Effect of Substituents on Aggregation Behaviors of Stilbazole and Stilbazolium Derivatives

Stilbazole and stilbazolium derivatives with dialkylaniline as electron pushing substituents and pyridine or pyridinium substituents as electron drawing substituents were synthesized and their aggregation behaviors were studied by UV‐visible, fluorescence and surface photovoltage spectroscopies. Experimental results indicate that the formation of aggregates can be mediated by the electron drawing ability of the substituents introduced for those compounds all bearing strong electron pushing substituent. When the electron drawing substituent is pyridine group, the molecules tend to aggregate in head‐to‐tail manner, whereas the substituent is pyridinium group, the molecules tend to aggregate in face‐to‐face manner. The effect of electron pushing and drawing ability of the substituents on the aggregation behavior is further analyzed in terms of frontier orbital interactions.     

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.     

Organocatalyzed Aldol Reaction between Pyridine‐2‐carbaldehydes and α‐Ketoacids: A Straightforward Route towards Indolizidines and Isotetronic Acids

Enantioselective aldol reactions between substituted pyridine carbaldehydes and α‐ketoacids were shown to provide isotetronic acids or their corresponding pyridinium salts, depending on the nature of the substituents on the pyridine ring. The pyridinium salts were generated through nucleophilic attack of the pyridine nitrogen atom onto the reactive keto functional group. Moderate‐to‐good yields of both compounds were typically obtained and high levels of enantioselectivity were observed by using benzimidazole pyrrolidine I as a catalyst. Hydrogenation of the resulting pyridinium salts led to new indolizidines with high ee values and diastereocontrol. X‐ray diffraction studies allowed the determination of the relative configuration of the products. Finally, DFT calculations were performed to rationalize the divergent pathway as a function of the pyridine substituents.     

Multinuclear 1H, 13C and 15N NMR study of some substituted 2-amino-4-nitropyridines and their N-oxides

1H, 13C and 15N NMR chemical shift assignments based on pulsed field gradient selected PFG 1H,X (X = 15C and 15N) HMQC and HMBC experiments are reported for three 4-nitropyridine N-oxides and four 4-nitropyridines. It was found that an ortho effect of a methyl group inhibits the deshielding effect of the 4-nitro group and that this effect and the so-called back donation is influenced by electronegativity and position of substituents in the multisubstituted pyridine N-oxides. The shielding effect of N-oxide group is most pronounced in the 15N NMR chemical shifts of the studied compounds. This effect is further modified by methylamino, methylnitramino, 5- or 3-methyl and 4-nitro groups. Among them the 4-nitro group exerts the highest influence on the shielding effect of the N-oxide functionality. Experimental 1H, 13C and 15N NMR chemical shifts and GIAO/DFT theoretical calculations are consistent with each other and supported by the reactivity on nucleophilic substitution, the UV spectral and the dipole moment data.     

An unprecedented radical ring closure on a pyridine nitrogen

An unprecedented radical ring-closure onto the pyridine nitrogen was observed when certain types of substituents were present around the pyridine nucleus.     

Selective lithiation of 4-(1H-1-pyrrolyl)pyridine. access to new electron-releasing ligands

The first lithiation of 4-(1H-1-pyrrolyl)pyridine has been realized. The use of BuLi-containing lithium aggregates induced the selective pyridine ring functionalization by taking advantage of the electron-donor effect of the pyrrole nucleus. Opportune substituents were introduced alpha to the pyridine nitrogen leading to new electron-enriched pyridylphosphine, bipyridine, and terpyridine ligands.     

Crystal solvates of carboxy-substituted Zn(II) phthalocyaninates with pyridine

Physicochemical characteristics (composition, energetic and chemical stability) of the molecular complexes of carboxy-substituted Zn(II) phthalocyaninates with pyridine were determined. It was found that the carboxyl substituents in positions 4 and 5 of the complex Zn(4,5-COOH)₈Pc favor the formation of the most stable molecular complexes with pyridine. The carboxyl substituents in the composition of Zn(3-COOH)₄Pc are not solvated with pyridine due to steric hindrances and the formation of the hydrogen bond between the carboxyl H atom and the meso-nitrogen atom; the molecular complexes of a tetrasubstituted Zn(4-COOH)₄Pc with pyridine are unstable.     

Structure and nitrogen basicity of pyridine metal extractants

The effect of various substituents in pyridine derivatives upon the basicity of the nitrogen was discussed. The dissociation constants were correlated with the Hammett parameters and compared with copper extraction data. The analysis of nitrogen basicity in pyridine derivatives permits to select possible structures of extractants for metal extraction from chloride solutions.     

