Palladium-Catalyzed Oxidative Carbonylation of the Alkenyl CH Bonds of Enamides: Synthesis of 1,3-Oxazin-6-ones

Synthesis of 2,2-difunctionalized 2H-azirines via I2-mediated annulation of enamines

Various 2,2-difunctionalized 2H-azirines were synthesized via I2-mediated annulation reactions of readily accessible enamines in the presence of nitrogen or non-nitrogen nucleophiles. The features of the present synthesis process also include no use of transition metals, simple operation, mild reaction conditions, a broad substrate scope, and gram-scale synthesis.

Rhodium-catalyzed aminoacylation of alkenes via carbonylative C–H activation toward poly(hetero)cyclic alkylarylketones

This work discloses the facile construction of polyheterocyclic alkylarylketones by the rhodium-catalyzed carbonylative aminoacylation of alkenes involving C–H activation, which provides molecules as candidates for the screening of antitumor agents.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Electrochemical Synthesis of Polysubstituted Oxazoles from Ketones and Acetonitrile

Herein we report an electrochemical strategy for the construction of polysubstituted oxazoles from easily available ketones and acetonitrile under room temperature. The method shows high efficiency, a broad substrate scope, and requires no external chemical oxidant. Mechanistic investigations suggest that the reaction proceeds through a Ritter-type reaction/oxidative cyclization using acetonitrile as the reactant and solvent. X-ray crystallographic analysis reveals that the reaction involves the attack of acetonitrile on the carbonyl carbon instead of the α-carbon.

Different Lewis Acid Promotor-Steered Highly Regioselective Phosphorylation of Tertiary Enamides

Different Lewis acid promotor-steered highly regioselective phosphorylation of tertiary enamides with diverse H-phosphonates or H-phosphine oxides was developed. Under the catalysis of iron salt, the phosphonyl group was introduced into the α-position of tertiary enamides, affording various α-phosphorylated amides in high efficiency. On the other hand, the β-phosphorylated tertiary enamides were efficiently obtained as the products in the presence of manganese(III) acetylacetonate.

Manganese(III)-promoted highly stereoselective phosphorylation of acyclic tertiary enamides to synthesize E-selective β-phosphoryl enamides

A concise manganese(III)-promoted stereoselective β-phosphorylation of acyclic tertiary enamides and diverse H-phosphine oxides was achieved. This reaction proceeds with absolute E-selectivity in contrast to Z-selectivity obtained in other previous works and affords various E-selective β-phosphorylated tertiary enamides in high efficiency. To the best of our knowledge, this is the first case of E-selective β-phosphorylation of tertiary enamides through C-H functionalization. In addition, the method features broad substrate scope, good functional group compatibility and efficient scale-up.

Stannyl radical-mediated synthesis of 6H-1,3-oxazin-6-ones from 2-acyloxyazirines or whether free radicals can open the azirine ring?

A fundamentally new radical cascade reaction of 2-acyloxyazirines underlays an effective one-step method for the preparation of 5-hydroxy-6H-1,3-oxazin-6-ones from methyl 2-acyloxy-2H-azirine-2-carboxylates using Bu3SnH/ACHN system as a source of stannyl radicals....

Visible-light-initiated regio- and stereoselective C(sp2)–H phosphorylation of enamides under transition-metal-free conditions

A visible-light-induced stereo- and regioselective phosphorylation of enamides with phosphine oxides under transition-metal-free conditions has been disclosed.

Difluorocarbene-enabled access to 1,3-oxazin-6-ones from enamides

1,3-Oxazin-6-ones as important structural scaffolds widely exist in many bioactive or therapeutic agents.

Rhodium(iii)-catalyzed annulation of enamides with sulfoxonium ylides toward isoquinolines

An efficient rhodium(iii)-catalyzed C-H activation followed by intermolecular annulation between enamides and sulfoxonium ylides has been developed. The transformation proceeds smoothly with a broad range of substrates, affording a series of isoquinoline derivatives in moderate to good yields under additive-free conditions.

Visible-light-induced iodine-anion-catalyzed decarboxylative/deaminative C–H alkylation of enamides

An efficient method for photo-induced iodine-anion-catalyzed C–H alkylation of enamides has been developed. Redox-active esters and Katritzky salts of amino acids are amenable, successfully delivering various functionalized enamides.

