Visible-Light Photoredox-Catalyzed Synthesis of Nitrones: Unexpected Rate Acceleration by Water in the Synthesis of Isoxazolidines

Palladium-Catalyzed Synthesis of Nitrones Via Redox Cross-Coupling of Nitro Compounds and Alcohols

A novel methodology for the synthesis of nitrones via palladium-catalyzed redox cross-coupling of nitro compounds and alcohols is established. The protocol is a mild, convenient, ligand-free, and scalable synthesis method that can be compatible with various nitro compounds and alcohols. Nitrone is a significant multifunctional platform synthon which can be synthesized directly and efficiently via this tactic from commercially available and cheap raw materials.

Brønsted acid-catalyzed annulation reaction of hydroxamic acids: synthesis of cyclopentane-fused isoxazolidines and their benzilic amide rearrangement

Readily available hydroxamic acids were leveraged to access challenging nitrones in the presence of H3PO4 as a Brønsted acid catalyst and engaged in an intramolecular (3+2) annulation reaction to make valuable cyclopentane-fused isoxazolidines with high yields and excellent diastereoselectivity. The products were further utilized in a unique base-promoted benzilic amide rearrangement to provide cyclopentane-fused γ-lactams bearing three contiguous stereocenters as a single diastereomer.

Unravelling the Effect of Water Addition in Consecutive Photocatalysis with Naphthalene Diimide

We describe the effect of water addition in the catalyst performance for a C-H functionalization of benzene. Improved yields and selectivity were achieved in a consecutive photoredox catalysis in contrast to the reaction without water. Mechanistic analysis demonstrated this is due to a better catalyst stability and faster kinetic rather than a change in the different steps of the mechanism. The addition of water constitutes a convenient approach to improve catalyst performance, and it was also observed with other catalysts.

Leveraging Alkyne-Oximium Cyclization for the Stereoselective Synthesis of Isoxazolidine

A TMSOTf-mediated highly diastereoselective synthesis of isoxazolidine bearing three contiguous stereocenters is described. This method utilizes O-propargyl hydroxylamines as a novel building block for the rapid assembly of the isoxazolidines via alkyne-oximium cyclization. The strategy could be used in the synthesis of enantiomerically enriched isoxazolidines. Reductive cleavage of the N-O bond efficiently gives the corresponding 1,3-aminodiols with precise control over all stereocenters formed.

Visible-Light Photoredox Catalysis in Water

The use of water in organic synthesis draws attention to its green chemistry features and its unique ability to unveil unconventional reactivities. Herein, literature about the use of water as a reaction medium under visible-light photocatalytic conditions is summarized in order to highlight challenges and opportunities. Accordingly, this Synopsis has been divided into four different sections focused on (1) the unconventional role of water in photocatalytic reactions, (2) in-/on-water reactions, (3) water-soluble photocatalysts, and (4) photomicellar catalytic systems.

Synthesis of spirocyclic Δ4-isoxazolines via [3 + 2] cycloaddition of indanone-derived ketonitrones with alkynes

A [3 + 2] cycloaddition of indanone-derived nitrones and alkynes under mild conditions is developed, allowing facile synthesis of spirocyclicindenyl isoxazolines with structural diversity. The sequential protocol of generated in situ ketonitrone from unsaturated ketones and N-alkylhydroxylamines is also achieved successfully, affording the desired products in considerable yield with moderate to good diastereoselectivity. Moreover, the spirocyclic product can be conveniently transformed into indenyl-based allylic alcohol and enamide.

A [3 + 2] cycloaddition of indanone-derived nitrones with alkynes under mild conditions has been developed. It is a highly efficient and straightforward method for the synthesis of diverse spirocyclicindenyl isoxazolines.

Visible-Light-Mediated α-Amino Alkylation of Azomethine Imines: An Approach to N-(β-Aminoalkyl)pyrazolidinones

Herein, a mild and robust photocatalytic protocol for the combination of amino and pyrazolidinone functionalities through a radical α-amino alkylation of azomethine iminium ions is demonstrated. This method presents a high functional group tolerance providing direct access to a large family of N-(β-aminoalkyl)pyrazolidinones in good to excellent yields, including the late-stage incorporation of the pyrazolidinone moiety to pharmaceutical ingredients. We propose a plausible scenario for the C-C bond-forming step which involves radical addition followed by a spin-center-shift event.

One-electron oxidative dehydrogenative annulation and cyclization reactions

This review focuses on the recent advances in one-electron oxidation involved oxidative dehydrogenative annulations and cyclizations for the intermolecular and intramolecular construction of valuable ring structures.

Synthesis and Transformations of Nitrones for Organic Synthesis

Nitrones are important compounds and are highly useful in many aspects. The first part describes the methods for synthesis of nitrones, which are useful and environmentally friendly. Catalytic oxidations, condensations, and other useful reactions are described. The nitrones thus obtained are key intermediates for the synthesis of biologically important nitrogen compounds. The second part describes the fundamental transformations of nitrones, which will provide the strategies and means for the construction of nitrogen compounds. The reactions with nucleophiles or radicals, C-H functionalization, and various addition reactions are described. The last reactions are particularly important for highly selective carbon-carbon bond formations. 1,3-Dipolar cycloaddition reactions are excluded because the size of the review is limited and excellent reviews have been published in Chemical Reviews.

