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.

Synthesis of acyl fluorides through deoxyfluorination of carboxylic acids

A direct deoxyfluorination of carboxylic acids by utilizing inorganic potassium fluoride (KF) as a safe and inexpensive fluoride source has been developed. Both aryl carboxylic acids and cinnamyl carboxylic acids could be efficiently transformed into valuable acyl fluorides in moderate to high yields with good functional group tolerance. A scale-up reaction could be carried out smoothly under solvent-free conditions, which further demonstrated the practicality of this reaction in organic synthesis.

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.

Nickel-Catalyzed Decarbonylative Reductive Alkylation of Aroyl Fluorides with Alkyl Bromides

This paper describes the nickel-catalyzed reductive alkylation of aroyl fluorides with alkyl bromides in a decarbonylative manner. In this reaction, various functional groups are well tolerated and the C(sp²)-C(sp³) bond can be constructed directly without the use of organometallic reagents. The present reaction is a cross-electrophile coupling via the radical pathway, affording the corresponding alkylarenes in moderate to good yields.

Mechanism-Driven Development of Group 10 Metal-Catalyzed Decarbonylative Coupling Reactions

Transition-metal-catalyzed cross-coupling reactions are widely used in both academia and industry for the construction of carbon-carbon and carbon-heteroatom bonds. The vast majority of cross-coupling reactions utilize aryl (pseudo)halides as the electrophilic coupling partner. Carboxylic acid derivatives (RC(O)X) represent a complementary class of electrophiles that can engage in decarbonylative couplings to produce analogous products. This decarbonylative approach offers the advantage that RC(O)X are abundant and inexpensive. In addition, decarbonylative coupling enables both intramolecular (between R and X of the carboxylic acid derivative) as well as intermolecular bond-forming reactions (in which an exogeneous nucleophile is coupled with the R group derived from RC(O)X). In these intermolecular reactions, the X-substituent on the carboxylic acid can be tuned to facilitate both oxidative addition and transmetalation, thus eliminating the need for an exogeneous base. This Account details our group's development of a diverse variety of base-free decarbonylative coupling reactions catalyzed by group 10 metals. Furthermore, it highlights how catalyst design can be guided by stoichiometric organometallic studies of these systems.Our early studies focused on intramolecular decarbonylative couplings that transform RC(O)X to the corresponding R-X with extrusion of CO. We first identified Pd and Ni monodentate phosphine catalysts that convert aryl thioesters (ArC(O)SR) to the corresponding thioethers (ArSR). We next expanded this reactivity to fluoroalkyl thioesters, using readily available fluoroalkyl carboxylic acids as the fluoroalkyl (RF) source. A Ni-phosphinoferrocene catalyst proved optimal, and the large bite angle bidentate ligand was necessary to promote the challenging RF-S bond-forming reductive elimination step.We next pursued intramolecular decarbonylative couplings of aroyl halides. Palladium-based catalysts bearing dialkylbiaryl ligands (e.g., BrettPhos) were identified as optimal for converting aroyl chlorides (ArC(O)Cl) to aryl chlorides (ArCl). These ligands were selected based on their ability to facilitate the key C-Cl bond-forming reductive elimination step of the catalytic cycle. In contrast, all attempts to convert aroyl fluorides [ArC(O)F)] to aryl fluorides (ArF) were unsuccessful with either Pd- or Ni-based catalysts. Organometallic studies of the Ni-system show that C(O)-F oxidative addition and CO deinsertion proceed smoothly, but the resulting nickel(II) aryl fluoride intermediate fails to undergo C-F bond-forming reductive elimination.In contrast to its inertness to reductive elimination, this nickel(II) aryl fluoride proved highly reactive toward transmetalation. The fluoride ligand serves as an internal base, such that no additional base is required. We leveraged this "transmetalation active" intermediate to achieve base-free Ni-catalyzed intermolecular decarbonylative coupling reactions between aroyl fluorides and boron reagents to access both biaryl and aryl-boronate ester products. By tuning the electrophile, transmetalating reagent, and catalyst, this same approach also proved applicable to base-free intermolecular decarbonylative fluoroalkylation (between difluoromethylacetyl fluoride and arylboronate esters) and aryl amination (between phenol esters and silyl amines).Moving forward, a key goal is to identify catalyst systems that enable more challenging bond constructions via this manifold. In addition, CO inhibition remains a major issue leading to the requirement for high temperatures and high catalyst loadings. Identifying catalysts that are resistant to CO binding and/or approaches to remove CO under mild conditions will be critical for making these reactions more practical and scalable.

