Complex-induced proximity effects in proton transfer and halogen-metal exchange reactions
Musick, Timothy Joseph
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https://hdl.handle.net/2142/20653
Description
Title
Complex-induced proximity effects in proton transfer and halogen-metal exchange reactions
Author(s)
Musick, Timothy Joseph
Issue Date
1989
Doctoral Committee Chair(s)
Beak, Peter
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
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 exchange and deuterium loss for $N$-deutero-$N$-alkyl-2-, 3-, and 4-halobenzamides (39a-f) are not consistent with that mechanism. An alternative mechanism in which initial loss of the acidic deuterium is followed by halogen-lithium exchange to give a dilithiated intermediate (41) is suggested. Deuterium transfer to the site of halogen-lithium exchange then occurs by intermolecular reaction of the dilithiated species with unreacted $N$-deuterated amide as it diffuses into the solution. 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 as the organolithium is added to the amide substrate. Differentiation between these two possibilities is achieved by employing reverse addition of the amide substrate to a solution of the organolithium reagent. In cases where halogen-lithium exchange occurs through a complex, no change in product ratios is observed.
Further evidence for halogen-lithium exchange occurring through a complex is acquired by comparing product ratios obtained by the use of monomeric lithium reagents and aggregated lithium reagents. Aggregated lithium reagents are necessary to form a complex on the deprotonated amide.
Other substrates containing both an acidic proton and an aromatic halogen which are treated with $n$-butyllithium include ortho- and para-halogenated benzoic acids (46), anilines (48), phenols (50), $N$-methyl-$N$-benzylamines (44), cinnamic acids (54), and benzanilides (57). Under all conditions it appears that halogen-lithium exchange occurs more slowly than removal of the acidic proton.
An investigation into the nature of the complex which effects halogen-lithium exchange is accomplished by intermolecular competition studies between secondary and tertiary amides, between secondary amide anions and secondary amides, between amides and benzoic acids, and by intramolecular competitions using substituted benzanilides.
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