Frédérique Barbosa

Memory of Chirality in Photochemistry

The helical-chiral character of the diradical intermediate 2, which cyclizes quicker than it equilibrates, explains the memory effect of chirality that occurs during the enantioselective photocyclization of alanine derivative 1 to give the proline derivative 3. Ts=H(3)CC(6)H(4)SO(2).

Violene/cyanine hybrids as electrochromics part 2: tetrakis(4-dimethylaminophenyl)ethene and its derivatives

The general structure of violene/cyanine hybrids (see below) is exemplified by tetrakis(4-dimethylaminophenyl)ethene 1(RED) its vinylogue 2(RED) and its diazavinylogue 3(RED). As judged from their cyclic voltammograms and spectroelectrograms, oxidation occurs perfectly reversible by loss of two electrons creating closed shell systems 1-3(OX)+2 with strong bathochromic shifts (Michlers hydrol blue moieties). ESR spectra indicate only minor amounts of radical cations. At much higher potentials by another reversible loss of two electrons (-->1-3(OX)+4) the long wavelengths absorptions are replaced by shorter ones. In system 4, containing two 4-dimethylaminophenyl units only, the violene character is better preserved since oxidation occurs stepwise by single electron transfer up to 4(OX)+4. These results are backed by theoretical calculations for 1-4, demonstrating the strong geometrical differences between the various oxidation levels. Besides, new types of cyclic structures for 1-4(OX)+4 are indicated by these calculations: For systems 1-3 cyclic structures for tetracations have been found to be more stable by 3-20 kcalmol(-1) than acyclic structures, whereas for system 4 the acyclic structure is more stable by about 22 kcalmol(-1). The redox behavior of systems 1-4 is of general importance for electrochromic systems.

Generation of Free Radicals by Emodic Acid and its [d-Lys6]GnRH-conjugate¶

In an attempt to develop an efficient chemotherapeutic agent targeted at malignant cells that express receptors to gonadotropin releasing hormone (GnRH) we coupled [d‐Lys6]GnRH covalently to an emodin derivative, i.e. emodic acid (Emo) to yield [d‐Lys6(Emo)]GnRH. Emodin is a naturally occurring anthraquinone which is widely used as a laxative and has other versatile biological activities. Physico‐chemical studies employing electron paramagnetic resonance and electrochemistry of the conjugate as well as the (Emo) moiety showed that these compounds could be easily reduced either chemically, photochemically or enzymatically to their corresponding semiquinones. In the presence of oxygen the semiquinones generated reactive oxygen species (ROS), mainly superoxide and hydroxyl radicals, which were detected by the spin trapping method. Moreover, upon irradiation with visible light these compounds produced ROS and a highly reactive excited triplet state of Emo, which by itself may cause the oxidation of certain electron acceptors such as amino acids and bases of nucleic acids. Thus, [d‐Lys6]GnRH‐photosensitizer conjugates may be potentially used for targeted photodynamic chemotherapy aimed at treating cancer cells that carry GnRH receptors. These conjugates may also induce cytotoxicity in the dark similar to common conventional chemotherapeutic agents. The peptidic moiety, [d‐Lys6]GnRH, was found to be stable toward highly reactive ROS generated either from enzymatic reduction or upon photoirradiation. The physico‐chemical properties of Emo were only marginally influenced by the peptidic [d‐Lys6]GnRH carrier.

Memory effect of chirality in the photocyclization of modified dipeptides

Abstract Photocyclizations of dipeptides 5 and 8 occur mainly with retention of configuration at the stereogenic center of alanine. This memory effect of chirality is explained by a hindered rotation in the two intermediate biradicals 9 and 10 that slows down the equilibration of the radicals.

Chiralitäts‐Memory‐Effekt in der Photochemie

Der helical-chirale Charakter der Diradikal-Zwischenstufe 2, die rascher cyclisiert als aquilibriert, erklart den Memory-Effekt der Chiralitat, der bei der enantioselektiven Photocyclisierung des Alaninderivats 1 zum Prolinderivat 3 auftritt. Ts=H3CC6H4SO2.

Hypericin Derivatives: Substituent Effects on Radical-anion Formation

The electron‐transfer properties of the hypericin derivatives, dibromo‐, hexaacetyl‐, hexamethyl‐ and desmethylhypericin, were studied. Cyclovoltammetric measurements revealed that dibromo‐ and desmethylhypericin have almost the same redox potentials as the parent hypericin. Substitution of the hydroxyl groups by acetoxy leads to less negative E½ values, whereas methoxy substitution induces more negative values. Electron paramagnetic resonance (EPR)/electron nuclear double resonance/general TRIPLE spectroscopy and quantum mechanical calculations were used to establish the structure of the one‐electron reduced stages of hypericin derivatives. Proton loss in the bay region, already demonstrated for hypericin, was also found for dibromo‐ and desmethylhypericin. The spin and charge of the radical ions are predominately confined to the central biphenoquinone moiety of the hypericin skeleton. Generation of the radical ions by in situ electrolysis indicates that the redox potentials of hypericin, dibromo‐ and desmethylhypericin, containing hydroxyls at the 1, 3, 4, 6, 8 and 13 positions, largely depend on the solvent. With phosphate‐buffered saline (pH 7.4)/dimethylsulfoxide (DMSO) as the solvent the EPR spectra of the corresponding radical ions appear at markedly lower potentials than in pure DMSO and N,N′‐dimethylformamide. However, this effect is not observable for hexaacetyl‐ and hexamethylhypericin‐lacking hydroxyl groups. In all cases the EPR data and calculations revealed the presence of 7,14 tautomers.

1,2,3,4-Tetra-tert-butyl-4-trimethylsilyl-4-sila-2-cyclobuten-1-yl: A Quantum Mechanical and ESR Study

Trisilane 7 has been synthesized as a potential photochemical precursor of tetra-tert-butylsilatetrahedrane (5) and/or tetra-tert-butylsilacyclobutadiene (6). Astonishingly, only one Si,Si bond is broken upon irradiation of 7 and the silacyclobutenyl radical 9 can be identified as the reaction product. The structure of radical 9, which in the absence of oxygen is persistent even at room temperature, has been elucidated by comparison of its experimental and calculated ESR spectra.

Stereoselective C,C-bond formation. Cyclizations of biradicals

The formation of C,C-bonds via photolytically generated biradicals can occur with high stereoselectivity. If triplet biradicals are involved, chiral induction is highly likely. Syntheses that occur only via singlet biradicals have a good chance to show a memory effect of chirality.

Ion pairing in radical cations: the example of 9,9′-bianthryl

9,9′-Bianthryl (1) was one-electron oxidized under several different conditions. Depending on the counter anion and the solvent, two structures of the corresponding radical cation were established. The temperature behaviour of the EPR spectra is interpreted in terms of a loose ion pair formed preferentially at low temperatures with the spin and the charge delocalised within the entire π system of 1˙+. At increased temperatures and with CF3COO− as the counterion and trifluoroacetic acid or 1,1,1,3,3,3-hexafluoropropanol as the solvent, a tight ion pair is formed which resembles the anthracene radical cation. The distinction between the two species can also be drawn from the optical spectra. Quantum mechanical calculations indicate that the formation of the radical cation is guided with a change of the twist angle between the two anthracene planes from 90° to ca. 74°.