Comparison Between PdCl 2 (PPh 3 ) 2 -and NiCl 2 (PPh 3 ) 2 -Catalyzed Cross-Coupling of Aryl Halides by Intramolecularly Stabilized Dialkylaluminum Reagents.-Aluminum complexes (I) are used as alkylating compounds in the title reaction of various aryl halides. For comparison, the reaction of naphthyl chloride and bromide is studied using either the nickel or the palladium catalyst.. It is found, that the nickel complex catalyzes cross-alkylation of both aryl chlorides and bromides with aluminum complexes that do not have β-hydrogen atoms. With diethyl and diisobutyl complexes (Id) and (Ie) in the presence of nickel catalyst substantial hydrogenolysis of the aryl halides occurs. The palladium catalyst promotes clean cross-alkylation of aryl bromides, but does not activate aryl chlorides. -
Mass spectrometric studies show that hydrogen/deuterium exchange procaw precede the hydrogenation of the title com-
pound.In a recent study') we have shown that both homogeneous (1) and silica-bound (2) (+ )-dicarbonyl-p-chloro-[p-[2-(triethoxysilyl)-ethanethiolato-S,S]bis( [S-methyl-2-(1 -methylethyl)cyclohexyl]dipheny1phosphine)dirhodium catalyze the asymmetric hydrogenation of methyl (Z)-a-acetamidocinnamate (3). The optical purity of the N-acetylphenylalanine methyl ester (4) formed, was found to be up to 98% ee at low conversions, but it dropped gradually when the reduction advanced. Loss in optical purity was also observed upon (i) lowering the hydrogen pressure below 7 atm, (ii) decreasing the catalyst: substrate ratio, and (iii) increasing the reaction tempera t urc.Since the homogeneous dirhodil;m catalyst decomposes in part during the hydrogenation process, we have initially attributed the diminishing in optical purity to a competing nonasymmetric hydrogenation induced by some of the liberated metallic rhodium. However, as the reduction of 3 by the silicon-bound catalyst (which does not deteriorate) was also found to give, with increasing reaction time, increasing amounts of racemic product, we considered hydrogen scrambling at the chiral carbon atom in 4 as an alternative cause for gradual loss in optical activity. Therefore, we decided to investigate the possibility of H/D exchange during the course of the hydrogenation process by using Dz in C H 3 0 H or C D 3 0 D as sol-
vents.In a series of experiments, 0.42 mmol of 3 in a mixture of either 2.5 ml of C H 3 0 H and 5.0 ml of C6H6, or of 2.5 ml of CD@D and 5.0 ml of C6H6 was hydrogenated in the presence of 2.21 x lo-' mmol offully activated 3') at 120°C and 7.14 atm (initial pressure) of either H2 or D2. Mass spectral analyses of the Tesulting reaction mixtures after 4 and 16 h, revealed that both the product and the recovered starting material had undergone extensive H/D exchange both by D2 and by the labeled solvent.The 70-eV EI mass spectrum of the starting material 3 that consisted of peaks at rn/z (YO) = 219 (6), 177 (60), 176 (7), 118 (13) (52), 88 (loo), is also in line with well-known mass spectrometric fragmentation patterns, and the major fragmentations of 4+' are those given in Scheme 2. In addition. both 3+' and 4+' were found to form the C7H: ion (I) by several different fragmentation routes.
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