A new class of organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines: synthesis, structure, pK(BH+) measurements, and properties

A series of stable organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines, were efficiently synthesized from N,N'-dialkylthioureas and 3-hydroxy-2-butanone and their basicities were measured in acetonitrile. The derivatives with tert-alkyl groups on the imino nitrogen were found to be more basic than the tBuP(1) (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by (13)C?NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino-substituents, including electron-acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas-phase basicity becomes more dependent on the field-inductive effect, polarizability, and resonance effects of the substituent.     

Influence of solvent on the ortho substituent effect in the alkaline hydrolysis of phenyl esters of substituted benzoic acids

The second‐order rate constants k (dm³ mol^?1 s^?1) for the alkaline hydrolysis of phenyl esters of meta‐, para‐ and ortho‐substituted benzoic acids in aqueous 5.3 M NaClO₄ and 1.0 M Bu₄NBr were measured by UV/Vis spectrophotometry at 25 °C. The variations in the ortho inductive, ortho resonance, as well as meta and para polar effects with solvent parameters were studied using data for the alkaline hydrolysis of phenyl esters of substituted benzoic acids in various media. The dependence of the ortho substituent effect on solvent can be precisely described with the following equation: Δlog k_ortho = log k_ortho ? log k_H = 0.059 + 2.19σ_I + 0.304σ°_R + 2.79E ? 0.016ΔEσ_I ? 0.085ΔEσ°_R, where ΔE is the solvent electrophilicity, ΔE = E_S ? E_H2O, characterizing the hydrogen‐bond donating power of the solvent. The increase in the meta and para polar substituent effects with decrease in the solvent hydrogen‐bond donor capacity (electrophilicity) was approximately to the same extent (?0.068ΔEσ°_m,p) as the resonance term for the ortho substituents. The steric term of ortho substituents was independent of the solvent parameters. The variations in the ortho inductive, ortho resonance, as well as meta and para polar substituent effects with the solvent electrophilicity were to the same extent as in phenyl benzoates containing the substituents in the phenyl part. The substituent effects in the alkaline hydrolysis of ethyl benzoates appeared to vary with the solvent electrophilicity nearly to the same extent as in the alkaline hydrolysis of substituted phenyl esters of benzoic acids. Copyright © 2009 John Wiley & Sons, Ltd.     

Prediction of ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous acetonitrile

The second-order rate constants k for the alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in aqueous 50.9% acetonitrile have been measured spectrophotometrically at 25°C. The log k values for meta and para derivatives correlated well with the Hammett σ_m,p substituent constants. The log k values for ortho-substituted phenyl benzoates showed good correlations with the Charton equation, containing the inductive, σ_I, resonance, σ^○ _R, and steric, E _s ^B, and Charton υ substituent constants. For ortho derivatives the predicted (log k _X)_calc values were calculated with equation (log k _ortho)_calc = (log k ^H _AN)_exp + 0.059 + 2.19σ_I + 0.304σ^○ _R + 2.79E _s ^B ? 0.0164ΔEσ_I — 0.0854ΔEσ^○ _R, where DE is the solvent electrophilicity, ΔE = E _AN — E _H20 = ?5.84 for aqueous 50.9% acetonitrile. The predicted (log k _X)_calc values for phenyl ortho-, meta- and para-substituted benzoates in aqueous 50.9% acetonitrile at 25°C precisely coincided with the experimental log k values determined in the present work. The substituent effects from the benzoyl moiety and aryl moiety were compared by correlating the log k values for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in various media with the corresponding log k values for substituted phenyl benzoates, C₆H₅CO₂C₆H₄-X.     

