σ-Hole mechanism in chalcogen-bond catalyzed aza-Diels-Alder reaction

Chalcogen bonding (ChB) catalysis has emerged as a promising non-covalent strategy for activating substrates in cycloaddition reactions, yet its mechanistic details in aza‑Diels-Alder transformations remain underexplored. This study systematically investigates the catalytic role of a homologous series of bis(pentafluorophenyl) chalcogenides, (C₆F₅)₂Ch (Ch = S, Se, Te), in the model aza‑Diels-Alder reaction between formaldehyde imine and 1,3‑butadiene. We demonstrate that catalytic activity increases in the order Te > Se > S, directly correlating with the σ‑hole electrophilicity of the chalcogen atom. The catalysts promote reaction asynchronicity by stabilizing the imine LUMO and selectively polarizing its π‑system, thereby lowering the activation barrier through enhanced charge transfer from the diene. Energy decomposition analysis reveals an evolution in the ChB character from dispersion‑dominated (S) to electrostatically driven (Te), rationalizing the observed trend. The catalytic hierarchy persists across solvents of varying polarity, confirming the robustness of this activation mode in solution.

All calculations were performed using Gaussian 09 software. Geometries of stationary points along the reaction pathway were optimized, and vibrational frequencies were computed at the M06‑2X/aug‑cc‑pVDZ level of theory. Intrinsic reaction coordinate (IRC) calculations verified transition‑state connectivity. Solvent effects were incorporated via the SMD continuum model for n‑heptane, THF, DMSO, and water. Molecular electrostatic potentials were analyzed on the 0.001 a.u. electron‑density isosurface using Multiwfn and VMD. Bonding characteristics were examined using Bader's quantum theory of atoms in molecules (AIM) and the non‑covalent interaction (NCI) index via Multiwfn. Natural bond orbital (NBO) analysis was carried out with NBO 5.0, and energy decomposition analysis (EDA) was employed to quantify interaction energy components.