Electrochemistry of Quinones with Respect to their Role in Biomedical Chemistry.

Quinones are ubiquitous in nature and form one of the largest class of antitumor agents approved for clinical use. They are known to be efficient in inhibiting cancer cells growth. Under physiological conditions they can undergo non-enzymatic one-electron reduction to give the moderately toxic species of semiquinone radical-anion. Thus, electrochemical study of quinones might provide a basic knowledge on semi-quinone radicals formation in both in vivo and in vitro under different media. Several processes are outlined briefly and discussed in the present article. Previously we investigated the electrochemical and spectral properties of ω-N-quinonyl amino acids. Such quinone-bearing peptides are known to be cytotoxic and of potential clinical significance. We were able to prove that the ω-amino quinonyl compounds are very effective in producing stable semiquinone radicals. Moreover, a direct relation was found between the first reduction potentials of the quinonyl moiety and their reactivity towards the ω-amino acids. In order to increase our knowledge of such amino quinonyl compounds and enlarge the arsenal of such cytotoxic compounds, a series of N,N-diquinonyl amines (1-6) bearing an internal proton (stems from the NH moiety) were synthesized. Their electron-transfer capabilities were probed by cyclic voltammetry measurements, in dichloromethane. It was found that the acidic NH group linking the two quinonyl moieties undergoes an initial electrochemical reduction step and generates a nitride anion. This step is followed by further reductions to yield quasi-stable semiquinone radicals and polyanions, Since these acidic diquinones (1-6) serve also as a source of internal proton donors even in non-polar medium, they might cause protonation of basic radical-anions and polyanion intermediates during the various electrochemical stages. The processes are demonstrated and discussed by analyzing different mechanistic schemes. The successful generation of relatively stable semiquinone radicals is a prerequisite for the manifestation of site directed antitumor activity by these bis-quinonyl amino derivatives. Based on the values of their redox potentials some of them could be promising candidates for clinical development.

Anodic oxidation of bisamides from diaminoalkanes by constant current electrolysis.

In general, bisamides derived from diamines and involving 3 and 4 methylene groups as spacers between the two amide functionalities behave similar to monoamides upon anodic oxidation in methanol/LiClO4 because both types undergo majorly mono- and dimethoxylations at the α-position to the N atom. However, in cases where the spacer contains two methylene groups only the anodic process leads mostly to CH2-CH2 bond cleavage to afford products of type RCONHCH2OCH3. Moreover, upon replacing LiClO4 with Et4NBF4 an additional fragmentation type of product was generated from the latter amides, namely RCONHCHO. Also, the anodic process was found to be more efficient with C felt as the anode, and in a mixture of 1:1 methanol/acetonitrile co-solvents.

Electrochemical oxidation of amides of type Ph2CHCONHAr.

Anodic oxidation of N-aryl-2,2-diphenylacetamides in acetonitrile undergoes three types of bond-cleavage, one between the benzylic carbon and the carbonyl group, the second between a carbonyl and 'N', and the third between the 'N' atom and aryl group. The selectivity of the cleavage and nature of emerged products is highly dependent on the nature of substituent attached to the aryl group. For example, electron-withdrawing groups direct the benzyl-carbonyl bond-breaking whereas electron-donating substituents favor the N-aryl bond cleavage. The type of products obtained involve benzophenone, 2,2-diphenylacetamide, N-(diphenylmethylene)acetamide, N-(diphenylmethyl)acetamide, α-lactam (1-acetyl-3,3-diphenylaziridin-2-one, as a 1 : 1 complex with 2,4-dinitroaniline) and aniline derivatives.

Electrochemical synthesis of 1,2-disilylethanes from α-silylacetic acids.

The synthesis of 1,2-disilylethanes [R(1)R(2)R(3)Si(CH(2))(2)SiR(1)R(2)R(3)] is usually conducted by using noble metal reagents or catalysts. This work describes a new electrochemical synthetic method for their preparation in good yields by oxidation of α-silylacetic acids at Pt anodes (Kolbe electrolysis). Most of the reported synthesized 1,2-disilylethanes in this work are unknown.

Electrochemical properties of a disilene, a tetrasila-1,3-butadiene, and their germanium analogues.

