Towards Sustainable Amino Acid Derived Organocatalysts for Asymmetric syn -Aldol Reactions

Catalytic Efficiency of Primary α-Amino Amides as Multifunctional Organocatalysts in Recent Asymmetric Organic Transformations

Chiral primary α-amino amides, consisting of an adjacent enamine bonding site (Bronsted base site), a hydrogen bonding site (Bronsted acid site), and flexible bulky substituent groups to modify the steric factor, are proving to be extremely valuable bifunctional organocatalysts for a wide range of asymmetric organic transformations. Primary α-amino amides are less expensive alternatives to other primary amino organocatalysts, such as chiral diamines and cinchona-alkaloid-derived primary amines, as they are easy to synthesize, air-stable, and allow for the incorporation of a variety of functional groups. In recent years, we have demonstrated the catalytic use of simple primary α-amino amides and their derivatives as organocatalysts for the aldol reaction, Strecker reaction, Michael tandem reaction, allylation of aldehydes, reduction of N-Aryl mines, opening of epoxides, hydrosilylation, asymmetric hydrogen transfer, and N-specific nitrosobenzene reaction with aldehydes.

Comparison of mesoporous fractal characteristics of silica-supported organocatalysts derived from bipyridine-proline and resultant effects on the catalytic asymmetric aldol performances

Three kinds of the bipyridine-proline chiral ligands as highly active species were successfully introduced on Zn-modified mesoporous silica nanomaterials (BMMs, MCM-41, and SBA-15) via the covalent attachment and coordination methods. Their microstructural features and physicochemical properties were extensively characterized via XRD patterns, SEM/TEM images, TGA profiles, FT-IR and UV-Vis spectra. In particular, their fractal features, the pair distance distribution function, and the Porod plots were evaluated thoroughly on the basis of the SAXS data. Meanwhile, their catalytic performances for asymmetric aldol reactions between p-nitrobenzaldehyde and cyclohexanone were evaluated. The results indicated that the bimodal mesoporous BMMs-based samples with short worm-like mesoporous channels possessed both mass and surface fractal features, whereas the MCM-41- and SBA-15-based samples with long-range ordered structures only showed surface fractal features. The influences of various reaction parameters, including the textures of the mesoporous silicas, the structures of the used chiral ligands, and the molecular volumes of aldehydes, on the catalytic activities (yield) and stereoselectivities (dr and ee) were investigated thoroughly. The results showed satisfactory activities (yields) and better stereoselectivity (dr and ee) in comparison with the homogeneous catalytic system using Z as the catalysts. In particular, the 3^rd recycle catalytic performances of the Z-immobilized heterogeneous catalysts retained high catalytic yields (around 80%) and ee values of 28%. These phenomena were well interpreted by the essential relationships between the fractal characteristics of these heterogeneous catalysts and their catalytic activities.

The relationships between the fractal evolution and catalytic behavior of the resultant catalysts were deeply explored via SAXS method and other characterizations. The applicability and reliability of fractal theory are therefore proved.

Amino Acylguanidines as Bioinspired Catalysts for the Asymmetric Aldol Reaction

The binding and stabilizing effect of arginine residues in certain aldolases served as inspiring source for the development of a family of amino acylguanidine organocatalysts. Screening and optimization led to identify the threonine derivative as the most suitable catalyst for the asymmetric aldol addition of hydroxyacetone, affording the syn diastereomer in high ee. In contrast, the proline derivative yielded the anti diasteromer. MMFF models suggest the presence of an extensive hydrogen bonding network between the acylguanidinium group and the reaction intermediates.

New Mesoporous Silica-Supported Organocatalysts Based on (2S)-(1,2,4-Triazol-3-yl)-Proline: Efficient, Reusable, and Heterogeneous Catalysts for the Asymmetric Aldol Reaction

Novel organocatalytic systems based on the recently developed (S)-proline derivative (2S)-[5-(benzylthio)-4-phenyl-(1,2,4-triazol)-3-yl]-pyrrolidine supported on mesoporous silica were prepared and their efficiency was assessed in the asymmetric aldol reaction. These materials were fully characterized by FT-IR, MS, XRD, and SEM microscopy, gathering relevant information regarding composition, morphology, and organocatalyst distribution in the doped silica. Careful optimization of the reaction conditions required for their application as catalysts in asymmetric aldol reactions between ketones and aldehydes afforded the anticipated aldol products with excellent yields and moderate diastereo- and enantioselectivities. The recommended experimental protocol is simple, fast, and efficient providing the enantioenriched aldol product, usually without the need of a special work-up or purification protocol. This approach constitutes a remarkable improvement in the field of heterogeneous (S)-proline-based organocatalysis; in particular, the solid-phase silica-bonded catalytic systems described herein allow for a substantial reduction in solvent usage. Furthermore, the supported system described here can be recovered, reactivated, and reused several times with limited loss in catalytic efficiency relative to freshly synthesized organocatalysts.

Recent advances in reactions promoted by amino acids and oligopeptides

During the last 20 years, Organocatalysis has become one of the major fields of Catalysis. Herein, we provide a recent overview on reactions where the use of amino acids and peptides as the organocatalysts was employed. All aspects regarding aldol reactions, Michael reactions, epoxidation, Henry reactions and many others that are crucial for the reaction conditions and reaction mechanisms are discussed.