Towards the activity of twisted acyclic amides

Abstract

N,N-Boc₂ amides have emerged as the most common class of acyclic twisted amides that have been engaged in a range of C–N activation and cross-coupling processes of ubiquitous amide bonds. These amides are readily synthesized from primary amides through a site-selective tert-butoxycarbonylation. Due to the steric bulk of di-tert-butoxy groups, these amides exhibit significant CN bond twisting, which promotes N–C bond cleavage, facilitating their use in cross-coupling reactions. Herein, we present a computational blueprint for the CN bond rotation in N,N-Boc₂ amides, revealing that the rotational barrier and twist angle (τ) are influenced by the nature of the substituents at the sp² carbon position. Sterically hindered substituents exhibit the highest distortions, leading to lower rotation barriers. Rotation along the CN bond is accompanied by phenyl ring rotation to minimize steric clashes. A strong correlation between the rotational barriers and the HOMO energies is observed. These findings provide key insights into the fundamental role of amide bond distortion in C–N activation processes.