Transmission Of Substituent Effects Research Articles

Transmission of substituent effect through π-conjugation by cyclobutane ring

Transmission of substituent effect through π-conjugation by cyclobutane ring

Computational evidence for back donation in an N → O group based on modes of transmission of substituent effects in 3-(4′-substituted) phenylfuroxans

The N-oxide nitrogen in C-4′ substituted 3-phenyl furoxans occupies a position analogous to C-β in 4-substituted styrenes that have been examined for modes of transmission of substituent effects from the C-4 substituent to C-β. From geometry optimizations through high-level MO theory calculations, it was first ensured that the N-2–C-3 liaison in 3-(4′-substituted)phenyl furoxans retains as much double-bond character as it does in the case of furoxan bearing no substituents and that the para-substituted phenyl and furoxan rings maintain near uniplanarity. The calculations, carried out for such furoxans, chosen to represent a spectrum of effects from electron-donor to electron-acceptor, showed how the change in the 4′-substituent affects electron redistribution within N-oxide group in the way expected: while the residual positive charge at N increases the residual negative charge at O decreases. An increase in the N-oxide bond order (as measured by the Wiberg bond index), together with a small reduction in the N-2–O-6 bond length, was also found. That these effects were not artefacts of the calculation procedure was ensured when the calculations, repeated using a different functional, showed not only inverse dependence of positive N-2 and negative O-6 net charges on N-2–O-6 bond lengths but also confirmatory evidence from N-oxide bond dissociation and second-order perturbation energies. These results are interpreted as demonstrating graded back donation from O to N within the N → O group caused by a combined action of mesomerism and π-polarisation involving the substituent at the para-position of the phenyl group offering a spectrum of effects from electron-releasing to electron-withdrawing. Calculations at the B3LYP/6-31++G** level have shown that back donation from O to N in N → O in 3-(4′-X-phenyl) furoxans increase with a change of X from electron-donor to electron-acceptor.

Can Non-bonded Pair of Electrons of Sp3 Nitrogen with Two Single σ-Bonds on Either Side Still Transmit Substituent Electronic Effects to the Reaction Site? Reversal of Attenuation Effect by Sp3 Nitrogen–A Chemical Education Perspective

Dependence of reactivity of organic molecules on substituents was a well-established phenomenon in terms of Hammett and Taft equations in physical-organic chemistry. The well-known Hammett and Taft linear free energy relationships were extensively used in elucidating the organic reaction mechanisms. The exponential depletion of Hammett reaction constant (ρ), as a function of distance in terms of increasing the number of sp3 carbon atoms (-CH2-) between the reaction center and the substituent, is understood from our laboratory experiments. But, introduction of sp2 carbon atoms (-CH=CH-) between the reaction center and the substituent enhances the Hammett reaction constant (ρ). In the present work, we have tried and observed the same and even little more effective transmission of substituent effect through sp3 nitrogen (-NH-). However, the presence of a sp3 carbon by the side of sp3 nitrogen (-NH-CH2-) depletes the substituent effect in the usual manner in N-phenyl glycines. Probable explanations were presented in support of our observation. In the present work, the observations were manifested in terms of Taft ρ* values instead of Hammett ρ value as the pKa values of only 4-nitopheylcarbamic acid and carbamic acids and the Taft σ* values of 4-nitrophenyl and H are available from literature.

Substituent Effect Transmission Power of Alkyl, Alkenyl, Alkynyl, Phenyl, Thiophenyl, and Polyacene Spacers.

The transmission of substituent effect through a variety of spacers, that is to say, alkyl, alkenyl, alkynyl, phenyl, thiophenyl, and polyacene has been studied by modeling Y-G-X type molecular systems (Y: reaction center; G: spacer moiety; X: substituent) using B3LYP/6-31G(d,p) density functional theory calculations. The reaction center is always kept as a C=C double bond and the molecular electrostatic potential (MESP) minimum (Vmin ) observed for this bond showed subtle variation with respect to the changes in the spacer unit and the nature of substituent. Strong linear correlations are observed between Hammett substituent constants (σI and σp ) and Vmin , which recommend the aptness of Vmin as an electronic descriptor to quantify the substituent effect. Since Vmin offers an alternative measure of substituent effect, the correlation between Vmin and σp has been used for assessing the transmission of substituent effect through a variety of spacer moieties. The highest transmission coefficient (γ) is always observed for smaller spacer length. Among all the spacers, alkenyl showed the highest and alkyl showed the lowest transmission power. The study recommends the use of short chains of C=C double, C≡C triple or a combination of both as spacers for the effective transmission of substituent effect to the reaction center.

