Specifics of the photolysis of alkylated hydroxy-1,2-dihydroquinolines in water and methanol

Comparative study of photoinduced reactions of 6-and 8-hydroxy-2,2,4-trimethyl-1,2-dihydroquinolines (6-OH-DHQ and 8-OH-DHQ) in water and methanol was carried out by continuous and pulse photolysis. The products of MeOH addition to the dihydroquinolines under study were isolated and identified. The spectral and kinetic parameters of active transient species generated in the photolysis were measured. The rate constants of the transient carbocations strongly depend on the position of the hydroxy group on the aromatic ring and are an order of magnitude lower for 6-OH-DHQ than 8-OH-DHQ (80 versus 870 s^?1, respectively). Two factors that play a significant part in the photolysis mechanism were revealed, namely, an increase in the acidity of the phenolic group in the excited state and the possibility of intramolecular hydrogen bonding between phenolic and amino groups in 8-OH-DHQ, which are in close proximity. The former factor causes the formation of by-products from the deprotonated excited state of OH-DHQ, which increases substantially in water. The latter affects the structure and the composition of transient species in the case of 8-OH-DHQ.     

Reactivity of the carbocation generated in the photolysis of 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline toward azide ion

The reaction of the azide ion with the carbocation generated in the photolysis of 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline in methanol was studied by pulse (conventional and laser) and steady-state photolysis techniques. The adduct of the azide ion was characterized by ¹H NMR spectrum. Experimental results were interpreted taking into account a competition between the addition of methanol and azide ion to the carbocation. The rate constants for the reaction of the azide ion with the carbocation (k _Az) were measured at 2—48 °C in a wide range of [N₃ ^–]₀ concentrations from 2·10^–7 to 0.1 mol L^–1 at different ionic strengths () of the solution. The resulting k _Az values are more than an order of magnitude lower than those for diffusional-controlled reactions and vary from 3.2·10⁸ ( = 0) to 4.5·10⁶ L mol^–1 s^–1 ( = 0.8 mol L^–1) in the presence of NaClO₄ (18 °C). The activation energy of addition of the azide ion to the carbocation is 21 kJ mol^–1, which is by 12 kJ mol^–1 lower than the activation energy of the reaction of the carbocation with methanol. The features of the reaction under study are discussed from the viewpoint of the structures of carbocations generated in the photolysis of dihydroquinolines.     

Laser flash photolysis study of the decay kinetics of carbocations of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in the porous glass in the presence of methanol

The efficiency of formation and the decay kinetics of carbocations formed under the photolysis of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in methanol and in a porous glass filled with methanol or dried in air or in vacuo were studied by the laser flash photolysis techniques. In MeOH, the carbocations recombine via the second-order law in the reaction with the MeO⁻ anion formed in an equimolar amount and decay via the first-order law in the reaction with the solvent with rate constants of 3·10⁸ L mol⁻¹ s^{− 1} and 1.4·10³ s⁻¹, respectively. When the solution is placed into the porous glass, no recombination of the carbocations with MeO⁻ is observed, and the reaction with the solvent is somewhat inhibited (rate constant 8·10² s⁻¹). More than tenfold inhibition of the reaction of the carbocations with methanol is observed on going to a monolayer of MeOH on the surface. The main route of carbocation decay in the porous glass dried in vacuo is the geminate recombination with the SiO groups. The corresponding kinetics is described in terms of the model of freely diffusing reactants. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2239–2243, October, 2005.     

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λ_pump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λ_max = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.     

