Acid–base and coordination properties of <Emphasis Type="Italic">Meso</Emphasis>-substituted porphyrins in nonaqueous solutions

Acid–base and coordination properties of alkyl and aryl meso-substituted porphyrins are studied spectrophotometrically in nonaqueous solutions. It is found that the nature of the substituent greatly affects the basicity of ligands for porphyrins characterized by a flat structure of macrocycle. The electronic effects of substituents have a much weaker influence on the kinetics of complexing. These effects could be due to the opposite orientation of some factors: an increase in the basicity and stability of the N–H bonds of porphyrin reaction centers. Dissociation constants pK _b of the cationic forms of meso-substituted derivatives of porphyrin are measured. The values of pK _b are in good agreement with classic concepts of the nature of substituents, particularly those indirectly included in the macrocycle through phenyl buffer rings.     

The kinetics of radical polymerization—XXV: Study of the radical reactivity of substituted nitrobenzenes—I

A study was made on the effect of some para- and meta-substituted nitrobenzenes on the radical polymerization of vinyl acetate. It has been shown that −CH₂X and −SO₂X type substituents have merely an inductive effect on the reactivity of nitro groups. It has also been established that the inductive effect of substituents is more intensive when they are in the para position than when in the meta position, i.e.: λ = 1·15.     

Displacement of neutral nitrogen donors by chloride in [Pt(SNS)(R-py)](ClO4)2 (SNS = 2,6-bis(methylthiomethyl)pyridine; R-py = pyridines): A reactivity comparison between meta- and para-substituted pyridines

The kinetics of the process [Pt(SNS)(R-py)]²⁺ + Cl⁻ → [Pt(SNS)Cl]⁺ + R-py {SNS = 2,6-bis(methylsulfanylmethyl)pyridine; R-py = meta- or para-substituted pyridines covering a wide range of basicity} were studied in methanol at 25 °C. The reactions obey the usual two-term rate law observed in the substitution reactions of square-planar d⁸ complexes. The plots of log k₂ {k₂ = second-order rate constants} against the pK_a of the heterocycles conjugate acids highlighted a different sensitivity of the two groups of N-donors to changes in basicity, thepara-substituted pyridines (4R-py) showing a weaker dependence on pK_a than the meta-substituted (3R-py). The results have been explained on the basis of a π-acidity difference between 3R-py and 4R-py which influences the reaction ground state.     

Substituent effects of phosphonate groups electronic repartition of π-conjugated ferrocene analogues of stilbene

The synthesis of para-substituted ferrocene analogues of stilbene was performed by using the Heck reaction, starting from vinylferrocene. The variation of the electronic density of these compounds with the electronic withdrawing strength of the substituents was studied using ¹³C NMR spectroscopy, absorption spectra and cyclic voltammetry. The correlation of Hammett constants with the redox properties of the substituted compounds using Nagy's method allowed us to revisit the determination of the Hammett constants of diethyl phosphonate ester and phosphonic acid substituents. Our measurements were in agreement with the literature except for the diethyl phosphonate group.     

The thermodynamic studies of the molecular interactions of methyltin(IV) tribromide with free base meso-tetraarylporphyrins

The thermodynamic parameters for the interactions of MeSnBr₃ with para- and meta-substituted meso-tetraphenylporphyrins (H₂T(4-X)PP; X=OCH₃, CH₃, H, Cl, NO₂ and H₂T(3-X)PP; X=CH₃, Cl) have been studied. The formation constants have been elucidated by using spectrophotometric titration and computer squad program data refinement. The adducts of composition 2:1 and 1:1 of MeSnBr₃ to H₂T(4-X)PP with stability constants K₁ and K₂ coexist in solution, but meta-substituted porphyrins form adducts of the composition 1:1 of MeSnBr₃ to H₂T(3-X)PP with stability constant K₁. Formation constants decrease with decreasing electron donation of para-substituted porphyrins as follow: H₂T(4-CH₃O)PP>H₂T(4-CH₃)PP>H₂TPP>H₂T(4-Cl)PP>H₂T(4-NO₂)PP. Despite the electron donation of the methyl group, H₂T(3-Cl)PP forms more stable adducts than H₂T(3-CH₃)PP with MeSnBr₃.     

