Selective Synthesis of Secondary Amines from Nitriles by a User‐Friendly Cobalt Catalyst

Insight into the MOtBu (M=Na, K)-Mediated Dehydrogenation of Dimethylamine-Borane and Transfer Hydrogenation of Nitriles to Primary Amines

Selective synthesis of primary amines from nitriles is challenging in synthetic chemistry due to the possible en-route generation of various amines and imines. Herein, we report a practical and operationally simple MOtBu-mediated (M=Na, K) transfer hydrogenation of nitriles to the corresponding primary amines with a relatively unexplored sacrificial hydrogen source (dimethylamine borane). The strategy encompasses a broad substrate scope under transition metal-free conditions and does not require any solvent. The mechanistic investigation was performed with the aid of control experiments and spectroscopic studies. The GC analysis of the reaction mixture exhibited the evolution of the H2 gas. Additionally, detailed computational calculations were undertaken to shed light on the possible intermediates and transition states involved during the present protocol.

Electrohydrogenation of Nitriles with Amines by Cobalt Catalysis

Catalytic hydrogenation of nitriles represents an efficient and sustainable one-step synthesis of valuable bulk and fine chemicals. We report herein a molecular cobalt electrocatalyst for selective hydrogenative coupling of nitriles with amines using protons as the hydrogen source. The key to success for this reductive reaction is the use of an electrocatalytic approach for efficient cobalt-hydride generation through a sequence of cathodic reduction and protonation. As only electrons (e- ) and protons (H+ ) as the redox equivalent and hydrogen source, this general electrohydrogenation protocol is showcased by highly selective and straightforward synthesis of various functionalized and structurally diverse amines, as well as deuterium isotope labeling applications. Mechanistic studies reveal that the electrogenerated cobalt-hydride transfer to nitrile process is the rate-determining step.

Kinetics and Pathway Analysis Reveals the Mechanism of a Homogeneous PNP-Iron-Catalyzed Nitrile Hydrogenation

Nitrile hydrogenation via the in situ-generated PNP-Fe^II(H)₂CO (1) catalyst leads to a previously inexplicable loss of mass balance. Reaction kinetics, reaction progress analysis, in situ pressure nuclear magnetic resonance, and X-ray diffraction analyses reveal a mechanism comprising reversible imine self-condensation and amine-imine condensation cascades that yield >95% primary amine. Imine self-condensation has never been reported in a nitrile hydrogenation mechanism. The reaction is first order in catalyst and hydrogen and zero order in benzonitrile when using 2-propanol as the solvent. Variable-temperature analysis revealed values for ΔG_298 K^⧧ (79.6 ± 26.8 kJ mol^-1), ΔH^⧧ (90.7 ± 9.7 kJ mol^-1), and ΔS^⧧ (37 ± 28 J mol^-1 K^-1), consistent with a solvent-mediated proton-shuttled dissociative transition state. This work provides a basis for future catalyst optimization and essential data for the design of continuous reactors with earth-abundant catalysts.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

A cyanide-catalyzed imino-Stetter reaction enables the concise total syntheses of rucaparib

Two routes toward the synthesis of rucaparib, an FDA-approved drug used for the treatment of ovarian and prostate cancers, have been developed from commercially available starting materials utilizing the cyanide-catalyzed imino-Stetter reaction as the key step for the construction of the indole motif bearing all the desired substituents in their correct positions. In the first-generation synthesis, meta-fluorobenzoate, the starting material currently used in the process chemistry route of rucaparib, was converted into 4,6-disubstituted 2-aminocinnamic acid derivatives (ester or amide). The cyanide-catalyzed imino-Stetter reaction of aldimines derived from the resulting 2-aminocinnamic acid derivatives and a commercially available aldehyde afforded the desired indole-3-acetic acid derivatives. The final azepinone formation completed the total synthesis of rucaparib in 27% overall yield. To resolve the issues raised in the first-generation synthesis, we further developed a second-generation synthesis of rucaparib. The Heck reaction of a commercially available ortho-iodoaniline derivative with acrylonitrile provided 4,6-disubstituted 2-aminocinnamonitrile, which was subjected to the imino-Stetter reaction with the same aldehyde to provide the desired indole-3-acetonitrile product. Subsequent construction of the azepinone scaffold completed the total synthesis of rucaparib in 59% overall yield over three separation operations. The synthetic strategy reported herein can provide a highly practical route to access rucaparib from commercially available starting materials (5.2% overall yield in the current process chemistry route vs. 59% overall yield in the second-generation synthesis).

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.

Switching Selectivity in Copper-Catalyzed Transfer Hydrogenation of Nitriles to Primary Amine-Boranes and Secondary Amines under Mild Conditions

A simple and efficient copper-catalyzed selective transfer hydrogenation of nitriles to primary amine-boranes and secondary amines with an oxazaborolidine-BH₃ complex is reported. The selectivity control was achieved under mild conditions by switching the solvent and the copper catalysts. More than 30 primary amine-boranes and 40 secondary amines were synthesized via this strategy in high selectivity and yields of up to 95%. The strategy was applied to the synthesis of ¹⁵N labeled in 89% yield.

Reductive Alkylation of Azides and Nitroarenes with Alcohols: A Selective Route to Mono- and Dialkylated Amines

Herein, we demonstrated an efficient protocol for reductive alkylation of azides/nitro compounds via a borrowing hydrogen (BH) method. By following this protocol, selective mono- and dialkylated amines were obtained under mild and solvent-free conditions. A series of control experiments and deuterium-labeling experiments were performed to understand this catalytic process. Mechanistic studies suggested that the Ir(III)-H was the active intermediate in this reaction. KIE study revealed that the breaking of the C-H bond of alcohol might be the rate-limiting step. Notably, this solvent-free strategy disclosed a high TON of around 5600. Based on kinetic studies and control experiments, a metal-ligand cooperative mechanism was proposed.

Amine-Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Transfer hydrogenation (TH) has historically been dominated by Meerwein-Ponndorf-Verley (MPV) reactions. However, with growing interest in amine-boranes, not least ammonia-borane (H3 N⋅BH3 ), as potential hydrogen storage materials, these compounds have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chemistry using H3 N⋅BH3 and their analogues (amine-boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: 1) classical TH, 2) nonclassical TH, and 3) hydrogenation. Simple experimental mechanistic probes can be employed to distinguish which pathway is operating and computational analysis can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temperature, solvent, amine-borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.

Reductive Alkylation of Quinolines to N-Alkyl Tetrahydroquinolines Catalyzed by Arylboronic Acid

A boronic acid catalyzed one-pot tandem reduction of quinolines to tetrahydroquinolines followed by reductive alkylation by the aldehyde has been demonstrated. This step-economcial synthesis of N-alkyl tetrahydroquinolines has been achieved directly from readily available quinolines, aldehydes, and Hantzsch ester under mild reaction conditions. The mechanistic study demonstrates the unique behavior of organoboron catalysts as both Lewis acids and hydrogen-bond donors.

One-pot, chemoselective synthesis of secondary amines from aryl nitriles using a PdPt–Fe3O4 nanoparticle catalyst

We have developed a new catalytic method for the one-pot, cascade synthesis of unsymmetrical secondary amines via the reductive amination of aryl nitriles with nitroalkanes using a PdPt–Fe₃O₄ nanoparticle (NP) catalyst.

Ammonia borane as a reducing agent in organic synthesis

Ammonia borane is gaining increasing attention as a sustainable and atom-economical winning reagent for the reduction of several substrates.