Advances and Challenges in Development of Transition Metal Catalysts for Heterogeneous Hydrogenation of Organic Compounds.

Actual problems of development of catalysts for hydrogenation of heterocyclic compounds by hydrogen are summarized and discussed. The scope of review covers composites of nanoparticles of platinum group metals and 3d metals for heterogeneous catalytic processes. Such problems include increase of catalyst activity, which is important for reduction of precious metals content; development of new catalytic systems which do not contain metals of platinum group or contain cheaper analogues of Pd; control of factors which make influence on the selectivity of the catalysts; achievement of high reproducibility of the catalyst's performance and quality control of the catalysts. Own results of the authors are also summarized and described. The catalysts were prepared by decomposition of Pd0 and Ni0 complexes, pyrolysis of Ni2+ and Co2+ complexes deposited on aerosil and reduction of Ni2+ in pores of porous support in situ. The developed catalysts were used for hydrogenation of multigram batches of heterocyclic compounds.

Redox Behavior of Cobalt-Phosphine Complexes vs Their Catalytic Activity in Organozinc Compound Formation: Background for Mechanistic Investigations.

Catalytic activity in arylzinc compound formation was studied for eight Co complexes with phosphines along with their redox properties for implementing the idea of rational design. It was found that Co(XantPhos)Cl2 and Co(N-XantPhos)Cl2 demonstrated distinct reversible CoII/CoI redox processes and acted as efficient catalysts of arylzinc compound formation. Meanwhile, for Co(DPEphos)Cl2, Co(dppf)Cl2, Co(dppb)Cl2, Co(PPh3)2Cl2, and Co(XantPhos)(Piv)2 (the latter one without the addition of LiCl), reversible redox processes were not observed. These catalysts did not act efficiently for the model process of organozinc compound formation. Co4(dppe)5Cl8 was the only exception, explained by a completely different structure (CoP4Cl and CoPCl3) of donor sets instead of CoP2X2 (X = Cl or O). The stability of complexes in tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) solutions was studied by UV-vis spectroscopy. Previously unknown X-ray structures for Co(XantPhos)(Piv)2, Co(N-XantPhos)Cl2, and {Co(DMF)6}{(CoCl3)2(dppb)} were determined. The use of pivalate counterions instead of chloride for Co(XantPhos)2+ led to a significant (ca. 20 times) increase of the kinetic solubility in THF compared to Co(XantPhos)Cl2, preserving high catalytic productivity upon the addition of LiCl. This allowed the latter to be efficiently used in combination with LiCl as the catalyst for arylzinc compound formation on a 2 g scale. The data obtained in this work can be regarded as experimental confirmation of the first and last stages of the plausible reaction pathway of arylzinc compound formation, involving CoII → CoI and CoI → CoII transformations, which could be a significant framework for further mechanistic investigations.

Screening of Palladium/Charcoal Catalysts for Hydrogenation of Diene Carboxylates with Isolated-Rings (Hetero)aliphatic Scaffold.

A series of seven palladium-containing composites, i.e., four Pd/C and three Pd(OH)2/C (Pearlman's catalysts), was prepared using modified common approaches to deposition of Pd or hydrated PdO on charcoal. All the composites were tested in the catalytic hydrogenation of diene carboxylates with the isolated-ring scaffold, e.g., 5,6-dihydropyridine-1(2H)-carboxylates with 2-(alkoxycarbonyl)cyclopent-1-en-1-yl and hex-1-en-1-yl substituents at the C(4)-position. The performance of the composites was also studied via the hydrogenation of quinoline as a model reaction. The composites were characterized by transmission and scanning electron microscopy (TEM and SEM), powder X-ray diffraction, and low-temperature N2 adsorption. It was found that the composites containing Pd nanoparticles (NPs) of 5-40 nm size were the most efficient catalysts for the hydrogenation of dienes, providing the reduced products with up to 90% yields at p(H2) = 100 atm, T = 30 °C for 24 h. The method of Pd NPs formation had more effect on the catalyst performance than the size of the NPs. The catalytic performance of Pearlman's catalysts (Pd(OH)2/C) in the hydrogenation of dienes was comparable to or lower than the performance of the Pd/C systems, though the Pearlman's catalysts were more efficient in the hydrogenation of quinoline.