Enantioselective metallaphotoredox catalysis, which combines photoredox catalysis and asymmetric transition-metal catalysis, has become an effective approach to achieve stereoconvergence under mild conditions. Although many impressive synthetic approaches have been developed to access central chirality, the construction of axial chirality by metallaphotoredox catalysis still remains underexplored. Herein, we report two visible light-induced cobalt-catalyzed asymmetric reductive couplings of biaryl dialdehydes to synthesize axially chiral aldehydes (60 examples, up to 98% yield, >19:1 dr, and >99% ee). This protocol shows good functional group tolerance, broad substrate scope, and excellent diastereo- and enantioselectivity.

Photoinduced Cobalt-Catalyzed Desymmetrization of Dialdehydes to Access Axial Chirality.

The arylethylamine pharmacophore is conserved across a range of biologically active natural products and pharmaceuticals, particularly in molecules that act on the central nervous system. Herein, we present a photoinduced copper-catalyzed azidoarylation of alkenes at a late stage with arylthianthrenium salts, allowing access to highly functionalized acyclic (hetero)arylethylamine scaffolds that are otherwise difficult to access. A mechanistic study is consistent with a rac-BINAP-CuI-azide (2) as the photoactive catalytic species. We show the utility of the new method by the expedient synthesis of racemic melphalan in four steps through C–H functionalization.

https://pubs.acs.org/doi/pdf/10.1021/jacs.3c04016

Photoinduced Copper-Catalyzed Late-Stage Azidoarylation of Alkenes via Arylthianthrenium Salts

Copper-catalyzed enantioselective coupling has been widely investigated, which allows rapid construction of various chiral molecules. Despite important advances via polar and radical mechanisms, exploring general and practical strategies for the regio-, enantio- and diastereoselective assembly of stereogenic centers is of significant value but remains highly problematic. The integration of photocatalysis with asymmetric copper catalysis could provide appealing access to the development of new reaction pathways and structurally diverse chiral compounds, and extend the boundaries of radical chemistry. This review summarizes recent advances in photoinduced copper-catalyzed enantioselective coupling reactions, and discusses the mechanistic aspects.

Photoinduced copper-catalyzed enantioselective coupling reactions.

Recent developments in mass spectrometry (MS) analyses have seen a concerted effort to reduce the complexity of analytical workflows through the simplification (or removal) of sample preparation and the shortening of run‐to‐run analysis times. Ambient ionization mass spectrometry (AIMS) is an exemplar MS‐based technology that has swiftly developed into a popular and powerful tool in analytical science. This increase in interest and demonstrable applications is down to its capacity to enable the rapid analysis of a diverse range of samples, typically in their native state or following a minimalistic sample preparation approach. The field of AIMS is constantly improving and expanding, with developments of powerful and novel techniques, improvements to existing instrumentation, and exciting new applications added with each year that passes. This annual review provides an overview of applications of AIMS techniques over the past year (2020), with a particular focus on the application of AIMS in a number of key fields of research including biomedical sciences, forensics and security, food sciences, the environment, and chemical synthesis. Novel ambient ionization techniques are introduced, including picolitre pressure‐probe electrospray ionization and fiber spray ionization, in addition to modifications and improvements to existing techniques such as hand‐held devices for ease of use, and USB‐powered ion sources for on‐site analysis. In all, the information provided in this review supports the view that AIMS has become a leading approach in MS‐based analyses and that improvements to existing methods, alongside the development of novel approaches, will continue across the foreseeable future.

Applications of ambient ionization mass spectrometry in 2020: An annual review

Thianthrenium-Enabled Phosphorylation of Aryl C-H Bonds via Electron Donor-Acceptor Complex Photoactivation.

The development of a mild and general method for C(sp3)-H functionalization of cyclic amines has been an ongoing challenge. In this work, we described the copper-catalyzed enantioselective C(sp3)-H alkynylation of unactivated cyclic 2-iodo-benzamide under photo-irradiation by intramolecular 1,5-hydrogen atom transfer (HAT). The employment of a new bisoxazoline diphenylamine ligand, in conjunction with 1,1'-bi-2-naphthol, which significantly improved the reduction potential of the copper complex, was the key to success of this chemistry. Mechanistic and computational studies supported that the new copper complex served dual role as a photoredox and coupling catalyst, the reaction went through a radical process, and the intramolecular 1,5-HAT process was involved in the rate-limiting step. Apart from broad substrate scope including unprecedented benzocyclic amines, this method also showed excellent diastereoselectivity in 2-monosubstituted cyclic amines via substrate control.

Photoinduced Copper-Catalyzed Asymmetric C(sp3)-H Alkynylation of Cyclic Amines by Intramolecular 1,5-Hydrogen Atom Transfer.

Great success in synthetic chemistry is motivated by the development of novel and reactive linchpins for carbon-carbon and carbon-heteroatom bond formation reactions, which has dramatically altered chemists' approach to building molecules. Herein, we report the ready synthesis of aryl sulfonium salts, a versatile electrophilic linchpin, via a novel Cu-mediated thianthrenation and phenoxathiination of commercially available arylborons with thianthrene and phenoxathiine, providing a series of aryl sulfonium salts in high efficiency. More importantly, by leveraging the sequential Ir-catalyzed C-H borylation and Cu-mediated thianthrenation of arylborons, the formal thianthrenation of arenes is also achieved. The Ir-catalyzed C-H borylation with undirected arenes normally occurred at the less steric hindrance position, thus providing a complementary method for thianthrenation of arenes in comparison with electrophilic thianthrenation. This process is capable of late-stage functionalization of a series of pharmaceuticals, which might find wide synthetic applications in both industry and academic sectors.

Cu-Mediated Thianthrenation and Phenoxathiination of Arylborons.

The nickel-catalyzed cross-electrophile coupling of 1,2,3-benzotriazin-4(3H)-ones with aryl bromides to generate a diverse array of ortho-arylated benzamide derivatives has been developed. The reaction displayed good functional group tolerance with Zn as the reductant. The key to this transformation is the ring opening of benzotriazinones, which undergo a denitrogenative process to obtain various benzamide derivatives (29 examples, 42-93% yield). The scalability of this transformation was demonstrated.

Nickel-Catalyzed Cross-Electrophile Coupling of 1,2,3-Benzotriazin-4(3H)-ones with Aryl Bromides.

Transition-metal clusters have attracted great attention in catalysis due to their unique reactivity and electronic properties, especially for novel substrate binding and activation modes at the bridging coordination sites of metal clusters. Although palladium complexes have demonstrated outstanding catalytic performance in various transformations, the catalytic behaviors of polynuclear palladium clusters in many important synthetic methodologies remain much less explored so far. Herein, we disclose the use of an atomically defined tri-nuclear palladium (Pd3Cl) species as a catalyst precursor in Ag(I)-assisted direct C-H arylation with aryl iodides under mild conditions. This catalyst system leads to the formation of synthetically important biaryls in good yields with high site selectivities without the assistance of directing groups.

Non-directed C-H arylation of electron-deficient arenes by synergistic silver and Pd3 cluster catalysis.

Carbon fiber paper spray ionization mass spectrometry.

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