Effect of the solvent composition in methanol-pentane and methanol-acetonitrile mixtures on the spectral and luminescent properties of 1,2-dihydroquinolines

The dependence of absorption and fluorescence spectra, quantum yields, and lifetimes of fluorescence on the solvent composition in the MeOH-C₅H₁₂ and MeOH-MeCN mixtures was studied for 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMDHQ). The variations in the parameters of deconvolution of the absorption and fluorescence spectra by the Gaussian functions in the MeOH-C₅H₁₂ mixtures of various compositions indicate the specificity of methanol clustering in saturated hydrocarbons and hydrogen bonding between TMDHQ and the methanol clusters of different compositions. At low MeOH concentrations (∼0.2 vol %), TMDHQ molecules are practically completely bound with the MeOH molecules by hydrogen bonds. In the MeOH-MeCN mixtures, the changes in the absorption and fluorescence spectra are observed at a substantially higher MeOH concentration (≥10 vol %) and monotonically change at the further increase in the MeOH concentration that is caused by the peculiarities of MeOH clustering in acetonitrile and the distribution of the TMDHQ molecules between the solvent components. At 50–95 vol % of MeOH in the mixture with MeCN, the fluorescence decay kinetics is described by the biexponential curve with the lifetime of the major component (τ₁) decreasing from 7.5 to 1.1 ns in pure MeCN and MeOH, respectively, and the lifetime of the minor component τ₂ ≈ 4 ns corresponding to the fluorescence lifetime in the solution containing 50 vol % MeOH. This indicates the existence of the free TMDHQ molecules, which are not bound with MeOH molecules or their clusters.     

Effect of solvent nature on the photoreduction kinetics of substituted benzoquinones

Effect of the solvent nature on the kinetics of photoreduction of substituted benzoquinones in the presence of hydrogen donors has been studied. It has been found that the effective photoreduction rate constant (k_H) for quinones decreases with an increase in solvent polarity. For the 3,6-di-tert-butylbenzoquinone–1,2-N,N-dimethylaniline pair, the dependence of ln k_H on the difference of the reciprocals of optical and static solvent permitivities (1/ε _∞–1/ε₀) is stepwise with a break point corresponding to CH₂Cl₂. A similar relationship lnk_H = f(1/ε _∞–1/ε₀) is observed for the p-chloranil–mesitylene pair. In the study of the photoreduction kinetics for a series of seven o-benzoquinones in the presence of p-derivatives of N,N-dimethylaniline in CH₂Cl₂, it has been found that the dependence of k_H on the free energy of electron transfer (ΔG_e) has a maximum for the 3,6-di-tert-butylquinone-1,2–N,N-dimethylaniline pair at ΔG_e = 0.11 eV.     

Effect of pH on kinetics of the decay of the radicals formed during photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in aqueous and micellar solutions

Kinetics of the decay of the transient radicals formed from 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMQ) in aqueous and micellar solutions of sodium dodecyl sulfate were studied by flash photolysis as a function of pH. In aqueous and micellar solutions of TMQ the mechanism of the decay of the transient species and the reaction products are different from those in homogeneous organic solutions. The decay of the transient radicals follows first-order kinetics in the entire range of pH under consideration in both aqueous and micellar solutions. In aqueous solutions at pH 9–12, the decay rate constant decreases from 25.3 to 3.7 s⁻¹. In micellar solutions at different pH, different types of micellar catalysis were observed. At pH 1, the rate constant in a micellar solution is slightly lower than that in an aqueous solution. At pH 3–11, the decay rate constant increases (positive micellar catalysis). The apparent rate constant depends linearly on the concentration of TMQ in micelles. The rate constant for the reaction of the transient radical cation with TMQ was determined (200 L mol⁻¹ s⁻¹). At pH>13, the decay rate constant in micelar solutions is lower than that in aqueous solutions (negative micellar catalysis). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 703–708, April, 1997.     

Reactivity of carbocations from 1,2-dihydroquinolines in binary mixtures of methanol with pentane and acetonitrile

The rate constants and activation parameters of the reactions of the carbocation resulting from 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline photolysis with methanol (k ₁) and the methoxide ion (k ₂) have been measured by flash photolysis in binary mixtures of methanol with inert solvents (nonpolar pentane and polar acetonitrile) in wide composition ranges. The changes in the activation parameters for k ₁ at different solvent compositions show that the increase in the rate constant in the pentane mixtures is mainly deter-mined by the increase in the preexponential factor. The decrease in k ₁ in the acetonitrile mixtures is deter-mined by the decrease in the methanol concentration and by the increase in the activation energy. The different roles of the methoxide ion in the reaction are demonstrated. They depend on the nature of the inert solvent in the mixture. The results of this study are considered in terms of methanol clustering in pentane and acetonitrile, the different solubilities of 6-ethoxy-1,2,2,4-tetramethyl-1,2-dihydroquinoline in the components of the binary mixtures, and the difference in distribution and solvation between the carbocation and the methoxide ion in the mixtures.     

