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Title:Endocyclic restriction test: Determination of a geometrical dependance for an aryl bromide-alkyllithium exchange reaction
Author(s):Allen, David John
Doctoral Committee Chair(s):Beak, Peter
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Organic
Abstract:The geometrical dependance for an aryl bromide-alkyllithium exchange reaction has been investigated by use of the endocyclic restriction test.
There have been four mechanistic alternatives presented for the aryl bromide-alkyllithium exchange reaction: a four-centered transition structure mechanism; a single-electron mediated mechanism; an S$\sb{\rm N}$2 attack at bromine; and a pathway involving the formation of a bromine ate complex intermediate. The four-center mechanism proceeds with a small C-Br-C transition structure angle, whereas the S$\sb{\rm N}$2 and ate complex formation mechanisms proceed with a large C-Br-C angle. The single-electron transfer mediated mechanism involves dissociation to give bromide and therefore should not have a geometrical dependance in respect to a C-Br-C transition structure angle.
Through double double-label experiments, we have determined that the aryl bromide-alkyllithium exchange of 52 to 53 proceeds intermolecularly in systems in which an intramolecular exchange through a five-, six-, and eight-membered endocyclic ring is available. The exchange will proceed intramolecularly through an eighteen-membered endocyclic ring. This is the first determination of a geometrical dependance for an aryl bromide-alkyllithium exchange.
Through these results, we have ruled out the four-center mechanism, which should proceed intramolecularly though the smaller endocyclic ring systems. The observation of a geometrical dependance also rules out the single-electron transfer mechanism. The mechanistic alternatives which involve large C-Br-C transition structure angles, the S$\sb{\rm N}$2 attack at bromine and the bromide ate complex intermediate mechanisms, are supported by our observations.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)
Issue Date:1991
Rights Information:Copyright 1991 Allen, David John
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9136527
OCLC Identifier:(UMI)AAI9136527

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