Kinetics of dissociative electron transfer to ascaridole and dihydroascaridole-model bicyclic endoperoxides of biological relevance

Activation of O-Electrophiles via Structural and Solvent Effects: SN2@O Reaction of Cyclic Diacyl Peroxides with Enol Acetates

The reactions of O-electrophiles, such as organic peroxides, with carbon nucleophiles are an umpolung alternative to the common approaches to C-O bond formation. Nucleophilic substitution at the oxygen atom of cyclic diacyl peroxides by enol acetates with the following deacylation leads to α-acyloxyketones with an appended carboxylic acid in 28-87% yields. The effect of fluorinated alcohols on the oxidative functionalization of enol acetates by cyclic diacyl peroxides was studied experimentally and computationally. Computational analysis reveals that the key step proceeds as a direct substitution nucleophilic bimolecular (S_N2) reaction at oxygen (S_N2@O). CF₃CH₂OH has a dual role in assisting in both steps of the reaction cascade: it lowers the energy of the S_N2@O activation step by hydrogen bonding to a remote carbonyl and promotes the deacylation of the cationic intermediate.

Tubular Perylene Bisimide Macrocycles for the Recognition of Geometrical Isomers of Azobenzenes

Perylene bisimide-based materials are good candidates for photosensitive applications. Herein, we report synthesis, characterization, and complexation studies of perylene bisimide macrocycles obtained through bayside coupling. The isomeric macrocycles incorporated with interesting optical properties and tubular-shaped cavities are able to recognize geometric isomers of azobenzenes and aromatic amines. Such selective recognition is useful toward developing potential sensors for interesting isomeric pairs in the future.

Combinations of ascaridole, carvacrol, and caryophyllene oxide against Leishmania

To date there are no vaccines against Leishmania and chemotherapy remains the mainstay for the control of leishmaniasis. The drugs currently used for leishmaniasis therapy are significantly toxic, expensive, and result in a growing frequency of refractory infections. In this study, we evaluated the effect of combinations of the main components of essential oil from Chenopodium ambrosioides (ascaridole, carvacrol, and caryophyllene oxide) against Leishmaniaamazonensis. Anti-leishmanial effects of combinations of pure compounds were evaluated in vitro and the fractional inhibitory concentration (FIC) indices were calculated. BALB/c mice infected with L. amazonensis were treated with different concentrations of ascaridole-carvacrol combinations by intralesional doses every 4 days. Disease progression and parasite burden in infected tissues were determined. In vitro experiments showed a synergistic effect of the combination of ascaridole-carvacrol against promastigotes of Leishmania with a FIC index of 0.171, while indifferent activities were observed for ascaridole-caryophyllene oxide (FIC index=3.613) and carvacrol-caryophyllene oxide (FIC index=2.356) combinations. The fixed ratio method showed that a 1:4 ascaridole-carvacrol ratio produced a better anti-protozoal activity on promastigotes, lower cytotoxicity, and synergistic activity on intracellular amastigotes (FIC index=0.416). Significant differences (p<0.05) in lesion size and parasite burden were demonstrated in BALB/c mice experimentally infected and treated with the ascaridole-carvacrol combinations compared with control animals. Carvacrol showed significant higher anti-radical activity in the DPPH assay compared with caryophyllene oxide. Electron spin resonance spectroscopy in combination with spin trapping suggested the presence of carbon-centered radicals after activation of ascaridole by Fe(2+). The intensity of the signals is preferably decreased upon addition of carvacrol. The ascaridole-carvacrol combination could represent a future alternative to monotherapeutic anti-leishmanial agents.

