N-Methylphenothiazine S-Oxide Enabled Oxidative C(sp2)-C(sp2) Coupling of Boronic Acids with Organolithiums via Phenothiaziniums

Phenothiazine Sulfoxides as Active Photocatalysts for the Synthesis of γ-Lactones

N-substituted phenothiazines are prominent and highly effective organic photoredox catalysts, particularly known for their strong reducing capabilities. Despite their wide utility, the closely related phenothiazine sulfoxides, which easily form upon oxidation, have been largely overlooked and have not been explored in the context of photocatalysis. Herein, we describe the discovery and application of N-phenylphenothiazine sulfoxide as a photocatalyst for the reductive activation of cyclic malonyl peroxides, giving access to complex γ-lactones starting from simple olefins. Detailed mechanistic studies were carried out to better understand the in situ formation of the active catalyst species from a commercial precursor, as well as the catalyst species interconversion and the photocatalytic mechanism for the formation of γ-lactone products. Specifically, we employed a broad range of mechanistic tools, including time-resolved spectroscopy, spectroelectrochemistry, transient UV-vis absorption spectroscopy, cyclic voltammetry, isotopic labeling, radical trapping experiments, NMR spectroscopy, and density functional theory (DFT) calculations. The synthetic utility of this protocol is demonstrated in a substrate scope study, highlighting the facile access to complex spirocyclic γ-lactones, which are widely recognized for their biological importance.

Photoredox Catalysts Based on N-(Hexyl)benzothioxanthene-3,4-dicarboximide for Photopolymerization and 3D Printing Under Visible Light

Photoredox catalytic systems are widely used in free radical polymerization as an important photoinitiating approach. However, many reported photoredox catalytic systems are limited by their low stabilities, high excitation powers, and low initiating efficiencies upon excitation in the visible region. Therefore, it is still a great challenge to develop efficient photoinitiating systems for photopolymerization under visible light. In this work, three new effective photosensitizers from N-(hexyl)benzothioxanthene-3,4-dicarboximide derivatives, namely 2-hexyl-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione (BTXI), 5-bromo-2-hexyl-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione (BTXI-Br) and 2-hexyl-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione 6,6-dioxide (BTXIO), were designed by density functional theory calculation and synthesized as photoredox catalysts for visible light induced photopolymerization. When combined with initiators such as oxidants, that is, bis(4-tert-butylphenyl)iodonium hexafluorophosphate or sulfonium salts (i.e., thianthrenium salts, phenoxathiinium salt, phenothiazinium salt, dibenzothiophenium salt) and the reductant ethyl dimethylaminobenzoate to form three-component initiating systems, they showed good to high performance in visible light photo polymerizations with LED@405 nm and LED@450 nm. In addition, these photoinitiating systems enable the successful digital light processing and direct laser writing of 3D structures with high resolution, demonstrating a promising strategy for 3D printing applications.

Stereoretentive Conversion to C-Glycosides from S-Glycosides via Ligand-Coupling on Sulfur(IV)

A novel strategy is reported for the stereoselective synthesis of C(sp2)-C(sp3) C-glycosides, which converts heteroaryl S-glycosides into heteroaryl C-glycosides with retention of configuration through a sequential process involving oxidation and Grignard reagent attack. The new method involves the generation of a S(IV) intermediate, followed by ligand coupling of the glycosyl and heteroaryl groups to yield heteroaryl C-glycosides. The diverse heteroaryl C-glycosides were achieved with good efficiency.

Alkyl sulfinates as cross-coupling partners for programmable and stereospecific installation of C(sp3) bioisosteres

In recent years, a variety of cycloalkyl groups with quaternary carbons, in particular cyclopropyl and cyclobutyl trifluoromethyl groups, have emerged as promising bioisosteres in drug-like molecules. The modular installation of such bioisosteres remains challenging to synthetic chemists. Alkyl sulfinate reagents have been developed as radical precursors to prepare functionalized heterocycles with the desired alkyl bioisosteres. However, the innate (radical) reactivity of this transformation poses reactivity and regioselectivity challenges for the functionalization of any aromatic or heteroaromatic scaffold. Here we showcase the ability of alkyl sulfinates to engage in sulfurane-mediated C(sp3)-C(sp2) cross-coupling, thereby allowing for programmable and stereospecific installation of these alkyl bioisosteres. The ability of this method to simplify retrosynthetic analysis is exemplified by the improved synthesis of multiple medicinally relevant scaffolds. Experimental studies and theoretical calculations for the mechanism of this sulfur chemistry reveal a ligand-coupling trend under alkyl Grignard activation via the sulfurane intermediate, stabilized by solvation of tetrahydrofuran.

Transition-Metal-Free O-Arylation of Alcohols and Phenols with S-Arylphenothiaziniums

Herein, we report the transition-metal-free O-arylation of alcohols and phenols with S-arylphenothiaziniums, which can be easily synthesized from boronic acids. Aryl substituents derived from arylboronic acids were selectively introduced into the hydroxy groups in alcohols and phenols, and a variety of aryl ethers were synthesized. This selectivity is supported by theoretical calculations.

Visible-Light-Initiated Catalyst-Free Trifluoromethylselenolation of Arylsulfonium Salts with [Me4N][SeCF3]

The redox potential gap between arylsulfonium salt and [Me4N][SeCF3] has been clearly disclosed by CV measurements. Construction of carbon-selenium bond by overcoming this gap without using catalysts and additives is...