2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1- H-imidazole-3-oxide-1-oxyl, a hydrogen-bonded organic quasi-1D ferromagnet

The title radical (F4BImNN) is a stable nitronylnitroxide that forms hydrogen-bonded NH... ON chains in the solid state. The chains assemble the F4BImNN molecules to form stacked contacts between the radical groups, in a geometry that is expected to exhibit ferromagnetic (FM) exchange based on spin polarization (SP) models. The experimental magnetic susceptibility of F4BImNN confirms the expectation, showing 1-D Heisenberg chain FM exchange behavior over 1.8-300 K with an intrachain exchange constant of Jchain/k = +22 K. At lower temperatures, ac magnetic susceptibility and variable field heat capacity measurements show that F4BImNN acts as a quasi-1-D ferromagnet. The dominant ferromagnetic exchange interaction is attributable to overlap between spin orbitals of molecules within the hydrogen-bonded chains, consistent with the SP model expectations. The chains appear to be antiferromagnetically exchange coupled, giving cusps in the ac susceptibility and zero field heat capacity at lower temperatures. The results indicate that the sample orders magnetically at about 0.7 K. The magnetic heat capacity ordering cusp shifts to lower temperatures as external magnetic field increases, consistent with forming a bulk antiferromagnetic phase below a Néel temperature of TN(0) = 0.72 K, with a critical field of Hc approximately 1800 Oe. The interchain exchange is estimated to be zJ/k congruent with (-)0.1 K.     

Isometric embeddings of 2-spheres into Schwarzschild manifolds

Let g be a Riemannian metric on the 2-sphere S². Results on isometric embeddings of (S², g) into a fixed model manifold often have implications in quasi-local mass related problems in general relativity. In this paper, motivated by the definitions of the Brown–York and the Wang–Yau mass, we consider isometric embeddings of (S², g) into conformally flat spaces. We prove that if g is close to the standard metric on S², then (S², g) admits an isometric embedding into any spatial Schwarzschild manifold with small mass. We also give a sufficient condition that ensures isometric embeddings of perturbations of a Euclidean convex surface into \({\mathbb{R}^3}\) equipped with a conformally flat metric.     

Metal-free and PdII-promoted [2+3] cycloadditions of a cyclic nitrone to phthalonitriles: syntheses of oxadiazolines as well as phthalamide-PdII and dihydropyrrolyl-iminoisoindolinone-PdII complexes with high catalytic activity in Suzuki-Miyaura cross-coupling reactions

The previously unknown reactions between phthalonitriles, 1,2-(CN)2(C6)R1R2R3R4 1 (1 a, R1=R2=R3=R4=H; 1 b, R1=R2=R4=H, R3=CH3; 1 c, R1=R4=H, R2=R3=Cl; 1 d, R1=R2=R3=R4=Cl; 1 e, R1=R2=R3=R4=F), and a cyclic nitrone, -O+N==CHCH2CH2CMe2 2, proceed under heating in a sealed tube to give phthalimides 3, 2-oxadiazolyl-benzonitriles 4 or ortho-bis(oxadiazolyl)tetrafluorobenzene 4 e'. In the presence of palladium(II) chloride, phthalonitriles 1 react with 2 at room temperature, to give bis(pyrrolidin-2-ylidene)phthalamide PdII complexes 5 via metal-promoted rupture of the N--O bond of the oxadiazoline ring. The ketoimine ligands thus generated can be liberated from the metal by displacement with a diphosphine. Although the first [2+3] cycloaddition of 2 to 1 can occur in the absence of the metal to give the mono-cycloadducts 4, the second [2+3] coupling at the still-unreacted cyano group requires its activation by coordination to PdII, affording complexes 6 containing two ligated oxadiazolyl-benzonitriles. These ligands undergo either i) further cycloaddition with 2 to afford ultimately (upon rearrangement) the bis(pyrrolidinylidene)phthalamide complexes 5 or ii) N--O bond cleavage in the oxadiazoline ring with intramolecular attack of the imine nitrogen on the cyano carbon and bridging to a second PdII center to afford dimeric palladium(II) complexes 7, with chloride bridges, that bear a dihydropyrrolyl-iminoisoindolinone, a new type of ligand.The compounds were characterized by IR, 1H, and 13C NMR spectroscopy, ESI MS or FAB+ MS, elemental analyses and, in the case of 4 c, 5 a, 5 c, and 7 c, also by X-ray diffraction analysis. Complexes 5 a and 7 c show high catalytic activity for the Suzuki-Miyaura cross-coupling reaction of bromobenzene and phenylboronic acid and give biphenyl in high yields with turnover frequencies (TOFs) of up to 9.0x10(5) h(-1).     

