Rhodium(III)-Catalyzed Switchable β-C(sp2)-H Alkenylation and Alkylation of Acyclic Enamides with Allyl Alcohols

Herein, rhodium(III)-catalyzed β-C(sp2)-H alkenylation and alkylation of enamides are presented using readily accessible allylic alcohols by switching the reaction conditions. This tunable transformation has been applied to a wide range of substrates and typically proceeded with excellent regioselectivity and stereoselectivity as well as with good functional group tolerance. The catalytic system offers an efficient approach for synthesizing various functionalized enamides bearing N-(2Z,4E)-butadiene and (Z)-β-C(sp2)-H alkylated enamides. In addition, mechanistic experiments suggest that Rh(III)-catalyzed C-H activation is not related to the critical step.

Asymmetric Organocatalyzed Cyclization Cascade Reactions of 3,3-Difluoro-2-aryl-3H-indoles and Enamides

The direct functionalization of β-C(sp2)-H bonds in enamides has garnered increasing attention within the realm of organic synthesis. However, these remarkable advancements are predominantly dependent on transition metals; limited success has been achieved via organocatalytic catalysis. Herein, we report a CPA-catalyzed β-C(sp2)-H functionalization of enamides cascade intramolecular cyclization to synthesize the chiral dihydropyrimido[1,6-a]indoles bearing gem-difluoromethylene. Moreover, this methodology enables the synthesis of diverse chiral dihydropyrimido[1,6-a]indoles with outstanding enantioselectivities in moderate to high yields.

Selective Cross-Dehydrogenative Coupling of Various Acyclic Enamides with Heteroarenes via Rh(III)-Catalyzed C-H Activation

The developed methodology describes an efficient Rh(III)-catalyzed oxidative C-H/C-H cross-coupling between acyclic enamides and heteroarenes. This cross dehydrogenative coupling (CDC) reaction offers advantages, including excellent regioselectivity and stereoselectivity, good functional group compatibility, and a broad substrate scope. Mechanistically, Rh(III)-catalyzed β-C(sp2)-H activation of acyclic enamides is proposed to be the critical step.

Photoredox/Nickel Dual-Catalyzed Stereoselective Synthesis of Distal Cyano-Substituted Enamides

Herein, the efficient photoredox/nickel dual-catalyzed cyanoalkylation reaction of enamides is illustrated. A wide scope of enamides and cycloketone oxime esters was well-tolerated, affording the synthetically versatile and geometrically defined β-cyanoalkylated enamide scaffolds. The synthetic practicality of this protocol was revealed by gram-scale reactions, further transformations of enamides, and late-stage modifications of biologically active molecules.

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.

Additive-Free, Visible-Light-Enabled Decarboxylative Alkylation of Enamides

Enamides are versatile precursors for synthesizing bioactive compounds. As their alkylations often require perstoichiometric amounts of oxidants, transition metals, or photocatalysts, we herein report a simple alternative for their alkylations by just using visible light to irradiate the mixture of the readily available N-hydroxyphthalimide esters and enamides without an additive. The reaction involves the photoactivation of a π-π stacking EDA complex between the substrates.

Electrochemical sulfonylation of enamides with sodium sulfinates to access β-amidovinyl sulfones

The electrochemical sulfonylation of enamides with sodium sulfinates was developed in an undivided cell in constant current mode, leading to the formation of β-amidovinyl sulfones in moderate to good yields. The catalyst-, electrolyte- and oxidant-free protocol features good functional group tolerance and employs electric current as a green oxidant. Mechanistic insights into the reaction indicate that the reaction may proceed via a radical mechanism.

Atroposelective Synthesis of Conjugated Diene-Based Axially Chiral Styrenes via Pd(II)-Catalyzed Thioether-Directed Alkenyl C-H Olefination

The efficient stereoselective synthesis of conjugated dienes, especially those with axial chirality, remains a great challenge. Herein, we report the highly atroposelective synthesis of axially chiral styrenes with a conjugated 1,3-diene scaffold via a Pd(II)-catalyzed thioether-directed alkenyl C-H olefination strategy. This strategy features easy operation, mild reaction conditions, high functional group tolerance (69 examples), complete Z-selectivity, and excellent enantioselectivities (up to 99% ee). Notably, the highly enantioselective synthesis of atropisomers with two stereogenic axes were also achieved using this strategy (up to 99% ee and 97:3 dr). Moreover, the reaction could be scaled up, and the resulting axially chiral styrenes could be easily oxidized into chiral sulfoxide derivatives with high diastereoselectivities, which showed great promise as a new type of sulfur-olefin ligand.

K2S2O8-mediated regio- and stereo-selective thiocyanation of enamides with NH4SCN

A direct and straightforward thiocyanation of enamides with NH₄SCN under metal-free conditions has been accomplished. A variety of (E)-β-thiocyanoenamides are readily produced in a regio- and stereo-selective manner. The protocol features mild reaction conditions, good functional group tolerance and operational simplicity. The potential utility of this strategy was further demonstrated by transformation of thiocyanate into thiotetrazole-containing compounds and a Pd-catalyzed cross-coupling reaction to afford six- or seven-membered sulfur-containing heterocycles. Mechanistic insights into the reaction indicate that the reaction may proceed via a radical mechanism.

