A practical approach for ambient-pressure hydrogenations using Pd on porous glass

Cobalt-Catalyzed Borylative Reduction of Azobenzenes to Hydrazobenzenes via a Diborylated-Hydrazine Intermediate

Cobalt-catalyzed borylative reduction of azobenzenes using pinacolborane is developed. The simple cobalt chloride catalyst and reaction conditions make this protocol attractive for hydrazobenzene synthesis. This borylative reduction shows good functional group compatibility and can be readily scaled up to the gram scale. Preliminary mechanistic studies clarified the proton source of the hydrazine products. This cobalt-catalyzed azobenzene borylative reaction provides a practical protocol to prepare synthetically useful diborylated hydrazines.

Catalytic Amination of Alcohols Using Diazo Compounds under Manganese Catalysis Through Hydrogenative N-Alkylation Reaction

Sustainable chemical production requires fundamentally new types of catalysts and catalytic technologies. The development of coherent and robust catalytic systems based on earth-abundant transition metals is essential, but highly challenging. Herein, we systematically explored a general hydrogenative cleavage/N-alkylation tandem of cyclic and acyclic diazo (N=N) compounds to value-added amines under manganese catalysis. The reaction is catalyzed by a single-site molecular manganese complex and proceeds via tandem dehydrogenation, transfer hydrogenation, and borrowing hydrogenation strategies. Interestingly, the reaction involves abundantly available renewable feedstocks, such as alcohols, that can act as (transfer)hydrogenating and alkylating agents. The synthetic application of our approach in large-scale pharmaceutical synthesis and easy access to highly demanding N-CH3 /CD3 derivatives are also demonstrated. Kinetic studies show that the reaction rate depends on the concentration of alcohol and Mn-catalyst and follows fractional orders. Several selective bond activation/formation reactions occur sequentially via amine-amide metal-ligand cooperation.

Reusable Cobalt-Catalyzed Selective Transfer Hydrogenation of Azoarenes and Nitroarenes

Herein, control transfer hydrogenation (TH) of azoarenes to hydrazo compounds is established employing easy-to-synthesize reusable cobalt catalyst using lower amounts of N₂H₄·H₂O under mild conditions. With this effective methodology, a library of symmetrical and unsymmetrical azoarene derivatives was successfully converted to their corresponding hydrazo derivatives. Further, this protocol was extended to the TH of nitroarenes to amines with good-to-excellent yields. Several kinetic studies along with Hammett studies were carried out to understand the plausible mechanism and the electronic effects in this transformation. This inexpensive catalyst can be recycled up to five times without considerable loss of catalytic activity.

Visible-Light-Promoted Hydrogenation of Azobenzenes to Hydrazobenzenes with Thioacetic Acid as the Reductant

A catalyst- and metal-free hydrogenation of azobenzenes to hydrazobenzenes in the presence of thioacetic acid was achieved under visible light irradiation. The transformation was carried out under mild conditions in an air atmosphere at ambient temperature, generating a variety of hydrazobenzenes with yields up to 99%. The current process is compatible with a variety of substituents and is highly chemoselective for azo reduction when other unsaturated functionalities (carbonyl, alkenyl, alkynyl, etc.) are contained. Preliminary mechanistic study indicated that the transformation could be a radical process.

N,N-Diisopropylethylamine-Mediated Electrochemical Reduction of Azobenzenes in Dichloromethane

We report a cathodic reduction-dominated electrochemical approach for the hydrogenation of azobenzenes in dichloromethane. With cheap and readily available N,N-diisopropylethylamine as a catalytic mediator, the reaction proceeded smoothly in a simple undivided cell under constant-current electrolysis. A series of azobenzenes were successfully reduced to the corresponding hydrazobenzenes in moderate to high yields at room temperature. Preliminarily mechanistic studies indicate that solvent dichloromethane acts as a hydrogen source. The use of a common solvent as a hydrogen source, no need for stoichiometric mediators or metallic reductants, and mild conditions make this work a more straightforward and sustainable protocol for hydrogenation of azobenzenes.

Convenient semihydrogenation of azoarenes to hydrazoarenes using H2

The high atom-economical and eco-benign nature of hydrogenation reactions make them much more superior to conventional reduction and transfer hydrogenation. Herein, a convenient and highly selective hydrogenation reaction of azoarenes using molecular hydrogen to access diverse hydrazoarenes is reported. The present catalytic method is general and operationally simple, and it operates under exceedingly mild conditions (room temperature and 1 atm of hydrogen pressure). The reusability of catalysts used in this method is also successfully demonstrated.

Visible-Light-Promoted Diboron-Mediated Transfer Hydrogenation of Azobenzenes to Hydrazobenzenes

A visible-light-promoted transfer hydrogenation of azobenzenes has been developed. In the presence of B₂pin₂ and upon visible-light irradiation, the reactions proceeded smoothly in methanol at ambient temperature. The azobenzenes with diverse functional groups have been reduced to the corresponding hydrazobenzenes with a yield of up to 96%. Preliminary mechanistic studies indicated that the hydrogen atom comes from the solvent and the transformation is achieved through a radical pathway.