Dissociative electron transfer to diphenyl-substituted bicyclic endoperoxides: the effect of molecular structure on the reactivity of distonic radical anions and determination of thermochemical parameters

The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with an observed peak potential of −1.4 V at 0.2 V s⁻¹. The major product (90% yield) resulting from the heterogeneous bulk electrolysis of 4 at −1.4 V with a rotating disk glassy carbon electrode is 1,4-diphenyl-cyclopent-2-ene-cis-1,3-diol with a consumption of 1.73 electrons per mole. In contrast, 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1), undergoes a two-electron reduction mechanism in quantitative yield. This difference in product yield between 1 and 4 is suggestive of a radical-anion mechanism, as observed with 1,4-diphenyl-2,3-dioxabicyclo-[2.2.2] octane (2) and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane (3). Convolution potential sweep voltammetry is used to determine unknown thermochemical parameters of 4, including the O-O bond dissociation energy and the standard reduction potential and a comparison is made to the previously studied bicyclic endoperoxides 1–3 with respect to the effect of molecular structure on the reactivity of distonic radical anions.

Electron transfer catalysis with monolayer protected Au₂₅ clusters

Au₂₅L₁₈ (L = S(CH₂)₂Ph) clusters were prepared and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au₂₅L₁₈⁺ quantitatively. The kinetics and thermodynamics of the corresponding electron transfer (ET) reactions were characterized via electrochemistry and thermochemical calculations. Au₂₅L₁₈⁺ was used in homogeneous redox catalysis experiments with a series of sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au₂₅L₁₈/Au₂₅L₁₈⁻ and the Au₂₅L₁₈⁺/Au₂₅L₁₈ redox couples as redox mediators. Simulation of the CV curves led to determination of the ET rate constant (k(ET)) values for concerted dissociative ET to the peroxides. The ET free energy ΔG° could be estimated for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log k(ET)vs.ΔG°) relationship. Comparison with the k(ET) obtained using a ferrocene-type donor with a formal potential similar to that of Au₂₅L₁₈/Au₂₅L₁₈⁻ showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.

The endoperoxide ascaridol shows strong differential cytotoxicity in nucleotide excision repair-deficient cells

Targeting synthetic lethality in DNA repair pathways has become a promising anti-cancer strategy. However little is known about such interactions with regard to the nucleotide excision repair (NER) pathway. Therefore, cell lines with a defect in the NER genes ERCC6 or XPC and their normal counterparts were screened with 53 chemically defined phytochemicals isolated from plants used in traditional Chinese medicine for differential cytotoxic effects. The screening revealed 12 drugs that killed NER-deficient cells more efficiently than proficient cells. Five drugs were further analyzed for IC(50) values, effects on cell cycle distribution, and induction of DNA damage. Ascaridol was the most effective compound with a difference of >1000-fold in resistance between normal and NER-deficient cells (IC(50) values for cells with deficiency in ERCC6: 0.15μM, XPC: 0.18μM, and normal cells: >180μM). NER-deficiency combined with ascaridol treatment led to G2/M-phase arrest, an increased percentage of subG1 cells, and a substantially higher DNA damage induction. These results were confirmed in a second set of NER-deficient and -proficient cell lines with isogenic background. Finally, ascaridol was characterized for its ability to generate oxidative DNA damage. The drug led to a dose-dependent increase in intracellular levels of reactive oxygen species at cytotoxic concentrations, but only NER-deficient cells showed a strongly induced amount of 8-oxodG sites. In summary, ascaridol is a cytotoxic and DNA-damaging compound which generates intracellular reactive oxidative intermediates and which selectively affects NER-deficient cells. This could provide a new therapeutic option to treat cancer cells with mutations in NER genes.

Generation of naphthoquinone radical anions by electrospray ionization: solution, gas-phase, and computational chemistry studies

Radical anions are present in several chemical processes, and understanding the reactivity of these species may be described by their thermodynamic properties. Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry studies. In this work, we report on the generation of radical anions of quinonoid compounds (Q) by electrospray ionization mass spectrometry. The balance between radical anion formation and the deprotonated molecule is also analyzed by influence of the experimental parameters (gas-phase acidity, electron affinity, and reduction potential) and solvent system employed. The gas-phase parameters for formation of radical species and deprotonated species were achieved on the basis of computational thermochemistry. The solution effects on the formation of radical anion (Q(•-)) and dianion (Q(2-)) were evaluated on the basis of cyclic voltammetry analysis and the reduction potentials compared with calculated electron affinities. The occurrence of unexpected ions [Q+15](-) was described as being a reaction between the solvent system and the radical anion, Q(•-). The gas-phase chemistry of the electrosprayed radical anions was obtained by collisional-induced dissociation and compared to the relative energy calculations. These results are important for understanding the formation and reactivity of radical anions and to establish their correlation with the reducing properties by electrospray ionization analyses.

