Effect of electron-withdrawing substituents on the electrophilicity of carbonyl carbons

The substituent effects on the carbonyl carbon atom for a series of twelve substituted phenyl acetates have been rationalized using a global electrophilicity index. This index is linearly correlated with the experimental reaction rate coefficients. We found that, in contrast to the proposed interpretation based on experimental ¹³C NMR chemical shifts and ground state destabilization calculations, the electrophilicity of carbonyl compounds increases due to the effect promoted by electron-withdrawing groups in these systems.     

Structure and photochromic properties of the crystalline 1,3-dimethyl-2-(3-phenylnorbornadienyl)benzimidazolium tetrafluoroborate

Molecular and crystal structures, as well as photochemistry in the solid state were studied for the norbornadiene, whose one double bond was modified by the simultaneous introduction of an electron-donating and electron-withdrawing substituents. Effects of the nature of substituents on the distance of the substituted C=C bond were analyzed by quantum chemical calculations.     

Trifluoroacetylsulfenyl trifluoroacetate,CF(3)C(O)-S-O-C(O)CF(3), a novel compound with a symmetrically substituted S-O bond: synthesis,spectroscopic characterization,and quantum chemical calculations

The new compound trifluoroacetylsulfenyl trifluoroacetate, CF(3)C(O)SOC(O)CF(3), which possesses two identical carbonyl substituents attached to the S-O bond, has been synthesized. The IR and UV spectra of the gas phase as well as the (13)C NMR spectrum of the solution in CDCl(3) were recorded and assigned. Quantum chemical calculations were performed with the ab initio methods HF and MP2 and the density functional approach B3LYP. The 6-31G basis set was chosen in all calculations. The molecule possesses a skew structure, and according to all computational methods, the syn-syn structure (C=O bonds of both C(O)CF(3) groups synperiplanar to S-O bond) represents the most stable conformer. In agreement with the quantum chemical calculations, the presence of small amounts (< or =5%) of a second conformer (anti-syn) cannot be excluded on the basis of the IR spectrum. The calculated values for the torsional angle around the S-O bond (delta(C-S-O-C)) of the syn-syn form are smaller than 80 degrees (72-78 degrees). Comparison with theoretical results for the corresponding disulfide CF(3)C(O)SSC(O)CF(3) and peroxide CF(3)C(O)OOC(O)CF(3) indicates that the structural properties of sulfenyl compounds are more similar to those of disulfides than to those of peroxides.     

Intrinsic conformational preferences of C(alpha,alpha)-dibenzylglycine

The intrinsic conformational preferences of C (alpha,alpha)-dibenzylglycine, a symmetric alpha,alpha-dialkylated amino acid bearing two benzyl substituents on the alpha-carbon atom, have been determined using quantum chemical calculations at the B3LYP/6-31+G(d,p) level. A total of 46 minimum energy conformations were found for the N-acetyl- N'-methylamide derivative, even though only nine of them showed a relative energy lower than 5.0 kcal/mol. The latter involves C 7, C 5, and alpha' backbone conformations stabilized by intramolecular hydrogen bonds and/or N-H...pi interactions. Calculation of the conformational free energies in different environments (gas-phase, carbon tetrachloride, chloroform, methanol, and water solutions) indicates that four different minima (two C 5 and two C 7) are energetically accessible at room temperature in the gas phase, while in methanol and aqueous solutions one such minimum (C 5) becomes the only significant conformation. Comparison with results recently reported for C (alpha,alpha)-diphenylglycine indicates that substitution of phenyl side groups by benzyl enhances the conformational flexibility leading to (i) a reduction of the strain of the peptide backbone and (ii) alleviating the repulsive interactions between the pi electron density of the phenyl groups and the lone pairs of the carbonyl oxygen atoms.     

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.     

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.     

Structure-reactivity correlation in the pyridinolysis of substituted phenyl acetates

The pyridinolysis of substituted phenyl acetates in acetonitrile was investigated with an electric conductivity method. The rates of these reactions were increased with electron-donating group in pyridine and electron-withdrawing group in phenyl acetate, ϱ_x (X;substituents in phenyl ring) values increase gradually according to the electron-donating ability in pyridine substituents, but the |ϱ_y|(Y;substituents in pyridine ring) values decrease with that of substituents. The β values increase with increasing electron-withdrawing ability of the substituents in the phenyl ring, it can be inferred that N---C bond formation increases progressively p-methyl to p-nitrophenyl acetates. This is in agreement with prediction of substituent effects for a simple S_N2 displacement reaction. The sensitivity parameters, β and ϱ, are inter-related and are themselves sensitive to the reactivity of the system. All above results are interpreted in terms of a dissociative S_N2 mechanism involving a metastable tetrahedral intermediate.     

