Borrowing Hydrogen Process Research Articles

Redox-enabled cooperative catalysis by activating secondary alcohols using low-valent Zn complexes.

Inspired by nature's redox management in bioinorganic systems, we developed various Zn-complexes to catalyze a radical-mediated borrowing hydrogen process for producing β-disubstituted ketones. A diverse range of secondary alcohols, including fatty alcohols, terpenoids and steroid analogs, were successfully utilized for the chemoselective functionalization of ketones. Several organometallic and control studies suggest that coordinatively unsaturated radical species operate as active catalysts to promote alcohol activation and initiate the HAT process.

Methanol as a Potential Hydrogen Source for Reduction Reactions Enabled by a Commercial Pt/C Catalyst

Catalytic reduction reactions using methanol as a transfer hydrogenating agent is gaining significant attention because this simple alcohol is inexpensive and produced on a bulk scale. Herein, we report the catalytic utilization of methanol as a hydrogen source for the reduction of different functional organic compounds such as nitroarenes, olefins, and carbonyl compounds. The key to the success of this transformation is the use of a commercially available Pt/C catalyst, which enabled the transfer hydrogenation of a series of simple and functionalized nitroarenes-to-anilines, alkenes-to-alkanes, and aldehydes-to-alcohols using methanol as both the solvent and hydrogen donor. The practicability of this Pt-based protocol is showcased by demonstrating catalyst recycling and reusability as well as reaction upscaling. In addition, the Pt/C catalytic system was also adaptable for the N-methylation and N-alkylation of anilines via the borrowing hydrogen process. This work provides a simple and flexible approach to prepare a variety of value-added products from readily available methanol, Pt/C, and other starting materials.

Fe-Catalyzed Amidation of Allylic Alcohols by Borrowing Hydrogen Catalysis.

Allylic amines are useful building blocks in organic synthesis, so the development of green and efficient methods for the preparation of allylic amines are of great importance. An Fe-catalyzed amidation of allylic alcohols with chiral tert-butylsulfinamide has been developed. With water as the only by-product, a range of synthetically useful chiral sulfinamide olefin derivatives (30 examples) were obtained under mild reaction conditions. The reaction can be performed on a gram-scale, and the products could serve as chiral ligands for asymmetric catalysis. Mechanistic studies suggest that the reaction proceeds by an Fe-catalyzed borrowing hydrogen process, which is different from most of the reported allylic amination reactions.

Efficient Synthesis of C3-Alkylated and Alkenylated Indoles via Manganese-Catalyzed Dehydrogenation.

The catalytic dehydrogenation of alcohols is essential for the sustainable production of valuable products. This provides a new strategy for green organic synthesis in chemical industries. Herein, we describe a manganese-based catalytic system that enables the efficient synthesis of C3-alkylated indoles from benzyl alcohols and indoles via the borrowing hydrogen process. Furthermore, dehydrogenative coupling of 2-arylethanols and indoles yields C3-alkenylated indoles. Meanwhile, reacting 2-aminophenethanol instead of indoles can also obtain the corresponding indole products with high selectivity under the same conditions.

Iridium complexes with a new type of N^N′‐donor anionic ligand catalyze the N‐benzylation of amines via borrowing hydrogen

The development of efficient and eco‐friendly methods for the synthesis of elaborate amines is highly desired as they are valuable chemicals. The catalytic alkylation of amines using alcohols as alkylating agents, through the so‐called borrowing hydrogen process, satisfies several of the principles of green chemistry. In this paper, four neutral half‐sandwich complexes of Ru(II), Rh(III), and Ir(III) have been synthesized and tested as catalysts in the N‐benzylation of amines with benzyl alcohol. The new derivatives contain a N^N′ anionic ligand derived from 5‐(pyridin‐2‐ylmethylene)hydantoin (Hpyhy) that has never been tested in metal complexes with catalytic applications. In particular, the Ir derivatives, [(Cp*)IrX(pyhy)] (X = Cl or H), exhibit high activity along with good selectivity in the process. Indeed, the scope of the optimized protocol has been proved in the benzylation of several primary and secondary amines. The selectivity towards monoalkylated or dialkylated amines has been tuned by adjusting the amine:alcohol ratio and the reaction time. Experimental results support a mechanism consisting of three consecutive steps, two of which are Ir catalyzed, and a favorable condensation step without the assistance of the catalyst. Moreover, an unproductive competitive pathway can operate when the reaction is performed in open‐air vessels, due to the irreversible release of H2. This route is hampered when the reaction is carried out in close vessels, likely because the release of H2 is reversed through metal‐based heterolytic cleavage. From our viewpoint, these results show the potential of the new catalysts in a very attractive and promising methodology for the synthesis of amines.

