Formation of Keteniminium Salts: Mechanistic Aspects and Substituent Effects

Keteniminium salts (KIs) are versatile intermediates in synthetic organic chemistry. Elucidation of the mechanistic aspects of KI formation reactions facilitates the design of KI intermediates that give access to complex compounds. In this study, in order to provide a comprehensive understanding of KI formation, various mechanisms were investigated using a density functional theory approach. Particularly, Ghosez’s KI formation mechanism, by activation of an amide with triflic anhydride, was extensively elaborated, since this procedure occurs under mild conditions and is, by far, the most frequently used method. Moreover, a broad range of substituents was examined to give insight on their potential contributions to the ease of formation of KIs. The effect of substituents on the reactivity of the corresponding starting amides was inspected by means of energetics, population analysis, frontier molecular orbitals (FMO) and reactivity descriptors. Computed data shows that electron donating groups lower the activation barrier by increasing the electron density of the amide carbonyl oxygen. Additionally, distortion/interaction model also confirmed the energetic outcomes. In addition, investigation of KI reactivity using FMO, and reactivity descriptors displayed that KI reactivity is inversely correlated with amide reactivity. Lastly, experimental outcomes are in line with computational predictions. We suggest that the reactivity of the amide has a crucial impact on the ease of KI formation and the reactivity of the corresponding KIs. This study gives pivotal insights into mechanistic aspects of KI formation and the role of the substituents.