Michele Zema

A new and highly efficient catalyst for the enantioselective Mukaiyama–Michael reaction between ( E )-3-crotonoyl-1,3-oxazolidin-2-one and 2-trimethylsilyloxyfuran

Abstract The Mukaiyama–Michael reaction between 2-trimethylsilyloxyfuran and (E)-3-crotonoyl-1,3-oxazolidin-2-one has been stereoselectively catalysed by several optically active complexes based on bis(oxazoline) (box) or pyridine bis(oxazoline) (pybox) chiral ligands and metal cations. The catalysts derived from the newly synthesised 2,6-bis[(4′R,5′R)-diphenyl-1,3-oxazolin-2′-yl]pyridine and the triflates of EuIII, LaIII, CeIV were highly efficient: the diastereoselectivity was entirely anti and the enantioselectivity was excellent (ranging from 98 to >99%). A mechanistic insight into the nature of the activated substrate–catalyst complex was inferred studying the lanthanum complexes with 1H and 13C NMR spectroscopy. Based on these results and on the crystallographic structure of the complex between pybox and La(OTf)3, a stereochemical model is proposed to rationalise the crucial role of the substituent in position 5, suitably placed to blind the Si-face of the coordinated reagent.

CHIRAL AMPLIFICATION IN DIELS-ALDER AND 1,3-DIPOLAR CYCLOADDITIONS CATALYZED BY BIS(OXAZOLINE)-ZN(II)-BASED CHIRAL COMPLEXES

Abstract A Zn(II)-bis(oxazoline) chiral complex catalyzes the cycloaddition reactions of alkenoyl-oxazolidinones with both cyclopentadiene and diphenyl nitrone. The 1,3-dipolar cycloaddition becomes exo selective with an ee of up to 86%. While the Mg(II)-based catalysts give a linear relationship between the ligands optical purity and the ee of the reaction product, a remarkable positive nonlinear effect was observed in both cycloadditions by using Zn(II) as the cationic core of the catalyst. The result may be rationalized by the formation of a stable, insoluble, and catalytically inactive heterochiral complex.

Electrochemically Controlled Assembling/Disassembling Processes with a Bis‐imine Bis‐quinoline Ligand and the CuII/CuI Couple

The easily attainable quadridentate ligand bis-N,N′-(2-methylquinoline)-1,2-trans-cyclohexanediimine forms double helical dimeric complexes with CuI and monomeric complexes with CuII, which are air- and moisture-stable and whose structures have been determined through X-ray diffraction. In an acetonitrile solution, it is possible to cycle electrochemically between the two forms, through a fast process which can be repeated at will (see diagram).

Light-emitting molecular machines: pH-induced intramolecular motions in a fluorescent nickel(II) scorpionate complex.

A cyclam-like macrocycle has been synthesized with a pendant arm containing a dansylamide group. In the corresponding nickel(II) complex, binding of the pendant arm to the metal is pH controlled. In particular, at pH 4.3, the sulfonamide group deprotonates and coordinates the NiII center, giving rise to a complex of trigonal bipyramidal stereochemistry, as shown by X-ray diffraction studies performed on the crystalline complex salt. At pH 7.5, an OH- ion binds the metal and a six-coordinate species forms. The binding-detachment of the pendant arm to/from the NiII center is signaled by changes in the emission properties of the dansyl subunit in the side chain; the fluorescence of this side chain is high when the pendant arm is not coordinated and low when the sulfonamide group is bound to the metal. The system investigated represents the prototype of a light-emitting molecular machine, driven by a pH change.

