Mechanisms of conformational chirality inversion in bicyclo[4.2.1]nonan‐9‐one and bicyclo[4.2.2]decane as studied in two‐parametric torsional energy surfaces

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Abstract
With the purpose of deciphering conformational inversion processes of typical mobile bicyclic molecules, torsional energy surfaces near the enantiomers of bicyclo[4.2.1]nonan‐9‐one (1) and bicyclo‐[4.2.2]decane (2) were prepared using molecular mechanics with an improved two‐bond drive technique. Inversion of 1 takes place most favorably via a Cs transition state with the tetramethylene chain over the ethano bridge [1B, ΔH± 6.1 (calculated) vs. 6.8 (observed) kcal/mol]. An alternative pathway involving a Cs local energy minimum (1C), in which the tetramethylene chain is bent over the carbonyl, has a barrier 2.4 kcal higher than 1B. The global energy minimum conformation of 2 has boat–chair cyclooctane and twist–boat cyclohexane rings (BCTB), and enantiomerizes into its mirror image (BCTB') via three intermediates: TCTB, CB, and TCTB'. The highest point in the proposed pathway, a saddlepoint CB, is calculated to lie 8.0 kcal/mol above BCTB (observed ΔH± 7.8 kcal/mol). The advantage of the two‐parametric over the one‐parametric torsional energy surface is discussed.

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