光氧化反应的研究 V: 氰基蒽敏化苊烯的电子转移光氧化反应

对于容易发生单线态氧(^1O2)反应的稠环烯烃能否在氰基蒽敏化下发生电子转移光氧化研究甚少. 作者曾报道了氰基蒽敏化的9-本甲叉芴的ET光氧化过程. 本文首次探讨了非交替稠环烃, 苊烯(AN), 在9,10-二氰蒽(DCA)或9-氰基蒽(CNA)敏化下的光氧化反应及其机理.     

Amino-indole radical cations—II : Stability and electron transfer reactions

2-Phenyl-3-aryl-amino indole and bis-indolyl-amine radical cations show different stabilities depending on the possibility of their being convertible into the corresponding imino-compounds. The route of this decomposition is demonstrated and the synthesis of some new amines and the corresponding radical cations is reported. Electron transfer reactions between amino-indoles and tris-(p-bromophenyl-)amminium perchlorate are also reported.     

Degenerate electron exchange reaction of n-alkane radical cations in solution

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate: (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed.     

Kinetics of radical formation and decay in photooxidation of 4-halophenols sensitized by 4-carboxybenzophenone in aqueous solutions

Kinetics of formation and recombination of radicals formed by quenching of the triplet state of 4-carboxybenzophenone (CB) with para-substituted phenol derivatives RC₆H₄OH (R = OMe, H, Cl, Br, I) in aqueous solutions was studied by nanosecond laser photolysis. At pH ≥ 5.4, quenching proceeds with high rate constants ((1–3)⋅10⁹ L mol⁻¹ s⁻¹) through electron transfer to form the radical anion CB^⋅− and radical cation RC₆H₄OH^⋅+. The latter is transformed into the phenoxyl radical within ≤10 ns. At pH ≤ 8, the CB^⋅− radical anion is protonated in a phosphate buffer with the rate constant increasing from 4⋅10⁶ to 15⋅10⁶ s⁻¹ with a decrease in the pH from 8 to 5.4. The yield of radicals decreases from 100 to 13% as the atomic weight of halogen in the RC₆H₄OH molecule increases due to an increase in the probability of recombination of the primary triplet radical pair in the solvent cage and partial intersystem crossing in an encounter complex (³CB, RC₆H₄OH). The effect of heavy atom is also observed in the kinetics of volume recombination of the radicals, the magnitude of effect corresponds to the acceleration of the primary recombination of the triplet radical pair. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1397–1402, June, 2005.     

Electron transfer between guanosine radical and amino acids in aqueous solution. 1. Reduction of guanosine radical by tyrosine

As a model of chemical DNA repair, the reductive electron transfer from the aromatic amino acid tyrosine to the radical of the purine base guanosine monophosphate (GMP) was studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP). The guanosyl radicals were photochemically generated in the quenching reaction of the triplet excited dye 2,2'-dipyridyl. Depending on the pH of the aqueous solution, four different guanosyl radicals were observed. The identification of the radicals was possible because of the high sensitivity of CIDNP to distinguish them through their ability or disability of participating in the degenerate electron hopping reaction with the diamagnetic molecules of guanosine monophosphate in the ground state. The CIDNP kinetics in this three-component system containing the dye, GMP, and N-acetyl tyrosine is strongly dependent on the efficiency of the electron-transfer reaction from tyrosine to the nucleotide radical. Quantitative analysis of the CIDNP kinetics obtained at different concentrations of the amino acid, together with the comparison with the CIDNP kinetics of the two-component systems (dipyridyl/tyrosine and dipyridyl/GMP) allowed for the determination of the rate constant ke of the reductive electron-transfer reaction for five pairs of reactants, with different protonation states depending on the pH: GH++*/TyrOH (pH 1.3), G+*/TyrOH (pH 2.9), G(-H)*/TyrOH (pH 7.5), G(-H)*/TyrO- (pH 11.3), and G(-2H)-*/TyrO- (pH 13.3). The rate constant ke varies from (7.1 +/- 3.0) x 10(8) M-1 s-1 (pH 1.3, 2.9) to less than 6 x 10(6) M-1 s-1 (pH 13.3).     

On the repair of thymine and thymidine bromohydrins: a possible new model for the damage and repair of nucleic acids

Bromohydrins (2, 3, 4, and 5), oxidatively damaged products of thymine bases, were repaired on exposure to sunlight, heat, and/or some reagents to regenerate the thymine bases. A radical mechanism is proposed for the repair reaction with sunlight and heat. A hypothesis concerning the biological significance of thymidine in deoxyribonucleic acid is presented.     

