Two-electron Reduction Product Research Articles

Reductive Activation of Arenes: XV. Anionic Products of m-Tolynitrile Reduction with Sodium in Liquid Ammonia, and Their Alkylation

From results of oxidation, protonation, and alkylation of the products arising in one- or two- electron reduction of m-tolunitrile with sodium in liquid ammonia followed a conclusion that these products are respectively anion-radical of the compound and 3-methyl-1-cyano-2,5-cyclohexadienyl anion. The reaction of both reduction products with alkyl halides gives rise to compounds of ipso-alkylation with respect to cyano group: the corresponding alkyltoluenes and 1-alkyl-3-methyl-1-cyclohexadienes. The ratio of these products depends on the structure of alkyl halide. The possibility to prepare selectively m-alkyltoluenes by reaction of the product of two-electron reduction of m-tolunitrile with alkyl halides was demonstrated.

The kinetics of neutral methyl viologen in acidic H 2O+DMF mixed solutions studied by cyclic voltammetry

The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V 2+) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V 0) in the presence of H + (from HCl), CH 3COOH (p K a=4.75), ClCH 2CH 2COOH (p K a=4.00), HCOOH (p K a=3.75) in aqueous media was examined by cyclic voltammetry according to the EEC i mechanism. To avoid the electrodeposition of V 0, we used a 9:1 (v/v%) H 2O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V 2+, the ratio of the anodic and cathodic peak current ( I pa2/ I pc2) corresponding to V 0–e −⇄V + was plotted against log τ, where τ is the time between E 1/2 and the switching potential, at various scan rates of 0.02–3.5 V s −1. The chemical reaction was found to be a parallel reaction consisting of H +-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as k H + =3.5×10 3 M −1 s −1, k CH 3COOH =5.7 M −1 s −1, k HCOOH=4.6×10 1 M −1 s −1, and k ClCH 2CH 2COOH =3.2×10 1 M −1 s −1 using the Nicholson–Shain method and k H 2O was estimated as <3×10 −6 M −1 s −1. The CVs were digitally simulated under the assumption of a two-step reaction of V 0 following the two-step electrode reactions of V 2+ to V 0. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V 0 is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V 0 is in pseudo-equilibrium with H + or HA to produce VH + which undergoes a reaction with H 2O.

Mixed halide/phosphine complexes of the dirhenium core. Part 7. Reactions of [Re 2I 8] 2− with monodentate phosphines

Reactions of the octaiododirhenate anion [Re 2I 8] 2− with monodentate phosphines (PR 3=PMe 3 ( 1), PMe 2Ph ( 2), PEt 3 ( 3), and PEt 2Ph ( 4)) have been studied in two different solvents, ethanol and benzene, at room temperature. The reactions in ethanol resulted in two-electron reduction products having the Re 2 4+ core, Re 2I 4(PR 3) 4 (R 3=Me 3 ( 1a) and Me 2Ph ( 2a)), for which X-ray diffraction study revealed a 1,3,6,8 isomer type. The reaction of [Re 2I 8] 2− with PMe 3 in benzene led to an intermediate edge-sharing bioctahedral dirhenium(III) complex, Re 2(μ-I) 2I 4(PMe 3) 4 ( 1b), with no metal–metal bond. The interaction of PEt 3 with [Re 2I 8] 2− in benzene again yielded a kinetic product with a different stoichiometry and a Re 2 5+ core, [Bu n 4N][Re 2I 6(PEt 3) 2] ( 3c). The analogous reactions of [Re 2I 8] 2− with PMe 2Ph and PEt 2Ph in benzene involved two-electron reductions and gave complex 2a and Re 2I 4(PEt 2Ph) 4 ( 4a), as products. Reduction of 1b with KC 8 gave 1a as the main product along with a few crystals of 1,3,6-Re 2I 5(PMe 3) 3 ( 1d). Mixed iodide/phosphine complexes of the dirhenium core Re 2 n+ , where n=4 ( 1a, 2a, 4a), 5 ( 1d, 3c), and 6 ( 1b) have for the first time been characterized by X-ray crystallography. In addition, these products have been further identified by a combination of mass spectrometry and cyclic voltammetric measurements.

