Nickel-Catalyzed Decarbonylative Cyanation of Acyl Chlorides

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

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.

Formation of C(sp2)-C(sp3) Bonds Instead of Amide C-N Bonds from Carboxylic Acid and Amine Substrate Pools by Decarbonylative Cross-Electrophile Coupling

Carbon-heteroatom bonds, most often amide and ester bonds, are the standard method to link together two complex fragments because carboxylic acids, amines, and alcohols are ubiquitous and the reactions are reliable. However, C-N and C-O linkages are often a metabolic liability because they are prone to hydrolysis. While C(sp2)-C(sp3) linkages are preferable in many cases, methods to make them require different starting materials or are less functional-group-compatible. We show here a new, decarbonylative reaction that forms C(sp2)-C(sp3) bonds from the reaction of activated carboxylic acids (via 2-pyridyl esters) with activated alkyl groups derived from amines (via N-alkyl pyridinium salts) and alcohols (via alkyl halides). Key to this process is a remarkably fast, reversible oxidative addition/decarbonylation sequence enabled by pyridone and bipyridine ligands that, under reaction conditions that purge CO(g), lead to a selective reaction. The conditions are mild enough to allow coupling of more complex fragments, such as those used in drug development, and this is demonstrated in the coupling of a typical Proteolysis Targeting Chimera (PROTAC) anchor with common linkers via C-C linkages.

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.

Photochemically Enabled, Ni-Catalyzed Cyanation of Aryl Halides

A light-promoted Ni-catalyzed cyanation of aryl halides employing 1,4-dicyanobenzene as a cyanating agent is reported. A broad array of aryl bromides, chlorides, and druglike molecules could be converted into their corresponding nitriles (65 examples). Mechanistic studies suggest that upon irradiation, the oxidative addition product Ni(II)(dtbbpy)(p-C₆H₄CN)(CN) undergoes homolytic cleavage of the Ni-aryl bond to generate an aryl radical and a Ni(I)-CN species, the latter of which initiates subsequent cyanation reactions.

Decarbonylative Fluoroalkylation at Palladium(II): From Fundamental Organometallic Studies to Catalysis

This Article describes the development of a decarbonylative Pd-catalyzed aryl-fluoroalkyl bond-forming reaction that couples fluoroalkylcarboxylic acid-derived electrophiles [R_FC(O)X] with aryl organometallics (Ar-M'). This reaction was optimized by interrogating the individual steps of the catalytic cycle (oxidative addition, carbonyl de-insertion, transmetalation, and reductive elimination) to identify a compatible pair of coupling partners and an appropriate Pd catalyst. These stoichiometric organometallic studies revealed several critical elements for reaction design. First, uncatalyzed background reactions between R_FC(O)X and Ar-M' can be avoided by using M' = boronate ester. Second, carbonyl de-insertion and Ar-R_F reductive elimination are the two slowest steps of the catalytic cycle when R_F = CF₃. Both steps are dramatically accelerated upon changing to R_F = CHF₂. Computational studies reveal that a favorable F₂C-H---X interaction contributes to accelerating carbonyl de-insertion in this system. Finally, transmetalation is slow with X = difluoroacetate but fast with X = F. Ultimately, these studies enabled the development of an (SPhos)Pd-catalyzed decarbonylative difluoromethylation of aryl neopentylglycol boronate esters with difluoroacetyl fluoride.

Cu2 O-Catalyzed Conversion of Benzyl Alcohols Into Aromatic Nitriles via the Complete Cleavage of the C≡N Triple Bond in the Cyanide Anion

Nitrogen transfer from cyanide anion to an aldehyde is emerging as a promising method for the synthesis of aromatic nitriles. However, this method still suffers from a disadvantage that a use of stoichiometric Cu(II) or Cu(I) salts is required to enable the reaction. As we report herein, we overcame this drawback and developed a catalytic method for nitrogen transfer from cyanide anion to an alcohol via the complete cleavage of the C≡N triple bond using phen/Cu₂ O as the catalyst. The present condition allowed a series of benzyl alcohols to be smoothly converted into aromatic nitriles in moderate to high yields. In addition, the present method could be extended to the conversion of cinnamic alcohol to 3-phenylacrylonitrile.

Palladium-catalyzed decarbonylative and decarboxylative cross-coupling of acyl chlorides with potassium perfluorobenzoates affording unsymmetrical biaryls

This paper describes the synthesis of unsymmetrical biaryls by the palladium-catalyzed cross-coupling reaction of acyl chlorides with potassium perfluorobenzoates. This transformation is unique in that it involves simultaneous decarbonylation and decarboxylation under redox-neutral conditions. Compared to conventional cross-coupling protocols for the synthesis of unsymmetrical biaryls, the two reactants in this synthetic strategy can be readily prepared from abundant and inexpensive aromatic carboxylic acids.

Manganese-Catalyzed Electrochemical Tandem Azidation-Coarctate Reaction: Easy Access to 2-Azo-benzonitriles

A one-pot cascade transformation consisting of an electrochemically driven azidation of 2H-indazole followed by coarctate fragmentation is developed to synthesize the 2-azo-benzonitrile motif. This manganese-catalyzed transformation is external-chemical-oxidant-free and operates at ambient temperature under air. This methodology exhibits good functional group tolerance, affording a broad range of substrate scopes of up to 89% isolated yield. Diverse derivatization of the 2-azo-benzonitrile product resulted in other valuable scaffolds.

The ammoxidation of alcohols over heterogeneous catalysts for the green synthesis of nitriles

This is the first review on the ammoxidation of alcohols over heterogeneous catalysts, in which issues and potential solutions are demonstrated.

Decarbonylative Suzuki-Miyaura Cross-Coupling of Aroyl Chlorides

Herein, we report a catalyst system for Pd-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of aroyl chlorides with boronic acids to furnish biaryls. This strategy is suitable for a broad range of common aroyl chlorides and boronic acids. The synthetic utility is highlighted in the direct late-stage functionalization of pharmaceuticals and natural products capitalizing on the presence of carboxylic acid moiety. Extensive mechanistic and DFT studies provide key insight into the reaction mechanism and high decarbonylative cross-coupling selectivity.

Synthesis of Indolizine Derivatives Triggered by the Oxidative Addition of Aroyl Chloride to Pd(0) Complex

An efficient synthesis of indolizine derivatives from propargylic pyridines and aroyl chlorides was developed. The 5-endo-dig cyclization was initiated by the in situ formed acylpalladium species from the facile oxidative addition of aroyl chloride to Pd(0) complex. This transformation successfully occurred in the presence of an N-nucleophilic moiety and acid chlorides, a good electrophilic partner, affording highly functionalized indolizines in good-to-excellent yields.

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.