74132-79-9Relevant articles and documents
Aromaticity as a Function of the Ion Pair Character: Acceptor-substituted Cyclononatetraenyl Anions, Enolate Anions with Variable Charge Distribution, and Unusual Conformational Properties
Boche, Gernot,Heidenhain, Frank,Thiel, Walter,Eiben, Robert
, p. 3167 - 3190 (2007/10/02)
In contrast to normal enolates like 1-Li+ the conformational properties of the acceptor-substituted cyclononatetraenyl anions 2a-, 2b-, 2c-, amd of the dianion 2d-, are strongly determined by the gegenion, the solvent, and the temperature.Thus, the contact ion pairs 2b-Li+ and 2a-Na+ have properties similar to the ones of the nonafulvene silyl enol ether 7, as shown by the 1H- and 13C NMR spectra and by the facile valence isomerization to the corresponding dihydroindenes.Contrary to the contact ion pairs, the solvent separated ion pairs 2'a- Na+(THF, -45 deg C), 2'a- K+(THF), 2'b- Li+(THF, HMPT), 2'c- Li+(THF), and 2'd- 2Li+(THF), e.g., are substituted annulene anions, again revealed by the 1H and 13C NMR spectra and by the reluctance towards valence isomerization.Therefore, without assoziation of the gegenion (especially of the small Li+) to the potential enolate oxygen atom, the negative charge is preferentially delocalized in the annulene anion ring; 13C NMR spectra and MNDO calculations even indicate that the orthogonal conformation of the annulene anion ring and the substituent is the most favourable one! Comparable ion pair equilibria have been noticed before; the unusual conformational changes transforming reversibly an olefinic into an aromatic compound, which are connected with these ion pair effects, however, are without precedence.Likewise, "orthogonal enolates" are unknown.
