Sterically Hindered Ketones via Palladium-Catalyzed Suzuki–Miyaura Cross-Coupling of Amides by N–C(O) Activation

Classes of Amides that Undergo Selective N-C Amide Bond Activation: The Emergence of Ground-State Destabilization

Ground-state destabilization of the N-C(O) linkage represents a powerful tool to functionalize the historically inert amide bond. This burgeoning reaction manifold relies on the availability of amide bond precursors that participate in weakening of the nN → π*C=O conjugation through N-C twisting, N pyramidalization, and nN electronic delocalization. Since 2015, acyl N-C amide bond activation through ground-state destabilization of the amide bond has been achieved by transition-metal-catalyzed oxidative addition of the N-C(O) bond, generation of acyl radicals, and transition-metal-free acyl addition. This Perspective summarizes contributions of our laboratory in the development of new ground-state-destabilized amide precursors enabled by twist and electronic activation of the amide bond and synthetic utility of ground-state-destabilized amides in cross-coupling reactions and acyl addition reactions. The use of ground-state-destabilized amides as electrophiles enables a plethora of previously unknown transformations of the amide bond, such as acyl coupling, decarbonylative coupling, radical coupling, and transition-metal-free coupling to forge new C-C, C-N, C-O, C-S, C-P, and C-B bonds. Structural studies of activated amides and catalytic systems developed in the past decade enable the view of the amide bond to change from the "traditionally inert" to "readily modifiable" functional group with a continuum of reactivity dictated by ground-state destabilization.

Amide N-C Bond Activation: A Graphical Overview of Acyl and Decarbonylative Coupling

This Graphical Review provides an overview of amide bond activation achieved by selective oxidative addition of the N-C(O) acyl bond to transition metals and nucleophilic acyl addition, resulting in acyl and decarbonylative coupling together with key mechanistic details pertaining to amide bond distortion underlying this reactivity manifold.

Cu-catalysed transamidation of unactivated aliphatic amides

Direct transamidation is gaining prominence as a ground-breaking technique that generates a wide variety of amides without the requirement of acid-amine coupling or other intermediate steps. However, transamidation of unactivated aliphatic amides, on the other hand, has been a long-standing issue in comparison to transamidation of activated amides. Herein, we report a transamidation approach of an unactivated aliphatic amide using a copper catalyst and chlorotrimethylsilane as an additive. In addition, we used transamidation as a tool for selective N-C(O) cleavage and O-C(O) formation to synthesise 2-substituted benzoxazoles and benzothiazoles. The reactions were carried out without using any solvents and offered wide substitution scope.

Transamidation of thioamides with nucleophilic amines: thioamide N-C(S) activation by ground-state-destabilization

Thioamides are 'single-atom' isosteres of amide bonds that have found broad applications in organic synthesis, biochemistry and drug discovery. In this New Talent themed issue, we present a general strategy for activation of N-C(S) thioamide bonds by ground-state-destabilization. This concept is outlined in the context of a full study on transamidation of thioamides with nucleophilic amines, and relies on (1) site-selective N-activation of the thioamide bond to decrease resonance and (2) highly chemoselective nucleophilic acyl addition to the thioamide CS bond. The follow-up collapse of the tetrahedral intermediate is favored by the electronic properties of the amine leaving group. The ground-state-destabilization concept of thioamides enables weakening of the N-C(S) bond and rationally modifies the properties of valuable thioamide isosteres for the development of new methods in organic synthesis. We fully expect that in analogy to the burgeoning field of destabilized amides introduced by our group in 2015, the thioamide bond ground-state-destabilization activation concept will find broad applications in various facets of chemical science, including metal-free, metal-catalyzed and metal-promoted reaction pathways.

A Metal-Free Path to 2-Iodo-3-alkyl-1-arylbut-2-en-1-ones and Their Application to the Domino Synthesis of Functionalized 2H-Pyran-2-ones

We report a metal-free selective synthesis of 2-iodo-3-alkyl-1-arylbut-2-en-1-ones from propargylic alcohols that is enabled by N-iodosuccinimide. A variety of substituted propargylic alcohols are amenable to delivering the selective 2-iodoenone products in very good yields. The utility of the α-iodoenone derivatives is further extended by developing an efficient, novel, and new synthetic methodology for the synthesis of 3,5,6-trisubstituted 2H-pyran-2-ones. To the best of our knowledge, this protocol is the first of its kind to accomplish 3,5,6-trisubstituted 2H-pyran-2-ones through an unprecedented domino (formation of two C-C bonds and one C-O bond) one-pot process via intermolecular Heck coupling, base-driven Michael addition, and base-mediated double bond isomerization followed by cyclo-condensation. This protocol showed good compatibility with a wide range of iodoenones (18 examples) and 2H-pyran-2-ones (42 examples). Mechanistic studies indicate that palladium is only involved in the Heck coupling; the base solely drives the rest of the steps.

