Hexene-1 polymerization by homogeneous zirconocene and heterogeneous-supported TiCl3 catalysts

MgCl₂-supported TiCl₃ catalysts, with and/or without electron donor modifier (internal B_i or external B_e), were compared with rac-ethylenebis(indenyl)zirconium dichloride ( 1 ) activated with either MAO or the cation forming agent, triphenyl carbenium tetrakis(pentafluorophenyl)borate ( 2 ), with triethylalumium (TEA). The activities of the heterogeneous catalysts depend on the presence or absence of the Lewis base, were relatively insensitive to the temperature of polymerization, and produce poly(hexene) with molecular weights up to 10⁶. The 1 /MAO catalyst has about five times higher activity at 50°C but is almost inactive at ?30°C; the overall activation energy is 12.4 kcal mol^?1. In contrast, the activity for hexene polymerization by the 1/2 /TEA catalyst is actually slightly greater at lower temperature. The MW's of poly(hexene) obtained with the zirconocenium catalysts are only in the tens of thousands because of rapid β-hydride elimination by the electrophilic cationic Zr center. © 1993 John Wiley & Sons, Inc.     

Flow injection-mini-column technique with ICP-AES detection for the isolation and preconcentration of the fast reactive aluminium fraction in waters

A flow injection mini-column system based on short reaction times with 8-hydroxyquinoline (oxine) with ICP-AES detection is described for the isolation and preconcentration of the fast reactive or toxic aluminium fraction in water samples. Using a 3 s reaction time with oxine (5 × 10^–4 mol/l) at pH 5.0, the fast reactive aluminium fraction is shown not to include the non-toxic AlF²⁺ species at low F^–: Al³⁺ molar ratios (0.3 : 1). The complexed aluminium is isolated in a stable and recoverable form on mini-columns of Amberlite XAD-2 (0.3 cm × 5.0 cm, resin particle size range 0.08 mm–0.16 mm)). The retained aluminium is recovered by back-flushing the analytical column with 1 mol/l HCl for final element specific detection by ICP-AES. Detection limits (after preconcentration) of 2 g/l, a linear range of 0–500 g/l, and possible preconcentration factors of up to 18 times are demonstrated with the present system. Implications for the possible solution of sample stability problems encountered with labile aluminium species analysis and the development of a field sampling technique are discussed, where the desired Al fraction is quantitatively retained in a stable form on mini-columns.     

Separation, recovery and recycling of a fluorous-tagged nickel catalyst using fluorous solid-phase extraction

Highly efficient separation, recovery and recycling of the fluorous-tagged catalyst, [Ni[F13C6C(O)CHC(O)C6F13]2], can be achieved with fluorous solid-phase extraction, after the Lewis acid catalysed synthesis of enaminodiones in conventional organic solvents.     

Unprecedented copper-catalyzed asymmetric conjugate addition of organometallic reagents to alpha,beta-unsaturated lactams

For the first time, an excellent enantioselectivity has been obtained in the conjugate addition of hard organometallic reagents to alpha,beta-unsaturated lactams bearing appropriate protecting-activating groups on the nitrogen.     

Remarkable solvent-dependent excited-state chirality: a molecular modulator of circularly polarized luminescence

The photochemical control of ground- and excited-state chirality of (M)-cis-(1) and (P)-trans-(2)-2-nitro-7-(dimethylamino)-9-(2',3'-dihydro-1'H-naphtho[2,1-b]-thiopyran-1'-ylidene)-9H-thioxanthene is described. It is shown that while ground state chirality can be controlled photochemically by irradiation with light of different wavelengths, the excited state chirality can be tuned either photochemically in a similar way or by appropriate choice of solvent. In benzene solution, circularly polarized luminescence of the two isomers with opposite ground-state helicity, (M)-cis-1 and (P)-trans-2, revealed corresponding excited states of opposite helicity. On the contrary, in n-hexane solution, circularly polarized luminescence was identical for the two forms indicating identical excited state chirality. Circularly polarized luminescence (CPL), steady-state and time-dependent fluorescence, and time-resolved microwave conductivity (TRMC) measurements in both n-hexane and benzene are reported, which provide an explanation for the remarkable solvent dependence of excited-state chirality.     

Mössbauer spectroscopic study of FeAl1−x Co x

We report the results of a Mössbauer study of the alloy sytem FeAl_1?x Co _x forx ≥ 0.3 at temperatures down to 83 K. Magnetic splitting is observed forx ≥ 0.35 at all temperatures. However, forx=0.3, no splitting is observed at room temperature, and superparamagnetic behavior occurs at LN₂ temperature. The magnetically split spectra are fitted each with a distribution of hyperfine fields and the average hyperfine field \(\bar B_{hf} \) as a function of temperature is obtained. The variation of \(\bar B_{hf} \) withT is explained using the model of magnetic clusters with collective magnetic excitations from which the saturation hyperfine field and the magnetic anisotropy energy for these clusters are obtained. Also, the results are discussed using the model of random atomic distributions, and the agreement between the calculated and the experimentally obtained distributions of hyperfine fields is found improve asx increases.     

Thermodynamic exploitation of the liquidus curves in the MIPO3–Pb(PO3)2, MIPO3–Cu(PO3)2 and MIPO3–Ce(PO3)3 systems (M I=Li, Na, K, Rb, Cs, Ag, Tl)

The thermodynamic exploitation of the solid–liquid equilibria in the M^IPO₃–Pb(PO₃)₂, M^IPO₃–Cu(PO₃)₂ and M^IPO₃–Ce(PO₃)₃ systems (with M ^I=Li, Na, K, Rb, Cs, Ag, Tl) is carried out using a semi-empirical equation of the liquidus curves already used with success for similar binary systems. The enthalpy of fusion is calculated for each pure polyphosphate on the assumption that the liquid solution is ideal and only formed by M^IPO₃ and M(PO₃)_q entities (q=2 for Pb and Cu, q=3 for M=Ce). In the most binary systems, a wide difference between the calculated values of the melting enthalpies of these polyphosphates and the measured ones determined from the DTA curves, was observed. This difference is probably due to the existence of some molecular associations in the liquid phase.     

Formation d''etats triplets du biacetyle dans sa reaction en phase gazeuse avec un plasma froid d''azote

The behaviour of the biacetyl molecule (BA) in a cold plasma, obtained by extraction of reactive species from a nitrogen plasma generated in a microwave discharge, is discussed. When the partial pressures of nitrogen and biacetyl are approximately equal (p_N2 ≈ 0.60 Torr, p_BA ≈ 0.55 Torr), the emission of the ³A_u---¹A_g transition of biacetyl is observed with a vibrational structure. On reaction of biacetyl with nitrogen cold plasma preferentially enriched in N₂(A³Σ⁺_u), an enhancement of the emission intensity of the phosphorescence of biacetyl is observed. The initiator of this transition is the triplet state BA(³B_u) which originates from an isoenergetic transfer from N₂(A³Σ⁺_u) according to the overall mechanism

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Sequential Pd(II)-Pd(0) catalysis for the rapid synthesis of coumarins

Electrophilic palladium-catalyzed cycloisomerization of brominated aryl propiolates produces brominated coumarins. The brominated coumarins can be diversified by reduction of the Pd(II) catalyst to Pd(0) followed by Suzuki, Sonogashira, Heck, or Hartwig-Buchwald coupling. Thus, a single loading of precatalyst can be used to conduct sequential reactions, allowing the synthesis of functionalized coumarins. Extension of this methodology toward the synthesis of coumarin libraries is discussed.