Application of pyranoside phosphite-pyridine ligands to enantioselective metal-catalyzed allylic substitutions and conjugate 1,4-additions

A series of glucopyranoside phosphite–pyridine ligands have been applied in the metal-catalyzed allylic substitution and conjugate 1,4-addition reactions of several substrate types. We have been able to identify ligands that provided promising enantioselectivities in the Pd-catalyzed intermolecular allylic substitution of cyclic substrates (ee’s up to 86%) and desymmetrization (ee’s up to 94%) and in the Cu-catalyzed conjugate addition of challenging aliphatic enones (ee’s up to 90%).     

Studies on the chemical stability and synthetic utility of an oxazolidine linker for solid-phase chemistry

A chemical stability study on the oxazolidine linker system has been carried out using a dual-linker analytical construct within a parallel reaction scan. The study established the compatibility of the oxazolidine platform with a wide range of commonly employed synthetic reaction conditions including nucleophilic, oxidizing, and reducing conditions. The scan was further used to probe and optimize acidic conditions under which the oxazolidine could release the substrate from the solid-support and to identify reagents that could cleave while retaining other acid-labile groups. The solid-phase synthesis of a small molecular array established the utility of oxazolidine aldehyde 1 as a building block for asymmetric chemistry while exploiting the data generated by the reaction scan.     

Enantioselective synthesis of (−)-barrenazines A and B

A short enantioselective synthesis of barrenazines A and B is described. Barrenazines A and B are prepared following a common synthetic route in nine steps (19% overall yield) and eight steps (21% overall yield), respectively, from readily available 4-methoxy-3-(triisopropylsilyl)pyridine. The synthesis relies on a highly diastereoselective nucleophilic addition of a Grignard reagent to a chiral acylpyridinium salt, a radical azidation of a silyl enol ether and the assembly of the pyrazine ring by reductive dimerization of a functionalized 5-azidopiperidin-4-one.     

Archaeometric Analysis of Ancient Copper Artefacts by Laser-Induced Breakdown Spectroscopy Technique

Twelve archaeological copper objects from the burial site of “Fontino” cave, near Grosseto, (around 2500–2000 B.C.) were analysed using laser-induced breakdown spectroscopy. Qualitative results and a preliminary study of the samples’ composition are reported and used to make a quantitative estimate; based on these results, the samples were classified using principal components statistical analysis. The perspectives of using laser-induced breakdown spectroscopy for archaeometric analysis are also discussed.     

Synthesis and reactivity of the Tl1 salts of

Synthons Tl1[TCNE]*- (1) and Tl12[TCNE]2- (2), for [TCNE]*- and [TCNE]2-, respectively, in metathesis reactions have been quantitatively prepared and characterized. The structure of 1 was solved and refined in a monoclinic unit cell at 27 degrees C [C2/c, a = 12.6966 (12) angstroms, b=7.7599 (7) angstroms, c=15.5041 (15) angstroms, beta = 96.610 (5) degrees , V= 1517.4 (2) angstroms3, Dcalcd = 2.911 gcm-3, Z=8, R1 = 0.0575, omegaR2=0.0701] and exhibits nuCN absorptions at 2,191 (s) and 2,162 (s) cm-1 consistent with metal-bound [TCNE]*-. The structure of 1 consists of a distorted square antiprismatic octacoordinate Tl1 bound to six monodentate [TCNE]*-s with TlN separations ranging from 2.901 to 3.171 angstroms averaging 3.020 angstroms, and one bidentate [TCNE]*- with TlN separations averaging 3.279 angstroms. The TlN bonding is attributed to electrostatic bonding. The [TCNE]*-s form dimerized zigzag chains with intra- and interdimer separations of 2.87 and 3.29 angstroms, respectively. The tight pi-[TCNE](2)2- dimer is diamagnetic and has the shortest intradimer [TCNE]*- distance reported. These synthons for [TCNE]*- and [TCNE]2- in metathesis reactions lead to the precipitation of, for example, TlIX (X = Cl, Br, OAc). Reaction of 1 with MnIII(porphyrin)X (X = Cl, OAc) forms the molecule-based magnets of [MnIII(porphyrin)][TCNE] composition, while the reaction of [CrI(C6H6)2]Br and (Me2N)2CC(NMe2)2Cl2, [TDAE]Cl2, with 1 forms [CrI(C6H6)2] [TCNE] and [TDAE][TCNE]2, respectively. The structure of [TDAE][TCNE]2.MeCN was solved and refined in an orthorhombic unit cell at 21 degrees C [I222, a = 10.2332(15), b = 13.341(6), c = 19.907(8) angstroms, V= 2717.7 angstroms3, Z = 4; Dcalcd = 1.216 gcm-3, R=0.083, Romega = 0.104] and exhibits upsilonCN absorptions at 2,193 (m), 2,174 (s), and 2,163 (s) cm-1 consistent with isolated [TCNE](2)2- , in contrast to the aforementioned TlI bound [TCNE](2)2-. The reaction of 2 with [TDAE]Cl2 forms [TDAE]2+[TCNE]2-.     

Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain**

Azobenzene and its derivatives are one of the most widespread molecular scaffolds used in a range of modern applications, as well as in fundamental research. After photoexcitation, azo-based photoswitches revert back to the most stable isomer on a timescale ( ) that determines the range of potential applications. Attempts to bring to extreme values prompted the development of azobenzene and azoheteroarene derivatives that either rebalance the E- and Z-isomer stabilities, or exploit unconventional thermal isomerization mechanisms. In the former case, one successful strategy has been the creation of macrocycle strain, which tends to impact the E/Z stability asymmetrically, and thus significantly modify . On the bright side, bridged derivatives have shown an improved optical switching owing to the higher quantum yields and absence of degradation. However, in most (if not all) cases, bridged derivatives display a reversed thermal stability (more stable Z-isomer), and smaller than the acyclic counterparts, which restricts their potential interest to applications requiring a fast forward and backwards switch. In this paper, the impact of alkyl bridges on the thermal stability of phenyl-azoheteroarenes is investigated by using computational methods, and it is revealed that it is indeed possible to combine such improved photoswitching characteristics while preserving the regular thermal stability (more stable E-isomer), and increased values under the appropriate connectivity and bridge length.     

Hot atom effect on the leachability of elements in solid waste

The effect of neutron irradiation on the leachability of elements in solid wastes comprising of coal fly ash, hospital and municipal incinerator ashes was studied. There was a marked increase in leachability in the neutron irradiated wastes compared to non-irradiated wastes especially for elements such as As, Cd, Co, Cr, Sm and Zn. For elements such as Fe and Sm there was no significant difference in the leachabilities in the irradiated and non-irradiated wastes. The possible causes of this scenario and implications are discussed.     

Effect of the organic fragment on the mesogenic properties of a series of organogold(I) isocyanide complexes. X-ray crystal structure of [Au(CCC5H4N)(CNC6H4O(O)CC6H4OC10H21)]

Rod-like organogold(I) complexes [AuR(CNC₆H₄O(O)CC₆H₄OC₁₀H₂₁-p)] were prepared and their liquid crystal behaviour was studied. Depending on the nature of R, the synthetic methodology was different. Thus, for R = substituted alkynyl ligands, the new compounds were prepared in two steps:(i) reaction of [AuCl(tht)] (tht = tetrahydrothiophene) with R′CCH(R′ = C₅H₄N, C₆H₄CN, C₆H₄CCC₅H₄N) in the presence of NaOAc to give insoluble [Au(CCR′)]_n; (ii) reaction of the latter polymers with the isonitrile CNC₆H₄O(O)CC₆H₄OC₁₀H₂₁-p.For R = fluorinated aryls, the complexes were prepared by displacement of tht from the compounds [AuR(tht)] (R = C₅F₄N, C₆F₄C₅H₄N, C₆F₅) with isonitrile.In addition, an unexpected ionic derivative [Au(CCC₅H₄NC₁₀H₂₁)₂][Au(CCC₅H₄N)₂] was formed in the reaction between [PPh₄][Au(CCC₅H₄N)₂] and C₁₀H₂₁I. All these compounds have been characterized by IR and NMR spectroscopy and mass spectrometry. The X-ray crystal structure of the compound with R = CCC₅H₄N shows a linear molecule in which the gold atom is surrounded by the pyridine-containing acetylene and the isonitrile ligand, and no direct gold-gold interaction occurs. Six of the neutral compounds are liquid crystals and their optical, thermal and thermodynamic data were analyzed and compared in terms of molecular polarizability.