Quantum calculations on the acid catalyzed rearrangements of norborn-5-en-2-one, 7-oxanorborn-5-en-2-one and 7-azanorborn-5-en-2-one

Protonation of norborn-5-en-2-one gives 6-oxo-2-norbornyl cation (2) as most stable C7H9O+ cation. Quantum calculations at B3LYP/6-31G(d) level (gas phase, 1 atm, 25 °C) predict a CS-structure for 2 that can be represented as an intramolecular π-complex (Dewar π-complex model) of an acetylium ion with an alkene (cyclopent-3-enyl moiety). Isomeric 5-oxo-2-norbornyl (1) cation is calculated to be 16.7 kcal/mol less stable than 2. Part of the relatively high stability of 2 arises from the electron-releasing carbonyl group through n(CO) ↔ σ ↔ 2p(C+) hyperconjugation. This effect is also present in more stable 6-oxo-3-oxa (14) and 6-oxo-3-aza-2-norbornyl cation (19) as these ions are calculated to be 7.4 and 3.3 kcal/mol, respectively, more stable than their 5-oxo isomers.