Hammett Substituent Constants Research Articles

Benzoquinone based chemodosimeters for selective and sensitive colorimetric and turn-on fluorescent sensing of cyanide in water

Dibromobenzoquinone-indole ensembles containing different substituents (OMe, Me, H, Br, and NO2) have been designed, synthesized and characterized using spectral techniques. These chemodosimeters selectively sense cyanide ion colorimetrically with turn-on fluorescence in aqueous HEPES buffer. Absorption and emission spectral studies suggest that the interaction between the receptors and cyanide is strong (binding constant in the order of 104M−1) with 1:1 stoichiometry. The detection limit for these receptors to sense cyanide is found to be in the nano molar range which is far below the permissible limit recommended by WHO for cyanide ion in drinking water. 1H and 13C NMR titration results indicate that the mechanism of sensing of cyanide by these receptors is through nucleophilic addition of cyanide to the CC bond of the indole ring. The electro-reduction potentials for the quinone moiety before and after the addition of cyanide substantiate the proposed mechanism. Mulliken charges computed, using DFT method, suggest that these receptors possess an electropositive C-atom for the addition reaction to occur, while such charge polarization is not there in similar compounds containing benzoquinone or naphthoquinone. The effect of substituents on the λICT, binding constant and δNH of indole has been explained using Hammett's substituents constants (σp).

General C-H Arylation Strategy for the Synthesis of Tunable Visible Light-Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores.

Herein we report the discovery of the benzo[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emission covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a chemoselective process involving Tf2O-mediated amide cyclodehydration, followed by intramolecular C-H arylation. Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent access to varied substitution. The effects of said substitution on the compounds' photophysical properties were rationalized by density functional theory calculations. For some compounds, emission wavelengths are directly correlated to the substituent's Hammett constants. Easily introduced nonconjugated reactive functional groups allow the labeling of biomolecules without modification of emissive properties. This work provides a straightforward platform for the synthesis of new moderately bright fluorescent dyes remarkable for their chemical stability, predictability, and unusually high excitation-emission differential.

Synthesis, spectral studies and insect antifeedant activities of some 4-substituted 1-naphthacyl bromide and its esters

<p>About nine 4-substituted 1-naphthacyl bromides and its esters have been synthesized by greener synthetic method using fly-ash catalyzed water mediated reaction. These acyl bromides and esters have been characterized by their physical constants, Mass, IR and NMR spectral data. These carbonyl frequencies(cm-1) of existed rotomers of these compounds have been assigned and correlated with Hammett substituent constants, F, R and Swain-Lupton’s parameters. The insect antifeedant activities of the synthesized acyl bromide and esters have been evaluated using 4<sup>th</sup> instar larvae <em>Achoea Janatha L</em>.</p>

Synthesis, Evaluation of Substituent Effect and Antimicrobial Activities of Substituted (E)-1-(3-bromo-4-morpholinophenyl)-3-phenylprop-2-en-1-one Compounds

A series of ten substituted (E)-1-(3-bromo-4-morpholinophenyl)-3-phenylprop-2-en-1-one compounds were synthesized by Crossed-Aldol condensation of 3-bromo-4-marpholino acetophenone with various substituted and unsubstituted benzaldehydes in presence of sodium hydroxide. The entire ten compounds are novel and these have been newly synthesized compounds. The synthesized substituted 3-phenylprop-2-en-1-one were characterized by their physical constants and UV, IR, NMR spectral data. These observed UV absorption maximum (λ max nm) value. The group frequencies of infrared absorption (cm -1 ) of νCO s-cis and s-trans, deformation modes of νCH out of plane and in-plane, νCH=CH out of plane, ν>C=C<out of plane values, 1 H chemical shifts (ppm) vinyl protons, 13 C chemical shifts (ppm) carbonyl carbons and vinyl carbons values were correlated with various Hammett substituent constants, and Swain-Lupton parameters using single and multi-regression analyses. From the results of statistical analysis, the effects of substituents on the functional group frequencies were studied. The antimicrobial activities of these synthesized substituted (E)-1-(3-bromo-4-morpholinophenyl)-3-phenylprop-2-en-1-ones have been screened using Kirby-Bauer method. DOI: http://dx.doi.org/10.17807/orbital.v9i1.852

Towards a physical interpretation of substituent effect: Quantum chemical interpretation of Hammett substituent constants

