Decarbonylative Cyanation of Amides by Palladium Catalysis

Ligand-Enabled Gold-Catalyzed Cyanation of Organohalides

Herein, we disclose the first report on gold-catalyzed C(sp2)-CN cross-coupling reaction by employing a ligand-enabled Au(I)/Au(III) redox catalysis. This transformation utilizes acetone cyanohydrin as a nucleophilic cyanide source to convert simple aryl and alkenyl iodides into the corresponding nitriles. Combined experimental and computational studies highlighted the crucial role of cationic silver salts in activating the stable (P,N)-AuCN complex towards the oxidative addition of aryl iodides to subsequently generate key aryl-Au(III) cyanide complexes.

Green and efficient synthesis of dibenzyl cyanamides and ureas with cyanamide as a block

A method for the two-step synthesis of dibenzyl cyanamide and dibenzyl urea via cyanamide is presented. This approach is both efficient and environmentally friendly. Various N,N-dibenzyl ureas could be obtained by reactions of N,N-dibenzyl cyanamides and N,N-dibenzyl cyanamides as intermediates formed from cyanamide. In the absence of metal, ligand and hydrogen peroxide as the oxidant, products with moderate yields have been obtained under mild conditions. Key features include the use of widely available and easily handled cyanamide sources as starting materials.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Palladium-Catalyzed Decarbonylative Coupling of (Hetero)Aryl Boronate Esters with Difluorobenzyl Glutarimides

This report describes the Pd-catalyzed decarbonylative coupling of difluorobenzyl glutarimides with (hetero)aryl boronate esters to yield difluorobenzyl-substituted (hetero)arene products. The use of PAd2Bu as the phosphine ligand in combination with neopentylboronate ester nucleophiles proved critical for the selective formation of the decarbonylative coupling product versus analogous difluorobenzyl ketone. This transformation is effective for electronically diverse (hetero)aryl boronate esters and substituted difluorobenzyl glutarimides.

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.

Palladium-Catalyzed Direct Decarbonylative Cyanation of Aryl Carboxylic Acids

The direct transformation of aryl carboxylic acids to aryl nitrile compounds is an interesting topic because carboxylic acids are not only abundant in nature but are also inexpensive and stable. Here, the synthesis of a series of aryl nitriles by palladium-catalyzed decarbonylative cyanation of carboxylic acids without base has been achieved. The successful decarbonylative cyanation of drug molecules and Gram-scale reaction to verify the practicality and operability of this method are analyzed.

Palladium-Catalyzed Decarbonylative Cyanation of Carboxylic Acids with TMSCN

The direct decarbonylative cyanation of benzoic acids with TMSCN was achieved through palladium catalysis. By this strategy, a wide range of nitriles including those with functional groups was synthesized in good to high yields. Moreover, this reaction applied to modifying bioactive molecules such as adapalene, probenecid, telmisartan, and 3-methylflavone-8-carboxylic acid. These results demonstrate that this new reaction has potential synthetic value in organic synthesis.

Bulky Di(1-adamantyl)phosphinous Acid-Ligated Pd(II) Precatalysts for Suzuki Reactions of Unreactive Aryl Chlorides

Di(1-adamantyl)phosphine oxide (SPO-Ad: Ad2P(V)(=O)H), a stable tautomer of di(1-adamantyl)phosphinous acid (PA-Ad: Ad2P(III)-OH), was employed to synthesize two new PA-Ad-coordinated complexes, POPd-Ad and POPd2-Ad. POPd-Ad was easily transformed from POPd2-Ad in acetonitrile, and the [M - H]- ion of the deprotonated POPd-Ad was observed in the electrospray ionization-mass spectrum of POPd2-Ad. Both complexes are effective precatalysts for the Suzuki reaction of aryl chlorides. The reduction of Pd(II) in POPd-Ad and POPd2-Ad by arylboronic acid was examined, and the ideal Pd-to-PA ratio in the Suzuki reaction was found to be 1:1. The effect of temperature on the catalytic yields was studied to examine the possible ligation state of the active species and the dimer-to-monomer process of POPd2-Ad. Mononuclear and mono-ligated Pd species was assumed to be catalytically active. The electronic and steric effects of PA-Ad were slightly better than those reported for PA-tBu ( t Bu2P(III)-OH). Density functional theory calculations were performed to evaluate the formation of mono-ligated and mononuclear Pd species from POPd-Ad and POPd2-Ad. Furthermore, the reaction time and catalyst loading could be reduced for the reported POPd1-tBu precatalyst using the optimized reaction conditions for POPd-Ad. The complexes synthesized in this extensive study will complement the existing SPO-coordinated POPd series of precatalysts.

