Substituent effect on the 1H NMR Spectra of trans aryl methyl oxiranes and arylpropenes

The ¹H NMR spectra of 1-aryl-2-methyl oxiranes substituted in the phenyl ring and of the corresponding vinyl derivatives have been analysed. The substituent effect on oxiran protons seems mostly polar in character, as shown by correlations with substituent constants, solvent effects and the fact that perturbation decreases with increasing distance from the substituent. The comparison with the corresponding vinyl derivatives, in which conjugation effects are present, confirms this point. The results also seem to exclude the possibility of substituents causing significant changes on the preferred conformation of the phenyl ring. Ring current contributions on oxirane protons, evaluated by SCF procedure, show that their change with substituents is very small and does not represent a significant part of the change of proton chemical shift with substituents.     

Substituent cross-interaction effects on the electronic character of the C=N bridging group in substituted benzylidene anilines--models for molecular cores of mesogenic compounds. A 13C NMR study and comparison with theoretical results

13C NMR chemical shifts delta(C)(C=N) were measured in CDCl3 for a wide set of mesogenic molecule model compounds, viz. the substituted benzylidene anilines p-X-C6H4CH=NC6H4-p-Y (X = NO2, CN, CF3, F, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, F, Cl, H, Me, MeO, or NMe2). The substituent dependence of delta(C)(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on delta(C)(C=N): electron-withdrawing substituents cause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominating over the resonance effects. In contrast, the aniline substituents Y exert normal effects: electron-withdrawing substituents cause deshielding, while electron-donating ones cause shielding of the C=N carbon, the strengths of the inductive and resonance effects being closely similar. Additionally, the presence of a specific cross-interaction between X and Y could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene or aniline ring substituents. Electron-withdrawing substituents on the aniline ring decrease the sensitivity of delta(C)(C=N) to the substitution on the benzylidine ring, while electron-donating substituents have the opposite effect. In contrast, electron-withdrawing substituents on the benzylidene ring increase the sensitivity of delta(C)(C=N) to the substituent on the aniline ring, while electron-donating substituents act in the opposite way. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects). The present NMR characteristics are discussed as regards the computational literature data. Valuable information has been obtained on the effects of the substituents on the molecular core of the mesogenic model compounds.     

Comparison of the electronic structures of imine and hydrazone side-chain functionalities with the aid of 13C and 15N NMR chemical shifts and PM3 calculations. The influence of C=N-substitution on the sensitivity to aromatic substitution

Benzaldehyde derivatives possessing a C=N double bond in the side-chain of the aromatic ring exhibit a reverse dependence of the (13)C NMR chemical shifts of the C=N carbon on the benzylidenic substituents X. Thus, electron-withdrawing substituents cause shielding (shift is reduced), while electron-donating ones cause deshielding. The origin of this phenomenon, which is in contrast with the idea of the generalized electronic effect, is extensively studied here by comparing the behavior of sets of benzaldehyde derivatives bearing various substitutents Y on the C=N nitrogen (Y-N=CH-C(6)H(4)-X). The effects of substituents X on the C=N unit change when Y is varied. Combination of the influences of the substituents X and Y gives a sensitive balance between the different resonance structures of the compounds. Our graphical treatment, where the rho(I) and rho(R) values observed for substituent X are plotted against the sigma(p)(+) value of substituent Y, is a novel use of Hammett-type substituent parameters. The justification of this method and our conclusions could be verified, for instance, by the fair correlation between the rho(I) or rho(R) values and the atomic charges of the imine carbon of the unsubstitued phenyl derivatives as well as by the correlations of the relevant bond orders and/or bond lengths both with the substituent parameters and with the atomic charges.     

Evidence of substituent-induced electronic interplay. Effect of the remote aromatic ring substituent of phenyl benzoates on the sensitivity of the carbonyl unit to electronic effects of phenyl or benzoyl ring substituents

