Photocatalytic Atom‐Transfer Radical Addition of Activated Chlorides to Alkenes

A General Platform for Copper-Catalyzed Atom Transfer Radical Addition with Electron-Deficient Olefins

We disclose a broad platform for copper-catalyzed atom transfer radical addition (ATRA) of electron-deficient olefins. Catalytic Cu(dtbbpy)2(OTf)2 enables radical addition of electron-deficient alkyl halides to acrylates, acrylamides, and vinyl sulfones in fair to excellent yields. The resultant ATRA products can be used in a variety of telescoped reactions, including substitution with basic amine nucleophiles to afford α-amino esters. The synthetic utility of the products is further demonstrated through derivatization to give substituted cyclopropanes and a γ,δ-unsaturated ester.

Dual ligand-enabled iron and halogen-containing carboxylate-based photocatalysis for chloro/fluoro-polyhaloalkylation of alkenes

Herein, we demonstrate a practical dual ligand-enabled iron photocatalysis paradigm-converting all kinds of halogen-containing carboxylates (C n X m COO-, X: F, Cl, Br) into C n X m radicals for the valuable chloro/fluoro-polyhaloalkylation of non-activated alkenes with easily available trichloroacetonitrile/Selectfluor as the electrophilic halogenation reagent. The modular in situ assembly of the effective iron and C n X m COO--based light-harvesting species using the two ligands-OMe/CF3-substituted bipyridine and acetonitrile/trichloroacetonitrile is evidenced by detailed mechanistic studies. The late-stage modification, low loading amount of iron (TON: 257) and feasible gram-scale synthesis show the utility of this protocol. We thus anticipate that the dual ligand-enabled iron photocatalysis paradigm may facilitate activation and transformation of inert bulk chemicals.

Copper-catalysed bromine atom transfer cyclisation in SDS micelles

The atom transfer radical cyclisation (ATRC) of non-activated alkyl bromides was realized under blue light irradiation in carbonate-buffered aqueous SDS solution using a catalytic system of CuBr2, Me6-TREN and ascorbic acid. The beneficial effect of SDS micelles can be accounted for by the activation of the C-Br bond as well as by the suppression of competitive reductive cyclisation.

Sodium Hypophosphite as a Halogen Atom Transfer (XAT) Agent under Photocatalytic Conditions

The ability of sodium hypophosphite to generate the phosphorus-centered radical, which can activate the carbon-halogen bond via the halogen atom transfer (XAT) is described. The hydroalkylation of nonactivated alkenes with methyl bromoacetate was performed using sodium hypophosphite as reducing agent under photocatalytic conditions. The key phosphorus centered radical is formed from the hypophosphite anion by hydrogen atom abstraction.

Modulating β-Keto-enamine-Based Covalent Organic Frameworks for Photocatalytic Atom-Transfer Radical Addition Reaction

The atom-transfer radical addition (ATRA) reaction simultaneously forges carbon-carbon and carbon-halogen bonds. However, frequently-used photosensitizers such as precious transition metal complexes, or organic dyes have limitations in terms of their potential toxicity and recyclability. Three β-ketoenamine-linked covalent organic frameworks (COFs) from 1,3,5-triformylphloroglucinol and 1,4-phenylenediamines with variable transient photocurrent and photocatalytic activity have been prepared. A COF bearing electron-deficient Cl atoms displayed the highest photocatalytic activity toward the ATRA reaction of polyhalogenated alkanes to give halogenated olefins under visible light at room temperature. This heterogeneous photocatalyst exhibited good functional group tolerance and could be recycled without significant loss of activity.

Catalytic Photoredox Carbobromination of Unactivated Alkenes with α-Bromocarbonyls via the Mechanistically Distinct Radical-Addition Radical-Pairing Pathway

We disclose a catalytic photoredox carbobromination of unactivated alkenes with α-bromocarbonyl compounds under a blue LED light. The reaction proceeds with α-bromoesters, α-bromonitriles and α-bromo-γ-lactones along with terminal and 1,2-disubstituted internal alkenes. Reactions with indenes and 1,1-disubstituted alkenes generate alkylated alkenes. Mechanistic studies by product selectivity and three-way competitive crossover experiments suggest that the reaction operates by a radical-addition radical-pairing (RARP) mechanism. The catalytic turnover is achieved by a single electron reduction of PC•+ by Br- (or Br3 -), rather than by alkyl radical (R•), and the product is generated by the pairing of Br• (or Br2•-) and R•, instead of the combination of Br- and a carbocation (R+).

Copper-mediated bromine atom transfer radical cyclisation of unactivated alkyl bromides

The atom transfer radical cyclisation of unactivated alkyl bromides was realized by using a catalytic system of CuBr and Me₆-TREN. This protocol is applicable to the preparation of five-membered rings from unsaturated primary and secondary bromides.

Dual Photoredox/Copper Catalyzed Fluoroalkylative Alkene Difunctionalization

A method for performing radical difunctionalization of alkenes using fluorinated halides and a nucleophilic component (thiolate and iodide anions) is described. These nucleophilic fragments serve as redox active groups for performing subsequent transformations. The difunctionalization reaction involves photoredox generation of fluorinated radicals, their addition to the double bond followed by copper-promoted C-S or C-I bond formation. The primary products can be further coupled with silyl enol ethers, α-(trifluoromethyl)styrenes, and trimethylsilyl cyanide affording a variety of fluorinated compounds.

Aminals as powerful XAT-reagents: activation of fluorinated alkyl chlorides

Readily available 1,3,5-trimethyl-1,3,5-triazinane serves as an efficient reagent for halogen atom transfer. Under photocatalytic conditions, the triazinane generates an α-aminoalkyl radical, which can activate the C-Cl bond of fluorinated alkyl chlorides. The hydrofluoroalkylation reaction between fluorinated alkyl chlorides and alkenes is described. The efficiency of the diamino-substituted radical derived from the triazinane is associated with stereoelectronic effects defined by a six-membered cycle forcing the anti-periplanar arrangement of the radical orbital and lone pairs of adjacent nitrogen atoms.

Syn-Selective Chlorosulfonylation of Alkynes via a Copper-Powder-Initiated Atom Transfer Radical Addition Reaction and Mechanistic Studies

Copper-powder-catalyzed syn-selective chlorosulfonylation of readily available alkynes by an atom transfer radical addition (ATAR) process has been developed, providing straightforward access to a broad range of (Z)-β-chlorovinylsulfones in good yields under mild conditions. In addition, this method is ligand-free and features excellent stereoselectivity and high atom economy. Moreover, the product was obtained without an apparent loss of yield when the reaction was performed on the gram scale at a low catalyst loading. In this reaction, the copper powder not only acts as a sulfone radical initiator but also produces the catalytically active CuCl species. Mechanistic investigations and DFT calculation studies revealed that the stereoselectivity is controlled by the thermodynamic stabilities of the in situ-generated cyclic alkenyl Cu^II complex intermediate, which can serve as a chlorine atom transfer agent.

Organophotoredox-Catalyzed Reductive Tetrafluoroalkylation of Alkenes

A method for the hydroperfluoroalkylation of alkenes with 1,2-dibromotetrafluoroethane leading to tetrafluorinated bromides is described. The reaction is conveniently performed under blue light irradiation using an organic photocatalyst and ascorbic acid as a reducing agent. Primary products can be further functionalized via radical pathways affording various tetrafluorinated compounds.