Tetrabenzo fluorene

Tetrabenzo[a,c,g,i]fluorene has been used to selectively link synthetic intermediates to charcoal, for the purpose of their purification. In polar solvents, the tetrabenzo-fluorene is strongly adsorbed by charcoal this enables efficient separation of the intermediate from reagents. After centrifugation and washing, the intermediate is displaced from the charcoal and released into solution by addition of a non-polar solvent, and a new synthetic operation in solution can be conducted (Figure 3.43). Tetrabenzo-fluorene has also been used for the purification of peptides [849] and oligonucleotides [850], and for the synthesis of quinolones [851]. 

The use of tetrabenzo[a,c,g,i]fluorene (Tbf) derivatives as tagging reagents, such as 8.70 (224), was reported for acids each Tbf-tagged reagent is purified by specific adsorption on charcoal, typically stirring for 20 min in an ice bath, and subsequent washing with lipophilic solvents to recover pure intermediates, typically stirring with toluene aliquots at 40 °C until no more UV adsorbance is detected in the toluene aliquots. 

A related strategy was suggested in which one of the reactants is attached via a linker to tetrabenzo[a,c,g,i]fluorene (Tbf). The reaction product can now be purified by taking advantage of the high affinity of Tbf to charcoal in polar solvents (mixture of DCM and methanol). Desorption can be affected with non polar solvents such as toluene [144]. The method was exemplified by the synthesis of a quinolone carboxylic acid derivative (ciprofloxacin). 

The tetrabenzo[ ,c,g,/]fluorene moiety has not been widely used in organometallic chemistry so far. By thermal heating or by reacting IVa with bases (NEts in CH2CI2), 17H-tetrabenzo[< < g,i]fluorene (IVb) is formed (Scheme 6.5). 

The 17H-isomer IVb is the thermodynamically preferred one, compared to TbfH (rVa) as the kinetic one [30]. The former can be easily obtained as needle-shaped monocUnic crystals, suitable for X-ray diffraction [25]. 17H-Tetrabenzo[a,< g,i]fluorene (FVb) crystallizes in space group P2/c with four molecules per unit cell (Fig. 6.1). 

Directly reacting TbfLi(THF)4 (V) with TiCl4(THF)2 ended up in the reduction of the metal center and in the coinstantaneous oxidative coupling of the ligand. As a natural consequence, the synthesis of Tbf titanium trichloride via a silylated tetrabenzo[fl,c,g,i]fluorene derivative was approached, but it did not yield any identifiable products so far [25]. For these reasons, the more redox stable Ti(III) complexes were prepared first [31]. 

Hay, A. M., Hobbs-Dewitt, S., MacDonald, A. A., Ramage, R. Use of tetrabenzo[aic,g,f]fluorene as an anchor group for the solid/solution phase synthesis of the quinolone antibacterial, ciprofloxacin. Synthesis, 1979-1985 (1999). 

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