Guangrong Meng

Synthesis of Biaryls through Nickel-Catalyzed Suzuki-Miyaura Coupling of Amides by Carbon-Nitrogen Bond Cleavage.

The first Ni-catalyzed Suzuki-Miyaura coupling of amides for the synthesis of widely occurring biaryl compounds through N-C amide bond activation is reported. The reaction tolerates a wide range of electron-withdrawing, electron-neutral, and electron-donating substituents on both coupling partners. The reaction constitutes the first example of the Ni-catalyzed generation of aryl electrophiles from bench-stable amides with potential applications for a broad range of organometallic reactions.

General Olefin Synthesis by the Palladium‐Catalyzed Heck Reaction of Amides: Sterically Controlled Chemoselective NC Activation

Metal-catalyzed reactions of amides proceeding via metal insertion into the N-CO bond are severely underdeveloped due to resonance stabilization of the amide bond. Herein we report the first Heck reaction of amides proceeding via highly chemoselective N-CO cleavage catalyzed by Pd(0) utilizing amide bond ground-state destabilization. Conceptually, this transformation provides access to a myriad of metal-catalyzed transformations of amides via metal insertion/decarbonylation.

Sterically Controlled Pd-Catalyzed Chemoselective Ketone Synthesis via N-C Cleavage in Twisted Amides.

Highly chemoselective, palladium(0)-catalyzed, direct cross-coupling between boronic acids and geometrically activated amides is reported. The reaction proceeds via selective activation of the N-C(O) bond, shows excellent functional group tolerance, and delivers the versatile ketone products in high yields. The observed reactivity is consistent with a decrease of nN → π*C═O conjugation resulting from destabilization of the amide ground state. Notably, the method provides direct access to acyl-metal intermediates from sterically distorted, bench-stable amide precursors under mild catalytic conditions.

Rhodium-Catalyzed C–H Bond Functionalization with Amides by Double C–H/C–N Bond Activation

The first C-H bond functionalization with amides as the coupling partners via selective activation of the amide N-C bond using rhodium(I) catalysts under highly chemoselective conditions is reported. Notably, this report constitutes the first catalytic activation of the amide N-C(O) bond by rhodium. We expect that this concept will have broad implications for using amides as coupling partners for C-H activation beyond the work described herein.

N-Acylsaccharins: Stable Electrophilic Amide-Based Acyl Transfer Reagents in Pd-Catalyzed Suzuki-Miyaura Coupling via N-C Cleavage.

The development of efficient catalytic methods for N-C bond cleavage in amides remains an important synthetic challenge. The first Pd-catalyzed Suzuki-Miyaura cross-coupling of N-acylsaccharins with boronic acids by selective N-C bond activation is reported. The reaction enables preparation of a variety of functionalized diaryl and alkyl-aryl ketones with broad functional group tolerance and in good to excellent yields. Of general interest, N-acylsaccharins serve as new, highly reactive, bench-stable, economical, amide-based, electrophilic acyl transfer reagents via acyl-metal intermediates. Mechanistic studies strongly support the amide N-C(O) bond twist as the enabling feature of N-acylsaccharins in the N-C bond cleavage.

Ground-State Distortion in N-Acyl-tert-butyl-carbamates (Boc) and N-Acyl-tosylamides (Ts): Twisted Amides of Relevance to Amide N-C Cross-Coupling.

Amide N-C(O) bonds are generally unreactive in cross-coupling reactions employing low-valent transition metals due to nN → π*C═O resonance. Herein we demonstrate that N-acyl-tert-butyl-carbamates (Boc) and N-acyl-tosylamides (Ts), two classes of acyclic amides that have recently enabled the development of elusive amide bond N-C cross-coupling reactions with organometallic reagents, are intrinsically twisted around the N-C(O) axis. The data have important implications for the design of new amide cross-coupling reactions with the N-C(O) amide bond cleavage as a key step.

Structures of Highly Twisted Amides Relevant to Amide N-C Cross-Coupling: Evidence for Ground-State Amide Destabilization.

Herein, we show that acyclic amides that have recently enabled a series of elusive transition-metal-catalyzed N-C activation/cross-coupling reactions are highly twisted around the N-C(O) axis by a new destabilization mechanism of the amide bond. A unique effect of the N-glutarimide substituent, leading to uniformly high twist (ca. 90°) irrespective of the steric effect at the carbon side of the amide bond has been found. This represents the first example of a twisted amide that does not bear significant steric hindrance at the α-carbon atom. The (15) N NMR data show linear correlations between electron density at nitrogen and amide bond twist. This study strongly supports the concept of amide bond ground-state twist as a blueprint for activation of amides toward N-C bond cleavage. The new mechanism offers considerable opportunities for organic synthesis and biological processes involving non-planar amide bonds.

A General Method for Two-Step Transamidation of Secondary Amides Using Commercially Available, Air- and Moisture-Stable Palladium/NHC (N-Heterocyclic Carbene) Complexes

The first general method is reported for transamidation of secondary carboxamides catalyzed by Pd-NHC (NHC = N-heterocyclic carbene) complexes. Commercially available, air- and moisture-stable (NHC)Pd(R-allyl)Cl complexes can effect C-N cross-coupling of a wide range of N-Boc and N-Ts amides, obtained by selective amide N-functionalization, with non-nucleophilic anilines and sterically hindered amines in very good yields. The first use of versatile Pd-NHC complexes as catalysts is represented for transition-metal-catalyzed C(acyl)-N amination of amides by N-C activation.

Resonance Destabilization in N-Acylanilines (Anilides): Electronically-Activated Planar Amides of Relevance in N–C(O) Cross-Coupling

Transition-metal-catalyzed activation of amide N-C(O) bonds proceeds via selective metal insertion into the carbon-nitrogen amide bond. Herein, we demonstrate that N-acylanilines (anilides), the first class of planar amides that have been shown to undergo selective amide N-C cross-coupling reactions, feature a significantly decreased barrier to rotation around the amide N-C(O) bond. Most significantly, we demonstrate that amide nN → π*C═O resonance in simple anilides can be varied by as much as 10 kcal/mol. The data have important implications for the design of N-C(O) amide cross-coupling reactions and control of the molecular conformation of anilides by resonance effects.

Pd-PEPPSI: Pd-NHC Precatalyst for Suzuki–Miyaura Cross-Coupling Reactions of Amides

Pd-PEPPSI-IPr serves as a highly reactive precatalyst in the direct Suzuki-Miyaura cross-coupling of amides. An array of amides can be cross-coupled with a range of arylboronic acids in very good yields using a single, operationally simple protocol. The studies described represent the first use of versatile PEPPSI type of Pd-NHC complexes as catalysts for the cross-coupling of amides by N-C bond activation. The method is user-friendly, since it employs a commercially available, air- and moisture-stable Pd-PEPPSI-IPr complex. Pd-PEPPSI-IPr provides a significant improvement over all current Pd/phosphane catalysts for amide N-C bond activation. Mechanistic studies provide insight into the reaction rates of Pd-NHC-catalyzed cross-coupling of different amides, with Pd-PEPPSI-IPr being particularly effective for the cross-coupling of N-Boc carbamates under the developed conditions.