Investigation of the mechanisms of ene reactions using kinetic isotope effects

The ene reaction is one important type of pericyclic reaction in organic chemistry. To investigate the mechanisms of different ene reactions, a combination of natural-abundance kinetic isotope effect (KEE) measurements with ab initio calculations has been used, and new mechanisms have been established for a few ene reactions.;Through the use of a combination of experimental isotope effects and theoretical calculations a detailed experiment-based transition state geometry was established for the thermal ene reaction of allyl benzene with malefic anhydride.;Based on natural-abundance KIE measurements and the intermolecular KIE using d0/d12-2,3-dimethyl-2-butene, it is established that proton transfer is the rate-limiting step in the dialkylaluminum chloride-catalyzed ene reaction of formaldehyde, in contrast to previously reported results. Based on model calculations, it is proposed that the reaction proceeds through a reversible formation of a cation intermediate followed by rate-limiting proton abstraction.;A dynamic effect was used to resolve the conflicting data on the singlet oxygen ene reaction. Based on the energy surface from high-level theoretical calculations for this reaction, it is discovered that this reaction is a single step reaction. The transition state is perepoxide-like and therefore gives small symmetrical olefinic carbon isotope effects and insignificant allylic hydrogen effects. However, the perepoxide intermediate is not formed and ene products are formed from a valley-ridge inflection point.;Theoretical results have predicted a new mechanism for ene reactions of triazolinediones involving an open biradical as the key intermediate. This biradical may either form the ene product or reversibly form an intermediate aziridinium imide. The formation of the aziridinium imide would thus be a shunt off the main ene mechanistic pathway. A variety of observations associated with these reactions is found to support the new mechanism.;The allylic hydroxylation reaction of selenium dioxide has been studied. A concerted ene reaction with SeO2 as the active oxidant appears to be the major mechanistic pathway operative in these reactions.;Finally, experimental KIEs for the Diels-Alder reaction of PTAD with 2-tertbutyl-1,3-butadiene qualitatively support a synchronous process. However, a close examination of these data with the aid of theoretical calculations revealed a mixture of two highly asynchronous transition states.