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Mechanisms for Solvolytic Elimination and Substitution Reactions Involving Short-lived Carbocation Intermediates
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Chemistry, Department of Chemistry, Organic Chemistry.
2002 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solvolysis reactions of a range of tertiary substrates in largely aqueous solvents were studied in such respects as β-deuterium kinetic isotope effects, linear free energy relationships and stereochemistry.

Solvolysis of the fluorene derivatives 9-methyl–9-(2´-X-2´-propyl)fluorene (1-X, X = Cl, Br, OOCCF3) involves a very short-lived carbocation intermediate. The fraction of alkene is increased by addition of general bases, which can be expressed by a Brφnsted parameter β = 0.07. The kinetic deuterium isotope effects vary with solvent composition in a way which is not consistent with a common carbocation intermediate which has time to choose between dehydronation and addition of a solvent water molecule.

In the absence of bases, the reaction of 4-chloro-4-(4´-nitrophenyl)pentan-2-one (2-Cl) proceeds through a short-lived carbocation intermediate yielding 4-(4´-nitrophenyl)-2-oxopent-4-ene (2-t-ne)as the main elimination product. Addition of acetate ion and other weak bases results in the base-promoted E2 (or E1cb) reaction to give (E)-4-(4´-nitrophenyl)-2-oxopent-3-ene (2-E-ne) and (Z)-4-(4´-nitrophenyl)-2-oxopent-3-ene(2-Z-ne). There is no evidence for a water-promoted E2 (or E1cb) reaction.

The stereochemistry studies of elimination from (R,S and S,R)-[1-(3´-fluoro)phenyl-2-methyl]cyclopentyl-p-nitrobenzoate (3-PNB) and its (R,R and S,S)isomer 3´-PNB and (R,S and S,R)-[1´-(3´´-fluoro)phenyl-2´-methylcyclopentyl]-2,2,2-trifluoroacetate(3-OOCCF3) exclude the concerted pericyclic elimination mechanism for formation of the alkene 1-(3´-fluoro)phenyl-2-methylcyclopentene(3-m-ne). The effects of added thiocyanate ion and halide ions on the solvolysis reaction are discussed.

Mass spectrometry analysis showed complete incorporation of the labeled oxygen from solvent water into the product 2-hydroxy-2-phenyl-3-butene (4-OH), confirming that it is the tertiary carbon-oxygen bond that is broken in the acid-catalyzed solvolysis of 2-methoxy-2-phenyl-3-butene (4-OMe). The mechanism for the dominant formation of the less stable 4-OH is discussed.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2002. , p. 45
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 743
Keywords [en]
Organic chemistry
Keywords [sv]
Organisk kemi
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-2565ISBN: 91-554-5389-9 (print)OAI: oai:DiVA.org:uu-2565DiVA, id: diva2:161900
Public defence
2002-09-25, The Svedberg Lecture Hall, Uppsala, 10:15
Opponent
Available from: 2002-09-02 Created: 2002-09-02Bibliographically approved

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