Visible-Light-Driven Reductive Carboxylation of Benzyl Bromides with Carbon Dioxide Using Formate as Terminal Reductant.

Cheap and available formate can be seen formally as a carbon dioxide radical anion (CO2•-) combined with a hydrogen atom, where the CO2•- is not only a highly active radical but also a very powerful reductant. In this paper, we successfully realized a visible-light-driven carboxylation of benzyl bromides with carbon dioxide to prepare high-value arylacetic acids using potassium formate as a terminal reductant. This reaction is characterized by mild reaction conditions and a wide range of substrates. Moreover, under nitrogen atmosphere, the reaction can also achieve the carboxylation of benzyl bromides utilizing an excess of potassium formate. Mechanistic experiments indicate this carboxylation proceeded through CO2•-, which was generated from the oxidation of 1,4-diazabicyclo[2.2.2]octane with excited photosensitizer Ir(ppy)2(dtbbpy)PF6 in the presence of the potassium formate.

Photoredox-catalyzed fluorodifluoroacetylation of alkenes with FSO2CF2CO2Me and Et3N·3HF.

Photoredox-catalyzed three-component fluorodifluoroacetylation of aromatic alkenes is reported, which features a wide substrate scope and functional group tolerance. An advantage of the reaction is the use of a nucleophilic fluoride source and a general difluoroacetylation reagent for the fluorodifluoroacetylation of alkenes.

MeOTf/KI-catalyzed efficient synthesis of 2-arylnaphthalenes via cyclodimerization of styrene oxides.

The MeOTf/KI-catalyzed synthesis of 2-arylnaphthalene derivatives from aryl ethylene oxides in alcohol under ambient conditions is described. The present protocol has a higher atom efficiency and wider substrate applicability with excellent yields. The reaction proceeded using the aryl ethylene oxides to give 2-arylnaphthalenes either in homo-coupling or in cross-coupling. The reaction could also be carried out at the gram scale in minutes.

Recent advances in nucleophile-triggered CO2-incorporated cyclization leading to heterocycles.

Carbon dioxide (CO2) has emerged as a sustainable, feasible, abundant one-carbon synthon and displays great potential in the synthesis of heterocycles such as lactones, lactams, and 2-oxazolidinones, which are privileged motifs in pharmaceutical chemistry demonstrating bioactivities. Although the fixation of CO2 is restricted due to its thermodynamic stability and kinetic inertness, multiple breakthroughs have been realized in annulation chemistry. This review concentrates on the advances made in the last five years in CO2-incorporated cyclization triggered by N-, O-, and C-nucleophiles. Three transformation modes of CO2 including carboxylative cyclization, carbonylative cyclization, and reductive cyclization have been summarized. Moreover, typical mechanisms and significant applications of these reactions are also described.

α-Methylation of 2-Arylacetonitrile by a Trimethylamine-Borane/CO2 System.

A highly selective monomethylation of 2-arylacetonitrile using CO2 is described. The utilization of trimethylamine-borane facilitates the six-electron reduction of CO2. This reaction is the first selective six-electron reductive functionalization of CO2 faciliated by C(sp3)-H bonds. A variety of 2-arylpropionitrile was obtained in good yields. The reaction could also be applied at the gram scale.

Reduction of CO2 with NaBH4/I2 for the Conversion of Thiophenols to Aryl Methyl Sulfides.

We report a tandem reaction to realize reduction of carbon dioxide with thiophenols to generate aryl methyl sulfides under the NaBH4/I2 system with 18-crown-6 as the solvent. Thiophenols bearing electron-donating and electron-withdrawing groups are feasible in this reaction. Controlled experiment results indicate that 18-crown-6 plays a critical role in six-electron reduction of carbon dioxide.

Substrate-Controlled Transformation of Azobenzenes to Indazoles and Indoles via Rh(III)-Catalysis.

Rh(III)-catalyzed substrate-controlled transformation of azobenzenes to indazoles and 2-acyl (NH) indoles is achieved via C-H functionalization. Generally, good functional groups tolerance, satisfying yields, and excellent regio-selectivity are achieved in this reaction. Mechanistically, the reaction with acrylates undergoes ß-hydride elimination, while the reaction with vinyl ketones or acrylamides undergoes nucleophilic addition. Copper acetate was supposed to play different roles in the ß-hydride elimination to furnish indazoles and nucleophilic addition of C-Rh bond to deliver 2-acyl (NH) indoles.