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).     

密度泛函理论研究温度和取代基对4-亚芳基亚氨基-1,2,4-三唑-3(2H)-酮气相热分解的影响

采用密度泛函理论(DFT)方法, 在B3LYP/6-31＋＋G(d,p)水平上研究了4-亚芳基亚氨基-1,2,4-三唑-3(2H)-酮及其类似物的气相热分解反应. 从热力学性质、几何结构参数、自然电荷分布、温度效应等角度探讨不同取代基对标题物热分解反应的影响. 结果表明: R (R＝H, CH2CH2CN)取代基对反应影响很小; 而苯环上Y (Y＝MeO, Me, H, Cl和NO2)取代基对活化自由能(ΔG≠)、键长(ΔR≠)、键角(Δα≠)和自然电荷(Δq≠)变化的影响与Hammet常数(σ)呈现线性关系ΔP≠＝ρσ＋C (P＝G, R, α和q, r＞0.95). 给电子基团使得相应的活化自由能降低, 键长变短, 键角变大, 自然电荷增加; 吸电子反之.     

Spectroscopic characterization of the 1-substituted 3,3-diphenyl-4-(2'-hydroxyphenyl)azetidin-2-ones: application of (13)C NMR, (1)H-(13)C COSY NMR and mass spectroscopy

The article deals with spectroscopic characterization of azetidin-2-ones. The presence of substituents like hydroxyl, fluoro, methoxy and benzhydryl, etc., on the azetidin-2-one ring significantly affects the IR absorption and (13)C NMR frequencies of the carbonyl group present in these compounds. The presence of an ester carbonyl group or too many methine protons in the molecule has been observed to limit the scope of IR and (1)H NMR spectroscopy in unambiguous assignment of the structure. The application of (13)C NMR, 2D NMR ((1)H-(13)C COSY) and mass spectroscopy in characterization of complex azetidin-2-ones is discussed. An application of the latter two techniques is described in deciding unequivocally between an azetidin-2-one ring and chroman-2-one ring structure for the product obtained by treatment of the 1-substituted 3,3-diphenyl-4-[2'-(O-diphenylacyl)hydroxyphenyl]-2-azetidinones with ethanolic sodium hydroxide at room temperature.     

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.     

13C NMR spectra of 1-(N-arylidene)amino-1,2,3-triazoles

The ¹³C nmr spectra of several 1-(N-arylidene)amino-1,2,3-triazoles were determined. The long range influence of substituents on ¹³C resonances of triazole and C = N carbon atoms was examined and a correlation of their chemical shifts with net charge densities (q_{π + σ}) calculated by the CNDO/2 method was made. It was also shown that the dependence of the coupling constants ¹J(CH) for C = N carbon on the substituents is better correlated using electrons in bond values instead of charge densities.     

13C and 1H chemical shifts in 2-benzylidene[3]ferrocenophane-1,3-diones. Elucidation of the substituent dependence of carbonyl chemical shifts in different carbonyl derivatives

The ¹³C and ¹H chemical shifts of the ferrocene moiety, as well as the carbonyl carbons and styrene moiety, of substituted 2-benzylidene[3]ferrocenophane-1,3-diones have been assigned. Correlations of ¹³C substituent chemical shifts of both carbonyl carbons with the Hammett constants have been found, and the effect of the transmission of substituent effects on these chemical shifts through the styrene moiety is discussed. An explanation is given for the different sensitivities of the carbonyl carbon chemical shifts to the electronic effect of substituents in mono- and dicarbonyl derivatives.     

Density functional theory for adsorption of HCHO on the FeO（100） surface

The density functional theory (DFT) and periodic slab model were used to get information concerning the adsorption of HCHO on the FeO(100) surface. A preferred η2-(C,O)-di-σ four-membered ring adsorption conformation on the Fe-top site was found to be the most favorable structure with the predicted adsorption energy of 210.7 kJ/mol. The analysis of density of states, Mulliken population, and vibrational frequencies before and after adsorption showed clear weakening of the carbonyl bond, and high sp3 charact...     

