Functionalisation of ethereal-based saturated heterocycles with concomitant aerobic C–H activation and C–C bond formation

Herein we disclose a novel method for the aerobic C–H activation of ethereal-based heterocycles to generate various α-functionalised building blocks.

Visible Light-Enabled sp3 -C-H Functionalization with Chloro- and Bromoalkynes: Chemoselective Route to Vinylchlorides or Alkynes

An unprecedented direct atom-economic chemo- and regioselective hydroalkylation of chloroalkynes and an sp3 -C-H alkynylation of bromoalkynes was achieved. The reaction partners are unfunctionalized ethers, alcohols, amides, and even non-activated hydrocarbons. We found that a household fluorescent bulb was able to excite a diaryl ketone, which then selectively abstracts a H-atom from an sp3 -C-H bond. The product of a formal alkyne insertion into the sp3 -C-H bond was obtained with chloroalkynes, providing valuable vinyl chlorides. The photo-organocatalytic hydrogen atom transfer strategy gives rise to a broad range of diversely functionalized olefins. When bromoalkynes are applied in the presence of a base, a chemoselectivity switch to an alkynylation is observed. This reaction can even be performed for the alkynylation of unactivated sp3 -C-H bonds, in this case with a preference of the more substituted carbon. Accompanying quantum chemical calculations indicate a vinyl radical intermediate with pronounced linear coordination of the carbon radical center, thus enabling the formation of both diastereoisomers after H-atom abstraction, suggesting that the (Z)-diastereoisomer is preferred, which supports the experimentally observed (E/Z)-distribution.

Visible Light-Induced Direct S0 → Tn Transition of Benzophenone Promotes C(sp3)-H Alkynylation of Ethers and Amides

Benzophenone has an S₀ → S₁ absorption band at 365 nm. However, the rarely reported S₀ → T_n transition occurs upon irradiation at longer wavelengths. Herein, we employed benzophenone as a catalyst and exploited its S₀ → T_n transition in C(sp³)-H alkynylations with hypervalent iodine reagents. The selective benzophenone excitation prevented alkynylating reagent decomposition, enabling the reaction to proceed under mild conditions. The reaction mechanism was investigated by spectroscopic and computational studies.

Synthetic Applications of Sulfonium Salts

This minireview aims to cover the developments over the past two decades in the chemistry of sulfonium salts. Specifically, insight is provided into the synthetic strategies available for the preparation of these compounds, the different reactivity patterns that are expected depending on their structural features or the reaction conditions applied, and the diversity of organic scaffolds that can thereby be synthesized. Additionally, the pros and cons derived from the use of sulfonium salts are presented and critically compared, when possible, in relation to reagents not based on sulfur but depicting similar reactivity.

Copper-Catalyzed Disulfonation of Terminal Alkynes with Sodium Arylsulfinates

A novel and direct disulfonylation reaction of commercially available terminal alkynes under copper(I)/bromodifluoroacetate cocatalyst has been realized. This protocol provides a facile and practical pathway to selectively access (E)-1,2-disulfonylethenes, in which features good functional group compatibility, easily available starting materials, and excellent stereoselectivity with good yields.

Blue light photoredox-catalysed acetalation of alkynyl bromides

Herein, we report an organo-photoredox-based protocol using 2,2-diethoxyacetic acid as the acetal source to achieve acetalation of alkynyl bromides to afford various alkynyl acetal products. In addition to arylethynyl bromides, substrates bearing heteroaryl rings (thiophene, pyridine, and indole) smoothly gave the corresponding acetalation products. This mild protocol has potential utility for the synthesis of aldehydes by further protonization.

Base-promoted domino radical cyclization of 1,6-enynes

A good regioselective, high atom-economical and transition-metal-free method for the synthesis of α-functionalized ether derivatives via the domino radical cyclization of 1,6-enynes is described. A series of α-functionalized ether derivatives could be easily obtained in good yields with wide functional group tolerance by using less toxic and inexpensive Cs₂CO₃ as the base. The control experiment results show that the reaction involves a radical process. This strategy provides a regioselective way toward the formation of dual C-C bonds in one step.

Visible light promoted coupling of alkynyl bromides and Hantzsch esters for the synthesis of internal alkynes

A metal-free visible light promoted C(sp3)-C(sp) coupling reaction of alkynyl bromides and Hantzsch esters was developed, giving internal alkynes with primary, secondary, tertiary alkyl or other functional groups in good to high yields.

5-(Alkynyl)dibenzothiophenium Triflates: Sulfur-Based Reagents for Electrophilic Alkynylation

The synthesis of a series of 5-(alkynyl) dibenzothiophenium triflates, prepared from dibenzo[b,d]thiophene 5-oxide and the corresponding trimethylsilyl-substituted alkynes is reported. Their structures were determined by X-ray crystallography, and their reactivities as electrophilic alkynylation reagents evaluated. Their broad substrate scope and functional-group tolerance illustrate their potential to become an alternative to the broadly used EBX reagents. Isotope labeling studies reveal that alkynyldibenzothiophenium salts may undergo attack by nucleophiles at either the α- or β-carbon atom depending on the nature of their substitution pattern. Subsequent elimination of the dibenzothiophene unit and 1,2-migration of one of the groups (in case of β-attack) affords the desired alkynes.

Copper-catalyzed oxidative intermolecular 1,2-alkylarylation of styrenes with ethers and indoles

A new copper-catalyzed oxidative three-component intermolecular 1,2-alkylarylation of styrenes with ethers and indoles is disclosed.

Acid-mediated sulfonylation of arylethynylene bromides with sodium arylsulfinates: synthesis of (E)-1,2-bis(arylsulfonyl)ethylenes and arylacetylenic sulfones

A solvent-dependent sulfonylation of arylethynylene bromides with sodium arylsulfinates to afford (E)-1,2-bis(arylsulfonyl)ethylenes and arylacetylenic sulfones is described.

Dimethylzinc-Initiated Radical Coupling of β-Bromostyrenes with Ethers and Amines

A new coupling reaction has been developed in which β-bromostyrenes react with ethers and tertiary amines to introduce the styryl group in the α-position. The transformation is mediated by Me2 Zn/O2 with 10 % MnCl2 and is believed to proceed by a radical addition-elimination mechanism. The ether and the amine are employed as solvent and the coupling takes place through the most stable α radical for unsymmetrical substrates. The products are obtained in moderate to good yields as the pure E isomers. The coupling can be achieved with a range of smaller cyclic and acyclic ethers/amines as well as various substituted β-bromostyrenes.

Visible-Light-Promoted Vinylation of Tetrahydrofuran with Alkynes through Direct C-H Bond Functionalization

Mild and direct C-H bond functionalizations and vinylations of tetrahydrofuran with alkynes have been accomplished through visible light photocatalysis, yielding a range of vinyl tetrahydrofurans under the synergistic actions of organic dye-type photocatalyst eosin Y, tert-butyl hydroperoxide (t-BuOOH), and a 45 W household lightbulb. A significant kinetic isotope effect (KIE) was recorded, which helps shed light on the mechanistic course.