An Efficient and Straightforward Approach for Accessing Thioesters via Palladium-Catalyzed C-N Cleavage of Thioamides

We first report the coupling of activated thioamides with alcohols to efficiently form thioesters via palladium-catalyzed C-N cleavage strategy. The new approach employs the thioamides as thioacylating reagent to give...

Nickel-Mediated Photoreductive Cross Coupling of Carboxylic Acid Derivatives for Ketone Synthesis

A simple visible light photochemical, nickel-catalyzed synthesis of ketones from carboxylic acid-derived precursors is presented. Hantzsch ester (HE) functions as a cheap, green and strong photoreductant to facilitate radical generation and also engages in the Ni-catalytic cycle to restore the reactive species. With this dual role, HE allows for the coupling of a large variety of radicals (1°,2°, benzylic, α-oxy & α-amino) with aroyl and alkanoyl moieties, a new feature in reactions of this type. With both precursors deriving from abundant carboxylic acids, this protocol is a welcome addition to the organic chemistry toolbox. The reaction proceeds under mild conditions without the need for toxic metal reagents or bases and shows a wide scope, including pharmaceuticals and complex molecular architectures.

Acylation of electrophilic bicyclo[1.1.0]butanes via Co/Ni-catalyzed reductive cross-coupling

Cyclobutane scaffold is appreciated in medicinal chemistry for its strained, well-defined three-dimensional structure. Consequently, methods for the synthesis of cyclobutyl derivatives have become highly desired, particularly those offering access to compounds with new patterns of substituents. Herein, an acylation of electrophilic strained molecules at the bridgehead carbon with [Formula: see text]-acyl-glutarimides is reported. For this, the polarity-reversal strategy based on cobalt catalysis that enables the generation of cyclobutyl radicals in a strain release event was harnessed. These nucleophilic species, in the presence of a Ni-complex, couple with [Formula: see text]-acyl-glutarimides to give cyclobutyl ketones in decent yields.

Bimetallic Cooperative Catalysis for Decarbonylative Heteroarylation of Carboxylic Acids via C-O/C-H Coupling

Cooperative bimetallic catalysis is a fundamental approach in modern synthetic chemistry. We report bimetallic cooperative catalysis for the direct decarbonylative heteroarylation of ubiquitous carboxylic acids via acyl C-O/C-H coupling. This novel catalytic system exploits the cooperative action of a copper catalyst and a palladium catalyst in decarbonylation, which enables highly chemoselective synthesis of important heterobiaryl motifs through the coupling of carboxylic acids with heteroarenes in the absence of prefunctionalization or directing groups. This cooperative decarbonylative method uses common carboxylic acids and shows a remarkably broad substrate scope (>70 examples), including late-stage modification of pharmaceuticals and streamlined synthesis of bioactive agents. Extensive mechanistic and computational studies were conducted to gain insight into the mechanism of the reaction. The key step involves intersection of the two catalytic cycles via transmetallation of the copper-aryl species with the palladium(II) intermediate generated by oxidative addition/decarbonylation.

Aerobic Copper-Catalyzed Salicylaldehydic Cformyl -H Arylations with Arylboronic Acids

We report a challenging copper-catalyzed C_formyl -H arylation of salicylaldehydes with arylboronic acids that involves unique salicylaldehydic copper species that differ from reported salicylaldehydic rhodacycles and palladacycles. This protocol has high chemoselectivity for the C_formyl -H bond compared to the phenolic O-H bond involving copper catalysis under high reaction temperatures. This approach is compatible with a wide range of salicylaldehyde and arylboronic acid substrates, including estrone and carbazole derivatives, which leads to the corresponding arylation products. Mechanistic studies show that the 2-hydroxy group of the salicylaldehyde substrate triggers the formation of salicylaldehydic copper complexes through a Cu^I /Cu^II /Cu^III catalytic cycle.

Iodine Promoted Conversion of Esters to Nitriles and Ketones under Metal-Free Conditions

We report a novel strategy to prepare valuable nitriles and ketones through the conversion of esters under metal-free conditions. By using the I₂/PCl₃ system, various substrates including aliphatic and aromatic esters could react with acetonitrile and arenes to afford the desired products in good to excellent yields. This method is compatible with a number of functional groups and provides a simple and practical approach for the synthesis of nitrile compounds and aryl ketones.

Nickel-Catalyzed Conversion of Amides to Carboxylic Acids

We report the conversion of amides to carboxylic acids using nonprecious metal catalysis. The methodology strategically employs a nickel-catalyzed esterification using 2-(trimethylsilyl)ethanol, followed by a fluoride-mediated deprotection in a single-pot operation. This approach circumvents catalyst poisoning observed in attempts to directly hydrolyze amides using nickel catalysis. The selectivity and mildness of this transformation are shown through competition experiments and the net-hydrolysis of a complex valine-derived substrate. This strategy addresses a limitation in the field with regard to functional groups accessible from amides using transition metal-catalyzed C-N bond activation and should prove useful in synthetic applications.