Iridium-Catalyzed Cyclization of Isoxazolines and Alkenes: Divergent Access to Pyrrolidines, Pyrroles, and Carbazoles

α-Nucleophilic Addition to α,β-Unsaturated Carbonyl Compounds via Photocatalytically Generated α-Carbonyl Carbocations

We report the photocatalytic oxidation of α-carbonyl radicals of amides or esters to the corresponding α-carbonyl carbocations through super photoreductant CBZ6 induced redox-neutral photocatalysis. The α-carbonyl radicals are formed by the β-addition of alkyl radicals generated in situ by the photocatalytic fragmentation of N-hydroxyphthalimide esters to the α,β-unsaturated amides and esters. This method enables the α-nucleophilic addition of hydroxyl or alkoxyl radicals to amides and esters without any prefunctionalization.

Photoreductive N-N Homocoupling Catalyzed by a Superphotoreductant

Azines are valuable synthons or target molecules in organic synthesis. In this work, we report that CBZ6 could work as a photoreductive catalyst for the N-N homocoupling of oximes in high efficiency. This therefore enabled convenient access to a large variety of azines from the corresponding aryl and alkyl ketones, as well as aryl aldehydes in up to 99% yield. 2,4-Dinitrophenoxyl was used as the recyclable auxiliary and the CBZ6 photocatalyst could also be recycled. These together with the low catalyst loading (2 mol % of CBZ6), the short reaction time (4 h), and the nontoxic DMSO solvent make the current process a sustainable and practical alternative to the classic methods.

Photocatalytic low-temperature defluorination of PFASs

Polyfluoroalkyl and perfluoroalkyl substances (PFASs) are found in many everyday consumer products, often because of their high thermal and chemical stabilities, as well as their hydrophobic and oleophobic properties1. However, the inert carbon-fluorine (C-F) bonds that give PFASs their properties also provide resistance to decomposition through defluorination, leading to long-term persistence in the environment, as well as in the human body, raising substantial safety and health concerns1-5. Despite recent advances in non-incineration approaches for the destruction of functionalized PFASs, processes for the recycling of perfluorocarbons (PFCs) as well as polymeric PFASs such as polytetrafluoroethylene (PTFE) are limited to methods that use either elevated temperatures or strong reducing reagents. Here we report the defluorination of PFASs with a highly twisted carbazole-cored super-photoreductant KQGZ. A series of PFASs could be defluorinated photocatalytically at 40-60 °C. PTFE gave amorphous carbon and fluoride salts as the major products. Oligomeric PFASs such as PFCs, perfluorooctane sulfonic acid (PFOS), polyfluorooctanoic acid (PFOA) and derivatives give carbonate, formate, oxalate and trifluoroacetate as the defluorinated products. This allows for the recycling of fluorine in PFASs as inorganic fluoride salt. The mechanistic investigation reveals the difference in reaction behaviour and product components for PTFE and oligomeric PFASs. This work opens a window for the low-temperature photoreductive defluorination of the 'forever chemicals' PFASs, especially for PTFE, as well as the discovery of new super-photoreductants.

Photocatalytic Redox-Neutral Alkoxyacylation of Alkenes

β-Alkoxyketones are important building blocks in organic synthesis. By utilizing CBZ6, with an oxidative potential of -2.16 V (vs the saturated calomel electrode), as a redox-neutral photocatalyst, alkoxyacylation of olefins was accomplished under the irradiation of visible light via a cationic intermediate. It involves the addition of an acyl radical to olefin to form a radical intermediate and the following oxidation of the radical intermediate to the benzyl cationic intermediate that is captured by alkoxy anions. This process provides concise and practical access to the β-functionalized ketones.

Synthesis of α-Hydroxyl and α-Amino Pyridinyl Esters via Photoreductive Dual Radical Cross-Coupling

A method for the synthesis of α-hydroxyl and α-amino pyridinyl esters via photoreductive dual radical cross-coupling catalyzed by the super-organoreductant CBZ6 has been developed. A wide range of 2-pyridinylation and 4-pyridinylation of either α-ketoesters or imine derivatives has been achieved. The applications in the synthesis of pyridinyl amino-hydroxyl acids as well as a new chiral oxazoline ligand have also been accomplished.

