A visible light-driven photoredox-catalyzed and copper(II)-assisted three-component radical addition/hydroxylation reaction of alkenes, sulfur ylides, and water is reported. This process shows broad substrate scope and high functional group tolerance, with respect to both readily available sulfur ylides and alkenes, providing high-yielding and practical access to valuable γ-hydroxy carbonyl compounds. Key to the success of the reaction is the controlled generation of α-carbonyl carbon radicals from sulfur ylides via sulfonium salts by a visible-light-driven proton-coupled electron transfer (PCET) strategy in a mixture of 2,2,2-trifluoroethanol/CH2Cl2. Addition of Cu(TFA)2·H2O helps to accelerate the radical-cation crossover to improve the reaction efficiency. Mechanistic studies suggest that the hydroxy moiety in the products stems from water. This study also builds up a platform for further investigation into the radical synthetic chemistry of sulfur ylides.
A visible-light-driven and room temperature photo-Wolff-Kischner reaction of sulfur ylides and Ntosylhydrazones has been developed for the first time to provide modular access to alkene synthesis. The high functional group tolerance and broad substrate scope were demonstrated by more than 60 examples. Both E-and Zolefinic stereochemistry in the products could be controlled with excellent stereoselectivity. A series of mechanistic studies support that the reaction should proceed through a radical-carbanion crossover pathway, specifically involving addition of photo-generated sulfur ylide radical cations to N-tosylhydrazones to form carbanions and subsequent Wolff-Kischner process.Sulfur ylides, firstly reported in 1930 by Ingold and Jessop, [1] are a class of zwitterionic compounds characterized by a carbanion and a neighboring positively charged sulfur atom. Since the pioneering establishment of the synthetic prowess of these compounds in the Johnson-Corey-Chaykovsky epoxidation and cyclopropanation reactions, [2] the application of sulfur ylides as versatile one-carbon synthons in organic chemistry has undergone tremendous growth over the past decades, owing to their inherent unique chemical properties. [3] Many of these classical reactions have become textbook knowledge. Surprisingly, in sharp contrast to these ionic chemistry, the use of sulfur ylide radical cations, generated from the corresponding sulfur ylides by one-electron oxidation, in radical synthetic chemistry has been largely ignored. [4,5] The main reason may be attributed to the fact that the chemical properties of sulfur ylide radical cation depend primarily on the nature of the substituents on the sulfur, which often results in the formation of complex mixtures through various decomposition pathways. For example, Schuster was the first to investigate the reactivity modes of phenacyl sulfur ylides under photosensitization (λ > 400 nm) with 9,10-dicyanoanthracene (DCA) as the photocatalyst. [6] It was found that the outcome of the reactions depends remarkably on the structures of sulfur ylides because sulfur ylide radical cations were involved as the same intermediates (Scheme 1A). For instance, the DCA-sensitized photolysis of dimethylsulfur phenacylide (DMSY) led to formation of trans-[a] P.
A selective three-component 1,4-difluoroalkylesterification of 1-aryl-1,3-dienes enabled by dual photoredox and copper catalysis is described. This protocol uses commercially available CF2-reagents as radical precursors and carboxylic acids as oxygen-based nucleophiles, providing access to difluoroalkylated allylic esters. This protocol could be extended to intramolecular two-component 1,4-difluoroalkylesterification to access 3-substituted benzobutyrolactones. Preliminary mechanistic studies support a radical process.
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