Kumada Arylation of Secondary Amides Enabled by Chromium Catalysis for Unsymmetric Ketone Synthesis under Mild Conditions

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.

Meerwein Arylation of Aryl(alkyl)idenemalononitriles and Diazonium Salts for the Synthesis of 2-(Aryl(alkyl)/arylmethylene)malononitrile Derivatives

A metal-free Meerwein arylation reaction from aryl(alkyl)idenemalononitriles and diazonium salts for the synthesis of 2-(aryl(alkyl)/arylmethylene)malononitrile derivatives under mild conditions was well developed. Different from the general addition reactions between alkenes and diazonium salts, this study performed the traditional coupling reaction for the formation of C(sp2)-C(sp2) bond arylation products. The radical reaction mechanism was well verified in the control experiments. The other advantages of the approach are broad-scope substrates and good group tolerance. Moreover, the obtained products can be readily converted into high-value asymmetric ketones and hydrogenation reactions.

Tamarindus indica seed ash extract for C-C coupling under added organics and volatile organic solvent-free conditions: a waste repurposing technique for Suzuki-Miyaura reaction

A tremendous research has been appeared on Pd-catalyzed Suzuki-Miyaura cross-coupling (SMC) during the last four decades due to its high prominence in constructing biaryl motifs of several complexes as well as simple organic compounds of high biological and commercial significance. The use of organic solid waste-derived materials for SMC in benign solvents like water/aqueous media is a very good achievement in these cases. We report in this article the usability of water extract of Tamarindus indica seeds ash (WETS) as a renewable base and reaction medium for Pd(OAc)₂-catalyzed SMC reaction at room temperature (RT). The WETS has been characterized using powder XRD, EDAX, SEM, and FTIR analysis. Furthermore, this process is highly environmentally beneficial by the waste repurposing to prominent chemical transformation along with the advantages such as ambient condition and avoids non-renewable chemicals like volatile organic solvents, ligands, promoters, and bases. Based on these merits and the quick reactions with high yields of products, this method can attain the interest of the scientific community in exploring the waste-derived ashes to significant chemical transformations. Tamarindus indica seed ash extract for C-C coupling under added organics and volatile organic solvent-free conditions: a waste repurposing technique for Suzuki-Miyaura reaction.

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.

Sonogashira Cross-Coupling of Aryl Ammonium Salts by Selective C-N Activation Catalyzed by Air- and Moisture-Stable, Highly Active [Pd(NHC)(3-CF3-An)Cl2] (An = Aniline) Precatalysts

We report the Sonogashira cross-coupling of aryl ammonium salts catalyzed by air- and moisture-stable [Pd(NHC)(3-CF₃-An)Cl₂] (An = aniline). This highly active Pd(II)-NHC complex features broad scope and excellent C-N activation selectivity in the challenging alkynylative cross-coupling of aryl ammonium salts. Full structural characterization and computational studies demonstrate the effect of pyridine to aniline replacement as highly effective stabilizing ancillary ligand in well-defined Pd(II)-NHCs. Considering the high reactivity and the recent commercialization of [Pd(NHC)(3-CF₃-An)Cl₂] (Millipore Sigma, no. 915165), this catalyst represents an attractive approach to the activation of C-N bonds of broad synthetic interest.

Dihydroquinazolinones as adaptative C(sp3) handles in arylations and alkylations via dual catalytic C-C bond-functionalization

C–C bond forming cross-couplings are convenient technologies for the construction of functional molecules. Consequently, there is continual interest in approaches that can render traditionally inert functionality as cross-coupling partners, included in this are ketones which are widely-available commodity chemicals and easy to install synthetic handles. Herein, we describe a dual catalytic strategy that utilizes dihydroquinazolinones derived from ketone congeners as adaptative one-electron handles for forging C(sp³) architectures via α C–C cleavage with aryl and alkyl bromides. Our approach is achieved by combining the flexibility and modularity of nickel catalysis with the propensity of photoredox events for generating open-shell reaction intermediates. This method is distinguished by its wide scope and broad application profile––including chemical diversification of advanced intermediates––, providing a catalytic technique complementary to existing C(sp³) cross-coupling reactions that operates within the C–C bond-functionalization arena.

Although derived from feedstock chemicals and therefore in principle abundant, ketones are not widely used as cross-coupling partners in organic synthesis. Herein, the authors use ketone derivatives as one-electron handles for forging C(sp3) architectures via dual photo- and nickel catalysis.