Synthesis and Characterization of Push-Pull Porphyrins

A series of “push-pull” porphyrins with 4-nitrophenyl and 4-aminophenyl substituents were synthesized and separated by flash column chromatographic techniques. They were fully characterized by elemental analysis, FAB-MS, FTIR, UV-visible, and ¹H NMR spectroscopies. The unsymmetrical π-electron distribution of the porphyrins caused by the donor (amino) and acceptor (nitro) substituents were investigated by ¹H NMR technique. The pyrrole-H resonance positions can be correlated to the Hammett σ constants of the substituents. Although with strong donor and acceptor substituents, UV-visible spectra show the push-pull porphyrins have rather weak solvatochromism and hence limited intramolecular charge-transfer character.     

Synthesis and spectroscopic characterization of meso-tetraarylporphyrins with fused phenanthrene rings

A series of meso-tetraaryl porphyrins with fused phenanthrene rings have been synthesized from boron trifluoride-catalyzed Lindsey condensation of phenanthro[9,10-c]pyrrole with various para-substituted arylaldehydes at low temperature. Their structures were characterized by UV-vis, ¹H NMR, and mass spectroscopies. The UV-vis spectra of these compounds showed remarkable bathochromic shift of the Soret band to the wavelength around 577 nm and Q-bands into the near-infrared region.     

THE EFFECTS OF PERIPHERAL SUBSTITUENTS ON THE KINETICS OF ZINC ION INCORPORATION AND ACID CATALYZED REMOVAL FROM WATER SOLUBLE SULFONATED PORPHYRINS

Abstract

The kinetics of Zn²⁺ and Zn(OH)⁺ incorporation into and the kinetics of the acid catalyzed removal of Zn(II) from twelve water-soluble, sulfonated derivatives of tetraphenylporphyrin with alkyl or halogen groups in the para, ortho or di-ortho positions were investigated. While the incorporation reactions showed little dependence on porphyrin basicity, the Zn-P (P = porphyrin derivative) acid solvolysis reactions were faster the higher the basicity of the free base (H₂-P) compound. Equilibrium constants for the formation of cadmium porphyrins decreased with an increase in porphyrin basicity. The predeformed tetrakis(4-sulfonatophenyl)-β-octabromo-porphyrin reacted with Zn²⁺ about 10³ times faster than porphyrins of similar basicity. These results indicate how substituents on the phenyl and beta-pyrrole rings influence the solution chemistry of water soluble porphyrins.     

Quantum chemical studies on para-substituted styrenyl fullerene. Study of substitution effects on structural and electronic properties

Quantum chemical investigations on para-substituted styrenyl fullerene derivatives with substituents linked to para position of styrenyl group have been carried out. The density functional theory (DFT) have been used in this study with B3LYP functional and the modest 3-21G* basis sets. The substituents were variable donating and withdrawing to study the effect of such structural change on the structural and electronic properties of the studied molecules.     

Use of two-parameter correlations for studying the radical reactivity of aromatic compounds

Two-parameter equation correlations are reported for radical reactions of aromatic compounds. In these correlations polar and resonance substituent constants identical with the substituent constants of aliphatic compounds were used. The equations correlate the rate constants for H abstraction reactions and for the addition of a variety of free radicals to the ortho-, meta-, and para-substituted aromatic compounds. Besides, they correlate parameters of the spectra for substituted aromatic radicals. The correlations show that the effects of para substituents on the reactions studied are nearly entirely resonance effects, whereas for the meta- and ortho-substituted compounds polar (inductive) effects become essential. Application of the two-parameter correlations permits also to determine the structure of transition states (σ or π-complex) in free-radical reactions.     