Molecular structure, characterization and reactivity of dioxo complexes formed by vanadium(V) with α-hydroxycarboxylate ligands

Abstract The structure of the dioxovanadium(V) complexes of glycolic, lactic and malic acids Rb2[{VO2(glyc)}2] [glyc=glycolato(2−)] (1), Cs2[{VO2(mal)}2] [mal=malato(2−)] (2), and Cs2[{VO2(lact)}2]·2H2O [lact=lactato(2−)] (3) has been studied by single-crystal X-ray diffraction. In order to assign the features of the peculiar arrangement adopted by these kinds of complexes, the metrical details of these compounds have been examined and compared to the data available in literature for analogous complexes. The reactivity of the compounds has also been studied. In particular, the reduction of the metal ion by a biological reductant like cysteine, relevant to the role of these complexes in the constitution of the cofactor of the active site of vanadium nitrogenase, has been followed by EPR spectroscopy in an aqueous solution. In addition, the effect of peroxo complex formation on the structure of the dinuclear V2O2 cage has been analyzed by comparison with previously published results.

Thermal behavior of a Ti-rich phlogopite from Mt. Vulture (Potenza, Italy): An in situ X-ray single-crystal diffraction study

Abstract The structural evolution of a trioctahedral mica from Cava St. Antonio, Mt. Vulture, Potenza, Italy, has been studied in the temperature range 100-1023 K using in situ single-crystal X-ray diffraction techniques. The sample used shows a Ti-rich composition close to the phlogopite-annite join with the following crystal-chemical formula: (K0.85Na0.11Ba0.03)(Al0.17Mg1.98Mn0.01Fe2+0.29Fe3+0.37Ti0.18)(Si2.75Al1.25) O10.66(F0.13OH1.20Cl0.01). In the present study, the chemical and structural changes and the deprotonation process involved during heating have been characterized. Analysis of the data showed that a, b, and c cell parameters expand almost linearly up to 823 K, while the β angle remains constant. A sharp decrease in the unit-cell dimensions was observed at 873 and 1023 K. Linear trends obtained during reversal experiments from 1023 K to room temperature demonstrated the irreversibility of these events. Structure refinements of single-crystal XRD data collected at 100, 200, 298, 473, 673, 873, and 1023 K converged to 2.14 ≤ R (%) ≤ 8.47, 2.47 ≤ Rw (%) ≤ 10.83. In the temperature range 100-673 K, the thermal expansion along the c direction is mainly due to interlayer thickness dilation. The tetrahedral ring approaches the ideal hexagonal shape with increasing temperature to match the expanding octahedral sheet. In the range 873-1023 K, a strong shrinking of the interlayer is associated with the shortening of the M1-O4 and M2-O4 distances and to the consequent reduction of octahedral thickness. Such structural features indicate the occurrence of Fe oxidation process, involving loss of structural H, which is responsible for a phase transition. Mössbauer spectroscopy supported this hypothesis.

Kinetics of Fe2+-Mg order-disorder in P21/c pigeonite

Abstract The kinetics of the Fe-Mg intracrystalline exchange reaction in P21/c pigeonite (Wo10En47Fs43) free of exsolved augite, from the Paraná rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe2+. The linear regression of ln kD* vs. 1/T yielded the following equation: ln kD* = -2925(± 110)/T(K) + 0.574(± 0.111); (R2 = 0.997) The Tc values calculated using this equation were 566 (±6) and 571 (±6) °C for the two crystals. Analysis of the kinetic data was performed according to Muellers model, which allowed retrieval of the disordering rate constants C0Kdis+ for all four temperatures. The Arrhenius relation: ln Kdis+ = ln K0 - Q/(RT) = 20.45(± 1.91) . 25191 (± 1900)/T(K); (R2 = 0.989) yielded an activation energy of 50.03 (±3) kcal/mol for the Fe-Mg exchange process. Cooling time constants, calculated at the QMF buffer conditions of the host rock were, for the two crystals, η = 0.94 × 10-1 K-1 year-1 and η = 1.10 × 10-1 K-1year-1, and gave cooling rates on the order of 10 °C/h consistent with very fast lava cooling.

