Iron-catalyzed three-component coupling of aldehyde, alkyne, and amine under neat conditions in air

Rapid Access to Divergent Fused Polycycles Via One-Pot A3 Coupling and Intramolecular Diels-Alder Reaction

Divergent nitrogen-containing fused polycyclic ring systems are constructed from simple starting materials via a one-pot aldehyde-alkyne-amine (A3) coupling and intramolecular Diels-Alder reaction. This domino reaction directly furnishes linear 5/5/5 and 5/5/6, or nonlinear 5/5/6/5, polycyclic rings containing an oxa-bridged fused 5/5 bicycle and a 1,6-enyne substructure. One-step derivation of the oxa-bridged 5/5 bicycle leads to a polyfunctionalized 5/5 bicycle with tetrahydrofuran fused back-to-back to pyrroline or a 6/5 bicycle with the hexahydro-1H-isoindole structure, while cycloisomerizing the enyne substructure adds an extra fused 5-membered ring to afford functionalized linear 5/5/5/5 or 5/5/5/5/5 fused ring systems from selected substrates. In addition, the one-pot product can be designed so that the alkyne moiety is hydroalkoxylated to form an additional heterocyle in a linear 5/5/5/6 or nonlinear 5/5/6/5/5 ring system. This diversity-oriented synthetic approach thus allows rapid access to an under-explored structural space for discovery of new biological or non-biological activities or functions.

Metal-Catalysed A3 Coupling Methodologies: Classification and Visualisation

The multicomponent reaction of aldehydes, amines, and alkynes, known as A3 coupling, yields propargylamines, a valuable organic scaffold, and has received significant interest and attention in the last years. In order to fully realise the potential of the metal-based catalytic protocols that facilitate this transformation, we summarise substrates, in situ and well-characterised synthetic methods that provide this scaffold and attempt a monumental classification considering several variables (Metal, Coordinating atom(s), Ligand type and name, in-situ or well-characterised, co-catalyst, catalyst and ligand Loading (mol%), solvent, volume, atmosphere, temperature, microwave, time, yield, selectivity (e.e. d.r.), substrate name, functionality, loading (amines, aldehydes, alkynes), and use of molecular sieves). This pioneering work creates a valuable database that contains 2376 entries and allows us to produce graphs and better visualise their impact on the reaction.

Ultra-fast Cu-based A3-coupling catalysts: faceted Cu2O microcrystals as efficient catalyst-delivery systems in batch and flow conditions

Cu(I) catalysts were studied for the synthesis of a propargylamine via A3-coupling of aldehyde, amine, and alkyne, under solvent-free and low loading conditions, using batch microwave or flow thermal heating. We explored ultra-low loading conditions with Cu(I) salts as fast and active catalysts featuring turnover frequencies (TOFs) above 105 h−1. Well-defined octahedral and cubic Cu2O microcrystals were also successfully applied and compared with this reaction. Both types of microcrystals exhibited excellent catalytic activities within minutes, via in situ generation of low dose of Cu(I) ions within the reaction medium, to achieve TON beyond 2000 and recycling up to 10 times in a flow reactor. The study of the catalytic system demonstrated that the activity was surface-structure dependent and allowed for the design of low Cu contamination A3-coupling systems, affording a product at the decigram scale, with Cu contamination below FDA recommendations for drug synthesis, without the need for a purification procedure.

Modern Catalysts in A3- Coupling Reactions

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Propargylamines are an important constituent of diverse, biologically active and industrially valuable compounds. These useful, convenient and effective compounds can be synthesized via the A3-coupling reactions between an aldehyde, amine, and alkyne in the presence of a catalyst. In the past years, most of the catalysts containing transition metals were applied in these reactions, but today, various heterogeneous catalysts, especially nanocatalysts are used. The purpose of this review was to introduce some modern catalysts for the A3-coupling reaction.