Reductive Activation of Arenes: XIII. Reaction with Butyl Bromide of Anionic Products Formed by Reduction of 9,10-Dicyanoanthracene with Potassium in Liquid Ammonia

Reduction of 9,10-dicyanoanthracene with one equivalent of metallic potassium in liquid ammonia at -33°C gives a product which almost does not react with butyl bromide. By contrast, the two-electron reduction product reacts with butyl bromide to form, depending on the reaction conditions and procedure for treatment of the reaction mixture, one of the three compounds: 10-butyl-9-cyanoanthracene, 10-butyl-10-cyano-9,10-dihydroanthracen-9-one, or 9,10-dibutyl-9,10-dicyano-9,10-dihydroanthracene. Relatively stable 10-butyl-9,10-dicyano-9,10-dihydro-9-anthryl anion was presumed to be a common precursor of these products.

3-amino-1,2,4-benzotriazine 4-oxide: characterization of a new metabolite arising from bioreductive processing of the antitumor agent 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine).

Tirapazamine (1) is a promising antitumor agent that selectively causes DNA damage in hypoxic tumor cells, following one-electron bioreductive activation. Surprisingly, after more than 10 years of study, the products arising from bioreductive metabolism of tirapazamine have not been completely characterized. The two previously characterized metabolites are 3-amino-1,2,4-benzotriazine 1-oxide (3) and 3-amino-1,2,4-benzotriazine (5). In this work, 3-amino-1,2,4-benzotriazine 4-oxide (4) is identified for the first time as a product resulting from one-electron activation of the antitumor agent tirapazamine by the enzymes xanthine/xanthine oxidase and NADPH:cytochrome P450 oxidoreductase. As part of this work, the novel N-oxide (4) was unambiguously synthesized and characterized using NMR spectroscopy, UV-vis spectroscopy, LC/MS, and X-ray crystallography. Under conditions where the parent drug tirapazamine is enzymatically activated, the metabolite 4 is produced but readily undergoes further reduction to the benzotriazine (5). Thus, under circumstances where extensive reductive metabolism occurs, the yield of the 4-oxide (4) decreases. In contrast, the isomeric two-electron reduction product 3-amino-1,2,4-benzotriazine 1-oxide (3) does not readily undergo enzymatic reduction and, therefore, is found as a major bioreductive metabolite under all conditions. Finally, the ability of the 4-oxide metabolite (4) to participate in tirapazamine-mediated DNA damage is considered.

NADPH:cytochrome c (P450) reductase activates tirapazamine (SR4233) to restore hypoxic and oxic cytotoxicity in an aerobic resistant derivative of the A549 lung cancer cell line.

Tirapazamine (TPZ, SR4233, WIN 59075) is a bioreductive drug that is activated in regions of low oxygen tension to a cytotoxic radical intermediate. This labile metabolite shows high selective toxicity towards hypoxic cells, such as those found in solid tumours. Under aerobic conditions, redox cycling occurs with subsequent generation of superoxide radicals, which are also cytotoxic. NADPH:cytochrome c (P450) reductase (P450R) is a one-electron reducing enzyme that efficiently activates TPZ. Recently a derivative of the A549 non-small cell lung cancer cell line (A549c50) was generated that showed substantially reduced P450R activity compared to its parental line (Elwell et al (1997) Biochem Pharmacol54: 249–257). Here, it is demonstrated that the A549c50 cells are markedly more resistant to TPZ under both aerobic and hypoxic conditions. In addition, these cells have a dramatically impaired ability to metabolize TPZ to its two-electron reduction product, SR4317, under hypoxic conditions when compared to wild-type cells. P450R activity in the A549c50 cells was reintroduced to similar levels as that seen in the parental A549 cells by transfection of the full-length cDNA for human P450R. These P450R over-expressing cells exhibit restored sensitivity to TPZ under both aerobic and hypoxic conditions, comparable to that found in the original parental A549 cells. Further, the ability of the transfected cells to metabolize TPZ to SR4317 under hypoxic conditions is also shown to be restored. This provides further evidence that P450R can play an important role in the activation, metabolism and toxicity of this lead bioreductive drug. © 2000 Cancer Research Campaign