Interaction of some complementary constituents of nucleic acids in aqueous solution: UV and IR hypochromicities

Ultraviolet spectra of mixtures of monomeric units of nucleic acids and dinucleoside monophosphates, containing the complementary nucleobases adenine and uracil, have been obtained in aqueous solution. Hypochromicity of spectra of the mixtures with respect to free components has been determined, introducing the correction corresponding to selfassociation of these compounds. Hypochromicity of mixtures can be considered an experimental evidence of the existence of heteroassociation processes.Infrared spectra of some complementary constituents of nucleic acids in aqueous solution also reveal hypochromic effects, which provide information about the molecular groups involved in the heteroassociation.     

Role of sulfide radical cations in electron transfer promoted molecular oxygenations at sulfur

The methylene blue, N-methylquinolinium tetrafluoroborate, and pyrylium-cation-sensitized photooxygenations of 5H, 7H-dibenzo[b,g] [1,5]dithiocin, 1, and 1,5-dithiacyclooctane, 2, have been investigated. The methylene blue sensitized reactions exhibit all of the characteristics of a singlet oxygen reaction including isotope effects for the formation of a hydroperoxysulfonium ylide and the ability of 1 and 2 to quench the time-resolved emission of singlet oxygen at 1270 nm. The product compositions in the N-methylquinolinium tetrafluoroborate and pyrylium-cation-sensitized reactions are dramatically different and are both different from that anticipated for the participation of singlet oxygen. This argues for different reaction mechanisms for all three sensitizers. However, both the quinolinium and pyrylium-cation-sensitized reactions display all of the characteristics of electron-transfer-initiated photooxygenations. Both sensitizers were quenched at nearly diffusion-limited rates by 1 and 2. Laser flash photolysis of mixtures of either sensitizer and 1 or 2 resulted in direct observation of the reduced sensitizer and the sulfide radical cation. In addition, electron-transfer reactions involving both sensitizers were shown to be exergonic. These results are consistent with the previously proposed outer sphere electron-transfer mechanism for N-methylquinolinium tetrafluoroborate and were used to argue for a new inner sphere mechanism for the pyrylium cation reactions.     

Cation-independent electron transfer between ferricyanide and ferrocyanide ions in aqueous solution

Electron transfer between Fe(CN)(6)(3-) and Fe(CN)(6)(4-) in homogeneous aqueous solution with K(+) as the counterion normally proceeds almost exclusively by a K(+)-catalyzed pathway, but this can be suppressed, and the direct Fe(CN)(6)(3)(-)-Fe(CN)(6)(4-) electron transfer path exposed, by complexing the K(+) with crypt-2.2.2 or 18-crown-6. Fe((13)CN)(6)(4-)-NMR line broadening measurements using either crypt-2.2.2 or (with extrapolation to zero uncomplexed [K(+)]) 18-crown-6 gave consistent values for the rate constant and activation volume (k(0) = (2.4 +/- 0.1) x 10(2) L mol(-1) s(-1) and Delta V(0) = -11.3 +/- 0.3 cm(3) mol(-1), respectively, at 25 degrees C and ionic strength I = 0.2 mol L(-1)) for the uncatalyzed electron transfer path. These values conform well to predictions based on Marcus theory. When [K(+)] was controlled with 18-crown-6, the observed rate constant k(ex) was a linear function of uncomplexed [K(+)], giving k(K) = (4.3 +/- 0.1) x 10(4) L(2) mol(-2) s(-1) at 25 degrees C and I = 0.26 mol L(-1) for the K(+)-catalyzed pathway. When no complexing agent was present, k(ex) was roughly proportional to [K(+)](total), but the corresponding rate constant k(K)' (=k(ex)/[K(+)](total)) was about 60% larger than k(K), evidently because ion pairing by hydrated K(+) lowered the anion-anion repulsions. Ionic strength as such had only a small effect on k(0), k(K), and k(K)'. The rate constants commonly cited in the literature for the Fe(CN)(6)(3-/4-) self-exchange reaction are in fact k(K)'[K(+)](total) values for typical experimental [K(+)](total) levels.     

