Unusual Intramolecular Kinetic Isotope Effects: Selectivity beyond Transition State Theory’s Jurisdiction

Abstract

The nature of non-statistical dynamic Kinetic Isotope Effects for a series of ordinary organic reactions is studied throughout this dissertation. A combination of experimental intramolecular KIEs using NMR methodology with computational calculations was employed in order to shed light about the selectivity in the Diels-Alder dimerization of methacrolein and cyclopentadienone, the iminium catalyzed Diels-Alder between cyclopentadiene and trans-cinnamaldehyde and the cleavage of alkoxy radicals.

Newtonian isotope effects as the underlying physical phenomena behind the KIEs in the thermal dimerization of methcrolein was studied. The temperature dependence of these novel form of KIEs was investigated by analyzing the isotopic distribution at dicyclopentadienone prepared at low and high temperatures. It was shown that Newtonian KIEs are temperature independent over a range of 100 degrees.

The non-statistical behavior in the cleavage of alkoxy radicals has been investigated. These highly reactive intermediates are prone to behave in a non-statistical fashion due to their excess energy. The opening of 1-methylcyclopentoxy radical has shown that heavy-atom tunneling can play an important role even at elevated temperatures. As the barrier for the cleavage for alkoxy radicals is decreased, other dynamic effects start to impact the selectivity of this reaction in ways that TST cannot account for.

On the iminium catalyzed Diels-Alder reaction between cyclopentadiene and transcinnamaldehyde, computational calculations support a two steps cycloaddition on the free energy surface and a “seemingly concerted” reaction in potential energy. The role of non-statistical recrossing for this cycloaddition was investigated experimentally by measuring intramolecular KIEs.