Transition-metal-catalyst-free reaction of amides and acetonitriles: synthesis of β-ketonitriles

The first example of the coupling reaction between amide and acetonitrile for the synthesis of β-ketonitriles was developed. Various amides provide the corresponding β-ketonitriles in good yields.

Continuous Flow Acylation of (Hetero)aryllithiums with Polyfunctional N,N-Dimethylamides and Tetramethylurea in Toluene

The continuous flow reaction of various aryl or heteroaryl bromides in toluene in the presence of THF (1.0 equiv) with sec -BuLi (1.1 equiv) provided at 25 °C within 40 sec the corresponding aryllithiums which were acylated with various functionalized N,N-dimethylamides including easily enolizable amides at -20 °C within 27 sec, producing highly functionalized ketones in 48-90% yield (36 examples). This method was well suited for the preparation of α-chiral ketones such as naproxene and ibuprofen derived ketones with 99% ee . A one-pot stepwise bis-addition of two different lithium organometallics to 1,1,3,3-tetramethyurea (TMU) provided unsymmetrical ketones in 69-79% yield (9 examples).

Mechanistic Diversity of Low-Valent Chromium Catalysis: Cross-Coupling and Hydrofunctionalization

ConspectusTransition-metal catalysis has traditionally been dominated by precious metals because of their high reactivity toward chemical transformations. As a cost-effective alternative, catalysis by earth-abundant group 6 metal chromium is underdeveloped, and its reactivity remains largely unexplored, although the industrially important Phillips catalyst, which is composed of Cr as the active metal, is currently used to supply almost 40% of the total world demand for high-density polyethylene. Cr has traditionally served in organoreagents with high-valent states (≥2+), which are typified by reactions involving Nozaki-Hiyama-Kishi (NHK) and Takai-Utimoto one-electron transfer processes. Given that low-valent metals usually facilitate the process of oxidative addition (OA), studying the catalysis of Cr in the low-valent state provides the opportunity to develop new transformations. However, probably because of the low stability of reactive low-valent Cr or the lack of catalytic activity of structurally stable complexes, there has been limited success with respect to developing catalysis promoted by low-valent Cr. In recent years, our group has probed the reactivity of low-valent Cr in catalysis by adopting a strategy of forming reactive Cr in situ. In this Account, we detail our efforts to study the catalytic behavior and mechanism of low-valent Cr in challenging transformations, such as the cleavage of chemically inert bonds for the cross-coupling and hydrofunctionalization of arenes and nitro motifs, by developing strategies to address the prominent selectivity issues. We highlight the finding that low-valent Cr, being formed in situ, possesses the intriguing ability to promote the catalytic cleavage of unactivated C-O, C-N, and C-H bonds to achieve the Kumada couplings and even to enable challenging cross-coupling between two unactivated C(aryl)-O/C(aryl)-N bonds. During these catalytic processes, Cr usually adopts a high-spin state to interact with chemicals, allowing for insertion into unactivated σ-bonds. The OA catalytic model involving a two-electron process for the cleavage of unactivated bonds has rarely been considered for Cr. We highlight the finding that Cr allows for the breakage of two chemically inert bonds in one catalytic cycle. This ability is intriguing because most transition metals are suitable only for the cleavage of one unactivated bond in catalysis. Mechanisms involving two-electron OA for Cr are unusual, with processes involving one-electron transfer more often proposed, as exemplified in the NHK reactions. These reactions provide efficient strategies for forming functionalized benzaldehydes, amides, anilines, and amines, usually with high levels of selectivity. We hope that this account will extend the scope of cognition to Cr catalysis.

Designing a Cr-catalyst bearing redox non-innocent phenalenyl-based ligand towards hydrosilylative CO2 functionalization

Herein, we report the synthesis of a Cr(iii)-complex bearing a redox non-innocent phenalenyl-based ligand and its use as a catalyst for SET mediated hydrosilylative reduction of carbon dioxide towards formylation of primary amides under mild conditions. A preliminary mechanistic picture for this transformation has been proposed by isolation and characterization of several reactive intermediates.

Rh-Catalyzed Base-Free Decarbonylative Borylation of Twisted Amides

We report the rhodium-catalyzed base-free decarbonylative borylation of twisted amides. The synthesis of versatile arylboronate esters from aryl twisted amides is achieved via decarbonylative rhodium(I) catalysis and highly selective N-C(O) insertion. The method is notable for a very practical, additive-free Rh(I) catalyst system. The method shows broad functional group tolerance and excellent substrate scope, including site-selective decarbonylative borylation/Heck cross-coupling via divergent N-C/C-Br cleavage and late-stage pharmaceutical borylation.