Self-protonation mechanism in the electrochemical reduction of Jatropholone

Abstract The electrochemical reduction mechanism of jatropholone (JOH) and derivatives (JOAc) was investigated by cyclic voltammetry, polarography, coulometry and controlled potential electrolysis. It involves at the first wave potential, a self-protonation mechanism, whereby the two-electron reduction product (JOHH − ), an aromatic ketone, is formed together with the conjugate base of the former compound, JO − . The unusual stoichiometry of the cathodic process at the first wave, 1 mol electron mol −1 , indicates that JOHH − is not further protonated. The decreased basicity of the anion is due to resonance stabilisation. The second wave is related to the reduction of JO − and further reduction of JOHH − , that has a reminiscent electroactive group. In the latter case, the background electrolyte is liable to be the proton donor, through Hoffman elimination. The complete reduction involves 4 e − and 4 protons. Electrolysis performed near the first wave potential led to the almost exclusive reduction of the exocyclic double bond, without dimerization.

The role of protonation, hydration, elimination, and ring opening in the electroreduction of hexazinone

The herbicide hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (lH,3H)-dione) is a 1,3,5-triazine derivative which exists in a single cyclic tautomeric form. In the presence of water its azomethine bond is covalently hydrated (Kd=0.79, in acetonitrile as a predominant co-solvent) and protonated (pKa=1.2). Following dehydration, the protonated form of the azomethine bond undergoes a two-electron reduction in wave I1. The decrease in current at pH>2.5 results from the decrease in the rate of protonation (kr=6.3×106 mol−1 s−l). The current at pH 2 to 4 is smaller than theoretical due to a competitive hydration. The two-electron reduction product formed in waves undergoes elimination of dimethylamine. The resulting azomethine bond in the product of elimination is reduced in wave I2. The oxidized form of hexazinone undergoes an acid-catalyzed ring opening at pH<1.3. The acid catalysis is attributed to a protonation of the hydrated form of the heterocycle. The formyl group of the open-chain form is reduced in a more negative wave I1a. Ac polarography indicated adsorption of the oxidized form of hexazinone, dc polarography and voltammetry show also an adsorption of its reduced form.

Synthesis and evaluation of nitro 5-deazaflavin-pyrrolecarboxamide(s) hybrid molecules as novel DNA targeted bioreductive antitumor agents

A series of 6-nitro-5-deazaflavins bearing at N(3) or N(10) position the pyrrolecarboxamide(s) group as DNA minor groove binder has been synthesized. These hybrid molecules show similar redox properties to those of 6-nitro-5-deazaflavins with no pyrrolecarboxamide(s) group, suggesting that they generate stable one- and two-electron reduction product(s). Electrolytic reductions of the hybrid molecules were carried out at a controlled potential under anaerobic conditions in the presence of plasmid pBR322 DNA. Significant conversions of the supercoiled circular pBR322 DNA (form I) to the open circular DNA (form II) have been found by treatment with the reductively activated 6-nitro-5-deazaflavin derivatives. Their DNA damaging effects have been found to be enhanced as the number of pyrrolecarboxamide group as the DNA binder increases. Antitumor activities of the hybrid molecules towards KB and L1210 cells were evaluated in vitro. It has been found that the antitumor effects of the compounds on KB cells slightly change and those on L1210 cells decrease as the number of the pyrrolecarboxamide group increases. These results reveal that the combination of 6-nitro-5-deazaflavin molecule with the pyrrolecarboxamide(s) group increase the DNA binding properties of the compounds, giving rise to promoted DNA damaging effects, and also suggest that the combination would affect the capacity of the compounds to act as the substrate for intracellular reductases and/or the cellular uptake of the compounds.

Electrochemical investigation of lanthanide octa-tert-butylsubstituted diphthalocyanine complexes in solutions. Approximation of their redox transitions by semiempirical calculations for yttrium diphthalocyanine

The electrochemical potentials of seven redox transitions for green forms and eight redox transitions for blue forms of neutral octa-tert-butylsubstituted diphthalocyanine cornplexes of lanthanides Pct2Ln (Ln=Pr, Sm, Dy, and Lu) in solutions were measured by cyclic voltammetry and rotating disc electrode techniques. The spectroelectrochemical investigation of the products of two-electron oxidation and reduction of the green form of Pct2Lu was performed. The frontier molecular orbitals, total charge densities, total spin densities, electrostatic potentials, and heats of ion formation for (Pc2Y)m+,n− (m=0, 1, 2 and 3;n=1, 2, 3 and 4), which can model the products of the redox transitions of the diphthalocyanines under study, were calculated using the semiempirical ZINDO/1 method. The calculations for (Pc2Y)m+,n− and absorption spectra show that the electron changes in all redox transitions of the green forms of Pct2Ln are mainly localized on the ligands.

