Borrowing Hydrogen in Water and Ionic Liquids: Iridium-Catalyzed Alkylation of Amines with Alcohols

Fentanyl-Type Antagonist of the μ-Opioid Receptor: Important Role of Axial Chirality in the Active Conformation

In recent years, synthetic opioids have emerged as a predominant cause of drug-overdose-related fatalities, causing the "opioid crisis." To design safer therapeutic agents, we accidentally discovered μ-opioid receptor (MOR) antagonists based on fentanyl with a relatively uncomplicated chemical composition that potentiates structural modifications. Here, we showed the development of novel atropisomeric fentanyl analogues that exhibit more potent antagonistic activity against MOR than naloxone, a morphinan MOR antagonist. Derivatives displaying stable axial chirality were synthesized based on the amide structure of fentanyl. The aS- and aR-enantiomers exerted antagonistic and agonistic effects on the MOR, respectively, and each atropisomer interacted with the MOR by assuming a distinct binding mode through molecular docking. These findings suggest that introducing atropisomerism into fentanyl may serve as a key feature in the molecular design of future MOR antagonists to help mitigate the opioid crisis.

Ir-Catalyzed Synthesis of Functionalized Pyrrolidines and Piperidines Using the Borrowing Hydrogen Methodology

The Ir(III)-catalyzed synthesis of 3-pyrrolidinols and 4-piperidinols combining 1,2,4-butanetriol or 1,3,5-pentanetriol with primary amines was carried out. This borrowing hydrogen methodology was further extended to the sequential diamination of triols leading to amino-pyrrolidines and amino-piperidines.

Water as a solvent: transition metal catalyzed dehydrogenation of alcohols going green

The long-established practice of using organic solvents in synthetic chemistry is currently becoming a major focus of environmental alarms as many of the chemical wastes are generated in the form of organic solvents. Recently, various alternative solvents have been recognized by the scientific community, including water, ionic liquids, supercritical fluids, glycerol, polyethylene glycol, etc. Among these alternatives, water is unquestionably an ideal solvent as it is abundant, cheap, non-toxic, and non-flammable. In the last few decades, a breakthrough has been achieved in the field of transition metal-catalyzed dehydrogenation of alcohols and the related chemistry for the sustainable synthesis of a wide range of valuable compounds. Although a large number of reports with new potential are published every year following this alcohol dehydrogenation strategy, the utilization of water as a solvent in alcohol dehydrogenation and related coupling reactions is yet to be highlighted properly. This review summarizes the advances in metal-catalyzed dehydrogenative functionalization of alcohols using water as a solvent.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

Tandem Catalytic Indolization/Enantioconvergent Substitution of Alcohols by Borrowing Hydrogen to Access Tricyclic Indoles

An efficient tandem catalysis method is achieved for the direct conversion of alcohol-containing alkynyl anilines to valuable chiral 2,3-fused tricyclic indoles. This method relies on a tandem indolization followed by enantioconvergent substitution of alcohols via borrowing hydrogen to construct two rings in one step, enabled by relay and cooperative catalysis of a chiral iridium complex with a chiral phosphoric acid. Highly diastereoselective transformations of the tricyclic indole products also provide efficient access to a diverse array of complex polycyclic indoline compounds.

Direct N-alkylation of sulfur-containing amines

An efficient ruthenium-catalyzed method has been developed for the direct N-alkylation of sulfur-containing amines with alcohols, for the first time, by a step-economical and environmentally friendly hydrogen borrowing strategy. The present methodology features base-free conditions and broad substrate scope, with water being the only by-product. Moreover, this protocol has been applied to the synthesis of the pharmaceutical drug Quetiapine.

Mimicking transition metals in borrowing hydrogen from alcohols

Borrowing hydrogen from alcohols, storing it on a catalyst and subsequent transfer of the hydrogen from the catalyst to an in situ generated imine is the hallmark of a transition metal mediated catalytic N-alkylation of amines. However, such a borrowing hydrogen mechanism with a transition metal free catalytic system which stores hydrogen molecules in the catalyst backbone is yet to be established. Herein, we demonstrate that a phenalenyl ligand can imitate the role of transition metals in storing and transferring hydrogen molecules leading to borrowing hydrogen mediated alkylation of anilines by alcohols including a wide range of substrate scope. A close inspection of the mechanistic pathway by characterizing several intermediates through various spectroscopic techniques, deuterium labelling experiments, and DFT study concluded that the phenalenyl radical based backbone sequentially adds H+, H˙ and an electron through a dearomatization process which are subsequently used as reducing equivalents to the C-N double bond in a catalytic fashion.

