Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)-alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO(2)Me; R(1), R(2)=4-OCH(3), 4-CH(3), 2,6-(CH(3))(2).

The structure and dynamics of the lanthanide(III) complexes of DTPA-BGLUCA3- (DTPA-bis(glucamide)), DTPA-BENGALAA3- (DTPA-bis(ethylenegalactamine−amide)), DTPA-BEA3- (DTPA-bis(ethanolamide)), and DTPA-BPDA3- (DTPA-bis(propanediolamide)) in water have been investigated. These complexes are of relevance as potential MRI contrast agents. 13C relaxation times of the Nd(III) complexes show octadentate binding of the organic ligand via the three amines, the three carboxylates, and the two amide oxygens. 17O NMR measurements indicate that the coordination sphere is completed by one water ligand. Eight diastereomeric pairs of isomers are observed for the DTPA-bis(sugaramides). Data sets obtained from variable-temperature and -pressure 17O NMR at 9.4 T and variable-temperature 1H nuclear magnetic relaxation dispersion (NMRD) on the Gd(III) complexes were fitted simultaneously to give insight into the parameters governing the water 1H relaxivity. The water exchange rates, k ex 298, on [Gd(DTPA-BPDA)(H2O)], [Gd(DTPA-BGLUCA)(H2O)] and [Gd(DTPA-BENGALAA)(H2O)] are 3.6 ± 0.3, 3.8 ± 0.2, and 2.2 ± 0.1 × 105 s-1, and the activation volumes are +6.7, +6.8, and +5.6 cm3 mol-1 (±0.2 cm3 mol-1), respectively, indicating a strongly dissociatively activated mechanism. The sugar moieties have no significant influence on the coordination of the Gd(III) ion and on the parameters governing the relaxivity, apart from the expected increase in the rotational correlation time. The relaxivity under the usual MRI conditions is limited by the water exchange rate and the electronic relaxation. The data obtained are used to explain the relaxivity of conjugates of polysaccharides and Gd(DTPA).

A series of zerovalent palladium complexes Pd(NN)(alkene) of bi-or tridentate nitrogen ligands of the general formula 6-R′′-C 5 H 3 N-(C(R′)dNR)-2 (R′′ ) H, Me, CHdNR′; R′ ) H, Me; R ) alkyl, aryl, or amino group) and dimethylfumarate (dmfu) have been prepared and were subsequently employed as precatalysts in the homogeneous stereoselective semihydrogenation of 1-phenyl-1-propyne. An X-ray structure of Pd(C 5 H 4 N-(C(Me)dNi-Pr)-2)(dmfu) was obtained. Whereas only relative small changes in substituents apply, the various complexes show very different stabilities under hydrogenation conditions. The complex Pd-(C 5 H 4 N-(C(H)dN(CH 2 ) 2 OH)-2)(dmfu) exhibits a good selectivity for the (Z)-alkene but decomposes just before full conversion of the alkyne, whereas the complex Pd(C 5 H 4 N-(C(H)d Ni-Pr)-2)(dmfu) exhibits a good selectivity and stability under hydrogenation conditions and is a suitable catalyst for the stereoselective hydrogenation of 1-phenyl-1-propyne to (Z)-1phenyl-1-propene.