Experimental and Computational IR Spectroscopic Studies on the Transmission of Substituent Effects on N–H, C=N, B–N, and B–O Bond Stretching Frequencies in 3,5-Disubstituted-1,2,4,5-Oxadiazaboroles

Single and dual substituent correlation analyses are applied to study the transmission of substituent effects on IR stretching frequencies of N–H, C=N, B–N, and B–O bonds in 3,5-disubstituted-4,5-dihydro-1,2,4,5- oxadiazaboroles (1a-1r). For the correlation study σ, F, R, and R– parameters are used as constants. The substituent effects are estimated based on the results of the statistical analysis. The differences among the regression coefficients are discussed in terms of the relative importance of the substituent field and resonance effects. For a better understanding of the results, density functional theory (DFT) calculations are performed to determine the preferred geometry and to calculate the theoretical stretching frequencies of N–H, C=N, B–N, and B–O bonds.

Experimental and computational IR spectroscopic studies on the transmission of substituent effects on N—H, C=N, B—N and B—O bond stretching frequencies in 3,5-disubstituted-1,2,4,5-oxadiazaboroles

Experimental and computational IR spectroscopic studies on the transmission of substituent effects on N—H, C=N, B—N and B—O bond stretching frequencies in 3,5-disubstituted-1,2,4,5-oxadiazaboroles

13C NMR spectroscopy of heterocycles: 1-phenyl-3-aryl/t-butyl-5-arylpyrazoles

Abstract A series of chalcones 1–12 were converted to pyrazolines (1Pi–12Pi) by reaction with phenylhydrazine followed by DDQ oxidation to produce the corresponding pyrazoles (1Pz–12Pz). Three 1-phenyl-3-t-butyl-5-arylpyrazoles (13Pz–15Pz) were synthesized using an analogous approach. Molecular modeling studies predicted the 5-aryl group of the pyrazoles for both series to have a torsion angle of 52°–54° whereas the 1-phenyl group was predicted to have 35°–37° torsion angles. The 3-aryl group was predicted to be essentially coplanar (−3°) with the pyrazole system in the first series. 13C NMR data for both series, 1Pz–12Pz and 13Pz–15Pz, were collected in DMSO-d 6 at 50°C. A plot of the C4 chemical shifts for 1Pz–12Pz versus Hammet constants for 5-aryl substituents yielded a very good linear correlation (R2=0.96) with a slope of 1.5. The chemical shift data for C4 showed little or no dependence on 3-aryl substituents. The result for 13Pz–15Pz, despite only three points, was consistent with the first series results and yielded a ρ value of 2.0. Distal transmission of substituent effects (5-aryl groups) to C4 of the pyrazole system was reduced by roughly 50–60% of that of the analogous planar isoxazole system, but are not consistent with results for the similarly twisted 4-bromoisoxazoles.

Solvatochromism of Thiouracil Azo Dyes

A series of sixteen synthesized thiouracil azo dyes was obtained by condensation of arylazo acetoacetic esters with thiourea. Purified products were characterized by UV–Vis, FT–IR, 1H and 13C NMR spectroscopy. The UV–Vis absorption spectra of all synthesized compounds were recorded in nineteen solvents of different properties. The study and analysis of the spectral data of azo dyes generally indicate that the tautomeric equilibrium is shifted to the hydrazone tautomeric form in the solid state. Linear solvation energy relationships describe solvent influence on solvatochromism of the hydrazone form with the use of the Catalan model. Linear free energy relationships were applied in the form of single substituent parameter model to analyze substituent electronic effects on the spectral behavior of the compounds by Hammett’s equation. The correlations obtained with aid of theoretical calculations give insight into the influence of molecular conformation on the transmission of substituent effects, as well as different solvent–solute interactions.

New azo-decorated N-pyrrolidinylthiazoles: synthesis, properties and an unexpected remote substituent effect transmission.