Electronic properties of furyl substituents at phosphorus and their influence on 31P NMR chemical shifts

The electronic properties of 2-furyl and 3-furyl substituents attached to phosphanes and phosphonium salts were studied by means of IR spectroscopy and experimental and computational (31)P NMR spectroscopy. The heteroaromatic systems proved to be electron withdrawing with respect to phenyl substituents. However, phosphorus atoms with attached furyl substituents are strongly shielded in NMR. The reason for this phenomenon was studied by solid state (31)P MAS NMR experiments. The chemical shift tensor was extracted, and the orientation within the molecules was determined. The tensor component sigma(33), which is effected the most by furyl systems, is oriented perpendicular to the P-C bonds of the substituents. P-furyl bonds are shorter than P-phenyl bonds. We assume therefore a lower ground-state energy of the molecules, because of the electron withdrawing properties of the 2-furyl systems. The sigma(para) component of the (31)P NMR magnetic shielding is therefore smaller, which results in an overall increase of the magnetic shielding.     

29Si NMR Shielding Tensors in Triphenylsilanes – 29Si Solid State NMR Experiments and DFT‐IGLO Calculations

²⁹Si NMR shielding tensors of a series of triphenylsilanes Ph₃SiR with R = Ph, Me, F, Cl, Br, OH, OMe, SH, NH₂, SiPh₃, C≡CPh were determined from ²⁹Si CP/MAS spectra recorded at low spinning rates. In addition the principal components of the shielding tensor were calculated employing the DFT‐IGLO method. For most silanes experimental and calculated values are in good accordance. Larger differences were observed for systems with hydrogen bridge forming substituents and the halides bromide and chloride. In some of the spectra the shielding information interfered with residual dipolar couplings. The different contributions of the various substituents to the principal components of the shielding tensor and the orientation of the tensor within the molecules are discussed and compared for the compounds under investigation.     

Porphyrin-based organogels: control of the aggregation mode by a pyridine-carboxylic acid interaction

To find a new strategy for designing porphyrin-based organogelators, hydrogen-bond-donating (carboxylic acid)/accepting (pyridine) substituents or electron-donating (dialkylamino)/withdrawing (pyridine) substituents were introduced into peripheral positions of a porphyrin (1cp or 1ep, respectively), and the gelation properties were compared with those of symmetrical reference compounds bearing two pyridyl substituents or two ester groups (1pp or 1ee, respectively). It was found that the symmetrical molecules show a solubility that is quite inferior to that of 1cp and 1ep having a dipole moment and precipitate from most organic solvents. 1cp, 1ep, and 1ee formed gels with cyclohexane, methylcyclohexane, and several alcoholic solvents, but scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that their superstructures constructed in the organogels are very different. In cyclohexane, 1cp resulted in a sheetlike structure, whereas 1ep and 1ee resulted in a fiberlike structure. The difference is attributed to the two-dimensional interactive forces in 1cp consisting of the porphyrin-porphyrin pi-pi stacking and the carboxylic acid-pyridine hydrogen bonding. In fact, when the hydrogen-bonding interaction was weakened by alcoholic solvents or by adding pyridine or N,N-(dimethylamino)pyridine, the sheetlike structure was transfigured to the fiberlike structure. Further detailed analyses of their aggregation modes were conducted by spectroscopic methods such as ultraviolet-visible (UV-vis) absorption, Fourier transform infrared (FT-IR), and X-ray diffraction (XRD). On the basis of these findings, the influence of these peripheral substituents on the gel formation and the aggregation mode was discussed.     

A theoretical investigation on the effect of π–π stacking interaction on 1H isotropic chemical shielding in certain homo- and hetero-nuclear aromatic systems

Significant changes in the proton chemical shielding (and hence the chemical shift) are predicted in going from the monomer to the dimer of benzene, naphthalene, pyridine and quinoline systems and also the trimer of benzene and pyridine. The computed NMR spectra show additional splitting in going from the monomer to the dimer and the trimer of different species. The aromatic protons show a significant upfield shift due to the enhancement of anisotropic shielding by the π electron cloud of the neighboring molecule(s). The nature of the NMR spectra also changes with the orientation of the stacked conformers. The results obtained using M?ller-Plesset second-order perturbation theory along with the GIAO method show the changes in isotropic shielding, in a reasonable basis set independent fashion.     

Interactions of different heterocyclic compounds with monoionic forms of montmorillonite

Interactions of 3-R-and 2-R pyridine (R=CH₃, Cl, NH₂) with Ni(II)-exchanged montmorillonite have been studied. Thermal and X-ray analyses indicate that pyridine derivatives are intercalated into the interlayer spaces of montmorillonite. Infrared spectral data shown that the Lewis and/or Brönsted type of interactions of pyridine derivatives is connected with different steric and inductive effects of the substituents (R) on the pyridine ring. The alkylpyridines increase the electron density on the donor nitrogen atom and support the coordination to the central atom. The halogen substituents have a negative inductive effect (–I), so that those ligands show a lower basicity and weaker σ-bonding properties than pyridine and also the lower possibility of the coordination.     

