Conformational analysis of asymmetric ethanes : Extraction of acyclic conformational ligand constants from time-averaged geminal fluorine anisochronism |
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Authors: | Peter Capriel Gerhard Binsch |
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Institution: | Institut für Organische Chemie, Universität Munchen, Karlstrasse 23, 8000 München 2, Germany |
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Abstract: | The syntheses and ambient-temperature 19F NMR data are reported for 27 asymmetric ethanes of the general formula RCF2CXYZ, including a complete series of 10 compounds with R = Cl and all combinations of the 5 ligands H, F, Cl, Br and Ph. Within the theoretical framework of a previously proposed heuristic mathematical model the geminal 19F chemical shift differences are fitted to chirality functions X = ?R (λx - λy) (λy - λz) (λz - λx) to yield acyclic conformational ligand constants λ and substituent parameters ?. It is demonstrated that the λ constants already reported for an analogous series of 10 compounds with R = Br are transferable to the Cl series with ?Cl = 0.63 ± 0.07. Crude first approximations are also reported for the normalised (according to ?Br = 1) ligand constants λCh3, λOH, λOCH3 and λ1 and for the substituent parameter λH. It is argued that the λ values thus ext play a role in the conformational analysis of asymmetric ethanes that is conceptually analogous to the conformational free energies in monosubstituted cyclohexanes. |
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