed.     

Substituent effects on meta-proton chemical shifts in substituted benzenes

The chemical shifts at infinite dilution and coupling constants of some meta-substituted nitrobenzenes (C₆H₄NO₂X) were determined in CDCI₃ solutions (X = CHO, OCH₃, CI, Br, NH₂, NO₂) and in CCI₄ solutions (X = OCH₃, CI, Br). Substituent effects on the chemical shift of the meta-hydrogen are analysed in terms of existing theories.     

A correlation of substituent effects with proton chemical shifts in aromatic tetrazolic acids

The effect of substituents on the proton chemical shifts and spin–spin coupling constants in ortho-, meta- and para-substituted 5-phenyltetrazoles (tetrazolic acids) in DMSO–CH₃CN (1:1, v/v) was studied. With the meta- and para- substituted compounds the additivity rule of chemical shifts was obeyed, thereby enabling increments characterizing the effects of individual substituents in monosubstituted benzenes to be determined. By employing the Smith and Proulx equation, the chemical shifts of the aromatic protons were correlated with the F, R and Q substituent constants. The values of these constants are 1.02, ?0.004 and 5.49, respectively, for the tetrazolyl substituent.     

σ-Correlations and steric effects

Carbon-1 shifts in 1-dimethylimino-3-dimethylamino-1-(4-substituted)phenylprop-2-ene perchlorates show high correlation and negative dependence v. a σ set having high f character, similar to α and carbonyl carbon shifts in 4-substituted styrenes and N,N-dimethylbezamides, respectively. In sharp contrast with the β-carbons of the styrenes and the ¹⁵N absorptions of benzamides, whose shifts show high positive correlations with specific ratios of f and R characters, the C-2 shifts remain invariant. In the correlation of the barriers about the N-CO bond in N,N-dimethylbenzamides v. f/R, the R effect has been reported to increase dramatically for the meta-series when values determined in acetonitrile are exchanged with those determined in chloroform and, in the para series, to be higher than (normal) in both solvents. α-Carbon shifts in tris-para-substituted triphenylcarbonium ions correlate with a σ set having exceptionally high R character (c. 98 %). Explanations, based mainly on the likely action of steric factors, are suggested.     

Synthesis of P‐Unsymmetrically Substituted Derivatives of NAPHOS

2, 2′‐Bromomethyl‐1, 1′‐binaphthyl reacted with di‐tert‐butylphosphine to form (R, S)‐4, 4‐di‐tert‐butyl‐4, 5‐dihydro‐3Hdinaphtho[2, 1‐c:1′, 2′‐e] phosphepinium bromide 5a . The di‐iso‐propyl‐ ( 5b) and the phenyl‐ethyl ( 5c ) analogue of compound 5a were prepared by similar routes. Treatment of 5a with potassium diphenylphosphide, KPPh₂, afforded the corresponding bis‐phosphine, 2‐di‐tert‐butylphosphino‐methyl‐2′‐diphenylphosphino‐methyl‐1, 1′‐binaphthyl 6 . An attempt at the synthesis of the first example of a bis‐phosphonite ligand with a 2, 2′‐dimethyl‐1, 1′‐binaphthyl backbone unexpectedly led, in the first step, to 2, 2′‐bis[diethylamino‐methoxy‐phosphino]‐1, 1′‐binaphthyl 9 . X‐ray crystal structure analyses were carried out for the phosphepinium bromides 5a and 5c , and for the bis‐phosphines 6 and 9 . In compounds 5a and 5c the interplanar angle between the two parts of the binaphthyl group is 65.8° and 64.5°, respectively, as reflected in the conformation of the seven‐membered ring. In 5a the bromide and methanol residues are hydrogen‐bonded to form Br^— (···HOCH₃)₂ units. In 6 the binaphthyl interplanar angle is 86.1°; the two halves of the molecule show appreciably different conformations of the ring substituents, as do those of 9 (binaphthyl angle 78.6°).     

Origin of the Regioselectivity in the Gas‐Phase Aniline+CH3+ Electrophilic Aromatic Substitution

Nonadiabatic ab initio molecular dynamics simulations are carried out to monitor the attack of CH₃⁺ on aniline in the gas phase to form the corresponding σ complexes. The reaction is ultrafast and is governed by a single electron transfer within 30 fs, which involves two sequential conical intersections and finally produces a radical pair. Positive‐charge allocation in the aromatic compound is found to govern the substitution pattern in ortho, meta, or para position. Although the major products in the first step of the electrophilic aromatic substitution are the ortho and para σ complexes, initially 26 % of the simulated trajectories also form meta complexes, which then undergo H shifts, mainly to the para position.     

Formation of basic compounds during the indole cyclization of ketone phenylhydrazones

On the basis of previous occasional findings, the Fischer indole cyclization of ten ketone phenylhydrazones containing moieties of increasing bulkiness was investigated in order to isolate eventual side products. In the cases of the three 2-, 3- and 4-acetylpyridine phenylhydrazones the corresponding 2-pyridylindoles were the sole compounds so far isolated. In all the remaining cases, beside the indoles a mixture of basic compounds was obtained. In all cases aniline and a 2-substituted (2-methyl or 2-phenyl)benzimidazole were formed, the last resulting through an apparent ortho-semidinic rearrangements of phenylhydrazones. Starting from methyl isopropyl ketone phenylhydrazone a compound of formula C₁₁H₁₅NO was also isolated, to which the structure of 3-(4-aminophenyl)-3-methylbutanone was assigned on the basis of ir, nmr spectra and of the chemical reactivity. The formation of this compound seems related to a para-benzidine-like rearrangement of phenyl hydrazone.     

