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Title:Studies of Nucleophilic Substitution at Neutral Nitrogen
Author(s):Li, Jinglin
Doctoral Committee Chair(s):Beak, Peter
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Inorganic
Chemistry, Organic
Chemistry, Physical
Abstract:The mechanism of nucleophilic substitution at a formally neutral nitrogen has been investigated. The two limiting mechanisms are a concerted reaction in which the transition state has the nucleophile and leaving group associated with nitrogen and an ionization process involving the initial formation of a nitrenium ion-pair which is subsequently attacked by the nucleophile.
An oxygen-18 labeling experiment has been used to address the possibility of reversible formation of a symmetrical nitrenium intermediate 81-${\bf\sp }$O. The recovered oxygen-18 labeled substrate 13-${\bf\sp }$O from a substitution reaction has the oxygen-18 label unscrambled. This result rules out the existence of a symmetrical ion-pair and supports the concerted reaction pathway. The endocyclic restriction test has been used to evaluate the transition state geometry. A double-labeling experiment shows that the substitution proceeds by an intermolecular process in the short-chain 95, and by an intramolecular process in the long-chain 113. These results suggest that there is a geometrical requirement for the substitution in which the entering and leaving groups must be at a large angle of ca. 180$\sp\circ$ for the reaction to proceed. The most likely mechanism for the substitution at nitrogen of 13 is a concerted S$\sb{\rm N}$2 process via a trigonal bipyramidal transition state with nucleophile attack from back-side of the leaving group.
The scope of the formal displacement of an alkoxyl group on nitrogen has been studied for the dialkylmethoxylamine 119. The reaction has been found to be limited to tert-butyl-lithium and sec-butyllithium reagents. Although a slow radical probe test failed to detect a nitrogen radical intermediate, the reactivity of these organolithium reagents suggests a mechanism consistent with a SET process.
Issue Date:1992
Description:107 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1992.
Other Identifier(s):(UMI)AAI9305601
Date Available in IDEALS:2014-12-17
Date Deposited:1992

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