Studies of nucleophilic displacement at nitrogen of hydroxylamine derivatives
Conser, Kathryn Rose
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Permalink
https://hdl.handle.net/2142/22098
Description
Title
Studies of nucleophilic displacement at nitrogen of hydroxylamine derivatives
Author(s)
Conser, Kathryn Rose
Issue Date
1995
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
We have investigated the mechanism of electrophilic amination reactions in which both anionic and neutral nitrogen sources were used. For nucleophilic displacement at anionic nitrogen, we examined the substituent effects adjacent to the central atom and the nucleophile and compared our results to S$\sb{\rm N}$2 displacements at carbon. We found that as the size of the substituents increased around nitrogen, the relative rate decreased. This result is consistent with the substituent effects observed for S$\sb{\rm N}$2 reactions at carbon. In contrast, we found that more hindered nucleophiles were the most reactive towards amination with the reactivity trend t-BuLi $>$ s-BuLi $>$ n-BuLi. Although this trend is inconsistent with the carbon S$\sb{\rm N\/}$2 model, our results may be rationalized by a mechanism which involves steric interactions within a lithium aggregate.
To further examine the transition state geometry required for displacement reactions at anionic nitrogen, we synthesized several substrates for use in the endocyclic restriction test. These molecules contained long tethers which would allow intramolecular backside attack within an endocyclic ring. However, we failed to detect any of the desired products after performing the endocyclic restriction test reactions. Authentic products were synthesized to confirm the absence of the desired materials in each system. Our results indicate that either nucleophile generation or reactivity is problematic. We have proposed other nucleophiles which can be tested in these long chain systems.
Studies of electrophilic amination using neutral nitrogen sources have shown that an aryl ring within the aminating agent does not act as an electron acceptor to promote a single electron transfer mechanism. In addition, the starting aggregation state of the organolithium reagent does not determine the relative reactivity of these nucleophiles. For aminations at neutral nitrogen, our results are consistent with either a mechanism involving steric hindrance within a lithium aggregate or a single electron transfer mechanism with an extremely short lived nitrogen radical intermediate.
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