HOW ADVANTAGEOUS IS THE INTRAMOLECULAR AGGREGATION OF 1,4-ORGANODILITHIO COMPOUNDS

io-5,5,7,7-tenamethyl-5,7-dihydrodibenz[c,e]oxepin (3) is a conformati onally confined analogue of o,o'-dilithiobiphenyl (1). A temperature v ariable nmr study of this model compound, monitoring the coalescence o f its diastereotopic methyl 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 intramolecu lar aggregation of o,o'-dilithiobisaryls is found to be significantly smaller than predicted by ab initio calculations.