Chelate von Chinolin-8-ol-Derivaten. VIII. Säure- und Komplexstabilitätskonstanten alkyl- bzw. alkenylsubstituierter Chinolin-8-ole

Chelates of 8-Quinolinol Derivatives. VIII. Acid and Complex Stability Constants of Alkyl and Alkenyl Substituted 8-Quinolinols For a series of 7-substituted alkyl and alkenyl 8-quinolinols as well as 2-, 5-, and 7-methyl-8-quinolinols the acid constants and the stability constants of the nickel and zinc chelates were determined potentiometrically in 75 vol-% dioxane-water mixture. Methyl groups, as expected, increase the basicity of both donor atoms of the 8-quinolinols, whereas alkyl substituents in 7-position cause a decrease in basicity of the quinoline N-atom. The influence of substituents on the complex stability is only small.     

13C-NMR-Spektren von γ-substituierten pentamethincyanin-farbstoffen mit indolenin-endgruppen

The ¹³C-resonances of fourteen γ-substituted pentamethincyanine dyes 1 with indolenine nuclei have been measured and assigned. The effect of the γ-substituents R on the ¹³C chemical shifts is compared with the corresponding effect of substituents R of monosubstituted benzenes and 1-substituted trans-butadienes. The chemical shifts of the C²-methincarbons, which are separated by four bonds from R, correlate with Hammett's substituent parameters σ_para. A significant substituent effect through nine covalent bonds has been observed.     

A correlation of substituent effects with proton chemical shifts in aromatic tetrazolic acids

The effect of substituents on the proton chemical shifts and spin–spin coupling constants in ortho-, meta- and para-substituted 5-phenyltetrazoles (tetrazolic acids) in DMSO–CH₃CN (1:1, v/v) was studied. With the meta- and para- substituted compounds the additivity rule of chemical shifts was obeyed, thereby enabling increments characterizing the effects of individual substituents in monosubstituted benzenes to be determined. By employing the Smith and Proulx equation, the chemical shifts of the aromatic protons were correlated with the F, R and Q substituent constants. The values of these constants are 1.02, ?0.004 and 5.49, respectively, for the tetrazolyl substituent.     

Synthesis of some multi-β-substituted cationic porphyrins and studies on their interaction with DNA

A series of multi-β-substituted cationic porphyrins, 2,3,12,13-tetraphenyl-5-(N-trimethyl-4-ammoniumphenyl)-10,15,20-triphenylporphyrin 2; 2,3,12,13-tetramethyl-5-(N-trimethyl-4-ammoniumphenyl)-10,15,20-triphenylporphyrin 3; 2,3,7,8,12,13,17,18-octaphenyl-5-(N-trimethyl-4-ammoniumphenyl)-10,15,20-triphenylporphyrin 4, and 2,3,7,8,12,13,17,18-octaphenyl-5,10-di(N-trimethyl-4-ammoniumphenyl)-15,20-diphenylporphyrin 5, have been synthesized. Their photooxidative abilities and interaction with DNA were investigated by UV, fluorescence, CD, and gel electrophoresis. It is found that substituents at β-position of the porphyrins have significant effect on interactions and binding mode of the porphyrins with DNA. Increasing positive charges in the porphyrins strengthen their interactions with DNA.     

Composites based on phenyl substituted cobalt porphyrins with Nafion as catalysts for oxygen electroreduction

The electrochemical behavior of composite materials based on phenyl substituted cobalt porphyrins and Nafion is studied. Several cobalt porphyrins with presumably predictable variation of their hydrophilic/hydrophobic properties due to different donor and acceptor substituents in the para position of phenyl rings are synthesized and studied. It is shown that introduction of Nafion into a system with acceptor substituents results in a significant acceleration of the model oxygen reduction reaction. This allows assuming that a bond between a proton of the Nafion sulfogroup with the porphyrin active center is most probable in this very group of porphyrins, which facilitates the protonation step required for activation of the oxygen molecule. A certain correlation is found between the model reaction of oxygen electroreduction (halfwave potential, reaction rate constant) and Hammett constant varying as dependent on the nature of peripheric substituents.     

