A new specific mechanism for thioacid/azide amidation: electronic and solvent effects

Detailed theoretical studies of azide/thioacid amidation are performed using density functional theory. The calculated results indicate that electronic properties of azide have significant effects on reaction pathways, which result in two distinct mechanisms for electron-rich and electron-poor azide coupling in the base-promoted amidation. For electron-rich azide amidation, after the concerted 3+2] cycloaddition of azide/thiocarboxylate, a new reaction channel is found challenging that recently mentioned, which follows two consecutive, unimolecular reactions with very low activation barriers (< 1.6 kcal mol⁻¹) to give an anionic amide and a nitrous sulfide (N₂S). Distinct from electron-rich azide amidation, electron-poor azide first couples with thiocarboxylate to form a linear stable adduct, and then passes through the transition state of the rate-controlling step to afford the anionic amide, rather than the thiatrazoline. The free energy barrier of this step is 4.2 kcal mol⁻¹ lower than that previously proposed. Comparatively, the azide/thioacid amidations undergo the concerted 3+2] cycloaddition and the subsequent retro-3+2] cycloaddition process to give cis-enol form of the amide, which have higher activation barriers than those in the based-promoted amidation. Solvent effects investigated indicate that non-polar solvents, such as chloroform, are more preferable for the base-promoted thioacid/azide amidation.