Photoinitiated stereoselective direct C(sp2)–H perfluoroalkylation and difluoroacetylation of enamides

Photoinitiated regio- and stereoselective C(sp²)–H perfluoroalkylation and difluoroacetylation of enamides are developed, furnishing biologically and physiologically privileged fluoro-containing enamide scaffolds.

Recent advances in chelation-assisted site- and stereoselective alkenyl C–H functionalization

This review highlights recent advances in vicinal- and geminal-group-directed alkenyl C–H functionalizations which proceeded by endo- and exo-cyclometallation.

Visible-Light-Induced Regio- and Stereoselective C(sp2)-H Trifluoroethylation of Enamides with 2,2,2-Trifluoroethyl Iodide

A photoredox-catalyzed regio- and stereoselective trifluoroethylation reaction of enamides using commercially available 2,2,2-trifluoroethyl iodide as trifluoroethylating agents has been developed, furnishing geometrically defined and synthetically and physiochemically pivotal β-trifluoroethylated enamides bearing a diverse range of functional groups.

Palladium-Catalyzed Enantioselective Heck Carbonylation with a Monodentate Phosphoramidite Ligand: Asymmetric Synthesis of (+)-Physostigmine, (+)-Physovenine, and (+)-Folicanthine

Reported herein is the development of the first enantioselective monodentate ligand assisted Pd-catalyzed domino Heck carbonylation reaction with CO. The highly enantioselective domino Heck carbonylation of N-aryl acrylamides and various nucleophiles, including arylboronic acids, anilines, and alcohols, in the presence of CO was achieved. A novel monodentate phosphoramidite ligand, Xida-Phos, has been developed for this reaction and it displays excellent reactivity and enantioselectivity. The reaction employs readily available starting materials, tolerates a wide range of functional groups, and provides straightforward access to a diverse array of enantioenriched oxindoles having β-carbonyl-substituted all-carbon quaternary stereocenters, thus providing a facile and complementary method for the asymmetric synthesis of bioactive hexahydropyrroloindole and its dimeric alkaloids.

Synthesis of indolo- and pyrrolo[1,2-a]quinoxalinones through a palladium-catalyzed oxidative carbonylation of the C2 position of indole

A methodology that involves the Pd-catalyzed direct C(sp2)–H bond carbonylation of the C₂ position of indole has been introduced for the synthesis of indolo[1,2-a]quinoxalin-6(5H)-ones.

A Rh(iii)-catalyzed regioselective intermolecular oxa-Pauson–Khand reaction of alkynes, arylboronic acids and CO to form butenolides

A Rh(iii)-catalyzed regioselective intermolecular oxa-Pauson–Khand reaction of arylboronic acids, alkynes and CO for the synthesis of butenolides has been developed.

Perfluorocarboxylic Anhydrides Triggered Cyclization: Access to 4-Perfluoroalkylpyridines

A double nucleophilic addition-cyclization-elimination cascade is developed, that allows various 2,6-diaryl-4-perfluoroalkylpyridines to be synthesized in one step from easily available enamides and perfluorocarboxylic anhydrides. The procedure is also operationally simple and scalable and provides access to the facial construction of 4-fluoroalkylpyridines, which are of great interest in medicinal chemistry.

Hypervalent iodane mediated reactions of N-acetyl enamines for the synthesis of oxazoles and imidazoles

A hypervalent iodane reagent used for the intramolecular cyclization of N-acetyl enamines and intermolecular cyclocondensation of enamines and nitriles was investigated. The reaction was performed under mild conditions and gave oxazoles and imidazoles, respectively, in moderate to excellent yields. This transformation exhibits good reactivity, selectivity and functional group tolerance. The selectivity of the intra- or intermolecular reaction is dependent on the structure of N-acetyl enamines.

Cobalt-Catalyzed Direct Carbonylative Synthesis of Free (NH)-Benzo[cd]indol-2(1H)-ones from Naphthylamides

A cobalt-catalyzed C-H carbonylation of naphthylamides for the synthesis of benzo[cd]indol-2(1H)-one scaffolds has been developed. The reaction employs a traceless directing group and uses benzene-1,3,5-triyl triormate as the CO source, affording various free (NH)-benzo[cd]indol-2(1H)-ones in moderate to high yields (up to 88%). Using this protocol, the total synthesis of BET bromodomain inhibitors A and B was accomplished as well.