Quantitative description of substituent effects is of a great importance especially in organic chemistry and QSAR-type treatments. The proposed approaches: substituent effect stabilization energy (SESE) and charge of the substituent active region (cSAR) provide substituent effect characteristics, physically independent of the Hammett's substituent constants, σ. To document abilities of these descriptors the B3LYP/6-311++G(d,p) method is employed to examine changes in properties of a reaction center Y (Y = COOH or COO− groups) and a transmitting moiety (benzene ring) due to substituent effects in a series of meta- and para-X-substituted benzoic acid and benzoate anion derivatives (X = NMe2, NH2, OH, OMe, CH3, H, F, Cl, CF3, CN, CHO, COMe, CONH2, COOH, NO2, NO). The transmitting moiety is described by aromaticity indices HOMA and NICS(1). Furthermore, an advantage of the cSAR characteristic is the ability to use it to describe both electron donating/accepting properties of a substituent as well as a reaction center. It allows demonstration of the reverse substituent effects of COOH and COO− groups on substituent X.

Synthesis of Perfluoro-tert-butyl Tyrosine, for Application in 19F NMR, via a Diazonium-Coupling Reaction.

A practical synthesis of the novel highly fluorinated amino acid Fmoc-perfluoro-tert-butyl tyrosine was developed. The sequence proceeds in two steps from commercially available Fmoc-4-NH2-phenylalanine via diazotization followed by diazonium coupling reaction with perfluoro-tert-butanol. In peptides, perfluoro-tert-butyl tyrosine was detected in 30 s by NMR spectroscopy at 500 nM peptide concentration due to nine chemically equivalent fluorines that are a sharp singlet by 19F NMR. Perfluoro-tert-butyl ether has an estimated σp Hammett substituent constant of +0.30.

A Theoretical Study of the Relationship between the Electrophilicity ω Index and Hammett Constant σp in [3+2] Cycloaddition Reactions of Aryl Azide/Alkyne Derivatives.

The relationship between the electrophilicity ω index and the Hammett constant σp has been studied for the [2+3] cycloaddition reactions of a series of para-substituted phenyl azides towards para-substituted phenyl alkynes. The electrophilicity ω index—a reactivity density functional theory (DFT) descriptor evaluated at the ground state of the molecules—shows a good linear relationship with the Hammett substituent constants σp. The theoretical scale of reactivity correctly explains the electrophilic activation/deactivation effects promoted by electron-withdrawing and electron-releasing substituents in both azide and alkyne components.

Synthesis, Electrochemical Characterization, and Linear Free Energy Relationship of 1,3-Diphenyl-6-alkyl/arylfulvenes

A series of 1,3-diphenyl-6-alkyl/arylfulvenes was prepared, and the electrochemical properties were investigated. The addition of phenyl groups about the fulvene raised the reduction potential and helped to stabilize the electrochemically generated radical anion. The addition of various functional groups onto the phenyl ring at the 6-position of 1,3,6-triphenylfulvene results in a linear free energy relationship between reduction potential and the Hammett substituent constant, σ. Further extending the conjugation at the 6-position of 1,3-diphenyl-6-arylfulvenes increases the reversibility of the redox reactions, but does not appear to further stabilize the generated radical anion. This in-depth investigation provides evidence that the compounds studied may have utility in light-harvesting applications.

Enantioselective hydroformylation of 2- and 4-substituted styrenes with PtCl2[(R)-BINAP] + SnCl2 ‘in situ’ catalyst

Two sets of styrenes possessing various substituents either in ortho or para position were hydroformylated in the presence of ‘in situ’ catalyst formed from PtCl2[(R)-BINAP] and tin(II) chloride. The reversal of the absolute configuration of the preferred enantiomers was observed using both sets of substrates by the variation of the reaction temperature in the range of 40–100 °C. In case of the 4-substituted styrenes, the reversal temperature of the enantioselectivity shows correlation with the Hammett substituent constants, i.e., with the electron donor or electron acceptor properties of the para-substituents. This phenomenon was explained by the reversible formation of the Pt-branched alkyl intermediates, leading to the corresponding (R)- and (S)-enantiomers of 2-arylpropanals.Strong substituent effect on the regioselectivity was observed in the hydroformylation of 2-substituted styrenes: the presence of substituents characterised by larger steric parameter resulted in the highly favoured formation of the linear aldehyde. For instance, regioselectivities of 45%, 22% and 7% towards branched aldehyde were obtained with styrene, 2-fluoro- and 2-bromostyrene, respectively, at 80 °C reaction temperature. In addition to the characteristic change of regioselectivity, the reversal of absolute configuration as a function of reaction temperature was also observed.

