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Das s‐cis‐/s‐trans ‐Konformerengleichgewicht bei Allencarbonsäureestern
Author(s) -
Lang Robert W.,
Hansen HansJürgen
Publication year - 1980
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19800630512
Subject(s) - chemistry , stereochemistry , cis–trans isomerism , conjugated system , nuclear magnetic resonance spectroscopy , organic chemistry , polymer
The s‐cis/s‐trans Conformational Equilibrium of Allenic Esters The following UV. and 1 H‐NMR. spectroscopic investigations of the allenic compounds 1–9 ( cf. Scheme 2 ) refer for the first time on a concrete experimental base to the preferred s‐ trans conformation of the allenic esters 1–6. With an additional allenic increment of 12 nm, the well established Woodward‐Fieser rules in UV. spectroscopy could be extended on conjugated allenic compounds for the calculation of the (π→π*)‐type band position as well as for the prediction of the s‐ cis /s‐ trans conformational fraction ( cf. Scheme 1 ). From 1 H‐NMR. shift experiments we conclude, that even small structural changes at the molecular framework have a measurable influence on the conformational equilibrium. In contrast to the situation in similarly substituted α, β‐unsaturated compounds ( 14 und 15 ), the allenic esters ( 1 and 4 ) exhibit preferentially s‐ trans conformations ( cf. Scheme 3 ), but in both cases, the equilibrium is displaced toward s‐ cis conformation, when the α‐position is methyl substituted ( e.g. 14 → 15 and 1 → 4 ).