Electrochemical reduction of Schiff base ligands H 2salen and H 2salophen

The electrochemical reduction of the Schiff base ligands N, N′-1,2-ethylenebis( salicylideneimine) and N, N′-1,2-phenylenebis( salicylideneimine) has been investigated in DMF by cyclic voltammetry, coulometry and controlled potential electrolysis. The process involves a self-protonation mechanism whereby the two-electron reduction product, a cyclic derivative, is formed together with the conjugate base of the substrate, as a consequence of proton transfer from the substrate itself to the basic intermediates.

A semiempirical computational study of electron transfer reactivity of one‐ vs. two‐ring model systems for anthracycline pharmacophores. I. A rationale for mode of action

The redox capacities of 5-hydroxy-1,4-naphthaquinone (VI), 5-8-dihydroxy-1,4-naphthaquinone (VII), and 5,8-dihydroxy-1,4-naphthaquinone imine (VIII) as model systems for the pharmacophore of aclacinomycin A, adriamycin/daunomycin, and 5-iminodaunomycin (5IDN), respectively, along with 1,4-naphthaquinone (V), 1,4-benzoquinone (IV), and 1,4-benzoquinone imine (IX), have been investigated by the AM1 semiempirical method. The reduction activation of the parent (Q) model systems to their various redox states [quinone radical anion (Q−2), semiquinone (QH˙), semiquinone anion (QH−), and hydroquinone (QH2)], the redox capacities of the redox states, and the intermolecular electron self-exchange processes between the redox states and electron transfer reactions from the redox states to molecular oxygen have been examined using reaction enthalpies, adiabatic ionization potentials and electron affinities, and absolute and adiabatic electronegativities. Keto—enol transformations and the effects of solvation and H bonding on keto–enol tautomers of VI and of the hydroquinones of VI and VIII have also been assessed. The results indicate that the reactivity of VIII, relative to that of VII, may not be diminished. VI, however, appears to be less reactive than VII, and this suggests clues for the reduced toxicities of aclacinomycin A. Overall, the results suggest that the experimentally observed reduced cardiotoxic effects of 5IDN may be explained by changes in electron configuration and/or electron density and in geometry, such as changes in planarity that accompany enol to keto transformations in the reduction by product of 5IDN (Bird et al.7)—that is, between the hydroquinone (II) of naphthacenedione (I) and naphthacenone (III). Moreover, the results suggest that the two-electron reduction product, Q−2, of the drugs can be the reductant that produces reactive dioxygen species, such as O2−˙ and HO2˙, via electron transfer to molecular oxygen as opposed to QH˙ and QH2, which have been postulated to be responsible for electron transfer. This possibly new role for Q−2 may be important in cardioxicity, particularly in aprotic and/or hydrophobic media. © 1996 by John Wiley & Sons, Inc.

Spectroscopic study on the photoreduction of hypocrellin A: generation of semiquinone radical anion and hydroquinone

Hypocrellin A (HA) is an efficient phototherapeutic agent. Illumination of HA in dimethylsulphoxide (DMSO) or DMSO-buffer (1:1 by volume, pH > 6.7) generated a strong electron paramagnetic resonance (EPR) signal. This EPR signal was intensified in the presence of reductants. The EPR spectrum obtained was assigned to the semiquinone radical anion of HA (HA .−) based on a series of experimental results. Decay of HA .−, attributed to a radical-radical reaction, follows second-order kinetics. In acidic DMSO-buffer (1:1 by volume) solution, no EPR signal of HA .− was detected in the absence or presence of reductant. This was explained by a fast disproportionation of radicals, facilitated by protonation of radicals. The spectrophotometric measurements indicated that on illumination HA was directly reduced to its two-electron reduction product, i.e. hydroquinone, in acidic solution. The absorption maximum of the hydroquinone of HA which is at 496 nm at pH 5.8 shifts bathochromically with increase in pH of the medium. In DMSO or DMSO-buffer (1:1 by volume, pH > 6.7) solutions the semiquinone radical anion of HA was also observed spectrophotometrically. The absorption maximum of HA .− is at around 628 nm. Strong intramolecular hydrogen bonding was considered to exist in the chromophore of HA .−.

