Breaking Amides using Nickel Catalysis

Nickel-catalyzed transamidation of aliphatic amide derivatives

Transamidation, or the conversion of one amide to another, is a long-standing challenge in organic synthesis. Although notable progress has been made in the transamidation of primary amides, the transamidation of secondary amides has remained underdeveloped, especially when considering aliphatic substrates. Herein, we report a two-step approach to achieve the transamidation of secondary aliphatic amides, which relies on non-precious metal catalysis. The method involves initial Boc-functionalization of secondary amide substrates to weaken the amide C-N bond. Subsequent treatment with a nickel catalyst, in the presence of an appropriate amine coupling partner, then delivers the net transamidated products. The transformation proceeds in synthetically useful yields across a range of substrates. A series of competition experiments delineate selectivity patterns that should influence future synthetic design. Moreover, the transamidation of Boc-activated secondary amide derivatives bearing epimerizable stereocenters underscores the mildness and synthetic utility of this methodology. This study provides the most general solution to the classic problem of secondary amide transamidation reported to date.

Suzuki-Miyaura cross-coupling of amides and esters at room temperature: correlation with barriers to rotation around C-N and C-O bonds

The Suzuki-Miyaura cross-coupling has been widely recognized as one of the most important methods for the construction of C-C bonds. However, in contrast to traditional aryl halide or pseudohalide electrophiles, coupling reactions with unactivated C-N and C-O electrophiles have proven significantly more challenging. Here we report the first general palladium-catalyzed Suzuki-Miyaura cross-coupling of both common amides and aryl esters through the selective cleavage of the C-N and C-O bonds under exceedingly mild conditions. Notably, for the first time we demonstrate selective C(acyl)-N and C(acyl)-O cleavage/cross-coupling under the same reaction conditions. The reaction uses a commercially available, bench-stable and operationally-convenient (η3-1-t-Bu-indenyl)Pd(IPr)(Cl) precatalyst. Furthermore, we demonstrate that the reactivity of generic amides and aryl esters can be correlated with barriers to isomerization around the C(acyl)-X (X = N, O) bond, thus providing a blueprint for the development of a broad range of novel coupling reactions of ester and amide electrophiles by the selective activation of C-O and C-N bonds.

N-Methylamino Pyrimidyl Amides (MAPA): Highly Reactive, Electronically-Activated Amides in Catalytic N-C(O) Cleavage

Despite recent progress in catalytic cross-coupling technologies, the direct activation of N-alkyl-N-aryl amides has been a challenging transformation. Here, we report the first Suzuki cross-coupling of N-methylamino pyrimidyl amides (MAPA) enabled by the controlled n_N → π_Ar conjugation and the resulting remodeling of the partial double bond character of the amide bond. The new mode of amide activation is suitable for generating acyl-metal intermediates from unactivated primary and secondary amides.

Hydroxamic Acids as Chemoselective (ortho-Amino)arylation Reagents via Sigmatropic Rearrangement

The use of readily available hydroxamic acids as reagents for the chemoselective (ortho-amino)arylation of amides is described. This reaction proceeds under metal-free, mild conditions, displays a very broad scope, and constitutes a direct approach for the metal-free attachment of aniline residues to carbonyl derivatives.

Kinetic Modeling of the Nickel-Catalyzed Esterification of Amides

Nickel-catalyzed coupling reactions provide exciting tools in chemical synthesis. However, most methodologies in this area require high catalyst loadings, which commonly range from 10-20 mol % nickel. Through an academic-industrial collaboration, we demonstrate that kinetic modeling can be used strategically to overcome this problem, specifically within the context of the Ni-catalyzed conversion of amides to esters. The successful application of this methodology to a multigram-scale coupling, using only 0.4 mol % Ni, highlights the impact of this endeavor.

Direct Conversion of Carboxylic Acids to Alkyl Ketones

An efficient and mild method for acyl-C_sp3 bond formation based on the direct conversion of carboxylic acids has been established. This protocol is enabled by the synergistic, Ir-photoredox/nickel catalytic cross-coupling of in situ activated carboxylic acids and alkyltrifluoroborates. This versatile method is amenable to the cross-coupling of structurally diverse carboxylic acids with various potassium alkyltrifluoroborates, affording the corresponding ketones with high yields. In this operationally simple cross-coupling protocol, aliphatic ketones are obtained in one step from bench stable, readily available carboxylic acids.

