A variation of titanium-induced carbonyl coupling - Synthesis and conformations of terfluorenyl

Reaction of fluorenone (4) with a low-valent titanium reagent generated from TiCl₄ and Zn in THF in the presence of pyridine gave terfluorenyl (6) in 71% yield, bifluorenylidene (2), the conventional McMurry reaction product, in 2% yield, and additional reduction products. The reductive 'trimerization' was rationalized in terms of an attack of the intermediate fluorenone dianion on bifluorenylidene. The molecular structure of a single crystal of 6 indicated an approximately C₂ conformation, with a slightly twisted central ring, and two equally folded side moieties with dihedral angles of 58.1°and 58.0°between the central and side five-membered rings. The AM1 and PM3 calculations showed C₂ global minima, similar to the conformation in the crystal. The calculated C(s) transition state conformations were found to be 21.3 (AM1) and 19.9 (PM3) kcal/mol higher in energy than the global C₂ minima. A 2D-NMR NOESY experiment on 6 supported the C₂ (+sc,+sc) or C₂ (-sc,-sc) conformation in solution.