Order-disorder kinetics in orthopyroxene with exsolution products

Abstract The equilibrium behavior and kinetics of the Fe-Mg intracrystalline exchange reaction in an orthopyroxene sample containing exsolution products were studied by X-ray diffraction (XRD). Isothermal annealing experiments were performed on an orthopyroxene crystal from the Johnstown diogenite, which shows coherent (100) augite lamellae and Guinier-Preston zones. The kinetic experiments were carried out at 700, 800, and 850 °C until Fe-Mg exchange equilibrium was reached. Oxygen fugacity was controlled by the WI buffer. Equilibrium conditions were also confirmed by “reversal” experiments. After each annealing run single-crystal XRD data were collected, and the orthopyroxene phase (Pbca) was refined after subtraction of the contribution of the exsolved C2/c phase to the observed structure factors. The fraction of augite was ~2%. The low values of the disordering rate constants K+, calculated using Mueller’s equation, and the surprisingly high value of the activation energy (102.3 kcal/mol) for the Fe-Mg disordering reaction are ascribed to the presence of exsolution products in the orthopyroxene. Exsolution products seem not to affect the equilibrium behavior.

Thermal history of Acapulco and ALHA81261 acapulcoites constrained by Fe2+Mg ordering in orthopyroxene

The quenched Fe2+Mg ordering states of four orthopyroxene crystals from the Acapulco meteorite and two from the Antarctic ALHA81261 acapulcoite were determined from X-ray single-crystal diffraction and electron-microprobe analysis. The closure temperatures of cation ordering were 49028 and 46543°C for Acapulco and ALHA81261, respectively. Kinetic analysis of the Fe2+Mg intracrystalline reaction, on the basis of Muellers theory [1,2], was carried out using the method developed by Ganguly [3] to measure the cooling rates of the host rock, assuming both asymptotic and exponential models. The determined ordering states yield cooling rates, near their closure temperatures, of ∼ 0.17 and ∼ 0.05°C/day for Acapulco and ALHA81261 respectively, slightly slower than that measured for the FRO90011 lodranite [4]. A break-up of the A-L parent body, with subsequent radiative heat loss of small ejected fragments, may explain such rapid cooling. The closure temperatures obtained for the two acapulcoites were then compared with those calculated, using the same method, for other achondrites and their cooling rates with those measured for The FRO90011 lodranite [4] and the Estherville mesosiderite [5]. The sequence of decreasing closure temperatures from the FRO90011 lodranite to the Landes IAB iron meteorite reflects decreasing cooling rates. Silicates enclosed in iron meteorites, and thus involved in core formation processes, deeper in the interior of a parent asteroid, seem to have cooled more slowly than meteorites that were affected by partial melting events.

Ca in orthopyroxene: structural variations and kinetics of the disordering process

The kinetics of the disordering process in Ca-rich orthopyroxenes was studied. The substitution of Ca for Mg and Fe in the orthopyroxene structure was investigated in order to evaluate the influence of this cation on the disordering process. Geometrical parameters obtained by X-ray single-crystal diffraction on different Ca-rich orthopyroxenes showed that Ca enters the octahedral M2 site of orthopyroxene, causing an enlargement of this polyhedron. As a consequence, both tetrahedral chains extend to allow matching between the tetrahedral and octahedral layers. The kinetic study was carried out at T = 730°C on an orthopyroxene from the volcanic rock sample L3. This orthopyroxene, with composition Wo 4 En 60 Fs 36 , contains thin exsolved augite lamellae but no Guinier-Preston zones. A series of isothermal annealing runs were performed on a single crystal in the presence of IW buffer followed by quenching. The ordering degree after each annealing run was determined by X-ray single-crystal diffraction. Analysis of the kinetic data using Mueller9s (1967) theory yielded a disordering rate constant of 2.69(8)·10 -2 min -1 . This value is in perfect agreement with that expected for a “normal” kinetic behaviour. The presence of Ca and augite lamellae in the orthopyroxene matrix do not seem to be responsible for the drastic change in the activation energy of the Fe-Mg exchange process observed in the Ca-rich orthopyroxene from the Johnstown meteorite.