Nickel-Catalyzed Cyanation of Aryl Thioethers

A nickel-catalyzed cyanation of aryl thioethers using Zn(CN)₂ as a cyanide source has been developed to access functionalized aryl nitriles. The ligand dcype (1,2-bis(dicyclohexylphosphino)ethane) in combination with the base KOAc (potassium acetate) is essential for achieving this transformation efficiently. This reaction involves both a C-S bond activation and a C-C bond formation. The scalability, low catalyst and reagents loadings, and high functional group tolerance have enabled both late-stage derivatization and polymer recycling, demonstrating the reaction's utility across organic chemistry.

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.

Diversity-Oriented Synthesis of Thiazolidine-2-imines via Microwave-Assisted One-Pot, Telescopic Approach and Its Interaction with Biomacromolecules

In this work, a one-pot, telescopic approach is described for the combinatorial library of thiazolidine-2-imines. The synthetic manipulation proceeds smoothly via the reaction of 2-aminopyridine/pyrazine/pyrimidine with substituted isothiocyanates followed by base catalyzed ring closure with 1,2-dibromoethane to obtain thiazolidine-2-imines with broad substrate scope and high functional group tolerance. The synthetic strategy merges well with the thiourea formation followed by base catalyzed ring closure reaction for the thiazolidine-2-imine synthesis in a more modular and straightforward approach. The synthetic procedure reported herein represents a cleaner route toward thiazolidine-2-imines as compared to traditional methodologies. Moreover, the biological significance of combinatorially synthesized thiazolidin-2-imines has been investigated for their use as possible inhibitors for acetyl cholinesterase through molecular docking studies.

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.

Nickel or Palladium-Catalyzed Decarbonylative Transformations of Carboxylic Acid Derivatives

Carboxylic acid derivatives containing acyl halides, anhydrides, esters, amides and acyl nitriles are highly appealing electrophiles in transition-metal-catalyzed carbon-carbon bond-forming reactions due to their ready availability and low cost, which can provide divergent transformations of carboxylic acids into other value-added products. In this Minireview, we focus on the recent advances of decarbonylative transformations of carboxylic acid derivatives in carbon-carbon bond formations using Ni or Pd catalysts. A series of reaction types, product classifications and reaction pathways are presented herein, which show the advantageous features of carboxylic acid derivatives as alternative to aryl or alkyl halides in terms of reactivity and compatibility. The well-accepted mechanism of nickel- or palladium-catalyzed decarbonylative transformations involves initial oxidative addition of carboxylic acid derivatives, followed by decarbonylation or transmetalation (or insertion), and reductive elimination to generate the products, thereby regenerating the catalysts.

Ni-Catalyzed Suzuki-Miyaura Cross-Coupling of Aliphatic Amides on the Benchtop

Suzuki-Miyaura cross-couplings of amides offer an approach to the synthesis of ketones that avoids the use of basic or pyrophoric nucleophiles. However, these reactions require glovebox manipulations, thus limiting their practicality. We report a benchtop protocol for Suzuki-Miyaura cross-couplings of aliphatic amides that utilizes a paraffin capsule containing a Ni(0) precatalyst and NHC ligand. This methodology is broad in scope, is scalable, and provides a user-friendly approach to convert aliphatic amides to alkyl-aryl ketones.

Selective C–C bond cleavage of amides fused to 8-aminoquinoline controlled by a catalyst and an oxidant

Herein, copper-catalyzed direct C–C bond cleavage of amides fused to 8-aminoquinoline as a directing group to form urea in the presence of amines and dioxygen is reported.

Resonance promoted ring-opening metathesis polymerization of twisted amides

The living ring-opening metathesis polymerization (ROMP) of an unsaturated twisted amide using the third-generation Grubbs initiator is described. Unlike prior examples of ROMP monomers that rely on angular or steric strain for propagation, this system is driven by resonance destabilization of the amide that arises from geometric constraints of the bicyclic framework. Upon ring-opening, the amide can rotate and rehybridize to give a stabilized and planar conjugated system that promotes living propagation. The absence of other strain elements in the twisted amide is supported by the inability of a carbon analogue of the monomer to polymerize and computational studies that find resonance destabilization accounts for 11.3 kcal mol-1 of the overall 12.0 kcal mol-1 ring strain. The twisted amide polymerization is capable of preparing high molecular weight polymers rapidly at room temperature, and post-polymerization modification combined with 2D NMR spectroscopy confirms a regioirregular polymer microstructure.