Photolytic decomposition of dibenzylic sulfites

The photolytic decay of a library of para-substituted dibenzylic sulfites has been evaluated by UV radiation in a Srinivasan-Griffin-Rayonet photochemical reactor in various deuterated solvents. The decay for each dibenzylic sulfite was examined with respect to Swain and Lupton’s field constant, F. The rate of photolytic decay varies depending on the identity of the benzyl substituents. Furthermore, it has been observed that the solvent affects both the rate of sulfite photolytic decay as well as final product distribution.     

Nucleophilicity and Basicity in the Quaternization of 4-Substituted Quinuclidines. Polar effects. Part III

The second order rate constants (k) for quaternization of thirty 4-substituted quinuclidines ( 1 ) by methyl iodide in methanol have been measured. The effects of the substituents on rate are relatively small and indicate an early transition state with little charge separation. They also indicate less involvement of the nitrogen atom, i.e. a looser transition state, for quaternization by allyl halides than for the less reactive propyl halides. The log k values of 4-substituted quinuclidines correlate moderately well with their pK_a values, the main reason being the imperfect correlation between nucleophilicity and basicity. The assumption that substituents exert equal effects on rates and equilibria is therefore only partly justified. Certain irregularities in the correlation can be attributed to a reduction of the nucleophilicity of nitrogen in 1 due to hydrogen bonding to the solvent methanol. Furthermore, quaternization is faster and responds more strongly to substituent effects in the aprotic solvent acetonitrile.     

Solvent effect on thermal degradation of plasticized para-substituted polystyrenes

The thermal effect on stability of a series of para-substituted polystyrenes with methyl, methoxy and α-methyl substituents in various solvents was studied in the temperature range of 298-363 K. They gave a monomer fluorescence as a minor part and excimer fluorescence as a major part. Thermal heating of para-substituted polystyrenes shows a decrease in both monomer and excimer fluorescences in all used solvents. Thermal heating causes a small fluorescence quenching effect at lower temperatures in solution but becomes very dominant at higher temperatures. Added terephthalate and phthalate plasticizers to these para-substituted polystyrenes caused a quenching of both monomer and excimer fluorescences without the formation of exciplex emission. The thermal quenching processes of the plasticized polymers were accompanied by a change in the structure of the fluorescence spectra at high heating temperatures. This may indicate that thermodestruction of these polymers starts from a random chain scission. The change in solvent polarity has considerable effect on fluorescence quenching but it has a minor effect on the thermal degradation of these polymers. The binding energies for excimer formation were calculated in the used solvents.     

Carbon Participation in the Solvolysis of 6-endo-Substituted 2-exo-Norbornyl Toluenesulfonates. Norbornanes part 6

The solvolysis rate constants k for the 6-endo-substituted 2-exo-norbornyl toluenesulfonates 7 have been determined. Values of logk correlate well with the respective inductive constants of the substitutents except when the latter are nucleophilic and therefore lead to endo-cyclization, or when they are n-electron donors and cause concerted fragmentation. In general 6-endo-substituted tosylates 7 react somewhat more slowly than their 6-exo-epimers. Identical or different mixtures were obtained from the C(6)-epimers 7 and 1 depending on whether the substituent was an electron donor or acceptor. It is concluded that donor substituents at C(6) enhance 1,3-bridging in the intermediate epimeric cations and lead to their rapid and complete equilibration, and that electron acceptors reduce bridging and hence their equilibration rates.     

Manganese triacetate oxidation of methyl 1-hydroxy-2-naphthalene carboxylates

Manganese-triacetate mediated oxidation of 1-hydroxy-2-napthalene carboxylates in benzene under anhydrous conditions delivers the dimerized product. However, acetoxylation on the ortho- or para-position, or oxidation to quinones occurs on the 1-hydroxy-3-substituted 2-napthalene carboxylates depending on the nature of the substituents when the reaction is carried out in a mixture of acetic acid/acetonitrile.