Role of steric hindrance in photoinduced proton transfer from solvent to excited molecules of 1,2-dihydroquinolines

It was shown that the photolysis of 1,2,6-trimethyl-1,2-dihydroquinoline (126TMDHQ) in water, methanol, ethanol, and isopropanol affords the corresponding adducts of water and the alcohols, unlike the case of 2,2,4-trimethyl-1,2-dihydroquinolines bearing the methyl, alkoxyl, and hydroxyl substituents in the 1-, 6-, and 8-positions, which were previously found to form adducts only in the presence of water and MeOH. The quantum yield of the 126TMDQ photolysis (Φ) in this solvent series changes as Φ_MeOH:Φ_EtOH:Φ_PrOH = 10:3:1. The results were rationalized in terms of the effect of steric hindrance caused by substituents on the heterocycle and increasing size of the alcohol alkyl group on proton transfer from the solvent to the 1,2-dihydroquinoline molecule in the excited singlet state. The existence of two adduct isomers was revealed. The preferential formation of one of the isomers was considered from the standpoint of carbocation accessibility to the solvent by nucleophilic attack.     

The effect of the nature of the surfactant on the location of 6-R-2,2,4-trimethyl-1,2-dihydroquinolines in micelles

The locus of solubilization of 6-R-2,2,4-trimethyl-1,2-dihydroquinoline molecules (R=Me, OEt) in sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) has been determined by comparing the UV spectra of micellar solutions of the dihydroquinolines and their solutions in solvents of various polarities. The parameterR _pv (defined as the ratio of the absorbance of the long-wave band maximum to that at the adjacent valley) decreases with an increase in the solvent polarity in the order:n-heptane > 2-propanol > ethanol > H₂O. In SDS micellar solutions,R _pv is close to the corresponding value in water and does not depend on [SDS]. In CTAB micellar solutions,R _pv is essentially greater than in water and increases with [CTAB]. Thus, the solubilized dihydroquinoline molecules in SDS micelles reside in the Stern layer, and in CTAB micelles they are located both in the interior of the micelle and in the Stern layer; in this case the micelle packing begins from the core.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 948–950, May, 1994.     

The scaled-particle theory and solvent isotope effects on the thermodynamic properties of inert solutes in water and methanol

The scaled-particle theory has been applied to the calculation of the thermodynamic changes associated with the formation of a cavity in several isotopic varieties of liquid water and methanol. From these results, the thermodynamic functions for the transfer of a cavity (or a hard-sphere solute) have been computed for the following solvent pairs: H₂OD₂O, H₂OH₂ ¹⁸O, H₂ ¹⁸OD₂ ¹⁸O, D₂OD₂ ¹⁸O, CH₃OHCH₃OD. For the last two of these solvents, density measurements required for the calculations were carried out as a function of temperature. The calculated deuterium solvent isotope effect on the heats and entropies of hard-sphere solutes in water is much greater than the¹⁸O isotope effect; the former also exhibits a more pronounced temperature dependence. The transfer functions computed for hard-sphere solutes are compared to experimental data on the transfer of various solutes from H₂O to D₂O and from CH₃OH to CH₃OD. In most of the cases examined, the cavity effect accounts for a large part of the transfer quantities measured for rare gases, hydrocarbons, and solutes containing a significant hydrocarbon substituent.     

Acid-base properties of N-methyl-substituted 1,2-dihydroquinolines in the ground and excited states

Changes in the absorption and fluorescence spectra of 1,2,2,3-tetramethyl-(1), 1,2,2,4-tetramethyl-(2), 6-ethoxy-1,2,2,4-tetramethyl-(3), and 1,2,6-trimethyl-1,2-dihydroquinolines (4) were studied in aqueous solution over a wide pH range from 1.0 to 12.0. The quantum yields of fluorescence and the values of pK _a of dihydroquinolines (DHQs) under study in the ground and excited states were determined, pK _a = 4.5, 3.8, 4.5, and 4.2 for the ground state of compounds 1–4, respectively, and pK _a ∼ 1.7 for the S ₁^* state for all DHQs.     

