Bifunctional metal-free photo-organocatalysts for enantioselective aerobic oxidation of β-dicarbonyl compounds

Synthesis of New Amino-Functionalized Porphyrins:Preliminary Study of Their Organophotocatalytic Activity

The design, synthesis, and initial study of amino-functionalized porphyrins as a new class of bifunctional catalysts for asymmetric organophotocatalysis is described. Two new types of amine-porphyrin hybrids derived from 5,10,15,20-tetraphenylporphyrin (TPPH₂), in which a cyclic secondary amine moiety is covalently linked either to a β-pyrrolic position (Type A) or to the p-position of one of the meso phenyl groups (Type B), were prepared by condensation, reductive amination, or amidation reactions from the suitable porphyrins (either formyl or methanamine derivatives) with readily available chiral amines. A preliminary study of the possible use of Type A amine-porphyrin hybrids as asymmetric, bifunctional organophotocatalysts was performed using the chiral, imidazolidinone-catalyzed Diels-Alder cycloaddition between cyclopentadiene 28 and trans-cinnamaldehyde 29 as a benchmark reaction. The yield and the stereochemical outcome of this process, obtained under purely organocatalytic conditions, under dual organophocatalysis, and under bifunctional organophotocatalysis, were compared.

Ion-Pairing Catalysis in Stereoselective, Light-Induced Transformations

With the rapid development of photoredox catalysis, numerous concepts for asymmetric induction were successfully and broadly adapted from polar two-electron transformations to radical chemistry. While this applies to organocatalysis or transition metal chemistry, asymmetric ion-pairing catalysis remains a niche application within light-driven reactions today. This perspective gives an overview of recent examples, strategies, and their application in stereoselective transformations at the interface of ion-pairing and photo(redox) catalysis.

Biocompatible Porphyrin-Peptide Conjugates as Theranostic Agents Targeting the Epstein-Barr Virus

Epstein-Barr virus (EBV) is a common human-infected virus related to many diseases and cancers. Recently, some peptides have been found to serve targeting and therapeutic roles by inhibiting EBNA1, an oncoprotein of the EBV. We herein report the conjugation of the EBNA1-targeting peptides and porphyrins which can bring synergistic effects by both introducing more specific treatments (photodynamic therapy) and improving the biocompatibility of the photosensitizer and the peptides. One of our compounds exhibited significant photo-cytotoxicity where the Lethal Concentration 50 (LC50 )=6.1 μM in EBV-positive cells. Besides, in vitro cell imaging and co-staining can also be achieved simultaneously and suggested the binding inside nucleus.

Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

Asymmetric catalysis is a major theme of research in contemporary synthetic organic chemistry. The discovery of general strategies for highly enantioselective photochemical reactions, however, has been a relatively recent development, and the variety of photoreactions that can be conducted in a stereocontrolled manner is consequently somewhat limited. Asymmetric photocatalysis is complicated by the short lifetimes and high reactivities characteristic of photogenerated reactive intermediates; the design of catalyst architectures that can provide effective enantiodifferentiating environments for these intermediates while minimizing the participation of uncontrolled racemic background processes has proven to be a key challenge for progress in this field. This review provides a summary of the chiral catalyst structures that have been studied for solution-phase asymmetric photochemistry, including chiral organic sensitizers, inorganic chromophores, and soluble macromolecules. While some of these photocatalysts are derived from privileged catalyst structures that are effective for both ground-state and photochemical transformations, others are structural designs unique to photocatalysis and offer insight into the logic required for highly effective stereocontrolled photocatalysis.

Aerobic Oxidations in Asymmetric Synthesis: Catalytic Strategies and Recent Developments

Despite the remarkable advances in the area of asymmetric catalytic oxidations over the past decades, the development of sustainable and environmentally benign enantioselective oxidation techniques, especially with the efficiency level similar to natural enzymes, still represents a challenge. The growing demand for enantiopure compounds and high interest to industry-relevant green technological advances continue to encourage the research pursuits in this field. Among various oxidants, molecular oxygen is ubiquitous, being available at low cost, environmentally benign and easy-to-handle material. This review highlights recent achievements in catalytic enantioselective oxidations utilizing molecular oxygen as the sole oxidant, with focus on the mechanisms of dioxygen activation and chirogenesis in these transformations.