Mizoroki-Heck Cyclizations of Amide Derivatives for the Introduction of Quaternary Centers

We report non-decarbonylative Mizoroki-Heck reactions of amide derivatives. The transformation relies on the use of nickel catalysis and proceeds using sterically hindered tri- and tetrasubstituted olefins to give products containing quaternary centers. The resulting polycyclic or spirocyclic products can be obtained in good yields. Moreover, a diastereoselective variant of this method gives access to an adduct bearing vicinal, highly substituted sp3 stereocenters. These results demonstrate that amide derivatives can be used as building blocks for the assembly of complex scaffolds.

Synergistic Visible-Light Photoredox/Nickel-Catalyzed Synthesis of Aliphatic Ketones via N-C Cleavage of Imides

An electrophilic, imide-based, visible-light-promoted photoredox/Ni-catalyzed cross-coupling reaction for the synthesis of aliphatic ketones has been developed. This protocol proceeds through N-C(O) bond activation, made possible through the lower activation energy for metal insertion into this bond due to delocalization of the lone pair of electrons on the nitrogen by electron-withdrawing groups. The operationally simple and mild cross-coupling reaction is performed at ambient temperature and exhibits tolerance for a variety of functional groups.

σ N-C Bond Difunctionalization in Bridged Twisted Amides: Sew-and-Cut Activation Approach to Functionalized Isoquinolines

A rare example of highly selective σ N-C bond difunctionalization in bridged twisted lactams through N-C cleavage has been achieved. In combination with the intramolecular Heck cyclization, this method affords a two-step bond reorganization event ("sew-and-cut") to access functionalized isoquinoline ring systems directly with high atom economy. C-H bond functionalizations directed by a weakly coordinating bridged amide bond increase scaffold diversity. Preliminary mechanistic studies on the effect of amide distortion and the role of electrophile in this unusual σ N-C amide difunctionalization are described.

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.

Nickel-Catalyzed Reduction of Secondary and Tertiary Amides

The nickel-catalyzed reduction of secondary and tertiary amides to give amine products is reported. The transformation is tolerant of extensive variation with respect to the amide substrate, proceeds in the presence of esters and epimerizable stereocenters, and can be used to achieve the reduction of lactams. Moreover, this methodology provides a simple tactic for accessing medicinally relevant α-deuterated amines.

Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling of N-Mesylamides by N-C Cleavage: Electronic Effect of the Mesyl Group

A general Pd-catalyzed Suzuki-Miyaura cross-coupling of N-mesylamides with arylboronic acids by selective N-C cleavage has been developed. The presented results represent the first example of a transition-metal-catalyzed cross-coupling of amides activated by an atom-economic, cheap, and benign mesyl group. The reaction delivers arylated products featuring a range of useful functional groups by chemoselective cleavage of the amide N-C bond with high efficiency. Both the scope and the origin of high selectivity are discussed. A beneficial effect of the N-mesyl substituent on the bond activation in acyclic amides is presented.

Evidence and isolation of tetrahedral intermediates formed upon the addition of lithium carbenoids to Weinreb amides and N-acylpyrroles

Trapping tetrahedral intermediates from Weinreb amides andN-acylpyrroles.

Sc(OTf)3-catalyzed synthesis of anhydrides from twisted amides

A novel, one-step synthesis of anhydrides from twisted amides is reported.

Ni-Catalyzed cross-coupling reactions of N-acylpyrrole-type amides with organoboron reagents

We report the first Ni/bis-NHC-catalyzed cross-coupling of N-acylpyrrole-type amides with arylboronic esters to obtain diarylketones.

Theoretical study on the reaction mechanism of “ligandless” Ni-catalyzed hydrodesulfurization of aryl sulfide

The reaction mechanism of Ni(COD)₂ catalyzed hydrodesulfurization of aryl sulfide PhSMe with HSiMe₃ has been predicted to have two competitive reaction pathways, with or without PhSMe spectator ligand, by using density functional theory methods.

N-Acylsuccinimides: twist-controlled, acyl-transfer reagents in Suzuki–Miyaura cross-coupling by N–C amide bond activation

The palladium-catalyzed Suzuki–Miyaura cross-coupling of N-acylsuccinimides as versatile acyl-transfer reagents via selective amide N–C bond cleavage is reported.