Laser photolysis studies on competing processes of ionic dissociation and hydrogen abstraction in benzophenone-N,N-diethylaniline system

The quenching behaviour of triplet benzophenone by N,N-diethylaniline has been directly demonstrated with the laser photolysis method. In acetonitrile solution, ionic dissociation to the benzophenone anion and N,N-diethylaniline cation occurred, while a ketyl radical was formed in benzene solution. Encounter collisional ionic dissociation and hydrogen abstraction compete each other, depending on the solvent polarity.     

Generation and reactivity of ketyl radicals with lignin related structures. On the importance of the ketyl pathway in the photoyellowing of lignin containing pulps and papers

[reaction: see text] Ketyl radicals with lignin related structures have been generated by means of radiation chemical and photochemical techniques. In the former studies ketyl radicals are produced by reaction of alpha-carbonyl-beta-aryl ether lignin models with the solvated electron produced by pulse radiolysis of an aqueous solution at pH 6.0. The UV-vis spectra of ketyl radicals are characterized by three main absorption bands. The shape and position of these bands slightly change when the spectra are recorded in alkaline solution (pH 11.0) being now assigned to the ketyl radical anions and a pKa = 9.5 is determined for the 1-(3,4,5-trimethoxyphenyl)-2-phenoxyethanol-1-yl radical. Decay rates of ketyl radicals are found to be dose dependent and, at low doses, lie in the range (1.7-2.7) x 10(3) s(-1). In the presence of oxygen a fast decay of the ketyl radicals is observed (k2 = 1.8-2.7 x 10(9) M(-1) s(-1)) that is accompanied by the formation of stable products, i.e., the starting ketones. In the photochemical studies ketyl radicals have been produced by charge-transfer (CT) photoactivation of the electron donor-acceptor salts of methyl viologen (MV2+) with alpha-hydroxy-alpha-phenoxymethyl-aryl acetates. This process leads to the instantaneous formation of the reduced acceptor (methyl viologen radical cation, MV+*), as is clearly shown in a laser flash photolysis experiment by the two absorption bands centered at 390 and 605 nm, and an acyloxyl radical [ArC(CO2*))(OH)CH2(OC6H5)], which undergoes a very fast decarboxylation with formation of the ketyl radicals. Steady-state photoirradiation of the CT ion pairs indicates that 1-aryl-2-phenoxyethanones are formed as primary photoproducts by oxidation of ketyl radicals by MV2+ (under argon) or by molecular oxygen. Small amounts of acetophenones are formed by further photolysis of 1-aryl-2-phenoxyethanones and not by beta-fragmentation of the ketyl radicals. The high reactivity of ketyl radicals with oxygen coupled with the low rates of beta-fragmentation of the same species have an important bearing in the context of the photoyellowing of lignin containing pulps and papers.     

Electrode reduction mechanism of aromatic nitriles in aprotic solvents: Benzonitrile

The electrode reaction mechanism of benzonitrile in anhydrous DMF has been studied by polarography, potential sweep voltammetry, macroscale electrolysis, e.p.r. spectrometry and kinetic analysis. The relatively stable radical anion formed by one-electron addition decays according to a first order kinetic law, with formation of benzene and CN^? as final products together with substantial amounts of the alkene and alkylamine corresponding to the tetraalkylammonium salt used as background electrolyte. The results are interpreted in terms of a mechanism involving protonation of the anion radical by the solvent and the back-ground electrolyte, followed by dismutation of the resulting neutral radical with the anion radical and elimination of CN^? ions from the anionic species thus formed. By phenol addition CN^? elimination is prevented and the reduction proceeds to 1-cyclohexene-1-nitrile or to cyclohexane-nitrile, depending on the reduction potential.     

Phototriggered Release of a Leaving Group in Ketoprofen Derivatives via a Benzylic Carbanion Pathway,But not via a Biradical Pathway

