Surface nickel (NiOx) species, surface NiAlxOy compound, and NiO crystallites are present on the Ni/Al2O3 catalysts, and the ratio of these nickel species is dependent on the nickel loading. Surface nickel interacts with the TiO2 support to form a surface nickel titanate compound (NiTiOx) which has a lower reducibility. The weak interaction between the surface nickel and the silica support results in the formation of NiO crystallites on the SiO2 surface. The Ni/Al2O3 and Ni/TiO2 catalysts contain new surface Lewis acid sites and the amount of surface Lewis acid sites increases with increasing nickel concentration. The Ni/SiO2 catalysts have no sign of the presence of the surface Lewis acid sites. Only the Ni/Al2O3 catalysts have shown the ammonia adsorption at temperature of 200°C. Supported nickel on alumina catalysts possess the highest amination conversion, and the amine yield increases with increasing nickel loading up to 15% and starts to level off. By comparing amination catalysis with quantitatively TPR studies of the H2 consumed of the Ni/Al2O3 catalysts, it appears that the dispersed nickel species are the active sites for amination. In addition, the amination product is mainly the secondary amine due to the presence of water. 相似文献
Allylic amination of methyl oleate with bis(N-p-toluenesulfonyl) sulfodiimide results in a mixture of methyl 11-amino-(N-p-toluenesulfonyl)-9-E-octadecenoate and methyl 8-amino-(N-p-toluenesulfonyl)-9-E-octadecenoate in 58% yield. These novel products were isolated and characterized by nuclear magnetic resonance, infrared
spectroscopy, mass spectrometry, and melting point. The reaction was analyzed by highperformance liquid chromatography and
thin-layer chromatography. 相似文献
Substrate binding pockets of ω‐transaminase (ω‐TA) consist of a large (L) pocket capable of dual recognition of hydrophobic and carboxyl substituents, and a small (S) pocket displaying a strict steric constraint that permits entry of a substituent no larger than an ethyl group. Despite the unique catalytic utility of ω‐TA enabling asymmetric reductive amination of carbonyl compounds, the severe size exclusion occurring in the S pocket has limited synthetic applications of ω‐TA to access structurally diverse chiral amines and amino acids. Here we report the first example of an ω‐TA whose S pocket shows a non‐canonical steric constraint and readily accommodates up to an n‐butyl substituent. The relaxed substrate specificity of the (S)‐selective ω‐TA, cloned from Paracoccus denitrificans (PDTA), afforded efficient asymmetric syntheses of unnatural amino acids carrying long alkyl side chains such as L ‐norvaline and L ‐norleucine. Molecular modeling using the recently released X‐ray structure of PDTA could pinpoint an exact location of the S pocket which had remained dubious. Entry of a hydrophobic substituent in the L pocket was found to have the S pocket accept up to an ethyl substituent, reminiscent of the canonical steric constraint. In contrast, binding of a carboxyl group to the L pocket induced a slight movement of V153 away from the small‐pocket‐forming residues. The resulting structural change elicited excavation of the S pocket, leading to formation of a narrow tunnel‐like structure allowing accommodation of linear alkyl groups of carboxylate‐bearing substrates. To verify the active site model, we introduced site‐directed mutagenesis to six active site residues and examined whether the point mutations alleviated the steric constraint in the S pocket. Consistent with the molecular modeling results, the V153A variant assumed an elongated S pocket and accepted even an n‐hexyl substituent. Our findings provide precise structural information on substrate binding to the active site of ω‐TA, which is expected to benefit rational redesign of substrate specificity of ω‐TA.