New 5-azo substituted thiazole derivatives have been obtained, under mild conditions and in good yields, by the reaction between 2-N-pyrrolidinylthiazole and a series of aryldiazonium salts bearing, mainly in the para position, groups with different electronic effects. The NMR spectra of the products show broad signals of the methylene groups in the alpha position to the pyrrolidinyl nitrogen, suggesting a hindered rotation around the C2-N bond, of which a double bond character was also evidenced by X-ray diffraction analyses. The free energies of activation for the rotational processes have been obtained from (1)H NMR experiments and computer simulations at different temperatures and provided good correlation with the σ constants of the substituents on the 'remote' benzene ring. This represents an unexpected and peculiar result since the restricted rotation around the C2-N bond was shown to be influenced by a substituent situated very far away. 2,4-Di-N-pyrrolidinylthiazole showed a much greater reactivity than 2-N-pyrrolidinylthiazole with aryldiazonium salts.

13C NMR spectroscopy of heterocycles: 3,5-diaryl-4-bromoisoxazoles

Abstract A series of 3,5-diarylisoxazoles (1–9) underwent reaction with N-bromosuccinimide in acetic acid to yield the corresponding 3.5-diaryl-4-bromoisoxazoles (1Br–9Br) in low to moderate yield. The X-ray structure of 3-(4-chlorophenyl)-5-phenyl-4-bromoisoxazole showed torsion angles of ~40° and ~36° between the 3- and 5-aryl groups and the central ring system, respectively. Molecular modeling studies predicted the 3,5-phenylisoxazole ring system to be essentially coplanar and the 3,5-diphenyl groups of the 4-bromoisoxazole derivative to be twisted with torsional angles of 50° and 37°, respectively. Carbon-13 nuclear magnetic resonance (13C NMR) data of the 3,5-diaryl-4-bromoisoxazoles were obtained in dimethyl sulfoxide-d 6 (DMSO-d 6) at 50°C. Plots of the C4 chemical shift data for isoxazoles (1–9) vs. those for 4-bromoisoxazoles (1Br–9Br) showed a good linear correlation (r 2 = 0.974) with a slope of 0.96. Substitution on the 3-aryl group had essentially no effect on the chemical shift for C4 of the 4-bromoisoxazole, whereas that on the 5-aryl group showed an excellent correlation with σ+ values. Despite the predicted torsion angle differences, distal transmission of substituent effects in the 4-bromo ring system was essentially analogous to that in the planar 3,5-diarylisoxazoles.

A Robust, “One-Pot” Method for Acquiring Kinetic Data for Hammett Plots Used To Demonstrate Transmission of Substituent Effects in Reactions of Aromatic Ethyl Esters

A “one-pot” method for acquiring kinetic data for the reactions of a series of substituted aromatic esters with potassium hydroxide using 13C NMR spectroscopy is described, which provides an efficient way to obtain sufficient data to demonstrate the Hammett equation in undergraduate laboratories. The method is time-efficient and robust, and lends itself to group work. The high throughput nature of the method allows collection of sufficient data to explore the transmission of electronic effects between the aromatic moiety and the ester functionality with varying chain lengths.

An Anomalous Hammett Correlation for a Series of Substituted 3-Benzyl-2-phenyl-1,3-thiazolidin-4-ones

A series of 3-benzyl-2-phenyl-1,3-thiazolidin-4-one derivatives, with substituents at the N-benzyl site, was synthesized and characterized. Excluding the p -MeO and m -Br derivatives, distinct correlations between Hammett constant ? and 13 C substituent chemical shifts were observed in the C2, C4, and C5 carbons in the thiazolidin-4-one ring. This was unexpected because the transmission of substituent effects appears to be occurring via the sp 3 hybridized N-benzyl carbon. No discernible correlation was seen for the N-benzyl carbon; the carbon through which the effects were occurring. Correlations for substituent chemical shifts to Swain Lupton substituent parameters r and f constants were also attempted. Similar to the Hammett correlations, C2, C4, and C5 showed a reasonable degree of correlation with minimal to no improvement over Hammett constants; similarly, as with the Hammett correlations, the benzyl carbon exhibited no correlation with Swain Lupton parameters. Normal 0 7.8 ? 0 2 false false false EN-US ZH-CN X-NONE

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 Catalan-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.