Coupling of amides with ketones via C–N/C–H bond cleavage: a mild synthesis of 1,3-diketones

Amides react with enolizable ketones to give 1,3-diketones via C–N cleavage of amides and deprotonation of 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.

Palladium-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of amides by carbon-nitrogen bond activation

Palladium-catalyzed Suzuki-Miyaura cross-coupling or aryl halides is widely employed in the synthesis of many important molecules in synthetic chemistry, including pharmaceuticals, polymers and functional materials. Herein, we disclose the first palladium-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of amides for the synthesis of biaryls through the selective activation of the N-C(O) bond of amides. This new method relies on the precise sequence engineering of the catalytic cycle, wherein decarbonylation occurs prior to the transmetallation step. The reaction is compatible with a wide range of boronic acids and amides, providing valuable biaryls in high yields (>60 examples). DFT studies support a mechanism involving oxidative addition, decarbonylation and transmetallation and provide insight into high N-C(O) bond activation selectivity. Most crucially, the reaction establishes the use of palladium catalysis in the biaryl Suzuki-Miyaura cross-coupling of the amide bond and should enable the design of a wide variety of cross-coupling methods in which palladium rivals the traditional biaryl synthesis from aryl halides and pseudohalides.

i-Pr2NMgCl·LiCl Enables the Synthesis of Ketones by Direct Addition of Grignard Reagents to Carboxylate Anions

The direct preparation of ketones from carboxylate anions is greatly limited by the required use of organolithium reagents or activated acyl sources that need to be independently prepared. Herein, a specific magnesium amide additive is used to activate and control the addition of more tolerant Grignard reagents to carboxylate anions. This strategy enables the modular synthesis of ketones from CO₂ and the preparation of isotopically labeled pharmaceutical building blocks in a single operation.

Iron(0)-Mediated Reformatsky Reaction for the Synthesis of β-Hydroxyl Carbonyl Compounds

An efficient, economical, and practical Reformatsky reaction of α-halo carbonyl compounds with aldehydes/ketones by using cheap and commercial iron(0) powder as reaction mediator is developed. The reactions proceeded effectively in the presence of a catalytic amount of iodine (20 mol %) to afford the synthetically useful β-hydroxyl carbonyl compounds in moderate to good yields.

Carbon-Carbon Bond Formation of Trifluoroacetyl Amides with Grignard Reagents via C(O)-CF3 Bond Cleavage

The reaction of trifluoroacetyl amides with Grignard reagent for the substitution of CF₃ group with various alkyl or aryl groups is described. A variety of aryl, quinolin-8-yl, and (hetero)alkyl functional groups as well as F, Cl, and Br atoms are well tolerated. These moisture-stable and easily available trifluoroacetyl amides can be conveniently obtained and used as new versatile precursors for isocyanates. The control experiments show that the reaction proceeds via an isocyanate intermediate and/or alkoxide/amide dual anionic intermediate.

[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.

Robust Chromium Precursors for Catalysis: Isolation and Structure of a Single-Component Ethylene Tetramerization Precatalyst

We have introduced a new class of stable organometallic Cr reagents (compounds 1-4) that are readily prepared, yet reactive enough to serve as precursors. They were used for ethylene tetramerization catalysis following stoichiometric activation by in situ protonation. This study highlights the importance of balancing stability with reactivity in generating an organometallic precursor that is useful in catalysis. Moreover, precursor 4 allowed for the isolation and crystallographic characterization of a room-temperature stable cationic species, (PNP)CrR₂⁺ (R = o-C₆H₄(CH₂)₂OMe, PNP = ⁱPrN(PPh₂)₂). This complex (5) may be used as a single component precatalyst, without any alkylaluminum reagents. This result provides an unprecedented level of insight into the kind of structures that must be produced from more complicated activation processes.

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.

Recent Advances in Acyl Suzuki Cross-Coupling

Acyl Suzuki cross-coupling involves the coupling of an organoboron reagent with an acyl electrophile (acyl halide, anhydride, ester, amide). This review provides a timely overview of the very important advances that have recently taken place in the acylative Suzuki cross-coupling. Particular emphasis is directed toward the type of acyl electrophiles, catalyst systems and new cross-coupling partners. This review will be of value to synthetic chemists involved in this rapidly developing field of Suzuki cross-coupling as well as those interested in using acylative Suzuki cross-coupling for the synthesis of ketones as a catalytic alternative to stoichiometric nucleophilic additions or Friedel-Crafts reactions.