Unlocking Epoxide Radical Anion Reactivity for Asymmetric Cross-Coupling by Copper Metallaphotoredox

The ability to tame radical anions that feature both an unpaired spin and a charge is critical for synthetic chemistry, enabling the construction of diverse chemical bonds via unique reaction pathways and promoting advances in the area of synthesis. In this regard, C(sp3)-rich epoxide radical anions are particularly attractive but often elusive, highly reactive intermediates. Classic methods to access epoxide radical anions exploit single-electron chemistry by using dissolving alkali, sacrificial electrodes, or redox metals. However, these methods are often prohibitive because of reagent safety issues and over-reduction, limiting their wide implementation, especially in asymmetric synthesis. Herein, we realize a copper metallaphotoredox platform to unlock epoxide radical anion reactivity, allowing the controlled generation of epoxide radical anions and their enantioconvergent cross-coupling with diverse, readily available partners via distonic radical anion intermediates. These discoveries permit highly regio-, chemo-, and enantioselective hydrocyanation and hydroalkynylation, thereby providing a general solution to the challenge of epoxide radical anion-mediated enantioselective chemical diversification.

The reactivity of alkenyl boron reagents in catalytic reactions: recent advances and perspectives

Recent advances focusing on novel reactivity of alkenyl boron reagents in polar or radical pathways within catalytic reactions by employing transition metal catalysis, organocatalysis have been summarized and discussed.

Photoexcited Palladium-Initiated Remote Desaturation of N-Alkoxypyridinium Salts

1,5-Hydrogen atom transfer (HAT) is an effective strategy to achieve remote desaturation of nonfunctionalized alkanes. Herein, we report a photoinduced remote desaturation reaction of N-alkoxypyridinium salts, which serve as alkoxyl radical precursors. Mechanistic studies show that a single electron transfer between the excited palladium complex and a N-alkoxypyridinium salt initiates a radical chain process leading to desaturation of N-alkoxypyridinium salts. This chain mechanism is supported by the measurement of the quantum yield of this reaction (Φ = 82). This reaction is applicable to a range of N-alkoxypyridinium salts, including some complex molecule-derived ones.

Metal-catalyzed reactions of organic nitriles and boronic acids to access diverse functionality

The nitrile or cyano (-CN) group is one of the most appreciated and effective functional groups in organic synthesis, having a polar unsaturated C-N triple bond. Despite sufficient stability and being intrinsically inert, the nitrile group can be easily transformed into many other functional groups, such as amines, carboxylic acids, ketones, etc. which makes it a vital group in organic synthesis. On the other hand, despite several boronic acids having a low level of genotoxicity, they have found wide applicability in the field of organic synthesis, especially in transition metal-catalyzed cross-coupling reactions. Recently, transition-metal-catalyzed cascade additions or addition/cyclization processes of boronic acids to the nitrile group open up exciting and useful strategies to prepare a variety of functional molecules through the formation of C-C, C-N and CO bonds. Boronic acids can be added to the cyano functionality through catalytic carbometallation or through a radical cascade process to provide newer pathways for the rapid construction of various important acyclic ketones or amides, carbamidines, carbocycles and N,O-heterocycles. The present review focuses on various transition-metal-catalyzed additions of boronic acids via carbometallation or radical cascade processes using the cyano group as an acceptor.

Palladium-catalyzed cyclizative cross coupling of ynone oximes with 2-haloaryl N-acrylamides for isoxazolyl indoline bis-heterocycles

Metal-catalyzed cyclizative cross-coupling reactions have attracted enormous attention due to their unique cascade nature. We demonstrated, herein, a dual-cyclizative coupling of ynone oxime ethers with acrylamides for the synthesis of methylene-linked isoxazolyl 2-oxindoles. The cascade was triggered by a palladium(II)-catalyzed ynone oxime ether cyclization, which underwent a Heck-type coupling intercepted by an aryl iodide insertion. Control experiments were carried out to understand the mechanism.

Merging strain-release and copper catalysis: the selective ring-opening cross-coupling of 1,2-oxazetidines with boronic acids

An unprecedented ring-opening cross-coupling of 1,2-oxazetidines with readily available arylboronic acids is achieved for the first time by copper catalysis. Unlike the known electrophilic oxygen reactivity in coupling with organometallic reagents, 1,2-oxazetidines were utilized as formaldimine precursors in this protocol. Remarkable features of this reaction include simple operation, inexpensive catalyst, broad scope and high regioselectivity, delivering a wide array of aminomethylation products. The practicality of this reaction was validated in the one-step downstream transformation of the obtained products into synthetically important molecules and late-stage modification of bioactive acids.