Electronic effect on the photophysical properties of 2-(2-hydroxyphenyl)benzothiazole-based excited state intramolecular proton transfer fluorophores synthesized by sonogashira-coupling reaction

For excited-state intramolecular proton transfer (ESIPT) fluorophores, the intramolecular hydrogen bond strength has decisive influence on their photophysical properties. Herein, a series of 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives as new ESIPT fluorophores are synthesized through Sonogashira-coupling reaction, and their electronic structures are systematically tuned through various substitutions with different Hammett substituent constants (σ) in the 5-position of the phenol ring. Theoretical calculations and crystal analysis show that the HBT derivative with larger σ contains the stronger intramolecular hydrogen bond, which is less likely to be damaged by external factors, and that it has larger energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The experimentally observed trends are well in line with the theoretical calculations. More importantly, unlike other HBT derivatives and parent HBT displaying anion emission in PBS medium, the two HBT derivatives bearing methyl ester and cyano substitutions, respectively, emit as keto tautomer, which are expected to be applied in biological aspects.

H-bond and dipole–dipole interactions between water and [sbnd]COO[sbnd] functional group in methyl benzoate derivatives: Substituent and heteroatom effects

Interactions of COOCH3 functional group of methyl benzoate (MB) and its derivatives can substantially be affected by substituents and heteroatoms. The substituent and heteroatom effects on the intermolecular interactions in the complexes of MB and its derivatives with the water molecule have been investigated by the quantum mechanical calculations in the present study. Electron donating (ED)/electron withdrawing (EW) substituents increase/decrease the binding energy (ΔEbin) between mentioned functional group and hydrogen bond donor, and the N heteroatom in the six membered ring weakens the effect. The effects of the N heteroatoms depend on their positions in the ring, and the effects of substituents depend on the number and the positions of heteroatoms. Some relations were obtained between the dipole moment of carbonyl group, results obtained from the natural bond orbital (NBO) and the atoms in molecules (AIM) analysis, binding energy and Hammett constants of substituents. The results can be used to predict the factors affect the intermolecular interactions, and to improve the ligand-protein bindings in biological systems.

Synthesis and Metalation of Doubly o-Phenylene-Bridged Cyclic Bis(dipyrrin)s with Highly Bent Skeleton of Dibenzoporphyrin(2.1.2.1).

Facile synthesis of dibenzoporphyrins(2.1.2.1) has been successfully reported by the simple condensation reaction of o-dipyrrolylbenzene with various aldehydes in the presence of a Lewis acid. This reaction enables the preparation of various dibenzoporphyrin(2.1.2.1) derivatives with p-substituted phenyl groups, five-membered heterocycles, and ethynyl groups at the meso-positions. Dibenzoporphyrins(2.1.2.1) consist of two dipyrrin units that are connected by o-phenylene bridges, which adopt highly bent saddle-shaped structures; this was confirmed by X-ray diffraction analysis. We found that dibenzoporphyrin(2.1.2.1) can be described as a 20π antiaromatic conjugated system, but practically, it is not an antiaromatic macrocycle, which we revealed by (1) H NMR spectroscopy. The redox potentials had good correlations with Hammett substituent constant (σp ) of the substituents at the meso-positions. The free-base dibenzoporphyrin(2.1.2.1) was able to form the metal complexes with nickel(II), copper(II), palladium(II), platinum(II), and tin(IV) ions. These results suggested that dibenzoporphyrin(2.1.2.1) derivatives can be utilized as novel macrocyclic dianionic tetradentate ligands for various metal ions to give complexes with varying optical and electrochemical properties.

