Iminyl Radical-Triggered 1,5-Hydrogen-Atom Transfer/Heck-Type Coupling by Visible-Light Photoredox Catalysis

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Formal γ-C(sp3)-H Activation of Ketones via Microwave-Promoted and Iminyl-Radical-Mediated 1,5-Hydrogen Atom Transfer

Microwave irradiation of O-phenyloximes triggers N-O homolysis and 1,5-hydrogen atom transfer (HAT), resulting in formal γ-C-H functionalization of ketones after trapping of the radical intermediate and in situ imine hydrolysis. The Lewis acid InCl3·H2O facilitated HAT, enabling functionalization of benzylic and nonbenzylic secondary carbon atoms. Functionalization of primary carbons was feasible but afforded low yields, requiring ClCH2CO2H instead of InCl3·H2O as an additive. C-O and C-C bond formation could both be accomplished by this method.

Visible-Light-Mediated Energy Transfer Enables the Synthesis of β-Lactams via Intramolecular Hydrogen Atom Transfer

The synthesis of 2-azetidinones (β-lactams) from simple acrylamide starting materials by visible-light-mediated energy transfer catalysis is reported. The reaction features a C(sp³ )-H functionalization via a variation of the Norrish-Yang photocyclization involving a carbon-to-carbon 1,5-hydrogen atom transfer (supported by deuterium labelling and DFT calculations) and can be used for the construction of a diverse range of β-lactam products.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Iminyl radical-triggered relay annulation for the construction of bridged aza-tetracycles bearing four contiguous stereogenic centers

Bridged tetracyclic nitrogen scaffolds are found in numerous biologically active molecules and medicinally relevant structures. Traditional methods usually require tedious reaction steps, and/or the use of structurally specific starting materials. We report an unprecedented, iminyl radical-triggered relay annulation from oxime-derived peresters and azadienes, which shows good substrate scope and functional group compatibility, and can deliver various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation. This transformation represents the first example of trifunctionalization of iminyl radicals through simultaneous formation of one C–N and two C–C bonds. DFT calculation studies were conducted to obtain an in-depth insight into the reaction pathways, which revealed that the reactions involved an interesting 1,6-hydrogen atom transfer process.

A novel radical relay annulation is realized for the construction of various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation.

Recent Advance in Iminyl Radical Triggered C–H and C–C Bond Functionalization of Oxime Esters via 1,5-HAT and β-Carbon Scission

The direct functionalization of C(sp3)–H and C(sp3)–C(sp3) bonds is considered as one of the most valuable synthetic strategies because of its high efficiency and step-economy for the rapid assembly of complex molecules. However, the relatively high bond disassociation energies (BDEs) and similar chemical environment lead to large obstacles in terms of low reactivity and selectivity. Using a radical-based strategy has proved to be an efficient approach to overcome these difficulties via a hydrogen atom transfer (HAT) process for selective C(sp3)–H functionalization and β-carbon scission for C(sp3)–C(sp3) bond derivatization. Oxime esters have emerged as outstanding precursors of iminyl radicals for versatile chemical transformations. This short review summaries the recent advances in site-specific C(sp3)–H functionalization and C(sp3)–C(sp3) bond cleavage starting from oxime esters by our group and pioneering work by others, mainly focusing on the reaction design as well as the reaction mechanism.

1 Introduction

2 C(sp3)–H Bond Functionalization via 1,5-HAT of Acyclic Oxime Esters­

2.1 1,5-HAT/Cyclization

2.2 1,5-HAT/C–C or C–Heteroatom Bond Formation

3 C(sp3)–C(sp3) Bond Functionalization via β-Carbon Scission of Cyclic Oxime Esters

3.1 β-Carbon Scission/C–C or C–Heteroatom Bond Formation

3.2 β-Carbon Scission/Cyclization

4 Conclusion and Outlook

Visible Light Driven and Copper-Catalyzed C(sp3)-H Functionalization of O-Pentafluorobenzoyl Ketone Oximes

The C(sp³)-H functionalization of O-pentafluorobenzoyl ketone oximes was implemented under visible light irradiation with copper complexes as catalysts. The reactions involve iminyl-radical-mediated intramolecular hydrogen atom transfer as the key step, with the iminyl radicals being generated via copper-effected N-O cleavage. The reaction afforded 3,4-dihydro-2H-pyrroles under the conditions of [Cu(DPEphos)(bcp)]PF₆ and DABCO, while γ-pentafluorobenzoyloxy ketones were produced predominantly when [Cu(dpp)₂]PF₆ and InCl₃·4H₂O were used as catalysts.

Visible-light-mediated synthesis of quinolines

Visible light promoted synthetic routes of quinolines using different strategies are hereby documented.