Effect of solvent on the methanol JHCOH value

³J_HCOH, which depends on solvent dielectric constant (ε), has been calculated by the INDO MO finite perturbation method incorporated with “Solvaton Theory”. The calculation results agree with the experimental finding that ³J_HCOH decreases with increase of ε.The respective contributions of conformational change and dielectric constant variation to the effect of solvent on experimental values of ³J_HCOH are discussed separately because ³J_HCOH, the vicinal coupling constant, is known to be sensitive to conformational change. It is concluded that ε contributes more to the change of the experimental value of ³J_HCOH by solvents.     

Photolysis of 1,2-dihydroquinolines in micellar solutions of anionic and cationic surfactants

The kinetics of the photolysis of substituted 1,2-dihydroquinolines (DHQ) in micellar solutions was studied by steady-state and flash photolysis. The photolysis mechanism depends dramatically on the location of DHQ molecules in micelles, which is governed by the surfactant nature. In micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), where the DHQ molecules are located in the Stern layer, the intermediate species decay kinetics follows a first-order law. When DHQ is in neutral form (pH 4–12), the rate constant of the intermediate carbocation decay increases from 25 to 198 s^?1 with an increasing concentration of DHQ in micelles. The positive micellar catalysis is caused by the acceleration of the final product formation with the DHQ molecule via proton abstraction from the intermediate cation. The formation of several types of intermediate species—carbocations in the aqueous phase and aminyl radicals in micelles—is observed in micellar solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) due to the preferential location of DHQ molecules in the micellar core. The carbocation decays via a pseudofirst-order reaction with a rate constant close to that in the aqueous solution. The lifetime of the DHQ aminyl radicals in the micellar solutions is longer by several orders of magnitude than the lifetime observed for homogeneous solutions of hydrocarbons and alcohols.     

Effect of the substituent position at the heterocycle on proton transfer from alcohols to the excited molecules of 1,2-dihydroquinolines

A novel compound 1,2,2,3-tetramethyl-1,2-dihydroquinoline (1223TMDQ) was synthesized, and the products of its steady-state photolysis in water, MeOH, EtOH, trifluoroethanol (TFE), PrⁿOH, and BuiOH were analyzed by ¹H NMR and mass-spectrometry. The corresponding adducts with water and alcohols were identified. The presence of the adducts for alcohols with a number of carbon atoms n > 1 distinguishes 1223TMDQ from 2,2,4-trimethyl-1,2-dihydroquinolines with methyl-, alkoxy-, and hydroxy-substituents at positions 1, 6, and 8, for which the formation of adducts was observed only in the presence of water and MeOH. The results were interpreted in terms of the effect of steric hindrance caused by substituents in the heterocycle and increasing size of the alkyl group of alcohol on proton transfer from a solvent to the molecule of 1,2-dihydroquinoline, which occurs in the complex between the solvent and the dihydroquinoline molecule in the excited singlet state. It was shown that the main steric hindrance for the photoinduced proton transfer in 2,2,4-substituted 1,2-dihydroquinolines is the substituent at position C(4) of the heterocycle.     

Effect of isotope substitution on the magnitude of nonequilibrium nuclear polarization in photolysis of acetone in methanol

Polarization of nuclei in both the products of the reactions and in the CHD₂OD proton without polarization in the CH₃OH protons is observed in irradiation of a solution of acetone in CD₃OD in the presence of CHD₂OD and CH₃OH. Polarization of the protons of the products is strongly dependent on the temperature of the solution and arises in radical pairs; polarization of the proton of partially deuterated methyl alcohol is due to a mechanism of optical nuclear polarization. It was hypothesized that the isotope effect is due to a difference in proton and electron relaxation and to a difference in the rates of cross-relaxation transitions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2683–2687, December, 1989.