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Palladium pi-allyl intermediates: The mechanism of elimination and the intramolecular ene reaction
Abstract
Research in the Takacs group has been focused on developing catalytic palladium mediated cyclization methodology for the purpose of natural product synthesis. Two mechanistic questions arose in the course of this research. The first is related to the mechanism of the elimination from a palladium $\pi$-allyl intermediate to form a 1,3-diene. The palladium-catalyzed eliminations of allylic acetates and allylic carbonates to form 1,3-dienes were examined. Three possible mechanistic pathways were examined. They are the commonly accepted $\beta$-hydride elimination, a specific base promoted elimination, and a general base promoted elimination. Through examination of deuterium isotope effects and the stereospecificity of the elimination reaction it has been concluded that the palladium-catalyzed elimination of allylic carbonates proceeds via the general base promoted pathway. The second question arose from an observation made in the Takacs group. It was noticed if a $\alpha,\beta,\gamma,\delta$-unsaturated ester tethered to another diene was subjected to a palladium catalyst in THF and 5.0 equivalents of acetic acid the major product was a bicyclic carbocycle. This bicyclic product could arise from the typical palladium-catalyzed cyclization reaction developed in the Takacs group followed by an intramolecular palladium-ene reaction. This led to the examination of the intramolecular palladium-ene reaction with an $\gamma$-acetoxy-$\alpha,\beta$-unsaturated ester tethered to an alkene. Using this substrate, an investigation was undertaken in search of cyclization conditions milder than those previously employed. Although we were unsuccessful in our investigation some understanding of the examples in the literature came from this study. The optimal conditions for the intramolecular palladium-ene reaction were found to be dependent on the presence or absence of substituents capable of coordinating palladium. A study of rhodium-catalyzed tetraene carbocyclizations has also been completed and published in Organometallics 1994, vol. 13, pages 44787-4793. A study of enantioselective room temperature Diels-Alder reactions employing chiral bis(oxazoline)-zinc, -magnesium, and -copper triflate catalysts was also completed. This latter work has been submitted for publication. These two studies will not be discussed herein.
Subject Area
Organic chemistry
Recommended Citation
Lawson, Edward Charles, "Palladium pi-allyl intermediates: The mechanism of elimination and the intramolecular ene reaction" (1997). ETD collection for University of Nebraska-Lincoln. AAI9734623.
https://digitalcommons.unl.edu/dissertations/AAI9734623