Concerted heavy-atom bond cleavage and proton and electron transfers illustrated by proton-assisted reductive cleavage of an O-O bond

Electron transfer may be concerted with proton transfer. It may also be concerted with the cleavage of a bond between heavy atoms. All three events may also be concerted. A model is presented to analyze the kinetics of these all-concerted reactions for homogeneous or electrochemical reduction or oxidation processes. It allows the estimation of the kinetic advantage that derives from the increase of the bond-breaking driving force resulting from the concerted proton transfer. Application of the model to the electrochemical reductive cleavage of the O-O bond of an organic peroxide in the presence of a proximal acid group illustrates the applicability of the model and provides an example demonstrating that electron transfer, heavy-atom bond breaking, and proton transfer may be all concerted. Such analyses are expected to be useful for the invention, analysis, and optimization of reactions involved in contemporary energy challenges as well as for the comprehension of major biochemical processes, a number of which involve electron and proton transfer together with cleavage of bonds between heavy atoms.

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES OF 2-AMINOPHENOTHIAZINE

Phenothiazines derivatives are versatile compounds that are used in many fields, depending on the type and position of the substitution on the parent molecule. The photochemical, photophysical and electrochemical properties of several phenothiazine derivatives have been previously reported in detail. However, no reports have been presented for 2-aminophenothiazine (APH), a candidate that provides for the further chemical modification and the introduction of specific substituents. In this work, the photophysical and electrochemical properties of APH were measured in acetonitrile. The APH ground state absorption and fluorescence spectrum (phi(f) < 0.01) are similar to the corresponding that of PH parent molecule. A mono exponential decay fluorescence lifetime of 0.65 ns was determined for APH in acetonitrile. Characterization of the 355 nm nanosecond laser flash photolysis transient species reveals the presence of the triplet-triplet transient intermediate with a high intersystem crossing quantum yield (phi(T) = 0.72 +/- 0.07), indicating that the APH main excited state deactivation channel is intersystem crossing. The oxidation potential of APH is lower than phenothiazine parent molecule ((0.38 V vs 0.69 V vs Ag/AgCl(sat)). Altogether, these results show that APH has photochemical and photophysical properties similar to the phenothiazine parent molecule, but with the possibility of providing an amino functionality at 2-position for further chemical modification.

Electron Transfer to Sulfides and Disulfides: Intrinsic Barriers and Relationship between Heterogeneous and Homogeneous Electron-Transfer Kinetics

The electron-acceptor properties of series of related sulfides and disulfides were investigated in N,N-dimethylformamide with homogeneous (redox catalysis) and/or heterogeneous (cyclic voltammetry and convolution analysis) electrochemical techniques. The electron-transfer rate constants were determined as a function of the reaction free energy and the corresponding intrinsic barriers were determined. The dependence of relevant thermodynamic and kinetic parameters on substituents was assessed. The kinetic data were also analyzed in relation to corresponding data pertaining to reduction of diaryl disulfides. All investigated reductions take place by stepwise dissociative electron transfer (DET) which causes cleavage of the C(alkyl)--S or S--S bond. A generalized picture of how the intrinsic electron-transfer barrier depends on molecular features, ring substituents, and the presence of spacers between the frangible bond and aromatic groups was established. The reduction mechanism was found to undergo a progressive (and now predictable) transition between common stepwise DET and DET proceeding through formation of loose radical anions. The intrinsic barriers were compared with available results for ET to several classes of dissociative- and nondissociative-type acceptors, and this led to verification that the heterogeneous and the homogeneous data correlate as predicted by the Hush theory.