The redox properties of aryl-substituted disilene (1), tetrasila-1,3-butadiene (2), digermene (3), and tetragerma-1,3-butadiene (4) (in which the aryl group is 2,4,6-triisopropylphenyl) have been studied by cyclic voltammetry (CV) in two different solvents. In o-dichlorobenzene, disilene 1 exhibited reversible redox couples in both oxidation and reduction processes, whereas tetrasilabutadiene 2 showed a reversible behavior only in its oxidation and an irreversible wave in its reduction. Tetragermabutadiene 4 afforded a reversible couple in its reduction and an irreversible wave in its oxidation, whereas digermene 3 showed only an irreversible oxidation wave but no reduction one. In comparison, all observed oxidation and reduction waves in THF were irreversible. In both solvents, the same trend of decreasing ease of oxidation has been observed: 2>4>3>1. The calculated vertical ionization energies (E(i)) for the 2,6-dimethylphenyl-substituted model compounds 9-12 afforded a trend of ease of oxidation that decreases in the series tetrasilabutadiene 10>tetragermabutadiene 12>disilene 9>digermene 11. This predicted trend parallels the measured oxidation potentials in the sense that the heavy butadienes are easier to oxidize than the heavy alkenes. The trend in the reduction direction has been found to be different in the two solvents studied. The computed electron affinities (E(ea)) for compounds 9-12 indicated that the ease of reduction decreased in the sequence tetragermabutadiene 12>tetrasilabutadiene 10>digermene 11>disilene 9. However, no straightforward relation between the computed electron affinities and the experimentally measured reduction potentials has been found.

Electrochemical properties and computations of stable radicals of the heavy group 14 elements (Si, Ge, and Sn).

A series of stable radicals centered on persilyl-substituted heavy Group 14 elements, (tBu(2)MeSi)(3)E(*) (E = Si, Ge, Sn), was studied by cyclic voltammetry in different solvents to determine their first oxidation and reduction potentials and to compare their ease of oxidation and reduction with known experimental and computed ionization energies (E(i)) and electron affinities (E(ea)), respectively. It has been observed that all of the first oxidation and reduction potentials for the three radicals studied are irreversible in o-dichlorobenzene (o-DCB), whereas the reduction waves are quasi-reversible in THF. A good correlation has been found between measured oxidation potentials and ionization energy values, but no correlation between reduction potentials and electron affinity values was found, probably due to kinetic and surface effects.

One-Pot Anodic Conversion of Symmetrical Bisamides of Ethylene Diamine to Unsymmetrical gem-Bisamides of Methylene Diamine.

Symmetrical bisamides of ethylene diamine of type ArCONHCH2CH2NHCOAr undergo anodic C-C bond cleavage in acetonitrile-LiClO4 under controlled-potential electrolysis. The electrogenerated carbocation intermediates react with the solvent acetonitrile to afford unsymmetrical gem-bisamides of type ArCONHCH2NHCOMe in a one-pot reaction. The yields of the latter products are moderate (up to 60%). Other minor products involve two symmetrical gem-bisamides of type ArCONHCH2NHCOAr and MeCONHCH2NHCOMe and fragmentation products (e.g., ArCONHCHO, ArCONH2, and ArCN).

Freestanding Gold/Graphene-Oxide/Manganese Oxide Microsupercapacitor Displaying High Areal Energy Density.

Microsupercapacitors are touted as one of the promising "next frontiers" in energy-storage research and applications. Despite their potential, significant challenges still exist in terms of physical properties and electrochemical performance, particularly attaining high energy density, stability, ease of synthesis, and feasibility of large-scale production. We present new freestanding microporous electrodes comprising self-assembled scaffold of gold and reduced graphene oxide (rGO) nanowires coated with MnO2 . The electrodes exhibited excellent electrochemical characteristics, particularly superior high areal capacitance. Moreover, the freestanding Au/rGO scaffold also served as the current collector, obviating the need for an additional electrode support required in most reported supercapacitors, thus enabling low volume and weight devices with a high overall device specific energy. Stacked symmetrical solid-state supercapacitors were fabricated using the Au/rGO/MnO2 electrodes in parallel configurations showing the advantage of using freestanding electrodes in the fabrication of low-volume devices.

Anodic oxidation of mono- and disubstituted 1,4-dimethoxybenzenes.

The electrochemical oxidation of mono- and disubstituted 1,4-dimethoxybenzene derivatives with zero, one, and two benzylic CH(2)X groups (X = OAc, Cl, OH) (5a-c and 6a-c) has been carried out by using both constant-current and controlled-potential techniques in methanol and in the presence of different electrolytes and working electrodes. Constant-current electrolysis in KOH-methanol solutions yielded mostly the corresponding 1,4-quinone derivatives from 5a-c and 6b, whereas the disubstituted 1,4-dimethoxybenzenes 6a,c underwent side-chain oxidation to form 2,5-dimethoxyterephthalaldehydes. Upon alteration of the medium from the commonly used basic KOH-methanol to neutral LiClO(4)-methanol, a new spectrum of products was achieved in most cases, involving novel coupling products from monosubstituted substrates and quinone derivatives from disubstituted ones. Controlled-potential oxidation at the glassy carbon anodes and in a neutral LiClO(4)-methanol medium led to more complex mixtures of products, namely, polymers and new coupling products from monosubstituted substrates and quinones and side-chain oxidation (or substitution) products from the disubstituted ones. Three new coupling products were isolated and characterized by X-ray measurements.