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

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Abstract

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. A Cs-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 η5-(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.

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