Visible Light Promoted, Catalyst-Free Radical Carbohydroxylation and Carboetherification under Mild Biomimetic Conditions

Ferrocene catalyzed carbohydroxylation of alkenes using H2O and cycloketone oxime esters

A cyanoalkyl-hydroxylation reaction of aryl alkenes has been successfully devised, employing ferrocene as a catalyst for the addition of a cycloketone oxime ester and H2O across the double bond of the alkene. This environmentally friendly approach employs a solvent mixture consisting of water and demonstrates redox neutrality, along with exceptional regio- and chemoselectivity, leading to the formation of diverse distal hydroxy-nitrile compounds. Moreover, this research presents noteworthy contributions in terms of late-stage functionalization of complex molecules and offers valuable insights into the mechanistic aspects of the reaction.

Ligand-Enabled NiII -Catalyzed Hydroxylarylation of Alkenes with Molecular Oxygen

The use of molecular oxygen as the terminal oxidant in transition metal catalyzed oxidative process is an appealing and challenging task in organic synthetic chemistry. Here, we report a Ni-catalyzed hydroxylarylation of unactivated alkenes enabled by a β-diketone ligand with high efficiency and excellent regioselectivity employing molecular oxygen as the oxidant and hydroxyl source. This reaction features mild conditions, broad substrate scope and incredible heterocycle compatibility, providing a variety of β-hydroxylamides, γ-hydroxylamides, β-aminoalcohols, γ-aminoalcohols, and 1,3-diols in high yields. The synthetic value of this methodology was demonstrated by the efficient synthesis of two bioactive compounds, (±)-3'-methoxyl citreochlorol and tea catechin metabolites M4.

Cyanine- and Rhodamine-Derived Alkynes for the Selective Targeting of Cancerous Mitochondria through Radical Thiol-Yne Coupling in Live Cells

Despite their long history and their synthetic potential underlined by various recent advances, radical thiol-yne coupling reactions have so far only rarely been exploited for the functionalization of biomolecules, and no examples yet exist for their application in live cells - although natural thiols show widespread occurrence therein. By taking advantage of the particular cellular conditions of mitochondria in cancer cells, we have demonstrated that radical thiol-yne coupling represents a powerful reaction principle for the selective targeting of these organelles. Within our studies, fluorescently labeled reactive alkyne probes were investigated, for which the fluorescent moiety was chosen to enable both mitochondria accumulation as well as highly sensitive detection. After preliminary studies under cell-free conditions, the most promising alkyne-dye conjugates were evaluated in various cellular experiments comprising analysis by flow cytometry and microscopy. All in all, these results pave the way for improved future therapeutic strategies relying on live-cell compatibility and selectivity among cellular compartments.

Asymmetric Carbohydroxylation of Alkenes Using Photoenzymatic Catalysis

Alkene difunctionalizations enable the synthesis of structurally elaborated products from simple and ubiquitous starting materials in a single chemical step. Carbohydroxylations of olefins represent a family of reactivity that furnish structurally complex alcohols. While examples of this type of three-component coupling have been reported, catalytic asymmetric examples remain elusive. Here, we report an enzyme-catalyzed asymmetric carbohydroxylation of alkenes catalyzed by flavin-dependent "ene"-reductases to produce enantioenriched tertiary alcohols. Seven rounds of protein engineering reshape the enzyme's active site to increase activity and enantioselectivity. Mechanistic studies suggest that C-O bond formation occurs via a 5-endo-trig cyclization with the pendant ketone to afford an α-oxy radical which is oxidized and hydrolyzed to form the product. This work demonstrates photoenzymatic reactions involving "ene"-reductases can terminate radicals via mechanisms other than hydrogen atom transfer, expanding their utility in chemical synthesis.

