Date of Award
1996
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Abstract
The irradiation of cyclic enones with ultraviolet light in the presence of alkenes results in the formation of cyclobutane adducts possessing head-to-head and/or head-to-tail regiochemistry. The long-accepted Corey-de Mayo mechanism predicts that the regiochemistry of the cyclobutane adducts is controlled by the relative orientation of the enone and alkene in an exciplex intermediate. Weedon and co-workers have demonstrated that partitioning of 1,4-biradical intermediates between starting material and cyclobutane products controls the product regiochemistry in the intermolecular photocycloaddition reactions of 2-cyclopentenones with alkenes.;These earlier results have been extended. The 1,4-biradical intermediates formed in the photocycloaddition reactions of 2-cyclohexenone with alkenes, in the photocycloaddition reactions of 2-cyclopentenone and 2-cyclohexenone with allene, and in the photodimerization reaction of 2-cyclopentenone were trapped by hydrogen selenide. The relative yields and the structures of the trapped biradicals were used to deduce the structures and relative rates of formation of the biradical intermediates from which the trapped products originated. For the systems studied the regiochemistry of the cyclobutane adducts is not controlled by the relative rates of formation of their biradical precursors but rather by the manner in which they partition between closure to cycloadducts and reversion to starting materials. This finding is in disagreement with the Corey-de Mayo exciplex mechanism and helps to generalize the ideas proposed by Weedon and co-workers on the factors controlling the regiochemical outcome of enone-alkene photocycloaddition reactions.;Ultraviolet irradiation of cyclic enones possessing a tethered alkene substituent often proceeds to yield intramolecular cyclobutane adducts with a high degree of regioselectivity. The regiochemical outcome in these reactions can in many cases be predicted by the "rule of five" which states that the dominant products are those which could have been formed via a 1,4-biradical intermediate in which a 5-membered ring has been produced. In addition, it has been argued that reversion of the 1,4-biradical intermediates to ground state starting materials is an insignificant process in these systems. Despite the success of the rule of five for predicting the regiochemistry of the cycloadducts in these reactions, there has been no unambiguous evidence reported that confirms that the only important biradical intermediates involved are indeed those in which a five-membered ring has been formed.;In the work described in this thesis, the triplet 1,4-biradical intermediates formed in the intramolecular photocycloaddition reactions of 3-(4{dollar}\sp\prime{dollar}-pentenyl)-2-cyclopentenone, 3-(4{dollar}\sp\prime{dollar}-pentenyl)-2-cyclohexenone, 3-(3{dollar}\sp\prime{dollar}-butenyl)-2-cyclohexenone, 3-(5{dollar}\sp\prime{dollar}-hexenyl)-2-cyclohexenone and 3-allyloxy-2-cyclohexenone were trapped by hydrogen selenide. The biradical intermediates involved in the photocycloaddition reaction of carvone could not be trapped by this reagent. The structures and relative yields of the trapped 1,4-biradicals were used to deduce the structures and relative rates of formation of the biradical intermediates from which the trapped products originated. The results confirm the rule of five, and that the dominant 1,4-biradical intermediates produced are those in which a 5-membered ring has been formed. The reaction regiochemistry is determined by the rates of formation of the biradical intermediates rather than by the partitioning of the biradicals between starting material and cyclobutane adducts. This is opposite to the factors which control the regiochemical outcome in intermolecular photocycloaddition reactions. (Abstract shortened by UMI.)
Recommended Citation
Maradyn, David Jack, "1,4-biradicals In Intermolecular Photocycloaddition Reactions Of Cyclic Enones With Alkenes, And, 1,4-biradicals In Intramolecular Photocycloaddition Reactions Of Cyclic Enones With Alkenes" (1996). Digitized Theses. 2620.
https://ir.lib.uwo.ca/digitizedtheses/2620