A multireference configuration interaction study with singles and doubles of some mesoionic rings: reaction and activation free energies for the ring‐opening reaction

For the first time, multireference configuration interaction with singles and doubles (MR‐CISD) calculations (including extensivity corrections, MR‐CISD+Q) have been performed to study the ring‐opening reactions of the following mesoionic rings and their 2, 3, and 4 methyl‐substituted derivatives: (a) 1,3‐oxazol‐5‐one, (c) 1,3‐oxazol‐5‐thiolate, and (d) 1,3‐thiazol‐5‐thiolate. The ring‐opening reaction of the parent 1,3‐thiazol‐5‐one mesoionic ring (b) has also been studied. The effect of methyl and S replacement on the reaction and activation free energies (ΔG and ΔG†, respectively) is studied. For a, the effect of methyl replacement on C2 is almost negligible, while on N3 and C4, it is significantly larger, especially in the latter position. The open structure (ketene form) of a is considerably more stable, and the replacement of the exocyclic O by S stabilizes the ketene tautomer even more and increases ΔG† considerably. On the other hand, replacement of the endocyclic O by S (yielding b) completely prevents the formation of the open structure, while replacement of both endo and exo atoms strongly stabilizes the cyclic structure and leads to a remarkably high value of ΔG†. In some cases, the relative polarity of the stationary points is used to estimate how ΔG and ΔG† are expected to change as one goes from the gas phase to aprotic polar solvents. A linear correlation between the multiconfigurational character of the mesoionic rings and ΔG† has been observed, and such correlation is used to estimate the ΔG† values for the methyl‐substituted c and d. The largest barrier (13.78 kcal/mol) has been obtained for the 4‐methyl‐1,3‐thiazol‐5‐thiolate (4‐m‐d). This works shows that the replacement of both O atoms of mesoionic 1,3‐oxazolium‐5‐olates by S atoms is crucial to achieve thermodynamic and kinetic stability toward a ring‐opening reaction leading to the ketene form. This outcome comes from very accurate calculations never used before for these systems. Furthermore, the obtained results suggest that (to some extent) the free energy barrier can be tuned by the multiconfigurational character of the mesoionic.