Unusual Rate Acceleration in Brønsted Acid Catalyzed Dehydration Reactions: Local Hydrophobic Environment in Aggregated N‐(2,6‐diphenylphenyl)‐N‐mesitylammonium Pentafluorobenzenesulfonates

Bulky diarylammonium pentafluorobenzenesulfonates effectively promote dehydration reactions, such as condensation reactions to give esters and the dehydrative cyclization of 1,3,5‐triketones. In particular, N‐(2,6‐diphenylphenyl)‐N‐mesitylammonium pentafluorobenzenesulfonate shows much higher catalytic activity than C₆F₅SO₃H under reaction conditions without the removal of generated water, even though the former is a weaker acid. Its crystallization gives an aggregated cyclic ion pair, which is composed of two diarylammonium cations, four pentafluorobenzenesulfonate anions, and two oxonium cations. This ion pair is strongly stabilized by four intermolecular and two intramolecular π–π attractive interactions and 10 hydrogen bonds. The extremely high catalytic activity of N‐(2,6‐diphenylphenyl)‐N‐mesitylammonium pentafluorobenzenesulfonate in the dehydration reactions may be ascribed to the local hydrophobic environment of the tightly aggregated ammonium salts.     

Membrane properties of cationic liposomes composed of dipalmitoylphosphatidylcholine and dipalmityldimethylammonium bromide

Cationic liposomes composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmityldimethylammmonium bromide (DPAB) were prepared by the Bangham method and the effect of DPAB on the membrane properties was examined in terms of liposomal shape, particle size, trapping efficiency, surface potential and dispersibility. The dispersibility of the mixed DPPC/DPAB liposomes (the mole fraction of DPAB (X_DPAB)  0.05) was excellent and the dispersibility was maintained for 6 months, since the zeta-potential of the mixed liposomes was approximately +40 mV. The trapping efficiency of the mixed DPPC/DPAB liposomes (X_DPAB = 0.05) was 10 times greater than that of the DPPC liposomes, and the value was largest among the mixed liposomes (X_DPAB = 0–1.0). Freeze-fracture electron micrographs indicated that the shape of the mixed DPPC/DPAB liposomes (X_DPAB = 0.05) was that of large unilamellar vesicles (LUVs) with a diameter of approximately 2 μm, while the shape of the DPPC liposomes was that of multilamellar vesicles (MLVs). The mixed liposomes had, therefore, a high trapping efficiency. Furthermore, the shape of the mixed DPPC/DPAB liposomes (X_DPAB = 0.75) was also that of LUVs with a diameter of approximately 2 μm and these had a high trapping efficiency. Whereas, the particle size (500 nm) of the mixed DPPC/DPAB liposomes (X_DPAB = 0.25) was smaller than that of the former and had the minimum trapping efficiency. The phase transition temperature of the liposomal bilayer membranes indicated a maximum value at 0.25–0.30 mole fractions of DPAB. These facts were considered to be due to the fact that DPPC and DPAB, whose molar ratio was 7.5:2.5, were tightly packed in the liposomal bilayer membranes and that the curvature of the liposomal particle was resultantly large. Nevertheless, LUVs having a high trapping efficiency were easily obtained by mixing a small amount of DPAB with the DPPC.     

Asymmetric [2 + 1] Cycloaddition Reactions of 1-Seleno-2-silylethene

The reaction of (E)-1-(phenylseleno)-2-(trimethylsilyl)ethene (1) and vinyl ketones 2a-d in the presence of a chiral Lewis acid prepared from TiCl(4), Ti(O(i)Pr)(4), (R)- or (S)-1,1'-binaphthol (BINOL), and MS4A gave enantiomerically enriched cis cyclopropane products 3a-d. The enantiomeric excess and chemical yield varied depending on the ratio of TiCl(4) and Ti(O(i)Pr)(4) to 1. Reproducible results (43-47% ee/33-41% yields) for cis-1-acetyl-2-[(phenylseleno)(trimethylsilyl)methyl]cyclopropane (3a) were obtained using 1.1 equiv of TiCl(4), 0.54-0.65 equiv of Ti(O(i)Pr)(4), and 1.65 equiv of BINOL. The observed enantioselectivity was explained by consideration of the structure of the postulated intermediates, alkoxy titanium-carbonyl complexes, via ab initio MO calculations.     

Lewis acid-promoted cyclization of heteroatom-substituted enynes

Lewis acid-promoted cyclizations of heteroatom-substituted enynes have been examined. The reaction of enynes and bearing silicon substituents on an alkyne afforded the halogenated five-membered gamma-lactones and gamma-lactams as the main products. The reaction of substrates and having 2-phosphonoacrylate instead of malonate also gave halogenated five-membered cyclic compounds and as the major products. The cyclized products are highly substituted and potentially useful for further synthetic transformations.     

Is the Beckmann rearrangement a concerted or stepwise reaction? A computational study

[reaction: see text] RB3LYP calculations were performed on the Beckman rearrangement by the use of three substrates, acetone oxime (1), acetophenone oxime (2), and cyclohexanone oxime (3). Acidic solvents were modeled by H+ (CH3COOH)3 and H3O+ (H2O)6, and reaction paths were determined precisely. For 1, a two-step process involving a sigma-type cationic complex was obtained. For 2, a three-step process with pi- and sigma-type complexes was found in H+ (CH3COOH)3 and a two-step process involving a sigma-type cationic complex was obtained in H3O+ (H2O)6. However, for 3, a concerted process without pi and sigma complexes was calculated, which leads to the product, epsilon-caprolactam. Three different mechanisms were explained in terms of FMO theory.     

Reversible photoelectronic signal conversion based on photoisomerization-controlled coordination change of azobenzene-bipyridine ligands to copper

A new "molecular photoelectronic transducer", working in a cyclic manner totally powered with light irradiation, has been constructed. The UV/blue-controlled repetitive motion of azobenzene moieties in 6,6'-bis(4' '-tolylazo)-4,4'-bis(4-tertbutylphenyl)-2,2'-bipyridine causes reciprocal CuI translocation between two coordination environments, resulting in pumping of the redox potential of CuI. Therefore, UV/blue light information can be successfully transformed into an electrode potential change and positive/negative current response, which is closely related to natural visual transduction both functionally and mechanically. This result demonstrates a new strategy for artificial molecular machine assembly, i.e. forming a path with multistep chemical reactions between input/output couples at choice.     

Bulky diarylammonium arenesulfonates as selective esterification catalysts

More environmentally benign alternatives to current chemical processes, especially large-scale, fundamental reactions such as ester condensations, are highly desirable for many reactions. Bulky diarylammonium pentafluorobenzenesulfonates and tosylates serve as extremely active dehydration catalysts for the ester condensation reaction of carboxylic acids with equimolar amounts of sterically demanding alcohols and acid-sensitive alcohols. Typically, the esterification reaction is performed in heptane by heating at 80 degrees C in the presence of 1 mol % of the catalyst without removing water. Esterification with primary alcohols proceeds without solvents even at room temperature. Furthermore, 4-(N-mesitylamino)polystyrene resin-bound pentafluorobenzenesulfonate can be recycled more than 10 times without activity loss.     

Hydroxyl radical generation via photoreduction of a simple pyridine N-oxide by an NADH analogue

Photoreduction of pyridine N-oxide, which has a key structure of antitumor agents for hypoxic solid tumors, by 1-benzyl-1,4-dihydronicotinamide in deaerated aprotic media resulted in generation of hydroxyl radical, leading to the oxidation of salicylic acid to 2,3- and 2,5-dihydroxybenzoic acids, and catechol.