Density Functional Theory Calculations of the Barrier to Atropisomerism of a Dibenzo[d,f][1, 3, 2]dioxaphosphepin Moiety: a Tool for Rational Ligand Design

Density functional theory calculations yield reasonable values for the barrier to atropisomerism in a dibenzo[d, f][1, 3, 2]dioxaphosphepin moiety of diphosphite ligands and thus provide an energetic criterion for falsification of hypothetical stable stereoisomers in rational ligand design.     

Acrylamide in the Baylis-Hillman reaction: expanded reaction scope and the unexpected superiority of DABCO over more basic tertiary amine catalysts

DMAP, DBU, and quinuclidine efficiently promote novel hydroalkoxylation reactions of acrylamide in primary alcohol solvents. DABCO is a comparatively poor hydroalkoxylation promoter and can effect clean, selective Baylis-Hillman reactions between acrylamide and aldehydes in alcoholic/aqueous media in which more basic nucleophilic catalysts promote hydroalkoxylation preferentially. Optimization of the reaction conditions has allowed acrylamide to be reacted with a range of aromatic aldehydes in moderate to excellent yields, including the first examples involving deactivated, electron-rich substrates such as p-tolualdehyde and o-anisaldehyde.     

Chemical and photochemical electron transfer of new helianthrone derivatives: aspects of their photodynamic activity

Helianthrones 2-4 are a new class of synthetic photosensitizers, which have a molecular skeleton related to that of hypericin. We established that irradiation of heliantrones with visible light leads to the formation of semiquinone radicals and reactive oxygen species. The structures of the paramagnetic anion species produced by electron transfer were calculated on the density functional level and investigated by cyclovoltammetry, UV/vis, and EPR/ENDOR spectroscopy. As with hypericin, the pi system of the helianthrones was found to be considerably deviated from planarity, and, upon electron transfer, deprotonation in the bay region occurs. The structure of the semiquinone radicals was found to be identical in THF, DMF, and aqueous buffered solutions regardless of the means by which reduction was achieved. Semiquinone radicals can be formed via self-electron transfer between the excited state and the ground state or via electron transfer from an electron donor to the excited state of helianthrone. Therefore, the presence of an electron donor significantly enhanced the photogeneration of semiquinone and superoxide radical. The kinetic studies showed that no significant photochemical destruction of helianthrones occurred upon irradiation. Generation of superoxide and singlet oxygen upon irradiation of helianthrones was established by spin trapping techniques. This shows that both type I and type II mechanisms are of importance for the photodynamic action of these compounds.     

Übergangsmetall—methylen-komplexe: LIX. Ein neues darstellungsverfahren für vinyliden-komplexe

The diazo olefins N₂CCR₂ (R/R = CH₃/CH₃ and (CH₂)₅, respectively), generated in situ from the corresponding cyclic N-nitrosourethanes 2a, 2b, are suitable precursors for the corresponding vinylidene ligands 6CCR₂. Thus, treatment of the RhRh complex [(η⁵-C₅Me₅)Rh(μ-CO)]₂ (1) with 2a, 2b in the presence of lithium ethoxide yields the otherwise inaccessible μ-vinylidene complexes (μ-CCR₂)[(η⁵-C₅Me₅)Rh(CO)]₂ (3a, 3b). This procedure extends the well-documented diazoalkane method for synthesis of μ-alkylidene complexes to the less stable diazoalkenes*     

Thermoxidative degradation of poly(vinyl alcohol) under dynamic thermogravimetric conditions. II. Influence of the hydrolysis degree

The influence of the degree of hydrolysis on thermoxidative degradation of the poly(vinyl alcohol) was studied. The thermoxidative degradation was carried out under dynamic thermal conditions. Taking into account the dependence of the activation energy values on the heating rate, molecular weight, degree of conversion, and degree of hydrolysis, some conclusions were drawn about the decomposition mechanism and the variation of the thermal stability.     

Modulation with acetonitrile of the dynamics of guest binding to the two distinct binding sites of cholate aggregates

Bile salt aggregates are supramolecular systems containing two different binding sites. The effect of the addition of acetonitrile on the specificity and dynamics of guest binding to the two binding sites of cholate aggregates was studied. The protection of guests included in the aggregate from interaction with ions in the aqueous phase was evaluated from quenching of the singlet and triplet excited states of guest molecules bound to the cholate aggregates. The dynamics of guest binding to the primary and secondary binding sites of the cholate aggregates were determined at increasing acetonitrile mole fractions. The structure of the aggregates was not significantly altered provided the cholate concentrations were higher than 20 mM and the acetonitrile mole fraction did not exceed 0.033 (9.1% v/v). These results show that acetonitrile can be used to modulate the solubility of guests in the aggregates and to manipulate the residence time of guests in the primary and secondary binding sites.