Cu-Catalyzed Cross-Coupling of Benzylboronic Esters and Epoxides

A reaction between epoxides and benzylboronic acid pinacol esters is described. CuI was found to be an effective catalyst of this transformation upon activation of the benzylboronic ester with an alkyllithium reagent. The reaction was very efficient and a variety of substituted epoxides were found to be good substrates with good regioselectivity for substitution at the less substituted side of the epoxide. A reaction using an enantioenriched secondary benzylboronic ester was found to not be stereospecific.

Palladium-Catalyzed Tandem C-C Activation/Cyclization Induced by Carbopalladation of Functionalized Nitriles: Synthesis of Benzo Dipyrromethenes

A palladium-catalyzed carbon-carbon bond activation-initiated reaction of 2-(3-phenyloxiran-2-yl)benzonitriles with arylboronic acids is reported. Multiple chemical bonds were cleavaged and reconstructed via β-carbon elimination in this reaction, enabling the construction of valuable benzo-fused dipyrromethenes that are difficult to prepare by other methods. Additionally, a series of benzannulated boron dipyrromethenes are synthesized and show practical significance in terms of expanding the applications and types of fluorescent materials. The proposed mechanism is supported by preliminary mechanistic experiments.

Terpyridine-based Pd(ii)/Ni(ii) organometallic framework nano-sheets supported on graphene oxide-investigating the fabrication, tuning of catalytic properties and synergetic effects

Tailoring the structures of catalysts and the arrangement of organic bimetallic catalysts are essential in both fundamental research and applications. However, they still impose enormous challenges such as size and active species distribution, ordered uniformity, and controllable composition, which are critical in determining their specific activities and efficiency. Herein, a novel terpyridine-based hetero-bimetallic Ni/Pd nanosheet supported on graphene oxide (denoted as GO@Tpy-Ni/Pd) was fabricated, which exhibited higher catalytic activity, substrate applicability and recyclability for the Suzuki coupling reaction under mild conditions. The catalytic mechanism was heterogeneous catalysis at the interface and the synergetic effect between Pd and Ni resulted in a little Ni(0)/Pd(0) cluster including Pd(ii)/Ni(ii) as a whole being formed through electron transfer on the catalytic surface. This phenomenon could be interpreted as the nanoscale clusters of Ni/Pd being the real active centre stabilized by the ligand and GO and the synergetic effect. The absorption and desorption of different substrates and products on Ni/Pd clusters, as calculated by DFT, was proved to be another key factor.

The synergistic effect between Ni and Pd atom was the crucial factor for enhancing catalytic activity.

Regioselective Cross-Electrophile Coupling of Epoxides and (Hetero)aryl Iodides via Ni/Ti/Photoredox Catalysis

A cross-electrophile coupling reaction of epoxides and (hetero)aryl iodides that operates via the merger of three catalytic cycles involving a Ni-, Ti-, and organic photoredox catalyst has been developed. Three distinct classes of epoxides, styrene oxides, cyclic epoxides, and terminal aliphatic epoxides, all undergo coupling in moderate to good yield and high regioselectivity with the use of three different nitrogen-based ligands for Ni under otherwise identical reaction conditions. The mild reaction conditions accommodate a broad scope of abundant and complex coupling partners. Mechanistic studies suggest that when styrene oxides are employed radical intermediates are involved via Ti-radical ring-opening of the epoxide. Conversely, for terminal aliphatic epoxides, involvement of an iodohydrin intermediate enables the formation of the unexpected linear product.

Synthesis of gem-difluoroalkenes via nickel-catalyzed allylic defluorinative reductive cross-coupling of trifluoromethyl alkenes with epoxides

A nickel-catalyzed defluorinative reductive cross-coupling of trifluoromethyl alkenes with epoxides has been developed.

Palladium-catalyzed tandem reaction of epoxynitriles with arylboronic acids in aqueous medium: divergent synthesis of furans and pyrroles

A palladium-catalyzed controllable divergent synthesis of furans and pyrroles through a cascade reaction of epoxynitriles with arylboronic acids has been developed in the aqueous phase.