Fluorenyllithium ( 1 ) adds to tert -butyl isocyanate to yield 9-( N - tert -butylcarboxamido)fluorenyllithium ( 3 ). Its reduction with LiAlH 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 η 5 -azapentadienyllithium derivative 11 . The X-ray crystal structure analysis of 11 ·(Et 2 O) 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 η 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 1 H NMR time scale. It equilibrates the annulated arene moieties of 11 . The Gibbs activation energy of this symmetrization process is Δ G ⧧ enant = 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 η 5 -(1-azapentadienyl)lithium derivative 11 . Deprotonation of 6 with tert -butyllithium furnishes 1 and the dilithiated compound 10 . Single crystals of 1 ·Et 2 O were analyzed by X-ray diffraction. The compound shows a polymeric “super-sandwich” structure with an alternating fluorenyl-Li·OEt 2 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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Könemann et al. (1997) studied this question.
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