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
✍ Scribed by Eiji Ōsawa
- Publisher
- John Wiley and Sons
- Year
- 1982
- Tongue
- English
- Weight
- 565 KB
- Volume
- 3
- Category
- Article
- ISSN
- 0192-8651
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✦ Synopsis
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 C~s~ transition state with the tetramethylene chain over the ethano bridge [1B, Δ__H__^±^ 6.1 (calculated) vs. 6.8 (observed) kcal/mol]. An alternative pathway involving a C~s~ 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 Δ__H__^±^ 7.8 kcal/mol). The advantage of the two‐parametric over the one‐parametric torsional energy surface is discussed.