Prediction of ortho substituent effect in alkaline hydrolysis of substituted phenyl benzoates in aqueous acetonitrile

The second‐order rate constants k (dm³mol^?1s^?1) for alkaline hydrolysis of meta‐, para‐ and ortho‐substituted phenyl esters of benzoic acid, C₆H₅CO₂C₆H₄‐X, in aqueous 50.9% (v/v) acetonitrile have been measured spectrophotometrically at 25 °C. In substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X, the substituent effects log k_X ? log k_H in aqueous 50.9% acetonitrile at 25 °C for para, meta and ortho derivatives showed good correlations with the Taft and Charton equations, respectively. Using the log k values for various media at 25 °C, the variation of the ortho substituent effect with solvent was found to be precisely described with the following equation: Δlog k_ortho = log k_ortho ? log k_H = 1.57σ_I + 0.93σ°_R + 1.08E_s^B ? 0.030ΔEσ_I ? 0.069ΔEσ°_R, where ΔE is the solvent electrophilicity, ΔE = E_S ? E_H20, characterizing the hydrogen‐bond donating power of the solvent. We found that the experimental log k values for ortho‐, para‐ and meta‐substituted phenyl benzoates in aqueous 50.9% acetonitrile at 25 °C, determined in the present work, precisely coincided with the log k values predicted with the equation (log k^X)_calc = (log k^H_AN)_exp + (Δlog k^X)_calc where the substituent effect (Δlog k^X)_calc was calculated from equation describing the variation of the substituent effect with the solvent electrophilicity parameter, using for aqueous 50.9% CH₃CN the solvent electrophilicity parameter, ΔE = ?5.84. In going from water to aqueous 50.9% CH₃CN, the ortho inductive term grows twice less as compared with the para polar effect. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of ortho substituents on carbonyl carbon 13C NMR chemical shifts in substituted phenyl benzoates

¹³C NMR spectra of 37 ortho‐, meta‐, and para‐substituted phenyl benzoates, containing substituents in benzoyl and phenyl moiety, 4 ortho‐substituted methyl and 5 ethyl benzoates as well as 9 R‐substituted alkyl benzoates have been recorded. The influence of the ortho substituents on the carbonyl carbon ¹³C NMR chemical shift, δ_CO, was found to be described by a linear multiple regression equation containing the inductive, σ_I, resonance, σ°_R, and steric, E, or υ substituent constants. For all the ortho‐substituted esters containing substituents in the acyl part as well as the phenyl part, the substituent‐induced reverse inductive effect (ρ_I < 0), the normal resonance effect (ρ_R > 0), and the negative steric effect (δ_ortho < 0) with the E were observed. In the case of ortho substituents in the phenyl part, the resonance effect was negligible. Due to inductive effect, the ortho electron‐withdrawing substituents showed an upfield shift or shielding of the carbonyl carbon, while the electron‐donating substituents had an opposite effect. Because of the sterical consequences, ortho substituents revealed a deshielding effect on the ¹³C NMR chemical shift of the carbonyl carbon. For all the meta‐ and para‐substituted esters, the reverse substituent‐induced inductive and resonance effects (ρ_I < 0, ρ_R < 0) were found to be significant. In alkyl benzoates, the alkyl substituents showed the reverse inductive and steric effects. The log k values for the alkaline hydrolysis in water, aqueous 0.5 M Bu₄NBr and 2.25 M Bu₄NBr, and the IR frequencies, ν_CO, for the ortho‐, meta‐, and para‐substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding ¹³C NMR substituent chemical shifts, Δδ_CO. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermal behaviour of melamine-modified urea–formaldehyde resins

Thermal behaviour of industrial UF resins modified by low level of melamine was followed by TG-DTA technique on the labsys ^TM instrument Setaram together with the ¹³C NMR analysis of resin structure and testing boards in current production at Estonian particleboard factory Pärnu Plaaditehas AS. DTA curve of UF resin which has been cocondensed during synthesis with even low level of melamine shows the shift of condensation exotherm and water evaporation endotherm to considerable higher temperatures. The effect of melamine monomer introduced to UF resin just before curing was compared. The effect of addition of urea as formaldehyde scavenger was studied.     

