Mechanistic Diversity in Thermal Fragmentation Reactions: A Computational Exploration of CO and CO₂ Extrusions from Five-Membered Rings

The mechanisms of a variety of thermal pericyclic fragmentation reactions of five-membered heterocyclic rings are subjected to scrutiny at a density functional level by computation of transition state free energy barriers and intrinsic reaction coordinates (IRCs). The preferred computed products generally match those observed in flash vacuum thermolysis experiments. For certain reactions, which also have the highest reaction temperatures and computed barriers, a degree of multireference character to the wave function manifests in an overestimation of the DFT-computed barrier, with a more reasonable barrier obtained by a CASSCF single point energy calculation. Many of the IRCs exhibit “hidden intermediates” along the reaction pathway, but conversely reactions that could be considered to involve the formation of an intermediate nitrene prior to alkyl or aryl migration show no evidence of such an intermediate. Such exploration of the diversity of behavior in a class of compounds using computational methods with interactive presentation of the results within the body of a journal article is suggested as being almost a <i>sine qua non</i> for laboratory-based research on reactive intermediates