13C NMR study of the enol forms of β-diketones

Carbon-13 NMR spectra of a series of β-diketones in the enol form with various β-substituents have been studied. An additive influence of the β-substituents on the chemical shifts of the carbon atoms in the hydrogen bonded chelate ring has been found. It is shown that the α- and β-carbon chemical shifts can be calculated by means of a set of increments for arbitrary combinations of the X and Y substituents. Analysis of the experimental data enables the conclusions to be drawn that enol–enolic tautomerism with different populations of the forms (A) and (B) is absent in β-diketones and that the carbon chemical shift changes are caused by electron density redistribution in the hydrogen bonded chelate ring (C).     

Rigid backbone 1,8-anthracene-linked bis-oxazolines (AnBOXes): design,synthesis, application and characteristics in catalytic asymmetric aziridination

A series of rigid backbone 1,8-anthracene-linked bis-oxazolines (AnBOXes) have been designed, synthesized, and evaluated in the catalytic asymmetric aziridination with [N-(p-toluenesulfonyl)imino]phenyliodinane (PhINTs) as a nitrene source. The results indicate that highly enantioselective aziridination of chalcones catalyzed by an AnBOX and CuOTf complex with up to >99% ee and the opposite enantioselectivity, compared with the ligands of Evans et al., can be achieved. The enantioselectivity is substituent dependent with respect to chalcones. Chalcones with electron-donating substituents show higher enantioselectivities due to the stronger Lewis basicity of the oxygen of their carbonyl groups than those with electron-withdrawing substituents. The results also indicate that the coordination between the oxygen of the carbonyl group in chalcones and the ether group in alkenes with the copper in the catalyst is essential for high enantioselectivity, while the π–π stacking interaction between two reactants plays an importantly additional role for high enantioselectivity in asymmetric aziridination. An excellent backbone-controlled stereoselectivity was observed for the AnBOX ligands in asymmetric aziridination, as this will provide very important information for designing novel ligands.     

Use of 13C NMR Spectroscopy to Establish the Structure of 6(7)-R-Quinoxaline N,N′-Dioxides

The effect of substituents in position 6 on the positions of the signals of the carbon atoms in the ¹³C NMR spectra of substituted 1,2,3,4-tetrahydro-5,10-phenazine N,N′-dioxides has been analyzed, increments of substituents have been found, and a scheme has been proposed for the calculation of the chemical shifts of carbon atoms in the ¹³C NMR spectra of 6(7)-R-quinoxaline N,N′-dioxides. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, 1076–1080, July, 2005.     

Barriers to rotation of the dimethylamino group in some 2-amino-4-(N,N-dimethylamino)pyrimidines

The free energy of activation for rotation about the exocyclic C? N bond of the dimethylamino group of some 6-substituted 2-amino-4-(N,N-dimethylamino)pyrmidines has been determined using ¹H NMR line shape analysis. The results are discussed in terms of the relative electron-withdrawing and electron-releasing effects of the substituents.     

Palladium-catalyzed carbonylative annulation of internal alkynes: synthesis of 3,4-disubstituted coumarins

The palladium-catalyzed annulation of internal alkynes by o-iodophenols in the presence of CO results in exclusive formation of coumarins. No isomeric chromones have been observed. The best reaction conditions utilize the 2-iodophenol, 5 equiv of alkyne, 1 atm of CO, 5 mol % Pd(OAc)2, 2 equiv of pyridine, and 1 equiv of n-Bu4NCl in DMF at 120 degrees C. The use of a sterically unhindered pyridine base is essential to achieve high yields. A wide variety of 3,4-disubstituted coumarins containing alkyl, aryl, silyl, alkoxy, acyl, and ester groups have been prepared in moderate to good yields. Mixtures of regioisomers have been obtained when unsymmetrical alkynes are employed. 2-iodophenols with electron-withdrawing and electron-donating substituents and 3-iodo-2-pyridone are effective in this annulation process. The reaction is believed to proceed via (1) oxidative addition of the 2-iodophenol to Pd(0), (2) insertion of the alkyne triple bond into the aryl-palladium bond, (3) CO insertion into the resulting vinylic carbon-palladium bond, and (4) nucleophilic attack of the phenolic oxygen on the carbonyl carbon of the acylpalladium complex with simultaneous regeneration of the Pd(0) catalyst. This annulation process is the first example of intermolecular insertion of an alkyne occurring in preference to CO insertion.