Long‐range transmission of substituent effects on 13C NMR chemical shifts of imine carbon in benzylidene anilines

The 13C NMR chemical shifts of six kinds of substituted benzylidene anilines, with different backbone conjugation length, have been used as a probe to investigate the long‐range transmission of substituent effects. In this context, it was found that for substituents Y at the aniline unit, the transmission of the inductive and conjugative effects depend on the chemical bond numbers n(Y) between Y and the imine carbon, and the parameters n(Y)−2σF(Y) and n(Y)−2σR(Y) are suitable to scale the corrected inductive and conjugative effects, respectively. However, for substituents X, the chemical bond numbers n(X) between X and the imine carbon influences only the transmission of inductive effects of X, and the n(X)−2σF(X) item is appropriate to evaluate the modified inductive effects of X. Similarly, Δσ(cor)2 was proposed to describe the transmitted effect of the cross‐interaction effect. With the parameters n(X)−2σF(X), σR(X), n(Y)−2σF(Y), n(Y)−2σR(Y), Δσ(cor)2, and δC(parent), the δC(C = N) values of 181 samples can be well correlated. The correlation coefficient is 0.9957, and the standard derivation is only 0.23 ppm. Moreover, the multi‐parameter correlation equation is predicted well the δC(C = N) of other 25 samples of designed conjugated benzylidene anilines. Copyright © 2012 John Wiley & Sons, Ltd.

Transmission of Electronic Substituent Effects through a Benzene Framework: A Computational Study of 4-Substituted Biphenyls Based on Structural Variation

The transmission of substituent effects through a benzene framework has been studied by a novel approach, based on the structural variation of the Ph group in p-Ph-C(6)H(4)-X molecules. The molecular structures of many 4-substituted biphenyls were determined from MO calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. The twist angle between the phenyl probe (ring B) and the benzene framework carrying the substituent (ring A) was set at 90° to prevent π-electron transfer from one ring to the other and at 0° to maximize it. The structural variation of the probe is best represented by a linear combination of the internal ring angles, termed S(F)(BIPH(o)) and S(F)(BIPH(c)) for the orthogonal and coplanar conformations of the molecules, respectively. Regression analysis of these parameters using appropriate explanatory variables reveals a composite field effect, a substantial proportion of which is originated by resonance-induced π-charges on the carbon atoms of ring A. Field-induced polarization of the π-system of ring A also contributes to the structural variation of the probe. Thus, the S(F)(BIPH(o)) parameter is very well reproduced by a linear combination of the π-charges on the ortho, meta, and para carbons of ring A, an uncommon example of a quantitative relationship between molecular geometry and electron density distribution. Comparison of S(F)(BIPH(o)) with the gas-phase acidities of para-substituted benzoic acids shows that, while the deprotonating carboxylic probe is more sensitive to π-electron withdrawal than donation, the phenyl probe is equally sensitive to both. While the ability of the orthogonal biphenyl system to exchange π-electrons with the para substituent is the same as that of the benzene ring in Ph-X molecules, an increase by about 18% occurs when the conformation is changed from orthogonal to coplanar. The structural variation of the probe becomes more complicated, however. This is due to π-electron transfer from one ring to the other, which is shown to introduce quadratic terms in the regressions.

Natural bond orbital approach to the transmission of substituent effect through the fulvene and benzene ring systems

Electronic structure of 22 monosubstituted derivatives of benzene and exocyclically substituted fulvene with substituents: B(OH)(2), BH(2), CCH, CF(3), CH(3), CHCH(2), CHO, Cl, CMe(3), CN, COCH(3), CONH(2), COOH, F, NH(2), NMe(2), NO, NO(2), OCH(3), OH, SiH(3), SiMe(3) were studied theoretically by means of Natural Bond Orbital analysis. It is shown, that sum of π-electron population of carbon atoms of the fulvene and benzene rings, pEDA(F) and pEDA(B), respectively correlate well with Hammett substituent constants [Formula in text] and aromaticity index NICS. The substituent effect acting on pi-electron occupation at carbon atoms of the fulvene ring is significantly stronger than in the case of benzene. Electron occupations of ring carbon atoms (except C1) in fulvene plotted against each other give linear regressions with high correlation coefficients. The same is true for ortho- and para-carbon atoms in benzene. Positive slopes of the regressions indicate similar for fulvene and benzene kind of substituent effect - mostly resonance in nature. Only the regressions of occupation at the carbon atom in meta- position of benzene against ortho- and para-positions gives negative slopes and low correlation coefficients.