13C NMR shifts and conformations of substituted indans

¹³C NMR spectra of indan derivatives bearing substituents in the 1, 2, 5 and 6 positions are reported and assigned by LIS measurements and other techniques. Epimeric indanes bearing vicinal oxygen and phenyl or benzyl substituents show ring carbon shielding in the cis relative to the trans isomers, which is compared with corresponding cyclopentane shifts, and indicates the predominance of envelope conformations with pseudoaxial oxygen substituents for the cis isomers. Acetylation shifts show consistently larger shielding at C-β for the trans compounds. Introduction of oxygen at C-5 leads to asymmetric shielding effects at the ortho carbon atoms as soon as there is a substituent in the para position which can participate in mesomeric forms.     

New Heterocycles with a 3-Aminofurazanyl Substituent

New 3-aminofurazans containing 1,2,4- and 1,3,4-oxadiazole, pyridine, and 1,2,4-triazole substituents in the 4-position were synthesized.     

19F and199Hg NMR spectra of certain fluorine-containing arylmercury derivatives of acetylenes

Conclusions ¹⁹F and¹⁹⁹Hg NMR have been used to investigate the comparative influence of electronic effects of the substituents on the shielding of¹⁹F and¹⁹⁹Hg nuclei in ArHgC-CC₆H₄F-4 and 4-FC₆H₄HgC=CR. It has been found that the shielding of the fluorine decreases and the shielding of the mercury increases with increasing electron-acceptor properties of the Ar or R, or when the change is made from toluene to pyridine. The¹⁹F and¹⁹⁹Hg chemical shifts correlate well with the and ⁰ Hammett constants.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1524–1529, July, 1985.     

Syntheses of shuttlecock- and bowl-equipped phenylazopyridines and photomodulation of their coordination ability to Zn-porphyrin

Shuttlecock- and bowl-equipped 4-(phenylazo)pyridine derivatives, which bear substituents that allow the pyridine moiety to protrude in the trans form but hinder it in the cis form, have been designed and synthesized. These molecules show cis/trans photoisomerization despite the presence of bulky substituents. ¹H NMR titration with Zn-porphyrin showed that the trans isomers coordinate to Zn-porphyrin much stronger than the cis isomers.     

A base promoted cyclization of N-propargylaminopyridines. Synthesis of imidazo[1,2-a]pyridine derivatives

A base promoted cyclization of the protected N-propargylaminopyridines was shown to be an efficient method for the preparation of imidazo[1,2-a]pyridine derivatives. The reactions were carried out with a small excess of base, at room temperature or slightly above producing the heterocyclic products in moderate to good yields. The stereoelectronic properties of substituents on the pyridine ring were shown to influence the cyclization process.     

Exploring the Palladium? and Platinum?Bis(pyridine) Complex Motif by NMR Spectroscopy,X‐ray Crystallography, (Tandem) Mass Spectrometry,and Isothermal Titration Calorimetry: Do Substituent Effects Follow Chemical Intuition?

A series of ten palladium? bis(pyridine) complexes, as well as their corresponding platinum complexes, have been synthesized. The pyridine ligands in each series carried different σ‐donor and/or π‐acceptor/donor substituents at the para‐position of their pyridine rings. These complexes were analysed by NMR spectroscopy, X‐ray crystallography, (tandem) MS, and isothermal titration calorimetry (ITC) to validate whether these methods allowed us to obtain a concise and systematic picture of the relative and absolute thermodynamic stabilities of the complexes, as determined by the electronic effects of the substituents. Interestingly, the NMR spectroscopic data hardly correlated with the expected substituent effects but the heteronuclear platinum? phosphorus coupling constants did. Crystallographic data were found to be blurred by packing effects. Instead, tandem MS and ITC data were in line with each other and followed the expected trends.     

Influence of substituents on the through-space shielding of aromatic rings

A series of naphthalene derivatives, bearing a methyl group and a substituted phenyl ring in a 1,8-relationship, have been synthesized. The chemical shifts of the protons of the methyl group, which are pointed toward the shielding zone of the phenyl ring, were monitored as the phenyl substituents were varied. This work indicates that the shielding effect of the phenyl ring is not so severely altered by the substituents as to significantly influence the chemical shift of the methyl group. Nonetheless, within the small changes observed experimentally, there appears to be a tendency for electron-withdrawing X to shift the methyl signal downfield, whereas electron-donating X-groups cause a more upfield shift. Polarization and field effects are discussed as possible causes for this phenomenon. Chemical shifts computed for selected members of the series, using the recently published procedures of Rablen and Bally, are in agreement with the experimentally observed trends.