You-Quan Zou

Highly Efficient Aerobic Oxidative Hydroxylation of Arylboronic Acids: Photoredox Catalysis Using Visible Light

The development of green and sustainable methods for effective synthesis of fine chemicals is an important goal in our society. During the last decade, visible-light photoredox catalysis has shown great promise as a method to advance these goals. As a result of its high natural abundance, benign environmental impact, cleanliness, and sustainability, photocatalysis using visible light is a reliable and powerful tool. In this context, there are several examples of its usefulness, including asymmetric alkylation of aldehydes, [2+2] cycloaddition of enones, [3+2] cycloaddition of aryl cyclopropyl ketones, reductive dehalogenation, radical addition to unsaturated bonds, and coupling reactions. 9] Despite these advances, oxidative reactions initiated by visible light are largely unexplored. In 2003, Zen et al. reported a visible-light photocatalytic reaction for the oxidation of sulfides into sulfoxides. Later, Zhao et al. oxidized alcohols to aldehydes using visible light and dye-sensitized TiO2. [11] Recently, the groups of Blechert and Wang developed a metal-free photooxidative system to achieve the oxidation of amines and alcohols, and Jiao et al. described the use of a Ru polypyridine complex and 4-methoxypyridine to promote the conversion of a-aryl halogen derivatives into a-aryl carbonyl compounds. The molecular oxygen in air has been widely used as a green oxidant in synthesis. Consequently, the development of visible-light photooxidative reactions using air as the oxidant is highly desirable. We recently developed a visiblelight-induced oxidation/[3+2] cycloaddition/oxidative aromatization sequence for dihydroisoquinoline esters and electrondeficient alkenes or alkynes to construct pyrrolo[2,1-a] isoquinolines. 16] In this sequence, a superoxide radical anion, which was generated from molecular oxygen, plays a key role throughout the process. Based on the investigation of the mechanism, we envisioned that this kind of highly active species might have Lewis basicity and therefore react with the appropriate acidic components. We report herein the realization of this strategy for the direct aerobic oxidative hydroxylation of arylboronic acids to aryl alcohols using visible-light irradiation and air as the source of the terminal oxidant. In this reaction, we envisioned that the superoxide radical anion generated from the photoredox cycle could react with arylboronic acids because of its Lewis acidity, which arises from the vacant p orbital on the boron atom, followed by a series of rearrangements to provide aryl alcohols (Scheme 1).

An organocatalytic Michael-aldol cascade: formal [3+2] annulation to construct enantioenriched spirocyclic oxindole derivatives

An efficient organocatalytic Michael-aldol cascade reaction for the asymmetric synthesis of spirocyclic oxindole derivatives fused with tetrahydrothiophenes has been developed through a formal [3+2] annulation strategy.

Three-component coupling reaction triggered by insertion of arynes into the S═O bond of DMSO.

An unprecedented three-component coupling reaction of arynes, α-bromo carbonyl compounds, and DMSO triggered by insertion of arynes into the S═O bond of DMSO has been developed. The reaction can generate a wide range of multisubstituted aryl methyl thioethers in good yields, wherein DMSO serves as both methylthiolation reagent and oxygen source.

Novel thiourea-amine bifunctional catalysts for asymmetric conjugate addition of ketones/aldehydes to nitroalkenes: rational structural combination for high catalytic efficiency

A series of thiourea-amine bifunctional catalysts have been developed by a rational combination of prolines with cinchona alkaloids, which are connected by a thiourea motif. The catalyst 3a, prepared from L-proline and cinchonidine, was found to be a highly efficient catalyst for the conjugate addition of ketones/aldehydes to a wide range of nitroalkenes (up to 98/2 dr and 96% ee). The privileged cinchonidine backbone and the thiourea motif are essential to the reaction activity and enantioselectivity.

De Novo Synthesis of Imidazoles by Visible‐Light‐Induced Photocatalytic Aerobic Oxidation/[3+2] Cycloaddition/Aromatization Cascade

A visible-light-induced photocatalytic aerobic oxidation/[3+2] cycloaddition/aromatization cascade between secondary amines and isocyanides has been successfully developed. The reaction provides a general and efficient access to diversely substituted imidazoles and imidazo[1,5-a]quinoxalin-4(5 H)-ones in good yields under mild conditions.

Pyrrolidinyl-sulfamide derivatives as a new class of bifunctional organocatalysts for direct asymmetric Michael addition of cyclohexanone to nitroalkenes.

A series of chiral pyrrolidinyl-sulfamide derivatives have been identified as efficient bifunctional organocatalysts for the direct Michael addition of cyclohexanone to a wide range of nitroalkenes. The desired Michael adducts were obtained in high chemical yields and excellent stereoselectivities (up to 99/1 dr and 95% ee).

Visible-light-induced photocatalytic formyloxylation reactions of 3-bromooxindoles with water and DMF: the scope and mechanism

The formyloxylation reaction of 3-bromooxindoles with water and N,N-dimethylformamide (DMF) has been developed in the presence of the photoredox catalyst fac-Ir(ppy)3 under irradiation of visible light at ambient temperature. The reaction provides a straightforward approach to pharmaceutically and synthetically useful 3-formyloxyoxindoles in high yields. The mechanism of this transformation was investigated by fluorescence quenching experiments, “on–off” switching of the light source, labeling experiments, mass spectral analyses and in situ IR experiments.

Phototandem Catalysis: Efficient Synthesis of 3-Ester-3-hydroxy-2-oxindoles by a Visible Light-Induced Cyclization of Diazoamides through an Aerobic Oxidation Sequence

An unprecedented phototandem catalysis based on a single iridium photocatalyst has been successfully developed. This powerful strategy consists of two mechanistically distinct catalytic cycles, namely, photocatalytic energy transfer (ET) and single electron transfer (SET). The novel protocol allows a rapid and efficient construction of biologically and synthetically important 3-ester-3-hydroxy-2-oxindole derivatives from readily available diazoamides through a cyclization/aerobic oxidation sequence under very mild conditions.

Aerobic oxidative C–B bond cleavage of arylboronic acids mediated by methylhydrazines

The aerobic oxidative C–B bond cleavage and ipso-hydroxylation of arylboronic acids mediated by methylhydrazine have been developed under metal-free conditions. This transformation affords structurally diverse phenols in good to excellent yields (26 examples, 71–99%).

Visible‐Light‐Induced Organic Photochemical Reactions via Energy Transfer Pathways

Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible-light-induced photochemical reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.