Sulfonamide as Photoinduced Hydrogen-Atom Transfer Catalyst for Regioselective Alkylation of C(sp3)-H Bonds Adjacent to Heteroatoms

Based on the DFT calculations, the sulfonamide was explored as an efficient hydrogen-atom transfer catalyst for the C(sp³)-H alkylation. The combination of a metal-free photoredox catalyst and a sulfonamide catalyst enables highly regioselective alkylation of the C-H bonds adjacent to heteroatoms, which features broad substrate scope and excellent functional group compatibility. Remarkably, the sulfonamide catalyst was also applicable to the C(sp³)-C(sp³) couplings through the merger of photoredox, nickel, and HAT catalysis.

Visible-light-driven palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade

A visible-light-induced palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade is described. A range of six to nine-membered β-alkenylated cyclic ketones possessing a quaternary carbon center were accessed under mild conditions. Besides styrenes, the electron-rich alkenes such as silyl enol ethers and enamides were also compatible, providing the desired β-alkylated cyclic ketones in moderate to good yields.

Palladium-Catalyzed β-Arylation of Cyclic α,β-Unsaturated O-Methyl Oximes with Aryl Iodides

We report a Pd-catalyzed β-arylation of cyclic α,β-unsaturated O-methyl oximes with aryl iodides. This reaction shows complete regioselectivity and excellent functional group tolerance. β-Arylation of 2-cyclohexen-1-one O-methyl oxime (existing as 2 : 1 E/Z mixture) with certain aryl iodides such as 4-iodoanisole affords only β-arylated (E)-O-methyl oximes.

Functionalization of remote C(sp3)-H bonds enabled by copper-catalyzed coupling of O-acyloximes with terminal alkynes

Transition metal catalyzed Sonogashira cross-coupling of terminal alkynes with aryl(vinyl) (pseudo)halides has been successfully extended to alkyl halides for the synthesis of functionalized internal alkynes. The direct alkynylation of remote unfunctionalized sp3 carbon by terminal alkynes remains difficult to realize. We report herein an approach to this synthetic challenge by developing two catalytic remote sp3 carbon alkynylation protocols. In the presence of a catalytic amount of Cu(I) salt and a tridentate ligand (tBu3-terpyridine), O-acyloximes derived from cycloalkanones and acyclic ketones are efficiently coupled with terminal alkynes to afford a variety of γ- and δ-alkynyl nitriles and γ-alkynyl ketones, respectively. These reactions proceed through a domino sequence involving copper-catalyzed reductive generation of iminyl radical followed by radical translocation via either β-scission or 1,5-hydrogen atom transfer (1,5-HAT) and copper-catalyzed alkynylation of the resulting translocated carbon radicals. The protocols are applicable to complex natural products.

Transition-metal-free radical relay cyclization of vinyl azides with 1,4-dihydropyridines involving a 1,5-hydrogen-atom transfer: access to α-tetralone scaffolds

The remote C(sp³)–H functionalization enabled by a radical-mediated 1,5-hydrogen-atom transfer (HAT) process using vinyl azides and 1,4-dihydropyridines as precursors has been described.

A visible light-mediated, decarboxylative, desulfonylative Smiles rearrangement for general arylethylamine syntheses

A decarboxylative, desulfonylative Smiles rearrangement is reported for the synthesis of a wide range of biologically relevant arylethylamines, including fluorinated phenylethylamines, heterocyclic amphetamines and an unnatural amino acid.

Remote C(sp3 )-H Arylation and Vinylation of N-Alkoxypyridinium Salts to δ-Aryl and δ-Vinyl Alcohols

The reaction of readily available and bench-stable N-alkoxypyridinium salts with arylboronic and vinylboronic acids afforded δ-aryl and δ-vinyl alcohols, respectively, in the presence of fac-Ir(ppy)₃ and Cu(OTf)₂ dual catalysts. The reaction takes place through a domino process involving the reductive generation of alkoxyl radicals, 1,5-hydrogen atom transfer (1,5-HAT) and the copper-catalyzed cross-coupling reaction of the resulting translocated carbon radicals with boronic acids. Complementary to the Minisci reaction, this method allows for the arylation of nucleophilic alkyl radicals with both electron-rich and electron-poor arenes under mild reaction conditions.

Visible Light Driven Alkylation of C(sp3)-H Bonds Enabled by 1,6-Hydrogen Atom Transfer/Radical Relay Addition

A visible-light-driven sulfamate esters guided alkylation of unactivated C(sp³)-H bonds enabled by a 1,6-HAT/radical addition cascade is described. Not only structurally diverse Michael acceptors but also styrenes are amenable to this alkylation reaction. Notably, the N-H bonds activation radical relay refrained from prefunctionalization and using excess external oxidants.