Effects of Ion Pair Separation on Inversion and Rotation of Sulfur‐, Selenium‐, and Silicon‐Stabilized Organolithium Reagents

摘要: Most vinyl,['] cyclopropyl,[*' and a -a lko~y[~] lithium reagents are configurationally stable. Other types racemize too rapidly for trapping experiments[4". '1 or require very low temperatures to avoid rapid eq~ilibration.[~"' 61 Advances in the utility of chiral organolithium depend on detailed structural i n f ~ r m t o ] mechanistic understanding their racemization. One limiting mechanism reagent epimerization is bimolecular associative process through intermediate l.[4b1 An alternative ion pair dissociation via 2 has been frequently proposed since polar solvents usually accelerate racernization,"", 6a1 but this not directly testable status most known. Our hexamethylphosphoric triamide (HMPA) titration techn q e ' . *] addresses problem by allowing unequivocal identification contact separated pairs. The correlation between structure inversion barriers several can now be established.