MeOTf-Mediated Annulation of Alkylnitriles and Arylalkynes Leading to Polysubstituted NH-Pyrroles.

A metal-free multicomponent domino reaction for the highly regioselective synthesis of tetrasubstituted NH-pyrroles from readily available alkylnitriles, arylalkynes, and MeOTf has been developed. A variety of NH-pyrroles were obtained in moderate to good yields under mild conditions. In addition, the reactions using diarylalkynes with electron-rich aryl groups were found to afford isoquinolines.

Conversion of zirconacyclopentadienes into metalloles: Fagan-Nugent reaction and beyond.

Metalloles are derivatives of cyclopentadiene in which the methylene unit is replaced by a heteroatom, such as S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, and so on. Many metallole derivatives have been widely used as photovoltaic cells, organic light emitting diodes (OLEDs), chemical sensors, electrochromic devices, microelectronic actuators, and organic field effect transistors (OFETs). In the meantime, many of them showed promising biological actives. Due to the similarity to cyclopentadiene, the anionic forms of metalloles were also widely explored in coordination chemistry. As a result, development of a general method for the formation of metalloles from available starting materials is highly desired. In this Account, we outline formation of various p-block element metalloles from zirconacyclopentadienes. The zirconacyclopentadienes can be easily prepared from two molecules of alkynes and a low-valent zirconocene species "Cp2Zr(II)" (Cp = cyclopentadienyl). Fagan and Nugent first reported the formation of main group metalloles from zirconacyclopentadiene, which provided a versatile approach for the construction of metalloles, especially for the formation of metalloles in heavier p-block elements. To further expand the substrate scope, a number of stepwise conversions were developed, which involve 1,4-dimetallo- or dihalo-1,3-butadiene as intermediates from zirconacyclopentadienes. Here, four processes are classified based on direct and indirect conversion of zirconacyclopentadienes into metalloles. Direct reaction of zirconacyclopentadienes with element halides afforded heterocycles of main group elements, which provided a versatile method for the synthesis of metalloles. Nonetheless, the reaction scope was restricted to heavier p-block elements such as S, Se, P, As, Sb, Bi, Ge, Sn, Ga, and In. And these reactions usually suffered low yields and long reaction time. Transmetalation of zirconacyclopentadiene with copper chloride greatly enriched the zirconacyclopentadiene chemistry. The synthesis of stannoles and pyrroles from zirconacyclopentadienes has been developed in the presence of CuCl. The direct reaction of the zirconacyclopentadienes with SiCl4 or R2SiCl2 does not give the desired silacyclopendadiene derivatives, even in the presence of CuCl. It can be circumvented by using dilithiated dienes from diiododienes, which are easily prepared by the iodination of zirconacyclopentadienes using CuCl as an additive. Finally, an umpolung strategy, reaction of electrophilic 1,4-diiodo-1,3-butadiene with nucleophilic amine or sulfide reagents, was successfully used in the formation of pyrroles and thiophenes.

MeOTf- and TBD-Mediated Carbonylation of ortho-Arylanilines with CO2 Leading to Phenanthridinones.

Carbonylation of o-arylanilines utilizing CO2 as a carbonyl source for the synthesis of important phenanthridinones with a free (NH)-lactam motif has been described under metal-free condition. A range of o-arylanilines were transformed to the corresponding phenanthridinones in high yields.

Copper-catalyzed carboxylation reactions using carbon dioxide.

Carbon dioxide (CO2), as a waste of manufacture, is a cheap and abundant C1 source. Utilization of CO2 for synthesis of carboxylic acids has been developed. Transition-metal complexes play a key role in catalytic carboxylation reactions employing CO2. This review summarizes recent advances in copper-catalyzed carboxylation reactions using CO2. The contents are arranged based on various substrates: organometallic reagents, aryl iodides, sodium sulfinates, terminal alkynes, arenes, heteroarenes, and unsaturated substrates.

I2-Mediated 2H-indazole synthesis via halogen-bond-assisted benzyl C-H functionalization.

I2-Mediated benzyl C-H functionalization has been developed for the synthesis of 2H-indazoles, which features high efficiency, simple conditions and no need for metals. Mechanistic experiments and DFT calculations have revealed halogen bond assistance and a radical chain process for this reaction. The azo group and the bound iodine cooperate in the hydrogen abstraction step, which circumvents the thermodynamic disfavor of direct hydrogen abstraction by a simple iodine radical.