Carbonyl carbon (13)C NMR chemical shifts delta(C)(C[double bond]O) measured in this work for a wide set of substituted phenyl benzoates p-Y-C(6)H(4)CO(2)C(6)H(4)-p-X (X = NO(2), CN, Cl, Br, H, Me, or MeO; Y = NO(2), Cl, H, Me, MeO, or NMe(2) ) have been used as a tool to study substituent effects on the carbonyl unit. The goal of the work was to study the cross-interaction between X and Y in that respect. Both the phenyl substituents X and the benzoyl substituents Y have a reverse effect on delta(C)(C[double bond]O). Electron-withdrawing substituents cause shielding while electron-donating ones have an opposite influence, with both inductive and resonance effects being significant. The presence of cross-interaction between X and Y could be clearly verified. Electronic effects of the remote aromatic ring substituents systematically modify the sensitivity of the C[double bond]O group to the electronic effects of the phenyl or benzoyl ring substituents. Electron-withdrawing substituents in one ring decrease the sensitivity of delta(C)(C[double bond]O) to the substitution of another ring, while electron-donating substituents inversely affect the sensitivity. It is suggested that the results can be explained by substituent-sensitive balance of the contributions of different resonance structures (electron delocalization, Scheme 1).     

Structural variations and electronic substituent effects in phenylcubane derivatives: a quantum chemical study

Electronic substituent effects in 4-substituted 1-phenylcubane derivatives, Ph–C₈H₆–X, have been investigated from the structural changes caused by the substituent X. The molecular structures of 34 derivatives with charged or dipolar substituents have been determined from quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Geometrical variations caused by substitution appear both in the cubane framework and in the benzene ring, but the two kinds of changes show no correlation. The rather small changes in the benzene ring geometry are caused by long-range polar effects (field effects), while the larger changes in the cubane cage are controlled primarily by electronegativity effects. A structural parameter measuring the long-range polar effect of the substituent, S _F^CUB, has been derived from the geometry of the phenyl group acting as a probe. This parameter correlates well with the calculated gas-phase acidities of 4-substituted cubane-1-carboxylic acids, HOOC–C₈H₆–X, and with other indicators of long-range polar effects obtained from bicyclo[2.2.2]octane derivatives. The correlations can further be improved by introducing a resonance parameter as an additional explanatory variable. This indicates that the electron delocalization resulting from hyperconjugative interactions between substituent and cage modifies the long-range polar effect of the substituent. Strong hyperconjugative interactions between some charged substituents and the cubane cage result in remarkable variations in the cage geometry, superimposed onto those ascribed to electronegativity effects.     

Substituent influences on the stability of the ring and chain tautomers in 1,3-O,N-heterocyclic systems: characterization by 13C NMR chemical shifts, PM3 charge densities, and isodesmic reactions

Substituent effects on the stabilities of the ring and chain forms in a tautomeric equilibrium of five series of 2-phenyloxazolidines or -perhydro-1,3-oxazines possessing nine different substitutions at the phenyl moiety have been studied with the aid of 13C NMR spectroscopy and PM3 charge density and energy calculations. Reaction energies of the isodesmic reactions, obtained from the calculated energies of formation, show that electron-donating substituents stabilize both the chain and ring tautomers but the effect is stronger on the stability of the chain form than on that of the ring form. The 13C chemical shift changes induced by the phenyl substituents (SCS) were analyzed by several different single and dual substituent parameter approaches. The best correlations were obtained by equation SCS = rhoFsigmaF + rhoRsigmaR. In all cases the rhoF values and in most cases also the rhoR values were negative at both the C=N and C-2 carbons, indicating a reverse behavior of the electron density. This concept could be verified by the charge density calculations. The 13C chemical shifts of the C=N and C-2 carbons show a normal dependence on the charge density (q(tot)), but the charge density shows a reverse dependence on substitution. Correlation analysis of the 13C chemical shifts, solvent effect (CDCl3 vs DMSO-d6) on the NMR behavior as well as the effect of substituents on the electron densities and on the stabilities of the ring and chain tautomers show that the substituent dependence of the relative stability of the ring and chain tautomers in equilibrium is governed by several different electronic effects. At least intramolecular hydrogen bonding between the imine nitrogen and the hydroxyl group as well as polarization of the C=N bond seem to contribute in the chain form. Stereoelectronic and electrostatic effects are possible to explain the increase in stability of the ring form by electron-donating substituents.     

Substituent effects in the 13C NMR chemical shifts of para-(para-substituted benzylidene amino)benzonitrile and para-(ortho-substituted benzylidene amino)benzonitrile

¹³C NMR chemical shifts were measured in CDCl₃ for two series of substituted benzylidene anilines. The substituted benzylidene anilines p-X-C₆H₄CH=NC₆H₄-p-CN p-X-C₆H₄CH=NC₆H₄-o-CN (X = NO₂, F, Cl, Br, H, Me, MeO, NMe₂). The substituent dependence of δC(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on δC(C=N): electron-withdrawing substituents cause shielding, while electrondonating ones do the reverse, the resonance effects clearly predominating over the inductive effects. Additionally, the presence of a specific cross-interaction between X and C=N could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene substituents. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects).     

