Chengwei Liu

Twisted Amides: From Obscurity to Broadly Useful Transition‐Metal‐Catalyzed Reactions by N−C Amide Bond Activation

The concept of using amide bond distortion to modulate amidic resonance has been known for more than 75 years. Two classic twisted amides (bridged lactams) ingeniously designed and synthesized by Kirby and Stoltz to feature fully perpendicular amide bonds, and as a consequence emanate amino-ketone-like reactivity, are now routinely recognized in all organic chemistry textbooks. However, only recently the use of amide bond twist (distortion) has advanced to the general organic chemistry mainstream enabling a host of highly attractive N-C amide bond cross-coupling reactions of broad synthetic relevance. In this Minireview, we discuss recent progress in this area and present a detailed overview of the prominent role of amide bond destabilization as a driving force in the development of transition-metal-catalyzed cross-coupling reactions by N-C bond activation.

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.

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.

Chemoselective Ketone Synthesis by the Addition of Organometallics to N-Acylazetidines

A general and highly chemoselective method for the synthesis of ketones by the addition of organometallics to N-acylazetidines via stable tetrahedral intermediates is reported for the first time. The transformation is characterized by its wide substrate scope and exquisite selectivity for the ketone products even when a large excess of nucleophilic reagents is used. Even of broader interest is the use of N-acylazetidines as bench-stable, readily available amide acylating reagents, in which the reactivity is controlled by amide pyramidalization and strain of the four-membered ring to afford synthetically valuable building blocks.

Twisted N-Acyl-Hydantoins: Rotationally-Inverted Urea-Imides of Relevance in N–C(O) Cross-Coupling

We report a combined structural and computational study on the properties of twisted acyclic hydantoins. These compounds feature cyclic urea-imide moiety that is widely found in bioactive compounds and is structurally related to the classic bridged hydantoins proposed by Smissman more than 50 years ago. We demonstrate that C to N-substitution of the imide moiety in the succinimide ring to give hydantoin results in one of the most distorted acyclic amide bonds reported to date. The energetic properties of twisted acyclic hydantoins with respect to structures, resonance energies, barriers to rotation, and proton affinities are discussed. The energetic and structural properties of twisted acyclic hydantoins described provide a benchmark to facilitate the development of twisted amides based on the biorelevant cyclic urea-imide scaffold.