New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities.     

A six-minute determination of carbon and hydrogen with titrimetric finish

Summary A method is described for the C/H determination with a completely titrimetric finish. Both carbon and hydrogen are converted to an equivalent amount of carbon dioxide, which is titrated in a non-aquous medium. The procedure can be applied for the analysis of substances separated by a gas Chromatograph. The determination takes about six minutes.

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Zusammenfassung Ein Verfahren zur C-H-Bestimmung wird beschrieben, bei dem die Endbestimmung volumetrisch erfolgt. Hierbei werden sowohl Kohlenstoff als auch Wasserstoff in äquivalente Mengen Kohlendioxid übergeführt und dieses durch Titration in nichtwäßrigem Medium erfaßt. Die Methode kann auch für die Analyse gaschromatographisch getrennter Substanzen benutzt werden. Eine Bestimmung ist innerhalb von 6 min beendet.

     

High-yield solvothermal formation of magnetic CoPt alloy nanowires

One-dimensional (1D) magnetic nanomaterials have attracted much attention recently because of their applications in magnetic recording and spintronics. Nevertheless, it remains a challenge to prepare free-standing magnetic nanowires in high yield. This Communication reports the successful high-yield synthesis of an interesting 1D ferromagnetic CoPt alloy by direct decomposition of platinum acetylacetonate and cobalt carbonyl compound in ethylenediamine solvent through a solvothermal reaction. The CoPt alloy nanowires obtained have a tunable diameter of 10-50 nm and a length along the longitudinal axis of up to several microns, depending on crystallization temperature and reaction time. A unique formation mechanism involving coarsening and ripening under solvothermal conditions was discovered. This research opens new opportunities in synthesizing nanomaterials through low-temperature solvothermal processes.     

Microwave spectrum and dipole moment of glycolic acid

More than 200 rotational transitions of glycolic acid (CH₂OHCOOH) have been assigned in the frequency range 18–40 GHz. Rotational constants and centrifugal distortion constants for the ground state and two vibrationally excited states were deduced. From Stark splittings the dipole moment was determined: μ_a = 1.913(5), μ_b = 0.995(14) and μ_total = 2.156(9) D.     

Accurate measurement of the exciton diffusion length in a conjugated polymer using a heterostructure with a side-chain cross-linked fullerene layer

Exciton diffusion and photoluminescence quenching in conjugated polymer/fullerene heterostructures are studied by time-resolved photoluminescence. It is observed that heterostructures consisting of a spin-coated poly(p-phenylene vinylene) (PPV)-based derivative and evaporated C60 are ill-defined because of diffusion of C60 into the polymer, leading to an overestimation of the exciton diffusion length. This artifact is resolved by the use of a novel, thermally side-chain polymerizing and cross-linking fullerene derivative (F2D) containing two diacetylene moieties, forming a completely immobilized electron acceptor layer. With this heterostructure test system, an exciton diffusion length of 5 +/- 1 nm is derived for this PPV derivative from time-integrated luminescence quenching data.     

A redox-mediator pathway for enhanced multi-colour electrochemiluminescence in aqueous solution

The classic and most widely used co-reactant electrochemiluminescence (ECL) reaction of tris(2,2′-bipyridine)ruthenium(ii) ([Ru(bpy)₃]²⁺) and tri-n-propylamine is enhanced by an order of magnitude by fac-[Ir(sppy)₃]³⁻ (where sppy = 5′-sulfo-2-phenylpyridinato-C²,N), through a novel ‘redox mediator’ pathway. Moreover, the concomitant green emission of [Ir(sppy)₃]³⁻* enables internal standardisation of the co-reactant ECL of [Ru(bpy)₃]²⁺. This can be applied using a digital camera as the photodetector by exploiting the ratio of R and B values of the RGB colour data, providing superior sensitivity and precision for the development of low-cost, portable ECL-based analytical devices.

A water-soluble Ir(iii) complex is shown to enhance the ‘remote’ mechanism of the most widely used co-reactant ECL reaction of tris(2,2′-bipyridine)ruthenium(ii) with tripropylamine.