Experimental and GIAO 15N NMR study of substituent effects in 1H-tetrazoles

A series of 1-aryl/alkyl-1H-1,2,3,4-tetrazoles, 5-substituted 1H-tetrazoles, and 1,5- and 2,5-disubstituted 1H-tetrazoles were studied by a combination of experimental NMR (natural abundance (15)N, (15)N/(1)H HMBC, and (13)C) and computational GIAO-NMR techniques to explore substituent effects on (15)N (and (13)C) NMR chemical shifts in the tetrazole (TA) moiety. Computed (15)N chemical shifts via GIAO-B3LYP/6-311+G(2d,p) calculations gave satisfactory results in comparison with experimental data. Whereas N-alkylation leads to large (15)N chemical shift changes, changes in the N(1)-aryl derivatives bearing diverse substituent(s) are generally small except for polar ortho-substituents (COOH, NO(2)). Large Δδ(15)N values were computed in N(1)-aryl derivatives for p-COH(2)(+) and p-OMeH(+) as extreme examples of electron-withdrawing substituents on a TA moiety.     

Visualization and quantification of anisotropic effects on the H NMR spectra of 1,3-oxazino[4,3-a]isoquinolines—indirect estimates of steric compression

The anisotropic effects of the phenyl, α- and β-naphthyl moieties in four series of 1,3-oxazino[4,3-a]isoquinolines on the ¹H chemical shifts of the isoquinoline protons were calculated by employing the Nucleus Independent Chemical Shift (NICS) concept and visualized as anisotropic cones by a through-space NMR shielding grid. The signs and extents of these spatial effects on the ¹H chemical shifts of the isoquinoline protons were compared with the experimental ¹H NMR spectra. The differences between the experimental δ (¹H)/ppm values and the calculated anisotropic effects of the aromatic moieties are discussed in terms of the steric compression that occurs in the compounds studied.     

Use of two-parameter correlations for studying the radical reactivity of aromatic compounds

Two-parameter equation correlations are reported for radical reactions of aromatic compounds. In these correlations polar and resonance substituent constants identical with the substituent constants of aliphatic compounds were used. The equations correlate the rate constants for H abstraction reactions and for the addition of a variety of free radicals to the ortho-, meta-, and para-substituted aromatic compounds. Besides, they correlate parameters of the spectra for substituted aromatic radicals. The correlations show that the effects of para substituents on the reactions studied are nearly entirely resonance effects, whereas for the meta- and ortho-substituted compounds polar (inductive) effects become essential. Application of the two-parameter correlations permits also to determine the structure of transition states (σ or π-complex) in free-radical reactions.     

Solvent and structural effects in tautomeric 3-cyano-4-(substituted phenyl)-6-phenyl-2(1H)-pyridones: experimental and quantum chemical study

The tautomeric equilibria between 2-pyridone and 2-hydroxypyridine forms of methoxy, chloro, and nitro derivatives of 3-cyano-4-(2-, 3-, and 4-substituted phenyl)-6-phenyl-2(1H)-pyridones were evaluated from UV/Vis spectral data. Linear solvation energy relationships of Kamlet–Taft and Catalán-rationalized solvent have influence on tautomeric equilibria. Transmission of substituent effect was analyzed by the Hammett equation. Quantum chemical calculations were performed by density functional theory (B3LYP). The experimental data were interpreted with the aid of time-dependent density functional method. Electron density distribution was analyzed by Bader’s analysis. It was found that substituents of different electronic properties change the extent of conjugation, and affect intramolecular charge transfer character. Theoretical calculations and experimental results gave insight into the influence of the molecular conformation on the transmission of substituent effects, as well as on contribution of different solvent–solute interactions.     

