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.

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.

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.

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.

Cobalt-Catalyzed Highly Regioselective Three-Component Arylcarboxylation of Acrylate with Aryl Bromides and Carbon Dioxide.

Cobalt-catalyzed regioselective three-component arylcarboxylation of acrylate with aryl bromides and carbon dioxide has been developed. The reaction is carried out by using cobalt chloride as a precatalyst and zinc powder as a reducing reagent under CO2 (1 atm) at 40 °C. A range of aryl bromides are used for this reaction, leading to a series of valuable carboxylic acids with high regioselectivity and functional-group compatibility. Mechanistic experiments and DFT calculations indicate that this arylcarboxylation reaction involves the reaction of CO2 with a cobalt enolate intermediate to form the C-C bond.

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.

Visible-Light-Induced Catalyst-Free Carboxylation of Acylsilanes with Carbon Dioxide.

Intermolecular carbon-carbon bond formation between acylsilanes and carbon dioxide (CO2) was achieved by photoirradiation under catalyst-free conditions. In this reaction, siloxycarbenes generated by photoisomerization of the acylsilanes added to the C═O bond of CO2 to give α-ketocarboxylates, which underwent hydrolysis to afford α-ketocarboxylic derivatives in good yields. Control experiments suggest that the generated siloxycarbene is likely to be from the singlet state (S1) of the acylsilane and the addition to CO2 is not in a concerted manner.

Light-Mediated Carboxylation Using Carbon Dioxide.

Carbon dioxide is a green and sustainable one-carbon source, which could be utilized in the production of various fine chemicals. In recent studies, the light-mediated carboxylation employing CO2 has received considerable attention. The photocarboxylation of substrates with CO2 to build novel C-C bonds is introduced in this Minireview. The article is arranged based on the light-driven reactive intermediates, including CO2 radical anion, substrate radical anions, carbanions, and M-C species. Most of the cases are under the topic of photoredox catalysis, with single electron transfer as the main driving force. Some non-catalytic examples are also discussed to provide more mechanistic insights.

Concise and Efficient Synthesis of Indole-Indolone Scaffolds through MeOTf-Induced Annulation of N-(2-Cyanoaryl)indoles.

MeOTf-triggered annulation of N-(2-cyanoaryl)indoles to provide indolo[1,2-a]indol-10-imines and the corresponding indolo[1,2-a]indol-10-ones have been described under metal-free conditions. A variety of functional groups are tolerated in their scaffolds with good to excellent yields. The reaction could be carried to gram scale.

Visible-light-triggered direct keto-difluoroacetylation of styrenes with (fluorosulfonyl)difluoroacetate and dimethyl sulfoxide leads to α-difluoroacetylated ketones.

Photoredox-catalyzed direct keto-difluoroacetylation of styrenes with (fluorosulfonyl)difluoroacetate and dimethyl sulfoxide as an oxidant is disclosed. A variety of α-difluoroacetylated ketones bearing functional groups with good yields are obtained using fac-Ir(ppy)3 as a photocatalyst under visible light irradiation.

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.

Potassium complexes containing bidentate pyrrole ligands: synthesis, structures, and catalytic activity for the cyclotrimerization of isocyanates.

Bidentate pyrrolyl ligands, 2-(t-butyliminomethyl)pyrrole and 2-(t-butylaminomethyl)pyrrole, reacted with KH to give complexes [C4H3N(2-CH[double bond, length as m-dash]NtBu)K(THF)]n (1) and [C4H3N(2-CH2NHtBu)K]n (2), respectively. Each has been characterized by C, H and N microanalysis, NMR spectroscopy, and single crystal X-ray structural analysis. The X-ray structure of complex 1 (n≥ 1) demonstrated that it exists as a 1D zig-zag coordination polymer in the solid state. Conversely, the structure of complex 2 (n≥ 1) showed that it is a 2D supramolecular network. They proved to be an effective class of catalysts for cyclotrimerization of isocyanate in excellent yield under mild conditions.

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.

ROTf-induced annulation of heteroatom reagents and unsaturated substrates leading to cyclic compounds.

The development of metal-free organic reactions is one of the hotspots in the synthesis of cyclic compounds. ROTf (alkyl trifluoromethanesulfonates), due to their good electrophilicity, are powerful alkylating reagents at heteroatoms such as nitrogen, oxygen, sulfur and phosphorus to induce an electrophilic centre for carbon-carbon or carbon-heteroatom bond formation. Inspired by this chemistry, a variety of research concentrating on heterocycles synthesis has been carried out. In this review, we mainly summarize the ROTf-induced annulation of heteroatom reagents such as nitriles, carbodiimides, azobenzenes, isothiocyanates, aldehydes, isocyanates and phosphaalkene with themselves or alkynes to afford cyclic compounds.

Reduction of CO2 into Methylene Coupled with the Formation of C-S Bonds under NaBH4/I2 System.

A selective four-electron reduction of CO2 with thiophenol using NaBH4 as a reductant is described to access dithioacetals. This reaction provides a novel synthetic method for the highly selective conversion of CO2 into methylene, and a new access to molecular structures via formation of C-S bonds using CO2 as the C1 source.

Nickel-Catalyzed Arylative Carboxylation of Alkynes with Arylmagnesium Reagents and Carbon Dioxide Leading to Trisubstituted Acrylic Acids.

Nickel-catalyzed arylcarboxylation of alkynes with arylmagnesium reagents and carbon dioxide (CO2, 1 atm) was realized in one pot. Various trisubstituted acrylic acids within an aryl group at the ß-position have been prepared efficiently with good regioselectivity under mild conditions. The resulting products could be further transformed to benzoannelated cycles retaining CO2 as a one-carbon synthon.

1,4-Dioxane-Tuned Catalyst-Free Methylation of Amines by CO2 and NaBH4.

A catalyst-free reductive functionalization of CO2 with amines and NaBH4 was developed. The N-methylation of amines was carried out with CO2 as a C1 building block and 1,4-dioxane as the solvent. Notably, the six-electron reduction of CO2 to form the methyl group occurred simultaneously with formation of the C-N bond to give the N-methylated amine.

Triflates-Triggered Intermolecular Cyclization of Carbodiimides Leading to 2-Aminoquinazolinone and 2,4-Diaminoquinazoline Derivatives.

A triflate-triggered intermolecular cyclization of carbodiimides to provide 2-amino-4-imino-quinazolines, which could be easily transformed to 2-aminoquinazolinones and 2,4-diaminoquinazolines in high yield, has been reported. A variety of functional groups are tolerated in the 2-amino-4-iminoquinazoline scaffolds.