Recent Advances in C–F Bond Activation of Acyl Fluorides Directed toward Catalytic Transformation by Transition Metals, N-Heterocyclic Carbenes, or Phosphines

Numerous studies on the activation of carbon–fluorine bonds have been reported in recent years. For example, acyl fluorides have been utilized as versatile reagents for acylation, arylation, and even fluorination. In this review, we focus on acyl fluorides as compounds with carbon–fluorine bonds, and highlight recent advances in strategies for the activation of their C–F bonds via transition-metal catalysis, N-heterocyclic carbene (NHCs) catalysis, organophosphine catalysis, and classical nucleophilic substitution reactions.

1 Introduction

2 Transition-Metal-Mediated C–F Bond Activation

2.1 Acylation (Carbonyl-Retentive) Coupling Reactions

2.2 Decarbonylative Reactions

2.3 C–F Bond Activation by Other Transition Metals

3 C–F Bond Activation by N-Heterocyclic Carbenes (NHCs)

3.1 NHC-Catalyzed Cycloaddition of Acyl Fluorides

3.2 NHC-Catalyzed Radical Functionalization of Acyl Fluorides

3.3 NHC-Catalyzed Nucleophilic Fluorination of (Hetero)aromatics

4 C–F Bond Activation by Phosphines

4.1 Phosphine-Catalyzed Direct Activation of the C–F Bond of Acyl Fluorides

4.2 Phosphine-Catalyzed Indirect Activation of the C–F Bond of Acyl Fluorides

5 C–F Bond Activation by Classical Nucleophilic Substitution

6 Miscellaneous Examples

7 Summary and Perspective

Electrophilic N-trifluoromethylthiophthalimide as a fluorinated reagent in the synthesis of acyl fluorides

Herein we report the deoxygenated fluorination of readily available carboxylic acids. A series of acyl fluorides have been synthesized using shelf-stable N-trifluoromethylthiophthalimide as a fluorinated reagent for the first time.

Palladium/Copper-Cocatalyzed Arylsilylation of Internal Alkynes with Acyl Fluorides and Silylboranes: Synthesis of Tetrasubstituted Alkenylsilanes by Three-Component Coupling Reaction

In this Letter, the palladium/copper-cocatalyzed arylsilylation of internal alkynes with acyl fluorides and silylboranes is described. This is the first example in which acyl fluorides have been utilized for the three-component coupling reaction via decarbonylation, yielding a variety of tetrasubstituted alkenylsilanes in moderate to good yields.

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.

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.

Palladium-Catalyzed Decarbonylative Borylation of Aryl Anhydrides

A palladium-catalyzed base-free decarbonylative borylation of aryl anhydrides has been developed. Catalyst system consisting of Pd(OAc)₂/dppb enables readily available aryl anhydrides to be employed as electrophiles for the synthesis of versatile arylboronate esters via O-C(O) bond activation and decarbonylation. This method is characterized by an excellent functional group tolerance and broad substrate scope, using bench stable aryl anhydrides as aryl electrophiles in C-B bond formation. Mechanistic studies and functionalization of late-stage pharmaceutical molecules are disclosed.

Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel-Catalyzed Direct Decarbonylative Borylation

The Ni-catalyzed decarbonylative borylation of (hetero)aryl carboxylic acids with B₂ cat₂ has been achieved without recourse to any additives. This Ni-catalyzed method exhibits a broad substrate scope covering poorly reactive non-ortho-substituted (hetero)aryl carboxylic acids, and tolerates diverse functional groups including some of the groups active to Ni⁰ catalysts. The key to achieve this decarbonylative borylation reaction is the choice of B₂ cat₂ as a coupling partner that not only acts as a borylating reagent, but also chemoselectively activates aryl carboxylic acids towards oxidative addition of their C(acyl)-O bond to Ni⁰ catalyst via the formation of acyloxyboron compounds. A combination of experimental and computational studies reveals a detailed plausible mechanism for this reaction system, which involves a hitherto unknown concerted decarbonylation and reductive elimination step that generates the aryl boronic ester product. This mode of boron-promoted carboxylic acid activation is also applicable to other types of reactions.