The effect of substrate roughness on the static friction of CuO nanowires

The dependence of static friction on surface roughness was measured for copper oxide nanowires on silicon wafers coated with amorphous silicon. The surface roughness of the substrate was varied to different extent by the chemical etching of the substrates. For friction measurements, the nanowires (NWs) were pushed by an atomic-force microscope (AFM) tip at one end of the NW until complete displacement of the NW was achieved. The elastic bending profile of a NW during this manipulation process was used to calculate the ultimate static friction force. A strong dependence of static friction on surface roughness was demonstrated. The real contact area and interfacial shear strength were estimated using a multiple elastic asperity model, which is based on the Derjaguin–Muller–Toporov (DMT) contact mechanics. The model included vertical elastic flexure of NW rested on high asperities due to van der Waals force.     

Basicity of some P1 phosphazenes in water and in aqueous surfactant solution

The pKa values in water and in dilute surfactant solution for 15 ring-substituted phenyl P1 pyrrolidino phosphazenes PhN=P(NC4H8)3 and the phenyl P1 dimethylamino phosphazene PhN=P(NMe2)3 previously studied in acetonitrile (AN) and tetrahydrofuran (THF) are reported. The nonionic surfactant Tween 20 was used for the basicity measurements of some compounds to overcome the solubility problems. Measurements with a control group of phosphazenes in both media were used to validate the use of the obtained pKa values as estimates of aqueous values. The pK(a) values of the studied phosphazenes in aqueous medium vary from 6.82 (2,6-dinitro-) to 12.00 (4-dimethylamino-). The basicity span is 5.18 pKa units. The aqueous pKa values of the P1 phosphazenes were correlated with the respective basicity data in AN and THF and from these correlations the pK(a) values in water for the parent compounds HN=P(NC4H8)3 and HN=P(NMe2)3 were estimated as 13.9 and 13.3. Also a comparison of the basicity of phosphazenes and some guanidines, amines and pyridines was made. In water the parent phosphazenes and guanidines are the strongest of all the groups of bases studied. In AN and THF the parent phosphazenes are clearly the strongest bases followed by guanidines, amines and pyridines which are bracketed between the basicities of phenyl phosphazenes. In the gas phase the phosphazenes for which data are available are clearly more basic than the other compounds referred to here. Comparison of the basicity data of P1 phosphazenes and some guanidines confirms earlier conclusions about the partly ylidic character of the N=P double bond.     

17O NMR studies of ortho‐substituent effects in substituted phenyl tosylates

¹⁷O NMR spectra for 35 ortho‐, para‐, and meta‐substituted phenyl tosylates (phenyl 4‐methylbenzenesulfonates), 4‐CH₃‐C₆H₄SO₂OC₆H₄‐X, at natural abundance in acetonitrile at 50 °C were recorded. The ¹⁷O NMR chemical shifts, δ(¹⁷O), of the sulfonyl (SO₂) and the single‐bonded phenoxy (OPh) oxygens for para and meta derivatives correlated well with dual substituent parameter treatment using the Taft inductive, σ_I, and resonance, σº_R, constants. The influence of ortho substituents on the sulfonyl oxygen and the single‐bonded phenoxy oxygen chemical shifts, δ(¹⁷O), was found to be nicely described by the Charton equation: δ(¹⁷O)_ortho = δ(¹⁷O)_H + ρ_Iσ_I + ρ_Rσ°_R + δE_s^B when the data treatment was performed separately for electron‐donating +R substituents and electron‐attracting ?R substituents. Electron‐attracting meta and para substituents in the phenyl moiety caused deshielding while the electron‐donating meta, para and ortho +R substituents produce shielding effects on the sulfonyl (SO₂) and single‐bonded phenoxy (OPh) oxygens. The influence of ortho inductive and resonance effects in the case of +R substituents was found to be approximately twice higher than the corresponding influence from the para position. Due to the steric effect of ortho substituents a decrease in shielding of the oxygens at the sulfonyl group (δE_s^B > 0, E_s^B < 0) was detected. Copyright © 2012 John Wiley & Sons, Ltd.