Single‐Electron Transfer in σ‐Complexation Reactions of 2,6‐Dimethoxy‐3,5‐dinitropyridine with Para‐X‐phenoxide Anions in Aqueous Solution

ABSTRACTSecond‐order rate constants have been measured spectrophotometrically for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine 1 with 4‐X‐substituted phenoxide anions (X = OMe, Me, H, Cl, and CN) 2a–e in aqueous solution at various temperatures. The effect of phenoxide substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to nonlinear correlations of Δ≠H and Δ≠S with Hammett's substituent constants (σ). Each Hammett plots exhibits two intersecting straight lines for the reactions of 1 with the phenoxide anions 2a–e, whereas the Yukawa–Tsuno plots for the same reactions are linear. The large negative ρ values (−4.03 to −3.80) obtained for the reactions of 1 with the phenoxide anions possessing an electron‐donating group supports the proposal that the reactions proceed through a single‐electron transfer mechanism.

Quantitative structure-activity relationship analysis of substituted arylazo pyridone dyes in photocatalytic system: Experimental and theoretical study

A series of arylazo pyridone dyes was synthesized by changing the type of the substituent group in the diazo moiety, ranging from strong electron-donating to strong electron-withdrawing groups. The structural and electronic properties of the investigated dyes was calculated at the M062X/6-31+G(d,p) level of theory. The observed good linear correlations between atomic charges and Hammett σp constants provided a basis to discuss the transmission of electronic substituent effects through a dye framework. The reactivity of synthesized dyes was tested through their decolorization efficiency in TiO2 photocatalytic system (Degussa P-25). Quantitative structure-activity relationship analysis revealed a strong correlation between reactivity of investigated dyes and Hammett substituent constants. The reaction was facilitated by electron-withdrawing groups, and retarded by electron-donating ones. Quantum mechanical calculations was used in order to describe the mechanism of the photocatalytic oxidation reactions of investigated dyes and interpret their reactivities within the framework of the Density Functional Theory (DFT). According to DFT based reactivity descriptors, i.e. Fukui functions and local softness, the active site moves from azo nitrogen atom linked to benzene ring to pyridone carbon atom linked to azo bond, going from dyes with electron-donating groups to dyes with electron-withdrawing groups.

Effects of single bond-ion and single bond-diradical form on the stretching vibration of C[dbnd]N bridging bond in 4,4′-disubstituted benzylidene anilines

Fifty-seven samples of model compounds, 4,4′-disubstituted benzylidene anilines, p-X-ArCH=NAr-p-Y were synthesized. Their infrared absorption spectra were recorded, and the stretching vibration frequencies νCN of the CN bridging bond were determined. New stretching vibration mode was proposed by means of the analysis of the factors affecting νCN, that is there are mainly three modes in the stretching vibration of CN bond: (I) polar double bond form CN, (II) single bond-ion form C+–N− and (III) single bond-diradical form C–N. The contributions of the forms (I) and (II) to the change of νCN can be quantified by using Hammett substituent constant (including substituent cross-interaction effects between X and Y groups), whereas the contribution of the form (III) can be quantified by employing the excited-state substituent constant. The most contribution of these three forms is the form (III), the next is the form (II), whose contribution difference was discussed with the viewpoint of energy requirements in vibration with the form (III) and form (II).

Synthesis, assessment of substituent effect and antimicrobial activities of (4E)-4-(benzylideneamino)-1,2-dihydro-2,3-dimethyl-1-phenylpyrazol-5-one compounds

A series of substituted (4E)-4-(benzylideneamino)-1,2-dihydro-2,3-dimethyl-1-phenylpyrazol-5-one compounds has been synthesized from 4-aminoantipyrine and substituted benzaldehydes. Their structures have been confirmed by their physical constants, UV, IR and NMR spectral data. The observed UV absorption maximum λmax(nm), IR frequencies νCN(cm−1), NMR δ(ppm) of C–H & CN chemical shift values have been correlated with Hammett substituent constants and F and R parameters using single and multi-linear regression analyses. From the results of statistical analysis, the effect of substituents on the spectral data has been studied. The antimicrobial activities of all the Schiff bases synthesized have been studied using Bauer–Kirby method.

