Polymer-Supported Chiral Heterogeneous Copper Catalyst for Asymmetric Conjugate Addition of Ketones and Imines under Batch and Flow

On the Hunt for Chiral Single-Atom Catalysts

Enantioselective transformations are crucial in various fields, including chemistry, biology, and materials science. Today, the selective production of enantiopure compounds is achieved through asymmetric homogeneous catalysis. Single-atom catalysts (SACs) are emerging as a transformative approach in chemistry, enabling the heterogenization of organometallic complexes and effectively bridging the gap between homogeneous and heterogeneous catalysis. Despite their potential, the integration of SACs into enantioselective processes remains an underexplored area. This perspective offers a comprehensive analysis of possible strategies for the design of heterogeneous asymmetric catalysts, examining how chiral surfaces, chiral modifiers, grafted chiral complexes, and spatial confinement techniques can be effectively employed to enhance enantioselectivity. Each of these methods presents distinct advantages and challenges; for example, chiral surfaces and chiral modifiers offer potential for tailored reactivity but can suffer from limited stability and selectivity, while grafted chiral complexes provide robust platforms but may face issues related to scalability and synthesis complexity. Spatial confinement strategies show promise in enhancing catalyst efficiency but may be constrained by accessibility and reproducibility concerns. These strategies lay the groundwork for their adaptation to SACs, by providing innovative approaches to replicate the well-defined chiral environments of homogeneous catalysts while preserving the stability, reusability, and unique advantages of single-atom heterogeneous systems.

Polymeric Micellar Nanocatalysts for CuAAC Click Reaction in Water

Polymer-supported copper catalysts have attained a prominent status and continue to be a focal point of ongoing research and development due to their adaptable properties, which make them invaluable tools for diverse catalytic reactions in aqueous solutions. The objective of this investigation is to develop catalysts supported on a random copolymer that can be assembled in water. A series of random copolymer was prepared through postpolymerization modification of a polymer precursor, poly(pentafluorophenyl acrylate) (PPFPA), employing 1-amino-2-propanol and 1-(3-aminopropyl)imidazole via nucleophilic substitution. Following alkylation and copper insertion, it yielded a polymer-supported copper (Cu) catalyst on poly(N-(2-hydroxypropyl)acrylamide)-ran-poly(N-(3-(1-benzylimidazolium-3-yl)propyl)acrylamide) PHPAM76-ran-PILAM24(Cu(I)), capable of assembling into micellar catalysts in water with a diameter of 175 nm and low polydispersity. These developed self-assembled micelles can serve as nanocatalysts for the copper-catalyzed azide-alkyne cycloaddition (CuAAC) between alkyne and azide derivatives in an aqueous system. Employing PHPAM76-ran-PILAM24(Cu(I)) as the micellar catalyst with a 1 mol % Cu loading significantly enhances reaction yields (95-99%), achieving complete conversion at room temperature within 1-4 h, with minimal copper residue detected in the product (<0.06 ppm) after a straightforward extraction process. This research highlights the versatility of postpolymerization modification of the polymer precursor, PPFPA through nucleophilic substitution as a promising strategy for the development of tailored nanocatalysts for diverse chemical reactions in the future.

Fe3O4@HA-Cu(OAc)2 nanocomposite as a nanomagnetic water-compatible catalyst for efficient synthesis of 1,2,3-triazoles in water

A new humic acid-based nanomagnetic copper(II) composite was prepared and used as an eco-friendly recoverable catalyst for synthesizing 1,4-disubstituted 1,2,3-triazoles. The synthesis was done via the three-component click reaction of alkyl halide, sodium azide, and terminal alkyne with good to excellent yield. A simple magnetic copper acetate composite, Fe3O4@HA-Cu(OAc)2, was prepared using humic acid and characterized by SEM, TEM, XRD, EDX, EDS-mapping, VSM, TGA, AAS, and FT-IR. The catalyst showed high catalytic potential in a one-pot three-component click reaction in the synthesis of 1,2,3-triazoles. The nanomagnetic Fe3O4@HA-Cu(OAc)2 catalyst demonstrated high efficiency, stability, compatibility with oxygen/water and recyclable copper(II), with cost-effectiveness. The catalyst was easily recovered with an external magnetic field, and therefore, time and energy were saved as no filtration or decantation technique was needed. Due to the high dispersibility in water, nanomagnetic Fe3O4@HA-Cu(OAc)2 was utilized as a highly efficient catalyst for the click synthesis of triazoles.