2‐Acetoxymethyl‐2‐(3‐benzoylphenyl)propionic acid (KP‐OAc) was used as a model to elucidate the solvent‐mediated photochemistry mechanism of Ketoprofen (KP). In solutions with a low concentration of water, KP‐OAc exhibits a benzophenone‐like photochemistry, reacting with water molecules through some reaction to form a ketyl radical intermediate. In neutral solutions with a high concentration of water or acidic solutions, KP‐OAc undergoes a photodecarboxylation reaction with the assistance of water molecules or with the catalysis of perchloric acid to directly generate a biradical intermediate that cannot induce the phototrigger reaction to release the AcO^? group. Therefore, the lifetime of the biradical intermediate of KP‐OAc is almost same as that of the biradical intermediate formed from KP in the same kinds of solutions. However, the photodecarboxylation of KP‐OAc in phosphate buffer solution directly produces the benzylic carbanion intermediate, which can induce the phototrigger reaction to release the AcO^? group. Therefore, the lifetime of the biradical intermediate of KP‐OAc is significantly shorter than the lifetime of the biradical intermediate of KP in phosphate buffer solution. Interestingly, the investigation of the photochemistry of KP‐OAc not only verifies the solvent‐mediated photochemistry mechanism of KP but also provides some new insight into the potential of using this kind of platform for phototrigger applications. The biradical intermediate is not the key species leading to the phototrigger reaction but the benzylic carbanion species is the key reactive intermediate that can mediate the phototrigger reaction of KP‐OAc. Therefore, a change in the pH of the solutions can be utilized to switch on and switch off the photorelease reactions of KP derivative phototrigger compounds.     

Stoichiometry and mechanism of protonation of alkali metal salts of benzophenone radical anions by weak proton donors and its relevance to the base-catalyzed decomposition of benzopinacol

The Stoichiometry of the protonation of lithium and potassium salts of benzophenone radical anions and of the lithium salt of the fluorenone radical anion by methanol has been measured and found to be [(Ar₂C=O)^–]/[MeOH] =21. This result, which was obtained by the method of magnetic titration, implies that paramagnetism decays by the reaction between a ketyl anion and a ketyl radical (i.e., a protonated ketyl anion). The reactivities of alkali metal salts of fluorenone radical anions in relation to methanol exhibit a pronounced dependence on the nature of the counterion. No kinetic deuterium isotope effect has been found for the protonation of the lithium salt of the benzophenone radical anion in tetrahydrofuran (THF) bytert-pentyl alcohol. The lithium salt of the benzophenone radical anion inN,N,N,N-tetramethylethylenediamine (TMEDA) behaves markedly differently. Namely, its protonation by methanol exhibits 1 1 Stoichiometry and it reacts considerably more slowly withsec-butyl alkohol,K(THF)/K(TMEDA) = 2.5. Benzopinacol undergoes decomposition by an alkoxide base to diphenyl ketyl, which decays into an equimolar mixture of benzophenone and benzhydrol. The reaction follows second-order kinetics and the specific rate constants exhibit an inverse relationship with respect to the initial concentration of the alkoxide. With a very strong base benzopinacol decomposes into two diphenyl ketyl anions. On the basis of this information as well as on studies of products, relevant mechanisms are proposed for the protonation of ketyl anions and for the decomposition of aromatic pinacols in basic media.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 83–91, January, 1995.     

How and when does an unusual and efficient photoredox reaction of 2-(1-hydroxyethyl) 9,10-anthraquinone occur? A combined time-resolved spectroscopic and DFT study

The photophysics and photochemical reactions of 2-(1-hydroxyethyl) 9,10-anthroquinone (2-HEAQ) were studied using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy techniques and density functional theory (DFT) calculations. In acetonitrile, 2-HEAQ underwent efficient intersystem crossing to the triplet excited state ((2-HEAQ)(3)). A typical photoreduction reaction for aromatic ketones took place via production of a ketyl radical intermediate for 2-HEAQ in isopropanol. In water-containing solutions with pH values between 2 and 10, an unusual photoredox reaction reported by Wan and co-workers was detected and characterized. Observation of the protonated species in neutral and acidic aqueous solutions by fs-TA spectra indicated the carbonyl oxygen of (2-HEAQ)(3) was protonated initially and acted as a precursor of the photoredox reaction. The preference of the photoredox reaction to occur under moderate acidic conditions compared to neutral condition observed using ns-TR(3) spectroscopy was consistent with results from DFT calculations, which suggested protonation of the carbonyl group was the rate-determining step. Under stronger acidic conditions (pH 0), although the protonated (2-HEAQ)(3) was formed, the predominant reaction was the photohydration reaction instead of the photoredox reaction. In stronger basic solutions (pH 12), (2-HEAQ)(3) decayed with no obvious photochemical reactions detected by time-resolved spectroscopic experiments. Reaction mechanisms and key reactive intermediates for the unusual photoredox reaction were elucidated from time-resolved spectroscopy and DFT results. A brief discussion is given of when photoredox reactions may likely take place in the photochemistry of aromatic carbonyl-containing compounds and possible implications for using BP and AQ scaffolds for phototrigger compounds.     

