Papieruntersuchung

Analytical and Bioanalytical Chemistry -     

Stability and recovery of triazine and chloroacetamide herbicides from pH adjusted water samples by using empore solid-phase extraction disks and gas chromatography with ion trap mass spectrometry

Empore disks were used to successfully extract herbicide residues from a difficult-to-analyze surface water source and deionized water. Herbicide recoveries were lower in surface water at 7,14, or 21 days after fortification and storage at 4 degrees C, presumably due to chemical sorption onto precipitated organic particulates. The addition of acid to the samples, as recommended in EPA Method 525.2, did not affect recoveries of alachlor and metolachlor, but reduced recoveries of atrazine, simazine, and cyanazine. Treatment of water samples with sodium hypochlorite did not affect alachlor or metolachlor recoveries, but greatly reduced the recovery of all triazine herbicides. This indicates that addition of acid or sodium hypochlorite to water samples may be detrimental to triazine analysis.     

Palladium-catalyzed carbon-carbon bond-forming 1,2-ligand migration of organoalanes

A one-pot, two-step process that transforms terminal alkynes into ethyl methyl-substituted benzylic quaternary carbon centers is described. (E)-2,2-Disubstituted-1-alkenyldimethylalanes have been shown to participate in 1,2-alkyl migration from aluminum to carbon with concomitant arylation at the 2-position to furnish ethyl methyl-substituted benzylic quaternary carbon centers, when reacted intramolecularly with aryl halides and triflates in the presence of a Pd(0) catalyst. The protocol is initiated with Cp2ZrCl2-catalyzed methylalumination of terminal alkynes followed by palladium-catalyzed intramolecular arylation of the resulting (E)-2,2-disubstituted-1-alkenyldimethylalanes, leading to 1,2-methyl shift from aluminum to carbon. In that sequence, a total of three new C-C single bonds are made, and two of the three alkyl groups on Me3Al transferred to the substrate on vicinal carbons. This method was applied to a variety of substrates, and the mechanism was investigated by deuterium-labeling experiments, which revealed that protodealumination of the final dialkylaluminum triflate or halide intermediates by CH3CN results in the formation of the fourth bond in the course of the transformation.     

Reductive Amination of Aldehydes and Ketones with Sodium Triacetoxyborohydride. Studies on Direct and Indirect Reductive Amination Procedures(1)

Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4).     

Latex Fractionation by Sedimentation and Colloidal Crystallization: The Case of Poly(styrene-co-acrylamide)

Poly(styrene-co-acrylamide) (PS-AAM) latex was prepared, fractionated by sedimentation under gravity, and characterized by PCS, infrared spectra, secondary and backscattered electron imaging in the scanning electron microscope, and electron spectroscopy imaging in an analytical transmission electron microscope. Three latex fractions were obtained. The lower fraction was opalescent and its particles were the more uniform, concerning size, chemical composition, and topochemical features. This lower fraction was still further fractionated by zonal centrifugation in a density gradient, yielding two fractions with similar macrocrystal-forming abilities but different sizes and chemical compositions. These results confirm those previously obtained for the PS-HEMA latex. Copyright 2000 Academic Press.