Electrocatalytic reduction of benzoylformic acid mediated by methyl viologen

Benzoylformic acid is reduced electrocatalytically to mandelic acid in excellent yield in the presence of methyl viologen of which two-electron reduction product behaves as active reductant, and the presence of β-CD or its derivative induces enantioselectivity.

DT-diaphorase protects cells from the hypoxic cytotoxicity of indoloquinone EO9.

Aerobic sensitivity to indoloquinone EO9 has been shown to correlate with cellular levels of the two-electron reducing enzyme DT-diaphorase. However, little is known about the relative roles of one- and two-electron reducing enzymes in the hypoxic cytotoxicity of EO9. We have characterised a panel of 23 human tumour cell lines for both bioreductive enzyme activities and aerobic sensitivity to EO9. Eight cell lines were then selected for a comparison of aerobic and hypoxic sensitivities. Activities of DT-diaphorase showed a wide range (> 10,000-fold), while activities of the one-electron reducing cytochrome b5 and cytochrome P450 reductases were generally lower and showed only a 15- and 25-fold range respectively. The aerobic cytotoxicity of EO9 was clearly related to the cellular levels of DT-diaphorase (r = 0.87), with higher levels giving increased sensitivity, but not to the levels of one-electron reducing enzymes. In contrast, there was no relationship between sensitivity to BCNU, cisplatin or the bioreductive agent SR 4233 (tirapazamine) and activities of any of these reducing enzymes. Under hypoxic conditions sensitivity to EO9 was markedly increased in cell lines with low levels of DT-diaphorase activity, while cell lines with high levels show only a small increase in sensitivity. This is reflected by a clear correlation (r = 0.98) between cellular DT-diaphorase activity and the ratio of aerobic to hypoxic sensitivity to EO9. However, we have now for the first time demonstrated an inverse correlation (r = 0.93) between the cellular activity of DT-diaphorase and hypoxic sensitivity to EO9, that is sensitivity decreases with increasing DT-diaphorase activity. Moreover, this correlation was lost when cells were exposed to drug in the presence of dicoumarol, supporting an involvement of DT-diaphorase in this relationship. These observations question the previously straightforward role for DT-diaphorase in the metabolic activation of EO9. Whereas DT-diaphorase is associated with increased toxicity in air, it appears to reduce the cytotoxicity of EO9 in hypoxic conditions. This suggests either that the one-electron reduction product of EO9 metabolism, the semiquinone, is more toxic than the two-electron reduction product, the hydroquinone, or that the hydroquinone is not cytotoxic and aerobic toxicity is due to the transient appearance of the semiquinone upon back oxidation of the hydroquinone.

Direct measurement of pO 2 distribution and bioreductive enzymes in human malignant brain tumors

Purpose : To measure the oxygen status of human malignant brain tumors in vivo and to determine the activities and expression of bioreductive enzymes in these same human brain tumor samples, as a means of assessing their suitability as targets for bioreductive drug therapy. Methods and Materials : A polarographic oxygen electrode was used to measure the intratumoral oxygen tension in twenty patients with malignant brain tumors during open brain surgery, performed under standard anaesthetic conditions. Six different tracks, each with a path length of 22 mm, were recorded per patient representing 192 readings. Following pO 2 measurements the tumors were resected and stored in liquid N 2 for subsequent bioreductive enzyme analysis. Eight human malignant brain tumors were assessed, by enzyme activity and western blot expression, for the presence of various bioreductive enzymes. These enzymes included DT-diaphorase, NADH cytochrome b5 reductase, and NADPH cytochrome P-450 reductase. Of these eight gliomas analyzed six samples were incubated with the bioreductive drug tirapazamine, in the presence of cofactor(s), to establish whether human brain tumors could metabolize this compound. Results : Both the high grade intrinsic and metastatic brain tumors showed significant regions of hypoxia. All the tumors subjected to enzyme profiling contained the bioreductive enzymes, DT-diaphorase, NADH cytochrome b5 reductase and NADPH cytochrome P-450 reductase. Also all six of the brain tumors investigated could metabolize tirapazamine to the two-electron reduction product. Conclusion : These findings would favor primary brain tumors as suitable targets for bioreductive therapy.

