Cobalt-Catalyzed Selective Hydrogenation of Nitriles to Secondary Imines

Efficient and chemoselective imine synthesis catalyzed by a well-defined PN3-manganese(II) pincer system

The highly efficient reductive amination of aldehydes with ammonia (NH3) and hydrogen (H2) to form secondary imines is described, as well as the dehydrogenative homocoupling of benzyl amines. Using an air-stable, well-defined PN3-manganese(II) pincer complex as a catalyst precursor, various aldehydes are easily converted directly into secondary imines using NH3 as a nitrogen source under H2 in a one-pot reaction. Importantly, the same catalyst facilitates the dehydrogenative homocoupling of various benzylamines, exclusively forming imine products. These reactions are conducted under very mild conditions, without the addition of any additives, yielding excellent selectivities and high yields of secondary imines in a green manner by minimizing wastes.

Metal/Non-Metal Catalyzed Activation of Organic Nitriles

Nitrile activation is a prominent topic in recent developments in chemistry, especially in organic, inorganic, biological chemistry, as well as in the natural synthesis of products and in the pharmaceutical industry. The activation of nitriles using both metal and non-metal precursors has attracted several researchers, who are exploring newer ways to synthesize novel compounds. Nitrile activation can be achieved by combining various catalytic double hydroelementation reactions, such as hydrosilylation, hydroboration, and hydrogenation of organonitriles using silanes, pinacolborane, and other sources of hydrogen. These methodologies have garnered considerable attention since they are effective in the reduction of organonitriles, whose end products are extensively applied in synthetic organic chemistry. In this review, we summarize the development of selective hydroborylation, hydrosilylation, dihydroborysilylation, and hydrogenation of organonitriles, as well as their reaction mechanisms and the role of metal complexes in the catalytic cycles. This review article explains various synthetic methodologies applied toward the reduction of organonitriles into corresponding amines.

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

Catalytic and mechanistic studies of a highly active and E-selective Co(II) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Herein we report the application of a Co(II) PNNH pincer catalyst system (PNNH = 2-(5-(t-butyl)-1H-pyrazol-3-yl)-6-(dialkylphosphinomethyl)pyridine) for the highly E-selective transfer semihydrogenation of internal diaryl alkynes using methanol and ammonia borane...

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.

Facile Synthesis of Rh Anchored Uniform Spherical COF for One-Pot Tandem Reductive Amination of Aldehydes to Secondary Imines

The development of transition metal-based heterogeneous catalysts for economical and efficient synthesis of secondary imines remains both desirable and challenging. Herein, for the first time, we present two kinds of Rh nanoparticle anchored uniform spherical COF heterogeneous catalysts with well-defined crystalline structures for the effective one-pot tandem reductive amination of aldehydes on a gram scale. This reaction is carried out using ammonia as a nitrogen source and hydrogen gas as the source of hydrogen, which is not only an atom-economical but also an environmentally friendly process for the selective production of secondary imines. In particular, in the presence of the better-designed Rh nanoparticles anchored COF2 catalyst, the starting material aldehydes could be fully converted (99% conversion), and 95% selectivity of N-benzylidene(phenyl)methanamine is obtained under mild reaction conditions (2 MPa of H₂ and 90 °C). Additionally, the Rh/COF2 catalyst is also applied to a variety of substituted aromatic aldehyde compounds, manifesting good yields in corresponding secondary imines. This work not only expands the COF family but also offers economical and effective access to acquire various aromatic amine targets, especially secondary imines.

Aromaticity in catalysis: metal ligand cooperation via ligand dearomatization and rearomatization

Unlike the conventional model of transition metal catalysis, ligands in metal-ligand cooperative (or bifunctional) catalysis are involved in the substrate activations. Such processes have offered unique mechanistic understandings and led to new concepts for the catalyst design. In particular, unprecedented activities were discovered when the ligand could undergo dearomatization-rearomatization reactions during the catalytic cycle. Aromatization can provide an extra driving force to thermodynamics; consequently, it brings a new perspective to ligand platform design for catalysis. While numerous applications were demonstrated, the influences of changing ligand aromatic properties were often overlooked. In this article, representative ligand systems will be highlighted and a comparison between the Milstein and the Huang pincer systems will be discussed to provide theoretical and conceptual insights.