Synthesis of Chromenoisoxazolidines from Substituted Salicylic Nitrones via Visible-Light Photocatalysis

This effort reports the first redox-neutral visible-light photocatalytic intramolecular dipolar cycloaddition for the diastereoselective synthesis of chromenoisoxazolidines. The authors have found that alkenylphenyl nitrones with a diverse substitution pattern on the aromatic ring and the alkenyl substituent undergo visible-light-promoted cycloadditions in the presence of catalytic amounts of Ru(bpy)₃Cl₂ in high yields and selectivities. Evidence indicates that the proposed redox-neutral pathway is the predominant photoredox mechanism for this transformation.

Additive-free regio- and diastereoselective construction of fully-substituted isoxazolidines employing diazo compounds

An efficient additive-free three-component access to densely functionalized isoxazolidines using diazo compounds, nitrosoarene, and allenic esters has been uncovered.

Merging visible-light photoredox and micellar catalysis: arylation reactions with anilines nitrosated in situ

A micellar photocatalytic system has been disclosed, which enables arylation reactions with anilines nitrosated in situ to run in water.

Recent advances in visible-light-driven organic reactions

In recent years, visible-light-driven organic reactions have been experiencing a significant renaissance in response to topical interest in environmentally friendly green chemical synthesis. The transformations using inexpensive, readily available visible-light sources have come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In this review, we focus on recent advances in the development of visible-light-driven organic reactions, including aerobic oxidation, hydrogen-evolution reactions, energy-transfer reactions and asymmetric reactions. These key research topics represent a promising strategy towards the development of practical, scalable industrial processes with great environmental benefits.

Visible-Light Photoredox-Catalyzed Giese Reaction: Decarboxylative Addition of Amino Acid Derived α-Amino Radicals to Electron-Deficient Olefins

A tin- and halide-free, visible-light photoredox-catalyzed Giese reaction was developed. Primary and secondary α-amino radicals were generated readily from amino acids in the presence of catalytic amounts of an iridium photocatalyst. The reactivity of the α-amino radicals has been evaluated for the functionalization of a variety of activated olefins.

Visible light active CdS nanorods: one-pot synthesis of aldonitrones

A new CdS nanorod was synthesized and it was used in the one-pot synthesis of aldonitrones under blue LED irradiation.

Photoorganocatalysed and visible light photoredox catalysed trifluoromethylation of olefins and (hetero)aromatics in batch and continuous flow

Trifluoromethylation of olefins and (hetero)aromatics with sodium triflinate as CF₃source and readily accessible benzophenone derivatives as photosensitisers has been developed in batch and flow.

Recent synthetic additions to the visible light photoredox catalysis toolbox

The boom in visible light photoredox catalysis (VLPC) research has demonstrated that this novel synthetic approach is here to stay. VLPC enables reactive radical intermediates to be catalytically generated at ambient temperature, a feat not generally allowed through traditional pyrolysis- or radical initiator-based methodologies. VLPC has vastly extended the range of substrates and reaction schemes that have been traditionally the domain of radical reactions. In this review the photophysics background of VLPC will be briefly discussed, followed by a report on recent inroads of VLPC into decarboxylative couplings and radical C-H functionalization of aromatic compounds. The bulk of the review will be dedicated to advances in synergistic catalysis involving VLPC, namely the combination of photoredox catalysis with organocatalysis, including β-functionalization of carbonyl groups, functionalization of weak aliphatic C-H bonds, and anti-Markovnikov hydrofunctionalization of alkenes; dual catalysis with gold or with nickel, photoredox catalysis as an oxidation promoter in transition metal catalysis, and acid-catalyzed enantioselective radical addition to π systems.

Dehydrogenative TEMPO-Mediated Formation of Unstable Nitrones: Easy Access to N-Carbamoyl Isoxazolines

N-carbamoyl nitrones represent an important class of reagents for the synthesis of a variety of natural and biologically active compounds. These compounds are generally converted into valuable 4-isoxazolines upon cyclization reaction with dipolarophiles. However, these types of N-protected nitrones are highly unstable, which limits their synthesis, storage and practical use, enforcing alternative lengthy or elaborated synthetic routes. In this work, a 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated formal "dehydrogenation" of N-protected benzyl-, allyl- and alkyl-substituted hydroxylamines followed by in situ trapping of the generated unstable nitrones into N-carbamoyl 4-isoxazolines is presented. A plausible mechanism is also proposed, in which the dipolarophile shows an important assistant role in the generation of the active nitrone intermediate. This simple protocol avoids the problematic isolation of N-carbamoyl protected nitrones, providing new synthetic possibilities in isoxazoline chemistry.

A New Approach to Nitrones through Cascade Reaction of Nitro Compounds Enabled by Visible Light Photoredox Catalysis

A series of nitroalkanes were efficiently transformed to alkylnitrones using a visible light irradiation photocatalytic process. The mild, efficient, and environmentally benign reaction method, involving dynamic reciprocations of cascade pathways, comprises a mixture of a Ru(bpy)3Cl2 photoredox catalyst and DIPIBA or Hünig's base in CH3CN. Notably, DIPIBA was found to be the best additive for the cross condensation reaction of nitroalkanes with aldehydes. The structures of appropriate products were confirmed by X-ray analysis.

Visible-light photoredox catalysis: direct synthesis of fused β-carbolines through an oxidation/[3 + 2] cycloaddition/oxidative aromatization reaction cascade in batch and flow microreactors

Fused β-carbolines were synthesized via a visible light photoredox catalyzed oxidation/[3 + 2] cycloaddition/oxidative aromatization reaction cascade in batch and flow microreactors.