Induktiver Effekt, negative Hyperkonjugation und Gegenion sind nicht die Ursache der besonderen Eigenschaften des Bicyclo[3.2.1]octa‐3,6‐dien‐2‐yl‐Anions. — Eine NMR‐Studie mit phenylsubstituierten Modellen

Inductive Effect, Negative Hyperconjugation, and Gegenion do not Cause the Unusual Properties of the Bicyclo[3.2.1]octa‐3,6‐dien‐2‐yl Anion. An NMR Study with Phenyl‐Substituted Models The 13 C‐NMR spectra of the phenyl‐substituted bicyclo[3.2.1]oct‐3‐en‐2‐yl anions of 4 1‐K and 6 1‐Li show that in the corresponding dienyl anions of 3 1‐K and 5 1‐Li not only C‐6,7 have unusual chemical shifts but also C‐2,3,4. The values of the phenyl groups prove the larger negative charge in the allyl system of 4 1‐K and 6 1‐Li as compared to that of 3 1‐K and 5 1‐Li, respectively. Thus, the C6–C7 π bond in 3 1‐K and 5 1‐Li causes the reduction of the negative charge in the allyl subunit. Different from the effect of the C6–C7 π bond in 3 1‐K, the anellated three‐membered ring in the tricyclo[3.3.1.0 2,4 ]1‐nonenyl anion of 7 1‐K has no influence on the chemical shifts of the phenyl‐substituted allyl anion moiety. Experiments aimed at the exchange of the metal cations of 3 1‐K, 5 1‐Li, and 6 1‐Li for a tetramethylammonium ion were successful only with 5 1‐Li. The NMR data of 5 1‐Li and 5 1‐NMe 4 are virtually identical. In the case of 3 1‐Li and 6 1‐Li, the treatment with tetramethyl‐ammonium chloride caused an S N 2 process with formation of the methylbicyclooctadienes 8a and 10 , respectively. Also, 5 1‐Li underwent this reaction, however so slow that significant concentrations of 5 1‐NMe 4 could be observed in solution. 7 Li‐NMR spectra were taken from 5 1‐Li, 6 1‐Li, and the solution containing 5 1‐NMe 4 .