Electrochemical reduction of G3-factor endoperoxide and its methyl ether: evidence for a competition between concerted and stepwise dissociative electron transfer

The reduction of the bicyclic G-factor endoperoxides G3 and G3Me was studied in N,N-dimethylformamide using cyclic voltammetry and convolution analysis. Electron transfer leads to irreversible cleavage of the O--O bond. Detailed analysis of the voltammetry curves reveals a non-linear dependence on the transfer coefficient indicating a mechanistic transition from a stepwise mechanism to one with more concerted character with increasing potential. By using quantum calculations to estimate the O--O bond dissociation energies, the experimental data was used to evaluate the standard reduction potentials and other pertinent thermochemical information.

Controlled Modulation of Conductance in Silicon Devices by Molecular Monolayers

We have controllably modulated the drain current (I(D)) and threshold voltage (V(T)) in pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) by grafting a monolayer of molecules atop oxide-free H-passivated silicon surfaces. An electronically controlled series of molecules, from strong pi-electron donors to strong pi-electron acceptors, was covalently attached onto the channel region of the transistors. The device conductance was thus systematically tuned in accordance with the electron-donating ability of the grafted molecules, which is attributed to the charge transfer between the device channel and the molecules. This surface grafting protocol might serve as a useful method for controlling electronic characteristics in small silicon devices at future technology nodes.

Reactions of Artemisinin and Arteether with Acid: Implications for Stability and Mode of Antimalarial Action

The currently accepted mechanism of trioxane antimalarial action involves generation of free radicals within or near susceptible sites probably arising from the production of distonic radical anions. An alternative mechanistic proposal involving the ionic scission of the peroxide group and consequent generation of a carbocation at C-4 has been suggested to account for antimalarial activity. We have investigated this latter mechanism using DFT (B3LYP/6-31+G* level) and established the preferred Lewis acid protonation sites (artemisinin O5a>>O4a approximately O3a>O2a>O1a; arteether O4a>or=O3a>O5b>>O2a>O1a; Figure 3) and the consequent decomposition pathways and hydrolysis sites. In neither molecule is protonation likely to occur on the peroxide bond O1-O2 and therefore lead to scission. Therefore, the alternative radical pathway remains the likeliest explanation for antimalarial action.

Regioselective S—O vs. C—O bond cleavage in sulfenate ester radical anions

The electron transfer (ET) reduction of benzyl benzenesulfenate ester (1) and tert-butyl benzenesulfenate ester (2) was investigated using electrochemical techniques. Analysis of the cyclic voltammetry of each compound suggests that the ET reduction proceeds via a stepwise dissociative mechanism. The voltammograms of 1 are similar to those of diaryl disulfides and it was found through controlled potential electrolysis (CPE) product studies that ET reduction leads to S—O bond cleavage. The voltammograms of 2 are dramatically different with a sharper dissociative wave occurring at a more negative peak potential. CPE experiments indicate products that result from ET leading to C—O bond cleavage in this case. DFT calculations of the singly occupied molecular orbitals (SOMOs) of 1 and 2 were performed and offer a rationale for the different reactivity of the two radical anions.Key words: sulfenate esters, dissociative electron transfer, electrochemistry, radical anions.

On the Susceptibility of Organic Peroxy Bonds to Hydride Reduction

Reduction of organic molecules that contain a peroxy bond is broadly considered as a "risky" and uncertain operation when cleavage of the peroxy linkage is not desired. For this reason, such reduction steps are normally avoided at the planning stage of the synthesis when possible. As a natural consequence, the information in the literature about the susceptibility of organic peroxy bonds to reducing species is scant. In this work the tolerance of organic peroxy bonds to some common hydride reductants was examined systematically for the first time. Using reduction of ester group to alcohol as a probe, LiAlH(4), LiAlH(O(t)()Bu)(3), LiBHEt(3), and LiBH(4) were found to be significantly better than other reductants examined when taking into consideration both the completeness of the reduction of ester groups and the peroxy bond survival rate. LiBH(4) appeared to be the most suitable reductant for the reduction under discussion, not only because of the high reduction yields/excellent compatibility with peroxy bonds, but also because of the advantages in practical aspects. The results disclosed herein may (hopefully) provide a handy reference for dealing with reduction of other peroxy bond-containing molecules in the future.