Reductive Cleavage of C-X or N-S Bonds Catalyzed by Super Organoreductant CBZ6

The reductive cleavage of C(Ar)-X bonds is the key step for the cross coupling of Ar-X with other groups. In this work, under the irradiation of 407 nm LEDs using sodium formate as reductant and thiol as hydrogen atom transfer agent, a variety of (hetero)aryl chlorides, bromides, and iodides could be reduced to corresponding (hetero)arenes. The key intermediates, aryl radicals, could be trapped by either hydrogen, phosphite, or borates. The same reduction conditions can be extended to the deprotection of sulfonamides.

CBZ6 as a Recyclable Organic Photoreductant for Pinacol Coupling

A recyclable organic photoreductant (1 mol % CBZ6)-catalyzed reductive (pinacol) coupling of aldehydes, ketones, and imines has been developed. Irradiated by purple light (407 nm) using triethylamine as an electron donor, a variety of 1,2-diols and 1,2-diamines could be prepared. The oxidation potential of the excited state of CBZ6 is established as -1.92 V (vs saturated calomel electrode (SCE)). The relative high reductive potential enables the reductive coupling of carbonyl compounds and their derivatives. CBZ6 can be prepared in gram scale and is acid/base- or air-stable. It could be applied in large-scale photoreductive synthesis and recovered in high yield after the reaction.

Recent Advancements in Pyrrole Synthesis

This review article features selected examples on the synthesis of functionalized pyrroles that were reported between 2014 and 2019. Pyrrole is an important nitrogen-containing aromatic heterocycle that can be found in numerous compounds of biological and material significance. Given its vast importance, pyrrole continues to be an attractive target for the development of new synthetic reactions. The contents of this article are organized by the starting materials, which can be broadly classified into four different types: substrates bearing π-systems, substrates bearing carbonyl and other polar groups, and substrates bearing heterocyclic motifs. Brief discussions on plausible reaction mechanisms for most transformations are also presented.

Nitro-Substituted Benzaldehydes in the Generation of Azomethine Ylides and Retro-1,3-Dipolar Cycloadditions

1,3-Dipolar cycloaddition of 2- and 3-nitrobenzaldehydes with 2-aminomethylpyridine and ethyl (2E)-2-cyano-3-(4-nitrophenyl)prop-2-enoate yielded endo-cycloadducts as the sole products under various reaction conditions. Fortuitously, 4-nitrobenzaldehyde behaved differently in three- and four-component cascades to produce a mixture of endo- and exo'-cycloadducts. This reaction is solvent- and temperature-dependent, and consequently, both the endo- and exo'-cycloadducts were synthesized in an excellent regio-, stereo-, and chemoselective fashion. Retro-1,3-dipolar cycloadditions of the endo-cycloadducts were conducted under mild reaction conditions, and the generated syn-dipoles were stereomutated into anti-dipoles which recycloadded with the dipolarophiles to provide the exo'-cycloadducts. Mechanistic studies were carried out to support the proposed mechanisms. Unprecedentedly, particular arylidene scaffolds participated as aldehyde or activated methylene precursors. Density functional theory calculations were performed to shed light on the importance of AcOH in the generation and isomerization of dipoles and to explain the high selectivity and the possibility of retro-cycloaddition.

Sequential Cu(II)-promoted oxidation/[3 + 2] cycloaddition/aromatization tandem reaction for the synthesis of 2-substituted benzo[f]isoindole-4,9-dione

An efficient method for the synthesis of 2-substituted benzo[f]isoindole-4,9-dione derivatives from N-substituted iminodiacetates and quinones via a Cu(II)-promoted oxidation/[3 + 2] cycloaddition/aromatization tandem reaction was reported. This tandem reaction uses a wide range of N-substituted iminodiacetate derivatives that contain the chain-alkyl, cycloalkyl, and aryl group on the N-atom. Based on optimized reaction conditions, the desired product of 2-substituted benzo[f]isoindole-4,9-diones was obtained in moderate to excellent yields. Taken together, the promising results of this research would provide an especially efficient strategy to synthesize polysubstituted pyrroles from easy available starting materials and promoted by cheaper Cu(OAc)2.

Synthesis of Highly Functionalized Indoles and Indolones via Selectivity-Switchable Olefinations

Highly functionalized indoles and indolones were prepared via selectivity-switchable mono- or diolefinations. The Julia olefination of the products followed by a Brønsted acid-prompted cyclization afforded indolones, whereas the indoles were obtained by a sequential Wittig olefination and electrocyclization. This method opens divergent access to highly functionalized nitrogen-containing bicyclic or tricyclic heterocycles.