A high‐level theoretical study into the atmospheric phase hydration, bond dissociation enthalpies, and acidity of aldehydes

CBS‐Q//B3, G4(MP2), and G4 composite method calculations were used to estimate atmospheric phase standard state (298.15 K, 1 atm) free energies of hydration (ΔhydrG°(g)), hydration equilibrium constants (log Khydr,(g)), bond dissociation enthalpies (BDEs), and enthalpies (ΔdH°(g)) and free energies (ΔdG°(g)) of aldehydic proton acid dissociation for various substituted aldehydes with electron withdrawing and electron releasing groups. Good quality log Khydr,(g) correlations with the Swain–Lupton resonance effect parameters R and R+ were found, allowing extension of the model to predict log Khydr,(g) values for 487 substituted aldehydes having available R‐values and 108 substituted aldehydes having available R+ values. Good correlations were also found between experimental aqueous phase hydration equilibrium constants (log Khydr,(aq)) and summative R/R+ values for peripheral substituents on a range of carbonyl derivatives (aldehydes, ketones, esters, and amides), suggesting that the structure–reactivity modeling approach can be extended to include all possible combinations of R1C(O)R2 carbonyl substitution in both gas and aqueous systems. Computationally derived BDEs and ΔdH°(g)/ΔdG°(g) were in good agreement with the limited experimental and theoretical datasets. BDEs did not generally correlate with any of the Hammett substituent constants or Swain–Lupton parameters considered. Gas phase acidities exhibited high correlation coefficients with Hammett inductive substituent constants (σI) and field effect parameters (F), allowing these to be employed as surrogates for estimating the gas phase aldehydic proton acidities of a larger potential compound range. The resulting models will be of use in predicting the environmental behavior for a broad range of environmentally relevant compounds containing carbonyl functionalities. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Synthesis, Hammett Spectral Correlations and Antimicrobial Activities of (&lt;i&gt;E&lt;/i&gt;)-1-Benzylidene-2-(4-Chlorophenyl)Hydrazines

In the present study, a series of nine number of (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been synthesized by condensation reaction of meta and para substituted benzaldehydes with 4-chlorophenylhydrazine using acetic acid catalyst. They are characterized by their physical constants, UV (200-450 nm), Infra-Red (KBr, 4000-400 cm−1) and NMR (1H and 13C) spectral data. The observed UV, IR and NMR spectral data have been correlated with Hammett substituent constants and Swain-Lupton’s F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis the effects of substituents on the spectral data have been discussed. The antimicrobial activities of all the synthesized (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been studied using Bauer–Kirby method.

Investigation of substitution effect on fluorescence properties of Zn2+-selective ratiometric fluorescent compounds: 2-(2′-Hydroxyphenyl)benzimidazole derivatives

2-(2′-Hydroxyphenyl)benzimidazole derivatives (X-HBIs), modified by various substituents X (X=H, CH3, OH, OCH3, NO2, NHCOCH3, NH2, N(CH3)2), were synthesized and their fluorescent behaviors and equilibriums in aqueous solution were studied. Strong fluorescence attributed to the tautomer emission was observed in aqueous solution at pH 7.4. The fluorescence intensities of the X-HBIs were enhanced selectively by addition of Zn2+ but not by addition of Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, and Cu2+. Additionally, the effective ratiometric fluorescence response to Zn2+ addition was observed in 5-NH2-HBI and 5-N(Me)2-HBI. The pH-titration and speciation studies proved that the X-HBIs have two or three protonation equilibriums and one complexation equilibrium corresponding to the formation of the [Zn(X-HBI)]+ complex. Further structural studies using extended X-ray absorption fine structure analyses and density functional theory calculations identified the dominant Zn2+ species as the [Zn(HBI)(H2O)3]+ complex in aqueous solution. Based on the substituent effect on the fluorescence properties of X-HBIs and their Zn2+ complexes in aqueous solution, the maximum fluorescence excitation and fluorescence wavelengths of both the tautomeric form and the Zn2+ complexes were dependent on the Hammett substituent constants of X, which was attributed to the change of the π–π* energy gap of HBI by introduction of the substituent.

Synthesis, Hammett Spectral Correlations and Antimicrobial Activities of (&lt;i&gt;E&lt;/i&gt;)-1-Benzylidene-2-(4-Chlorophenyl)Hydrazines

In the present study, a series of nine number of (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been synthesized by condensation reaction of meta and para substituted benzaldehydes with 4-chlorophenylhydrazine using acetic acid catalyst. They are characterized by their physical constants, UV (200-450 nm), Infra-Red (KBr, 4000-400 cm−1) and NMR (1H and 13C) spectral data. The observed UV, IR and NMR spectral data have been correlated with Hammett substituent constants and Swain-Lupton’s F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis the effects of substituents on the spectral data have been discussed. The antimicrobial activities of all the synthesized (E)-1-benzylidene-2-(4-chlorophenyl) hydrazine compounds have been studied using Bauer–Kirby method.