Facile Synthesis of COF-Supported Reduced Pd-Based Catalyst for One-Pot Reductive Amination of Aldehydes

Dibenzylamine motifs are an important class of crucial organic compounds and are widely used in fine chemical and pharmaceutical industries. The development of the efficient, economical, and environmentally friendly synthesis of amines using transition metal-based heterogeneous catalysts remains both desirable and challenging. Herein, we prepared the covalent organic framework (COF)-supported heterogeneous reduced COF-supported Pd-based catalyst and used it for the one-pot reductive amination of aldehydes. There are both Pd metallic state and oxidated Pdσ+ in the catalysts. Furthermore, in the presence of the reduced COF-supported Pd-based catalyst, many aromatic, aliphatic, and heterocyclic aldehydes with various functional groups substituted were converted to their corresponding amines products in good to excellent selectivity (up to 91%) under mild reaction conditions (70 °C, 2 h, NH3, 20 bar H2). This work expands the covalent organic frameworks for the material family and its support catalyst, opening up new catalytic applications in the economical, practical, and effective synthesis of secondary amines.

Mild and selective hydrogenation of nitriles into primary amines over a supported Ni catalyst

The mesoporous Al₂O₃ supported Ni catalyst demonstrated a high activity and selectivity for the hydrogenation of nitriles into primary amines under the mild conditions (60–80 °C and 2.5 bar H₂) with ammonia as the additive.

Nickel(ii) and nickel(0) complexes as precursors of nickel nanoparticles for the catalytic hydrogenation of benzonitrile

The use of nickel(ii) and nickel(0) complexes as precursors of nickel nanoparticles with catalytic activity in the hydrogenation of benzonitrile is reported.

Iron-Catalyzed Nitrene Transfer Reaction of 4-Hydroxystilbenes with Aryl Azides: Synthesis of Imines via C═C Bond Cleavage

C═C bond breaking to access the C═N bond remains an underdeveloped area. A new protocol for C═C bond cleavage of alkenes under nonoxidative conditions to produce imines via an iron-catalyzed nitrene transfer reaction of 4-hydroxystilbenes with aryl azides is reported. The success of various sequential one-pot reactions reveals that the good compatibility of this method makes it very attractive for synthetic applications. On the basis of experimental observations, a plausible reaction mechanism is also proposed.

Selective Carbanion-Pyridine Coordination of a Reactive P,N Ligand to RhI

Ligands with reactive carbon sites in the periphery of a metal center have emerged as a powerful approach for metal-ligand bond activation. These reactive carbon sites are commonly generated by deprotonation strategies. Carbon-silicon bond cleavage is a potential alternative to access such constructs. Herein, the monodesilylation of bis-silyl-substituted P,N scaffold PNSi2 in the coordination sphere of [RhI (Cl)(CO)(PNSi2 )] (1) with sodium azide is disclosed. This affords a unique dinucleating anionic κ2 -C,N-κ1 -P ligand with a carbanionic methine carbon atom directly bound to rhodium as part of a four-membered Rh-N-C-C rhodacycle. This dimer undergoes meta-pyridine C-H activation facilitated by weak bases, which leads to a desymmetrization of the system and provides a σ,π-bridging 3-pyridyl fragment bound to RhI . The facile Si-C cleavage strategy may pave the way to studying the reactivity and functionalization of a variety of κ2 -C,N-coordinated pyridine scaffolds for selective transformations.

Cobalt pincer complexes for catalytic reduction of nitriles to primary amines

An efficient cobalt pincer type complex catalyzes the hydrogenation of nitriles to amines under mild conditions with high yields.