On the Regioselectivity Control in the Palladium-Catalyzed Hydro-alkoxycarbonylation of α,β-Unsaturated Esters

The regioselectivity of the hydro-alkoxycarbonylation of methyl acrylate, methacrylate, and crotonate catalyzed by [PdCl₂L₂] complexes (L = phosphine ligands) can be largely controlled by variation of the ligands. PPh₃, promotes preferential carbonylation at the α-position, whereas with [(2,2-dimethyl-1,3-dioxolane-4,5-diyl)bis-(methylene)]bis[diphenylphosphine] as ligand, the β-position is overwhelmingly carbonylated.     

Intramolecular and intermolecular diels-alder reactions of acylhydrazones derived from methacrolein and ethylacrolein

The intramolecular Diels-Alder reactions of hydrazones derived from methacrolein or ethylacrolein and terminally unsaturated N-acyl-N-methylhydrazines have been investigated. The hydrazones 7b and 7c derived from N-methyl-N-pent-4-enoylhydrazine 3b were found to undergo intramolecular [4 + 2] cycloaddition above 140 °C and the pyridopyridazines 12 were isolated. The corresponding hydrazones 8b and 8c from N-methyl N-pent-4-ynoylhydrazone 4a reacted similarly and gave as the final products the pyridines 13. The scope of the reaction is limited, as was shown by the failure of several other terminally unsaturated hydrazones of β-unsaturated aldehydes to undergo intramolecular cycloaddition. These hydrazones did, however, undergo intermolecular [4 + 2] cyctoaddition to N-phenylmaleimide. Other hydraiones 15 of methacrolein. including the benzoylhydrazone and the phenylhydrazone, also reacted with N-phenylmaleimide to give the pyridine 14b by way of an isolable dihydropyridine 16.     

Speciation of arsenic(V) and arsenic(III) by cathodic stripping voltammetry in fresh water samples

A voltammetric stripping procedure is described for the determination of arsenic(V) in a mannitol-sulphuric acid medium. The arsenic is coprecipitated with copper and selenium and reduced to arsine at the hanging mercury drop electrode. Using an accumulation time of 240 s, the detection limit is 0.52 μg L^–1, the determination limit is 0.9 μg L^–1. The method has been applied to the determination of arsenic in water samples. By varying the composition of the supporting electrolyte it is possible to differentiate between arsenic(III) and arsenic(V). As both oxidation states have different toxicological characteristics, the ability to discriminate between both is an distinct advantage of the proposed method. Received: 25 October 1996 / Revised: 7 February 1997 / Accepted: 12 February 1997     

Rheological investigation of starch polysaccharide gelation

Small-amplitude dynamic measurements of aqueous starch polysaccharide solutions are performed by a Bohlin controlled-stress rheometer with air bearing. Three classes of starch polysaccharides–native starches, fractions of starches and hydrolysed starches–are compared in their molecular composition and rheological properties during and after the gelation process. Viscoelastic properties of solutions and gels are recorded in dependence on temperature and time, yielding storage and loss moduli during and after sol-gel transition. Hot concentrated aqueous solutions are cooled down from 90 °C to 5 °C at a rate of 1 °C/min. Measurements are carried out at 0,1 Hz and 5% strain amplitude deformation. The typical course of the moduli of gelling starch polysaccharide solutions shows a liquid-like behavior (G” > G') in the upper temperature level between 60 and 90 °C, a jumpwise increase with ensuing intersection of G' and G” below 60 °C and a solid-like behavior (G' > G”) at lower temperatures with a slightly in time growing storage modulus. Storage and loss moduli depend on molecular composition and concentration of the substance. The process of starch polysaccharide aggregation is discussed with regard to theories of physical gelation by Ross-Murphy and Winter.