Acid-, base-, and lewis-acid-catalyzed heterolysis of methoxide from an alpha-hydroxy-beta-methoxy radical: models for reactions catalyzed by coenzyme B12-dependent diol dehydratase

[Reaction: see text].A model for glycol radicals was employed in laser flash photolysis kinetic studies of catalysis of the fragmentation of a methoxy group adjacent to an alpha-hydroxy radical center. Photolysis of a phenylselenylmethylcyclopropane precursor gave a cyclopropylcarbinyl radical that rapidly ring opened to the target alpha-hydroxy-beta-methoxy radical (3). Heterolysis of the methoxy group in 3 gave an enolyl radical (4a) or an enol ether radical cation (4b), depending upon pH. Radicals 4 contain a 2,2-diphenylcyclopropane reporter group, and they rapidly opened to give UV-observable diphenylalkyl radicals as the final products. No heterolysis was observed for radical 3 under neutral conditions. In basic aqueous acetonitrile solutions, specific base catalysis of the heterolysis was observed; the pK(a) of radical 3 was determined to be 12.5 from kinetic titration plots, and the ketyl radical formed by deprotonation of 3 eliminated methoxide with a rate constant of 5 x 10(7) s(-1). In the presence of carboxylic acids in acetonitrile solutions, radical 3 eliminated methanol in a general acid-catalyzed reaction, and rate constants for protonation of the methoxy group in 3 by several acids were measured. Radical 3 also reacted by fragmentation of methoxide in Lewis-acid-catalyzed heterolysis reactions; ZnBr2, Sc(OTf)3, and BF3 were found to be efficient catalysts. Catalytic rate constants for the heterolysis reactions were in the range of 3 x 10(4) to 2 x 10(6) s(-1). The Lewis-acid-catalyzed heterolysis reactions are fast enough for kinetic competence in coenzyme B12 dependent enzyme-catalyzed reactions of glycols, and Lewis-acid-catalyzed cleavages of beta-ethers in radicals might be applied in synthetic reactions.     

Formation of spectral intermediate G-C and A-T anion complex in duplex DNA studied by pulse radiolysis

The dynamics of electron adducts of 2'-deoxynucleotides and oligonucelotides (ODNs) were measured spectroscopically by nanosecond pulse radiolysis. The radical anions of the nucleotides were produced within 10 ns by the reaction of hydrated electrons (e(aq)(-)) and were protonated to form the corresponding neutral radicals. At pH 7.0, the radical anion of deoxythymidine (dT(*-)) was protonated to form the neutral radical dT(H)(*) in the time range of microseconds. The rate constant for the protonation was determined as 1.8 x 10(10) M(-1) s(-1). In contrast, the neutral radical of dC(H)(*) was formed immediately after the pulse, suggesting that the protonation occurs within 10 ns. The transient spectra of excess electrons of the double-stranded ODNs 5'-TAATTTAATAT-3' (AT) and 5'-CGGCCCGGCGC-3' (GC) differed from those of pyrimidine radicals (C and T) and their composite. In contrast, the spectra of the electron adducts of the single-stranded ODNs GC and AT exhibited characteristics of C and T, respectively. These results suggest that, in duplex ODNs, the spectral intermediates of G-C and A-T anions complex were formed. On the microsecond time scale, the subsequent changes in absorbance of the ODN AT had a first-order rate constant of 4 x 10(4) s(-1), reflecting the protonation of T.     

Biomimetic Ketone Reduction by Disulfide Radical Anion

The conversion of ribonucleosides to 2′-deoxyribonucleosides is catalyzed by ribonucleoside reductase enzymes in nature. One of the key steps in this complex radical mechanism is the reduction of the 3′-ketodeoxynucleotide by a pair of cysteine residues, providing the electrons via a disulfide radical anion (RSSR^•−) in the active site of the enzyme. In the present study, the bioinspired conversion of ketones to corresponding alcohols was achieved by the intermediacy of disulfide radical anion of cysteine (CysSSCys)^•− in water. High concentration of cysteine and pH 10.6 are necessary for high-yielding reactions. The photoinitiated radical chain reaction includes the one-electron reduction of carbonyl moiety by disulfide radical anion, protonation of the resulting ketyl radical anion by water, and H-atom abstraction from CysSH. The (CysSSCys)^•− transient species generated by ionizing radiation in aqueous solutions allowed the measurement of kinetic data with ketones by pulse radiolysis. By measuring the rate of the decay of (CysSSCys)^•− at λ_max = 420 nm at various concentrations of ketones, we found the rate constants of three cyclic ketones to be in the range of 10⁴–10⁵ M⁻¹s⁻¹ at ~22 °C.     

