Degree Type

Dissertation

Date of Award

1991

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Walter S. Trahanovsky

Abstract

[alpha]-Substituent effects on the dimerization of furan-based (Section 1) and benzene-based (Section 2) o-quinodimethanes (o-QDM's) are reported. The mechanistic implications of these results are discussed;3-Ethylidene-2-methylene-, 2-ethylidene-3-methylene-, and 2,3-diethylidene-2,3-dihydrofuran were prepared by fluoride induced 1,4-conjugative elimination of trimethylsilyl acetate from the appropriate precursors. The [superscript]1H NMR spectra of these furan-based o-quinodimethanes were obtained and the dimerization products of each were studied. It was found that a methyl group at the 3-methylene position retards the rate of dimerization which is consistent with the previously proposed dimerization mechanism, the two-step diradical mechanism. Structures were assigned to 6 dimers from 3-ethylidene-2-methylene-2,3-dihydrofuran, 7 dimers from 2-ethylid-ene-3-methylene-2,3-dihydrofuran and 5 dimers from 2,3-diethylidene-2,3-dihydro-furan. Most of these dimers are (4+4) and (4+2) cyclo dimers, but one, 2[superscript]'-ethyl-2-ethyl-enyl-3,3[superscript]'-bifurfuryl, is a disproportionation product derived from the diradical proposed in the dimerization of 2-ethylidene-3-methylene-2,3-dihydrofuran. Indentification of the disproportionation product provides additional support for the two-step diradical mechanism. From analysis of the stereochemistry and regiochemistry of the dimers, it is concluded that both cisoid and transoid conformations of the diradical intermediate are formed by cisoid and transoid encounters of two monomer molecules. Also, from analysis of the products it is concluded that the regiochemistry of the cyclization of the diradical intermediates is controlled mainly by the interaction of the active sites of the furan moieties in the cyclization step; the initial conformation of the intermediate and the conformational changes which occur in going to the conformation which cyclizes are less important;In section two, the preparation of [alpha]-methyl-, [alpha]-cyclopropyl-, [alpha]-tert-butyl-, and [alpha]-mesityl-o-xylylene and the kinetics and product analyses of their Diels-Alder reactions and dimerizations are reported. Trapping the reactive o-QDM's with methyl methacrylate gives more of the "ortho" than the "meta" isomers in all the cases we studied. The change from [alpha]-methyl- to [alpha]-cyclopropyl to [alpha]-tert-butyl-o-xylylene results in a decrease of the "ortho" to "meta" ratio from 8.8 to 4.3 to 2.2, respectively. The trend of the regioselectively is consistent with the concerted mechanism. Dimerization of the o-QDM's provided significant amounts of head-to-head dimers. The change of the [alpha]-substituent from methyl to cyclopropyl, to tert-butyl and to mesityl enhances the competitiveness of the head-to-head dimerization. In the dimerization of [alpha]-tert-butyl and [alpha]-mesityl-o-xylylene, each of which has a large enough [alpha]-substituent, only the head-to-head dimers were identified. This observation, which cannot be explained by the concerted mechanism, is consistent with a two-step diradical mechanism. Because of the similarity of the dimerization activation parameters, we proposed that the parent o-xylylene and other related o-xylylenes dimerize via a similar two-step, diradical mechanism.

DOI

https://doi.org/10.31274/rtd-180813-11518

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Man-kit Leung

Language

en

Proquest ID

AAI9207250

File Format

application/pdf

File Size

278 pages

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