Complex-Induced Proximity Effects and Dipole-Stabilized Carbanions: Kinetic Evidence for the Role of Complexes in the α'-Lithiations of Carboxamides

Article abstract of DOI:10.1021/ja00232a029

The kinetics of the α'-lithiations of N,N-dimethyl-2,4,6-triisopropylbenzamide (4) and N,N-diethyl-2,4,6-triisopropylbenzamide (5) by sec-butyllithium (s-BuLi) in cyclohexane have been investigated.Direct observation of the reactant amide, the intermediate amide-lithium reagent complex(es), and the α'-lithiated products by stopped-flow infrared spectroscopy was used to investigate the initial rate constants as a fuction of reactant concentrations.At constant amide concentration the initial rate constant decreases sharply with increasing concentration of s-BuLi.This behavior is also observed in the presence of tetramethylethylene diamine (TMEDA).The major interaction of TMEDA with s-BuLi in cyclohexane is shown by cryoscopic measurements to be an addition of TMEDA to aggregated s-BuLi.Semiquantitative schemes constructed on the basis of the kinetic data suggest the presence of at least two reactive complexes and that complexes with more ligands are more reactive.The implications of these results are discussed.