Vaclav Horak

Cyclic voltammetry of metal complexes of nitrones and nitroxides

Abstract Increasing interest in radicals bound to metals led to the present study of electron transfer to and from metal nitrone complexes and metal nitroxides. These are species involved in biological spin trapping and materials science applications, respectively. Cyclic voltammetry was used to study the redox behavior of complexes of the bidentate N - tert -butyl-α-(2-pyridyl)nitrone (2-PyBN) and the monodentate nitrone 2,5,5-trimethyl-1-pyrroline- N -oxide (M 3 PO) with metal hexafluoroacetylacetonates M(hfac) 2 in CH 2 Cl 2 . Bidentate complexes M(2-PyBN)(hfac) 2 with M=Mn II and Co II were easier to oxidize in CH 2 Cl 2 (less positive E pa ) than the corresponding M(hfac) 2 and harder to reduce (more negative E pc ). In MeCN, this behavior was also observed for M=Co II and Ni II which indicate more facile oxidation when M 3 PO is present as co-ligand compared with M(hfac) 2 alone. CVs of M(2-PyBN)(hfac) 2 and [M(M 3 PO)(hfac) 2 ] 2 in MeCN gave common reduction peaks due to hfac. Nitroxides (aminoxyls) 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-1-oxyl (CPNO) gave quasi-reversible oxidations to oxoammonium cations, while N - tert -butyl-(2-pyridyl)phenylmethanaminoxyl (2-PyBNO) showed only irreversible oxidations. CV of solutions of CPNO in the presence of M(hfac) 2 , where M is Mn II , Co II , or Ni II , indicated formation of metal–nitroxide complexes for Co II and Ni II . For stable metal nitroxides prepared independently, the CV of Mn(TEMPO) 2 (hfac) 2 showed dissociation to free nitroxide and Mn(hfac) 2 in CH 2 Cl 2 , while M(2-PyBNO)(hfac) 2 gave distinctive CVs for M=Mn II , Co II , and Ni II .

Spontaneous rearrangement in Corey's reaction

Abstract A 1 H-NMR study of the oxidative deamination of sec -alkyl primary amines with 3,5-di- tert -butyl-1,2-benzoquinone confirmed spontaneous rearrangement of the quinone-imines to the respective aromatic Schiff bases.

Voltammetric determination of a pesticide, sulfometuron methyl, via its N-chloroderivative

Abstract The reported method using differential pulse voltammetry on a platinum electrode allows selective determination of low concentrations of sulfometuron methyl (SMM, weed killer “Oust”, DuPont). An electroactive species is obtained in an aqueous solution of SMM with NaClO at pH 9.0. The reaction is characterized by a yellow coloration (λmax = 400 nm) with maximum absorbance measured after 30 min. The reaction product shows a cathodic peak in acidic medium, its current is linearly proportional to the SMM concentration. The potential range (Epc around 440 mV vs. Ag|AgCl) makes the monitoring highly selective. Oxidation of the iodide ion to iodine proves the presence of a ‘positive’ chlorine in SMM-Cl. Solid phase extraction of more hydrophobic SMM-Cl, when compared with SMM, from aqueous solutions allows a ten-fold increase in sensitivity of the method (ppb range or lower).

Generation and trapping of 2,3-naphthoquinone

Abstract 2,3-Naphthoquinone generated from 2,3-dihydroxynaphthalene was trapped with cyclopentadiene and its properties examined by dc polarography and cyclic voltametry.

The electrochemical oxidation of 2,6-dichloro-1,4-phenylenediamine

The electrochemical oxidation of 2,6-dichloro-1,4-phenylenediamine was studied at a glassy carbon electrode in a 50 vol% methanol-water mixture containing 0.1M HClO(4). Single sweep voltammograms are compared with curves obtained by digital simulation. Characteristic reaction parameters, such as formal potentials, charge transfer coefficients, rate constants of the electrochemical and the chemical steps, and diffusion coefficients, were determined from potential-step experiments in combination with a simulation-curvefitting routine.

POLAROGRAPHIC STUDY OF STRUCTURE-PROPERTIES RELATIONSHIP OF p-TOSYL SULFILIMINES

Abstract A structure-reactivity relationship for a series of eight N-p-tosyl sulfilimines and five related sulfoxides is investigated by polarographic reduction of the S-N and S-O bonds respectively. Conjugation of the tricoordinated sulfur is the dominant factor that controls the rate of the S-N and S-O bond cleavage, which follows a trend opposite to that of the oxidation process. A significant correlation between the E 1/2 values and the v S-N IR frequencies is presented.

Magnetic circular dichroism of cyclic π-electron systems: Part 30. Thialene and isothialene

Magnetic circular dichroism (MCD) spectra of thialene (1) and isothialene (2) were recorded and interpreted in terms of an aromatic (4N + 2) -electron [9] annulenide perimeter perturbed by a cross-link and a heteroatom. The B term sign sequence is the same for both heterocycles, B (L1) 0, suggesting that the frontier orbital energy differences are such that ΔHOMO ΔLUMO for 2. Isothialene thus represents a very rare example of an aromatic π system whose MCD signs for the L bands are predicted incorrectly at the usual PMO level. The correct answer, ΔHOMO < ΔLUMO, results when the PMO approach is carried beyond first order.

Chapter 6 Chemistry of Melanins

Publisher Summary This chapter provides a concise, yet comprehensive and up-to-date overview of the field of melanins, with particular emphasis on the chemical, physical, and biosynthetic aspects of these important and ubiquitous pigments. Tyrosinase is known to catalyze the biosynthesis of not only black but also brown, yellow, reddish brown, and carrot-red pigments. Melanin pigmentation is mainly determined by two chemically distinct but biogenetically related types of pigments. One of them is the dark, insoluble eumelanins that are produced from the tyrosinase-catalyzed oxidation of tyrosine, and the other is the alkalisoluble phaeomelanins that originate from an altered eumelanin pathway through the intervention of cysteine. Evidence suggests that certain heavy metal ions, commonly found in pigmented tissues, play an important role in melanogenesis. Of particular interest is the finding that copper and, to a lesser degree, zinc, cobalt, and iron have the ability to catalyze the rearrangement of dopachrome to 5,6-dihydroxyindole(s), which is a key regulatory step in the biosynthesis of eumelanins.