Divergent Conversion of N-Acyl-isoxazol-5(2 H)-ones to Oxazoles and 1,3-Oxazin-6-ones Using Photoredox Catalysis

The fragmentation of N-acyl-isoxazol-5-ones using visible light photoredox catalysis has been disclosed. The catalyst-controlled divergent mechanisms, namely the oxidative and reductive quenching catalytic cycle, are utilized. Various oxazoles and 1,3-oxazin-6-ones are selectively obtained from the same isoxazol-5-one skeleton under mild conditions.

Synthesis of Functionalized Imidazolium Salts via Iodine-Mediated Annulations of Enamines

A novel annulation reaction of two enamine molecules with iodine under basic conditions to form 4-functionalized imidazolium salts has been established. In this reaction, iodine acts as both an iodinating reagent and a Lewis acid catalyst. Features of this synthetic method include facilitative preparation of substrates, no use of transition metals, mild reaction conditions, simplicity of operation, and gram scale synthesis.

Pd/Cu Cocatalyzed Oxidative Tandem C-H Aminocarbonylation and Dehydrogenation of Tryptamines: Synthesis of Carbolinones

The Pd/Cu cocatalyzed oxidative tandem C-H aminocarbonylation and dehydrogenation was developed, affording carbolinones with molecular oxygen as the terminal oxidant. Natural product strychnocarpine and its derivatives were prepared conveniently using this strategy.

Cu(OTf)2-mediated C(sp2)–H arylsulfonylation of enamides via the insertion of sulfur dioxide

A stereoselective Cu(OTf)₂-mediated C(sp²)–H sulfonylation of enamides with arylsulfonyl radicals generated in situ from DABSO and diazonium salts is developed.

Recent advances in cobalt-catalysed C–H functionalizations

Ready availability, low cost and low toxicity of cobalt salts have redirected the attention of researchers away from noble metals, such as Pd, Rh, and Ir, towards Co in the field of C–H functionalisation.

Manganese(iii)-mediated selective phosphorylation of enamides: direct synthesis of β-phosphoryl enamides

Mn(iii)-Mediated radical oxidative selective phosphorylation ofN-styrylamides to afford β-phosphoryl enamides, which could be easily transformed to β-amino phosphonic acids, is described.

Rhodium-catalyzed alkenyl C–H functionalization with amides

Rhodium-catalyzed pyridyl-directed alkenyl C–H bond functionalization with aroyl- or acryl-amides via C–H/C–N activation was carried out.

Palladium catalyzed carbonylative annulation of the C(sp2)-H bond of N,1-diaryl-1H-tetrazol-5-amines and N,4-diaryl-4H-triazol-3-amines to quinazolinones

Pd(ii) catalyzed direct C-H carbonylative annulation of N,1-diaryl-1H-tetrazol-5-amines and N,4-diaryl-4H-1,2,4-triazol-3-amines gave the corresponding triazole and tetrazole fused quinazolinones in good yields. This methodology offers a convenient method for the synthesis of these important heterocyclic scaffolds in a highly atom economical process. On the mechanistic aspect weakly nucleophilic triazole and tetrazole moieties function as both directing as well as intramolecular nucleophiles. The catalytically active C-H activated intermediate dimeric Pd complex was isolated and characterized which on exposure to CO gas gave the corresponding tetrazole fused quinazolinone derivative. On the basis of isolation of the intermediate and observed kinetic isotope effects, a mechanism has been proposed for the C-H activated direct carbonylative annulation reaction.

Selective and Catalytic Hydrocarboxylation of Enamides and Imines with CO2 to Generate α,α-Disubstituted α-Amino Acids

The first catalytic hydrocarboxylation of enamides and imines with CO₂ to generate valuable α,α-disubstituted α-amino acids is reported. Notably, excellent chemo- and regio-selectivity are achieved, significantly different from previous reports on β-carboxylation of enamides, homocoupling or reduction of imines. Moreover, this transition-metal-free procedure exhibits low loading of an inexpensive catalyst, easily available substrates, mild reaction conditions, high efficiency, facile scalability and easy product derivatization, providing great potential for application in organic synthesis, pharmaceutical chemistry, and biochemistry.