Rhodium(III)-Catalyzed Direct C-H Arylation of Various Acyclic Enamides with Arylsilanes

The stereoselective β-C(sp²)-H arylation of various acyclic enamides with arylsilanes via Rh(III)-catalyzed cross-coupling reaction was illustrated. The methodology was characterized by extraordinary efficacy and stereoselectivity, a wide scope of substrates, good functional group tolerance, and the adoption of environmentally friendly arylsilanes. The utility of this present method was evidenced by the gram-scale synthesis and further elaboration of the product. In addition, Rh(III)-catalyzed C-H activation is considered to be the critical step in the reaction mechanism.

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.

Macrolactam Synthesis via Ring-Closing Alkene-Alkene Cross-Coupling Reactions

Reported herein is a practical method for macrolactam synthesis via a Rh(III)-catalyzed ring closing alkene-alkene cross-coupling reaction. The reaction proceeded via a Rh-catalyzed alkenyl sp² C-H activation process, which allows access to macrocyclic molecules of different ring sizes. Macrolactams containing a conjugated diene framework could be easily prepared in high chemoselectivities and Z,E stereoselectivities.

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.

Distal Alkenyl C-H Functionalization via the Palladium/Norbornene Cooperative Catalysis

A distal-selective alkenyl C-H arylation method is reported through a directed palladium/norbornene (Pd/NBE) cooperative catalysis. The key is to use an appropriate combination of the directing group and the NBE cocatalyst. A range of acyclic and cyclic cis-olefins are suitable substrates, and the reaction is operated under air with excellent site-selectivity. Preliminary mechanistic studies are consistent with the proposed Pd/NBE-catalyzed C-H activation instead of the Heck pathway. Initial success on distal alkylation has also been achieved using MeI and methyl bromoacetate as electrophiles.

Cu-catalyzed C–N bond cleavage of 3-aminoindazoles for the C–H arylation of enamines

We have presented a novel Cu-catalyzed stereoselective C–H arylation of enamines by using 3-aminoindazoles arylating agents via oxidative cleavage of two C–N bonds.

Recent advances in the direct β-C(sp2)–H functionalization of enamides

Recent advances in the direct β-C(sp²)–H functionalization of enamides, mainly including arylation, alkenylation, alkynylation, alkylation, acylation, sulfonylation, phosphorylation, and others, are reported.

Photoredox-catalyzed stereoselective alkylation of enamides with N-hydroxyphthalimide esters via decarboxylative cross-coupling reactions

Stereoselective β-C(sp2)-H alkylation of enamides with redox-active N-hydroxyphthalimide esters via a photoredox-catalyzed decarboxylative cross-coupling reaction is demonstrated. This methodology features operational simplicity, broad substrate scopes, and excellent stereoselectivities and functional group tolerance, affording a diverse array of geometrically defined and synthetically valuable enamides bearing primary, secondary or tertiary alkyl groups in satisfactory yields.

The ruthenium-catalyzed C-H functionalization of enamides with isocyanates: easy entry to pyrimidin-4-ones

Ruthenium-catalyzed heteroannulation between enamides and isocyanates has been realized as a complementary approach to conventional strategies for the synthesis of pyrimidin-4-ones. High step- and atom-economy was achieved for the rapid construction of such privileged scaffolds, which are found in a multitude of pharmaceutical compounds. The generality and practicability of this transformation were reflected by the broad scope of substrates with diverse functional groups, large-scale synthesis, and late-stage diversification.

Stereoselective synthesis of alkyl-, aryl-, vinyl- and alkynyl-substituted Z-enamides and enol ethers

Enamides and enol ethers are valuable building blocks in synthetic chemistry, yet their stereoselective synthesis can be challenging. Herein, we report a new stereoselective synthesis of vinyl, aryl, alkynyl, alkyl and thio-substituted Z-enamides and enol ethers based on the use of vinylbenziodoxolone (VBX) reagents. The stable VBX reagents were synthesized by stereoselective addition of N- or O-nucleophiles on the corresponding alkynyl reagents in the presence of a catalytic amount of cesium carbonate. The VBX reagents were used in palladium-catalyzed cross-couplings at room temperature to access Z-enamides and enol ethers.

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.

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.

Grignard-mediated rearrangement of trifluoroacetyl from dihydroisoquinoline enamides to afford tertiary trifluoromethylcarbinols

Treatment of the trifluoroacetyl enamides of dihydroisoquinolines 2 with diverse Grignard reagents afforded tertiary trifluoromethyl-carbinols 4 by facilitating the addition of tertiary carbinols to the β-carbon of enamides 2. Based on the confirmed formation of vinylogous amides 3, the transformation likely proceeds via unique acyl group rearrangement to the β-carbon of the enamide and subsequent nucleophilic addition of the Grignard reagent. Given the synthetic utility and novelty of this reaction, this work may open new avenues for the synthesis of pharmaceutically important tertiary trifluoromethylcarbinols on cyclic enamide systems.