Substituted dienes (Scheme 1, synthesized by C(sp²) – C(sp²) coupling of heteracycloalkene triflates and N-Boc tetrahydropyridine boronate) are suitable intermediates in the synthesis of the compounds with isolated (hetero)aliphatic rings. However, the hydrogenation of dienes to achieve formal C(sp³) – C(sp³) coupling connectivity is not a trivial task due to their high stability caused by the conjugation of the double bonds. Thus, the development of efficient catalysts for the hydrogenation of dienes is an important task of catalysis, physical and organic chemistry.
A series of composites were tested as catalysts for the hydrogenation of diene carboxylates 1 and 2 into N-Boc amino esters 3 and 4, respectively (Scheme 1). Four Pd/C and three Pd(OH)₂/C (Pearlman’s catalysts) were prepared using modified common approaches to the deposition of Pd nanoparticles or hydrated PdO on the charcoal. For the preparation of another series of the composites, Pd nanoparticles were generated by the decomposition of Pd₂(dba)₃ complex in presence of Norit activated carbon. The composites were characterized by transmission and scanning electron microscopy, powder X-ray diffraction, low-temperature N₂ adsorption, and room-temperature chemisorption of CO. As it was found, composites prepared by deposition of Pd via reduction of Pd²⁺, contained Pd nanoparticles of 5–40 nm size. Decomposition of Pd₂(dba)₃ complex in presence of Norit led to the formation of smaller nanoparticles with an average size of 2.9 nm, according to the data of CO chemisorption.
The catalytic performance of the Pd/C composites prepared by decomposition of Pd₂(dba)₃ in the hydrogenation of dienes 1 and 2 was comparable to those of the composite, prepared by reduction of PdCl₂ with formaldehyde followed by the addition of alkali, despite the different size of the Pd nanoparticles. At the same time, the composites prepared by reduction of Pd(OH)₂ with formaldehyde and Pearlman’s catalysts had comparably lower catalytic activity. Therefore, the method of Pd²⁺ reduction played a crucial role in the determination of the catalytic performance of the composite.

Eduard Verner

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