Extending the Chemistry of Hexamethylenetetramine in Ruthenium-Catalyzed Amine Oxidation

Cobalt-Catalyzed Acceptorless Dehydrogenation of Primary Amines to Nitriles

The direct double dehydrogenation from primary amines to nitriles without an oxidant or hydrogen acceptor is both intriguing and challenging. In this paper, we describe a non-noble metal catalyst capable of realizing such a transformation with high efficiency. A cobalt-centered N,N-bidentate complex was designed and employed as a metal-ligand cooperative dehydrogenation catalyst. Detailed kinetic studies, control experiments, and DFT calculations revealed the crucial hydride transfer, proton transfer, and hydrogen evolution processes. Finally, a tandem outer-sphere/inner-sphere mechanism was proposed for the dehydrogenation of amines to nitriles through an imine intermediate.

Ruthenium-Induced Decomposition of Hexamethylenetetramine as a Tool for the Acid-Free Sommelet Reaction in Aqueous Medium

Hexamethylenetetramine (HMTA) is one of the most versatile and most utilized nitrogen-containing heterocyclic compounds in academia and industry. Most of the reactions involving HMTA employ stoichiometric or excess amounts of acid, which hamper the sustainability of the reactions. Herein, we report the first example of the ruthenium-mediated decomposition of HMTA at room temperature, supported by a detailed mechanistic study using thermogravimetric analysis/differential thermal analysis, variable-temperature NMR, UV-vis spectroscopy, and density functional theory techniques. The mechanism study also involves a comparison of the decomposition of HMTA, protonated HMTA, [RuCl3(HMTA)], and [FeCl3(HMTA)], which revealed that [RuCl3(HMTA)] decomposes at the lowest temperature and has the lowest HOMO-LUMO band gap of 2.66 eV. The ruthenium-induced decomposition of HMTA is successfully used as a tool to increase the sustainability of the Sommelet reaction as it employs simple RuCl3·nH2O as a catalyst in as low as 0.5 mol % concentration in aqueous medium. The developed methodology is found to be very selective and efficient even for the very challenging substrates, namely, aliphatic aldehydes and substrates with electron-withdrawing substituents. The findings of this work are the first of its kind in which ruthenium is employed in the Sommelet reaction and also provide a possible platform to improve the sustainability of all reactions involving HMTA as a reactant/reagent.

Thermodynamic evaluations of the acceptorless dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles in acetonitrile

N-heterocycles are important chemical hydrogen-storage materials, and the acceptorless dehydrogenation and hydrogenation of N-heterocycles as organic hydrogen carriers have been widely studied, with the main focus on the catalyst synthesis and design, investigation of the redox mechanisms, and extension of substrate scope. In this work, the Gibbs free energies of the dehydrogenation of pre-aromatic N-heterocycles (YH2) and the hydrogenation of aromatic N-heterocycles (Y), i.e., ΔGH2R(YH2) and ΔGH2A(Y), were derived by constructing thermodynamic cycles using Hess' law. The thermodynamic abilities for the acceptorless dehydrogenation and hydrogenation of 78 pre-aromatic N-heterocycles (YH2) and related 78 aromatic N-heterocycles (Y) were well evaluated and discussed in acetonitrile. Moreover, the applications of the two thermodynamic parameters in identifying pre-aromatic N-heterocycles possessing reversible dehydrogenation and hydrogenation properties and the selection of the pre-aromatic N-heterocyclic hydrogen reductants in catalytic hydrogenation were considered and are discussed in detail. Undoubtedly, this work focuses on two new thermodynamic parameters of pre-aromatic and aromatic N-heterocycles, namely ΔGH2R(YH2) and ΔGH2A(Y), which are important supplements to our previous work to offer precise insights into the chemical hydrogen storage and hydrogenation reactions of pre-aromatic and aromatic N-heterocycles.

Nitrile Synthesis via Desulfonylative-Smiles Rearrangement

Herein, we designed a simple nitrile synthesis from N-[(2-nitrophenyl)sulfonyl]benzamides via base-promoted intramolecular nucleophilic aromatic substitution. The process features redox-neutral conditions as well as no requirement of toxic cyanide species and transition metals. Our process shows broad scope and various functional group compatibility, affording a variety of (hetero)aromatic nitriles in good to excellent yields.