Copper-catalyzed asymmetric allylic substitution of racemic/meso substrates

The synthesis of enantiomerically pure compounds is a pivotal subject in the field of chemistry, with enantioselective catalysis currently standing as the primary approach for delivering specific enantiomers. Among these strategies, Cu-catalyzed asymmetric allylic substitution (AAS) is significant and irreplaceable, especially when it comes to the use of non-stabilized nucleophiles (pK a > 25). Although Cu-catalyzed AAS of prochiral substrates has also been widely developed, methodologies involving racemic/meso substrates are highly desirable, as the substrates undergo dynamic processes to give single enantiomer products. Inspired by the pioneering work of the Alexakis, Feringa and Gennari groups, Cu-catalyzed AAS has been continuously employed in deracemization and desymmetrization processes for the synthesis of enantiomerically enriched products. In this review, we mainly focus on the developments of Cu-catalyzed AAS with racemic/meso substrates over the past two decades, providing an explicit outline of the ligands employed, the scope of nucleophiles, the underlying dynamic processes and their practical applications.

Copper-Catalyzed Continuous-Flow Transfer Hydrogenation of Nitroarenes to Anilines: A Scalable and Reliable Protocol

A robust supported catalyst that is made up of copper nanoparticles on Celite has been successfully prepared for the selective transfer hydrogenation of aromatic nitrobenzenes to anilines under continuous flow. The method is efficient and environmentally benign thanks to the absence of hydrogen gas and precious metals. Long-term stability studies show that the catalytic system is able to achieve very high nitrobenzene conversion (>99%) when working for up to 145 h. The versatility of the transfer hydrogenation system has been tested using representative examples of nitroarenes, with moderate-to-excellent yields being obtained. The packed bed reactor (PBR) permits the use of a setup that can provide products via simple isolation by SPE without the need for further purification. The recovery and reuse of either EG or the ion-exchange resin leads to consistent waste reduction; therefore, E-factor distribution analysis has highlighted the environmental efficiency of this synthetic protocol.

Copper-Metallized Porous N-Heterocyclic Carbene Ligand Polymer-Catalyzed Regio- and Stereoselective 1,2-Carboboration of Alkynes

Alkenylboronates are highly versatile building blocks and valuable reagents in the synthesis of complex molecules. Compared with that of monosubstituted alkenylboronates, the synthesis of multisubstituted alkenylboronates is challenging. The copper-catalyzed carboboration of alkynes is an operationally simple and straightforward method for synthesizing bis/trisubstituted alkenylboronates. In this work, a series of copper-metallized N-Heterocyclic Carbene (NHC) ligand porous polymer catalysts are designed and synthesized in accordance with the mechanism of carboboration. By using CuCl@POL-NHC-Ph as the optimal nanocatalyst, this study realizes the β-regio- and stereoselective (syn-addition) 1,2-carboboration of alkynes (regioselectivity up to >99:1) with satisfactory yields and a wide range of substrates. This work not only overcomes the selectivity of carboboration but also provides a new strategy for the design of nanocatalysts and their application in organic synthesis.

Highly enantioselective synthesis of both tetrahydroquinoxalines and dihydroquinoxalinones via Rh-thiourea catalyzed asymmetric hydrogenation

Chiral tetrahydroquinoxalines and dihydroquinoxalinones represent the core structure of many bioactive molecules. Herein, a simple and efficient Rh-thiourea-catalyzed asymmetric hydrogenation for enantiopure tetrahydroquinoxalines and dihydroquinoxalinones was developed under 1 MPa H2 pressure at room temperature. The reaction was magnified to the gram scale furnishing the desired products with undamaged yield and enantioselectivity. Application of this methodology was also conducted successfully under continuous flow conditions. In addition, 1H NMR experiments revealed that the introduction of a strong Brønsted acid, HCl, not only activated the substrate but also established anion binding between the substrate and the ligand. More importantly, the chloride ion facilitated heterolytic cleavage of dihydrogen to regenerate the active dihydride species and HCl, which was computed to be the rate-determining step. Further deuterium labeling experiments and density functional theory (DFT) calculations demonstrated that this reaction underwent a plausible outer-sphere mechanism in this new catalytic transformation.

Copper-catalyzed asymmetric allylic alkylation of racemic inert cyclic allylic ethers under batch and flow conditions

The Cu-catalyzed AAA reactions employing challenging racemic inert cyclic allylic ethers with sterically hindered Grignard reagents have been disclosed under batch and flow conditions.

Rapid mild macrocyclization of depsipeptides under continuous flow: total syntheses of five cyclodepsipeptides

A systematic investigation of the flow macrocyclization approaches for five destruxin analogues natural products at three different cyclization point has been reported.