Acyl fluorides as direct precursors to fluoride ketyl radicals: reductive deuteration using SmI2 and D2O

A highly chemoselective reductive deuteration of acyl fluorides to provide α,α-dideuterio alcohols with exquisite levels of deuterium incorporation was developed using SmI₂ and D₂O as the deuterium source. This method introduces acyl fluorides as attractive radical precursors for the generation of reactive acyl-type fluoride ketyls that should find widespread application in many synthetic strategies involving single electron transfer processes.

Deoxyfluorination of Carboxylic Acids with CpFluor: Access to Acyl Fluorides and Amides

3,3-Difluoro-1,2-diphenylcyclopropene (CpFluor), a bench-stable fluorination reagent, has been developed in the deoxyfluorination of carboxylic acids to afford various acyl fluorides. This all-carbon-based fluorination reagent enabled the efficient transformation of (hetero)aryl, alkyl, alkenyl, and alkynyl carboxylic acids to the corresponding acyl fluorides under the neutral conditions. This deoxyfluorination method was featured by the synthesis of acyl fluorides with in-situ formed CpFluor, as well as the one-pot amidation reaction of carboxylic acids via in-situ formed acyl fluorides.

Acyl Fluorides from Carboxylic Acids, Aldehydes, or Alcohols under Oxidative Fluorination

We describe a novel reagent system to obtain acyl fluorides directly from three different functional group precursors: carboxylic acids, aldehydes, or alcohols. The transformation is achieved via a combination of trichloroisocyanuric acid and cesium fluoride, which facilitates the synthesis of various acyl fluorides in high yield (up to 99%). It can be applied to the late-stage functionalization of natural products and drug molecules that contain a carboxylic acid, an aldehyde, or an alcohol group.

Decarboxylative Cross-Coupling of Acyl Fluorides with Potassium Perfluorobenzoates

We report the transition metal-free decarboxylative cross-coupling reactions of acyl fluorides with potassium perfluorobenzoates. Compared with traditional transition metal-catalyzed cross-couplings, this protocol presents an extremely environmentally benign pathway to afford unsymmetrical diaryl ketones. To install perfluorophenyl groups, this method highlights highly selective, inexpensive, and nontoxic conditions. The reaction system tolerates various functional groups in acyl fluorides. Remarkably, all of the starting materials can be prepared from abundant carboxylic acids and the reaction proceeds without any catalysts and additives.

Methodologies and Strategies for Selective Borylation of C-Het and C-C Bonds

Organoborons have emerged as versatile building blocks in organic synthesis to achieve molecular diversity and as carboxylic acid bioisosteres with broad applicability in drug discovery. Traditionally, these compounds are prepared by the substitution of Grignard/lithium reagents with electrophilic boron species and Brown hydroboration. Recent developments have provided new routes for the efficient preparation of organoborons by applying reactions using chemical feedstocks with leaving groups. As compared to the previous methods that used organic halides (I, Br, and Cl), the direct borylation of less reactive C-Het and C-C bonds has become highly important to get efficiency and functional-group compatibility. This Review aims to provide a comprehensive overview of this topic, including (1) C-F bond borylation, (2) C-O bond borylation, (3) C-S bond borylation, (4) C-N bond borylation, and (5) C-C bond borylation. Considerable attention is given to the strategies and mechanisms involved. We expect that this Review will inspire chemists to discover more efficient transformations to expand this field.

Methoxylation of Acyl Fluorides with Tris(2,4,6-trimethoxyphenyl)phosphine via C-OMe Bond Cleavage under Metal-Free Conditions

Acyl fluorides are subjected to methoxylation with tris(2,4,6-trimethoxyphenyl)phosphine (TMPP) to afford the corresponding methyl esters in good to excellent yields. This transformation is featured by C(sp²)-OMe bond cleavage under metal-free conditions. Unprecedented utilization of TMPP as a methoxylating agent realized the installation of an OMe group into the desired products.

