How advantageous is the intramolecular aggregation of 1,4-organodilithio compounds?

1,11-Dilithio-5,5,7,7-tetramethyl-5,7-dihydrodibenz[c,e]oxepin (3; shown as I) is a conformationally confined analog of o,o'-dilithiobiphenyl (1). A temp. variable NMR study of this model compd., monitoring the coalescence of its diastereotopic Me groups, has revealed a barrier of about 12 kcal/mol to planarization, just 2 kcal/mol less than that found with the metal-free heterocycle. Thus, the energetic benefit of intramol. aggregation of o,o'-dilithiobisaryls is significantly smaller than that predicted by ab initio calcns. [on SciFinder (R)]