Photooxidation of Methionine Derivatives by the 4-Carboxybenzophenone Triplet State in Aqueous Solution. Intracomplex Proton Transfer Involving the Amino Group

Abstract— Oxidation of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines and three methionine-containing dipeptides was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to follow the concentration profiles of the intermediates formed from the quenching events. From these concentration profiles, quantum yields for the intermediates were determined. Branching ratios were evaluated for the decay of the charge-transfer complex by the competing processes of back electron transfer, proton transfer and escape of radical ions. The relative prominence of these processes was discussed in terms of the proton-transfer tendencies of the nominal sulfur-radical-cationic species. A systematic decrease was observed in the quantum yields for the escape of radical ions along with a correlated increase in the proton-transfer yields. The enhanced propensity of the sulfur radical cations to depro-tonate is due to deprotonation at the carbons adjacent to the sulfur-cationic site and at the unsubstituted amino groups when present. This scheme was supported by an observed decrease in the yields of dimeric sulfur radical cations with an increase in the electron-withdrawing abilities of the substituents, making the radical-cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlation of the quantum yields of ketyl radical formation in the photochemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur-containing substrates in pulse radiolysis experiments.     

A New Samarium Diiodide Induced Reaction: Intramolecular Attack of Ketyl Radical Anions on Aryl Substituents with Formation of 1,4-Cyclohexadiene Derivatives

Beware of samarium diiodide and aryl ketones! If the ketyl radical anion which is formed by electron transfer finds a properly placed aryl group, a highly diastereoselective cyclization may occur. After the transfer of a second electron and protonation bi- and polycyclic products with a common 1,4-cyclohexadiene moiety may be isolated [Eq. (a)]. X=CHCO₂R, NCH₂Ph; HMPA=(Me₂N)₃PO.     

Stability and rate of decay of anion radicals in the nitrofuran series

A method has been developed for kinetic measurements on nitrofuran anion radicals generated electrochemically within the ESR cavity. The radical concentrations are determined by comparison with benzene solutions of DPPH. Radical loss at high pH in aqueous alcohol occurs as a second-order reaction, the rate being very much dependent on the pH, but only slightly on the proportion of alcohol. A protonation mechanism of radical loss is discussed.     

1,2-和1,4-萘醌248 nm激光光解的瞬态吸收光谱研究

利用纳秒级激光光解动态吸收光谱装置,研究了1,2-和1,4-萘醌中性水溶液的瞬态吸收光谱.发现1,2-萘醌及1,4-萘醌被光电离后形成的阳离子自由基在380nm均有最大吸收,但1,4-萘醌阳离子自由基在衰变过程中又形成了两种新的活性粒子,它们的最大吸收分别位于410和580nm,分析表明:410nm属于1,4-萘醌脱氢自由基的吸收,而580nm很可能归属由于电子转移而形成的瞬态产物.进一步研究发现,1,2-萘醌在中性水溶液中能被248nm激光单光子电离.     

Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones

Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino- 3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron (e_aq⁻) has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pK_a values of ketylanion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized.     

Reaction dynamics of excited ketoprofen with triethylamine

The reaction dynamics of ketoprofen (KP) with and without triethylamine (TEA) in methanol both in the ground and the excited states was studied by laser flash photolysis and the pump-probe emission spectroscopy. After the excitation, triplet KP abstracted a hydrogen atom from methanol to form KP ketyl radical (KPH). In the presence of TEA, the acid-base equilibrium state was found to be KP + TEA right arrow over left arrow KP- + TEAH+ in the ground state. The equilibrium constant was determined to be 32 +/- 7. Excited KP- rapidly underwent decarboxylation to form a carbanion resonant with the 3-ethylbenzophenone ketyl biradical anion (3-EBP-), followed by a proton-transfer reaction with TEAH+ to produce the 3-ethylbenzophenone ketyl biradical (3-EBPH). Furthermore, 3-EBPH was found to make a complex with TEA, whose equilibrium constant was obtained to be 18 +/- 2 M(-1). The complex formation ability of 3-EBPH was discussed compared with benzophenone ketyl radical (BPH).     

Excited-state proton transfer in complexes of poly(methacrylic acid) with dodecyltrimethylammonium chloride

Proton-transfer reactions in aqueous solutions of poly(methacrylic acid) (PMA) were studied using a fluorescent probe and Fourier transform infrared (FTIR) spectroscopy. Protolytic photodissociation of 1-hydroxypyrene (HP) in water was found to be very slow. The PMA polyanion appeared to be very inefficient as a proton acceptor in the excited-state reaction with HP. However, a drastic increase in the deprotonation efficiency was observed in PMA solutions with the same pH values close to neutral when dodecyltrimethylammonium chloride (DTAC) was added. The protonated form of HP, as well as its anion, was shown to be solubilized in polyion-covered micelles. Time-resolved fluorescence data suggested at least two localization sites with different reactivities toward PMA. FTIR spectroscopy was used to quantify the degree of ionization of PMA in PMA-DTAC mixtures. The IR data indicated that protolytic dissociation of PMA could be well described by the Henderson-Hasselbach equation with an apparent pK of 6.6. In contrast, the fluorescent data revealed cooperative protonation of the PMA groups interacting with HP localized within surfactant assemblies. This selective protonation at a pH close to neutral may be associated with a conformational transition in the polymer-surfactant complex.     