Shedding light on the use of Cu(ii)-salen complexes in the A3 coupling reaction

One Cu(ii) complex, {Cu(ii)L} (1S), has been synthesised, in two high yielding steps under ambient conditions, and characterised by single-crystal X-Ray diffraction (SXRD), IR, UV-Vis, circular dichroism (CD), elemental analysis, thermogravimetric analysis (TGA) and electron spray ionization mass spectroscopy (ESI-MS). This air-stable compound enables the generation, at room temperature and in open-air, of twenty propargylamines, nine new, from secondary amines, aliphatic aldehydes and alkynes with a broad scope with yields up to 99%. Catalyst loadings can be as low as 1 mol%, while the recovered material retains its structural integrity and can be used up to 5 times without loss of its activity. Control experiments, SXRD, cyclic voltammetry and theoretical studies shed light on the mechanism revealing that the key to success is the use of phenoxido salen based ligands. These ligands orchestrate topological control permitting alkyne binding with concomitant activation of the C-H bond and simultaneously acting as a template temporarily accommodating the abstracted acetylenic proton, and continuously generating, via in situ formed radicals and a Single Electron Transfer (SET) mechanism, a transient Cu(i) active site to facilitate this transformation. The scope and limitations of this protocol are discussed and presented.

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

A comprehensive account of recent advances in the synthesis of imidazopyridines, assisted through transition-metal-catalyzed multicomponent reactions, C-H activation/functionalization and coupling reactions are highlighted in this review article. The basic illustration of this review comprises of schemes with concise account of explanatory text. The schemes depict the reaction conditions along with a quick look into the mechanism involved to render a deep understanding of the catalytic role. At some instances optimizations of certain features have been illustrated through tables, i.e., selectivity of catalyst, loading of the catalyst and percentage yield with different substrates. Each of the reported examples has been rigorously analyzed for reacting substrates, reaction conditions and transition metals used as the catalyst. This review will be helpful to the chemists in understanding the challenges associated with the reported methods as well as the future possibilities, both in the choice of substrates and catalysts. This review would be quite appealing to a wider range of organic chemists in academia and industrial R&D sectors working in the field of heterocyclic syntheses. In a nutshell, this review will be a guiding torch to envisage: (i) the role of various transition metals in the domain dedicated towards method development and (ii) for the modifications needed thereof in the R&D sector.

Zn-Catalyzed Multicomponent KA2 Coupling: One-Pot Assembly of Propargylamines Bearing Tetrasubstituted Carbon Centers

Tetrasubstituted propargylamines comprise a unique class of highly useful compounds, which can be accessed through the multicomponent coupling between ketones, amines, and alkynes (KA² coupling), an underexplored transformation. Herein, the development of a novel, highly efficient, and user-friendly catalytic system for the KA² coupling, based on the environmentally benign, inexpensive, and readily available zinc acetate, is described. This system is employed in the multicomponent assembly of unprecedented, tetrasubstituted propargylamines derived from structurally diverse, challenging, and even biorelevant substrates. Notable features of this protocol include the demonstration of the enhancing effect that neat conditions can have on catalytic activity, as well as the expedient functionalization of hindered, prochiral cyclohexanones, linear ketones, and interesting molecular scaffolds such as norcamphor and nornicotine.

Exploration of new reaction tools for late-stage functionalization of complex chemicals

Chemistry has always had as a target the conversion of molecules into valuable materials. Nevertheless, the aim of past synthesis has primarily focused on achieving a given transformation, regardless of the environmental impact of the synthetic route. Given the current global situation, the demand for sustainable alternatives has substantially increased. Our group focuses on developing selective chemical transformations that benefit from mild conditions, improved atom economy, and that can make use of renewable feedstocks as starting materials. This account summarizes our work over the past two decades specifically regarding the selective removal, conversion, and addition of functional groups that can, later on, be applied at a late stage for the modification of complex molecules.