A concise and efficient synthesis of benzimidazo[1,2-c]quinazolines through CuI-catalyzed intramolecular N-arylations.

A series of functionalized benzimidazo[1,2-c]quinazoline derivatives was obtained in excellent yields under mild conditions through a CuI-catalyzed Ullmann N-arylation starting from easily available starting materials.

Copper-catalyzed domino reactions for the synthesis of cyclic compounds.

Copper-catalyzed domino reactions are one of the most useful strategies for the construction of various cyclic compounds. In this Synopsis, we mainly focus on the latest advances in copper-catalyzed cross-coupling or addition-initiated domino reactions in the synthesis of cyclic compounds, including double alkenylation of N- or S-nucleophiles, alkenylation or alkynlation followed by cyclization of amides or amines, addition and cyclization of heteroallenes affording heterocycles, and coupling and cyclization of 1,3-dicarbonyl compounds toward heterocycles.

Zirconoarylation of alkynes through p-chloranil-promoted reductive elimination of arylzirconates.

A novel method for the zirconoarylation of alkynes was developed. TCQ-promoted reductive elimination of arylzirconate [LiCp2ZrAr(RC≡CR)], which was prepared by the reaction of zirconocene-alkyne complexes with aryllithium compounds, afforded trisubstituted alkenylzirconocenes. This reaction can afford multi-substituted olefins with high stereoselectivity.

Organo-Photoredox Catalyzed C(sp3)-H Bond Arylation of Aliphatic Amides.

A C(sp3)-H bond arylation of aliphatic amides has been achieved via organophotoredox catalysis. The reaction could be realized at room temperature with visible light source and metal-free catalyst. Quinuclidine is employed as an efficient HAT reagent and a range of aliphatic amides is employed as both substrate and solvent in the reaction. This photocatalyzed transformation provides a convenient protocol to afford a board range of N-benzyl amides.

Cu-catalyzed synthesis of diaryl thioethers and S-cycles by reaction of aryl iodides with carbon disulfide in the presence of DBU.

Diaryl thioethers and S-cycles were obtained on the basis of the copper-catalyzed reaction of carbon disulfide and aryl iodides in the presence of DBU. This reaction enables the one-pot synthesis of diaryl thioethers by employing cheap, available, and easy-to-handle carbon disulfide with aryl iodides. The reaction was successfully employed in the construction of sulfur-containing cyclic molecules.

Direct vicinal disubstitution of diaryliodonium salts by pyridine N-oxides and N-amidates by a 1,3-radical rearrangement.

Paired off: The title reaction leads to a series of o-pyridinium phenols (1) and anilines (2). The experimental and computational studies indicate that the key step involves homolytic cleavage to give a radical pair, which undergoes solvent-cage recombination to give the product.

CO2-Promoted and Nickel-Catalyzed Direct Hydroallylation of Terminal Alkynes with Allylic Alcohols: Access to 1,4-Dienes.

CO2-promoted and Ni-catalyzed direct hydroallylation of terminal alkynes with allylic alcohols has been achieved. Various 1,4-dienes could be synthesized in good yield with excellent Markovnikov selectivity for alkyl- and aryl-substituted terminal alkynes under mild reaction conditions. A gram-scale reaction gives considerable yield. Preliminary mechanistic studies support the reaction pathway through sequential carboxylation/allylnickelation/lithium bicarbonate nickelation/transmetalation in the hydroallylation of alkynes with allylic alcohols in the presence of CO2.

Nickel-Catalyzed Reductive Allylation of Aldehydes with Allylic Alcohols in the Presence of CO2.

CO2-assisted and Ni-catalyzed direct reductive allylation of aldehydes utilizing allylic alcohols as allylic precursor has been reported. Various homoallyl alcohols could be synthesized in excellent yield with enhanced regioselectivity and stereoselectivity for alkyl- and aryl-substituted aldehydes under mild conditions. For different substrates, proper collocation of the catalytic precursor and ligand is crucial. Preliminary mechanistic studies supported the reaction pathway through a sequential allyl hydrocarbonate formation/allylnickelation/coordination insertion process by the Ni(I)/Ni(III) catalytic cycle, which has been proven by cyclic voltammetry analysis.