Decarbonylative Borylation of Amides by Palladium Catalysis

Decarbonylative borylation of aryl anhydrides via rhodium catalysis

Decarbonylative borylation of aryl anhydrides by rhodium catalysis has been reported. A base-free system with Rh(PPh3)3Cl as a catalyst enables the efficient synthesis of various arylboronate esters from readily available aryl anhydrides. The reaction involves the cleavage of C(O)-O bonds and the formation of C-B bonds. The experimental results demonstrated that compared with carboxylic acids, amides, and esters, anhydrides have higher reactivity in the decarbonylative borylation reaction under the current conditions. Furthermore, compared with the reported palladium-catalyzed borylation reaction of aryl anhydrides, the present rhodium-catalyzed method has the advantages of a shorter reaction time and a lower reaction temperature.

Recent Expedition in Pd- and Rh-Catalyzed C(Ar) -B Bond Formations and Their Applications in Modern Organic Syntheses

Transition metal-catalyzed borylation has emerged as a powerful and versatile strategy for synthesizing organoboron compounds. These compounds have found widespread applications in various aspects, including organic synthesis, materials science, and medicinal chemistry. This review provides a concise summary of the recent advances in palladium- and rhodium-catalyzed borylation from 2013 to 2023. The review covers the representative examples of catalysts, substrates scope and reaction conditions, with particular emphasis on the development of catalyst systems, such as phosphine ligands, NHC-carbene, and more. The diverse array of borylative products obtained for further applications in Suzuki-Miyaura coupling, and other transformations, are also discussed. Future directions in this rapidly evolving field, with the goal of designing more efficient, selective borylation methodologies are highlighted, too.

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.

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.

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.

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.

Nickel(II)-Catalyzed Borylation of Alkenyl Methyl Ethers via C-O Bond Cleavage

A new protocol has been developed for the borylation of conjugated alkenyl methyl ethers using B₂Pin₂ via C-O bond cleavage catalyzed by Ni(II). In this cross-coupling reaction, both E/Z isomers of alkenyl ethers are converted into (E)-alkenyl boronic esters with good reactivity. This transformation exhibits high chemoselectivity in the presence of competitive C-O bonds such as aryl ether, ester, amide, and thioether groups, thus providing a new method for the construction of various alkenyl boronates.

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.

Facile Synthesis of NH-Free 5-(Hetero)Aryl-Pyrrole-2-Carboxylates by Catalytic C-H Borylation and Suzuki Coupling

A convenient two-step preparation of NH-free 5-aryl-pyrrole-2-carboxylates is described. The synthetic route consists of catalytic borylation of commercially available pyrrole-2-carboxylate ester followed by Suzuki coupling without going through pyrrole N–H protection and deprotection steps. The resulting 5-aryl substituted pyrrole-2-carboxylates were synthesized in good- to excellent yields. This synthetic route can tolerate a variety of functional groups including those with acidic protons on the aryl bromide coupling partner. This methodology is also applicable for cross-coupling with heteroaryl bromides to yield pyrrole-thiophene, pyrrole-pyridine, and 2,3’-bi-pyrrole based bi-heteroaryls.

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.

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.

Nickel-Catalyzed Decarbonylative Stannylation of Acyl Fluorides under Ligand-Free Conditions

Nickel-catalyzed decarbonylative stannylation of acyl fluorides under ligand-free conditions was disclosed. A variety of aromatic acyl fluorides are capable of reacting with silylstannanes in the presence of cesium fluoride. A one-pot decarbonylative stannylation/Migita-Kosugi-Stille reaction of benzoyl fluoride, giving rise to the direct formation of the corresponding cross-coupled products, further demonstrated the synthetic utility of the present method. This newly developed methodology with a good functional-group compatibility via C-F bond cleavage and C-Sn bond formation under nickel catalysis opens a new area for the functionalization of acyl fluorides in terms of carbon-heteroatom bond formation.