Bicyclo[3.2.1]octa-2,6-dienyllithium and -potassium Chemistry Solvation, Aggregation, and 119Sn-7Li Coupling of Stannylated Derivatives

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The 13C NMR spectra of bicyclo[3.2.1]octa-2,6-dienyl anion (1) as the lithium (1 Li) and the potassium salt (1 K) were recorded at various temperatures and in different solvents. The results show 1K to exist as a contact ion pair (CIP), whereas for 1 Li solvent separated ions (SSIP) are also found as the temperature is lowered and when the solvating power of the solvent is increased. In the 7Li NMR spectra at very low temperature the signals of the SSIP disappear and only those of a monomeric CIP and a triple ion (1 Li 1) consisting of two carbanions and one lithium cation are present. For independent confirmation, the hitherto unknown 6-, 7-, and 3-trimethylstannyl substituted bicyclo[3.2.1]octa-2,6-dienyllithium derivatives 2 a Li, 2 b Li, and 3 Li were synthesized and also studied by 119Sn, 13C, and 1H NMR. According to the temperature dependence of the δ(13C)-values the CIP/SSIP-equilibrium of 2 a Li in THF/TMEDA is shifted to the CIP side. The observation of a unique 119Sn-7Li coupling (6.2 Hz) shows that at –90 °C a species containing 2 a and lithium exists under slow exchange conditions. A similar coupling of 3.5 Hz can be estimated for 3Li. A mixture of the epimeric carbanions 2a and 2 b yields two sets of 13C NMR signals which are due to the diastereomeric triple ions 2 aLi 2 a and 2 aLi 2 b.