Linear free energy relationships of the 13C NMR chemical shifts in 5-(3- and 4-substituted arylazo)-4,6-dimethyl-3-cyano-2-pyridones

The 13 C NMR chemical shifts of the C2 carbon atom in the heteroaromatic nuclei of seventeen 5(3- and 4-substituted arylazo)-4,6-dimethyl-3-cyano-2-pyridones were determined in deuterated DMSO solution. For quantitative assessment of the substituent effects on the 13 C NMR chemical shifts simple and extended Hammett equations and Swain-Lupton equation were used. The mode of transmission of substituent effects have been discussed in a relation to the geometry of investigated molecules. The geometry data were obtained using semiempirical MNDO-PM6 energy calculations. The observed 13 C NMR substituent chemical shifts were correlated with literature 13 C NMR data for the corresponding 3- and 4-substituted-2’,6’-dimethylazobenzenes and 3- and 4-substituted azobenzenes. A good linear dependence obtained in this way, provided a basis to discuss the influence of the ortho-methyl groups on the degree of coplanarity of azobenzene and pyridone systems, and on the efficiency of transmission of electronic substituent effect from one ring to the other one.

IR spectroscopic studies on the transmission of substituent effects to carbonyl and thiocarbonyl stretching frequencies in 4-substituted phenyl-4,5-dihydrobenzo [ f] [1,4] oxazepin-3 (2H)-ones (thiones)

Single and dual substituent correlation analysis were applied to study transmission of substituent effects on IR carbonyl and thiocarbonyl stretching frequencies of 4-substituted phenyl-4,5-dihydrobenzo [ f] [1,4] oxazepin-3 (2H)-ones ( 5a– n) and -thiones ( 6a– n). The substituent effects were estimated on the basis of results of the statistical analysis. The differences among the regression coefficients were discussed in terms of the relative importance of the substituent field and resonance effects. For a better understanding of the results, density functional theory (DFT) calculations were performed to determine the preferred geometry and to calculate the theoretical carbonyl and thiocarbonyl stretching frequencies.

Comparative NMR and IR spectral, X-ray structural and theoretical studies of eight 6-arylidenedibenzo[ b, e]thiepin-11-one-5,5-dioxides

Eight 6-arylidenedibenzo[ b, e]thiepin-11-one-5,5-dioxides are characterized by NMR and IR spectroscopy. Single crystal X-ray structures for three congeners are reported. In addition, the transmission of substituent effects in conjugated double bond system of 6-arylidenedibenzo[ b, e]thiepin-11-one-5,5-dioxide framework has been evaluated by calculating the correlations between selected 13C NMR chemical shifts and IR stretching wave numbers and Hammett constants of the substituents locating in the phenyl ring of the arylidene moiety.

E-2-Benzylidenebenzocycloalkanones. IV. Studies on transmission of substituent effects on 13C NMR chemical shifts of E-2-(X-benzylidene)-1-tetralones, and -benzosuberones. Comparison with the 13C NMR data of chalcones and E-2-(X-benzylidene)-1-indanones

Single substituent parameter (SSP) and dual substituent parameter (DSP) analyses were applied to study the transmission of substituent effects on selected 13C NMR chemical shifts of the cyclic chalcone analogues, E-2-(4′-X-benzylidene)-1-tetralones ( 2) and E-2-(4′-X-benzylidene)-1-benzosuberones ( 3). In order to study how the geometry of the cyclic chalcone analogues affects the transmission of substituent effects similar investigations with the respective chalcones ( 4) were also performed. The results obtained earlier with the five-membered analogue E-2-(4′-X-benzylidene)-1-indanones ( 1) were also involved in the comparisons. Geometry optimization of the unsubstituted 1a, 2a, 3a and 4a as well as the substituted 2 and 3 was performed by ab initio quantum chemical calculations. Both SSP and DSP analyses reflected that resonance effects contribute more to the chemical shift of C-α (C2), while inductive effects primarily affect that of C-β (C10) of the enone moiety of all the four series. This latter effect, however, is far not as pronounced as that of the former one. It was found that DSP analysis data ( ρ F and ρ R values) of transmission of substituent effects on the δC2 data can serve as a measure of choice to study the conformation (planarity) of the investigated enones in the four series.