B(C6F5)3-Catalyzed β-Functionalization of Pyrrolidines Using Isatins via Borrowing Hydrogen: Divergent Access to Substituted Pyrrolidines and Pyrroles.

We report herein a B(C6F5)3-catalyzed redox-neutral β-functionalization of pyrrolidines with isatins. Under transition-metal- and oxidant-free conditions at ambient temperature, a series of pyrrolidines bearing a functionalized exocyclic alkene are accessed in high efficiency through a borrowing hydrogen process. A simple switch to higher reaction temperature in a one-pot procedure also provides access to a diverse array of C(3)-functionalized pyrroles while liberating water and hydrogen gas as the only byproducts.

Redox-Neutral 1,3-Diol Synthesis by Base-Promoted Diastereoselective Alcohol-Aldolization.

In order to prepare more efficiently key 1,3-diol fragments, we have devised a base-promoted redox-neutral condensation of ketones with alcohols. This diastereoselective alcohol-aldolization enables bypassing the classical oxidation and reduction steps necessary for the preparation of this crucial backbone by an overall redox-neutral formal borrowing hydrogen process. The starting alcohols constitute both the precursors of the in situ generated reactive aldehydes and the hydride source necessary for the chemoselective reduction of the aldol adduct intermediates.

In water alkylation of amines with alcohols through a borrowing hydrogen process catalysed by ruthenium nanoparticles

Secondary amines are obtained with low environmental impact with a minimal amount of surfactants and organic solvents (biomass derived) employed only for product separation.

Retracted: Synthesis of 2-Arylisoindoline Derivatives Catalyzed by Reusable 1,2,4-Triazole Iridium on Mesoporous Silica through a Cascade Borrowing Hydrogen Strategy.

Covalent attachment of a 1,2,4-triazole iridium complex to mesoporous MCM-41 generated a heterogeneous catalyst that was found to be effective in the synthesis of 2-aryl isoindolines, quinolines, cyclic amines, and symmetrical secondary amines through a cascade borrowing hydrogen strategy. Interestingly, the supported heterogeneous iridium catalyst prepared from the 1,2,4-triazole iridium complex and mesoporous MCM-41 exhibited high catalytic activity in the preparation of 2-aryl isoindoline derivatives and symmetrical secondary amines. The catalyst system is highly recyclable for at least five times. Besides the important effect of the triazole, iridium sites grafted on siliceous supports can act as multifunctional catalytic centers and thus greatly enhance the catalytic activity of the catalysts. Furthermore, mechanistic experiments revealed that the reaction is initiated by an initial alcohol dehydrogenation and promoted by an iridium hydride intermediate. Importantly, the direct detection of a diagnostic iridium hydride signal confirmed that the synthesis of 2-aryl isoindolines occurs by a borrowing hydrogen process. This work provides an efficient example of isoindolines synthesis through a borrowing hydrogen strategy.

N-Methylation of Nitrogen-Containing Organic Substrates: A Comprehensive Overview

