Transition Metal-free Conditions Research Articles

Photoinduced Asymmetric Formal Siloxycarbene Insertion into sp3 C-H Bonds Enabled by Chiral Phosphoric Acid.

We disclosed herein an enantioselective formal siloxycarbene insertion reaction enabled by chiral phosphoric acid and blue LED irradiation. This is the first time the asymmetric siloxycarbene insertion into an sp3 C-H bond under transition-metal free conditions has been realized. The reaction features good isolated yields (up to 92%), high enantioselectivity (up to 99:1 er), mild reaction conditions, and good compatibility. Moreover, this method also provides a green and efficient method to construct a chiral quaternary carbon center.

Acid‐Mediated Condensation of 2‐Aminoaryl Cyclopropyl Ketones with Aldehydes for the Synthesis of 2,3‐Dihydrofuro[3,2‐c]quinolines

AbstractA method for the synthesis of 2,3‐dihydrofuro[3,2‐c]quinolines via TfOH or TMSOTf mediated condensation of 2‐aminoaryl cyclopropyl ketones with aldehydes has been developed. The reaction proceeds under transition metal or halogen free conditions, giving access to the target molecules in 27–90% isolated yields. Mechanistic study indicates that the reaction sequence involves imine formation, intramolecular Mannich condensation, Cloke‐Wilson rearrangement, and auto‐oxidation.

Insight into the MOtBu (M=Na, K)-Mediated Dehydrogenation of Dimethylamine-Borane and Transfer Hydrogenation of Nitriles to Primary Amines.

Selective synthesis of primary amines from nitriles is challenging in synthetic chemistry due to the possible en-route generation of various amines and imines. Herein, we report a practical and operationally simple MOtBu-mediated (M=Na, K) transfer hydrogenation of nitriles to the corresponding primary amines with a relatively unexplored sacrificial hydrogen source (dimethylamine borane). The strategy encompasses a broad substrate scope under transition metal-free conditions and does not require any solvent. The mechanistic investigation was performed with the aid of control experiments and spectroscopic studies. The GC analysis of the reaction mixture exhibited the evolution of the H2 gas. Additionally, detailed computational calculations were undertaken to shed light on the possible intermediates and transition states involved during the present protocol.

A four-component protocol for tandem C–S and C–C bond formation to access thienopyrrolediones under transition-metal free conditions

A mild and simple multicomponent procedure has been established for the preparation of substituted thienopyrrolediones. The advantages of this strategy include the use of a cheap, stable and smell-free in nature sulfur source, readily available raw materials, wide substrate scope, easily-handled protocol and a simple metal free procedure.

Selective Ring-Opening Amination of Isochromans and Tetrahydroisoquinolines.

The molecular structure-editing through selective C-C bond cleavage allows for the precise modification of molecular structures and opens up new possibilities in chemical synthesis. By strategically cleaving C-C bonds and editing the molecular structure, more efficient and versatile pathways for the synthesis of complex compounds could be designed, which brings significant implications for drug development and materials science. o-Aminophenethyl alcohols and amines are the essential key motifs in bioactive and functional material molecules. The traditional synthesis of these compounds usually requires multiple steps which could generate inseparable isomers and induce low efficiencies. By leveraging a molecular editing strategy, we herein reported a selective ring-opening amination of isochromans and tetrahydroisoquinolines for the efficient synthesis of o-aminophenethyl alcohols and amines. This innovative chemistry allows for the precise cleavage of C-C bonds under mild transition metal-free conditions. Notably, further synthetic application demonstrated that our method could provide an efficient approach to essential components of diverse bioactive molecules.