Syntheses and reactivity of meso-unsubstituted azuliporphyrins derived from 6-tert-butyl- and 6-phenylazulene

A series of six azuliporphyrins with substituents on the seven-membered ring were prepared by two different "3 + 1" routes from 6-tert-butyl- and 6-phenylazulene. The substituted azulenes can be converted into dialdehydes under Vilsmeier-Haack conditions, and these react with tripyrranes in the presence of TFA in CH2Cl2 to give azuliporphyrins in excellent yields. Alternatively, tripyrrane analogues can be prepared by reacting the substituted azulenes with an acetoxymethylpyrrole in the presence of acetic acid, and following a deprotection step, these condensed with a pyrrole dialdehyde to give the related azuliporphyrins in 45-51% yield. Five of the azuliporphyrins were sufficiently soluble in CDCl3 to afford high-quality proton and carbon-13 NMR data. The internal CH and NH resonances were observed near 3 ppm, although the precise values were dependent upon substituent effects. The presence of a tert-butyl group on the azulene moiety slightly enhanced the diatropicity of the macrocycle compared to the phenyl-substituted azuliporphyrins. Polar solvents also increased the downfield shifts to the external protons by stabilizing the dipolar resonance contributors that are responsible for the carbaporphyrinoid aromatic character. A tert-butyl-substituted azuliporphyrin also gave X-ray quality crystals, and this allowed the first structural analysis of a free base azuliporphyrin to be conducted. The macrocycle is near planar, and the azulene unit was only tilted out of the plane by 7.4 degrees. An analysis of the bond lengths suggests that a 17 atom delocalization pathway significantly contributes to the aromatic properties of this system. Protonation of azuliporphyrins affords dications with enhanced diamagnetic ring currents where the internal CH shifts to ca. -3 ppm. Again, the chemical shifts are influenced by the substituents and the presence of an electron-donating tert-butyl group on the azulene subunit increases the macrocyclic diatropicity. Two of the substituted azuliporphyrins were reacted with nickel(II) acetate or palladium(II) acetate in DMF to give the corresponding organometallic derivatives, and these stable complexes were isolated in excellent yields. Addition of pyrrolidine to NMR solutions of 23-substituted azuliporphyrins 19 demonstrated that nucleophilic addition products were present in equilibrium with the parent porphyrinoids, but these adducts are less favored than for azuliporphyrins lacking the 23-substituents. Although nucleophilic attack of a peroxide anion is believed to be the first step in the conversion of azuliporphyrins to benzocarbaporphyrins with t-BuOOH and KOH, the tert-butyl or phenyl substituents in azuliporphyrins 19a and 19b did not inhibit this chemistry. Two benzocarbaporphyrin products were isolated and characterized in each case, and mechanisms are proposed to explain the origins of these oxidative ring contraction products.     

Synthesis and antimicrobial properties of new 2-((4-ethylphenoxy)methyl)benzoylthioureas

New acylthiourea derivatives, 2-((4-ethylphenoxy)methyl)-N-(phenylcarbamothioyl)benzamides, were tested by qualitative and quantitative methods on various bacterial and fungal strains and proved to be active at low concentrations against Gram-positive and Gram-negative bacteria as well as fungi. These compounds were prepared by the reaction of 2-((4-ethylphenoxy)methyl)benzoyl isothiocyanate with various primary aromatic amines, and were characterised by melting point and solubility. The structures were identified by elemental analysis, ¹H and ¹³C NMR, and IR spectral data. The level of antimicrobial activity of the new 2-((4-ethylphenoxy)methyl)benzoylthiourea derivatives was dependent on the type, number and position of the substituent on the phenyl group attached to thiourea nitrogen. The iodine and nitro substituents favoured the antimicrobial activity against the Gram-negative bacterial strains, while the highest inhibitory effect against Gram-positive and fungal strains was exhibited by compounds with electron-donating substituents such as the methyl and ethyl groups.     

Synthesis of organoboron compounds bearing an azo group and substituent effects on their structures and photoisomerization

2-(Phenylazo)phenylboranes bearing several substituents were synthesized and substituent effects on their structures and photoisomerization behaviors were investigated to reveal the scope of the photoswitching of the coordination number of the boron by using an azobenzene-based photoresponsive ligand, 2-(phenylazo)phenyl group. ¹¹B NMR, X-ray crystallographic analysis, and UV-vis spectra revealed that electron-donating ability of the substituents at both the boron atom and the azobenzene moiety determined the strength of the interaction between the boron and the nitrogen of the azo group. Photoisomerization behaviors of 2-(phenylazo)phenylboranes are largely affected by the B-N interaction.     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : IV. Proton and carbon-13 NMR data for ortho-, 2,6-, and poly-substituted aryltrimethyltins

