Abstract
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 Cs 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.
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