Hydrogen peroxide and dioxygen activation by dinuclear copper complexes in aqueous solution: hydroxyl radical production initiated by internal electron transfer

Dinuclear Cu(II) complexes, CuII2Nn (n = 4 or 5), were recently found to specifically cleave DNA in the presence of a reducing thiol and O2 or in the presence of H2O2 alone. However, CuII2N3 and a closely related mononuclear Cu(II) complex exhibited no selective reaction under either condition. Spectroscopic studies indicate an intermediate is generated from CuII2Nn (n = 4 or 5) and mononuclear Cu(II) solutions in the presence of H2O2 or from CuI2Nn (n = 4 or 5) in the presence of O2. This intermediate decays to generate OH radicals and ligand degradation products at room temperature. The lack of reactivity of the intermediate with a series of added electron donors suggests the intermediate discharges through a rate-limiting intramolecular electron transfer from the ligand to the metal peroxo center to produce an OH radical and a ligand-based radical. These results imply that DNA cleavage does not result from direct reaction with a metal-peroxo intermediate but instead arises from reaction with either OH radicals or ligand-based radicals.     

Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether (1), 1,1,2,2-tetraphenylethane (5), 2-methyl-1,1,2-triphenylpropane (6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene (4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation. The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane. The carbocation fragment reacts with methanol to yield ether and/or acetals. The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed. The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations. In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the pbotosensitizer is the dominant reaction. The C—C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals. The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.     

Picosecond two-quantum UV photochemistry of thymine in aqueous solution

Two-quantum photochemical processes, taking place under high-intensity UV laser irradiation of biomolecules in aqueous solution, have been studied with thymine, one of the DNA bases, as an example. It has been found that two-step high-energy excitation of thymine results not only in its ionization and relaxation but also in electronic energy transfer to the solvent, water. The probabilities of primary photoprocesses from the high-lying electronic vibrational states of thymine have been measured. It has been shown that the radicals of the solvent, water, formed by its sensitized photodecomposition and also by direct two-photon ionization and dissociation make the basic contribution to the formation of the final chemically stable products in the picosecond UV photolysis of thymine in aqueous solution.     

Nucleophilic and electrophilic radical attack on maleic and fumaric acids in aqueous solution

Rate coefficients (k) of CH₂OH, , and radical addition to maleic and fumaric acids were investigated between pH 1 and 8. Strong pH dependences observed were attributed to changes in protonation states of acids: H₂X, HX⁻ and X²⁻. k of CH₂OH, , addition to fumaric acid decreased in the order k_H2F>k_HF^->k_F^2- in agreement with the nucleophilic character of reaction. The electrophilic radical showed opposite tendency. With maleic acid the monoanion had the highest reactivity towards nucleophilic and the lowest one towards electrophilic radicals. This is attributed to a prevalence of steric over polar effects for HM⁻.     

Spectroscopic detection, reactivity, and acid-base behavior of ring-dimethoxylated phenylethanoic acid radical cations and radical zwitterions in aqueous solution

A product and time-resolved kinetic study of the one-electron oxidation of ring-dimethoxylated phenylethanoic acids has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations or radical zwitterions depending on pH, and pK(a) values for the corresponding acid-base equilibria have been measured. The radical cations undergo decarboxylation with first-order rate constants (k(dec)) ranging from <10(2) to 5.6 x 10(4) s(-1) depending on radical cation stability. A significant increase in k(dec) (between 10 and 40 times) is observed on going from the radical cations to the corresponding radical zwitterions. The results are discussed in terms of the ease of intramolecular side chain to ring electron transfer required for decarboxylation, in both the radical cations and radical zwitterions.     

Comparison of photoelectron transfer in solution and the solid state for two intervalence radical cations

[structure: see text] The optical diffuse reflectance and solution spectra of two bis-hydrazine radical cationic intervalence compounds have been compared. The results are consistent with an ion-pairing increase and an "effective polarity" in these crystals that is not far from that of acetonitrile or other polar solvents.     

Effect of group electronegativity on electron transfer in bis(hydrazine) radical cations

The radical cation of 4,10-ditert-butyl-5,9-diisopropyl-4,5,9,10-tetraazatetracyclo[6.2.2.2]-tetradecane (sBI4T(+)), as well as its substituted bis(hydrazine) radical cations, is chosen for the investigation of the electronegativity dependence of its intramolecular electron transfer. To do so, two parameters, reorganization energy and electronic coupling, are calculated with several ab initio approaches. It is found that the electronic couplings decrease with the increase of the group electronegativity while the reorganization energies do not show an explicit dependency. Furthermore, Marcus formula is employed to reveal those effect on the electron transfer rates. The predicted rates of electron transfer generally decrease with increasing group electronegativity, although not monotonically.     

