Does Formal Intramolecular Transfer of an Acidic Deuterium to a Site of Halogen-Lithium Exchange Show That Lithium-Halogen Exchange Is Faster than Loss of the Acidic Deuterium? Evidence in Favor of an Alternative Mechanism

Article abstract of DOI:10.1021/ja00219a031

Reactions in which there is formal intramolecular transfer of an acidic deuterium to a site of halogen-lithium exchange could be interpreted to show that initial halogen-lithium exchange occurs faster than loss of the acidic deuterium.However studies of the competition between halogen-metal-deuterium exchange and deuterium loss for N-deuterio-N-alkyl-o, -m, and -p-halobenzimides are not consistent with that mechanism.We suggest an alternative in which initial loss of the acidic deuterium is followed by halogen-lithium exchange to give a dilithiated intermediate.Deuterium transfer to the site of halogen-lithium exchange then occurs by reaction of the dilithiated species intermolecularly with unreacted N-deuteriated amide.The halogen-lithium exchange is faster than complete mixing of the reactants and can occur either in an initially formed deprotonated complex or in a transient high local concentration of organolithium reagent.Evidence for both possibilities is provided.Two reactions from the literature in which halogen-lithium exchange appears to be faster than transfer of an acidic hydrogen have been reinvestigated and found to be interpretable in terms of similar sequences.