Structure and Chemical Behavior of an η5-(1-Azapentadienyl)lithium Derivative Generated by Deprotonation Reactions Starting from 9-(N-tert-Butylaminomethyl)fluorene or 6-(tert-Butylamino)dibenzofulvene

Fluorenyllithium (1) adds to tert-butyl isocyanate to yield 9-(N-tert-butylcarboxamido)fluorenyllithium (3). Its reduction with LiAlH4 followed by aqueous workup leads to the formation of the corresponding dibenzo-N-tert-butylaminofulvene 5. Treatment of 5 with sodium cyanoborohydride furnishes 9-(N-tert-butylaminomethyl)fluorene (6). Deprotonation of the fulvene 5 with n-butyllithium in ether yields the η5-azapentadienyllithium derivative 11. The X-ray crystal structure analysis of 11·(Et2O)2 reveals a nearly planar U-shaped 1-azapentadienyl moiety that exhibits five close contacts to the lithium atom located above the ligand plane. Computational chemistry (PM3) has identified the η5-(1-azapentadienyl)lithium derivative 11 as a global minimum. In solution compound 11 undergoes a dynamic degenerate rearrangement process that is rapid on the 1H NMR time scale. It equilibrates the annulated arene moieties of 11. The Gibbs activation energy of this symmetrization process is ΔG⧧enant = 11.8 ± 0.3 kcal mol-1. ...