Stereoretentive O-to-C rearrangement of vinyl acetals: solvent cage effects as a stereocontrol element

The Lewis acid-mediated rearrangement of chiral vinyl acetals may be induced to provide the product of stereoretention using Me3Al and BF3.OEt2 in concert. The selectivities obtained in this reaction (86:14 to 96:4) are complementary to that observed when relying on oxocarbenium facial bias to control the newly formed stereocenter. Evidence is presented that this reaction occurs by tight ion-pair binding in the solvent cage. The relay of C-O bond stereochemistry to a C-C bond stereocenter via ionic intermediates is an addition to similar established methods such as the Claisen rearrangement.     

A facile ketoxime preparation via the reduction of α,β-unsaturated nitroalkenes using sodium stannite

αβ-Unsaturated nitroalkenes are readily reduced by sodium stannite to ketoximes at room temperature.     

Levitation effect: size-dependent maximum in rotational diffusion in confined systems

Molecular dynamics of a model guest tetrahedral molecule AX(4) with differing bond lengths l(AX) for A-X bonds have been carried out in a sphere with different surface roughness. The rotational-diffusion coefficient D(R) shows a maximum for a particular value of l(AX). We show that this corresponds to the distance at which the interaction of the guest with the atoms of the host is most favorable. Although, the intensity of the maximum decreases with increase in the roughness of the confining surface, it is seen that the maximum exists even for a reasonably high degree of roughness. The observed maximum arises from the minimum in the torque on the tetrahedral molecule from its interaction with the confining medium due to mutual cancellation of forces. Activation energy for rotation is seen to be also a minimum for the bond length for which D(R) is a maximum. These results suggest that there is a maximum in the rotational-diffusion coefficient when the rotating molecule is confined to a sphere of comparable size similar to the maximum in translational diffusion coefficient seen in porous solids (the levitation effect) [Yashonath and Santikary J. Phys. Chem. 98, 6368 (1994)]. On increase in the roughness of the sphere surface, the value of l(AX) at which the maximum in D(R) is seen decreases.     

Single-crystal structure determination of a new polymorph (ε-Alq<Subscript>3</Subscript>) of the electroluminescence OLED (organic light-emitting diode) material,tris(8-hydroxyquinoline)aluminum (Alq<Subscript>3</Subscript>)

Tris(8-hydroxyquinoline) aluminum, Alq₃, is the most widely used electroluminescent material in organic light-emitting diode (OLED), chemistry. Single-crystal structure determination of a new polymorph of Alq₃, designated as -Alq₃, is described. There are three molecules of Alq₃ in the asymmetric unit. This octahedrally coordinated complex crystallizes in the triclinic space group P-1, lattice parameters a = 13.5190(5) Å, b = 15.8550(6)~Å, c = 18.7110(8) Å, = 95.4940(19), = 109.774(2), = 114.6270(16), Volume = 3296.8(2) ų, Z = 6, Z = 3.     

beta-acylvinyl anion and dianion equivalents: lithiation of 1-[(2EZ)-3-chloroprop-2-enyl]-1H-1,2,3-benzotriazole: preparation and elaboration of 1-(2-oxiranylvinyl)-1H-benzotriazoles

The allyllithium generated from 1-[(2EZ)-3-chloroprop-2-enyl]-1H-1,2,3-benzotriazole (5) and LDA, in the presence of HMPA, reacts with enolizable and nonenolizable carbonyls solely at the CCl terminus to give 1-(2-oxiranylvinyl)benzotriazoles 6a-g in 61-82% yields. Allyllithiums generated from 6a,c reacted exclusively at the CBt terminus to give 10a-d in 68-88% yields. Acidic hydrolysis of (oxiranylvinyl)benzotriazoles 6a-g and 10a-d provided 4-hydroxyalk-2-en-1-one derivatives 12a,b,c,e,g, 13a-d, and furan 14 in 54-86% yields.     

On existence of finite universal Korovkin sets in Segal algebras

We prove that there exists a finite universal Korovkin set w.r.t positive operators for the centre of a Segal algebra on a compact groupG if and only ifG is metrizable. As a consequence it follows that a Segal algebra on a compact abelian group admits a finite universal Korovkin set w.r.t. positive operators iff the group is metrizable.     

