Structural Distortions and Dynamic Behavior of the Elusive “L”-Shaped cis-Bicyclo[3.3.0]octenyllithium:  X-ray Crystallographic and NMR Studies

Substituted cis-bicyclo[3.3.0]octenyllithium prepared by addition of t-BuLi to 3-methylene-1,4-cyclooctadiene in the presence of TMEDA crystallizes as a dimer with one unsolvated Li⁺ sandwiched between the external faces of two allyl anions in a triple ion, and external to it the second Li⁺ is bidentately complexed to TMEDA, 8. Within each allyl unit, the allyl bonds have different lengths, and all four rings deviate from coplanarity which relieves strain in the rings despite introducing partial localization of the allyl anions. A similar structure prevails in solution as shown by ⁷Li NMR and the results of ⁷Li{¹H} HOESY and ¹H, ¹H NOESY experiments. Carbon-13 NMR line shape changes indicate that the system undergoes a fast allyl bond shift concerted with conformation shifts of the out of plane carbons, ca. ΔG^⧧ = 9 kcal·mol^-1. Cyclopentyllithium prepared by CH₃Li cleavage of the trimethylstannyl derivative slowly undergoes an allowed ring opening to pentadienyllithium as well as deprotonating the solvent. The different behavior of dienylic lithium species is attributed to the relative separation of their termini.