The present account surveys the results of the plethora of works on Nmethylation of nitrogen-containing substrates, mainly amines. The countless reports in the literature on this issue reveal the emergence of a set of methylating agents, which include: methanol, dimethyl carbonate, formaldehyde/formic acid, carbon dioxide/reductant, methyl iodide, dimethylsulfate, peroxides, dimethylsulfoxide, tetramethylammonium salts, and other unusual ones. Types of the methylating agents including, catalyst, solvent, base, ligand, reducing agent and other reaction conditions such as temperature and time would greatly affect the extent of selectivity of N-monomethylation vis-à-vis N,N-dimethylation. The degree of acidity or alkalinity of catalysts such as the solid catalysts (i.e. zeolites) showed a substantial impact on the selectivity and the course of methylation, leading to design adequate catalysts or to bring suitable modifications to the existing ones. Although this account takes into consideration all types of methylating agents, it is worthwhile to mention that the relatively recent works have been focused on the utilization of eco-friendly methylating agents, including carbon dioxide/reductant, methanol, and dimethylcarbonate. N-Methyl-containing drugs were successfully synthesized with some methylating agents under specified conditions. In some instances, unexpected products and events from the planned N-methylation of some nitrogen-containing molecules occurred. N-Formylation occurred as an intermediate or concomitant reaction when amines were subjected to catalyze methylation with methanol, formaldehyde/ formic acid, and carbon dioxide/reductant. The occasionally depicted mechanisms would elucidate the carbon and hydrogen sources of the affixing methyl group on the nitrogen site. Peculiarly, methylation involving methanol as a methylating agent and transition metal catalysis called for borrowing hydrogen process as a new mechanistic approach.

Divergent reactions of oxindoles with amino alcohols via the borrowing hydrogen process: oxindole ring opening vs. C3 alkylation

Oxindoles react with N-acetyl amino alcohols to form tryptamine-derived oxindoles, whereas analogous reactions with N-alkyl amino alcohols lead to lactam formation via a relatively mild borrowing hydrogen process.

Iron-Catalyzed Direct C3-Benzylation of Indoles with Benzyl Alcohols through Borrowing Hydrogen.

We present the coupling of primary and secondary benzyl alcohols with indoles to form 3-benzylated indoles and H2O that is catalyzed, for the first time, by a complex of earth-abundant iron. This transformation accommodates a variety of substrates and is distinguished by its operational simplicity, sustainability, high functional-group tolerance, and amenability to gram-scale synthesis. On the basis of the preliminary experimental observations, we propose that the reaction proceeds through a borrowing hydrogen process.

Acid-Assisted Ru-Catalyzed Enantioselective Amination of 1,2-Diols through Borrowing Hydrogen

Here, we present a highly enantioselective synthesis of 1,2-amino alcohols from readily available racemic 1,2-diols through a borrowing hydrogen process. An intriguing acid effect was discovered for this Ru-catalyzed amination reaction, which led to a significant improvement of the stereoselectivity of the process. Preliminary mechanistic studies suggest a scenario of Bronsted acid-assisted dynamic kinetic asymmetric amination of alcohols.

Observations concerning the synthesis of tryptamine homologues and branched tryptamine derivatives via the borrowing hydrogen process: synthesis of psilocin, bufotenin, and serotonin

Observations concerning the synthesis of substituted tryptamine derivatives starting from indoles and 1,n-amino alcohols via the borrowing hydrogen process are discussed. This catalytic, single-step, and modular approach to tryptamines and homotryptamines allows the synthesis of branched and nonbranched tryptamines as well as tryptamine-based natural products such as psilocin, bufotenin, and serotonin.

ChemInform Abstract: C‐Alkylation of Ketones and Related Compounds by Alcohols: Transition‐Metal‐Catalyzed Dehydrogenation

AbstractReview: 118 refs.

C-Alkylation of Ketones and Related Compounds by Alcohols: Transition-Metal-Catalyzed Dehydrogenation.

Transition-metal-catalyzed C-alkylation of ketones and secondary alcohols, with alcohols, avoids use of organometallic or environmentally unfriendly alkylating agents by means of borrowing hydrogen (BH) or hydrogen autotransfer (HA) activation of the alcohol substrates. Water is formed as the only by-product, thus making the BH process atom-economical and environmentally benign. Diverse homogeneous and heterogeneous transition-metal catalysts, ketones, and alcohols can be used for this transformation, thus rendering the BH process promising for replacing those procedures that use traditional alkylating agents. This Minireview summarizes the advances during the last five years in transition-metal-catalyzed BH α-alkylation of ketones, and β-alkylation of secondary alcohols with alcohols. A discussion on the application of the BH strategy for C-C bond formation is included.