Amavadin and other vanadium complexes as remarkably efficient catalysts for one-pot conversion of ethane to propionic and acetic acids

Synthetic amavadin Ca[V{ON[CH(CH(3))COO](2)}(2)] and its models Ca[V{ON(CH(2)COO)(2)}(2)] and [VO{N(CH(2)CH(2)O)(3)}], in the presence of K(2)S(2)O(8) in trifluoroacetic acid (TFA), exhibit remarkable catalytic activity for the one-pot carboxylation of ethane to propionic and acetic acids with the former as the main product (overall yields up to 93 %, catalyst turnover numbers (TONs) up to 2.0 x 10(4)). The simpler V complexes [VO(CF(3)SO(3))(2)], [VO(acac)(2)] and VOSO(4) are less active. The effects of various factors, namely, C(2)H(6) and CO pressures, time, temperature, and amounts of catalyst, TFA and K(2)S(2)O(8), have been investigated, and this allowed optimisation of the process and control of selectivity. (13)C-labelling experiments indicated that the formation of acetic acid follows two pathways, the dominant one via oxidation of ethane with preservation of the C--C bond, and the other via rupture of this bond and carbonylation of the methyl group by CO; the C--C bond is retained in the formation of propionic acid upon carbonylation of ethane. The reactions proceed via both C- and O-centred radicals, as shown by experiments with radical traps. On the basis of detailed DFT calculations, plausible reaction mechanisms are discussed. The carboxylation of ethane in the presence of CO follows the sequential formation of C(2)H(5) (*), C(2)H(5)CO(*), C(2)H(5)COO(*) and C(2)H(5)COOH. The C(2)H(5)COO(*) radical is easily formed on reaction of C(2)H(5)CO(*) with a peroxo V catalyst via a V{eta(1)-OOC(O)C(2)H(5)} intermediate. In the absence of CO, carboxylation proceeds by reaction of C(2)H(5) (*) with TFA. For the oxidation of ethane to acetic acid, either with preservation or cleavage of the C-C bond, metal-assisted and purely organic pathways are also proposed and discussed.     

OH radical reactions with phenylalanine in free and peptide forms

Density functional theory has been used to model the reaction of OH with l-phenylalanine, as a free molecule and in the Gly-Phe-Gly peptide. The influence of the environment has been investigated using water and benzene as models for polar and non-polar surroundings, in addition to gas phase calculations. Different paths of reaction have been considered, involving H abstractions and addition reactions, with global contributions to the overall reaction around 10% and 90% respectively when Phe is in its free form. The ortho-adducts (o-tyrosine) were found to be the major products of the Phe+OH reaction, for all the modeled environments and especially in water solutions. The reactivity of phenylalanine towards OH radical attacks is predicted to be higher in its peptidic form, compared to the free molecule. The peptidic environment also changes the sites' reactivity, and for the Gly-Phe-Gly+OH reaction H abstraction becomes the major path of reaction. The good agreement found between the calculated and the available experimental data supports the methodology used in this work, as well as the data reported here for the first time.     

Synthesis of core-shell composites using an inverse surfmer

Anilinium dodecylsulfate was prepared from aniline and sodium dodecylsulfate. The critical micellar concentration of the salt was determined using electrical conductimetry, which revealed that the change of countercation, sodium by anilinium, reduced the critical micellar concentration with respect to the conventional counterpart, sodium dodecylsulfate. The anilinium dodecylsulfate was used as the surfmer in the synthesis of polystyrene/polyaniline core-shell composites, first performing as the surfactant to stabilize the emulsion polymerization of styrene, and later as the monomer to synthesize polyaniline via oxidative polymerization. Here, the surfmer function was directed toward the external phase instead of to the internal phase, as with conventional surfmers with carbon-carbon double bonds. Consequently, the term inverse surfmer is proposed. Analyses of its composite microstructure using electron microscopy and thermogravimetric analysis confirmed the core-shell arrangement.     

Microwave-assisted C-3 selective oxidative radical alkylation of flavones

Flavones were directly alkylated at the C-3 position in moderate yields using a xanthate-based oxidative radical addition procedure. This methodology is a suitable synthetic tool for the direct substitution of the vinylic and unactivated C-H bond of the C ring of the flavone by an alkyl functionality under neutral conditions.     

Direct regiospecific and highly enantioselective intermolecular α-allylic alkylation of aldehydes by a combination of transition-metal and chiral amine catalysts

The first direct intermolecular regiospecific and highly enantioselective α-allylic alkylation of linear aldehydes by a combination of achiral bench-stable Pd(0) complexes and simple chiral amines as co-catalysts is disclosed. The co-catalytic asymmetric chemoselective and regiospecific α-allylic alkylation reaction is linked in tandem with in situ reduction to give the corresponding 2-alkyl alcohols with high enantiomeric ratios (up to 98:2 e.r.; e.r.=enantiomeric ratio). It is also an expeditious entry to valuable 2-alkyl substituted hemiacetals, 2-alkyl-butane-1,4-diols, and amines. The concise co-catalytic asymmetric total syntheses of biologically active natural products (e.g., Arundic acid) are disclosed.     

A novel synthetic strategy for monosubstituted cyclodextrin derivatives

A first solid phase approach to obtain monosubstituted CD conjugates to different labels has been developed. A new solid support has been designed to get a variety of C-6 monofunctionalized CDs (α, β, MeβCD and HPβCD) covalently linked through a phosphodiester bridge to different labels, in highly pure form and under very mild detachment conditions.