CHELATION-BASED STABILIZATION OF THE TRANSITION STRUCTURE IN A LITHIUM DIISOPROPYLAMIDE MEDIATED DEHYDROBROMINATION - AVOIDING THE UNIVERSAL GROUND-STATE ASSUMPTION

Dehydrobrominations of (+/-)-2-exo-bromonorbornane (RBr) by lithium di isopropylamide (LDA) were investigated to determine the roles of aggre gation and solvation. Elimination with LDA/n-BuOMe occurs by deaggrega tion of disolvated dimers via a monosolvated monomer transition struct ure (e.g., [i-Pr2NLi . n-BuOMe . RBr](double dagger)). In contrast, el imination by LDA-THF displays THF concentration dependencies that are consistent with parallel reaction pathways involving both mono- and di solvated monomer transition structures. Elimination is markedly faster by LDA-DME than by LDA with monodentate ligands and follows a rate la w consistent with a transition structure containing a chelated monomer ic LDA fragment. A number of hemilabile amino ethers reveal the capaci ty of different coordinating functionalities to chelate. A protocol ba sed upon kinetic methods affords the relative ligand binding energies in the LDA dimer reactants. Separating contributions of ground state f rom transition state stabilization allows us to attribute the stabiliz ing effects of chelation exclusively to the transition structure. The importance of chelating ligands in LDA-mediated dehydrobrominations, b ut not in previously studied reactions of LDA, sheds light on lithium ion chelation.