Directed Nickel-Catalyzed Selective Arylhydroxylation of Unactivated Alkenes under Air

An expeditious and novel nickel-catalyzed selective arylhydroxylation of unactivated alkenes with arylboronic acids was developed. This protocol is compatible with β,γ- and γ,δ-alkene amides, including traditionally challenging internal alkenes, to provide important β-arylethylalcohol scaffolds. The free hydroxyl group in the final product could be smoothly further transformed into other functional groups. Control experiments indicated that the oxygen atom of the hydroxyl group in the product is derived from the oxygen in the air.

Visible-Light-Mediated Radical Arylations Using a Fluorescein-Derived Diazonium Salt: Reactions Proceeding via an Intramolecular Forth and Back Electron Transfer

Functionalizations of arenes and alkenes via additive-free radical reactions using highly photosensitive, fluorescein-derived diazonium salts are described. The particular properties of the diazonium salts enable unique Meerwein-type carbohydroxylations of non-activated alkenes, which can be rationalized by a reaction mechanism involving forth and back electron transfer from and to the xanthene subunit of the fluorescein moiety.

Photoredox Hydroxy-arylation of the Terminal Double Bond of N-Substituted 3-Methyleneisoindolin-1-ones in Visible Light

Mild and efficient ruthenium-catalyzed hydroxy-arylation of the terminal double bond of N-substituted 3-methyleneisoindolin-1-ones is described. The reaction takes place with aryl diazonium salt as the arylating reagent and water as the hydroxyl source in visible light at ambient temperature. The strategy entails vicinal difunctionalization of alkene and enables construction of 3-benzyl-3-hydroxyisoindolin-1-one heterocyclic scaffolds in moderate to good yields. C-C and C-O bonds are formed in one pot without any external additive and oxidant through an in situ generation of a carbocation intermediate in green light.

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

Acyclic α-tertiary ethers represent a highly prevalent functionality, common to high-value bioactive molecules, such as pharmaceuticals and natural products, and feature as crucial synthetic handles in their construction. As such their synthesis has become an ever-more important goal in synthetic chemistry as the drawbacks of traditional strong base- and acid-mediated etherifications have become more limiting. In recent years, the generation of highly reactive intermediates via redox approaches has facilitated the synthesis of highly sterically-encumbered ethers and accordingly these strategies have been widely applied in α-tertiary ether synthesis. This review summarises and appraises the state-of-the-art in the application of redox strategies enabling acyclic α-tertiary ether synthesis.

Current Advances in Meerwein-type Radical Alkene Functionalizations

Alkene functionalizations via Meerwein arylations are becoming increasingly attractive, especially since a variety of mild and sustainable methods for aryl radical generation are available today. This entails a broad spectrum of substrates and radical scavengers, as well as convenient synthetic routes to relevant precursors for further transformations. The present review focuses on recent advances in Meerwein-type alkene functionalizations and gives insights into the key mechanistic details of the respective reactions.

1 Introduction

2 Hydroarylation and Carboarylation

3 Carboamination, Carbooxygenation, and Carbothiolation

4 Carbohalogenation

5 Conclusion and Outlook

Visible-Light-Induced Meerwein Fluoroarylation of Styrenes

An unprecedented approach for assembling a broad range of 1,2-diarylethane derivatives with fluorine-containing fully substituted carbon centers was developed. The protocol features straightforward operation, proceeds under metal-free condition, and accommodates a large variety of synthetically useful functionalities. The critical aspect to the success of this novel transformation lies in using aryldiazonium salts as both aryl radical progenitor and also as single electron acceptor which elegantly enables a radical-polar crossover manifold.

2-Fluoro-5-nitrophenyldiazonium: A Novel Sanger-Type Reagent for the Versatile Functionalization of Alcohols

As a novel Sanger-type reagent, 2-fluoro-5-nitrophenyldiazonium tetrafluoroborate enabled the versatile functionalization of primary and secondary aliphatic alcohols. Based on a mild nucleophilic aromatic substitution of the fluorine atom under unprecedented, base-free conditions, the diazonium unit on the aromatic core of the resulting aryl-alkyl ether could be employed for such diverse transformations as radical C-H activation and cyclization, as well as palladium catalyzed cross-coupling reactions.