Microwave assisted biomass derived carbon aerogel for highly efficient oxytetracycline hydrochloride degradation: Singlet oxygen mechanism and C vacancies accelerated electron transfer

With plant biomass α-cellulose and sodium lignosulfonate as starting materials, biochar aerogel catalysts were prepared with facile microwave assisted pyrolysis in 5 min; and the as-prepared biochars were utilized for efficient peroxymonosulfate (PMS) activation and oxytetracycline hydrochloride (OTC) degradation under transition metal free conditions. It was found that microwave treatment and sodium lignosulfonate were beneficial for thiophene S introduction, C = O groups and C vacancies enrichment, as well as specific surface area increase. The optimal α-cellulose and sodium lignosulfonate derived metal free biochar aerogel catalyst C3S2 displayed excellent ability to promote PMS activation for 99.9 % OTC degradation during 15 min with rate constant up to 0.818 min−1. Through correlation analysis, quenching tests as well as EPR characterization, C = O groups and thiophene S were considered as main sites for PMS activation to produce nonradicals singlet oxygen (1O2) in OTC degradation. Meanwhile, C vacancies enrichment further improved the pollutants degradation by means of electron transfer acceleration. This research demonstrated new insight in metal-free biochar catalyst design for highly efficient contaminants degradation with 1O2 pathway.

Electrochemical bromination of electron-rich compounds employing LiBr as a “Br” source

A direct electrochemical-mediated CH bromination strategy of electron-rich compound is established using LiBr as a “Br” source under transition metal-free and oxidant-free catalysis condition. This method is performed in a green and sustainable system. A wide range of electron-rich compounds were found to be compatible, providing the corresponding products with moderate yields.

Macrocyclization of carbon dioxide with 3-triflyloxybenzynes and tetrahydrofuran: straightforward access to 14-membered macrolactones.

A novel [2+2+5+5] macrocyclization of carbon dioxide with 3-triflyloxybenzynes and tetrahydrofuran has been disclosed for the first time under transition metal-free conditions. The reaction provides a facile method for the synthesis of a rare type of 14-membered macrocyclic lactone, which is potentially useful but difficult to access by existing methods.

Expedient deaminative phosphorylation and sulfonylation of benzylic tertiary amines enabled by difluorocarbene

A deaminative functionalization of aliphatic tertiary amines via the cleavage of C(sp3)–N bonds under transition metal-free conditions was disclosed, which goes through SN2 reaction with various phosphorus and sulfur nucleophiles.

(NH4)2S2O8 promoted tandem radical cyclization of quinazolin-4(3H)-ones with oxamic acids for the construction of fused quinazolinones under metal-free conditions.

A novel cascade radical addition/cyclization reaction of non-activated olefins and oxamic acids has been proposed. Under transition metal-free conditions, 36 quinazolinone derivatives containing an amide moiety were successfully synthesized, with the highest yield being 81%. This method involves the preparation of aminoacyl fused quinazolinone derivatives under mild conditions, offering advantages such as a high yield, a broad substrate compatibility, and a high atom economy.

Organocatalytic access to 3-pyridylphosphonates from vinyl phosphonates and aldehydes.

The preparation of 3-phosphorylated pyridines has remained the most challenging compared to the corresponding 2- or 4-functionalized pyridines. Herein, an unprecedented amino-organocatalyzed direct synthesis of 3-pyridylphosphonates from vinylphosphonates and aldehydes has been achieved. This allows access to a wide range of multi-substituted phosphorylated pyridines in excellent yields under transition metal-free conditions.

Esterification of N-Benzyl-N-t-butoxycarbonyl-amides and Unsaturated Alcohol under Transition Metal-Free Conditions

Esterification of <i>N</i>-Benzyl-<i>N</i>-<i>t</i>-butoxycarbonyl-amides and Unsaturated Alcohol under Transition Metal-Free Conditions

Ts2O mediated deoxygenative C2-dithiocarbamation of quinoline N-oxides with CS2 and amines.

A general, efficient and practical protocol for Ts2O promoted deoxygenative dithiocarbamation of quinoline N-oxides with in situ generated dithiocarbamic acids from CS2 and amines is reported. The reaction proceeded well under transition-metal free conditions to obtain a variety of novel quinoline-dithiocarbamate compounds with wide functional group tolerance and good to high yields.

Iron-Mediated Reductive Amidation of Triazine Esters with Nitroarenes.

A reductive amidation of triazine esters with nitroarenes by using cheap iron as a reducing metal in the presence of TMSCl in DMF was developed. The reactions proceeded efficiently under transition metal-free conditions to give the corresponding amides in moderate to good yields with good functional group compatibility. Preliminary mechanistic investigations indicated that nitrosobenzene, N-phenyl hydroxylamine, azoxybenzene, azobenzene, aniline, and N-arylformamide possibly served as the intermediates of the reaction.