Pd-catalyzed fluoro-carbonylation of aryl, vinyl, and heteroaryl iodides using 2-(difluoromethoxy)-5-nitropyridine

Acyl fluorides have recently gained a lot of attention as robust and versatile synthetic tools in synthetic chemistry. While several synthetic routes to acyl fluorides have been reported, a procedure involving direct insertion of the "fluoro-carbonyl" moiety using a single reagent has not yet been realized. Here we report the preparation of acyl fluorides by palladium-catalyzed fluoro-carbonylation of aryl, vinyl, and heteroaryl iodides using 2-(difluoromethoxy)-5-nitropyridine under CO-free conditions. 2-(difluoromethoxy)-5-nitropyridine is a stable, colorless solid that can be used as an alternative to the toxic gaseous formyl fluoride, which is commonly used under fluoride catalysis conditions. A wide variety of acyl fluorides are efficiently and safely obtained in high yield (up to 99%). A broad range of functional groups is tolerated under the optimized reaction conditions and the method can be applied to the late-stage fluoro-carbonylation of structurally complex C_sp2-iodides, including bioactive derivatives, such as Fenofibrate, Isoxepac, and Tocopherol. Furthermore, the one-pot transformation of aryl-iodides, including drug-like molecules, into the corresponding amides by successive fluoro-carbonylation/amidation reactions, demonstrates the potential synthetic utility of this strategy.

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.

Palladium/copper-cocatalyzed decarbonylative alkynylation of acyl fluorides with alkynylsilanes: synthesis of unsymmetrical diarylethynes

Alkynylation of acyl fluorides with alkynylsilanes via palladium/copper-cocatalysed carbon–fluorine bond cleavage and sequential decarbonylation, which provide an efficient route to various unsymmetrical diarylethynes, have been disclosed.

Mechanism and Scope of Nickel-Catalyzed Decarbonylative Borylation of Carboxylic Acid Fluorides

This Article describes the development of a base-free, nickel-catalyzed decarbonylative coupling of carboxylic acid fluorides with diboron reagents to selectively afford aryl boronate ester products. Detailed studies were conducted to assess the relative rates of direct transmetalation between aryl boronate esters and diboron reagents and a bisphosphine nickel(aryl)(fluoride) intermediate. These investigations revealed that diboron reagents undergo transmetalation with this Ni(aryl)(fluoride) intermediate at rates significantly faster than their aryl boronate ester congeners. Furthermore, the reactivity of both boron reagents toward transmetalation is enhanced with increasing electrophilicity of the boron center. These mechanistic insights were leveraged to develop a catalytic decarbonylative borylation of acid fluorides that proved applicable to a variety of (hetero)aryl carboxylic acid fluorides as well as diverse diboron reagents. The acid fluorides can be generated in situ directly from carboxylic acids. Furthermore, the mechanistic studies directed the identification of various air-stable Ni pre-catalysts for this transformation.

Redox-Neutral Decarbonylative Cross-Couplings Coming of Age

Major progress has recently been made in the challenging redox-neutral decarbonylative cross-coupling of carboxylic acids. For example, the use of acid fluorides as effective cross-coupling partners has been found to enable control of the decarbonylation selectivity and facilitates challenging Pd⁰ -catalyzed nucleophilic trifluoromethylation and exogenous base-free Suzuki cross-coupling reactions. In another recent advance, the use of acid chlorides in room temperature difluoromethylation and direct decarbonylative cross-coupling of carboxylic acids allows these classical substrates to be used as aryl electrophiles in cross-coupling reactions. Further challenges that are yet to be addressed in redox-neutral decarbonylative cross-couplings are also briefly summarized.

PPh3-Assisted Esterification of Acyl Fluorides with Ethers via C(sp3)–O Bond Cleavage Accelerated by TBAT

We describe the (triphenylphosphine (PPh3)-assisted methoxylation of acyl fluorides with cyclopentyl methyl ether (CPME) accelerated by tetrabutylammonium difluorotriphenysilicate (TBAT) via regiospecific C–OMe bond cleavage. Easily available CPME is utilized not only as the solvent, but a methoxylating agent in this transformation. The present method is featured by C–O and C–F bond cleavage under metal-free conditions, good functional-group tolerance, and wide substrate scope. Mechanistic studies revealed that the radical process was not involved.