Norbornenyl- and Nortricyclyl Free Radicals

<p>The number, symmetry, and product-forming capabilities of the intermediates in the photoinitiated reductions of endo- and exo-5- bromonorbornene and 2-bromonortricyclene with tri-n-butyltin hydride at temperatures between -10° and 22° were investigated.<br /> Three mechanisms we...

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Main Author: Firkins, John Lionel
Format: Others
Language:en
Published: 1970
Online Access:https://thesis.library.caltech.edu/9070/1/Firkins_jl_1970.pdf
Firkins, John Lionel (1970) Norbornenyl- and Nortricyclyl Free Radicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JFE9-5888. https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886 <https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886>
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spelling ndltd-CALTECH-oai-thesis.library.caltech.edu-90702020-12-03T05:02:39Z https://thesis.library.caltech.edu/9070/ Norbornenyl- and Nortricyclyl Free Radicals Firkins, John Lionel <p>The number, symmetry, and product-forming capabilities of the intermediates in the photoinitiated reductions of endo- and exo-5- bromonorbornene and 2-bromonortricyclene with tri-n-butyltin hydride at temperatures between -10° and 22° were investigated.<br /> Three mechanisms were evaluated:<br /> 1. The 5-norbornenyl- and 2-nortricyclyl radicals isomerize reversibly with the former producing nortricyclene by abstraction of hydrogen from tri-n-butyltin hydride.<br /> 2. The 5-norbornenyl- and 2-nortricyclyl radicals isomerize reversibly, but some norbornene can be formed from the 2-nortricyclyl radical or some nortricyclene can be formed from the 5-norbornenyl radical by abstraction of hydrogen.<br /> 3. There is intervention of a "bridged" radical which may be for med reversibly or irreversibly from the 5-norbornenyl- and 2-nortricyclyl radicals.</p> <p>Within small error limits, the ratios of norbornene to nortricyclene as a function of the concentration of tri-n-butyltin hydride are consistent with the first mechanism.</p> <p>In the reductions with tri-n-butyltin deuteride, primary deuterium isotope effects of 2. 3 and 2. 1 for the abstraction of deuterium by the 2-nortricyclyl- and 5-norbornenyl radicals, respectively, were found. The primary deuterium isotope effects were invariant with the concentration of tri-n-butyltin deuteride, although the ratios of norbornene to nortricyclene changed appreciably over this range. This is consistent with the first mechanism, and can accommodate the formation of either product from more than one intermediate only if the primary kinetic deuterium isotope effects are nearly equal for all reactions leading to the single product.</p> <p>The reduction of endo-5-bromonorbornene-5, 6, 6-d<sub>3</sub> with tri-n-butyltin hydride or tri-n-butyltin deuteride leads to both unrearranged and rearranged norbornenes. The ratios of unrearranged to rearranged norbornene require that the 5-norbornenyl-5, 6, 6-d<sub>3</sub> radical isomerize to an intermediate with the symmetry expected of a nortricyclyl free radical. The results are consistent with mechanism 1, but imply a surprising normal secondary kinetic deuterium isotope effect of about 1.25 for the abstraction of hydrogen by the 5-norbornenyl- 5, 6, 6-d<sub>3</sub> radical.</p> <p>Approximate calculations show that there does not appear to be any substantial difference in the stabilities of the 5-norbornenyl and 2-nortricyclyl radicals.</p> <p>Although the results can not exclude a small contribution by a mechanism other than mechanism 1, no such contribution is required to adequately explain the results.</p> 1970 Thesis NonPeerReviewed application/pdf en other https://thesis.library.caltech.edu/9070/1/Firkins_jl_1970.pdf Firkins, John Lionel (1970) Norbornenyl- and Nortricyclyl Free Radicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JFE9-5888. https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886 <https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886> https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886 CaltechTHESIS:07302015-142129886 10.7907/JFE9-5888
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description <p>The number, symmetry, and product-forming capabilities of the intermediates in the photoinitiated reductions of endo- and exo-5- bromonorbornene and 2-bromonortricyclene with tri-n-butyltin hydride at temperatures between -10° and 22° were investigated.<br /> Three mechanisms were evaluated:<br /> 1. The 5-norbornenyl- and 2-nortricyclyl radicals isomerize reversibly with the former producing nortricyclene by abstraction of hydrogen from tri-n-butyltin hydride.<br /> 2. The 5-norbornenyl- and 2-nortricyclyl radicals isomerize reversibly, but some norbornene can be formed from the 2-nortricyclyl radical or some nortricyclene can be formed from the 5-norbornenyl radical by abstraction of hydrogen.<br /> 3. There is intervention of a "bridged" radical which may be for med reversibly or irreversibly from the 5-norbornenyl- and 2-nortricyclyl radicals.</p> <p>Within small error limits, the ratios of norbornene to nortricyclene as a function of the concentration of tri-n-butyltin hydride are consistent with the first mechanism.</p> <p>In the reductions with tri-n-butyltin deuteride, primary deuterium isotope effects of 2. 3 and 2. 1 for the abstraction of deuterium by the 2-nortricyclyl- and 5-norbornenyl radicals, respectively, were found. The primary deuterium isotope effects were invariant with the concentration of tri-n-butyltin deuteride, although the ratios of norbornene to nortricyclene changed appreciably over this range. This is consistent with the first mechanism, and can accommodate the formation of either product from more than one intermediate only if the primary kinetic deuterium isotope effects are nearly equal for all reactions leading to the single product.</p> <p>The reduction of endo-5-bromonorbornene-5, 6, 6-d<sub>3</sub> with tri-n-butyltin hydride or tri-n-butyltin deuteride leads to both unrearranged and rearranged norbornenes. The ratios of unrearranged to rearranged norbornene require that the 5-norbornenyl-5, 6, 6-d<sub>3</sub> radical isomerize to an intermediate with the symmetry expected of a nortricyclyl free radical. The results are consistent with mechanism 1, but imply a surprising normal secondary kinetic deuterium isotope effect of about 1.25 for the abstraction of hydrogen by the 5-norbornenyl- 5, 6, 6-d<sub>3</sub> radical.</p> <p>Approximate calculations show that there does not appear to be any substantial difference in the stabilities of the 5-norbornenyl and 2-nortricyclyl radicals.</p> <p>Although the results can not exclude a small contribution by a mechanism other than mechanism 1, no such contribution is required to adequately explain the results.</p>
author Firkins, John Lionel
spellingShingle Firkins, John Lionel
Norbornenyl- and Nortricyclyl Free Radicals
author_facet Firkins, John Lionel
author_sort Firkins, John Lionel
title Norbornenyl- and Nortricyclyl Free Radicals
title_short Norbornenyl- and Nortricyclyl Free Radicals
title_full Norbornenyl- and Nortricyclyl Free Radicals
title_fullStr Norbornenyl- and Nortricyclyl Free Radicals
title_full_unstemmed Norbornenyl- and Nortricyclyl Free Radicals
title_sort norbornenyl- and nortricyclyl free radicals
publishDate 1970
url https://thesis.library.caltech.edu/9070/1/Firkins_jl_1970.pdf
Firkins, John Lionel (1970) Norbornenyl- and Nortricyclyl Free Radicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JFE9-5888. https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886 <https://resolver.caltech.edu/CaltechTHESIS:07302015-142129886>
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