Gram-Scale Synthesis of Flavoring Ketones in One Pot via Alkylation-Decarboxylation on Benzylic Carbon Using a Commercial Solid Acid Catalyst

The gram-scale synthesis of important flavoring ketones via alkylation of acetoacetic ester on substituted benzylic carbon followed by decarboxylation using a heterogeneous, commercial, solid acid catalyst is reported. The flavoring ketones were synthesized by the alkylation of acetoacetic ester, which proceeds through an S_N1-type reaction to generate an alkylated (β-ketoester) intermediate at the benzylic carbon, which is decarboxylated under the acidic condition. Among the solid acid catalysts used, Amberlyst-15 was found to be the best catalyst under the solvent-free condition. This protocol was successfully employed for the synthesis of various flavoring ketones such as raspberry ketone and ginger ketone with almost complete conversion and 82% isolated yield. The para-donating groups on the benzylic alcohol showed a high rate of reaction. The catalyst was easily recovered and reused 6 times without losing its activity and selectivity. Moreover, this reaction was demonstrated at a 10 g scale, which implicated the potential applicability of the protocol in the industry.

Partial and Total Solvent-Free Limonene’s Hydrogenation: Metals, Supports, Pressure, and Water Effects

Bio-based solvents menthene and menthane were obtained through limonene’s partial and total hydrogenation under various catalytic conditions. Heterogeneous catalysts based on different active metals and supports (carbon, alumina, and silica) were systematically tested for solvent-free total and partial hydrogenation of limonene under high and low hydrogen pressure. Influences of these catalysts on the formation of menthene, menthane, and cymene, a dehydrogenated product, were determined. The impact of water addition on the conversion and selectivity of the catalysts was also investigated. Amongst all tested catalysts, Rh/Alumina which was never tested for total and partial hydrogenation of limonene was the most effective as 1-menthene was quantitatively produced at low pressure (0.275 MPa) while menthane was mostly obtained at a higher pressure (2.75 MPa). Water addition on Rh/Alumina favoured menthene production even at high pressure. To propose menthane, menthene, and menthane/menthene mixture as an alternative to fossil-based solvents such as n-hexane for the extraction of natural products, β-carotene, vanillin, and rosmarinic acid solubilizations have been investigated. If a modeling approach using COSMO-RS software predicted a comparable solubilization of these 3 compounds for the 3 solvents, experimental assays revealed that menthene solubilizes β-carotene, vanillin, and rosmarinic acid three to five times better than n-hexane.

Photocatalytic hydrogenation of azobenzene to hydrazobenzene on cadmium sulfide under visible light irradiation

Visible light irradiation of commercially-available CdS in alcohol successfully promotes selective azobenzene-to-hydrazobenzene reduction, where surface S vacancies play a crucial role for activity and selectivity.

Immobilized ruthenium metal-containing ionic liquid-catalyzed dehydrogenation of dimethylamine borane complex for the reduction of olefins and nitroarenes

An efficient immobilized ruthenium metal containing ionic liquid (ImmRu-IL) catalyst has been developed for the transfer hydrogenation of olefins and nitroarenes. This methodology uses eco-friendly dimethylamine borane as a reducing agent.

Thermal behavior of pinan-2-ol and linalool

Linalool is an important intermediate for syntheses of isoprenoid fragrance compounds and vitamins A and E. One process option for its production is the thermal gas-phase isomerization of cis- and trans-pinan-2-ol. Investigations of this reaction were performed in a flow-type apparatus in a temperature range from 350–600 °C and a residence time range of 0.6–0.8 s. Rearrangement of the bicyclic alcohol led to linalool, plinols arising from consecutive reactions of linalool and other side products. Effects of residence time, temperature, surface-to-volume-ratio, carrier gas, and the presence of additives on yield and selectivity were studied. Furthermore, the effects of such parameters on ene-cyclization of linalool affording plinols were investigated. Results indicate that manipulation of the reaction in order to affect selectivity is difficult due to the large free path length to other molecules in the gas phase. However, conditions have been identified allowing one to increase the selectivity and the yield of linalool throughout pyrolysis of pinan-2-ol.

Microwave-assisted partial hydrogenation of citral by using ionic liquid-coated porous glass catalysts

The microwave-assisted hydrogenation of citral (3,7-dimethylocta-2,6-dienal) to citronellal with molecular hydrogen as the reducing agent was investigated. Several polar and non-polar solvents were screened and imidazolium-based ionic liquids were applied as modifiers for the palladium-containing porous glass catalysts (Pd/TP). The best results were obtained with N-ethyl-N'-methylimidazolium dicyanamide, N-ethyl-N'-methylimidazolium acetate, or N-ethyl-N'-methylimidazolium trifluoroacetate, which were used to prepare supported catalysts with an ionic liquid layer (SCILL) on Pd/TP by wet-impregnation. The influence of pressure and temperature when using these ionic liquid-containing catalysts, as well as their long-term stabilities, were examined. Working with microwave-assisted heating, high yields of citronellal were achieved under mild conditions within short reaction times. Catalyst characterization was carried out by means of BET measurements, X-ray photoelectron spectroscopy (XPS) and thermo-gravimetric analyses. The influences of the ionic liquid layer were derived from experiments carried out before and after the reactions.