Na2CO3-promoted thioesterification via N–C bond cleavage of amides to construct thioester derivatives

A mild, efficient, and transition-metal-free catalytic strategy is developed to construct thioesters via selective N–C bond cleavage of Boc2-activated primary amides. This strategy is successfully carried out with stoichiometric Na2CO3 as the base and provides the corresponding products in moderate to excellent yields.

Cesium Carbonate Catalyzed Esterification of N-Benzyl-N-Boc-amides under Ambient Conditions

We report a general activated amide to ester transformation catalyzed by Cs₂CO₃. Using this approach, esterification proceeds under relatively mild conditions and without the need for a transition metal catalyst. This method exhibits broad substrate scope and represents a practical alternative to existing esterification strategies. The synthetic utility of this protocol is demonstrated via the facile synthesis of crown ether derivatives and the late-stage modification of a representative natural product and several sugars in reasonable yields.

Highly-chemoselective step-down reduction of carboxylic acids to aromatic hydrocarbons via palladium catalysis

Aryl carboxylic acids are among the most abundant substrates in chemical synthesis and represent a perfect example of a traceless directing group that is central to many processes in the preparation of pharmaceuticals, natural products and polymers. Herein, we describe a highly selective method for the direct step-down reduction of carboxylic acids to arenes, proceeding via well-defined Pd(0)/(ii) catalytic cycle. The method shows a remarkably broad substrate scope, enabling to direct the classical acyl reduction towards selective decarbonylation by a redox-neutral mechanism. The utility of this reaction is highlighted in the direct defunctionalization of pharmaceuticals and natural products, and further emphasized in a range of traceless processes using removable carboxylic acids under mild, redox-neutral conditions orthogonal to protodecarboxylation. Extensive DFT computations were conducted to demonstrate preferred selectivity for the reversible oxidative addition and indicated that a versatile hydrogen atom transfer (HAT) pathway is operable.

[Pd(NHC)(acac)Cl]: Well-Defined, Air-Stable, and Readily Available Precatalysts for Suzuki and Buchwald-Hartwig Cross-coupling (Transamidation) of Amides and Esters by N-C/O-C Activation

A general class of well-defined, air-stable, and readily available Pd(II)-NHC precatalysts (NHC = N-heterocyclic carbene) for Suzuki and Buchwald-Hartwig cross-coupling of amides (transamidation) and esters by selective N-C/O-C cleavage is reported. Since these precatalysts are highly active and the easiest to synthesize, the study clearly suggests that [Pd(NHC)(acac)Cl] should be routinely included during the development of new cross-coupling methods. An assay for in situ screening of NHC salts in this cross-coupling manifold is presented.

Decarbonylative Borylation of Amides by Palladium Catalysis

The development of transition-metal-catalyzed borylation reactions is of significant importance for the fields of organic synthesis and medicinal chemistry because of the versatility of organoboron functional groups. Herein, we report the direct decarbonylative borylation of amides by highly selective carbon-nitrogen bond cleavage by palladium catalysis. The approach capitalizes on the ground-state destabilization of the amide bond in N-acyl glutarimides to achieve Pd-catalyzed insertion into the amide N-C bond and decarbonylation (deamidation). Mechanistic studies and the utility of this methodology in orthogonal sequential cross-couplings of robust, bench-stable amides are reported.

Triflamides: Highly Reactive, Electronically Activated N-Sulfonyl Amides in Catalytic N-C(O) Amide Cross-Coupling

The direct, highly chemoselective Suzuki-Miyaura cross-coupling of trifluoromethanesulfonamides (triflamides) by selective N-C(O) amide bond cleavage is reported. This operationally simple, mild, and user-friendly method accomplishes the direct synthesis of ketones from amides by a catalytic manifold as a powerful alternative to Weinreb amides. Mechanistic studies support rotational inversion and electronic activation, favoring selective insertion under mild conditions. Our data strongly suggest that triflamides should be routinely considered as precursors in amide bond cross-coupling.

Palladium/copper-catalyzed decarbonylative heteroarylation of amides via C–N bond activation

A novel strategy for the synthesis of 2-arylated oxazole derivatives via palladium/copper-catalyzed decarbonylative heteroarylation of amides via C–N bond activation by ground-state destabilization is reported.

Synthesis of 2-acetyl trisubstituted furans via copper-mediated deacylation cleavage of unstrained C(sp3)–C(sp2) bonds

A copper-mediated tandem addition/cyclization/carbon–carbon cleavage reaction for the convenient synthesis of 2-acyl trisubstituted furans has been developed.

Metal-free thioesterification of amides generating acyl thioesters

A base-initiated thioesterification of amides with various thiols is reported. This reaction can take place efficiently under metal-free and air-atmospheric conditions, and provides a facile and practically useful approach to the synthesis of valuable acyl thioesters.