Silver-halide/organic-composite structures: Toward materials with multiple photographic functionalities

We report the synthesis and structure of the novel silver-halide-based organic-inorganic hybrids Ag₂Br₆(PPD)₂, Ag₂Br₆(CD-2)₂·H₂O, Ag₂Br₄(TMBD), and Ag₂I₆(CD-2)₂·H₂O. 1,4-phenylenediammonium hexabromodiargentate(I) [Ag₂Br₆(PPD)₂] crystals are monoclinic (P2₁/n), with unit-cell dimensions, , , and β=93.109(1)°. N,N-diethyl-2-methyl-1,4-benzenediammonium hexabromodiargentate(I) monohydrate [Ag₂Br₆(CD-2)₂·H₂O] crystals are monoclinic (space group P2₁/c) with , , , and β=96.153(1)°. N,N,N′,N′-tetramethyl-1,4-benzenediammonium tetrabromodiargentate(I) [Ag₂Br₄(TMBD)] crystals are orthorhombic (space group Pbcn) with , , and . N,N-diethyl-2-methyl-1,4-benzenediammonium hexaiododiargentate(I) monohydrate, [Ag₂I₆(CD-2)₂·H₂O], are monoclinic (C2/c), with unit-cell dimensions, , , , and β=98.657(1)°. The novel structures are members of a class of silver-halide-based organic-inorganic hybrids based upon the assembly of [Ag_aX_b]ⁿ⁻ clusters and protonated organoamines in aqueous mineral acids. The clusters display short intracluster Ag-Ag distances, and computational methods are used to evaluate intracluster Ag-Ag bonding. The diverse stoichiometries and cluster connectivities observed suggest a rich compositional and structural chemistry based upon the general assembly method. We have extended the methodology to include a silver-halide-organoamonium chemistry in which the organic moiety is chosen to serve a specific photographic function and demonstrate the first examples of such materials. The methodology allows for the direct assembly of [Ag_aX_b]ⁿ⁻ clusters with commercial photographic color developer molecules, and we show that development is repressed but can later be “switched on” in a unique photographic scheme. The photographic properties of Ag₂Br₆(PPD)₂ are examined and show an extremely facile development rate owing to the fact that the developer molecules are within molecular proximity to the clusters. As a result of their molecular nature, we anticipate that such materials could enable conventional or completely new imaging technologies with very fast image access rates and very high resolution.     

Structure factor calculations of various DNA duplexes

Based upon a stereochemical guideline, both right- and left-handed duplexes were generated for A, B, and D forms of DNA using a mononucleotide as the repeat. Structure factor amplitudes were computed following two methods: (i) one employed an infinite helix as the motif and (ii) the other had an exact crystallographic repeat (e.g., 10 b.p. for B-DNA) as the motif. Both the procedures showed that DNA in either handedness for A, B, and D forms of DNA are consistent with the observed fiber data. This observation is attributed to the fact that fibre pattern (resolved only upto 3 Å) only gives the image of the gross structure of the molecule. Thus, when the gross structure is suitably fitted to match the observed pattern, it is immaterial as to what the precise stereochemistry of the repeating unit (fine structure) and handedness of a model are. Sequence specific helical conformations were obtained using dinucleotide as the repeating unit. Helices fell into two classes: (i) right-handed uniform (RU ) and (ii) left-handed zig-zag (LZ ) structures. A few aspects concerning the scattering profile of these helices are discussed.     

Chemistry of 5(2H)-isoxazolones: Novel conversion of positional isomers

Reactions of 4-arylidene-3-methyl-5(4H)-isoxazolones with nucleophiles lead to 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones. Formation of 5(4H)- and 5(2H)-isoxazolone derivatives have been observed when 4-substituted-arylidene-3-rriethyl-5(4H)-isoxazolones are reacted with methyl magnesium iodide. Oxidation of 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones with manganic acetate and pyridinium chlorochromate, gives 4,4′-bis[4-substituted-benzyl-3-methyl-5(4H)-isoxazolone]. Oxidation of 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones with metachloroperbenzoic acid gives 4-hydroxy-4-substituted-benzyl-3-methyl-5(4H)-isoxazolones and reactions of the same substrates with N-bromosuccinimide furnish 4-bromo-4-substitutedbenzyl-3-methyl-5(4H)isoxazolones.     

A novel and faster route for the synthesis of polyether-polycarbonate from carbon dioxide and epoxide through microwave irradiation

The purpose of this study was to investigate the utilization of CO₂ in order to obtain polyether-polycarbonate by the co-polymerization of epoxide with CO₂ using microwave irradiation. Double metal cyanide containing Zn with Co, Fe(II), Fe(III) and Ni has been utilized as a catalyst for the co-polymerization reaction. The effect of microwave power, catalyst amount and types of catalysts has been studied. The microwaves were found to be successful in the synthesis of polyether-polycarbonate in a shorter reaction time with higher turnover numbers than those in conventional methods.