Scope and stereochemistry of carbon-carbon bond formation via addition of carbon nucleophiles to nitrones
Tao, Ming
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Permalink
https://hdl.handle.net/2142/20828
Description
Title
Scope and stereochemistry of carbon-carbon bond formation via addition of carbon nucleophiles to nitrones
Author(s)
Tao, Ming
Issue Date
1992
Doctoral Committee Chair(s)
Coates, Robert M.
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
The scope and stereochemistry of nucleophilic addition of Grignard reagents and silyl ketene acetals to acyclic and cyclic nitrones have been studied. A 4:1 selectivity favoring axial addition was observed in the reactions of cis-3,5-dimethyl-2,3,4,5-tetrahydropyridine N-oxide with both methyl and phenyl Grignard reagents and the silyl ketene acetal of t-butyl acetate. However, similar additions of carbon nucleophiles to 4-t-butyl-2,3,4,5-tetrahydropyridine N-oxide afforded only the trans isomer from axial addition in moderate yields (60-68%). The stereoselectivity in these endocyclic nitrone additions can be explained by a stereoelectronic effect which favors axial approach of the nucleophile. Acyclic nitrones reacted with crotyl Grignard reagent to give different diastereoselectivity depending on the substrates. Syn-anti selectivities ranging from 7:1 to 1:3 were observed in the reactions of acyclic nitrones with E-silyl ketene acetals of t-butyl propionate. The addition of Grignard reagents to nitrones of glyoxylate esters provides a direct method for the preparation of N-($\alpha$-hydroxylamino) esters. The scope of this addition reaction was surveyed with a diversity of substrates and nucleophiles. Primary, secondary, tertiary, vinyl, and aryl Grignard reagents generally gave good yields (55-81%) in the nitrone addition reactions. High diastereoselectivity (13-15:1) was observed in Grignard addition to nitrones of (1R, 2S, 5R)-phenylmenthyl esters. The facial bias of the reaction can be explained by a chelation model. N-Deoxygenation of N,N-dialkylhydroxylamines was achieved by reduction with trivalent phosphorus reagents via desulfurization of their thionocarbonate derivatives. The mechanism of the desulfurization reaction has been studied by a double labelling experiment provide an intermolecular pathway. Alternatively, reduction of the N,N-dialkylhydroxylamine adducts was accomplished by either lithium-ammonia or catalytic hydrogenation of their hydroxyamino carbonates. N-Deoxygenation of silyl ketene acetal adducts was effected by catalytic hydrogenation using palladium as a catalyst.
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