Proton and carbon-13 NMR data recorded in the Fourier transform mode are reported for ten ortho-substituted, six 2,6-disubstituted, and six miscellaneous polysubstituted aryltrimethyltin compounds. Although ¦¹J(¹³C¹H)¦ and ¦²J(¹¹⁹SnC¹H)¦ coupling constants are rather insensitive to substituent variation, tin methyl proton chemical shifts reflect the increasing inductive effects as methyl-, chloro-, fluoro-, and trifluoromethyl-groups are brought into juxtaposition with the trimethyltin moiety. Resonances in the natural-abundance carbon-13 NMR spectra for the tin derivatives are assigned on the basis of additivity relationships, proton undecoupled spectra, and relative magnitudes of ¦J(¹¹⁹Sn¹³C)¦ and ¦J(¹³C¹⁹F)¦ coupling constants. Mutually deshielding γ-, δ-, and ?-effects in the carbon-13 chemical shifts of substituent carbons are rationalized in terms of steric crowding between the trimethyltin group and neighboring substituents. Deshieldings in ring carbons formally para- to conjugating substituents are discussed in terms of the steric inhibition of resonance model. Previous conclusions concerning lack of significant higher coordination at tin in aryltin derivatives bearing substituents with lone pair electrons are corroborated in this work.     

Induced Nematic Phase of New Synthesized Laterally Fluorinated Azo/Ester Derivatives

A new series of laterally fluorinated mesomorphic compounds, namely 2-fluoro-4-((4-(alkyloxy)phenyl)diazenyl)phenyl 4-substitutedbenzoate (In_x) were prepared and evaluated for their mesophase behavior. The synthesized series constitutes five members that possess different terminally attached polar groups (X). Their molecular structures were confirmed by elemental analyses and both FT-IR and NMR spectroscopy. Examination of the prepared derivatives was conducted via experimental and theoretical tools. Mesomorphic investigations were carried by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). DSC and POM measurements indicated that except for the un-substituted analogue, all other derivatives were purely nematogenic, possessing their nematic (N) mesophase enantiotropically. This is to say that insertions of terminal polar substituents on their mesogenic structures induced the N phase. In addition, the location of lateral and terminal polar moieties played a considerable role in achieving good thermal N stability. Computational calculations were investigated to determine the deduced optimized molecular structures. Theoretical data indicated that both size and polarity of the terminal substituent (X) have essential impact on the thermal parameters and optical properties of possible geometries.     

The distribution of stereochemical configurations in polystyrene as observed with 13C NMR

¹³C NMR configurational assignments are made for an amorphous polystyrene sample examined at 25.2 MHz and 120°C. The assignments are based strictly on a one-parameter Bernoullian fit that was in satisfactory agreement with the nine observed methylene relative intensities. The methylene regions of the ¹³C NMR spectra of a polystyrene were examined before and after hydrogenation of the side-chain phenyl substituents. It was concluded that ring current effects have influenced the ¹³C methylene chemical shifts substantially and are limited largely to contributions from adjacent phenyl substituents. In addition, aromatic substituent parameters are reported that can be used in conjunction with the Grant and Paul parameters for calculating chemical shifts in aromatic hydrocarbons and polymers. Finally, it is concluded that free-radical and n-butyllithium-prepared polystyrenes have essentially atactic structures with meso additions favored over racemic additions by approximately 55/45.     

Excellent correlation between substituent constants and pyridinium N-methyl chemical shifts

Substituents on the pyridinium ring of N-methylpyridinium derivatives, especially those on the 2- or 4-position, have a large effect on the ¹H and ¹³C NMR chemical shifts of the N-methyl group. Reasonable correlations between the chemical shift changes and the resonance substituent constants are observed. The dual substituent parameter approach provides an excellent correlation when a combination of polar and resonance substituent constants is employed.     

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.     

B环对位取代异黄酮化合物的核磁共振研究

对14个合成的B环对位取代异黄酮化合物核磁共振氢谱进行了研究.利用超导核磁共振归属了B环无取代异黄酮质子的化学位移,根据取代基化学位移的变化影响规律考察了取代基对分子的影响方式.研究结果表明,2'(6'),3'(5')位质子共振迁移分别与取代基参数σ_p和S_o线性相关,说明4'位取代基主要通过电子效应影响其间位质子,其磁各向异性仅影响邻位质子,该取代基对A环质子影响不大,而对C环尤其是对2-H影响较明显.