Dimethylamination of Primary Alcohols Using a Homogeneous Iridium Catalyst: A Synthetic Method for N,N-Dimethylamine Derivatives

A new catalytic system for N,N-dimethylamination of primary alcohols using aqueous dimethylamine in the absence of additional organic solvents has been developed. The reaction proceeds via borrowing hydrogen processes, which are atom-efficient and environmentally benign. An iridium catalyst bearing an N-heterocyclic carbene (NHC) ligand exhibited high performance, without showing any deactivation under aqueous conditions. In addition, valuable N,N-dimethylamine derivatives, including biologically active and pharmaceutical molecules, were synthesized. The practical application of this methodology was demonstrated by a gram-scale reaction.

Cyclometalated Iridium Complex-Catalyzed N-Alkylation of Amines with Alcohols via Borrowing Hydrogen in Aqueous Media

This paper develops a methodology for cyclometalated iridium complex-catalyzed N-alkylation of amines with alcohols via borrowing hydrogen in the aqueous phase. The cyclometalated iridium catalyst-mediated N-alkylation of amines with alcohols displays high activity (S/C up to 10,000 and yield up to 96%) and ratio of amine/imine (up to >99:1) in a broad range of substrates (up to 46 examples) using water as the green and eco-friendly solvent. Most importantly, this transformation is simple, efficient, and can be performed at a gram scale, showcasing its potential for industrially synthesizing N-alkylamine compounds.

Ruthenium Catalyzed Tandem Pictet-Spengler Reaction

We report a pyridyl-phosphine ruthenium(II) catalyzed tandem alcohol amination/Pictet-Spengler reaction sequence to synthesize tetrahydro-β-carbolines from an alcohol and tryptamine. Our conditions use a Lewis acid cocatalyst, In(OTf)₃, that is compatible with typically base catalyzed amination and an acid catalyzed Pictet-Spengler cyclization. This method proceeds well with benzylic alcohols, heterocyclic carbinols, and aliphatic alcohols. We also show how combining this reaction with a subsequent cycloamination enables a direct synthesis of tetracyclic alkaloids like harmicine.

A hydrogen borrowing annulation strategy for the stereocontrolled synthesis of saturated aza-heterocycles

A hydrogen borrowing annulation strategy enables the stereocontrolled synthesis of C2, C3 and C4-substituted saturated aza-heterocycles.

Borohydride stabilized gold-silver bimetallic nanocatalysts for highly efficient 4-nitrophenol reduction

Bimetallic Au _x Ag_1-x nanoparticles, prepared using sodium borohydride as the sole reducing and capping agent for various NaBH₄ : metal molar ratios, were investigated as catalysts for 4-nitrophenol reduction. This approach yielded the highest catalytic activities observed for this model reaction to date.

Stereoconvergent, Redox-Neutral Access to Tetrahydroquinoxalines through Relay Epoxide Opening/Amination of Alcohols

We present an economical catalytic procedure to convert readily available 1,2-diaminobenzenes and terminal epoxides into valuable 1,2,3,4-tetrahydroquinoxalines in a highly enantioselective fashion. This procedure operates through relay zinc and iridium catalysis, and achieves redox-neutral and stereoconvergent production of valuable chiral heterocycles from racemic starting materials with water as the only side product. The use of commercially available reagents and catalysts and a convenient procedure also make this catalytic method attractive for practical application.

The N-alkylation of sulfonamides with alcohols in water catalyzed by a water-soluble metal–ligand bifunctional iridium complex [Cp*Ir(biimH2)(H2O)][OTf]2

A water-soluble dinuclear Cp*Ir complex bearing 4,4′,6,6′-tetrahydroxy-2,2′-bipyrimidine as a bridging ligand was found to be a highly effective catalyst for the N-alkylation of ketones with alcohols in water.

Complexes of (η5-Cp*)Ir(iii) with 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone: catalyst for oxidation and 1,2-substituted benzimidazole synthesis

