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: The electron-releasing effect of homoconjugated carbonyl group

Abstract Protonation of norborn-5-en-2-one gives 6-oxo-2-norbornyl cation (2) as most stable C 7 H 9 O + cation. Quantum calculations at B3LYP/6-31G(d) level (gas phase, 1 atm, 25 °C) predict a C S -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 (C O) ↔  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.