Oxidize Amines to Nitrile Oxides: One Type of Amine Oxidation and Its Application to Directly Construct Isoxazoles and Isoxazolines

A facile oxidative heterocyclization of commercially available amines and tert-butyl nitrite with alkynes or alkenes leading to isoxazoles or isoxazolines is described. The unprecedented strategy of the oxidation of an amine directly to a nitrile oxide was used in this cyclization process. This reaction is highly efficient, regiospecific, operationally simple, mild, and tolerant of a variety of functional groups. Control experiments support a nitrile oxide intermediate mechanism for this novel class of oxidative cyclization reactions. Moreover, synthetic applications toward bioactive molecular skeletons and the late-stage modification of drugs were realized.

Aerobic oxidation of primary benzylic amines to amides and nitriles catalyzed by ruthenium carbonyl clusters carrying N,O-bidentate ligands

Four trinuclear ruthenium carbonyl clusters, (6-BrPyCHRO)₂Ru₃(CO)₈ (R = 4-OCH₃C₆H₄, 1a; R = 4-BrC₆H₄, 1b) and (2-OC₆H₄-HC[double bond, length as m-dash]N-C₆H₄R)₂Ru₃(CO)₈ (R = 4-OCH₃, 2a; R = 4-Br, 2b), were synthesized from the reactions of Ru₃(CO)₁₂ with the corresponding N,O-bidentate ligands (two pyridyl alcohols and two Schiff bases) respectively in a ratio of 1 : 2. Three new complexes 1b, 2a and 2b have been fully characterized by elemental analysis, FT-IR, NMR and X-ray crystallography. The catalytic activity of these ruthenium complexes for the aerobic oxidation of primary benzylic amines to amides and nitriles in the presence of t-BuOK was investigated, of which the Schiff base complex 2a was found to exhibit the highest activity.

Synthesis, characterization, catalytic and biological application of half-sandwich ruthenium complexes bearing hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

A series of cationic Ru(ii)(η⁶-p-cymene) complexes with thioether-functionalised N-heterocyclic carbene ligands have been prepared and fully characterized. Steric and electronic influence of the R thioether substituent on the coordination of the sulfur atom was investigated. The molecular structure of three of them has been determined by means of X-ray diffractrometry and confirmed the bidentate (κ²-C,S) coordination mode of the ligand. Interestingly, only a single diastereomer, as an enantiomeric couple, was observed in the solid state for complexes 1c, 1i and 1j. DFT calculations established a low energy inversion barrier between the two diastereomers through a sulfur pyramidal inversion pathway with R donating group while a dissociative/associative mechanism is more likely with R substituents that contain electron withdrawing group, thus suggesting that the only species observed by the ¹H-NMR correspond to an average resonance position of a fluxional mixtures of isomers. All these complexes were found to catalyse the oxydant-free double dehydrogenation of primary amine into nitrile. Ru complex bearing NHC-functionalised S-tBu group was further investigated in a wide range of amines and was found more selective for alkyl amine substrates than for benzylamine derivatives. Finally, preliminary results of the biological effects on various human cancer cells of four selected Ru complexes are reported.

Aerobic dehydrogenation of amines to nitriles catalyzed by triazolylidene ruthenium complexes with O2 as terminal oxidant

Pyridyl-substituted mesoionic triazolylidene ruthenium cymene complexes catalyze the oxidation of both aromatic and aliphatic amines to nitriles with high activity and selectivity under benign conditions using dioxygen as the terminal oxidant. Modification on the pyridyl moiety of the ligand scaffold has negligible effect on the catalytic performance, while substituents on the triazolylidene directly affect the catalytic fitness of the metal center, leading to distinct catalytic profiles. Pre-dissociation of the cymene ligand and formation of a solvento analogue further enhances the catalytic activity towards nitrile formation. Variation of reaction conditions provided valuable mechanistic insights and resulted in a highly efficient protocol for nitrile formation with maximum turnover numbers around 10 000. The turnover frequency reaches up to 400 h^-1, providing one of the fastest catalytic systems known to date for this transformation.