Structure and chemical behavior of an eta(5)-(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 LiAIH(4) 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 eta(5)-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 eta(5)-(1-azapentadienyl)lithium derivative 11 as a global minimum. In solution compound 11 undergoes a dynamic degenerate rearrangement process that is rapid on the H-1 NMR time scale. It equilibrates the annulated arene moieties of 11. The Gibbs activation energy of this symmetrization process is Delta G(enant)(double dagger) = 11.8 +/- 0.3 kcal mol(-1). A C-s-symmetric transition state geometry (11-TS) where the 1-azapentadienyl conjugation is interrupted between C9-C10 was identified by the PM3 calculation. Deprotonation of 9-(N-tert-butylaminomethyl)fluorene (6) gives rise to several competing pathways. Treatment with lithium hexamethyldisilazide leads to abstraction of the fluorenyl 9-H proton to produce the aminomethyl-substituted fluorenyl anion 7. Subsequent treatment of 7 with tert-butyllithium gives the dilithio compound 10. The reaction of 6 with n-butyllithium leads to the formation of a mixture of fluorenyllithium (1) and the eta(5)-(1-azapentadienyl)lithium derivative 11. Deprotonation of 6 with tert-butyllithium furnishes 1 and the dilithiated compound 10. Single crystals of 1 . Et2O were analyzed by X-ray diffraction. The compound shows a polymeric ''super-sandwich'' structure with an alternating fluorenyl-Li . OEt2 arrangement exhibiting inversion symmetry. The X-ray crystal structure analysis of 10 . (THF)(3) reveals an unsymmetrical arrangement of the lithium atoms at the amidomethylfluorenyl-''dianion'' framework with one THF molecule bridging between the metal centers. The bonding features of the dilithiated system 10 derived by double deprotonation of 6 were also analyzed by computational chemistry.