Photoinduced Remote C(sp3)-H Cyanation and Oxidation Enabled by a Vinyl Radical-Mediated 1,5-HAT Strategy

Selective Synthesis of Z-Michael Acceptors via Hydroalkylation of Conjugated Alkynes

Hydroalkylation of terminal alkynes is a powerful approach to the synthesis of disubstituted alkenes. However, its application is largely unexplored in the synthesis of α,β-unsaturated carbonyls, which are common among synthetic intermediates and biologically active molecules. The thermodynamically less stable Z-isomers of activated alkenes have been particularly challenging to access because of their propensity for isomerization and the paucity of reliable Z-selective hydroalkylation methods. We developed a highly Z-selective silver-catalyzed hydroalkylation of terminal conjugated alkynes using alkyl boranes as coupling partners. The reaction allows access to (Z)-α,β-unsaturated esters, secondary and tertiary alkyl amides, aryl amides, and alkyl and aryl ketones and tolerates a wide range of functional groups. The reaction can be performed successfully in the presence of alkyl and aryl halides, esters, protected alcohols, and amines. The hydroalkylation involves the formation of an alkynylboronate complex followed by a 1,2-metalate shift. This sequence of steps mechanistically constrains the stereochemical outcome, which, together with mild reaction conditions, ensures high Z-selectivity.

Direct Photochemical Synthesis of Substituted Benzo[b]fluorenes

We report a new and straightforward route toward substituted benzo[b]fluorenes via the direct photochemical conversion of alkynylated chalcones. This transformation exploits a high-power light-emitting diode emitting ultraviolet A light to enable the rapid formation of the target products (tres = 5 min). A continuous flow approach thereby facilitates reproducibility and scalability, granting streamlined access to these important scaffolds and their derivatives. A mechanistic proposal based on a biradical species is presented and supported by deuteration studies.

Direct C(sp3)-H functionalization with aryl and alkyl radicals as intermolecular hydrogen atom transfer (HAT) agents

Recent years have witnessed the emergence of direct intermolecular C(sp3)-H bond functionalization using in situ generated aryl/alkyl radicals as a unique class of hydrogen atom transfer (HAT) agents. A variety of precursors have been exploited to produce these radical HAT agents under photocatalytic, electrochemical or thermal conditions. To date, viable aryl radical precursors have included aryl diazonium salts or aryl azosulfones, diaryliodonium salts, O-benzoyl oximes, aryl sulfonium salts, aryl thioesters, and aryl halides; and applicable alkyl radical sources have included tetrahalogenated methanes (e.g., CCl3Br, CBr4 and CF3I), N-hydroxyphthalimide esters, alkyl bromides, and acetic acid. This review summarizes the current advances in direct intermolecular C(sp3)-H functionalization through key HAT events with in situ generated aryl/alkyl radicals and categorizes the procedures by the specific radical precursors applied. With an emphasis on the reaction conditions, mechanisms and representative substrate scopes of these protocols, this review aims to demonstrate the current trends and future challenges of this emerging field.

Metal-free radical selenothiocyanation of terminal and internal alkynes

We report herein a synthetic strategy for the generation of direct selenothiocyanation from both terminal and internal alkynes via a radical process. Alkynes derived from bioactive molecules, such as L(-)-borneol and L-menthol, are suitable for selenothiocyanation reaction. This method features metal-free conditions and readily available reagents.

Photoredox-Catalyzed Alkylamination of Alkenes via Oxidative Radical-Polar Crossover and Site-Selective 1,5-Hydrogen Atom Transfer

We reported the visible-light-mediated photoredox-catalyzed oxidative radical-polar crossover and 1,5-hydrogen atom transfer combined site-selective remote C(sp3)-N cross-coupling alkylamination of alkenes. Various anilines and hydroxamides (1,5-hydrogen atom transfer reagents) could be tolerated. The mechanistic studies indicated the radical nature of the reaction and the indispensability of light and photocatalyst. Stern-Volmer fluorescence quenching and cyclic voltammetry experiments have been used to outline the proposed reaction pathway.

An Expedient Radical Approach for the Decarboxylative Synthesis of Stereodefined All-Carbon Tetrasubstituted Olefins

We report a user-friendly approach for the decarboxylative formation of stereodefined and complex tri- and tetra-substituted olefins from vinyl cyclic carbonates and amines as radical precursors. The protocol relies on easy photo-initiated α-amino-radical formation followed by addition onto the double bond of the substrate resulting in a sequence involving carbonate ring-opening, double bond relay, CO2 extrusion and finally O-protonation. The developed protocol is efficient for both mismatched and matched polarity substrate combinations, and the scope of elaborate stereodefined olefins that can be forged including drug-functionalized derivatives is wide, diverse and further extendable to other types of heterocyclic and radical precursors. Mechanistic control reactions show that the decarboxylation step is a key driving force towards product formation, with the initial radical addition under steric control.