The treatment of 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone [L1-L4] with [(η⁵-Cp*)IrCl(μ-Cl)]₂ at 25 °C followed by NH₄PF₆ results in [(η⁵-Cp*)Ir(L)Cl][PF₆] (1-4 for L = L1 to L4), authenticated with high-resolution mass spectrometry (HR-MS) and multi-nuclei nuclear magnetic resonance (NMR) imaging (¹H, ¹³C{¹H} and ⁷⁷Se{¹H}). The structures of 1-4, established with single-crystal X-ray diffraction, reveal a "piano-stool" geometry around the Ir. The Ir-thio/selenoether (Ir-S/Ir-Se) bond distances (Å) are 2.347(18)-2.355(4)/2.4663(12)-2.4663(13) and Ir-thione/selenone (Ir-S/Ir-Se) distances are 2.4146(19)-2.417(2)/2.5141(16)-2.5159(12). The reaction of 1,2-phenylenediamine with benzylic alcohols and furfuryl alcohol under mild and ambient conditions, catalyzed efficiently with complexes 1-4, generates bisimine in situ. Cyclization and rearrangement via 1,3-hydride shift triggered by its electrophilic activation with Ir(iii) species finally results in 1,2-disubstituted benzimidazole. The yield of the heterocycles in this one-pot synthesis is excellent to good. The aldehydes generated in situ by aerial oxidation of alcohols in the presence of 1-4 as catalysts are precursors to the bisimine as the protocols of this heterocycle synthesis carried out in the absence of 1,2-phenylenediamine give them in excellent-to-good yield. The oxidation of alcohols by hydrogen transfer to acetone was catalyzed efficiently with complexes 1-4 and resulted in aldehyde/ketone in excellent-to-good yield. Each catalytic process is marginally more efficient with 1 than its counterparts.

Synthesis of chiral sultams with two adjacent stereocenters via palladium-catalyzed dynamic kinetic resolution

A palladium-catalyzed intramolecular asymmetric reductive amination of racemic α-branched ketones bearing the poorly nucleophilic sulfonamides has been successfully developed through dynamic kinetic resolution, providing chiral δ-sultams with two contiguous stereogenic centers.

Synthesis of chiral sultams via palladium-catalyzed intramolecular asymmetric reductive amination

A palladium-catalyzed intramolecular reductive amination of ketones with weakly nucleophilic sulfonamides has been developed, providing facile access to a range of chiral sultams with up to 99% enantioselectivity.

Alkylation of Sulfonamides with Trichloroacetimidates under Thermal Conditions

An intermolecular alkylation of sulfonamides with trichloroacetimidates is reported. This transformation does not require an exogenous acid, base, or transition metal catalyst; instead the addition occurs in refluxing toluene without additives. The sulfonamide alkylation partner appears to be only limited by sterics, with unsubstituted sulfonamides providing better yields than more encumbered N-alkyl sulfonamides. The trichloroacetimidate alkylating agent must be a stable cation precursor for the substitution reaction to proceed under these conditions.

C-Alkylation by Hydrogen Autotransfer Reactions

The development of practical, efficient, and atom-economical methods for the formation of carbon-carbon bonds remains a topic of considerable interest in current synthetic organic chemistry. In this review, we have summarized selected topics from the recent literature with particular emphasis on C-alkylation processes involving hydrogen transfer using alcohols as alkylation reagents. This review includes selected highlights concerning recent progress towards the modification of catalytic systems for the α-alkylation of ketones, nitriles, and esters. Furthermore, we have devoted a significant portion of this review to the methylation of ketones, alcohols, and indoles using methanol. Lastly, we have also documented recent advances in β-alkylation methods involving the dimerization of alcohols (Guerbet reaction), as well as new developments in C-alkylation methods based on sp (3) C-H activation.

Three-step synthesis of l-ido-1-deoxynojirimycin derivatives by reductive amination in water, “borrowing hydrogen” under neat conditions and deprotection

A three-step synthesis of l-ido-1-deoxynojirimycin derivatives, starting from readily available 2,3,4,6-tetra-O-benzyl-d-glucopyranose via Ir-catalyzed reductive amination in water, “borrowing hydrogen” under neat conditions, and Pd-catalyzed debenzylation, is reported.

DFT study on the iridium-catalyzed multi-alkylation of alcohol with ammonia

The catalytic mechanism for the multi-alkylation of benzyl alcohols with ammonia catalyzed by the water-soluble catalyst, [Cp*Ir^III(NH₃)₃][I]₂, is computationally investigated by density functional theory (DFT).

Iridium-catalyzed single-step N-substituted lactam synthesis from lactones and amines

Catalytic lactam synthesis was achieved directly from lactones and amines using an Ir catalyst. Three sequential transformations—aminolysis of lactone, N-alkylation of amine with hydroxyamide, and intramolecular transamidation of aminoamide—afforded the corresponding N-substituted lactams.

Mechanistic study on the Cp*iridium-catalyzed N-alkylation of amines with alcohols

Using DFT methods and the energetic span model, the most favorable hydrogen autotransfer pathway for the [Cp*IrCl₂]₂/K₂CO₃-catalyzed N-alkylation of amines with alcohols has been found.

An efficient heterogenized palladium catalyst for N-alkylation of amines and α-alkylation of ketones using alcohols

Silica supported palladium–NiXantphos complex is an efficient and high turnover heterogeneous catalyst for N-alkylation of amines and α-alkylation of ketones using alcohols under neat conditions at 120–140 °C following hydrogen borrowing strategy.