Rhodium Complexes Bearing 1,10-Phenanthroline Analogue Bis-NHC Ligands Are Active Catalysts for Transfer Hydrogenation of Ketones

Rhodium-Catalyzed Ring Expansion of Azetidines via Domino Conjugate Addition/N-Directed α-C(sp3)-H Activation

A facile synthetic method for 4-aryl-4,5-dihydropyrrole-3-carboxylates is developed, with a rhodium-catalyzed ring expansion strategy from readily available 2-(azetidin-3-ylidene) acetates and aryl boronic acids. Mechanistic investigations suggest a novel domino "conjugate addition/N-directed α-C(sp³)-H activation" process. The asymmetric catalytic synthesis of the 4-aryl-4,5-dihydropyrrole-3-carboxylate is realized by using QuinoxP* (91-97% ee). The synthetic utility of this protocol is demonstrated by the synthesis of 3,4-disubstituted or 2,3,4-trisubstituted pyrrolidines with excellent diastereoselectivities.

Photocatalytic hydrogenation of acetophenone on a titanium dioxide cellulose film

A previously developed sustainable immobilization concept for photocatalysts based on cellulose as a renewable support material was applied for the photocatalytic hydrogenation of acetophenone (ACP) to 1-phenyl ethanol (PE). Four different TiO₂ modifications (P25, P90, PC105, and PC500) were screened for the reaction showing good performance for PC25 and PC500. PC500 was selected for a detailed kinetic study to find the optimal operating conditions, and to obtain a better understanding of the photocatalytic pathway in relation to conventional and transfer hydrogenation. The kinetic data were analyzed using the pseudo-first-order reaction rate law. A complete conversion was obtained for ACP concentrations below 1 mM using a 360 nm filter and argon as the purge gas within 2–3 hours. High oxygen concentrations slow down or prevent the reaction, and wavelengths below 300 nm lead to side-products. By investigating the temperature dependency, an activation energy of 22 kJ mol⁻¹ was determined which is lower than the activation energies for conventional and transfer hydrogenation, because the light activation of the photocatalyst turns the endothermic to an exothermic reaction. PC500 was immobilized onto the cellulose film showing a 37% lower activity that remains almost constant after multiple use.

The photocatalytic hydrogenation of acetophenone to 1-phenylenthanol was investigated with cellulose-immobilized titanium oxide (TiO₂) particles.

Hydrogen Transfer Reactions of Carbonyls, Alkynes, and Alkenes with Noble Metals in the Presence of Alcohols/Ethers and Amines as Hydrogen Donors

Hydrogen transfer reactions have exceptional importance, due to their applicability in numerous synthetic pathways, with academic as well as industrial relevance. The most important transformations are, e.g., reduction, ring-closing, stereoselective reactions, and the synthesis of heterocycles. The present review provides insights into the hydrogen transfer reactions in the condensed phase in the presence of noble metals (Rh, Ru, Pd) as catalysts. Since the H-donor molecules (such as alcohols/ethers and amines (1°, 2°, 3°)) and the acceptor molecules (alkenes (C=C), alkynes (C≡C), and carbonyl (C=O) compounds) play a crucial role from mechanistic viewpoints, the present summary points out the key mechanistic differences with the interpretation of current contributions and the corresponding historical achievements as well.

Cobalt(ii)-catalysed transfer hydrogenation of olefins

Catalytic transfer hydrogenation of olefins is achieved by an earth-abundant metal cobalt catalyst. A range of alkene substrates including those with functional groups have been hydrogenated in high yields.

Transfer hydrogenation with abnormal dicarbene rhodium(iii) complexes containing ancillary and modular poly-pyridine ligands

Rhodium(iii) complexes with abnormal dicarbene and diimine ligands are transfer hydrogenation catalysts with visually distinct active and dormant states.