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Abstract A side selective single‐fluorination of pyridines is presented using commercially available AgF2 as fluorination agent and avoiding tedious multiple step procedures.
2,3,4,6-Tetra-O-benzyl-α-D-glucopyranosyl bromide (1) and, in one instance, its α-D-galacto isomer (2) were condensed under conditions of the Heiferich modification of the Koenigs–Knorr reaction with the four stereoisomeric methyl 4,6-O-benzylidene-3-deoxy-3-nitrohexopyranosides having the α-D-galacto (3), β-D-galacto (4), α-D-gluco (5), and β-D-gluco (6) configurations. Six new, 1 → 2 linked, fully blocked nitro disaccharides (7–12) were isolated. The configurations of the disaccharidic linkages were established by n.m.r. spectroscopy to be α in four of the products (8, 9, 11, and 12) and β in two (7 and 10). The steric course of the disaccharide bond formation was found to depend on the anomeric as well as the over-all configuration of the alcoholic components 3–6.
A series of monomeric palladium amido complexes of the form trans-(PPh3)2Pd(Ar)(NAr'2) and (DPPF)Pd(Ar)(NAr'2) (DPPF = 1,1'-bis(diphenylphosphino)ferrocene) and dimeric palladium amido complexes of the form {(PPh3)Pd(Ar)(μ-NHR)}2 (R = Ph, t-Bu) have been prepared by the reaction of lithium and potassium amides with palladium aryl halide complexes. An X-ray crystal structure of (DPPF)Pd(p-NMe2C6H4)[N(p-CH3C6H4)2] was obtained. Upon thermolysis in the presence of PPh3, serving as a trapping agent, both the monomeric and dimeric palladium amido complexes underwent C−N-bond-forming reductive elimination to form arylamines in high yields along with a Pd(0) species. Reductive elimination was also observed from azametallacycle (PPh3)Pd(η2-C6H4C6H4NH), to form carbazole and Pd(PPh3)4 at room temperature. Mechanistic studies on the reductive elimination reactions of the monomeric PPh3-ligated amido complexes indicated the presence of two competing pathways for the formation of amine. At low [PPh3], reductive elimination occurs via phosphine dissociation to form a three-coordinate intermediate; however, as [PPh3] is increased, a pathway of reductive elimination from a four-coordinate complex becomes dominant. The DPPF-ligated palladium amido complexes directly eliminated amine from the four-coordinate complex. The mechanism of the reductive elimination from dimeric palladium amido complexes was also studied. These complexes undergo reductive elimination of amine via dimer dissociation to generate a three-coordinate intermediate analogous to those formed by the PPh3-ligated monomeric amido complexes. The C−N-bond forming reductive elimination reactions were accelerated by electron-withdrawing groups on the Pd bound aryl group and by electron-donating groups on the amido ligand, suggesting that the aryl group acts as an electrophile and the amido ligand acts as a nucleophile.
Smoking is known to affect appetite and weight in both animals and humans. We examined the relation between smoking deprivation and caloric intake in five women with bulimia nervosa in whom the weight regulation aspects of smoking may be particularly salient
In compositionally complex refractory alloys, the formation of chemical short-range order has been calculated to flatten the spatial distribution of dislocation core energies. This study examines the extent to which such ``narrowing'' effects are unique to many-component systems, or if these results are more generally applicable. The authors computationally investigate how system chemistry, compositional complexity, and the presence of chemical short-range order affect dislocation energy distributions in subsystems of the NbMoTaW alloy. Order-induced narrowing is ultimately found to depend more on chemistry than the number of components.
Density functional theory has been used to investigate the thermodynamics and activation barriers associated with the direct oxidation of methane to acetic acid catalyzed by Pd2+ cation in concentrated sulfuric acid. Pd2+ cations in such solutions are ligated by two bisulfate anions and by one or two molecules of sulfuric acid. Methane oxidation is initiated by the addition of CH4 across one of the Pd-O bonds of a bisulfate ligand to form Pd(HSO4)(CH3)(H2SO4)2. The latter species will react with CO to produce Pd(HSO4)(CH3CO)(H2SO4)2. The most likely path to the final products is found to be via oxidation of Pd(HSO4)(CH3)(H2SO4)2 and Pd(HSO4)(CH3CO)(H2SO4)2 to form Pd(eta2-HSO4)(HSO4)2(CH3)(H2SO4) and Pd(eta2-HSO4)(HSO4)2(CH3CO)(H2SO4), respectively. CH3HSO4 or CH3COHSO4 is then produced by reductive elimination from the latter two species, and CH(3)COOH is then formed by hydrolysis of CH3COHSO4. The loss of Pd2+ from solution to form Pd(0) or Pd-black is predicted to occur via reduction with CO. This process is offset, though, by reoxidation of palladium by either H2SO4 or O2.