Elucidation of the Electron Transfer Reduction Mechanism of Anthracene Endoperoxides

The homogeneous and heterogeneous reductions of the endoperoxides 9,10-diphenyl-9,10-epidioxyanthracene (DPA-O2) and 9,10-dimethyl-9,10-epidioxyanthracene (DMA-O2) were investigated, and they were found to undergo a dissociative electron-transfer reduction of the O-O bond to yield a distonic radical anion, with no evidence for C-O bond dissociation. A number of thermochemical parameters for each were determined using Savéant's model for dissociative electron transfer (ET), including E degrees, DeltaG(o)++, and bond dissociation energies. The products of the ET are dependent on the mode of reduction, namely heterogeneous or homogeneous, and on the electrode potential or standard potential of the homogeneous donor, respectively. The dissociative reduction of DMA-O2 under heterogeneous and homogeneous conditions yields the corresponding 9,10-dihydroxyanthracene DMA-(OH)2, quantitatively, in an overall two-electron process. In the case of DPA-O2, ET reduction also yields the corresponding 9,10-dihydroxyanthracene DPA-(OH)2 from reduction of the distonic radical anion, but in competition with this reduction, an O-neophyl-type rearrangement occurs that generates a carbon radical with a minimum rate constant of 5.9 x 10(10) s(-1). In the presence of a sufficiently reducing medium, the carbon-centered radical is reduced (E degrees = -0.85 V vs SCE) and ultimately yields 9-phenoxy-10-phenyl anthracene (PPA). The observation of this product is remarkable. In the heterogeneous ET, the yield of DPA-(OH)2/PPA is 97:3 and allows an estimate of the rate constant for ET to the distonic radical anion. In homogeneous reductions, the O-neophyl rearrangement is quantitative, but the yield of PPA depends on the redox properties of the donor. A unified mechanism of reduction of DPA-O2 is presented to account for these observations.

Synthesis and chemistry of unusual bicyclic endoperoxides containing the pyridazine ring

Inverse-Diels-Alder reaction of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate with unsaturated bicyclic endoperoxides gave the bicyclic endoperoxides containing the pyridazine ring. The NEt(3) and CoTPP (TPP = tetraphenylporphyrin) catalyzed reaction of endoperoxide 8 resulted in the formation of hydroxy ketone 11 and cis-diol 9. Cleavage of the peroxide linkage in 8 with thiourea provided cis-diol 9. Oxidation of hydroxy ketone 11 and cis-diol 9 led to the phthalazine-5,8-dione 10. Furthermore, the various transformations of the other endoperoxides 19, 20, 22, 23, and 30 resulted in the formation of pyridazine derivatives.

Metallophthalocyanines photosensitize the breakdown of (hydro)peroxides in solution to yield hydroxyl or alkoxyl and peroxyl free radicals via different interaction pathways

Interactions of organic peroxides (R'OOR) and hydroperoxides (R'OOH), including H2O2, with excited triplet and singlet state metallophthalocyanines (MPc, M = Zn, Al) have been studied by T-T absorption decay and fluorescence quenching. The ensuing photochemical processes result in decomposition of (hydro)peroxides as assessed by photo-EPR (electron paramagnetic resonance) and spin trapping. In argon-saturated apolar solutions and low MPc concentrations, alkoxyl free radicals (*OR) were identified as the primary products of (hydro)peroxide breakdown. Similarly, photosensitized decomposition of symmetric disulfides results in the formation of sulfur-centered radicals. In air-free aqueous solutions, ROOH photosensitization always gave rise to a mixture of hydroxyl and peroxyl radical (*OOR) adducts in varying molar ratios. At high MPc concentrations, both in polar and in apolar solutions, the most abundant products of ROOH decomposition were identified as *OOR. This indicates a change in the predominant interaction pathway, most likely mediated by MPc exciplexes and involving H-atom abstraction from ROOH by MPc-cation radicals. The prevalence of MPc singlet vs. triplet state interactions was confirmed by the much higher singlet quenching rate constants (log kq up to 9.5; vs. log kT < or = 4.5). In contrast to the triplet quenching, singlet quenching rates were found to depend on the (hydro)peroxide structure, following closely the trend of varying *OR yields for different substrates. Thermodynamic calculations were performed to correlate experimental results with models for electronic energy and charge transfer processes in agreement with the Marcus theory (Rhem and Weller approximation) and Savéant's model for a concerted dissociative electron transfer mechanism.