Asymmetric α-Hydroxylation of α-Aryl-δ-lactams with Molecular Oxygen under Phase-Transfer Conditions

An asymmetric synthesis of α-aryl-α-hydroxy-δ-lactams via phase-transfer-catalyzed hydroxylation with molecular oxygen is described. High yields and high enantioselectivities were achieved using 2,2-diarylvinyl group as an achiral auxiliary. This strategy allows facile access to α-aryl-α-hydroxy-δ-lactam derivatives containing a chiral quaternary center.

Green Asymmetric Organocatalysis

Asymmetric organocatalysis is becoming one of the main tools for the synthesis of chiral compounds that are needed as medicines, crop protection agents, and other bioactive molecules. It can be effectively combined with various green chemistry methodologies. Intensification techniques, such as ball milling, flow, high pressure, or light, bring not only higher yields, faster reactions, and easier product isolation, but also new reactivities. More sustainable reaction media, such as ionic liquids, deep eutectic solvents, green solvent alternatives, and water, also considerably enhance the sustainability profile of many organocatalytic reactions.

Enantioselective synthesis enabled by visible light photocatalysis

Enantioselective photoreaction has been a synthetic challenge for decades. With the continuous development of modern visible light photocatalysis and asymmetric catalysis, remarkable advances have been achieved through the synergistic action of these catalytic reactions, allowing the construction of various enantiomerically enriched molecules that were once inaccessible using photocatalytic reactions. This review presents some of the contemporary developments in enantioselective visible-light photocatalysis reactions, covering the period from 2008 to March 2020, with the contents classified by catalysis type.

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

In this review we present relevant and recent applications of porphyrin derivatives as photocatalysts in organic synthesis, involving both single electron transfer (SET) and energy transfer (ET) mechanistic approaches. We demonstrate that these highly conjugated photosensitizers show increasing potential in photocatalysis since they combine both photo- and electrochemical properties which can substitute available metalloorganic photocatalysts. Batch and continuous-flow approaches are presented highlighting the relevance of enabling technologies for the renewal of porphyrin applications in photocatalysis. Finally, the reaction scale in which the methodologies were developed are highlighted since this is an important parameter in the authors' opinion.

Phase-transfer catalysis and the ion pair concept

This review outlines the recent advances in the field of asymmetric phase-transfer catalysis and the ion-pair concept including alkylation of amino acids and peptides, oxyindoles and other substrates, conjugate additions, fluorinations, photo-induced phase-transfer catalysis, Nitro-Mannich reactions, heterocyclizations and cycloadditions for the preparation of heterocycles, derivatization of isoxazoles, umpolung conjugate addition of imines and other three asymmetric reactions.

Nucleophilic Halogenation of Cyclic Nitronates: A General Access to 3-Halo-1,2-Oxazines

In this article, comprehensive studies on the nucleophilic chlorination and bromination of readily available six-membered cyclic nitronates (1,2-oxazine-N-oxides) are reported. Under optimized conditions (POCl₃ or (COBr)₂ with Hünig's base), 3-halo-substituted 1,2-oxazines, which are difficult to access by other routes, were obtained in good to high yields. The latter were shown to be convenient precursors to other 3-substituted 1,2-oxazine derivatives using Lewis/Brønsted acid-assisted substitution of the halide atom for C-, S-, and N-nucleophiles.

Visible light-mediated selective α-functionalization of 1,3-dicarbonyl compounds via disulfide induced aerobic oxidation

A visible light-mediated α-functionalization of 1,3-dicarbonyl compounds with switchable selectivity induced by disulfide is disclosed.