Approach to α-fluorinatedacetoxy ketones by fluoroacetoxylation of alkynes with hyper-valent iodine(III) reagents

An efficient method for the straightforward synthesis of α-fluorinatedacetoxy ketones has been described using fluoro-containing hyper-valent iodine(III) reagents and commercially available alkynes in transition metal-free conditions. This facile procedure exhibits wide substrate applicability and broad functional group tolerance under mild reaction conditions, delivering diverse fluoro-containing α-acetoxyketones in modern to excellent yields. In addition, gram-scale experiments have been accomplished, which promotes its potential applicability.

Photoredox-Catalyzed Phosphonocarboxylation of Allenes with Phosphine Oxides and CO2.

Phosphonocarboxylation of allenes with diarylphosphine oxides and CO2 via visible-light photoredox catalysis was developed for the first time. This work provided practical and sustainable access to highly valuable but otherwise difficult-to-access linear allylic β-phosphonyl carboxylic acids in moderate yields with exclusive regio- and stereoselectivity. This method was also characterized by step and atom economy and transition-metal free and mild conditions. Preliminary mechanistic studies suggested that allyl-methyl carbanion species are the key intermediates.

Photo-Induced Carbonylation of Aryl Bromides for the Synthesis of Aryl Esters and Amides Under Transition Metal-Free Conditions.

In this work, we developed a photo-induced carbonylation of aryl bromides under transition metal-free conditions. The reaction shows good activity with alcohol and amine nucleophiles. Various esters and amides were formed from aryl halides and alcohols and amines under mild conditions in moderate to good yields.

Visible-Light-Induced Desaturative β-Alkoxyoxalylation of N-Aryl Cyclic Amines with Difluoromethyl Bromides and H2O Via a Triple Cleavage Process.

A visible-light-driven desaturative β-alkoxyoxalyation of N-aryl cyclic amines with difluoromethyl bromides and H2O has been reported. This tandem reaction is triggered by homolysis of the C-Br bond to produce the difuoroalkyl radical, which undergoes the subsequent defluorinated β-alkoxyoxalylation cascades to afford a wide range of β-ketoester/ketoamides substituted enamines. The prominent feature of this reaction contains photocatalyst-free, transition-metal free, and mild conditions. The 18O labeling experiment disclosed that H2O is the oxygen source of the carbonyl unit.

Vinylation of N-Heteroarenes through Addition/Elimination Reactions of Vinyl Selenones.

A new protocol for the synthesis of N-vinyl azoles using vinyl selenones and azoles in the presence of potassium hydroxide was developed. This reaction proceeded under mild and transition metal-free conditions through an addition/elimination cascade process. Both aromatic and aliphatic vinyl selenones and various mono-, bi- and tri-cyclic azoles can be tolerated and give terminal N-vinyl azoles in moderate to high yields. A plausible mechanism is also proposed.

Photoredox-Catalyzed Decarboxylative Bromination, Chlorination and Thiocyanation Using Inorganic Salts.

Decarboxylative halogenation reactions of alkyl carboxylic acids are highly valuable reactions for the synthesis of structurally diverse alkyl halides. However, many reported protocols rely on stoichiometric strong oxidants or highly electrophilic halogenating agents. Herein, we describe visible-light photoredox-catalyzed decarboxylative halogenation reactions of N-hydroxyphthalimide-activated carboxylic acids that avoid stoichiometric oxidants and use inexpensive inorganic halide salts as the halogenating agents. Bromination with lithium bromide proceeds under simple, transition-metal-free conditions using an organic photoredox catalyst and no other additives, whereas dual photoredox-copper catalysis is required for chlorination with lithium chloride. The mild conditions display excellent functional-group tolerance, which is demonstrated through the transformation of a diverse range of structurally complex carboxylic acid containing natural products into the corresponding alkyl bromides and chlorides. In addition, we show the generality of the dual photoredox-copper-catalyzed decarboxylative functionalization with inorganic salts by extension to thiocyanation with potassium thiocyanide, which was applied to the synthesis of complex alkyl thiocyanates.