Insights into the free-energy dependence of intramolecular dissociative electron transfers

To study the relationship between rate and driving force of intramolecular dissociative electron transfers, a series of donor-spacer-acceptor (D-Sp-A) systems has been devised and synthesized. cis-1,4-Cyclohexanedyil and a perester functional group were kept constant as the spacer and acceptor, respectively. By changing the aryl substituents of the phthalimide moiety, which served as the donor, the driving force could be varied by 0.74 eV. X-ray diffraction crystallography and ab initio conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido)-axial(perester) conformation and the same D/A orientation. The intramolecular dissociative electron-transfer process was studied by electrochemical means in N,N-dimethylformamide, in comparison with thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular process consists of the electron transfer from the electrochemically generated phthalimide-moiety radical anion to the peroxide functional group. The electrochemical analysis provided clear evidence of a concerted dissociative electron-transfer mechanism, leading to the cleavage of the O-O bond. Support for this mechanism was obtained by ab initio MO calculations, which provided information about the LUMO of the acceptor and the SOMO of the donor. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter data being obtained by using the model molecules. As long as the effective location of the centroid of the donor SOMO does not vary significantly by changing the aryl substituent(s), the intramolecular dissociative electron transfer obeys the same main rules already highlighted for the corresponding intermolecular process. On the other hand, introduction of a nitro group drags the SOMO away from the acceptor, and consequently, the intramolecular rate drops by as much as 1.6 orders of magnitude from the expected value. Therefore, a larger solvent reorganization than for intermolecular electron transfers and the effective D/A distance and thus electronic coupling must be taken into account for quantitative predictions of intramolecular rates.

Theoretical and electrochemical analysis of dissociative electron transfers proceeding through formation of loose radical anion species: reduction of symmetrical and unsymmetrical disulfides

The dissociative reduction of a series of symmetrical (RSSR, R = H, Me, t-Bu, Ph) and unsymmetrical disulfides (RSSR', R = H, R' = Me and R = Ph, R' = Me, t-Bu) was studied theoretically, by MO ab initio calculations and, for five of them, also experimentally, by convolution voltammetry in N,N-dimethylformamide. The reduction is dissociative but proceeds by a stepwise mechanism entailing the formation of the radical anion species. The electrochemical data led to estimated large intrinsic barriers, in agreement with an unusually large structural modification undergone by the disulfide molecules upon electron transfer. The theoretical results refer to MP2/3-21G*//MP2/3-21G*, MP2/3-21*G*//MP2/3-21G*, CBS-4M, and G2(MP2), the latter approach being used only for the molecules of small molecular complexity. A loose radical-anion intermediate was localized and the dissociation pattern for the relevant bonds analyzed. For all compounds, the best fragmentation pathway in solution is cleavage of the S-S bond. In addition, S-S bond elongation is the major structural modification undergone by the disulfide molecule on its way to the radical anion and eventually to the fragmentation products. The calculated energy of activation for the initial electron transfer was estimated from the crossing of the energy profiles of the neutral molecule and its radical anion (in the form of Morse-like potentials) as a function of the S-S bond length coordinate. The inner intrinsic barrier obtained in this way is in good agreement with that determined by convolution voltammetry, once the solvent effect is taken into account.