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

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 loss for N-deuterio-N-alkyl-o, -m-, and -p-halobenzimides are not consistent with that mechanism. They 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