Generation and reactions of 3-alkylidene-1-pyrazoline radical cations by photoinduced electron transfer

The behavior of the 3-alkylidene-1-pyrazoline radical cations generated by photoinduced electron transfer reactions was examined. The nitrogen-retained radical cations have been detected using laser flash photolysis. The photochemical products indicate that E/Z isomerization, intramolecular cyclization, and solvent addition (acetonitrile) occurred.     

Time-resolved FT EPR and optical spectroscopy study on photooxidation of aliphatic alpha-amino acids in aqueous solutions; electron transfer from amino vs carboxylate functional group

Using time-resolved Fourier transform electron paramagnetic resonance, FT EPR, and optical spectroscopy, the photooxidation of glycine, alpha-alanine, alpha-aminoisobutyric acid, and model compounds beta-alanine, methylamine and sodium acetate, by excited triplets of anthraquinone-2,6-disulfonate dianion was studied in aqueous solutions in the pH range 5-13. Anthraquinone radical trianions showing strong emissive spin-polarization (CIDEP) were formed, indicating fast electron transfer from the quenchers to the spin-polarized quinone triplet as the primary reaction. None of the primary radicals formed upon one-electron oxidation of quenchers could be detected at the nanosecond time scale of FT EPR measurements because of their very fast transformation into secondary products. The latter were identified to be decarboxylated alpha-aminoalkyl radicals for alpha-amino acids anions and zwitterions, beta-aminoalkyl radicals for beta-alanine zwitterions, and methyl radicals for acetate anions; corresponding aminyl radicals were the first EPR detectable products from beta-alanine anions and methylamine. Thus, anthraquinone-2,6-disulfonate triplet can take an electron from both NH(2)- and -CO(2)(-) functional groups forming aminium ((+*)NH(2)-) and acyloxyl (-CO(2)(*)) radicals, respectively. Aminium radicals derived from beta-alanine anions and CH(3)-NH(2) stabilize by deprotonation into aminyl radicals, whereas these derived from alpha-amino acids anions are known to suffer ultrafast decarboxylation (tau approximately 10 ps). Analysis of the polarization patterns revealed that decarboxylation from acyloxyl radicals are considerably slower (ns < tau < 0.1 micros). Therefore, in the case of alpha-amino acids, the isoelectronic structures NH(2)-CR(2)-CO(2)(*) and (+*)NH(2)-CR(2)-CO(2)(-) probably do not constitute resonance mesomeric forms of one and the same species and the decarboxylation of aminium radicals is not preceded by the intramolecular carboxylate to amino group electron transfer. Absolute triplet quenching rate constants at zero ionic strength were in the range of 2 x 10(8) to 2 x 10(9) M(-1) s(-1) for R-NH(2) and 2 x 10(7) to 10(8) M(-1) s(-1) for R-CO(2)(-) type of electron donors, reflecting in principle their standard reduction potentials. The strengths of acids: (+)NH(3)-(*)CH(2), (+)NH(3)-(*)C(CH(3))H, and (+)NH(3)-(*)C(CH(3))(2), pK(a) <4, >6, and >7, respectively, were found to be remarkably strongly dependent on alpha-C substitution. The conjugate bases of these alpha-aminoalkyl radicals reduce anthraquinone-2,6-disulfonate dianion ground state with k(sec) = 3 x 10(9) M(-1) s(-1).     

Quinone transfer radical polymerization of styrene: Synthesis of the actual initiator

ortho‐Quinones, such as phenanthrenequinone and 3,6‐dimethoxyphenanthrenequinone, added with a catalytic amount of metal complexes, impart control to styrene polymerization via the previously reported quinone transfer radical polymerization (QTRP) process. In this study, compounds that mimic the dormant species proposed in the QTRP mechanism have been synthesized and tested as initiators in the presence of cobalt(II) acetylacetonate. These compounds, and particularly 3,6‐dimethoxy‐10‐hydroxy‐10‐(1‐phenyl‐ethyl)‐phenanthren‐9‐one, are effective control agents for the radical polymerization of styrene, in agreement with the recently proposed mechanism. Moreover, the induction period, which has been systematically reported in the presence of ortho‐quinones, is no longer observed. The end capping of the polystyrene chains by the control agent has been confirmed by ¹H NMR analysis. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1233–1244, 2006