Observation and Nature of Non-statistical Dynamic Effects in Ordinary Organic Reactions

Abstract

Statistical models like Transition State Theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) Theory have generally been successful in predicting the rates and selectivities of chemical reactions. However, these statistical models can fail to explain experimental results of ordinary organic reactions. For these reactions, consideration of nonstatistical dynamic effects or the detailed motion and momenta of the atoms is necessary to account for the experimental observations. Dynamic effects have been found to be important in a growing number of reactions and the nature of these effects can be varied.

One of the most interesting reactions investigated is the ozonolysis of vinyl ethers. Ozonolysis of a homologous series of vinyl ethers in solution exhibit experimental product ratios wherein the selectivity among cleavage pathways increases with the size of the alkyl group to an extent that is far less than RRKM theory would predict. Trajectory studies account for the observed selectivities and support a mechanism involving a competition between cleavage of the primary ozonide and intramolecular vibrational energy redistribution.

A recent theoretical study from our group predicted that a highly asynchronous organocatalytic Diels-Alder (DA) reaction, which is concerted in the potential energy surface, is stepwise in the free energy surface. Kinetic isotope effects (KIEs) were measured for three DA reactions. We envision that the entropic barrier may have several experimental consequences such as unusual isotope effects due to extensive recrossing. Preliminary results for the organocatalytic reaction show an intramolecular KIE close to unity that cannot be reconciled with statistical theories. This is in contrast with Lewis-acid catalyzed and thermal DA reactions, which exhibit substantial "normal" intramolecular KIEs that are in accord with TST predictions.

Finally, the Baeyer-Villiger oxidation of cylohexanone in water was investigated. KIEs were measured for the oxidation of cyclohexanone with peracetic acid and trifluoroperacetic acid. When using peracetic acid as the oxidant, the alkyl migration was determined to be the rate-determining step based on significant intermolecular KIEs on the carbonyl and alpha-methylene carbons. A change in the rate-determining step is seen when trifluoroperacetic acid is used. Only the carbonyl carbon exhibits a significant isotope effect. Theoretical predictions provide an experimental picture of the transition states and qualitatively support these conclusions.