Immobilized Gold Nanoparticles Prepared from Gold(III)-Containing Ionic Liquids on Silica: Application to the Sustainable Synthesis of Propargylamines

A cycloaurated phosphinothioic amide gold(III) complex was supported on amorphous silica with the aid of an imidazolium ionic liquid (IL) physisorbed in the SiO₂ pores (SiO₂⁻IL) and covalently bonded to the SiO₂ (SiO₂@IL). Gold(0) nanoparticles (AuNPs) were formed in situ and subsequently immobilized on the SiO₂⁻IL/SiO₂@IL phase. The resulting catalytic systems Au⁻SiO₂⁻IL and Au⁻SiO₂@IL promoted the solvent-free A³ coupling reaction of alkynes, aldehydes, and amines in high yields under solvent-free conditions with very low catalyst loading and without the use of additives. The Au⁻SiO₂@IL catalyst showed good recyclability and could be reused at least five times with yields of propargylamines of ≥80%. This synthetic method provides a green and low cost way to effectively prepare propargylamines. Additionally, 31P high resolution magic angle spinning (HRMAS) NMR spectroscopy is introduced as a simple technique to establish the Au loading of the catalyst.

Green synthesis of Au nanoparticles using an aqueous extract ofStachys lavandulifoliaand their catalytic performance for alkyne/aldehyde/amine A3coupling reactions

High reaction rate and easy availability make green synthesis of metal nanoparticles noticeable.

Amine-functionalized MIL-101(Cr) embedded with Co(ii) phthalocyanine as a durable catalyst for one-pot tandem oxidative A3coupling reactions of alcohols

In this study, we report a metal–organic framework (MIL-101(Cr)-NH₂) postsynthetically modifiedviacovalent immobilization of cobalt(ii) phthalocyanine as an efficient and mild catalytic system.

Recyclable Cu/C3N4 composite catalyzed AHA/A3 coupling reactions for the synthesis of propargylamines

The heterogeneous Cu/C₃N₄ catalyst was found to be efficient for the synthesis of propargylamines using a three-component coupling reaction of alkynes, CH₂Cl₂ and amines (AHA) without additional base. Moreover, the catalyst also showed highly catalytic activity in the synthesis of C1-alkynylated tetrahydroisoquinolines (THIQs) via an A³ reaction of alkynes, aldehydes and THIQ. The Cu/C₃N₄-catalyzed multicomponent reactions exhibited good functional group tolerance in most examples. Furthermore, the easily prepared Cu/C₃N₄ catalyst could be recovered and reused conveniently over 5 times without losing catalytic activities.

Recyclable Cu/C₃N₄ composite, a highly efficient catalyst for the synthesis of propargylamines via an AHA/A³ reaction.

Tunable Prussian blue analogues for the selective synthesis of propargylamines through A3 coupling

M¹[Co(CN)₆]_2/3-type Prussian blue analogues (M¹–Co PBAs) were studied as catalysts for the synthesis of propargylamines via A³ coupling of phenylacetylene, benzaldehyde and piperidine.

Magnetically recoverable Fe3O4@Au-coated nanoscale catalysts for the A3-coupling reaction

Magnetically recoverable and recyclable Fe₃O₄@Au nanocatalysts for A³-coupling reactions.

Facile synthesis of ZnAl2O4nanoparticles: efficient and reusable porous nano ZnAl2O4and copper supported on ZnAl2O4catalysts for one pot green synthesis of propargylamines and imidazo[1,2-a]pyridines by A3coupling reactions

A simple, facile and efficient route was developed for the synthesis of nano ZnAl₂O₄using ethanolamine.

Copper(ii) carboxymethylcellulose (CMC-CuII) as an efficient catalyst for aldehyde–alkyne–amine coupling under solvent-free conditions

A novel heterogeneous Cu(ii) carboxymethylcellulose catalyst was prepared and successfully employed in the A3 coupling without using any co-catalyst, other additives, bases under solvent-free conditions.