Biphotonic excitation of the fluorescence of benzophenone ketyl

Laser irradiation of benzophenone in isopropanol solution results in orange fluorescence, the yield of which depends on the laser pulse duration. This is shown to be due to benzophenone ketyl (hydroxydiphenyl methyl), formed as a photo-product. It is shown that simple time-resolved fluorescence measurements can yield basic information concerning the early photochemical history of such radical species.     

Reduction of activated olefins by SmI2. Detouring the classical Birch mechanism and a negative order in SmI2

The reaction of an excess of 1,1-diaryl-2,2-dicyanoethylenes (1) with SmI2 is biphasic for olefin with at least one available para position. The first phase is completed in less than 0.5 s with the second phase extending over a few hundred seconds. This phase is second order with respect to the radical anion, which is formed in the dead-time of the mixing in the stopped flow spectrophotometer and is overall of -1 order in the initial concentration of SmI2. In this phase, a dimer is formed between two radical anions with the formation of a C-C bond between a benzylic and a para position. The second phase is enhanced by proton donors and shows an H/D kinetic isotope effect with MeOH. Minute amounts of ethylene glycol accelerated the reaction to such an extent that the second phase is "absorbed" into the first, rendering it rate determining. In this phase, the dianionic dimer disproportionates after protonation to furnish the neutral species and the anion, which after second protonation provides the reduced product. When the two para positions are occupied by substituents, the reaction takes place by the traditional Birch reduction sequence of electron-proton-electron-proton-transfer steps. It is shown that the detour mechanism, coupling followed by disproportionation, should be typical of olefin but not of carbonyl reduction. This difference stems from the dissimilarity in protonation rate on carbon and oxygen.     

Reaction mechanisms and structural characterization of the reactive intermediates observed after the photolysis of 3-(hydroxymethyl)benzophenone in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions

Nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy was employed to investigate the photoinduced reactions of 3-(hydroxymethyl)benzophenone (1) in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions. Density functional theory calculations were utilized to help the interpretation of the experimental spectra. In acetonitrile, the neutral triplet state 1 [denoted here as (m-BPOH)(3)] was observed on the nanosecond to microsecond time scale. In 2-propanol this triplet state appeared to abstract a hydrogen atom from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3-(hydroxymethyl)phenyl)methylene)cyclohexa-2,5-dienyl)propan-2-ol. In 1:1 H(2)O:CH(3)CN aqueous solution at neutral pH, (m-BPOH)(3) reacted with water to produce the ArPK of 1 and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a novel photoredox reaction. TR(3) experiments in 1:1 H(2)O:CH(3)CN aqueous solution at pH 2 detected a new triplet biradical species, which is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face competition from the overall photohydration reaction at pH 0.     

Kinetics of the interaction of ketyl and semiquinone radicals with dioxygen. Concerted electron and proton transfer involving ketyl and semiquinone radicals

Kinetics of the interaction of ketyl and neutral semiquinone radicals with dioxygen was studied by the flash photolysis technique. The reactivity of neutral semiquinone radicals in the transfer of a hydrogen atom to O₂ is lower than that of ketyl radicals and increases as the reduction ability of the radicals increases, which give evidence for the charge transfer from the radicals to O₂ in the transition state of the reaction. The deuterium kinetic isotope effect of the reaction (up to 2.6) suggests considerable weakening of the O−H bond of the seminquinone radical in the transition state. A cyclic structure of the transition state similar to that in the reactions of ketyl radicals with hydrogen atom acceptors is proposed. In aprotic volvents, solvation has essentially no effect on the reactivity of neutral anthrasemiquinone radicals up to solvent nucleophilicityB≈240. In solvents with higher nucleophilicity and in protic solvents, their reactivity drops sharply. Hydrogen atom transfer reactions involving ketyl and neutral semiquinone radicals are shown to involve concerted electron and proton transfers, and to have transition states in which the partial transfer of an electron and a proton from the ketyl or semiquinone radical to an acceptor occurs. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1131–1137, June, 1997.