Visible Light-Induced Copper-Catalyzed Regio- and Stereoselective Difluoroalkylthiocyanation of Alkynes

The first regio- and stereoselective difluoroalkylthiocyanation of alkynes with BrCF2R and KSCN has been disclosed under visible light-induced copper catalysis. The copper complex photosensitizer formed in situ not only promotes the generation of CF2-alkyl radicals but also facilitates the construction of C-SCN bonds, allowing the reaction to proceed smoothly without any additional photocatalysts or radical initiators. Moreover, the challenging internal alkynes can also be transformed to deliver CF2-derived tetrasubstituted olefins with potential applications in agricultural and medicinal chemistry.

Photoinduced Regioselective Fluorination and Vinylation of Remote C(sp3)-H Bonds Using Thianthrenium Salts

Herein, a photoredox-driven practical protocol for fluorinated alkene synthesis using easily accessible and modular thianthrenium salts with electron-withdrawing alkynes or propargyl alcohols is reported. Vinyl radical intermediates, formed by the reaction between the alkyl or trifluoromethyl thianthrenium salts and electronically diverse alkynes, can mediate the key 1,5-HAT process of regioselective C(sp3)-H fluorination and vinylation. This protocol provides straightforward access to structurally diverse trifluoromethyl- or distally fluoro-functionalized alkene products in 21-79% yields with a broad substrate range under mild photocatalytic conditions.

Carbon quaternization of redox active esters and olefins by decarboxylative coupling

The synthesis of quaternary carbons often requires numerous steps and complex conditions or harsh reagents that act on heavily engineered substrates. This is largely a consequence of conventional polar-bond retrosynthetic disconnections that in turn require multiple functional group interconversions, redox manipulations, and protecting group chemistry. Here, we report a simple catalyst and reductant combination that converts two types of feedstock chemicals, carboxylic acids and olefins, into tetrasubstituted carbons through quaternization of radical intermediates. An iron porphyrin catalyst activates each substrate by electron transfer or hydrogen atom transfer, and then combines the fragments using a bimolecular homolytic substitution (SH2) reaction. This cross-coupling reduces the synthetic burden to procure numerous quaternary carbon---containing products from simple chemical feedstocks.

Visible-light-mediated synthesis of oxime esters via multicomponent reactions of aldehydes, aryl amines, and N-hydroxyphthalimide esters

Oxime esters are useful scaffolds in many organic chemistry transformations. Herein, a novel visible-light-mediated three-component reaction for synthesis of oxime esters is reported. Aldehydes, aniline, and N-hydroxyphthalimide (NHPI) esters were used as substrates in this three-component reaction, and eosin Y was used as a crucial photocatalyst for the reaction. Wide ranges of aldehydes and NHPI esters were well tolerated in this reaction method, generating various oxime esters with high efficiency under mild reaction conditions. This visible-light-mediated methodology will be a promising approach to synthesize useful oxime esters in a single step.

Cu(ii)-mediated direct intramolecular cyclopropanation of distal olefinic acetate: access to cyclopropane-fused γ-lactones

Cyclopropane-fused ring scaffolds represent one of the most appealing structural motifs in organic chemistry due to their wide presence in bioactive molecules and versatility in organic synthesis. These skeletons are typically prepared from olefinic diazo compounds via transition-metal catalysed intramolecular carbenoid insertion, which suffers from prefunctionalization of starting materials and limited substrate scope. Herein, we disclose a practical copper-mediated direct intramolecular cyclopropanation of distal olefinic acetate to synthesize cyclopropane-fused γ-lactones and lactams. This cascade reaction is postulated to proceed through a hydrogen atom transfer event induced radical cyclization and copper-mediated cyclopropanation sequence. The protocol features high atom- and step-economy, excellent diastereoselectivity, broad tolerance of functional groups, and operational simplicity.

Remote Radical 1,3-, 1,4-, 1,5-, 1,6- and 1,7-Difunctionalization Reactions

Radical transformations are powerful in organic synthesis for the construction of molecular scaffolds and introduction of functional groups. In radical difunctionalization reactions, the radicals in the first functionalized intermediates can be relocated through resonance, hydrogen atom or group transfer, and ring opening. The resulting radical intermediates can undertake the following paths for the second functionalization: (1) couple with other radical groups, (2) oxidize to cations and then react with nucleophiles, (3) reduce to anions and then react with electrophiles, (4) couple with metal-complexes. The rearrangements of radicals provide the opportunity for the synthesis of 1,3-, 1,4-, 1,5-, 1,6-, and 1,7-difunctionalization products. Multiple ways to initiate the radical reaction coupling with intermediate radical rearrangements make the radical reactions good for difunctionalization at the remote positions. These reactions offer the advantages of synthetic efficiency, operation simplicity, and product diversity.

Photoinduced Remote C(sp3)-H Imination Enabled by Vinyl Radical-Mediated 1,5-Hydrogen Atom Transfer

A vinyl radical-mediated 1,5-hydrogen atom transfer strategy for remote C(sp³)-H imination under visible-light-induced photochemical metal-free conditions afforded diverse γ-imino alkenes with excellent stereoselectivity. Oxime ester-based bifunctional reagents provided not only nucleophilic alkyl radicals for radical addition reactions with electron-deficient alkynes but also long-lived steady-state imine radicals for trapping alkyl radicals following the intramolecular 1,5-hydrogen migration of unstable olefin radicals.