Mixed carbonyl-thiocarbonyl dirhenium(III) complexes of the type [Re 2(μ-S)(μ-X)X 2(CS) (μ-dppm) 2(CO)]PF 6 (X  Cl or Br) and their conversion to the analogous μ-sulphur dioxide compounds

The reactions of Re 2(μ-S)(μ-X)X 3(CS)(μ-dppm) 2 (X  Cl or Br) with TlPF 6 and CO mixtures in CH 2Cl 2 afford green complexes of the type [Re 2(μ-S)(μ-X)X 2(CS)(μ-dppm) 2(CO)]PF 6. The treatment of these carbonyl-containing products with NOPF 6/O 2 mixtures leads to the formation of the corresponding sulphur dioxide complexes [Re 2(μ-SO 2)(μ-X)X 2(CS)(μ-dppm) 2(CO)]PF 6. The μ-S and μ-SO 2 complexes exhibit a rich redox chemistry, which in the case of [Re 2(μ-SO 2)(μ-X)(X 2(CS)(μ-dppm) 2(CO)]PF 6 has led to the isolation of the stable one- and two-electron reduction products Re 2(μ-SO 2)(μ-X)X 2(CS)(μ-dppm) 2(CO) and [(η 5-C 5H 5) 2Co][Re 2(μ-SO 2)(μ-X)X 2(CS)(μ-dppm) 2(CO)]. A comparison of the cyclic voltammetric and IR spectral properties of these carbonyl complexes with those of the previously isolated species Re 2(μ-S)(μ-X)X 3(CS)(μ-dppm) 2, [Re 2(μ-S)(μ-X)X 2(CS)(μ-dppm) 2(L)]PF 6 (L  RCN or RNC) and their μ-SO 2 analogues reveals a clear dependence of the potentials of the metal-based redox couples and the frequencies of the ν(CS) mode upon the electron-withdrawing ability of the ancillary ligand (CO, RCN, or RNC).

Reductive activation of arenes. VII. Alkylation of 9-cyanoanthracene two-electron reduction products in liquid ammonia.

The reactions of the 9-cyanoanthracene dianion generated by the action of two equivalents of potassium in liquid ammonia with primary alkyl iodides and bromides gave 9-cyano-9,10-dialkyl-9,10-dihydroanthracenes. The alkylation of the 9-cyano-9,10-dihydro-9-anthryl anion generated by the two-electron reduction of 9-cyanoanthracene in liquid ammonia in the presence of ammonium chloride gave 9-cyano-9-alkyl-9,10-dihydroanthracene. The results obtained by using cyclopropylmethyl bromide as model reagent suggest these reactions to proceed through S N mechanism. The spatial structure of 9,10-dihydroanthracene derivatives obtained is discussed.

Semiconductor Photocatalysis: Reaction Mechanisms for the Photoreductive cis–trans Isomerization of Electron-Deficient Alkenes Catalyzed by CdS Powder

Abstract Highly pure commercially available CdS powder (99.999%) catalyzes the effective cis–trans photoisomerization of electron-deficient alkenes under visible light irradiation using triethylamine (TEA) as an electron donor, accompanying the formation of the dihydro compound as a two-electron reduction product. The photoisomerization does not occur at all in the absence of TEA. Donor effect, solvent effect, deuterium incorporation experiments for photocatalysis, and MOPAC molecular orbital calculation (MNDO/PM3) of the intermediary radical anions from the alkenes were investigated in order to elucidate the mechanism of this photoisomerization. These results reveal that the CdS-catalyzed cis–trans photoisomerization should proceed through two pathways involving the photoreduction of alkenes: one through the back electron transfer from the radical anion of the alkene (alkene−•) towards the radical cation of TEA (TEA+•), both formed by photoexcited conduction band electrons and holes of CdS, respectively. The other is the reoxidation of a radical intermediate (alkyl•), formed by protonation of alkene−•, by TEA+•.

Photo-induced Electron Transfer Between Hypocrellins and BNAH

1 Introduction Much research work has been done on the mechanism of photo-induced reduction of quinone dyes by electron donors with visible light. Two ways to obtain the net two-electron reduction product QH~- bave been proposed. One is the direct two-electron transfer (electron-proton-electron transfer) within the complex and the other is the disproportionatioh of the semiquinone radical QH·formed by one-electron transfer.