Struktur-Reaktivitätsuntersuchungen mit heterosubstituierten Nitrilen—VII: 13C-, 14N- und 19F-NMR-Spektroskopische Untersuchungen an Verbindungen vom Typ Ar?X?CN

The chemical shifts of aromatic nitriles of the general structure para-Y? C₆H₄? X? CN with X = O, S, Se and N(CH₃) have been investigated by the ¹³C NMR technique. For cyanates (X = O) the ¹⁴N shifts and for Y = F the ¹⁹F shifts were likewise measured. The chemical shifts and the corresponding ¹³C shift increments Δ_n have been found to correlate with the appropriate substituent constants σ_R⁰, σ_p⁰ and σ_I, as well as with the π-electron densities calculated in the PPP approximation.     

13C NMR of substituted aryl carbenium ions: Continuous dependence of the ipso substituent shifts upon the carbenium character. Use of proportionality relationships for the evaluation of substituent–substituent interactions

¹³C substituent shifts in para-substituted phenyl and 2-phenylethenyl carbenium ions have been studied within the general model of para-disubstituted benzenes X? C₆H₄? Y. Large variations are observed for the ipso shifts induced by the Y substituents (Y = NO₂. … OCH₃) depending upon the electron demand at the remote carbenium centre. A good representation of these substituent-substituent interactions is obtained by a treatment derived from ‘proportionality relationships’, where the deviations from strict additivity are factorized into two terms characterizing, respectively, the influence of the fixed X group on the ring and the susceptibility of the incoming Y substituent. Continuity of the ipso shift variations in an extended range is demonstrated, without break-off between neutral polarized systems or ions as the electronic influence of the fixed X group increases.     

Carbon-13 NMR study of substituted benzyl N,N-dimethylcarbamates

The ¹³C NMR chemical shifts of m- and p-substituted benzyl N,N-dimethylcarbamates were measured in CDCl₃. The meta and para ¹³C substituent chemical shifts were analysed by means of dual substituent parameter (DSP) equations. Good correlations were obtained, especially for the para-carbon substituent chemical shifts. The computed transmission coefficients, ρ_I and ρ_R, are consistent with the general features of the fitting parameters. It has been shown that no significant electron demand is imposed by the ? CH₂OCON(CH₃)₂ substituent.     

Effect of exchange of terminal substituents on the mesophase behaviour of some azo/ester compounds

Six homologous series of 4-(4′-alkoxy phenylazo) phenyl 4″-substituted benzoates (In_a-f) were prepared in which, within each homologous series, the length of the terminal alkoxy chain varies between 6, 8, 10, and 12 carbons, while the other terminal substituent, X, is a polar group that alternatively changed from CH₃O, CH₃, H, Br, NO₂ and CN. Compounds prepared were characterised by spectroscopic methods, and their mesophase behaviour investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The results were discussed in terms of mesomeric and polarisability effects. In each group of compounds, bearing the same alkoxy substituent, the nematic-to-isotropic transition temperatures (T_C) were successfully correlated with the polarisability anisotropy of bonds to the substituent X. A comparative study was made between the investigated compounds and their previously prepared isomers, namely, 4-(4′-substituted phenylazo) phenyl 4″-alkoxybenzoates (IIn_a-f) in which the two terminal (alkoxy- and X) groups are exchanged.     

1H- und 13C-NMR-Untersuchungen zur Struktur N-substituierter Tetrazole—Die Strukturen des N-Methoxycarbonyl-, N-Acetyl- und N-(Tri-n-butylstannyl)-tetrazols sowie Substituenteneffekte auf δ13C und 1J(CH)

The ¹H and ¹³C NMR spectra of several isomeric N-substituted tetrazoles have been investigated. ¹³C NMR is shown to be more useful for distinguishing between structural isomers of N-substituted tetrazoles except for those carrying electropositive substituents like SnBu₃. Correlations of δC-5 (inverse) and ¹J(C-5,H) with s?₁ found for 1-substituted tetrazole allowed the identification of the N SnBu₃ derivative as 1-(tri-n-butylstannyl)tetrazole. The phenyl carbon chemical shift difference ΔC′ = δC-3′-δC-2′ is insignificant for structure elucidation and conformational studies of N-substituted 5-phenyltetrazoles; ΔH′ from ¹H NMR spectra seems to be more useful.     

Natural abundance 13C NMR investigation of substituted bromobenzene using noise decoupling of proton resonances

The natural abundance ¹³C NMR spectra of bromobenzene and ten derivatives have been obtained using the technique of noise modulated decoupling of the proton resonances. Assignments have been made for all ¹³C resonance signals using an additivity rule. The chemical shifts of the aromatic carbon nuclei in the para-substituted compounds are discussed in terms of the Taft parameters (σ_R, σ_I) and the electronegativity of the substituent.