Furfural from corn stover hemicelluloses. A mineral acid-free approach

Furfural was obtained from corn stover hemicelluloses by a microwave-assisted, green and heterogeneously catalyzed twostep cascade process as follows: first step, hydrothermal fractionation of corn stover hemicelluloses, and second step, hydrolysis/dehydration of soluble hemicellulosic sugars over niobium phosphate to yield furfural at moderate temperatures (<200 °C), with both steps being performed in water. Furfural yields of up to 23 mol% with respect to the starting raw biomass were reached

Novel alpha-nitroketonate nickel(II) complexes as homogeneous catalysts for ethylene oligomerization

The oxidative addition of alpha -nitroacetophenone (naph) to bis(cyclooctadienyl)nickel(0) [Ni(cod)(2)], in the presence or not of tricyclohexylphosphine (PCy3) was proven to give the eta (1),eta (2)-cyclooctenyl)(naph)nickel(II) complex (III). When this reaction was carried out in the presence of PCy3, further addition of methylalumoxane (MAO) or trialkylaluminium compounds afforded the corresponding alkyl(PCy3)(naph)nickel(II) complex. This was also obtained by an alternative synthetic procedure involving the addition of a trialkylaluminium to the equimolar adduct of Ni(naph)(2) with PCy3. When the above complexes, independently of the synthetic procedure adopted, were generated in situ by using MAO as a co-catalyst (Al/Ni=100), they resulted in being active in ethylene oligomerization. Whereas in the absence of PCy3, they gave mainly linear C-4-C-6 oligomers with turnover frequencies (TOFs) in the 4000-8000 h(-1) range, the presence of the phosphine ancillary Ligand not only produced a remarkable increase in activity (TOF=26,00012 1,000 h(-1)) but also afforded a completely different distribution of oligomers, mainly branched C-6-C-13 products being obtained. (C) 2001 Elsevier Science B.V. All rights reserved

Optically active polymers bearing side-chain photochromic moieties: synthesis and chiroptical properties of methacrylic homopolymers with pendant trans-azobenzene chromophores bound through L-leucine, L-valine and L-proline amino acid spacers

Full Paper: Novel optically active monomers, based on different L-amino acid residues such as trans-(S)-4-(2-methacryloylamino-3-methylbutanoylamino)azobenzene, trans-(S)-4-(2-methacryloylamino-4-methylpentanoylamino)azobenzene and trans-(S)-4-(N-methacryloyl-2-pyrrolidinoylamino)azobenzene, have been prepared and homopolymerized by free radical initiation. Circular dichroism spectra of the resulting polymers, as compared with those of the corresponding low molecular weight analogues, purposely synthesized, allow one to suggest that the macromolecules assume in solution achiral or chiral conformations with a prevailing screw sense, depending on the bulkiness and rigidity of the L-amino acid residue present in the side chains. The role of intra- and/or intermolecular hydrogen bonding between side-chain amido groups along the backbone and the structural requirements of the chiral groups in determining the macromolecular arrangement is also discussed

Selective propylene dimerization to 2,3-dimethylbutenes by homogeneous catalysts prepared by oxidative addition of alpha-nitroketones to nickel(0) complexes in the presence of phosphine ligands and organoaluminium co-catalysts

Novel nickel catalysts, prepared in situ by oxidative: addition of alpha -nitroketones to nickel(O) complexes, in the presence of a phophine ancillary ligand and activated by organoaluminium co-catalysts were investigated in propylene oligomerization with the aim to selectively obtain 2,3-dimethylbutenes (DMB). In particular, the effect of the nature of the alpha -nitroketonate ligand as well as of the basicity and bulkiness of the phosphine on catalyst performances were studied. Finally, the influence of the type of organoaluminium co-catalyst and reaction temperature were examined. In particular, when the Ni(cod)(2)/tricyclohexylphosphine (PCy3)/alpha -nitroacetophenone (naph)/methylalumoxane (MAO) catalytic system was employed, the highest up to now reported regioselectivity within C-6 cut (>90%) was achieved. Moreover, the use of balanced mixtures of MAO and Et2AlCl allowed to optimize the catalyst performances up to an overall yield to DMB of almost 70%, a significant productivity being also achieved (TOF = 4500 h(-1)). (C) 2001 Elsevier Science B.V. All rights reserved